The impact of pregnancy and associated hormones on the pharmacokinetics of Δ9-tetrahydrocannabinol.

THC undergoes a series of phase I oxidations and is sequentially metabolized by both phase I and II enzymes ( Figure 1 ). Over 80 metabolites have been identified in humans and in different preclinical species [ 70 ], but only some are detectable in circulation, are pharmacologically active, and/or are markers for usage levels. THC is mainly metabolized to pharmacologically active metabolite, 11-OH-THC by cytochrome P450 (CYP) 2C9 [ 71 – 73 ]. The fraction metabolized (f m ) of THC by CYP2C9 was assigned as 0.91 based on results of an in vitro study of THC depletion in human liver microsomes in the presence of bovine serum albumin (BSA) [ 74 ]. Following THC administration, 11-OH-THC circulates at concentrations that exhibit a pharmacological/psychoactive effect [ 23 , 75 ]. 11-OH-THC is sequentially metabolized to the inactive metabolite, (−)-11-nor-Δ 9 -carboxy-THC (11-COOH-THC) which undergoes glucuronidation to form (−)-11-nor-Δ 9 -THC-9-carboxylic acid glucuronide (11-COOH-THC glucuronide) ( Figure 1 ). Both 11-COOH-THC and its glucuronide are found in plasma and urine after THC administration. Despite being pharmacologically inactive, 11-COOH-THC and its glucuronide metabolite are frequently used as qualitative and quantitative markers for cannabis use [ 60 , 69 , 76 ]. Other hydroxylation pathways of THC include the formation of 8β-hydroxy-THC (8β-OH-THC) and 9α,10α-epoxyhexahydrocannabinol (9α,10α-EHHC [ 71 , 76 ]. The 8β-OH-THC and 9α,10α-EHHC metabolites are formed by CYP3A4 in human liver microsomes [ 71 , 77 ]. At high doses in rhesus monkeys, both 8β-OH-THC and 9α,10α-EHHC have been shown to have a similar pharmacodynamic effect as THC [ 2 ], but following administration of THC, the plasma 8β-OH-THC and 9α,10α-EHHC exposures are too minor to contribute to pharmacodynamic activity [ 2 ]. In vitro , 11-OH-THC is rapidly metabolized in human liver microsomes, cytosolic, and S9 fractions to form 11-COOH-THC [ 78 ]. 11-COOH-THC formation is greatest in the presence of cytosolic and microsomal alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), aldehyde oxidase (AO), and CYP2C9 enzymes [ 78 ]. 11-COOH-THC is glucuronidated to form 11-COOH-THC glucuronide [ 79 ]. In vitro experiments with individually expressed recombinant human uridine 5’-diphosphoglucuronosyltransferases (UGTs) showed that UGT1A3 had the greatest intrinsic clearance (CL int ) (10.0 μL/min/mg protein) toward 11-COOH-THC glucuronidation while UGT1A1 had minor affinity and thus lower CL int (1.29 μL/min/mg protein) [ 79 ]. 11-OH-THC can also be glucuronidated by UGT1A10, UGT1A9, and UGT2B7 contributing to 11-OH-THC depletion [ 74 , 80 ]. After THC administration, these glucuronide conjugates of 11-OH-THC have also been identified in feces and bile [ 80 , 81 ]. THC disposition following intravenous (iv) administration exhibits complex compartmental kinetics influenced by the interplay between THC clearance (CL) and distribution. THC is highly lipophilic and distributes extensively to peripheral tissues ( Table 1 ). The reported PK parameters show large inter-study variability likely due to different bioanalytical methods used and different sampling duration. Assessment of the true PK characteristics of THC and its metabolites requires meticulous review of the published work. Overall, 13 studies have been published to date describing THC and its metabolites’ disposition after iv administration of THC ( Table 2 ). These datasets provide an opportunity to evaluate the disposition characteristics and PK parameters of THC. Following iv administration, THC is predominantly eliminated via metabolism with minimal renal or biliary excretion of unchanged drug. After iv administration, less than 1% of THC was found unchanged in the urine or feces following 4 days sample collection, respectively (approximately 3–4 half-lives of THC) [ 97 ]. Following iv administration of radiolabeled THC, approximately a quarter of the total radioactivity was found in urine and half was recovered in feces [ 23 , 97 , 103 ]. In urine, overall conjugated metabolites are more abundant than unconjugated [ 96 , 101 ]. The major metabolites of THC found in urine are 11-COOH-THC glucuronide and 11-COOH-THC [ 96 , 101 ]. 11-OH-THC was excreted in the urine after THC administration but constituted a minor fraction of the total dose recovered in urine (5%) [ 23 ]. After iv administration of radiolabeled THC and 7-day collection of samples, 20% of the total dose was found as 11-OH-THC in feces [ 23 ]. In contrast to urine, the conjugated fraction in feces was nominal in comparison to the unconjugated. However, it is not known whether the metabolites are biliary secreted as unconjugated compounds or as glucuronide or sulfate conjugates that are then hydrolyzed in the gut. In deceased individuals with positive urine detection of cannabinoids, THC and its metabolites were also quantified in bile [ 81 ]. 11-COOH-THC glucuronide was in the greatest concentration, on average 18-fold greater than 11-COOH-THC and 33-fold greater than 11-OH-THC [ 81 ]. It is possible that metabolites are excreted to the bile as glucuronide conjugates and subsequently undergo deconjugation by gut bacteria. Enterohepatic recycling may also contribute to THC and its metabolite disposition, and all the conjugates may be excreted to the bile and then reabsorbed from the GI tract in the unconjugated form. Capturing enterohepatic recirculation in plasma concentration-time profiles is, however, complex due to the sampling conditions and inter-individual variability. In dogs following a laparotomy procedure, enterohepatic recirculation of iv administered THC was quantified [ 104 ]. Approximately 10–15% of the dose underwent enterohepatic recirculation [ 104 ]. Overall THC is predominately cleared by hepatic metabolism and THC has a high hepatic (metabolic) CL and extraction ratio (0.83, Table 1 ). After iv administration the CL is largely limited by hepatic blood flow. The blood-to-plasma (B:P) ratio of THC is 0.67, suggesting minimal partitioning of THC to blood cells [ 90 ], and making THC hepatic CL essentially plasma flow limited. Based on multiple studies that measured THC plasma concentrations beyond initial distribution phases (4–6 hours) and that used chromatographic separation methods that ensure separation of THC from its metabolites, THC plasma CL is approximately 50 L/h (blood CL 75 L/h, Table 1 ). THC is highly lipophilic and is extensively distributed to tissues and bound to plasma proteins with the estimated plasma unbound fraction (f u,p ) for THC being 1.1% and the distribution being sensitive to f u,p [ 74 , 105 ]. In plasma, approximately 63% of THC is bound to lipoproteins with the remaining fraction bound to albumin [ 106 ]. THC plasma concentration-time curve following iv administration is triphasic. The initial distribution is rapid with plasma concentrations declining. Within 30 minutes of iv administration, the plasma concentration-time profile begins to plateau toward the next phase of distribution [ 93 , 94 ]. This next phase of distribution most likely describes the distribution to slowly perfused tissues. The final elimination phase is reached within 8 h. The terminal half-life of THC is long, approximately 23 h due to the extensive distribution to tissues ( Table 1 ). The extensive distribution to peripheral tissues results in large steady-state volume of distribution (V ss ), ~470 L, which is also sensitive to adiposity [ 100 , 105 ]. In vitro THC, 11-OH-THC, and 11-COOH-THC bind to intracellular fatty acid binding proteins (FABP) [ 107 , 108 ], and it is likely FABPs also bind THC in vivo . FABPs are ubiquitously expressed and could influence the rate and extent of THC distribution [ 107 , 108 ]. THC is also a proposed substrate of efflux transporters P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) [ 109 , 110 ]. In one study with P-gp and Bcrp knockout mice, the THC brain/blood concentration ratio was higher in the knockout mice than that in wild-type mice at specific time points after intraperitoneal administration [ 109 ]. In another study the corresponding AUC ratio was significantly smaller in P-gp knockout mice (72% average decrease) but not in Bcrp knockout mice [ 111 ]. In Madin-Darby canine kidney-II (MDCK-II) cells expressing human P-gp or BCRP, THC was not a specific substrate or inhibitor for either efflux transporter [ 112 ]. This suggests P-gp and BCRP are not important for THC transport in humans. However, unbound THC was not measured and nonspecific binding to cells and plastics could confound the measurements. Further evaluation of THC transport is essential as it could impact intestinal absorption and brain, liver and placenta exposure, and biliary secretion of THC and its metabolites. Distribution data following tracer doses of radioactive THC in preclinical species exemplify the extensive distribution and slow redistribution of THC from peripheral tissues [ 113 – 115 ]. In samples from animals and from a postmortem individual, THC is extensively distributed to adipose [ 113 , 114 , 116 ]. In large white pigs following iv administration of THC, distribution of THC was quantified in different tissues including the adipose, liver, and brain [ 113 ]. Within 30 minutes of dosing, THC was quantifiable in all three tissues with the greatest amount per gram of tissue in liver, then adipose, and then the brain. THC was quickly eliminated from the liver and after 6 h was no longer detectable in the liver but continued to be slowly redistributed from the brain and adipose. Tissue distribution in a postmortem human was also assessed [ 114 ]. In this study, concentration of THC was greatest in adipose ( n = 1). In humans, 11-OH-THC and 11-COOH-THC circulate at appreciable concentrations after iv dosing of THC. 11-OH-THC has similar potency as THC toward CB receptors and its plasma concentrations