{"paper_id":"0888836c-6944-4147-8c12-43c73e60f04f","body_text":"Reliable family planning is an essential element of modern society, with unplanned\npregnancies inflicting a significant financial and emotional burden on both the\nindividual and societal levels. For much of human history, the socioeconomic burden\nof unintended pregnancies has fallen disproportionately on the mother, with its\nassociated financial consequences. The US News and World Report predicts an average\ncost of $267,000 to raise a child to 18 years of age. 1  There is also a significant\neconomic burden on the United States taxpayer from unplanned pregnancies, with\nestimates between $5.5 billion and $21 billion per year. 2 – 4\nOver the last century, there has been significant progress to ensure safe, reliable,\nand cost-effective female contraceptives, such as the birth control pill,\nintrauterine devices (IUDs), emergency contraceptives, diaphragms, vaginal rings,\nsubcutaneous injections, intramuscular (IM) injections, and implants. 5 , 6  Despite these efforts,\napproximately half of all pregnancies are unplanned. 7  Recent estimates suggest that\nthere are 121 million unintended pregnancies annually worldwide, with 61% of these\npregnancies ending in abortion. 8  Recent data in the United\nStates suggests that 45% of pregnancies are unplanned, and 40% of these pregnancies\nend in abortion. 9  With recent rulings in the United States Supreme Court casting\ndoubt on access to legal abortion, there has been a renewed need for reliable family\nplanning methods. 10\nThe ability of men to actively participate in family planning with male centered\ncontraceptives has been limited by the number of options, primarily condoms or\nvasectomy. Estimated numbers from the United Nations Department of Economic and\nSocial Affairs report in 2019 showed that male centered birth control accounted for\nonly 28% of worldwide contraceptive use (male condoms 21%, vasectomy 2%, and\nwithdrawal 5%). 6  These values are only slightly higher when looking at North\nAmerica (total 30.9%, male condom 17.6%, vasectomy 6.9%, and withdrawal 6.4%) and\nEurope (total 39.7%, male condom 29.2%, vasectomy 3%, withdrawal 7.5%). 6  Over the last\nhalf century, there has been an increased interest in developing reliable and safe\nmale birth control options other than the condom or vasectomy. Between 25% and 71%\nof men report that they would use a male birth control option analogous to the\nfemale pill if one was commercially available. 11 , 12  Throughout the last several\ndecades, there have been multiple hormonal and non-hormonal birth control options\nwith promising results; however, no option has garnered the US Food and Drug\nAdministration (FDA) approval. In this review, we examine the past and current\nagents of male birth control research.\n\nThe origin of male birth control is debated by scholars, but likely dates back to\ncondom use in ancient Egypt and Greece. The first recorded use of a condom is\nfound in the legend of Minos and Pasiphae, written by Antoninus Liberalis in the\nsecond century AD. Pasiphae is said to have used a goat’s bladder condom to\nprotect her from evil serpents and scorpions in her unfaithful husband’s\nsemen. 13  In the 10th century AD, the Persian physician\nAl-Akhawayni encouraged the use of animal gallbladder condoms. These ‘skin’\ncondoms were eventually replaced by rubber in 1844 and latex in the 1930s.\nMale condom use is associated with a 2–3% unintended pregnancy rate with perfect\nuse and 12% with typical use. 5 , 14  There have been many\npublic service campaigns to encourage consistent and proper condom use for the\nprevention of sexually transmitted infections (STIs) and unintended pregnancies.\nOne national study of 5865 US adolescents and adults aged 14–94 showed that only\n21.5% of men had used a condom at least once in the last 10 vaginal intercourse\nencounters. 15  Another cross-sectional national survey of adults aged\n18–44 showed an overall prevalence of condom use of 24.8% at their last sexual\nencounter. 16  In addition, the use of condoms has been shown to be\nassociated with mistrust in relationships or to interfere with\nintimacy. 17 , 18  There are also condoms developed with spermicidal\nagents, but poor evidence to suggest any greater benefit in preventing unwanted\npregnancies. 19\nThe first documented vasectomy was performed in 1823 by Sir Ashley Cooper on a\ndog. 20  Gosselin continued this research with human cadaver studies\nand further experimenting with vasectomy techniques on dogs in 1847. The first\nhuman vasectomy is credited to R. Harrison in London. 20  Vasectomy was not\ninitially seen for its value as a contraceptive. Multiple surgeons of the day\nemployed vasectomy with the intent of prostatic atrophy. The Austrian physician\nEugen Steinach also purported the use of unilateral vasectomy to restore vigor\nin older gentleman, a concept known as rejuvenescence. 21  The history of vasectomy\ntook a darker turn when it was identified as a means to forward the eugenics\nmovement. In 1897, A. J. Ochsner performed the first vasectomy in the United\nStates, which he saw as a means of sterilizing criminals and slowing ‘racial\ndegeneration’. 22  This disturbing work continued in 1902, when Harry C.\nSharp sterilized 42 inmates at the Indiana Reformatory to prevent the birth of\nfuture criminals. 23  It was around the time of the Second World War when the\nvasectomy was recognized as a viable form of consensual male\nsterilization. 20\nApproximately 500,000 vasectomies are performed annually in the United States,\nwith 5–10% of married men having undergone the procedure. 5 , 24  The\nunintended pregnancy rate in the first year following vasectomy is between 0.02%\nand 0.1%. 5 , 25  This is a\nrelatively quick and reliable method of sterilization with low complication\nrates of 1–2%. 