D-Amino Acids as Key Modulators in Neurology: Unlocking the Potential of D-Tryptophan and D-Phenylalanine

D-Amino Acids as Key Modulators in Neurology: Unlocking the Potential of D-Tryptophan and D-Phenylalanine | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 25 March 2025 V1 Latest version Share on D-Amino Acids as Key Modulators in Neurology: Unlocking the Potential of D-Tryptophan and D-Phenylalanine Authors : Yogita Dhurandhar , Shubham Tomar , and Kamta P. Namdeo 0000-0002-1319-0486 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.174293291.15732574/v1 773 views 183 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract [1]¿p1 [1]¿m1 Due to the lack of effective treatments, neurological diseases such as depression, Parkinson’s, and Alzheimer’s pose a serious threat to world health. D-amino acids (DAAs) have recently been shown to have the ability to modulate brain activity, providing new treatment avenues. D-amino acids, specifically D-tryptophan and D-phenylalanine, are examined in this review in relation to the treatment of neurological conditions. DAAs have unique pharmacological characteristics that affect different brain biological pathways, which sets them apart from their L-forms and makes them attractive options for targeted treatments. The history of neurological conditions and the ways that D-amino acids affect neurotransmission are the first things to look at. The review discusses D-tryptophan’s neuroprotective properties, such as its metabolism and role in serotonin synthesis, and provides information on how it may help treat mood disorders and neurodegeneration. Likewise, the processes and neuroprotective attributes of D-phenylalanine are examined, highlighting its involvement in dopamine synthesis and its therapeutic potential for disorders such as Parkinson’s disease. Furthermore, innovative tactics for improving drug delivery across the blood-brain barrier utilizing DAAs was investigated, including conjugation-based methods and solubility augmentation techniques. These strategies have the potential to transform the targeted administration of medicines. Future directions for DAAs in the treatment of neurological disorders are covered in the review’s conclusion, along with current research and possible therapeutic uses. In the end, D-amino acids offer a novel and distinctive approach to brain health and a viable path forward for improving treatment approaches in neurological illnesses. ”D-Amino Acids as Key Modulators in Neurology: Unlocking the Potential of D-Tryptophan and D-Phenylalanine” Yogita Dhurandhar 1 , Shubham Tomar 2 , Kamta P. Namdeo 1 * 1. Department of Pharmacy, Guru Ghasidas Vishwavidyalaya (A Central University), Bilaspur Chhattisgarh, India. Email- [email protected] . Orcid id: 0009-0003-9825-10262. Pharmacovigilance Programme of India, Indian Pharmacopoeia Commission, Ministry of Health & Family Welfare, Government of India, Ghaziabad, Uttar Pradesh, India. Email- [email protected] . Orcid Id: 0009-0008-9910-9300Postal address- Department of Pharmacy, Guru Ghasidas Vishwavidyalaya (A Central University), Bilaspur Chhattisgarh-495009, IndiaCorresponding author- Prof. (Dr.) Kamta P. Namdeo*Email id- [email protected] . Orcid id: 0000-0002-1319-0486 Funding We express our gratitude to the participants of the studies referenced in this review for their invaluable contributions. Nevertheless, it is imperative to recognise that we did not obtain any dedicated financing for the compilation of this review paper. Author contributions Yogita Dhurandhar contributed to the conceptualization, literature review, and drafting of the manuscript, focusing on the role of D-amino acids in neurological disorders. Shubham Tomar assisted in analyzing the pharmacological effects of D-tryptophan and D-phenylalanine and contributed to the writing and editing of the paper. Kamta P. Namdeo provided overall guidance, critical revisions, and coordinated the submission process, ensuring the accuracy and clarity of the review. Conflict of interest No potential conflict of interest relevant to this article were reported. Data availability statement No data was used for the research described in the article. not-yet-known not-yet-known not-yet-known unknown Ethics approval & Patient consent statement This review paper does not involve human or animal studies, clinical trials, or data collection from participants. Therefore, ethics approval is not applicable. not-yet-known not-yet-known not-yet-known unknown Permission to reproduce material from other sources Not Applicable [1]¿p1 [1]¿m1 ABSTRACT Due to the lack of effective treatments, neurological diseases such as depression, Parkinson’s, and Alzheimer’s pose a serious threat to world health. D-amino acids (DAAs) have recently been shown to have the ability to modulate brain activity, providing new treatment avenues. D-amino acids, specifically D-tryptophan and D-phenylalanine, are examined in this review in relation to the treatment of neurological conditions. DAAs have unique pharmacological characteristics that affect different brain biological pathways, which sets them apart from their L-forms and makes them attractive options for targeted treatments. The history of neurological conditions and the ways that D-amino acids affect neurotransmission are the first things to look at. The review discusses D-tryptophan’s neuroprotective properties, such as its metabolism and role in serotonin synthesis, and provides information on how it may help treat mood disorders and neurodegeneration. Likewise, the processes and neuroprotective attributes of D-phenylalanine are examined, highlighting its involvement in dopamine synthesis and its therapeutic potential for disorders such as Parkinson’s disease. Furthermore, innovative tactics for improving drug delivery across the blood-brain barrier utilizing DAAs was investigated, including conjugation-based methods and solubility augmentation techniques. These strategies have the potential to transform the targeted administration of medicines. Future directions for DAAs in the treatment of neurological disorders are covered in the review’s conclusion, along with current research and possible therapeutic uses. In the end, D-amino acids offer a novel and distinctive approach to brain health and a viable path forward for improving treatment approaches in neurological illnesses. Keywords- D-tryptophan, D-phenylalanine, Alzheimer’s disease, neuroprotection, blood-brain barrier, DAAs not-yet-known not-yet-known not-yet-known unknown Abbreviations AAs- Amino acids DAAs- D-amino acids LAAs- L-amino acids BBB- blood-brain barrier NMDA- N-methyl-D-aspartate AD- Alzheimer’s disease CNS- Central nervous system KP- kynurenine pathway CSF- cerebrospinal fluid BH4- tetrahydrobiopterin 3-HK-3-Hydroxykynurenine PAH- phenylalanine hydroxylase HD- Huntington’s disease D-Asn- D-Aspartic Acid QUIN -Quinolinic Acid ASD- autism spectrum