are high enough to contribute to pharmacological activity [ 23 ]. A unique aspect of THC metabolites is that both 11-OH-THC and 11-COOH-THC have been iv administered to humans, providing information about the PK characteristics of these two metabolites. A single study of 11-OH-THC PK after iv administration of 11-OH-THC was conducted in three male volunteers [ 23 ]. This study measured plasma concentrations of 11-OH-THC via thin layer chromatography (TLC) and liquid scintillation counting [ 23 ]. The majority of 11-OH-THC was cleared via metabolism with 5% of the iv-dose excreted unchanged in urine and up to 20% of the dose recovered unchanged in feces [ 23 ]. This supports the role of biliary secretion as a CL pathway for 11-OH-THC [ 23 ]. The B:P ratio of 11-OH-THC is 0.63 demonstrating similar lack of partitioning to red blood cells as THC [ 90 ]. The plasma CL of 11-OH-THC was 54 L/h (blood CL 85 L/h) demonstrating that 11-OH-THC has similarly high extraction ratio (ER = 0.95) as THC. 11-OH-THC is highly bound to plasma proteins with an f u,p 1.2% [ 74 , 105 ]. Similar to THC, 11-OH-THC is extensively distributed to peripheral tissues resulting in a large V ss (670 L). Similar to THC, 11-OH-THC followed 3-compartmental kinetics and is also extensively distributed to peripheral tissues like the brain and adipose [ 23 , 60 , 115 ]. The terminal elimination half-life of 11-OH-THC was 14.5 h when samples were collected for 72 h [ 23 ]. This half-life is shorter than that of THC, suggesting that 11-OH-THC will follow formation rate-limited kinetics as a metabolite of THC. Following iv administration of 11-COOH-THC to 12 male, Caucasian volunteers, blood and urine samples were collected for 96 h [ 89 ]. Unlike THC and 11-OH-THC, 11-COOH-THC has a low CL. The plasma CL of 11-COOH-THC was 5.5 L/h (blood CL 8.5 L/h). Less than 1% of 11-COOH-THC was excreted in the urine unchanged and the renal CL of 11-COOH-THC (0.0082 L/h) was lower than the overall total body CL [ 89 ]. The lower CL of 11-COOH-THC explains the high AUC of 11-COOH-THC in comparison to THC and 11-OH-THC following THC dosing. The distribution of 11-COOH-THC is also more restricted than THC and 11-OH-THC likely due to the charged and more polar nature of 11-COOH-THC ( Table 1 ) and it has lower distribution volume than THC and 11-OH-THC. In comparison to THC and 11-OH-THC, 11-COOH-THC is not as extensively bound to plasma proteins (f u,p = 8%) [ 91 ]. The V ss of 11-COOH-THC was 97.5 L [ 89 ]. Still, 11-COOH-THC also followed 3-compartmental kinetics and was extensively distributed to peripheral tissues [ 90 ]. The terminal elimination half-life of 11-COOH-THC was shorter than that of THC or 11-OH-THC, 18 h [ 89 ]. Hence, 11-COOH-THC disposition is also likely to be characterized by formation rate limited kinetics following dosing of THC. Despite having a minor renal CL, 11-COOH-THC is readily detected in the urine of frequent users and has been used as a quantitative and qualitative marker of THC usage [ 117 ]. Following iv administration of THC, 11-OH-THC area under the plasma concentration-versus time curve (AUC) is readily measured in plasma [ 75 ]. The formation of 11-OH-THC from THC predominately occurs by metabolism by CYP2C9 [ 71 , 73 , 74 ]. The in vitro CYP2C9 f m of 11-OH-THC was proposed to be 0.91 [ 74 ]. However, characterization of the PK of THC in human participants with different CYP2C9 genotypes suggested that the in vivo f m by CYP2C9 is between 0.75 and 0.8 [ 73 ]. Fraction metabolized can be estimated from the ratio of THC AUC in CYP2C9 poor metabolizers and extensive metabolizers. CYP2C9 polymorphism (CYP2C9* 3) that results in poor metabolizer phenotype does not cause complete ablation of protein and thus it is assumed that ~ 10% activity remains. In CYP2C9 poor metabolizers (CYP2C9*3/*3) the AUC of THC increased by 2.97-fold when compared to extensive metabolizers suggesting that individuals with CYP2C9 polymorphisms will likely experience higher exposure to THC after consumption of typical doses of cannabis. Another approach to assess f m is to use selective inhibitors of the enzymes of interest. THC exposure has been assessed following nasal mucosal spray formulation in the presence and absence of ketoconazole treatment (a strong inhibitor of CYP3A and a weak inhibitor of several other CYPs and transporters) [ 118 ]. THC exposure increased 1.8-fold in the presence of ketoconazole when compared to the control day [ 118 ]. This suggests that CYP3A4 likely contributes approximately 30% of THC elimination but interpreting specific in vivo CYP f m ’s is challenging due to the nonspecific inhibition of multiple pathways by ketoconazole. CYP2C19 inhibition with omeprazole did not impact THC disposition in vivo [ 118 ]. Cannabis products are typically used by inhalation (e.g. smoking or vaping) or orally in the form of edibles or as synthetic THC, Dronabinol. Pregnant individuals often use THC by smoking or vaping and less by oral route [ 20 ]. Oral route of administration results in more consistent exposures of THC than smoking, but the dose normalized AUC of THC is lower following oral consumption than after smoking [ 96 ]. Since THC is a high extraction ratio drug with high first pass metabolism and 11-OH-THC is formed from THC during first pass, the 11-OH-THC-to-THC AUC ratio is greater following oral administration than after iv administration. Due to this, 11-OH-THC constitutes a larger portion of circulating psychoactive cannabinoids following oral dosing than after smoking or vaping of THC. In fact, plasma AUC ratios of 11-OH-THC-to-THC after smoking have been reported to be as low as 1:20 [ 119 ], while after oral administration, this ratio was reported to increase to 1:1–1:2 [ 80 , 120 ]. THC is detectable in plasma within a minute after smoking and peak concentrations are observed as early as 3–10 min after smoking [ 121 ]. In subjects smoking THC by rolled cigarette, THC bioavailability in heavy cannabis users was on average 23%, while the bioavailability in light users was lower, 10% on average ( Table 1 ) [ 92 ]. This difference in bioavailability is attributed to smoking experience and varies according to depth of inhalation, puff duration, and breath-hold. Portions of smoked THC are lost in mainstream smoke (16–19%), pyrolysis (30%), and variable amounts are left in the cigarette butt [ 122 ]. In a controlled clinical study assessing the PK of inhaled THC, the average bioavailability after inhalation from a nebulizer was 28.7% [ 75 ]. One potential reason for the low bioavailability is metabolism in the lung. However, in human lung microsomes there was no notable depletion of THC [ 127 ]. Yet it is possible that this does not fully reflect the metabolism in lungs of heavy smokers as the control activity toward EROD in the human lung microsomes was very low when compared to human liver microsomes. The donors were smokers in the last 10 years, but they may not have been active smokers at the time of death. This may have resulted in lower CYP1A1 activity than what would be observed in the lungs of cannabis smokers as CYP1A1 is induced by smoking [ 123 ]. After oral administration, the oral bioavailability of THC is approximately 12% or less ( Table 1 ) [ 101 ]. The time to peak plasma concentrations (t max ) is variable with a range of 1–4 h and an average of 2 h [ 4 , 73 , 95 , 120 , 124 , 125 ]. There is a notable food effect with oral administration of THC. In comparison to fasted state, THC t max increased and apparent volume of distribution (V z /F) and oral CL (CL/F) decreased when oral doses of THC were consumed after a high-fat meal [ 120 , 125 ]. In the fed state, THC AUC increases 1.9- to 2.6-fold in comparison to fasted state suggesting an increase in extent of absorption. Based on PK calculations, in the fasted state, 50–60% of THC is absorbed from the GI tract while in the fed state approximately 90–95% is absorbed from the gut lumen [ 4 , 120 , 125 ]. This difference in the fraction absorbed from the gut lumen (F A ) is likely the reason for the food effect in THC exposure. It is difficult to deconvolute the specific contribution of absorption and gut metabolism in vivo to THC bioavailability. In intestinal microsomes there was notable depletion of both THC and 11-OH-THC suggesting some gut wall metabolism after oral dosing [ 126 ]. The predicted intestinal metabolism of THC was, however, minor in comparison to its hepatic CL [ 126 ], a finding consistent with the major contribution of CYP2C9 to THC CL and the lower expression of CYP2C9 in the intestine when compared to the liver. Studies assessing THC PK in humans have been predominately conducted in males while limited studies included female participants ( Table 1 ). Due to this, the information on potential sex differences in the PK of THC is very limited. THC distribution has been shown to be sensitive to body weight. On average, females have a smaller body weight than males and thus smaller volume of distribution of THC when compared to males [ 120 , 127 ]. After normalizing distribution volume to total body weight, there is no significant sex difference in THC volume of distribution [ 120 , 127 ]. No sex difference was observed in THC CL following iv administration of THC to six women taking oral contraceptives for at least 6 months and six men with sampling duration up to 48 h [ 80 ]. In the same study, THC disposition was also assessed following oral dosing of THC in sesame oil solution administered in soft gelatin capsule [ 80 ]. No sex difference was observed following the oral dosing either. Notably, the doses administered to females were lower than those given to male participants as the dosing was normalized to body weight. Similar lack of a sex difference was found in a study that assessed the food effect on orally administered THC capsules (5 and 10 mg administration to 13 women and 15 men) and administered the same dose to females and males [ 120 ]. Sex difference was only