26  One of the major barriers for widespread use of\nvasectomy is the permanence of the procedure, with the associated high cost and\nuncertainty of vasectomy reversal success. The complication rate of vasectomies\nis most determined by the procedure volume, with one study reporting that\ndoctors performing more than 50 procedures per year had one third the\ncomplication of their counterparts performing less than 10. 27  The most\ncommon complications are hematoma (2%), infection (3–4%), sperm granuloma (40%),\nand persistent post-vasectomy pain (1–14%). 28  The reversibility of the\nprocedure is an important consideration for many patients with the success of\nthe procedure being related to the method of vasectomy and the duration of\nobstruction. For men who underwent a reversal less than 3 years after their\ninitial operation, patency was reported as >95% and a pregnancy rate was\nreported at 75% with both rates decreasing as duration of obstruction\nincreased. 29\n\nNormal semen parameters have been established by international studies sponsored by\nthe World Health Organization (WHO), in which 3589 semen samples were analyzed. The\nselected men were all deemed fertile with a partner’s time to pregnancy of less than\n12 months. They defined a normal sperm concentration as greater than 15 million/ml\nof ejaculate. 30  With normal semen parameters, 75% of couples will achieve\npregnancy within 6 months, and 85% of couples will conceive in 1 year. 31 , 32  The perfect\nmale contraceptive would lead to azoospermia, where there are no motile sperm\nidentified on semen analysis. This ideal is often unachievable, and a more realistic\nreal-world goal would be a sperm concentration of less than 1 million/ml. The target\nof <1 million/ml has been shown to result in a pregnancy rate of less than 1% per\nyear, which is similar to female hormonal contraceptive pills. 33  This target\nsperm concentration has been deemed acceptable by the American Society of\nAndrology. 34  Beyond efficacy, the other main criteria for an acceptable\nmale contraceptive are reversibility and lack of systemic side effects. 35\nThe first attempts to develop male birth control by lowering sperm levels were\ntargeted at the hormonal axis between the hypothalamus, pituitary gland, and testes.\nIn healthy, eugonadal men, this axis operates with a negative feedback loop to\nregulate spermatogenesis and steroidogenesis, as shown in  Figure 1 . Pulsatile release of\ngonadotropin-releasing hormone (GnRH) from the hypothalamus is transported\n via  a portal vascular system to the anterior\npituitary. 36  The gonadotroph cells in the anterior pituitary respond to\nGnRH by releasing luteinizing hormone (LH) and follicle-stimulating hormone (FSH)\ninto the systemic circulation. FSH acts on the Sertoli cells to stimulate\nspermatogenesis in the seminiferous tubules. LH acts on Leydig cells to stimulate\nthe production of testosterone, subsequently increasing systemic levels of estradiol\nand 5-α-dihydrotestosterone (DHT). Testosterone from the Leydig cells and Inhibin\nfrom the Sertoli cells work  via  negative feedback to dampen\ngonadotropin release from the hypothalamus and pituitary. When functioning properly,\nthis process produces several million sperm per day and takes about 68 days for\nmaturation. 37\nHypothalamic-pituitary-gonadal axis.\nFigure modified with text, markings, and annotation after adaptation from\nServier Medical Art by Servier, licensed under a Creative Commons\nAttribution 3.0 Unported License\n\nMankind has known about the effects of androgen deprivation for centuries. Aristotle\nwrote about the effects of castration and the subsequent inability to reproduce,\nwhich is regarded as the first scientific article on infertility. 38  Arnold Adolph\nBerthold, the father of modern endocrinology, performed testicular transplantation\nexperiments in 1849 with roosters, where he noted the return of masculine\ncharacteristics after the transplantation of testicles into previously castrated\nroosters. 38 , 39  In 1935, testosterone was isolated from bull testes by Ernest\nLacquer and simultaneously synthesized in the laboratory by Adolf Butenandt and\nLeopold Ruzicka. 38  The idea of targeting the hypothalamic-pituitary-testes axis\nfor male birth control was formulated after witnessing the effects of testosterone\nsupplementation in healthy men. This was done initially with either injections, oral\nformulations, or topical applications.\nIn 1939, 4 years after the isolation of testosterone, Norris J. Heckel described\ndecreased spermatozoa count in a 67-year-old man with daily subcutaneous\ninjection of 10–25 mg of testosterone enanthate (TE). Sperm counts returned to\nnormal after the cessation of the injections. 40  The first clinical trials\nlooking at sperm count suppression with testosterone supplementation were\nconducted in the 1970s at the National Institutes of Health. These studies\nshowed efficacy in causing oligospermia or azoospermia in the majority of\nparticipants. 41 – 43  The WHO\nconducted two landmark studies in the 1980s and 1990s. The first was a\nmulticenter, international prospective trial with 271 healthy, fertile men. They\ninjected 200 mg TE IM weekly and noted 65% became azoospermic in 3 consecutive\nsemen samples with a mean time to azoospermia of 120 days. There was one\nrecorded pregnancy during 1486 months of the efficacy phase. Importantly, the\ntime to recovery (sperm concentration of at least 20 million/ml) was 3.7 months\nafter discontinuation and 6.7 months to baseline semen parameters. 