disorder INF-β- interferon-beta XA- xanthurenic acid PKU- phenylketonuria MDD- Major depressive disorder TRYCATs- TRP catabolites Aβ- amyloid-beta PTSD -post traumatic stress disorder MDT-1-Methyl-D-tryptophan TDO- tryptophan 2,3-dioxygenase HAA- hydroxyanthranilic acid RDS- reward deficiency syndrome Introduction All chiral compounds known as alpha amino acids apart from glycine are found exclusively in the L-forms of proteins and peptides found in mammals, as opposed to the D-forms [1]. As stereoisomers, L- and D-amino acids have different configurations (based on comparison with glyceraldehyde, which is designated as D). L-amino acids possess the amino group on the left side of the alpha carbon, while D-amino acids have it on the right. In addition to the L-isomeric form of amino acids (AAs), which are of significant nutritional value, the D-isomeric form of AAs is a common component of the cell wall of bacteria [2]. High temperatures, abnormal pH levels, adulteration, and microbiological contamination are all examples of harsh technical procedures that might encourage the existence of D-amino acids in food products. When it comes to a variety of brain illnesses, the detection of free AAs is also highly significant. The majority of the numerous methods that are available for the detection of AAs is complex and requires costly instrumentation, labor-intensive sample preparation, and proficient operators. With regard to chromatographic approaches, this is especially true [3]. In the late 1980s, it was revealed that free d-aspartate and d-serine operate as neurotransmitters in the brains of mammalian species [4]. Prior to this discovery, it was believed that D-amino acids, which are enantiomers of L-amino acids, carried no biological role in higher organisms. Recent pioneering research on D-tryptophan and D-phenylalanine has unveiled novel possibilities for the treatment of neurological illnesses, presenting intriguing alternatives to traditional therapy [5]. Amino acids predominantly reside in the L-form, which is naturally abundant in the body [6]. The D-form of these amino acids, which are enantiomers of their L-counterparts, has demonstrated distinctive features with potential therapeutic advantages in the treatment of numerous neurological disorders [7]. Serotonin’s precursor, L-tryptophan, is essential for mood regulation, sleep patterns, and brain health [8]. Recent research has brought attention to the fascinating prospect that D-tryptophan may be able to modulate neurotransmitter activity. This suggests that D-tryptophan may be useful in improving cognitive performance and managing mood disorders like anxiety and depression [9]. In a similar vein, D-phenylalanine, which is well-known for its function in dopamine synthesis, has shown promise in mitigating pain, elevating mood, and controlling stress reactions [10]. According to research, D-phenylalanine may also improve the efficacy of pain relief for people suffering from chronic pain and fibromyalgia [11]. The compelling nature of these D-amino acids lies in their superior capacity to traverse the blood-brain barrier compared to their L-isomers [2], a crucial aspect of their potential as neurotherapeutic agents. Ongoing research into D-tryptophan and D-phenylalanine may yield more precise and efficacious therapies for neurological conditions, hence enhancing patient outcomes and quality of life. Background behind Neurological Disorders Conditions that are classified as neurological illnesses typically impact not only the brain and spinal cord but also the nerves that are located throughout the body, weakness in the muscles, unconsciousness, seizures, loss of feeling, disorientation, pain, and even altered degrees of consciousness are some of the symptoms that may be encountered as a consequence of these illnesses [12]. There is evidence that diseases of this kind have been around since the beginning of human history. Nevertheless, the investigation of disorders of this kind did not begin to gain traction until a few decades ago. Genetic defects, congenital abnormalities, malnutrition, spinal cord injury, brain damage, nerve injury, disorders of the central or peripheral nervous system such as epilepsy, brain tumors, neuronal infections, and so on are some of the potential causes of neurological disorders [13]. Every year, stroke is responsible for the deaths of around six million individuals, with eighty percent of these deaths occurring in nations with low incomes [14]. Around the world, more than fifty million people suffer from epilepsy [15], forty-seven and a half million people are affected by dementia [16], and more than ten percent of people suffer from migraines. A major threat to public health, neurological diseases call for constant research and development of therapeutic approaches to fit their varied and complicated character. Role of D Amino Acids in Neurological Functioning D-amino acids, which represent comparatively low constituents relative to their L-amino acid counterparts. These L-amino acids have the potential to undergo racemization to D-isomers during the preparation of food. Previously believed to have a smaller biological impact than its L-enantiomers, D-amino acids are now known to have important roles in a variety of orgamism, including humans. The process of changing an amino acid entails replacing an amino-acid L with D. There are extremely few enzymes that can hydrolyze the amide bonds in humans’ very high concentration of D-amino acids [17] . The addition of non-natural amino acids into peptidomimetics enhances resistance to aminopeptidase degradation. Furthermore, when natural and synthetic amino acids are contrasted, the latter have lower oral bioavailability [18]. D-amino acids are used in biomaterials for obvious reasons: they are supposedly less susceptible to enzymatic digestion, which prolongs their stability and ability to function in a biological setting [19]. Studies use the term ”theoretical” because, despite the widespread belief that D-peptides are more resistant to protease activity than their L-analogues, there are surprisingly many instances of other enzymes that can operate on D-amino acids at a rate identical to that of L-amino acids [20]. D-serine functions in mammals as a co-agonist of NMDA receptors, which are involved in conditions including schizophrenia and neurodegeneration and are essential for learning, memory, and behavior [21]. In contrast, D-aspartate controls adult neurogenesis and is necessary for spermatogenesis and hormone synthesis in neuroendocrine and endocrine tissues [22]. Moreover, dietary proteins which have D-amino acids as a result of fermentation and heating during processing [23]. However, this process is only partially successful. The racemization process is affected by a number of parameters, including pH, time, and temperature, and the rate at which it occurs varies throughout