detected in C max and only in one occasion (females had greater C max than males) [ 120 ]. No other sex differences were observed in the other three treatment occasions nor in any PK parameters. In contrast, a sex difference was detected in a study in which THC or cannabis extracts were administered [ 128 ]. THC and 11-OH-THC exposures were greater in females ( n = 12) when compared to males ( n = 12) [ 128 ] suggesting a lower oral CL of THC in females. However, in this study same dose was given to females and males. The lower oral CL could be due to lower first pass metabolism or higher F A of THC in females or a lower CL in females due to lower body weight. This study may have been limited by the 9–24 h sampling duration which likely does not fully capture the terminal elimination phase of THC. As described in Figure 2 , different sampling durations result in different CL values if different terminal slopes are identified. The differences between the studies may be due to different formulations, dosing strategies in relation to food and different dose normalization strategies. It is possible that absorption processes are important contributors to the sexual dimorphisms in THC disposition. Further studies are needed to assess sex differences in THC disposition in humans especially in light of extrapolating the findings of sex differences and PK findings in females to predictions of THC PK in pregnancy. In rats, sex differences in the metabolism of THC contribute to sex differences observed in the behavioral effects of THC. In male rats, THC is predominately metabolized by the male specific Cyp, Cyp2c11, the homolog of human CYP2C9 [ 129 ], while the metabolism in female rats is less well characterized (females do not express Cyp2c11). Following injection of THC, females have greater plasma concentration of 11-OH-THC [ 130 ]. This sex difference in 11-OH-THC production is modulated by gonadal hormones [ 131 ], observed in adolescent as well as adult rats, and increases in magnitude with repeated THC treatment [ 130 ]. Greater production of 11-OH-THC by females appears to be due to different Cyp expression in the female vs male rat liver, such that females metabolize THC primarily to 11-OH-THC, whereas male rats produce less 11-OH-THC [ 132 , 133 ]. Sex differences in liver enzymes involved in THC metabolism have also been documented in mice [ 134 ]. These sex differences in rodents should not be extrapolated to humans as humans do not recapitulate the sex specific Cyp expression and rodent and human Cyps may form different metabolites confounding assessment of sex differences.

Across the globe there has been a notable increase in cannabis use and in the THC content of cannabis products. With the legalization in the US, and with the overall global change in culture around cannabis products, young populations and pregnant individuals use more of these products now when compared to previous generations. This has raised concerns about the fetal safety and potential effects of cannabis use on fetal development. However, due to legal concerns it has been exceedingly difficult to define cannabis exposures and usage patterns during pregnancy in different groups of pregnant individuals. For example, it is likely that individuals who consume cannabis for recreational purposes have different usage patterns and exposures than individuals who take cannabis for medicinal purposes. In both cases, the dose-effect relationship is paramount for driving usage levels as users likely self-titrate doses to the desired effect. Altered consumption patterns may alter fetal safety and methods of assessing cannabis use from presence of usage markers. Altered disposition of cannabinoids during pregnancy may also lead to effects that cannot be predicted from non-pregnant individuals due to the complex changes in the entire cannabis metabolome. This review strongly suggests that THC and its metabolite disposition is altered during pregnancy due to multiple mechanisms. Of interest are the predicted changes in THC distribution that will likely alter the time course and pharmacodynamic effects of THC. Physiological changes during pregnancy also likely alter THC CL. There are multiple physiological and hormonal changes that occur during pregnancy that can result in altered disposition of THC and its main metabolites. Of special interest to cannabinoid metabolism and overall drug disposition in pregnancy are the increased concentrations of hormones that can exert an effect on the expression of CYPs and UGTs commonly involved in THC CL. Increased maternal hormonal concentrations are potentially responsible for the altered hepatic and extrahepatic drug metabolism during pregnancy. Over the last decades, in vitro studies have suggested that pregnancy hormones (growth hormones, estradiol, progesterone, testosterone, and cortisol) regulate DMEs through interactions mediated by their canonical nuclear receptors and the orphan nuclear receptors. Improved understanding of the mechanism of induction of DMEs has advanced our ability to predict the inductive effect of hormones. However, limited studies have explored the impact of pregnancy hormones on PD of psychoactive drugs such as cannabinoids. Future studies are needed to evaluate how the dose-exposure-effect relationships of THC and its metabolites are affected during pregnancy and how such changes drive the usage patterns in pregnant individuals. The use of cannabis during pregnancy likely impacts the ECS, but limited studies have evaluated how the endocannabinoid signaling and concentrations are altered during human pregnancy. This is an important area of future research as it is likely that hormonal changes that alter THC disposition also play a role in modulating endocannabinoid concentrations. Altered endocannabinoid concentrations or signaling may have effects not only on fetal development but on maternal health as well. In this review, the PK characteristics of THC and its metabolites were reviewed in detail. However, it is important to consider the span of decades during which the PK data for THC and its metabolites have been collected. During this time window not only have cannabis products changed but also bioanalytical methods and PK analysis software have dramatically advanced. Hence, it is not surprising that results from different PK studies are somewhat discrepant. Previously used methods included high-performance liquid chromatography (HPLC) or thin layer chromatography (TLC) combined with liquid scintillation or gas chromatography-mass spectrometry (GC-MS). TLC is not a robust method for separating compounds from complex matrices like plasma due to the lack of specificity and separation power. Thus, if multiple analytes are incorporated in the same signal, the analyte concentration would be over-predicted together with an over-prediction in the AUC and under prediction of CL. This is exemplified in Table 2 , where the combination of TLC with liquid-scintillation leads to a drastically lower plasma CL value (0.76–15 L/h) when compared to GC-MS (36–59 L/h). Sampling duration is also an important consideration when evaluating THC disposition, CL, and V ss . A critical assumption to determine CL and half-life is that the sampling duration is long enough to capture the terminal phase. As described in Figure 2 , shorter sampling durations result in incomplete concentration-time profiles and thus overestimated CL values. Due to this, for our analysis CL and V ss were not assessed for concentration-time profiles that did not account for all phases (sampled for less than 8 h). After GC-MS analysis with an 8 h sampling window, plasma CL values of 47 ± 24 L/h were determined while in 2 studies CL were reported to be 57 ± 12 L/h and 59 ± 9 L/h ( Table 2 ). After iv administration, THC V ss is also impacted by analysis method ( Table 2 ). The determined V SS is on average greater in studies that used TLC analysis method when compared to those using GC-MS. Due to the many historical challenges in analysis of THC and its metabolites modern studies should be undertaken to define the plasma PK of THC, 11-OH-THC and 11-COOH-THC. This would be critical to enable fully capitalizing on the modern computational power and pharmacokinetic simulations. Such simulations are especially relevant for THC as there are limited possibilities for systematic analysis of THC PK in different sensitive populations such as pregnant individuals. The differences in THC metabolome between routes of administration have remained mechanistically unexplained in the literature. THC PK is sensitive to the route of administration. For example, the exposure of 11-OH-THC in relation to THC after oral administration is greater when compared to inhalation or iv dosing. This can impact the pharmacodynamic effects due to the contribution of 11-OH-THC as a psychoactive metabolite. In addition, as a high extraction ratio drug, THC is more sensitive to changes in hepatic blood flow than to increased hepatic DME activity after smoking or vaping. Yet, after oral ingestion, metabolism of THC is sensitive to altered intrinsic CL and thus changes in DME expression and activity. Altered DME activity will, however, likely impact the disposition of 11-OH-THC and 11-COOH-THC following any route of administration. THC metabolism and pharmacokinetics is complex. More detailed clinical epidemiological studies in humans during pregnancy and thorough in vitro studies of THC metabolism and induction of the enzymes are required to improve prediction of THC PK in special populations like pregnant individuals and to better assess THC safety during pregnancy. Such predictions can also be used to guide animal toxicological studies to assure exposures are relevant to human clinical situations. Better predictions and understanding of the dose-exposure-effect relationships can ultimately help provide better information to pregnant individuals about the risks of cannabis consumption during pregnancy.