44  The\nsecond WHO study included five additional centers, two additional countries, and\n399 healthy men. With the same regimen, they noted four pregnancies for 49.5\nperson-years involving men with oligozoospermia and 0 pregnancies during 230.4\nperson-years in azoospermic men. 45  These studies\ndemonstrated efficacy and reversibility of TE injections, but also highlighted\nseveral key limitations to this formulation. Azoospermia was only induced in\napproximately 65% of participants, the frequent injection schedule was not\ndesirable, and there were systemic side effects of unopposed testosterone\nsupplementation; acne, weight gain, altered mood, and changes in hematologic and\nlipid profiles. 35\nDue to the undesirable injection schedule of testosterone enanthate, there were\nattempts to study oral formulations of testosterone, namely testosterone\nundecanoate (TU). The first study with oral TU 80 mg three times daily showed\nthat this dose was insufficient to suppress gonadotropins for sustained\nazoospermia in six out of seven volunteers. 46  TU monotherapy was\nabandoned due to this sperm ‘rebound’.\n\nWith advancement of male endocrinology, attempts were made to improve sperm\nsuppression, time to suppression, and decrease side effects by using combination\nregimens of testosterone and progesterone. Multiple of the following regimens also\nintroduce alternate routes of delivery, namely topical gels and implantable\npellets.\nProgestin cyproterone acetate (CPA) is a ‘first-generation’ progestin with\nantiandrogen effects used for treatment of androgen driven processes such as\nadvanced prostate cancer, acne, precocious puberty, excess body hair growth,\nfemale birth control, and puberty blockers in transgender females. 47  CPA was\nfirst tested as a single agent for male contraception in 1980 in 25\nmen. 48  Doses of 0, 5, or 10 mg/day were given for 16 weeks, then\nparticipants were followed for 24 weeks with semen and serum assays. They noted\nsignificant decreases in sperm concentration, motility, morphology, serum\ntestosterone, and gonadotropins. The decrease in androgen levels made this an\nunlikely candidate for single agent use. Later studies looked at testosterone\nenanthate in combination with high (25 mg/day) and low (12.5 mg/day) dose CPA\nfor male contraception in the 1990s. 49  They studied 10 normal\nmen for 16 weeks and noted azoospermia in all men in the high-dose arm and 60%\nof the low-dose arm. Importantly, there were no changes in serum testosterone\nlevels, and gonadotropin drops were transient. Side effects include\ngynecomastia, sexual dysfunction, bone density loss, and depression. CPA is no\nlonger used in the United States and has an unlikely future as a component in\nmale birth control.\nLevonorgestrel (LNG) is a progestin commonly used in female birth control and\nemergency contraception. There are multiple routes of delivery for LNG,\nincluding oral, skin patch, IUD, and subcutaneous implant. 50  It was\nstudied as an implant for male contraception in combination with TU injections.\nThis study included 62 Chinese men at three dosage groups. Groups I and II\nreceived four LNG subcutaneous rods (75 mg each) followed by either 500 mg or\n1000 mg of TU IM every 8 weeks, respectively. Group III was a control, with only\n1000 mg TU IM every 8 weeks. They found rates of oligospermia (sperm\nconcentration < 3 million/ml) of 95%, 100%, and 86% in the three groups,\nrespectively. Group II trended toward the highest percentage of azoospermia\n(90%), but this was not significant. There were no significant side effects or\nchanges in baseline serum testosterone or gonadotropins.\nEtonogestrel (ENG) has been used since the 1990s as a long-acting, progestin\nfemale contraceptive in the form of subcutaneous implants or vaginal\nrings. 51  In the early 2000s, ENG was studied as a male\ncontraceptive in a double-blind, multicenter, placebo-controlled study at high\nand low doses in combination with TU injections. 52  There were 354 healthy\nmen treated for at least 42 weeks and followed for an additional 24 weeks post\ntreatment. They found that azoospermia was achieved in 89–94% of men by 16 weeks\nand sustained in 91% of men on treatment. Recovery to sperm\nconcentrations > 20 million/ml was 15 weeks. Side effects of this treatment\nincluded weight gain, mood changes, acne, sweating, and loss of libido.\nDesogestrel is an oral progestin used in combination with an estrogen agent or\nindividually in female birth control and to treat menopausal symptoms. 53 , 54  A\nmulticenter, prospective, randomized trial was conducted with 38 Caucasian and\n36 Chinese men to look at the use of low (150 µg daily) and high (300 µg daily)\ndose oral desogestrel and testosterone pellets (400 mg weeks 1 and\n12). 55  The high dose group achieved azoospermia at a significantly\nhigher rate than the low dose group (100%  versus  71%,\n p  < 0.05). The testosterone concentrations remained\nnormal, but side effects included lower HDL in caucasian participants and more\nweight gain in both groups.\nNorethisterone is a progestin with both oral (Norethisterone Acetate, NETA) and\nIM injection (Norethisterone Enanthate, NETE) formulations. A phase II clinical\ntrial was conducted looking at NETE or NETA in combination with TU\ninjections. 56  Groups I and II received low-dose (200 mg) or high-dose\n(400 mg) NETE every 6 weeks with 1000 mg TU every 6 weeks, respectively. Group\nIII received 10 mg daily NETA with 1000 mg TU every 6 weeks. These regimens\nproduced sperm concentrations < 1 million/ml in all participants with\nazoospermia in 13/14 (group I), 11/12 (group II), and 12/14 (group III). The\nside effects included increase in body weight, erythrocytes, hemoglobin, and\nhematocrit and decrease in HDL and alkaline phosphatase. They also noted an\nincrease in liver enzymes in the oral NETA group.\nMedroxyprogesterone acetate (MPA) is an oral or injectable progestin used\ncommonly in depot formulation for female birth control. It also treats\nmenopausal symptoms, endometriosis, abnormal uterine bleeding, certain cancers,\nprecocious puberty, and male paraphilia. 