the various L-amino acids that are found in proteins [24]. The existence of D-amino acids can hinder digestibility and nutritional quality by facilitating the production of D-peptide linkages and cross-linked amino acids, including lanthionine and lysinoalanine. There are two primary methods for employing D-amino acids: their conversion to L-isomers via racemases or epimerases, or their oxidative deamination by certain enzymes [25]. In mammals, the latter pathway is predominant; nonetheless, the effectiveness of these enzymes differs among species, potentially influencing the consumption of D-amino acids for growth. The amino acid content, digestibility, and physiological uptake efficiency of important D-amino acids determine their nutritional efficacy. Because D-amino acids are less susceptible to enzymatic degradation in proteins, their bioavailability can be greatly decreased, as they must be released through digestion in order to be absorbed nutritionally [26]. There are evidence that relates D-amino acids to a variety of pathological situations. D-amino acids are involved in several critical physiological processes; for instance, D-serine and D-aspartate function as co-agonists and agonists, respectively, at the NMDA receptor [27]. Changes in blood D-serine levels have been associated to a number of neurological and psychiatric conditions. In particular, amyotrophic lateral sclerosis, schizophrenia, and AD have all been linked to this biomarker. D-serine levels emerge as a consistent predictor of antidepressant responsiveness in major depressive disorder and posttraumatic stress disorder as well as a predictive biomarker for early cognitive impairment when examining both D-serine and D-proline levels concurrently. Furthermore, D-amino acids seem to serve as effective biomarkers for conditions such as pancreatic cancer and chronic renal illnesses that are not associated with the central nervous system [28]. not-yet-known not-yet-known not-yet-known unknown Table 1. Key roles of different DAAs 1 D-glutamate Influences learning and memory by modulating excitatory neurotransmission and synaptic plasticity. [29] 2 D-alanine Control of neurotransmitter synthesis [30] 3 D-Aspartate Influences synaptic transmission and plasticity via NMDA receptors. [31] 4 D-Serine Improves glutamatergic transmission and synaptic plasticity, especially in the hippocampus. [32] 5 D-Isoleucine Controlling brain energy metabolism and neuroprotection. [33] 6 D-Leucine Only anti seizure activity is known Responsible for protein synthesis and cell survival, potentially impacting neuroprotection and neuronal function. [34] 7 D-Phenylalanine Impacts mood, thought, and behavior by altering the metabolism of dopamine and serotonin. [35] 8 D-Threonine contributes to the control of neuronal cell signaling pathways and neurogenesis. [36] 9 D-Tyrosine Effects mood, stress response, and cognition by altering dopamine and norepinephrine synthesis. [37] 10 D-Arginine A precursor for nitric oxide (NO) synthesis, affecting vasodilation, neuroprotection, and communication pathways inside the brain. [38] 11 D-Cysteine Regulates redox and may protect neurons from oxidative damage. [39] 12 D-Histidine Histamine precursor implicated in neurotransmission, sleep-wake cycles, and immunity. [40] 13 D-Methionine Contributes to neuroprotection and myelin formation as a methyl donor. [41] 14 D-Lysine crucial for the production of proteins and the operation of neurons [42] 15 D-Proline contributing to the production of collagen and perhaps affecting the brain’s cellular matrix and neuroplasticity [43] 16 D-Valine Influences neurotransmitter equilibrium and neuronal health through amino acid metabolism. [44] 17 D-Tryptophan precursor of serotonin, which affects mood, sleep patterns, and emotional control. [8] 2. ”Why D-Amino Acids Are Preferred Over L-Amino Acids” There are several ways in which DAA might affect biological systems. They have two possible actions: they can function independently in a stereo-specific way, or they can be transformed into LAA, which serves as a reservoir of amino acids [45]. Furthermore, DAA may interact in particular processes but at a slower activation rate than LAA [46], or it may show non-specific actions akin to LAA. DAA may occasionally attach to sites antagonistically without activating them. These characteristics make DAA useful instruments for improving our understanding of metabolism by differentiating between specific and non-specific endogenous activities. The intricacy of their relationships, meanwhile, can make interpretations challenging [47], [48]. In some circumstances, D-amino acids are utilized more frequently than L-amino acids due to the distinctive biochemical features that they possess. D-amino acids offer improved stability, particularly in conditions where enzymatic degradation of peptides and proteins is a major issue. L-amino acids, on the other hand, are the primary amino acids found in natural proteins. Because of their resistance to degradation, D-amino acids are exceptionally useful in the creation of pharmaceuticals. They have the capacity to improve the stability and half-life of therapeutic peptides, so making them more efficient. Furthermore, D-amino acids have the ability to enhance the binding affinity of medications to their targets, notably in antimicrobial peptides [7]. This enhancement makes the pharmaceuticals more resistant to the enzymatic destruction that is caused by bacteria. When it comes to synthetic biology, the insertion of D-amino acids enables the production of new peptides and proteins that have their own distinct structures and applications. D-amino acids confer structural stiffness and stability to peptides, rendering them advantageous for the development of therapeutic medicines necessitating great stability, particularly in cancer and antiviral therapies. Their integration may result in peptides with less immunogenicity, since the immune system may not readily identify them, so averting undesirable immunological reactions. Moreover, dietary absorption of DAA is regulated differently from their peripheral use, with intestinal transporters optimized for maximal nutrient uptake. Adaptive advantages of utilizing DAA may be observed in organisms like Drosophila, which preferentially transport d-isomers, possibly due to symbiotic relationships with DAA-producing microbes [49]. 