There have been several reviews that detail pregnancy mediated physiological changes and how they may influence drug disposition [ 135 – 140 ]. Common physiological changes that occur during pregnancy that could lead to altered THC metabolome include increased cardiac output and hepatic blood flow, increased adiposity, and altered concentrations of plasma proteins. Along with these physiological changes, there are drastic changes in hormone concentrations during pregnancy that are often altered in a trimester dependent manner ( Table 3 ). In vitro , these ‘pregnancy hormones’ have been shown to induce different drug metabolizing enzymes (DMEs) including the CYPs and UGTs of interest for THC metabolism ( Table 4 ). Cardiac output is the volumetric flow rate of the heart’s pumping output and is influenced by stroke volume and heart rate. It is an important physiological function to understand as it determines the rate at which organs are perfused by blood and has a direct impact on organ CL and rate of distribution. As hepatic CL of high extraction ratio drugs such as THC is largely blood flow limited, changes in cardiac output and hepatic blood flow will have a direct impact on THC CL. Cardiac output increases steadily throughout gestation [ 155 – 157 ] with significant increases in the third trimester [ 155 ]. There are conflicting reports if this increase in cardiac output also leads to an increase in hepatic blood flow [ 155 – 158 ]. Following iv administration of nicotine, a high extraction ratio drug, the CL of nicotine increased up to 1.6-fold when compared to postpartum values suggesting that hepatic blood flow is increased [ 159 ]. Similar trend of increased CL was noted with iv administration of morphine at delivery [ 160 ]. Although it is difficult to determine the extent to which hepatic blood flow is altered during pregnancy, these data suggest that the increase in cardiac output during pregnancy does increase liver blood flow which in turn would increase THC CL during pregnancy. Plasma protein concentrations are altered during pregnancy often leading to changes in plasma protein binding of drugs. Changes in plasma protein binding will likely also alter the oral CL of THC, THC distribution kinetics, and the disposition of THC metabolites following any route of THC administration. THC and its metabolites, 11-OH-THC and 11-COOH-THC, are highly bound to plasma proteins, specifically albumin ( Table 1 ). THC has also been shown to be bound to both albumin and lipoproteins [ 107 ]. Albumin concentrations decrease during the 2 nd trimester and continue to decline throughout pregnancy reaching 70–80% of normal values at time of delivery [ 161 ]. Lipoproteins, on the other hand, increase gradually throughout pregnancy [ 162 , 163 ]. In one study, an average increase of 1.41-fold in lipoproteins was observed in the 3 rd trimester (323 ± 42 mg/dL) when compared to postpartum (229 ± 47 mg/dL) [ 163 ]. The extent of change in THC and metabolite plasma protein binding is dependent on the ratios of changes in albumin and lipoproteins and the affinities of the cannabinoids to these proteins. However, at present, the protein binding of THC and its metabolites during pregnancy has not been measured. THC and its metabolites are extensively distributed to peripheral tissues, specifically adipose. Gestational weight gain is highly variable and is influenced by factors such as multiple versus singlet pregnancy [ 164 ], maternal age [ 164 ], and gestational conditions such as preeclampsia [ 165 ]. During a normal, singlet pregnancy, the gestational weight gain is generally higher in the 2 nd and 3 rd trimester and can vary with maternal ethnicity and age [ 164 ]. Most of the increase in total body fat occurs during the 2 nd trimester with minor changes in 1 st and 3 rd trimesters [ 166 ]. In a meta-analysis of adipose changes in singlet pregnancies with no comorbidities, a steady increase in fat mass with the most notable changes at trimester 2 and 3 was observed [ 138 ]. Due to the lipophilic nature of THC and 11-OH-THC and the extensive distribution of the cannabinoids to adipose, the increase in adipose during pregnancy will likely increase THC and 11-OH-THC distribution volume. Biliary secretion and kidney function are important for THC metabolite CL. Gallbladder emptying has been reported to be incomplete and slower during pregnancy. The gallbladder emptying rate was decreased 53% during pregnancy when compared to non-pregnant individuals (0.022 ± 0.003 min −1 versus 0.041 ± 0.006 min −1 , respectively) [ 167 ]. These results were impacted by the gestational age of the pregnant individuals ranging from 8 to 36 weeks. In the 3 rd trimester, fasting and postprandial gallbladder volume increased when compared to postpartum values [ 168 ]. However, the ejection fraction decreased during pregnancy when compared to postpartum values (60.56 ± 18.80% versus 77.48 ± 13.37%) [ 168 ]. Based on these changes in gallbladder function, it is possible that the biliary secretion of THC and its metabolites is differentially affected during pregnancy. Renal CL depends on plasma protein binding, glomerular filtration rate (GFR), active tubular secretion, and/or reabsorption. GFR increased by 1.5-fold in the 1 st trimester and continued to increase throughout pregnancy when compared to postpartum values [ 169 ]. Filtration CL can also increase due to increased plasma unbound fraction and increased kidney blood flow. The effective renal plasma flow increases 80% by the 2 nd trimester and then decreases during the 3 rd trimester [ 170 ]. The effect of pregnancy on tubular secretion and expression of drug transporters has unfortunately not been well characterized and impact of pregnancy on passive reabsorption in the kidney is unknown. However, as renal CL is a minor pathway of THC elimination, it is unlikely that these changes are important for THC disposition. However, altered renal CL of 11-COOH-THC during pregnancy could impact assessment of THC exposure from urine drug screens. It is challenging to complete controlled clinical studies in pregnant individuals due to both ethical limitations and logistical challenges. Thus, most clinical studies completed in pregnant individuals have been opportunistic, i.e. studying drugs already prescribed for medical reasons, and have not used typical probe substrates to assess DME activity. This can complicate interpretation of the results as the drugs studied typically have more complex PK than probe drugs and patients may have comorbidities that affect interpretation of the results. In the context of understanding and predicting how THC disposition may change during pregnancy, knowledge of changes in CYP2C9 and CYP3A4 activity during pregnancy are most important. In addition, when the changes in exposure to 11-OH-THC and 11-COOH-THC are considered the changes in the activity of UGT1A and UGT2B7, as well as ADH, ALDH, and AO are most important. Currently, limited information is available regarding changes in CYP2C9 activity during pregnancy. Phenytoin PK in patients with epilepsy has been evaluated as a marker of CYP2C9 activity during pregnancy [ 171 – 174 ]. Interpretation of phenytoin PK is complicated as phenytoin exhibits non-linear kinetics due to saturation of CYP2C9. For drugs with non-linear kinetics, increases in V can result in an increase in CL due to the lower maximum concentrations reached and corresponding reduction in enzyme saturation. In addition, changes in plasma protein binding may complicate the interpretation of the findings on phenytoin disposition due to altered free concentrations and distribution characteristics. In two studies, phenytoin weight normalized unbound CL increased in trimester-dependent manner. On average, by the third trimester unbound CL increased by about 1.25-fold compared to post-partum [ 171 – 174 ]. A physiologically based pharmacokinetic (PBPK) model was developed and verified to assess CYP2C9 induction in trimester-dependent manner [ 175 ]. The model was developed with the trimester dependent changes of phenytoin plasma trough concentrations (concentration pre-dose) and verified with 3 rd trimester glyburide exposure [ 173 , 175 , 176 ]. The model predicted an increase in hepatic CYP2C9 of 1.4, 1.5, and 1.6-fold during trimesters 1, 2, and 3, respectively [ 175 ]. This suggests that following consumption of