47 , 57  The depot formulation\n(300 mg IM every 3 months) was studied in tandem with testosterone implants\n(every 3 or 4 months) in 55 healthy men as a form of long-acting male birth\ncontrol. 58  Once the men reached azoospermia, they entered a\n12 month efficacy period. They reported no pregnancies in 426 person-months.\nThere were two men (3.6%) who did not reach azoospermia. Once the efficacy\nperiod was terminated, there was complete recovery to sperm\nconcentration > 20 million/ml in 5 months.\n\nPrior testosterone and progesterone treatment options have been inconvenient for\nstudy participants due to the multiple routes of delivery and dosing regimens.\nCurrent hormonal options are not ideal for patients due to several factors including\nadverse side effect profiles of testosterone excess, failure rates of current\nhormonal agents, and rates of recovery of spermatogenesis. 59  Novel hormonal agents have\naimed to simplify the route of delivery and dosing regimen to make them more\nconvenient for patients.\nSegesterone acetate (SGA) is a progestin that has no androgenic, estrogenic, or\nglucocorticoid activity. Nestorone (NES), the brand name equivalent of SGA, has\nbeen formulated as a transdermal gel, making it a possible companion for\ncombined testosterone and progestin gel birth control. One of the first male\nbirth control studies to employ daily NES gel analyzed the effect on serum\ngonadotropin levels with and without combined testosterone gel in\n2009. 60  This was a randomized, unblinded, dual-center study with\n140 healthy men. There were seven groups of 20 men who received between 2 and\n8 mg of daily NES gel with or without 10 mg testosterone gel. They found the\nhighest levels of gonadotropin suppression (<0.5 IU/L) in the combined\ngroups, suggesting a need for further research into effects on sperm\nconcentration with the combined NES plus testosterone regimen.\nThe first study to look at the effects of NES gel plus testosterone gel on sperm\nconcentrations was in 2012. 61  This was a randomized,\ndouble-blind, dual-center study with 99 healthy men. There were three treatment\ngroups, each with 10 mg testosterone gel plus 0, 8, or 12 mg NES gel. These were\nadministered as two gels. The primary endpoint was azoospermia at 20–24 weeks.\nThey found similar rates of azoospermia in the testosterone plus 8 mg NES group\nand testosterone plus 12 mg NES group (89% and 88%, respectively). These were\nsignificantly higher than the placebo group (23%,  p  < 0.0001\nand  p  = 0.0002). The median serum free testosterone and total\ntestosterone were also maintained in the normal range. Only 57% adhered to the\nprotocol, prompting the group to look at patient satisfaction. Serious side\neffects were minimal, five men discontinued the trial due to irritability,\nnightmares, decreased libido, increased appetite, mood swings, and asthma\nexacerbation. The remaining participant discontinuations were due to\ninconvenience of the dosing regimen and frequency of study related visits. In\n2014, the group reported a 56% satisfaction with the regimen, and only 33% said\nthey would use the two gels as their primary form of contraception. 62\nIn order to improve compliance, a single gel formulation with combined NES and\ntestosterone was developed (NES-T). 63  This daily, single gel\nbirth control was studied in 44 healthy men in a double-blind, controlled\ntrial. 63  They found that 84% of the NES-T group had suppressed\nserum gonadotropins by day 28 compared to 16.7% in the T-only gel group\n( p  < 0.001). Importantly, 80% of the men were satisfied\nwith the regimen and over 50% said they would use the single gel formulation as\ntheir primary form of contraception if available on the market.\nDimethandrolone undecanoate (DMAU) is a synthetic precursor drug of\ndimethandrolone (DMA), which acts on both androgen and progesterone receptors in\nthe body. The dual androgen and progesterone activity have made it an appealing\ntarget for single agent male birth control. There are oral and IM formulations\nof this drug. A randomized, double-blind, phase I trial in 2014 looked at safety\nfor doses of DMAU between 25 and 800 mg  versus \nplacebo. 64  At doses above 200 mg of oral DMAU, serum gonadotropins\nand sex hormones were significantly depressed when taken with food. There were\nno serious adverse events, changes in vital signs, or laboratory changes up to\nconcentrations of 800 mg. In 2019, a double-blind, randomized,\nplacebo-controlled study with 100 healthy men looked at serum testosterone and\ngonadotropin levels at various doses of oral DMAU for 28 days. 65  They\nfound that doses of 200 mg per day were sufficient to markedly reduce serum\ntestosterone, LH, and FSH. Acceptability surveys showed that 87% of men\nreceiving the active medication were satisfied with this birth control method,\n91% reported no difficulty taking the pills with a high-fat meal, and more than\nhalf would use it as their primary form of contraception. 66\n7α-Methyl-19-nortestosterone (MENT) is a synthetic androgen with activity at both\nandrogen and progesterone receptors. 67  It is more potent than\ntestosterone and has more resistance to 5-α-reductase. 68  MENT acetate was\ndeveloped as a long-term birth control using ethylene vinyl acetate implants.\nThis drug was studied in the early 2000s in 35 men who received between one and\nfour subdermal implants (each implant released approximately 400 µg per day).\nThese were maintained for 6–12 months. They found an increase in MENT serum\nlevels with suppressed testosterone, LH, and FSH levels. Azoospermia occurred in\n67% (8/12) of the men in the four-implant group. In 2007, MENT acetate implants\nwere studied in combination with ENG implants  versus \ntestosterone pellets with ENG implants. 