3. D-Phenylalanine: Mechanisms and Benefits not-yet-known not-yet-known not-yet-known unknown 3.1. Biological Pathways and Metabolism of Phenylalanine Phenylalanine hydroxylase catalyzes the conversion of phenylalanine to tyrosine [50](figure 1). Phenylalanine is a crucial amino acid that facilitates both glucogenic and ketogenic pathways [51]. A number of different metabolic pathways are responsible for the phenylalanine metabolism in the body. Enzyme phenylalanine hydroxylase (PAH), which requires the cofactor tetrahydrobiopterin (BH4), is responsible for the conversion of phenylalanine to tyrosine, which is the major route [52]. The formation of tyrosine is accomplished through the hydroxylation of phenylalanine throughout this phase. A decrease in the conversion of phenylalanine occurs when there is sufficient tyrosine present in the diet [53]. Phenylalanine is converted to tyrosine in the liver when there is a deficiency in tyrosine levels. The disorder known as phenylketonuria (PKU) can also be caused by aberrant metabolism of phenylalanine, which can occur when there are deficiencies in the PAH enzyme [54]. In patients with PKU, phenylalanine is unable to be turned into tyrosine, and as a result, it builds up in the body [55]. In situations like these, phenylalanine undergoes a process of transamination with α-ketoglutarate, resulting in the formation of phenylpyruvate. This process might result in serious consequences, including mental impairment. Additionally, phenylalanine that is in excess can be transformed into a variety of different metabolites, such as phenylethylamine, phenylacetic acid, phenylacetylglutamine, and phenylactic acid [56]. Metabolites are produced by a variety of enzymatic processes, particularly when there is an abundance of phenylalanine in the body in comparison to the amount of tyrosine that is required by the body. Phenylalanine has the ability to attach to hydroxyl radicals, which allows it to block the inhibition of acetylcholinesterase activity in brain homogenates that is caused by hydroxyl radicals. Dopamine, norepinephrine, and epinephrine are some of the main neurotransmitters and hormones that are produced as a result of the conversion of tyrosine, which is something that is essential for the creation of tyrosine [8]. Catecholamines have significance for the proper operation of the neurological system, are produced as a result of a series of enzyme processes that bring about additional modifications of tyrosine [57]. not-yet-known not-yet-known not-yet-known unknown Figure 1. Metabolism of Phenylalanine 3.2. Neuroprotective Properties of D-Phenylalanine D-phenylalanine demonstrates significant potential in modulating neuronal activity, particularly through its role as an enkephalinase inhibitor, which contributes to its analgesic effects [58]. The evidence supports its use in pain management and suggests neuroprotective properties that may benefit conditions characterized by neurodegeneration [59]. Notably, a substantial increase in phenylalanine levels was observed, which may reflect its direct administration. Additionally, the metabolic study using D-[14C]-phenylalanine confirmed that D-phenylalanine does not changes to β-phenylethylamine, indicating that its potential antidepressant effects may not stem from modulation of this pathway [60]. D-phenylalanine, as an enkephalinase inhibitor, shows promise in enhancing analgesia and reducing inflammation. Its efficacy in treating chronic pain in patients suggests potential benefits for ”endorphin deficiency” conditions, like depression, schizophrenia, and arthritis, as well as in managing opiate withdrawal symptoms [61]. Research has shown that D-phenylalanine activates carbonic anhydrases. D-phenylalanine was used to study carbonic anhydrases in memory formation. This study demonstrated that hippocampal carbonic anhydrases enhance object recognition and fear extinction memory [3]. By blocking enkephalinase, DPA boosts brain endorphins. This eases stress and PTSD (posttraumatic stress disorder). DPA boosts immunological response and regulates pro-dopamine, which may improve dopamine balance in stressed people. A strong calpain-1 inhibitor, 1 c, was synthesized using D-phenylalanine, resulting in better cognitive function and stereoselective inhibition in amnestic mice. Adding D-phenylalanine to the peptidomimetic structure increased the inhibitor’s efficacy (IC50=78 nM) and neurodegenerative disorder therapy potential [62]. D-phenylalanine and other aromatic amino acids benefit Parkinson’s disease patients on a low-protein, high-carbohydrate (LPHC) diet. PD treatment may involve D-phenylalanine’s ability to mediate the gut-microbiota-brain axis by improving motor skills, dopaminergic function, and gut microbiota balance [63]. The amino acid D-phenylalanine may improve nutritional support for traumatic brain damage. D-phenylalanine may be part of a synergistic combination of nutrients and supplements that speed up TBI recovery and enhance outcomes as it repairs brain function [64]. D-phenylalanine may have a role in the metabolic disturbances that are brought about by SN-38, particularly in the metabolism of amino acids, which in turn affects the signalling pathways and enzymes that are involved in the brain’s energy metabolism [65]. D-phenylalanine may regulate amino acid metabolism and neuroprotection (figure 2). It may affect oxidative, apoptotic, and inflammatory responses in ischaemic stroke as part of the metabolic network, boosting Nateglinide (NAT)’s neuroprotective benefits by maintaining brain function and lowering neuroinflammation [66]. Figure 2. Phenylalanine and its role 4. D-Tryptophane: Mechanisms and Benefits 4.1. Biological Pathways and Metabolism of Tryptophane The serotonin route and the kynurenine pathway (figure 3) are the two primary enzymes that are responsible for the metabolism of tryptophan. The enzyme known as tryptophan hydroxylase is responsible for the initial conversion of tryptophan into 5-hydroxytryptophan which occurs in the serotonin pathway [67],[68]. After that, aromatic L-amino acid decarboxylase is responsible for converting this intermediate into serotonin. Under the action of norepinephrine and N-acetyltransferase, serotonin can undergo a further conversion into N-acetylserotonin. This conversion takes place in the absence of light [69]. The enzyme hydroxyindole-O-methyltransferase brings about the transformation of N-acetylserotonin into melatonin. During the kynurenine pathway, the enzyme tryptophan 2,3-dioxygenase is responsible for the conversion of tryptophan into kynurenine [70]. By utilising additional enzymes that are involved in the kynurenine pathway, quinolinic acid can be produced from kynurenine. After then, a small amount of quinolic acid can be transformed into niacin, which is an essential component in the production of NAD+. Furthermore, monoamine oxidase has the ability to convert serotonin into 5-hydroxyindole acetic acid [71]. This conversion can take place. In addition, the intermediate 5-hydroxyindole acetaldehyde can be turned into 5-hydroxytryptophol by use of aldehyde reductase, or it can be converted into 5-hydroxyindole acetic acid by means of aldehyde dehydrogenase. [1]¿p1 [1]¿m1 Figure 3. Metabolism of Tryptophane 4.2. Neuroprotective Properties of D-Tryptophane Alzheimer’s disease is categorized by the aggregation of amyloid-beta (Aβ) oligomers and plaques, resulting