edible THC, the oral bioavailability of THC may be decreased due to CYP2C9 induction. However, due to the high extraction ratio of THC, the systemic CL is unlikely to be affected by enzyme induction. The changes in CYP3A4 activity during pregnancy are perhaps the best studied for DMEs. Several clinical studies including evaluation of midazolam disposition during pregnancy have aimed to characterize both intestinal and hepatic CYP3A4 activity changes. Integration of these studies and PBPK simulations concluded that hepatic CYP3A4 activity is increased by 2-fold in the third trimester when compared to postpartum values while intestinal CYP3A4 is unchanged by pregnancy [ 177 , 178 ]. The induction of CYP3A4 across pregnancy is supported by a study using dextromethorphan urinary ratio as a probe substrate. This study suggested that CYP3A4 activity is induced in the first trimester and is maintained throughout the course of pregnancy [ 179 ]. However, dextromethorphan urinary ratio is not a robust CYP3A4 probe and more clinical studies are needed to confirm the gestational timeline of CYP3A4 induction. Regardless, the increase in CYP3A4 expression is unlikely to have a major impact on THC disposition during pregnancy due to the lack of induction of gut CYP3A4 and the major contribution of CYP2C9 to the hepatic CL of THC and the blood flow limitation of hepatic CL. Changes in glucuronidation and UGT enzyme activity during pregnancy have been difficult to assess due to the limited number of probe substrates available. The two subfamilies of UGTs, UGT1 and 2, have different physiological roles. UGT1 subfamily aids in glucuronidation of estrogens and bilirubin while UGT2 primarily catalyze glucuronidation of steroids and bile acids. Both contribute to glucuronidation of xenobiotics. Labetalol is glucuronidated in vitro by UGT1A1 and UGT2B7 and is predominately cleared by UGT1A1 and UGT2B7 in vivo [ 152 ]. The oral CL/F of labetalol increases in a gestational age dependent manner during pregnancy [ 180 ]. An increase in CL/F suggests a reduction in absorption, decrease in plasma protein binding, or an increase in hepatic CL int . In other populations, labetalol is nearly completely absorbed, and protein binding is not different between pregnant and non-pregnant individuals [ 180 ]. Thus, these data suggest that hepatic UGT1A1 and/or UGT2B7 activity increases during pregnancy. It is likely that the increase is in UGT1A1 rather than UGT2B7 as in pregnant individuals the induction of UGT2B7 is unclear. Zidovudine and morphine, two UGT2B7 substrates, are commonly administered to pregnant and non-pregnant individuals. Morphine CL is significantly greater when measured during 3 rd trimester of pregnancy when compared to non-pregnant individuals [ 160 ]. This may be explained by an increase in hepatic blood flow and not a change in UGT2B7 activity. Following iv administration of zidovudine, zidovudine CL during pregnancy was not significantly different when compared to non-pregnant individuals supporting lack of UGT2B7 induction [ 181 ]. In another study, zidovudine CL/F was increased in comparison to postpartum values, possibly explained by altered absorption characteristics [ 181 ]. The PK of lamotrigine, a UGT1A4 probe, has been studied extensively across gestational ages in pregnant patients with epilepsy [ 182 – 187 ]. Lamotrigine exposure decreases significantly during pregnancy. The plasma concentration and urinary ratios of the glucuronide metabolite-to-lamotrigine increased in trimester 2 and 3 up to 2-fold when compared to postpartum values [ 182 ]. This supports significant induction of UGT1A4. Taken together, these data suggest that the CL of 11-OH-THC and 11-COOH-THC mediated by UGT1A enzymes may increase during pregnancy but this hypothesis requires further study. Increase in glucuronidation of 11-COOH-THC may have implications for toxicological screening as 11-COOH-THC-glucuronide is the major THC metabolite typically detected in urine. Most CYP enzymes are inducible by xenobiotics and their constitutive expression and activity are typically regulated at the transcriptional level. Several excellent reviews exist on the mechanisms of regulation and induction of CYPs and include more details on the molecular mechanisms of induction [ 188 , 189 ]. The reader is referred to these reviews for in-depth analysis of the constitutive mechanisms of CYP regulation. Here we assess the potential impact of pregnancy related hormones on THC and metabolite disposition through altered DME activity. The expression of DMEs can be impacted by exogenous and endogenous ligands of the nuclear hormone receptors that regulate the transcription of these enzymes. The induction of CYP2C9 and CYP3A4 by xenobiotics is well established. For example, CYP2C9 mRNA is induced by xenobiotic ligands of nuclear receptor constitutive androstane receptor (CAR) such as phenobarbital [ 191 ]. CYP3A4 mRNA, on the other hand, is induced by ligands of pregnane X receptor (PXR) such as rifampin and phenytoin, but also by CAR ligands although to a smaller degree [ 190 ]. UGT canonical nuclear receptor regulation is not well defined. However, UGT mRNA expression is altered by pathways mediated by CAR, PXR, and peroxisome proliferator-activated receptor (PPAR) [ 191 , 192 ]. Interestingly, CYP2C9, CYP3A4, and UGT1A4 have been shown to be induced through glucocorticoid receptor (GR) or estrogen receptor (ER) activation and the HNF family of nuclear receptors that respond to variety of endogenous regulators. Pregnancy is associated with a plethora of hormonal changes ( Table 3 ), and it has long been hypothesized that some of these hormones also impact DME transcription and expression [ 135 , 136 ]. Although there are over a dozen hormones that are altered during pregnancy, only some have been associated with changes in activity or expression of CYP2C9, CYP3A4, and UGTs. Pregnancy hormones that have been shown to cause induction of DMEs are growth hormones, progesterone, estrogens, testosterone, and cortisol. Here, we briefly summarize some of the hormones known to impact the expression of CYP2C9, CYP3A4, or the UGTs of interest for THC and its metabolite disposition. Endogenous hormones typically interact with their canonical nuclear receptors and have the greatest affinity for them ( Table 3 ). However, there is evidence of ligand crosstalk and induction of nuclear receptor expression with different hormones. Indeed, CYPs and UGTs have been noted to increase in response to increasing steroid hormone concentrations both in vitro and in vivo . In studies in both HepG2 cells and human hepatocytes, CYP3A and CYP2C9 mRNA expressions were upregulated when cells were treated with estradiol, progesterone, cortisol, and/or a combination of these hormones [ 147 , 149 , 193 ]. During pregnancy, the liver and thus the hepatocytes, are exposed to a combination of hormones that change in a gestational-age dependent manner and hence the ratios of the hormones to each other likely also change. The combination of changes in pregnancy related hormones and growth factors could lead to a synergistic and/or antagonistic action, making it difficult to characterize individual mechanisms. There are several synthetic corticosteroids, such as dexamethasone and prednisolone, known for their in vivo inductive effects on DMEs [ 194 , 195 ]. Based on this, the increase in endogenous corticosteroid concentrations during human pregnancy ( Table 3 ) may also cause induction of DMEs during pregnancy. CYP2C9 regulatory regions have both GR-responsive and CAR-responsive response elements, based on a promoter deletion analysis [ 150 ]. However, the impact of increasing cortisol concentrations on CYP2C9 expression or activity in vivo remains to be established. Synthetic glucocorticoids, like dexamethasone, have been shown to induce CYP2C expression and activity in human hepatocytes [ 196 – 198 ]. These interactions likely occurred through indirect interactions with the PXR and CAR, as well as direct interactions through GR, the canonical nuclear receptor for glucocorticoids ( Table 3 ). Dexamethasone has been shown to upregulate CAR through interactions with GR [ 195 ]. Cortisol has been shown to exert similar effects through in vitro regulation of CYP3A4 gene (explained more in the following paragraphs) [ 199 ]. Thus, there is potential for increased cortisol