69  They achieved azoospermia\nin 80% of men in the MENT group within 12 weeks. Unfortunately, suppression was\nnot maintained due to decrease in MENT release from the implants over time. Side\neffects included loss of libido in 60% of men. They also noted decreased serum\nprostate-specific antigen (PSA) with no change in bone mass, which is due to the\nresistance to 5-α-reduction and lower activity in the prostate.\n11β-Methyl-19-nortestosterone-17β-dodecylcarbonate (11β-MNTDC) is a novel\nandrogenic steroid with progestogenic activity, identified for its specificity\nin suppressing spermatogenesis with a lack of systemic side effects. It is a\nprodrug of 11β-methyl-19-nortestosterone (11β-MNT), the biologically active\ncompound. 11β-MNTDC does not undergo aromatization and therefore has fewer\nexpected estrogenic effects. It is also more resistant to 5-α-reduction and\ntherefore has fewer expected androgenic side effects than testosterone. This\ndrug was studied in a randomized, double-blinded, placebo-controlled, phase I\nclinical trial in 2019. 70  Single oral doses (0, 100, 200, 400, and 800 mg) were\ngiven with or without food. They found that ingestion with food significantly\nincreased serum 11β-MNTDC and 11β-MNT levels. They found that doses up to 800 mg\nwere safe and doses between 200 and 800 mg were sufficient to significantly\nsuppress serum testosterone levels. Future studies are warranted to examine\ngonadotropin suppression and its effects on spermatogenesis.\nGnRH antagonists act by suppressing LH and testosterone through competitive\ninhibition of pituitary GnRH receptors. In a randomized clinical trial in 2004,\ndifferent GnRH antagonist preparations were found to cause azoospermia in 39 of\n47 subjects, and more recently, acycline has been tested in contraceptive\ntrails, although its primary use is in the treatment of prostate\ncancer. 71  Despite promising results, GnRH antagonists require\ndaily or weekly injections, and incur high costs, leading to many drug\ndevelopers to not pursue them further ( Table 1 ).\nSummary of novel hormonal agents.\nDMA, dimethandrolone; DMAU, dimethandrolone undecanoate; FSH,\nfollicle-stimulating hormone; LH, luteinizing hormone; NES,\nNestorone; NES-T, NES and testosterone.\n\nSince the 1950s, researchers have realized that spermatogenesis can be suppressed\nwithout directly acting on the hypothalamus-pituitary-testes axis. Multiple\nnon-hormonal agents have been identified that either irreversibly or reversibly\ndisrupt sperm maturation. These agents may be good candidates for male contraception\nwhen compared to hormonal agents, avoiding the side effects that occur when altering\nthe hormonal pathways, the stigma of hormone supplementation (anabolic steroid use\nin sports), and the difficulties of the dosing regimen or route of administration of\nhormonal agents.\nThese non-hormonal agents can theoretically act at any stage of spermatogenesis.\nCommon targets include the testicular retinoic acid receptor, Sertoli cell–germ cell\ninteractions, testicular epigenetics, and an assortment of small molecule\ninhibitors.\n\nRetinoic acid plays an essential role in spermatogenesis, assisting with\ndevelopment of the blood–testis barrier (BTB), spermatogonial differentiation,\nand spermiation. 72  Male mice with knock-outs of the retinoic acid receptor\nα (RARα) are infertile. 73  In the late 1950s, researchers studying antiparasitics\nnoted that rats receiving retinoic acid inhibitors developed the unintended\nconsequence of infertility. This realization led to human studies of retinoic\nacid inhibitors as the first potential non-hormonal male\ncontraceptives. 74  During the initial human studies at the Oregon State\nPenitentiary, approximately 60 inmates remained azoospermic for 1 year while\nreceiving the drug designated WIN 18,446. Unfortunately, when one of the inmates\nobtained access to contraband whiskey, they became incredibly ill with what was\nlater identified as a disulfiram reaction. This is due to the mechanism of the\ndrug, targeting aldehyde dehydrogenase 1A2 in the testes to block the production\nof retinoic acid. Off-target action on aldehyde dehydrogenase 2 in the liver\ncauses accumulation of serum acetaldehyde, with the associated systemic\nsymptoms. The use of WIN 18,446 was ultimately abandoned, but recent attempts\nhave been made to develop retinoic acid receptor antagonists more specific to\nthe testes.\nBristol-Myers-Squibb (BMS) has developed a series of retinoic acid receptor\nantagonists with varying degrees of specificity to the testes. BMS-189453 is a\npanretinoic acid receptor antagonist developed in the 1990s with action on the\nα, β, and γ receptors. 75 , 76  In rat models, administration of doses between 5 and\n240 mg/kg daily of BMS-189453 resulted in marked testicular degeneration and\natrophy. 75  Doses higher than 240 mg/kg resulted in severe toxicity\nand death. Other studies looked at lower doses of BMS-189453. Chung  et\nal. 77  used 5 mg/kg for 2 weeks or 2.5 mg/kg for 4 weeks and\nfound that all mice were sterile at 4 weeks with return of fertility by\n20 weeks. Even doses as low as 1 mg/kg for 4 to 16 weeks resulted in 100%\nsterility with a return of fertility after termination of the drug. 78\nOther studies have looked at more α-selective antagonists such as BMS-189532 and\nBMS-189614. 79 , 80  Despite promising  in vitro  studies,\nwhen given to mice at 2 and 10 mg/kg orally for 7 days, these medications showed\nlower potency than non-selective antagonists. 76 , 80  Normal spermatid\nformation and sperm release were observed when given orally, but expected\ndefects in spermatogenesis were seen with intravenous formulations. This\nsuggests that factors, such as hepatic metabolism, higher plasma protein\nbinding, or decreased testicular permeability are decreasing the bioavailability\nof the α-selective agents.\nYCT529 is an α-selective retinoic acid antagonist recently presented at the\nAmerican Chemical Society national meeting 2022. 