in neurodegeneration [72]. Aβ enhances the production of inflammatory cytokines and enzymes in the kynurenine pathway (KP) (figure 4), leading to modified concentrations of kynurenine metabolites such as 3-HK(3-Hydroxykynurenine), 3-HAA (3-Hydroxyanthranilic Acid), and QUIN (Quinolinic Acid). This mechanism leads to neurological tissue damage and establishes a loop that promotes more Aβ buildup, glial activation, heightened KP activity and exacerbating neurodegenerative diseases [73]. In Parkinson’s disease (PD)(figure 4), increased concentrations of TRP/KYN and KYNA/TRP are observed in the frontal brain, putamen, and substantia nigra, particularly in late stages, suggesting possible biomarkers for the condition. Urine sample is a non-invasive technique for detection. TRP supplementation exhibits neuroprotective properties through the inhibition of NF-κB, a mechanism that is counteracted by AhR pathway inhibitors. Elevated KYNA safeguards dopamine neurons against QUIN-induced injury, but diminished KYNA levels in PD are associated with decreased KAT-I and KAT-II activity [74]. KYNA concentrations are reduced in Huntington’s disease (HD) (figure 4), although levels of 3-HK and QUIN are elevated. Studies demonstrate that heightened vulnerability to NMDA-induced neurotoxicity in HD is associated with (KP) activity; The suppression of indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) reduces the production of 3-HK and QUIN [75]. TRP levels are decreased in serum and cerebrospinal fluid in multiple sclerosis (MS). Following interferon-beta (INF-β) administration, the L-kynurenine (L-KYN)/TRP ratio increases, suggesting that activated IDO-1 influences TRP metabolism in MS. Additionally, impaired immunosuppressive activity of T regulatory (Treg) cells against Th1 and Th17 cells contributes to the disease’s pathogenesis [76]. QUIN possess neurotoxic effects that overlap with mechanisms seen in amyotrophic lateral sclerosis (ALS) and motor neuron death. In ALS patients, QUIN levels were significantly increased in neurons and microglia in the cerebrospinal fluid, motor cortex, and spinal cord. Additionally, there were increases in cerebrospinal fluid levels of IDO, TRP, and KYN, along with elevated ratios of 3-HK/KYNA and QUIN/KYNA [77]. Both reduced and elevated NMDAR (N-methyl-D-aspartate receptor) functions are linked to autism spectrum disorder (ASD), indicating that maintaining normal NMDAR function is crucial. Modulators of NMDAR can help reduce repetitive and hyperactive behaviors as well as improve social behaviors. Additionally, TRYCATs, such as KYNA and QUIN, have regulatory effects on AMPAR and NMDAR, suggesting they play a role in influencing ASD-related behaviors [78]. In epilepsy models, IDO-1 levels, KYN/TRP ratios, and pro-inflammatory cytokines were elevated, reversed in IDO-1 knockout mice. Reducing IDO-1 metabolites can suppress epilepsy and neuronal damage. Kynurenine metabolites also modulate excitatory neurotransmission via glutamate receptors [79]. Vagal stimulation boosts AA levels, reducing seizure frequency and improving mood [80]. Changes in circulating TRP levels influence its availability for the production of 5-HT (serotonin) and melatonin in the brain, perhaps playing a role in many neuropsychiatric illnesses. Disruptions in 5-HT neurotransmission are associated with the pathophysiology of mental disorders [81]. Although several research have investigated blood and cerebrospinal fluid (CSF) TRP levels, few have analyzed their association. Declines in CSF TRP are associated with various psychiatric disorders, with depressed patients exhibiting greater reductions when comorbidities like suicidal behavior are present [82] .Interestingly, symptoms of obsessive-compulsive disorder (OCD) and attention deficit hyperactivity disorder (ADHD) may lead to increased TRP levels [83], [84], [85].Increased TRP catabolites (TRYCATs) can act as endogenous anxiogenics, potentially contributing to anxiety [86]. Research investigating peripheral TRP levels in depression frequently neglects comorbidities that may influence TRP levels, representing a considerable restriction. Although most studies suggest reduced plasma or serum tryptophan levels in depression, certain data contradict this pattern [87]. Bipolar disorder (BD) causes extreme mood swings and is linked to higher obesity rates, increasing the risk of metabolic and cardiovascular diseases, which can lower life expectancy. Obesity in BD patients is associated with cognitive deficits and increased suicide attempts [88]. Current BD diagnosis and treatment focus primarily on clinical symptoms, highlighting the need for effective biomarkers. TRP and its catabolic pathways are essential to the molecular underpinnings of BD, with TRYCATs implicated in immunological inflammation and neurodegeneration. A meta-analysis revealed a downregulation of the TRYCATs pathway in individuals with bipolar disorder; nevertheless, further investigation into peripheral and central TRYCAT levels is necessary. The Kynurenine axis, reliant on IDO-1 and TDO enzymes, is influenced by pro-inflammatory cytokines, particularly enhancing IDO-1 activity [89]. Schizophrenia is linked to altered TRP levels [90], with reduced plasma TRP concentrations observed in patients over 40. This decrease may result from increased conversion of TRP to kynurenine metabolites via the kynurenine pathway, regulated by IDO and TDO enzymes. IDO’s role in this pathway is crucial for activating myelin-specific T cells, which produce pro-inflammatory cytokines, contributing to demyelination and enhancing antibodies against myelin proteins. Evidence suggests that white matter abnormalities are key factors in the pathophysiology of schizophrenia, influencing its symptoms [91]. Major depressive disorder (MDD) is a predominant reason of worldwide impairment, and comprehending its etiology continues to be difficult. Serotonin has traditionally been linked to MDD, with the assumption of low central nervous system serotonin levels [92]. Studies show lower peripheral serotonin concentrations in MDD patients compared to controls. TRP, the precursor to serotonin, is essential for its synthesis, but the reasons for reduced peripheral serotonin in MDD are unclear, potentially due to low synthesis rates or high turnover. Investigating serotonin pathways, including precursors and metabolites, is crucial. Lower levels of kynurenic acid are associated with cognitive deficits in MDD, while xanthurenic acid can modulate synaptic transmission in the hippocampus, a key area in MDD [93]. ADHD is a common behavioral disorder marked by impulsivity, hyperactivity, and attention deficits, but its neural basis remains poorly understood. Initially, low serotonin (5-HT) levels were thought to increase impulsivity, but studies suggest a more complex relationship involving TRP metabolites and dopamine. Serotonin is associated with the