concentrations during pregnancy to induce CYP2C in the same manner as observed with dexamethasone. Progesterone and estradiol rapidly increase in a gestational-age dependent manner ( Table 3 ). In human hepatocytes treated with pregnancy levels of progesterone (0.1–10 μM) [ 147 ] CYP2A6, CYP2B6, CYP2C8, CYP3A4, and CYP3A5 mRNA expression and activity increased after treatment while CYP2C9 mRNA expression was not altered after progesterone treatment [ 147 ]. Both CYP2C8 and CYP2C9 are regulated by CAR activation, making it unusual that the CYPs were not induced together [ 200 ]. Studies with HepG2 cells and human hepatocytes have shown that the CYP2C9 promoter has a well-defined ERα response element and that CYP2C9 activity is inducible in HepG2 cells and human hepatocytes by estradiol and by estrogen receptor agonists [ 147 , 193 , 201 ]. Yet, the mechanisms of induction of CYP2C9 activity are unclear. In one study in human hepatocytes from female donors, up to 2-fold increase in CYP2C9 activity was observed based on 4-hydroxylation of diclofenac or tolbutamide but the CYP2C9 mRNA expression was unchanged [ 147 ]. The increase in activity was similar to the increase in activity after phenobarbital treatment, a classic CAR agonist [ 147 ]. The hepatocytes were treated with 1 μM estradiol, but the estradiol was rapidly metabolized (half-life: 0.57 ± 0.12 h), resulting in an average concentration of 157 ± 39 nM during the in vitro experiment [ 147 ]. In contrast, no change in CYP2C9 expression or activity after estradiol treatment was observed in human hepatocytes when treated with a cocktail of pregnancy hormones (progesterone, estrone, estradiol, estriol, 17-α hydroxyprogesterone, and growth hormone) at low and high concentrations [ 202 ]. The low and high cocktails included estradiol concentrations of 0.3 μM and 3 μM, respectively. The average estradiol concentration or exposure was not assessed during the treatment window. It is possible that the cocktail approach did not reproduce the earlier findings as treatment with a combination of hormones may lead to an antagonistic effects in which exposure to multiple compounds decreases the response. For example, in HepG2 cells, increased progesterone repressed estrogen-mediated CAR transactivation [ 203 ]. As such it may not be appropriate to consider the findings of the cocktail study as evidence of lack of effect of specific hormones. For example, when used in the cocktail approach growth hormones did not alter CYP2C9 mRNA. Yet based on these findings, it is not possible to conclude whether a direct link exists between elevated pregnancy hormone concentrations and CYP2C9 expression and induction. CYP3A4 response to pregnancy hormones has been studied in vitro . Estradiol induced CYP3A4 expression and activity after 0.1 μM and 1 μM estradiol treatment [ 147 ], but this finding has not been reproduced by others. In hepatic cell lines and human hepatocytes, CYP3A4 expression and activity were determined after different cocktail and individual hormone treatments [ 149 , 151 ]. Following treatments of native growth hormone (NGH), placental growth hormone (PGH), estradiol, estriol, cortisol, testosterone, and progesterone individually or as cocktails at pregnancy concentrations from third trimester unbound, total, and 10X total concentrations, cortisol was the only hormone to consistently induce CYP3A activity in all cocktails [ 149 ]. However, NGH, PGH, and progesterone did induce CYP3A activity when used at total third trimester concentrations. In a separate study in sandwich cultured human hepatocytes treated with a cocktail of pregnancy hormones, increasing cortisol concentrations led to increased CYP3A4 activity [ 151 ], supporting the predominant role of cortisol in inducing CYP3A4 during pregnancy. However, interpretation and translation of these studies to the specific effect of cortisol on CYP3A induction in vivo is convoluted by the cross talk of the different hormones and the basal concentrations of the hormones in vitro and in vivo . The findings of the role of cortisol in CYP3A4 regulation in human hepatocytes are supported by further mechanistic studies. In HepG2 cells CYP3A4 mRNA expression increased in response to increasing cortisol concentrations [ 199 ]. The greatest maximum fold induction (I max ) and potency (EC 50 ) were 18.8-fold and 0.27 μM, respectively, and occurred in cells transfected with PXR and GR [ 199 ]. In the PXR only and GR only transfected cells the I max values were 16.0-fold and 15.5-fold, while the EC 50 values were 0.35 μM and 1.18 μM, respectively [ 199 ]. This suggests that there is in vitro regulation of CYP3A4 expression that is receptor-dependent with the greatest effect in PXR and GR transfected cells. Taken together these studies suggest that the CYP3A4 mediated metabolism of THC may increase with increasing glucocorticoid concentrations such as observed during pregnancy switching the metabolism from 11-OH-THC to CYP3A4 mediated metabolites. Both 11-OH-THC and 11-COOH-THC undergo glucuronidation by UGTs. Specifically, 11-OH-THC is depleted by UGT1A9, UGT1A10 and UGT2B7 while 11-COOH-THC undergoes glucuronidation by UGT1A1 and UGT1A3 [ 74 , 79 ]. Thus altered activity of UGTs could result in altered THC metabolome. UGT1A1, UGT1A3, UGT1A9, and UGT2B7 are expressed in the liver [ 204 ]. Meanwhile, UGT1A10 is predominately expressed in extrahepatic tissues, specifically the intestine and may be less important for the systemic CL of 11-OH-THC [ 204 ]. Current data suggest that UGT1A enzymes are upregulated by some pregnancy hormones. In HepG2 cells treated with pregnancy concentrations of progesterone, UGT1A1 expression was upregulated via PXR-mediated mechanism [ 152 ]. Similarly, in sandwich cultured human hepatocytes, UGT1A1 and UGT1A4 expression increased in response to estradiol and a cocktail of hormones estradiol, estriol, estetrol, progesterone, and cortisol [ 153 ]. Increased UGT1A4 and UGT1A9 mRNA expression upon ERα activation has also been shown [ 154 , 205 ]. In contrast, UGT2B7 expression, however, did not increase in response to any hormone treatment [ 152 , 153 ]. Taken together these findings suggest that increasing estrogen concentrations during pregnancy will induce the expression and activity of the UGT1A enzymes that conjugate 11-OH-THC and 11-COOH-THC.

Cannabis is commonly used during pregnancy to mediate pregnancy related conditions such as body aches and nausea. The increasing acceptability of cannabis use during pregnancy warrants more research into the exposure and effects of cannabinoids during pregnancy. The main psychoactive component of cannabis, THC and its psychoactive metabolite 11-OH-THC, are partial agonists of the CB 1 receptor of the ECS. The ECS is critical for healthy pregnancy but little is known about how changes in the ECS together with PK changes modulated by hormonal and physiological changes during pregnancy alter THC pharmacology. THC is primarily cleared by hepatic metabolism by CYP2C9 to form 11-OH-THC which in turn is oxidized by multiple enzymes to 11-COOH-THC. Current evidence suggests that changes in pregnancy hormones are likely to cause the changes in drug metabolism and thus could also alter THC metabolism in pregnant individuals. Due to the high extraction ratio of THC, pregnancy and CYP2C9 activity are expected to impact THC disposition in a route dependent manner. Induction of CYP2C9 is only expected to alter the exposure to orally administrated THC. Yet, other pregnancy mediated changes such as increased liver blood flow and increased adiposity are predicted to affect both the PK and PD or THC during pregnancy. Pregnancy hormones may also have an impact on CB receptor expression as suggested by sex differences in THC PK, but current evidence for this is inconclusive. Better understanding of the regulatory mechanisms underlying the hormonal effects on expression and activity of DMEs and CB receptors is required to refine predictions of THC pharmacology and toxicology during pregnancy. As THC and its metabolites cross the placental barrier and the ECS is critical for fetal development, better understanding of the maternal-fetal transfer of cannabinoids is also needed. The mechanisms limiting cannabinoid distribution to the fetus are still unclear and how these mechanisms may vary in populations and with genetics should be assessed.