81  When given to mice orally\nfor 4 weeks, it reduced sperm counts and prevented 99% of pregnancy. There were\nno systemic side effects seen with less specific retinoic acid receptor\nantagonists. Importantly, mice could reproduce for 4–6 weeks after stopping the\nmedication. Reportedly, human clinical trials will begin this year.\n\nIndenopyridine derivatives, such as CDB-4022 and RTI-4587-073(l), have been shown\nto inhibit mature sperm production in rat, primate, and stallion\nmodels. 82 – 84  These\nmolecules work primarily by disrupting the attachment of immature spermatids\nfrom the seminiferous tubules, causing sloughing of immature, non-motile germ\ncells into the semen. Hild  et al. 82  studied the effects of a\nsingle dose of CDB-4022 on rat testicle ultrastructure. They noted degenerative\nchanges in both Sertoli cells and spermatids. The Sertoli cells had an increase\nin the number of vacuoles, cellular debris, swollen mitochondria, and swollen\nendoplasmic reticulum. The spermatids had diffuse chromatin and broken nuclear\nenvelopes. Primate studies with the same molecule showed sperm concentrations\nlower than 1 million/ml by 17 days and remained suppressed for\n6 weeks. 83  More interestingly, they found sperm motility had\ndropped to 0% with immature spermatids present. Serum Inhibin B was elevated,\nbut testosterone, LH, FSH, and estradiol were within normal ranges. All\nparameters of sperm health and serum markers returned to normal by 17 weeks.\nThe indenopyridine derivative RTI-4587-073(l) was studied in miniature\nstallions. 84  They noted severe oligoasthenozoospermia with high\nnumbers of immature germ cells in the ejaculate. They also noted increased FSH\nconcentrations in the treated stallions, reflecting the drugs activity on\nSertoli cells. The semen parameters and serum gonadotropin concentrations were\nfully reversible in about 70 days.\nAnalogues of Lonidamine, a chemotherapy drug, have been analyzed as potential\nreversible male birth control agents. Adjudin, one such Lonidamine analogue, is\nderived from 1H-indazole-3-carboxylic acid. It works by disrupting the\nSertoli-germ cell junction by targeting the apical ectoplasmic specialization\nproteins. 85 , 86  This leads to the exfoliation of immature spermatids.\nAdjudin was studied in a rat model with two doses of 50 mg/kg weekly, which\ncaused infertility in 100% of subjects. 87  Unfortunately, the target\nproteins of this drug were not specific to the gonads, and side effects included\nliver inflammation. This group attempted to increase testicular specificity by\nconjugating Adjudin with the carrier molecule FSH-β. They showed that this\napproach successfully increased gonadal selectivity and decreased the effective\ndose from 50 to 0.50 µg/kg in the rat model. 87\nThe BTB plays an important role in regulating all non-hormonal birth control\nagents. Recent rat studies have shown that overexpressing F5-peptide, an\nendogenous BTB modulator, can enhance the bioavailability of Adjudin in the\ntestis and lower the needed dose to induce reversible infertility. 88  By\nlowering the effective dose of Adjudin, this may lower the systemic side effects\nto an acceptable level. To date, there are no published  in vivo \nhuman studies with Adjudin.\nGamendazole is a Lonidamine analogue currently being studied for its\nantispermatogenic properties. This molecule works by inhibiting the heat shock\nprotein HSP90AB1 and the eukaryotic translation elongation factor EEF1A1,\nleading to increased Interleukin 1α production by Sertoli cells. 89 \nInterleukin 1α disrupts the Sertoli cell-germ cell junction, leading to\npremature exfoliation of immature spermatids, similar to Adjudin. There is also\na noted decrease in Inhibin B and associated increase in FSH. Initial rat\nstudies demonstrated 100% infertility 3 weeks after a single dose of\n6 mg/kg. 90  Unfortunately, fertility only returned to 57% of the\ninitial cohort. Doses of 3 mg/kg resulted in 66% infertility and 100% return of\nfertility. Further studies into dosing and reversibility need to be completed\nprior to human studies.\n\nCatSper is a calcium ion channel specific to sperm flagella and is essential for\nsperm motility. 91 , 92  It has roles in flagella hyperactivity, chemotaxis toward the\nfemale ovum, capacitation, and the acrosome reaction. 92  There are four CatSper\nchannels that have all been shown to be critical to male fertility in mouse\nknock-out models. 91 , 93 , 94  Below are several promising sperm CatSper blocker targets.\nOther sperm-specific ion channels, such as the potassium channels KSper and SLO3,\nare also being investigated as potential targets.\nRU1968 is a synthesized inhibitor of CatSper and SLO3. 95  Cross-species studies,\nincluding sperm from humans, mice, and sea urchins, showed inhibition of CatSper\nwith a 15-fold higher potency than SLO3. 95  No toxic side effects\nwere seen in human sperm. The mechanism of action is still being elucidated, but\nRU1968 was shown to inhibit the pro-motility response caused by progesterone in\nthe female reproductive tract. 95 , 96\nHC-056456 is a CatSper blocker shown to reversibly and selectively decrease ion\ntransit through the CatSper channel of patch-clamped sperm. 91  When\ntreated with HC-056456, human sperm loses flagellar hyperactivity. 91 \n In vivo  studies in a mouse model found decreased fertilization\nin HC-056456 treated sperm when inseminated into the uterus. 97  This was\nthe first  in vivo  study to look at CatSper blockers as a\npotential male contraceptive in a mammalian model.\nSLO3 is a sperm-specific potassium channel that regulates calcium entry through\nthe CatSper channel. 98  SLO3 works by hyperpolarizing the cell during\ncapacitation to allow for calcium entry  via  CatSper. It may\nalso work indirectly by altering the pH of sperm. Knock-out mice lacking SLO3\nare infertile. 