default mode network (DMN), which may be disrupted in ADHD. Differences in serum levels of TRP, kynurenic acid (KA), xanthurenic acid (XA), and hydroxyanthranilic acid (HAA) have been observed in adults with ADHD, indicating a connection between symptom severity and TRP metabolites. Positive correlations between TRP and kynurenine (KYN) levels suggest normal TRP conversion in ADHD patients. However, research on serotonin and TRP’s roles in aversion and ADHD is Limited and requires further investigation [94]. Figure 4. Role of tryptophane in different neurological disorders 5. Studies on the Efficacy of DAAs 5.1 Targeted Drug Delivery Across the Blood-Brain Barrier Using DAAs The blood-brain barrier poses important challenges for the delivery of medications to the brain. In a healthy brain, the blood-brain barrier stops most chemicals from moving from the bloodstream to the brain. Only tiny molecules can get through the blood-brain barrier, which is important for protecting normal brain function [95]. Some pathological situations, such as stroke, diabetes, seizures, multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease, compromise the integrity of the blood-brain barrier. Amino acids traverse the blood-brain barrier through the transport systems integrated within it [96]. An efficient technique for cerebral medication delivery utilises this knowledge to combine pharmaceuticals with amino acids that successfully enter the BBB. Three amino acid prodrugs of dopamine were made by Peura and his colleagues to help the brain better take in dopamine through the main amino acid transporter in the blood-brain barrier [97]. They synthesized three different prodrugs using an in-situ rat brain perfusion method . A mix of methotrexate (MTX) and lysine was made by Singh et al. to help MTX get into the brain better by using the brain’s own system for transporting lysine across the blood-brain barrier [98] . The MTX-lysine conjugate enhanced the efficiency of this prodrug’s brain transport. When administered, it also demonstrated good pharmacokinetics and biodistribution [99]. Furthermore, amino acids are naturally found in proteins, which makes poly (amino acid) carriers more biocompatible and lowers the risk of adverse immune reactions [100]. Pharmaceutical formulations may become more stable by adding amino acids. This could stop therapeutic substances from breaking down and increase the time they stay in circulation in the body. By modifying amino acids, specific receptors on cancer cells can be targeted, thereby enhancing the selectivity of drug delivery and simultaneously minimizing its impact on healthy tissue. The combination with amino acids might make it easier for cells to take in therapeutic medicines, which would improve how well these medicines work inside tumor cells. Amino acid carriers have the potential to improve patient quality of life by reducing the overall toxicity associated with current therapies. By boosting the targeted delivery and internalization of medications, it can be accomplished [101]. 5.2 Conjugation-Based Strategies for Targeted Drug Delivery via DAAs The FDA’s recent clearance of novel medications is evidence that peptide-derived drugs are getting substantial traction in the recent drug research process [102]. Conversely, over the past few decades, small-molecule-based medications have become a crucial component of pharmacological research. Biologics and small molecules sit between peptide-containing medications. Although peptides and tiny compounds, primarily heterocycles, have been successful in healing a wide range of ailments, they also have a number of disadvantages as treatments [103]. This gap may be addressed through the application of ”conjugation chemistry,” wherein the partners are united by a stable chemical bond, resulting in conjugates that display favourable attributes, so mitigating the stigma linked to the individual partners. The linking of small bioactive compounds to amino acids or peptides is a very effective and promising way to make new leads that are more potent [104]. An important thing to note is that many studies have shown that joining amino acids with peptides is important for making the bioactive molecule more stable, soluble, permeable, and selective. Coupled with amino acids or peptides, heterocyclics or bioactive compounds create conjugates with higher biological activities. This process also makes it easier for the conjugates to pass through cell membranes and enter the cytoplasm of the cells, where they exhibit increased activity. In earlier research [105], many examples of amino acids and peptides connected to bioactive scaffolds were found. These included prodrugs with different biological activities, lipopeptides, nucleic acid peptides, heterocyclic-conjugated peptides, and glycopeptides. Alkaloids exhibit a wide diversity of structural variants. What unites them is the existence of a fundamental nitrogen atom. An amine of the primary, secondary, or tertiary kind might be the nitrogen [106]. Despite their rapid excretion from the body, alkaloids possess unfavorable physical and chemical properties, including poor solubility, inadequate stability at normal pH, poor oral absorption, and diminished total bioavailability. These complications diminish the efficacy of alkaloids. In response to these issues, researchers have created and conjugated several alkaloid derivatives, including piperine, camptothecin, quinine, and matrine, with amino acids [107]. Natural substances can conjugate certain amino acids to produce bioactive molecules with pharmacological effects that surpass those of the original substances [108]. Conjugation was anticipated to modify both the pharmacological effects and the pharmacokinetic characteristics of the parent molecule. Structure categorises amino acids into two functional classes, each featuring reactive groups within the sidechain. To improve the chances of making new products that work better and have better physicochemical properties, conjugates can be made by mixing natural compounds with di-, tri-, or oligo-peptides along with amino acids. Amino acid conjugations of natural molecules have demonstrated enormous promise as novel active substances. Researchers may be particularly interested in a range of synthetic procedures for the conjugation of natural compounds with amino acids. Typically, researchers establish an ester bond or an amide connection to conjugate natural chemicals with amino acids. The amino acid structure comprises two reactive functional groups that facilitate the execution of this method. During the formation of an ester bond from the carboxylic group, the free amino group may act as a site of attack against the target protein. Typically, an ester bond or an amide connection may be established to conjugate natural chemicals with amino acids. The amino acid structure comprises two reactive functional groups that facilitate the execution of this method. During the formation of the ester bond from the carboxylic group to the amino group that is not bound, it may act as an active site against the target protein, and the same is true for the other way around. In general, the ways that the amide or ester counterparts were made led to the creation of analogues that were better for living things [109]. 