Cannabis has a long history of use for both medicinal and recreational purposes. The first documented case of its use was as early as 2800 BC in western China, but therapeutic indications of cannabis are mentioned throughout history across the globe [ 1 ]. In contemporary times, cannabis is used both recreationally and as a self-medication for variety of conditions. The main psychoactive component of cannabis is (−)-Δ 9 -tetrahydrocannabinol (THC, Figure 1 ). THC is responsible for the euphoria, enhancement of sensory perception, and antinociceptive effects of cannabis [ 2 ]. The structure of THC as the psychoactive component in cannabis was first solved in the early 1960s by Mechoulam and Gaoni [ 3 ]. Synthetic THC (Dronabinol) is an oral capsule formulation of THC, indicated to prevent nausea and vomiting due to chemotherapy or to increase appetite in patients with acquired immune deficiency syndrome (AIDS) [ 4 ]. Cannabis remains one of the most used illicit drugs, and many countries are shifting toward legalization of its medicinal and recreational use [ 5 ]. Globally, attitudes about cannabis are changing toward more acceptance of its use with increasing use by adolescents and young adults. According to the 2015 National Survey on Drug Use and Health, 22.2 million Americans currently use cannabis and 2.6 million individuals aged 12 or older tried cannabis for the first time in the last 12 months [ 6 , 7 ]. Many individual states in the United States (U.S.) have recently legalized cannabis use and increased its accessibility [ 8 ]. Simultaneously with the increase in legalization and usage, cannabis products have also tripled in the THC content [ 9 ]. Currently, 10 milligrams (mg) of THC in edibles is considered the standard serving in both U.S. and Canada, but many oral cannabis products contain up to 50 mg in a single package [ 10 , 11 ]. At these higher oral doses of THC (25 and 50 mg), significantly impaired cognitive and psychomotor functioning has been observed [ 12 ]. Increasing accessibility, usage, and THC content make it difficult to fully define the health risks that may be associated with contemporary cannabis use based on historical usage and safety profiles. This is particularly relevant for sensitive population subgroups such as pregnant individuals, children, and adolescents that may have not been exposed to high levels of cannabis in the past. In the U.S., cannabis is the most commonly used illicit drug among pregnant individuals. The reported cannabis usage during pregnancy more than doubled from 3 to 7% between 2002 and 2017 [ 13 ]. Socioeconomic status appears to impact the frequency of cannabis use during pregnancy. For example, 28% of young and socioeconomically disadvantaged women reported using cannabis throughout pregnancy [ 14 ]. During pregnancy cannabis products are reported to be used to alleviate body aches, nausea and stress and to improve mood [ 15 , 16 ]. In one study, up to 96% of women who used cannabis during pregnancy reported nausea as the reason for continued use of cannabis during pregnancy [ 17 ]. Prevalence of cannabis use is higher in pregnant individuals suffering from nausea and vomiting in pregnancy (11.3%) when compared to individuals who do not (4.5%) [ 18 ]. Taken together, these data demonstrate the increased need to define the safety of cannabis use during pregnancy. One reason for the increased use of cannabis during pregnancy is a perception of a lower risk than prescription drugs [ 19 ]. In one study, 78% of pregnant individuals perceived slight to no risk from cannabis use when used one-to-two times per week [ 20 ]. The trend in low-risk perception can be partially attributed to the increased cannabis legalization and the uncertainty about the perinatal adverse outcomes [ 19 ]. There is a perception that cannabis is not a ‘drug’ but instead is viewed as ‘natural,’ an ‘herb,’ and ‘vegan,’ and thus perceived safe to use during pregnancy [ 16 , 19 , 21 , 22 ]. Recreationally, cannabis is commonly used for the psychoactive effects [ 17 ]. The psychoactive effects of cannabis are mainly due to THC and its primary metabolite 11-hydroxy-THC (11-OH-THC) ( Figure 1 ) [ 23 ]. THC and 11-OH-THC are partial agonists of cannabinoid (CB) receptors 1 and 2. THC has greater affinity for CB 1 than CB 2 receptor [ 24 , 25 ]. CB receptors are G protein coupled receptors involved in the endocannabinoid system (ECS) and are ubiquitously expressed throughout the body [ 26 , 27 ]. CB 1 is abundantly expressed in the central nervous system in the neuronal and glial cell types [ 26 , 27 ]. CB 2 receptors are also expressed in select neurons but to lower level than CB 1 receptors [ 25 ]. The psychoactive effects of THC are a result of its binding to the CB 1 receptor in the central nervous system [ 25 ]. Both CB 1 and CB 2 receptors are expressed in peripheral tissues, including immune system, reproductive tissues, and in the developing fetal nervous system [ 25 , 28 ]. The ECS is composed of CB receptors, endocannabinoids that are the endogenous ligands for the CB receptors, and enzymes responsible for the synthesis and metabolism of endocannabinoids [ 26 ]. The endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), bind to CB receptors regulating their activity [ 26 ]. The activity of the ECS is modulated by the concentrations of endocannabinoids. Endocannabinoids are synthesized from phospholipids in different cell types via lipases. AEA is mainly inactivated through fatty acid amide hydrolase (FAAH)-mediated hydrolysis [ 29 , 30 ] while 2-AG is predominantly hydrolyzed by monoacylglycerol lipase (MAGL) [ 31 ]. Based on studies in rats, the brain concentrations of AEA and 2-AG are higher in females than males [ 28 ], suggesting that either the synthesis of these endocannabinoids is faster or their metabolism is slower in females than in males. Pregnant mice were shown to have decreased uteri FAAH activity and content in a gestational age-dependent manner and estradiol and progesterone treatment decreased FAAH activity in mouse uteri [ 32 ]. This suggests a potential impact of sex steroids on endocannabinoid synthesis and metabolism which could contribute to sex differences in ECS. Several studies in rodents have suggested that there are differences in CB receptor expression between males and females [ 28 ], but the results of sex differences in the expression of CB receptors have been conflicting and vary between different brain regions and between species [ 26 ]. Rodent studies have suggested that sex steroids and estrogens in particular may regulate CB 1 receptor expression in a brain region and species specific manner [ 33 , 34 ], but whether this translates to regulation during pregnancy is unknown. Few studies conducted in humans have suggested CB 1 receptor density is greater in women than men [ 28 ], but further studies are needed to determine whether sex differences exist in humans and how rodent studies translate to humans. The impact of sex on THC pharmacology has been extensively studied in various preclinical models of pain. Many studies have found THC to be more potent/efficacious in female than male rats [ 35 , 36 ] but the findings are often not reproduced in mouse models [ 28 , 37 , 38 ]. Some studies have suggested that in humans, women are more sensitive or responsive to the effects of THC than men [ 39 , 40 ] but not all studies have reproduced this finding [ 28 ]. Whether female hormones regulate some of these sex differences is unclear. In healthy, premenopausal women, THC effect was no different in the early and late follicular phases where estradiol and progesterone concentrations are different [ 41 ]. As noted by others [ 28 ], the findings on sex differences in THC effects, if observed, are likely dependent on sexual dimorphisms in receptor expression and sensitivity in the ECS, and in pharmacokinetics and exposures to cannabinoids. Whether the findings of sex differences can be extrapolated to pregnancy has not been studied and at present it is unknown whether pregnant individuals have different CB receptor expression or sensitivity and pharmacodynamic response to THC than non-pregnant women or men. Throughout pregnancy, the ECS plays an important role in regulating implantation, decidualization, placentation, and parturition [ 42 – 45 ]. The fetal ECS develops as early as 5 weeks of gestation and has been noted to play an essential role in early stages of neural development and cell survival in different animal models [ 46 , 47 ]. During pregnancy, plasma concentrations of AEA and 2-AG increased in humans and rodents, respective [ 48 , 49 ]. AEA has been shown to increase during labor [ 48 ]. AEA plasma concentrations were increased in patients with endometriosis and miscarriage, as well as in the fallopian tube of individuals with ectopic pregnancy [ 50 ]. In one study, both AEA and 2-AG concentrations were higher in individuals with Gestational Diabetes Mellitus when compared to healthy pregnancies [ 51 ]. As such, dysregulation of the ECS by cannabis use at any stage during pregnancy may lead to adverse pregnancy outcomes or harmful fetal outcomes. Several detailed reviews are available of the effects of cannabis on offspring following maternal use of cannabis and on the overall pharmacology of cannabis during pregnancy [ 26 , 52 – 59 ]. Currently, studies of the effects (therapeutic or adverse) of THC in different populations rely heavily on retrospective self-reporting. Many of the studies exploring the effect of cannabis use during pregnancy have relied on self-reporting of cannabis use to identify users. Self-reporting is subject to high false positive (5–36%) and false negative (1–7%) rates [ 60 ]. The high false positive and false negative rates may confound any study findings and will likely impact the reported variability in the presence and magnitude of effects of cannabis [ 61 – 64 ]. For example, one study using self-reporting found no significant impact of prenatal THC exposure on birth weight [ 65 ]. In contrast, another study found that birth weight was lower in babies born to cannabis users when compared to non-users when the study populations were categorized based on self-reporting and positive urine tests [ 66 ]. This simple categorization to users and non-users does not allow investigators to explore the influence of dose-dependency or frequency of usage although these effects are established in human and animal studies [ 12 , 67 ]. There are methods to qualitatively assess usage through different matrices (e.g. blood, serum, urine, hair, meconium). However, it is of interest to develop quantitative and noninvasive markers for THC usage to further study usage levels and effects [ 60 , 68 , 69 ]. The duration and magnitude of exposure to THC in maternal and fetal (including fetal brain) compartments are critical in driving THC pharmacology and the potential consequences of THC usage on fetal development. Yet it is neither ethical, feasible, nor legal to conduct controlled pharmacokinetic (PK) studies of maternal-fetal exposure to THC across gestational age. As such, information of PK of THC and its metabolites during human pregnancy is currently lacking, and it is unknown whether pregnancy results in significant changes in maternal THC disposition. To address this gap in knowledge, mechanistic understanding that allows predictions and extrapolations of THC disposition and exposure during pregnancy based on data collected in non-pregnant individuals is important. One such mechanistic approach is to better understand how metabolism of THC changes in response to the variable concentrations of pregnancy hormones. This review aims to describe and integrate the current knowledge on THC PK and the known mechanisms that alter drug disposition during pregnancy. We will further explore how this fundamental knowledge can be used to predict the changes in THC and its metabolite disposition during pregnancy. This review will first summarize what is currently known about the PK and pharmacological effects of THC and its in vivo metabolites in humans. We will then provide insight to the potential mechanisms of altered drug metabolism due to altered enzyme activity during pregnancy as regulated by hormonal changes and summarize how these changes are predicted to alter THC and its metabolite exposures and disposition.