98  Research is being done to look at potential SLO3\ninhibitors, such as quinine and quinidine, but to date there are no human\ntrials. 99\nPristimerin and lupeol are plant triterpenoids that are thought to inhibit sperm\nhyperactivation  via  the CatSper channel. Pristimerin is an\nisolate from  Tripterygium Wilfordii , and Lupeol is isolated\nfrom dandelion root, aloe vera, and mangos. They are thought to work by binding\nCatSper and blocking activation by progesterone and pregnenolone. 100  Further\nresearch has cast doubt on the efficacy of these medications at inhibiting sperm\nhyperactivation, and further research is still needed to determine their\npotential as male contraceptives. 96\n\nAn emerging area of interest is small molecule inhibitors of the spermatogenesis\npathway. These molecules act by blocking the action of target proteins, typically\nenzymes. Below are examples of small molecule inhibitors that act in the male\nreproductive tract.\nJQ1 is a small molecule inhibitor of a testicular bromodomain protein named\nBRDT. 101  Bromodomain proteins are involved in epigenetics by\nassisting with histone acetylation, chromatin remodeling, and recruiting\ntranscription factors. 102  BRDT knock-out mice are healthy but sterile. 103  Mouse\nmodels suggest that JQ1 leads to infertility by reducing the number of\nspermatozoa, sperm motility, and seminiferous tubule volume without altering\nserum hormone levels. 101  Male mice pretreated with JQ1 for 6 weeks were unable\nto procreate when caged with female mice continuously. 104  These effects were\nreversible, but JQ1 had off-target effects on non-testicular bromodomain\nproteins. 105  Future research will need to advance testicular\nspecificity.\nEP055 is a small molecule inhibitor of Eppin, an epididymal protease\ninhibitor. 106  Eppin is secreted by Sertoli cells and binds to the\nsperm surface in the testes. It is bound by semenogelin from the seminal vesicle\nduring ejaculation to modulate sperm motility. Eppin then regulates PSA enzyme\nactivity to hydrolyze semenogelin and induce progressive motility. 107  In male\nnonhuman primates immunized with Eppin, 78% developed high titers to Eppin and\nwere infertile. After cessation of immunocontraception, 71% recovered\nfertility. 108  Further primate studies showed that infusion with low\ndose EP055 (75–80 mg/kg) followed by high dose (125–130 mg/kg) led to no normal\nsperm motility 30 h after initial infusion and full recovery of sperm motility\nby 18 days. 106\nCyclosporine A and FK506 (tacrolimus) are calcineurin inhibitors used as\nimmunosuppressant drugs. 109  Their adverse effect on\nmale fertility has been observed and led to research into their efficacy as\nreversible non-hormonal male contraceptives. The sperm-specific calcineurin\nsubunits PPP3CC and PPP3R2 are potential targets for inhibition, as mice with\nknock-out in these genes are infertile with reduced sperm motility. 109 , 110  The\nmechanism is believed to be the inflexibility of the sperm midpiece. When\nhealthy mice are treated with Cyclosporine A or FK506, they develop defects of\nsperm motility and morphology within 4–5 days. These effects are reversible\nwithin 7 days of stopping the medication. 109  Testis-specific\ncalcineurin inhibitors may be a viable form of reversible male contraception.\nOther molecules essential to the sperm calcineurin pathway may also be\ntargetable. Inhibition of SPATA33, which localizes calcineurin to the\nmitochondria, has also led to infertile knock-out mice with reduced sperm\nmotility. 111\n\nInhibition of the peristalsis of sperm through the male reproductive tract is an\nadditional mechanism that has been explored for possible male contraceptives. Early\nstudies explored phenoxybenzamine, an alpha-1-adrengergic antagonist, for its\ncontraceptive characteristics through the selective blockade of the longitudinal\nmuscles of the vas deferens and inhibition of peristalsis. 112  It was previously shown to\ncause reversible infertility of rats and block ejaculation in a small cohort of\nadult males. 113 , 114  Studies have explored other alpha-1-adrenergic antagonists as\nwell, namely prazosin and tamsulosin. Despite prazosin’s debated efficacy as a\ncontraceptive agent, tamsulosin has previously been shown to decrease sperm\nconcentration; albeit with side effects such as dizziness and orthostatic\nhypotension. 115 – 119  To date, there have been no\nlarge-scale clinical trials evaluating alpha-1-adrenergic antagonists as potential\nmale contraceptives. Most recently, P2X1-purinoreceptor antagonists have been\npostulated as another option. P2X1-purinoreceptors are adenosine triphosphate (ATP)\nligand–gated cation channels expressed on the cell membranes of smooth muscle cells\nalong the vas deferens, and their absence leads to impaired peristalsis and reduced\nsperm concentration in the semen. 120 , 121  Eise  et\nal. 122  showed that a selective blockage of P2X1-purinoreceptors,\nthrough an oral agent in rats, led to a reduced number of pregnancies during mating.\nIt has yet to be further evaluated in large-scale clinical trials as well.\n\nTriptonide is a traditional Chinese herb produced from the vine of  T.\nWilfordii , also known as thunder god vine. Observational studies\nlooking at  Tripterygium  use for rheumatoid arthritis in humans\nshowed lower sperm counts. 123  Triptonide use in mice\nand primates caused morphological defects and sperm with no forward\nmotility. 124  Infertility was seen by 3–4 weeks in male mice and\n5–6 weeks in cynomolgus monkeys after initiation. Both species regained\nfertility in 4–6 weeks after stopping the medication. There were no serious\nsystemic effects or changes in serum markers in these studies. It appears to\ndisrupt spermatogenesis by targeting the interaction between junction\nplakoglobin and SPEM1 (spermatid maturation 1) receptor on the sperm. 