5.3 Solubility Enhancement Strategies in Drug Delivery through DAAs One of the main factors affecting a drug’s oral bioavailability is its solubility behavior. In the fields of pharmacological analysis and formulation development, it is common to practice to formulate medicines that don’t dissolve well in water. Nowadays, poorly water-soluble or lipophilic medicines make up 60% of all newly discovered pharmaceuticals [110]. Insoluble drugs need specialised formulations to enhance their solubility in the gastrointestinal system since they are not efficiently absorbed in their solid form when administered orally. Strategies to address poor aqueous solubility, dissolution rates, and not adequate bioavailability encompass decreased particle size via microsizing and nanosizing, salt formation, conjugate or prodrug formation, complexation, solid dispersions, surfactant utilisation, polymorphism, and micellization [111]. Attaching a drug to several carriers, which is sometimes called the ”prodrug method,” is an intriguing molecular change that can be used to change the pharmacokinetics, pharmacodynamics, and toxicity of a drug. Conjugates, or prodrugs, are often used to improve drugs that have problems like not dissolving well in water, being chemically unstable, not being absorbed well by the body, breaking down quickly before reaching the bloodstream, having a short half-life, being toxic, or causing irritation in the skin. Furthermore, by applying this methodology challenges associated with the formulation and distribution of pharmaceuticals can be tackled. Mixing amino acids and/or peptides can create a conjugate. Certain carrier proteins absorb this conjugate, which shares a structure with gut-derived nutrients. Compared to other promotions or carriers, amino acids are a natural dietary component and are not harmful in moderation. It is possible for amino acid prodrugs to help medicines that aren’t very soluble or permeable get into the body more easily [112]. People have long regarded natural products as a valuable source of effective medicinal medications. About 60% of the global population uses herbal and natural medicine to cure illnesses [113]. Natural products can be used as lead compounds in metabolic and structural research to better understand the relationship between biological activity and structure. This can lead to the synthesis of analogues and structural alteration, which can lower toxicity and increase medication efficacy. One significant category of bioorganic compounds is amino acids. It easily find them; they have a variety of structures, and they attach readily to other amino acids and biologically active compounds. They are able to significantly alter natural compounds low solubility and biological activity. Adding amino acids to natural products can often make them more soluble and increase their pharmacological activity [114]. Certain amino acids are used to make these compounds more selective. Researchers are thinking about incorporating amino acids into manufactured prodrugs to lessen toxicity in molecules with high activity and (less) toxicity [115]. Before altering a natural compound’s structure, researchers need to have a thorough understanding of all of its characteristics, including its solubility, known pharmacological activity, and structural change. By combining computer-simulated drug design with fast activity determination, researchers can quickly and effectively change the directional structure of natural chemicals. Amino acid use can enhance numerous features in natural product structure alteration. Amino acids therefore offer a broad range of potential applications in the structural alteration of natural goods [116]. 5.4 Formulations Utilizing DAAs for Enhanced Drug Delivery Formulations employing DAAs for improved drug delivery are attracting interest in pharmaceutical and biomedical research because of their capacity to enhance medication stability, bioavailability, and specificity. D-amino acids can be integrated into drug delivery systems to provide many advantages that mitigate the shortcomings of conventional medication formulations [117]. Chromium-D-phenylalanine (Cr (D-phe)) has a beneficial impact on the reproduction and development of Drosophila melanogaster. Treatment with Cr (D-phe) improved larval and pupal periods, egg hatching rates, fertility, fecundity, and lifespan. The compound also increased antioxidant enzyme levels, indicating a protective response to oxidative stress. These findings emphasise that Cr (D-phe) may enhance metabolic regulation and has potential applications in promoting reproductive health in various organisms [118]. The research indicates that 1-Methyl-D-tryptophan (MDT) and melatonin successfully counteract ketamine-induced schizophrenia-like behavioural and neurochemical changes in mice [119]. Both treatments alleviated positive, cognitive, and negative symptoms associated with schizophrenia, improving oxidative stress markers and inflammatory cytokine levels. In contrast to risperidone, MDT and melatonin were able to effectively address changes in myeloperoxidase activity and low glutathione levels. This showed that they could be used as therapeutic agents to target immune-inflammatory and oxidative pathways in schizophrenia [120]. As reported by Blum K, the combination of D-phenylalanine and N-acetyl-L-cysteine (NAC) presents a promising strategy for treating reward deficiency syndrome (RDS) by enhancing dopamine release and stabilization. The Brain Reward Cascade model shows how this approach aims to achieve dopamine homeostasis, showing how it might help regulate dopamine [121]. Research demonstrates that D-phenylalanine uses 𝜋 − 𝜋 stacking interactions to self-assemble into hydrogels. Spectroscopic studies have helped scientists make hydrogels based on D-phenylalanine by showing how concentration affects the structure and stability of gels. D-phenylalanine-based nanospheres (P(Lys-co-DPhe) and P(Glu-co-DPhe)) demonstrated stability, lack of cytotoxicity, and effective C-peptide delivery. These mixtures showed promise for long-lasting C-peptide release, which increased Na+/K+-ATPase activity outside of living things [122]. 