The psychoactive effects of cannabis follow a dose-effect relationship with increasing oral dosages of THC resulting in increasing psychological and physical effects [ 12 ]. Hence one may expect that if THC exposure is altered during pregnancy in comparison to non-pregnant state the pharmacological effects of THC will also be affected. The time course of plasma THC concentrations does not fully correspond to the time course and magnitude of the observed psychological effects resulting in a counterclockwise hysteresis for the concentration-effect relationship [ 206 – 208 ]. It is hypothesized that this hysteresis results from both distribution disequilibrium and accumulation of pharmacologically active metabolite, 11-OH-THC. The hysteresis collapses after weighting 11-OH-THC to have equipotency with THC but greater partitioning to the effect site than THC [ 206 ]. Both the distribution and exposure to 11-OH-THC together with THC may be altered during pregnancy, and hence, the time-course of effect for THC in pregnant individuals may also be different than that in non-pregnant individuals. To date, no studies have assessed THC PK during pregnancy in a way that would allow assessment of the dose-exposure relationship and the changes in THC PK during pregnancy. Many of the processes affecting THC PK-PD are altered during pregnancy. Following smoking of THC, a high extraction ratio drug, the CL of THC is expected to increase proportional to the increase in hepatic blood flow during pregnancy. Following oral administration of THC, the changes in protein binding and increases in CYP2C9 and CYP3A4 activity are predicted to lead to a decrease in THC bioavailability. Hence, following either oral administration or smoking of THC a decrease in AUC (exposure) of THC is expected leading to potentially diminished PD effect. It is possible that pregnant individuals will adjust their consumption levels to maintain a desired PD effect. Predicting changes in 11-OH-THC CL during pregnancy is more complicated due to the high extraction ratio of 11-OH-THC and the lack of knowledge in pregnancy mediated changes in ADH, ALDH, and AO activity and biliary secretion. As 11-OH-THC is formed in the liver, the metabolism of 11-OH-THC in situ prior to the metabolite escaping the liver may be increased, and hence, the apparent bioavailability of the metabolite decreased. In the absence of studies evaluating THC PK during pregnancy, an approach to characterizing fetal exposure and tissue concentrations in a gestational-stage specific manner is integrating results from in vitro work, animal studies, and human studies to inform a physiologically based pharmacokinetic (PBPK) model. This allows prediction of THC PK during pregnancy. Over the last decade there has been progress in assessing different drugs in pregnant individuals through PBPK simulations [ 138 , 175 , 178 ]. Only one model has assessed THC and 11-OH-THC exposures after inhalation in pregnant individuals [ 105 ]. As expected, the model simulates THC CL as hepatic blood flow limited and was not sensitive to changes in intrinsic CL or protein binding, two parameters that would change during pregnancy [ 105 ]. In contrast, sensitivity analysis predicted that THC V ss is sensitive to changes in adiposity and plasma protein binding [ 105 ]. Taken together this suggests volume of distribution of THC and its metabolites is altered during pregnancy. This may also impact THC half-life and duration of effects during pregnancy. Assessing the exposure of the fetus to THC during pregnancy is essential to understanding the effects of cannabis on fetal development. Yet, due to the potential negative side effects of THC, controlled clinical studies to assess the PK-PD of THC in pregnant individuals are not ethical. The potential of THC/11-OH-THC to produce short-term or long-term developmental toxicity can, however, be assessed through human epidemiological studies and in vitro experiments. Common, noninvasive methods to determine THC exposure in newborns is through measurement of umbilical cord concentrations or cannabinoids excreted into meconium [ 76 , 209 , 210 ]. Meconium analysis of cannabinoids, specifically 11-COOH-THC, appears to be more sensitive to usage than cord concentrations. In fact, 11-COOH-THC excreted in meconium is more sensitive means of ascertaining use that does not continue until the time of delivery [ 76 ]. This suggests that in the fetus THC and/or its metabolites are excreted into the fetal GI tract and remain in the meconium for prolonged periods. Cord blood concentrations on the other hand are limited to 3 rd trimester usage and do not provide good assessment of early pregnancy use. However, they do provide a valuable measurement of maternal-fetal distribution of cannabinoids [ 76 ]. In both nonhuman primates and humans, fetal exposure to THC is much lower than the corresponding maternal exposure. In a catheterized maternal-fetal macaque model the THC fetal-to-maternal plasma AUC ratio was 0.30 ± 0.17 and was consistent with the sparse human term umbilical vein-to-maternal plasma concentration ratio data, 0.26 ± 0.10 [ 211 , 212 ]. In perfused cotyledons from human term placentas, the steady-state plasma unbound cotyledon CL index (CL u,c,i ) ratio for THC was 0.45 ± 0.24 [ 213 ]. This was not inhibitable by P-gp or BCRP antagonist valspodar. This suggests that at term, the fetus has 1/3 of the maternal exposure. However, the mechanism that limits fetal THC exposure remains unclear. Due to its high passive permeability, THC would be expected to efficiently cross the placenta and have steady state fetal to maternal concentration ratios of unity. Yet, the fetal to maternal ratios are significantly below unity. The < 1 ratio suggests that there is active efflux transport of THC from the placenta to the maternal compartment or placental or fetal metabolism. Yet, no efflux transporters have been identified that would actively efflux THC. In pregnant mice, P-gp , Bcrp , and P-gp/Bcrp knockout had no impact on fetal-to-maternal THC AUC ratio [ 111 ]. Yet the AUC ratio was 0.31 [ 111 ] suggesting the presence of a process limiting fetal exposure to THC. Other factors that could limit fetal exposure could be metabolism of THC in the placenta or fetal liver. No notable depletion of THC or 11-OH-THC was observed in human placental microsomes consistent with the low CYP expression in the human placenta [ 126 ]. Depletion of THC was observed by human fetal liver microsomes [ 126 ] likely due to the presence of CYP3A7 and the substrate overlap between CYP3A4 and CYP3A7. The CYP3A7 metabolism may influence the fetal exposure to THC but it is predicted to have negligible impact on maternal cannabinoid CL. In fact, when accounting for the CL u,c,i ratio from the perfused placentas in conjuction with the previously calculated fetal metabolism, the estimated THC partition coefficient (K p ) was 0.28 ± 0.09 [ 213 ]. The predicted K p is similar to the previously observed human term umbilical vein-to-maternal plasma concentration ratio.