124  Mice\nwith SPEM1 gene knock-out have deformed sperm with a characteristic kinked head\nwrapped around by the mid-section of the tail. 125  Naturally occurring\ncompounds causing infertility may continue to offer insight into future male\ncontraceptive pathways.\nJusticia gendarussa  Burm f. (Acanthaceae), colloquially known as\nthe gendarussa leaf, is a medicinal plant local to tropical areas in southeast\nAsia and the Pacific islands and has been used as a method of male contraception\nby many from the region. 126  It has also been\nexplored for its potential use in a variety of conditions, including, but not\nlimited to, hypertension, AIDS, arthritis, and liver disease. 127 – 130  Specifically, the\nactive ingredient, flavonoid gendarusin A, has shown to reversibly inhibit the\nactivity of spermatozoa hyaluronidase, an enzyme crucial to sperm penetration of\nthe cumulus oophorus during the acrosome reaction. 131 – 133  The chemical make-up of\nthe leaf has been explored previously, but its potential use as a male\ncontraceptive has yet to be demonstrated in dedicated preclinic contexts and\nclinical trials. 134 – 136\nHistone demethylation plays a crucial role in germ cell regulation in both mice\nand humans. 137  KDM5B, a histone demethylase, and several other small\nmolecules that target histone demethylases (CPI-455, PBIT, and KDM5-C70) have\nbeen identified as potential targets for male contraception ( Table 2 ).\nSummary of novel non-hormonal agents.\nACS, American Chemical Society; BMS, Bristol-Myers-Squibb; FSH,\nfollicle-stimulating hormone; SPEM1, spermatid maturation 1.\n\nThere are more than 1500 human genes associated with male infertility, of which 200\nare believed to be reproductive tract specific. 138  These genes and proteins\noffer a host of enticing targets for future non-hormonal male contraceptives. Sinha\n et al. 139  have recently reported on\nthe development of a searchable database to help researchers identify high-quality\ncontraceptive targets (Contraceptive and Infertility Target DataBase,  https://www.citdbase.org ). Current advances in PROTAC (proteolysis\ntargeting chimera) and CRISPR (clustered regularly interspaced short palindromic\nrepeats) technologies have unlocked the ability to eliminate specific proteins and\nknockout specific genes, respectively. PROTACs eliminate proteins\n via  the ubiquitin-proteasome pathway and allow for the\ndegradation of enzymatic and non-enzymatic proteins along the spermatogenesis\npathway. CRISPR, on the contrary, allows us to study genes involved with the\ndifferent stages of spermatogenesis. The ideal targets are specific to the testes\nand have no paralogs. 138 \n Table 3  shows a subset\nof promising targets for these technologies that have been separated into the stage\nof spermatogenesis where they function. 138\nSelection of most promising non-hormonal target genes.\n\nThe Bimek SLV is an implantable device that occludes the vas with a reversible\nvalve mechanism. 140  The device is implanted in the outpatient setting, and\nthe patient is able to operate the device with a palpable switch. It was first\nimplanted in humans in 2009 and is currently undergoing further human clinical\ntrials. This device theoretically allows patients to switch their fertility on\nand off but would require a wash out period each time the device is closed,\nsimilar to the waiting period required after vasectomy.\nAnother area of active research is reversible vasal occlusion with polymer\ninjectables. RISUG ®  (reversible inhibition of sperm under guidance)\nis an ultrasound guided, sterile styrene maleic anhydride polymer injected into\nthe vas. RISUG is approved in India for permanent sterilization and has been\nshown to be reversible in animal models. 141  It is currently\nundergoing Phase III clinical trials. Vasalgel and Echo-V are two other polymer\ninjectables currently in development. 142  These are promising\nfuture products for reversible, procedural male sterilization.\n\nReliable family planning is an essential element of modern society. For much of human\nhistory, the burden of unintended pregnancies have fallen disproportionately on the\nmother. Recent concerns regarding access to legal abortion has sparked a renewed\ninterest for reliable male factor contraceptives beyond surgical sterilization and\ncondoms. Modern efforts to develop a male birth control agent date back to the\n1930s, and initially focused on altering the hypothalamic-pituitary-testes axis.\nHormonal contraceptives face multiple barriers including systemic side effects,\nchallenges with dosing regimens, route of delivery, and the public stigma of\nanabolic steroid abuse. Despite these challenges, some of the most widely studied\nand accepted potential male contraceptives are novel hormonal agents, such as DMAU\nand nestorone/testosterone gels. The recognition of retinoic acid receptors as\npotential reversible male contraceptives in the 1950s opened the door to exploring\nvarious non-hormonal agents. These novel non-hormonal contraceptives can target\nspermatogenesis at any stage of development, but common targets include Sertoli\ncell–germ cell interactions, sperm ion channels, testicular epigenetics, and an\nassortment of small molecule inhibitors. Several non-hormonal agents have entered\nhuman trials. The identification of reproductive tract–specific genes associated\nwith male infertility, coupled with advances in PROTACs and CRISPR, may lead to more\ntargeted drug development in the future. Despite multiple promising contenders over\nthe last half century, no novel birth control agents have garnered regulatory\napproval in the United States or abroad. Future research is needed to achieve novel\nforms of male birth control.","source_license":"CC-BY-4.0","license_restricted":false}