6. Discussion In the field of neurology, D-amino acids are becoming an interesting topic of research, particularly due to the fact that they have the ability to modulate brain activity and provide novel therapies for a wide range of brain diseases. Different from the more frequent L-amino acids, which are primarily engaged in the process of normal protein synthesis, DAAs have pharmacological qualities that are one of a kind, which enables them to interact with the biochemical pathways in the brain in a manner that is unlike any other. New therapeutic intervention options are made possible by DAAs’ special capacity to modify neurotransmitter networks, give neuroprotection, and maybe change the trajectory of neurodegenerative disorders. D-tryptophan and D-phenylalanine are two of the most researched DAAs; they are both essential for neurotransmission and brain function [123]. Because of its function in the formation of serotonin, D-tryptophan, a structural isomer of L-tryptophan, is especially significant. Dysregulation of serotonin, a mood-regulating neurotransmitter, is associated to anxiety and depression [124]. D-tryptophan’s ability to affect brain serotonin levels makes it a promising treatment for such disorders, and its neuroprotective characteristics may combat neurodegeneration [125]. Its ability to pass the BBB and adjust serotonin levels allows it to treat mood disorders with fewer negative effects than standard antidepressants. Analogously, D-phenylalanine is another D-amino acid that has promising therapeutic applications, especially when it comes to Parkinson’s disease and other movement disorders [58]. Dopamine is a neurotransmitter that is vital for motor function and whose deficiency is a defining feature of Parkinson’s disease. D-phenylalanine is necessary for its production. D-phenylalanine’s potential to cure a variety of neurological problems is demonstrated by its capacity to affect mood and movement abnormalities. A primary problem in using DAAs for therapeutic applications is ensuring their efficient delivery to the brain, considering the protective properties of the BBB. Recent improvements in drug delivery systems, including conjugation procedures and methods to enhance solubility, demonstrate potential in surmounting this obstacle. Enhancing the bioavailability and targeted distribution of DAAs may optimise their therapeutic benefits, hence creating new opportunities for their application in the treatment of neurological illnesses. The potential for DAAs to transform the treatment of neurological disorders is becoming more and more clear as research into these substances, especially D-tryptophan and D-phenylalanine, continues to advance. Their capacity to target certain brain biochemical pathways and the creation of sophisticated delivery systems may open the door to more individualised and successful therapies for people with diseases like Alzheimer’s, Parkinson’s, and depression. With more study and clinical approval, DAAs might revolutionise neurology in the future by providing more accurate and powerful treatment alternatives for intricate brain conditions. 7. Conclusion D-phenylalanine and D-tryptophan are two amino acids that have the potential to significantly advance the treatment of neurological conditions including Alzheimer’s disease, Parkinson’s disease, and psychological disorders like depression. DAAs, are characterized by their specific pharmacological properties, which allow them to control neurotransmission and exert an influence on brain activity in a manner that is distinct from that of their L-amino acids counterparts. As a result, they are appealing candidates for targeted therapies that are intended to address illnesses for which the medicines that are now available are insufficient. The neuroprotective properties of D-tryptophan, which include its role in the synthesis of serotonin, suggest that it may be beneficial for the treatment of mood disorders and neurodegeneration. On the other hand, the involvement of D-phenylalanine in the synthesis of dopamine highlights its potential for treating dopamine-related disorders such as Parkinson’s disease. Furthermore, new developments in drug delivery techniques, like conjugation-based approaches and solubility augmentation, may aid in overcoming obstacles like the BBB and boost the therapeutic use of DAAs. These creative methods may improve the availability and efficacy of D-tryptophan and D-phenylalanine in medical situations. It is evident that DAAs have therapeutic promise, but more study is necessary to completely comprehend their safety, effectiveness, and long-term implications in people. With the advancement of this science, D-amino acids present a unique and exciting avenue to improve brain function and open up new therapeutic options for neurological conditions. [1]¿p1 [1]¿m1 8. Future Directions and Implications for Neurological Disorder Management The study of D-tryptophan and D-phenylalanine as therapeutic agents for managing neurological disorders, particularly AD, presents several promising avenues for future research. First, conducting well-structured clinical trials is imperative to evaluate the safety and efficacy of these D-amino acids in diverse patient populations, focusing not only on cognitive improvements but also on behavioral and mood enhancements. To deepen our understanding, mechanistic studies are needed to elucidate their interactions with NMDA receptors and their effects on glutamatergic transmission and neurotrophic factor release, which may unveil new therapeutic pathways. Furthermore, investigating the potential for synergistic therapies combining D-tryptophan and D-phenylalanine with existing AD treatments could optimize therapeutic outcomes and improve patient quality of life. Formulation development is also crucial; exploring innovative delivery methods, such as nanoparticles or liposomal encapsulation, may enhance the bioavailability and effectiveness of these compounds in crossing the blood-brain barrier. Longitudinal studies that track cognitive decline over time in patients receiving D-amino acid treatments could provide valuable insights into their potential for delaying dementia symptoms. 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ACKNOWLEDGEMENTS We thank the Department of Pharmacy, Guru Ghasidas Vishwavidyalaya and groups for the resources and support. We also thank our peers and colleagues for their insightful comments and ideas that we greatly appreciate while preparing this manuscript. Information & Authors Information Version history V1 Version 1 25 March 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords alzheimer's disease blood-brain barrier d-phenylalanine d-tryptophan neuroprotection Authors Affiliations Yogita Dhurandhar Guru Ghasidas Vishwavidyalaya View all articles by this author Shubham Tomar Indian Pharmacopoeia Commission View all articles by this author Kamta P. Namdeo 0000-0002-1319-0486 [email protected] Guru Ghasidas Vishwavidyalaya View all articles by this author Metrics & Citations Metrics Article Usage 773 views 183 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Yogita Dhurandhar, Shubham Tomar, Kamta P. Namdeo. 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