Pharmacokinetics, metabolism and safety of deuterated L-DOPA (SD-1077)/carbidopa compared to L-DOPA/carbidopa following single oral dose administration in healthy subjects

Mitigating gut microbial degradation of levodopa and enhancing brain dopamine: Implications in Parkinson's disease

Parkinson's disease is managed using levodopa; however, as Parkinson's disease progresses, patients require increased doses of levodopa, which can cause undesirable side effects. Additionally, the oral bioavailability of levodopa decreases in Parkinson's disease patients due to the increased metabolism of levodopa to dopamine by gut bacteria, Enterococcus faecalis, resulting in decreased neuronal uptake and dopamine formation. Parkinson's disease patients have varying levels of these bacteria. Thus, decreasing bacterial metabolism is a promising therapeutic approach to enhance the bioavailability of levodopa in the brain. In this work, we show that Mito-ortho-HNK, formed by modification of a naturally occurring molecule, honokiol, conjugated to a triphenylphosphonium moiety, mitigates the metabolism of levodopa-alone or combined with carbidopa-to dopamine. Mito-ortho-HNK suppresses the growth of E. faecalis, decreases dopamine levels in the gut, and increases dopamine levels in the brain. Mitigating the gut bacterial metabolism of levodopa as shown here could enhance its efficacy.

Probing solution conformations of l-DOPA: Integration of experiment and simulation via vibrational optical activity

l-DOPA plays a critical role as a precursor to dopamine and is a standard treatment for Parkinson's disease. Recent research has highlighted the potential therapeutic advantages of deuterated l-DOPA analogs having a longer biological half-life. For their spectroscopic characterization, the in-detail characterization of l-DOPA itself is necessary. This article presents a thorough examination of the vibrational spectra of l-DOPA, with a particular emphasis on chirally sensitive VOA techniques. We successfully obtained high-quality Raman and ROA spectra of l-DOPA in its cationic form, under low pH conditions, and at a high concentration of 100 mg/ml. These spectra cover a broad spectral range, allowing for precise comparisons with theoretical simulations. We also obtained IR and VCD spectra, but they faced limitations due to the narrow accessible spectral region. Exploration of l-DOPA's conformational landscape revealed its intrinsic flexibility, with multiple coexisting conformations. To characterize these conformations, we employed two methods: one involved potential energy surface scans with implicit solvation, and the other utilized molecular dynamics simulations with explicit solvation. Comparing ROA spectra from different conformer groups and applying spectral decomposition proved crucial in determining the correct conformer ratios. The use of explicit solvation significantly improved the quality of the final simulated spectral profiles. The accurate determination of conformer ratios, rather than solely relying on the number of averaged spectra, played a crucial role in simulation accuracy. In conclusion, our study offers valuable insights into the structure and conformational behavior of l-DOPA and represents a valuable resource for subsequent spectroscopic studies of its deuterated analogs.

Deuterium in drug discovery: progress, opportunities and challenges

Substitution of a hydrogen atom with its heavy isotope deuterium entails the addition of one neutron to a molecule. Despite being a subtle change, this structural modification, known as deuteration, may improve the pharmacokinetic and/or toxicity profile of drugs, potentially translating into improvements in efficacy and safety compared with the non-deuterated counterparts. Initially, efforts to exploit this potential primarily led to the development of deuterated analogues of marketed drugs through a 'deuterium switch' approach, such as deutetrabenazine, which became the first deuterated drug to receive FDA approval in 2017. In the past few years, the focus has shifted to applying deuteration in novel drug discovery, and the FDA approved the pioneering de novo deuterated drug deucravacitinib in 2022. In this Review, we highlight key milestones in the field of deuteration in drug discovery and development, emphasizing recent and instructive medicinal chemistry programmes and discussing the opportunities and hurdles for drug developers, as well as the questions that remain to be addressed.

Deuterated driven new chemical entities: An optimistic way to improve therapeutic efficacy

In organic chemistry, the use of deuterium exchange as a tool to study the mechanism of chemical reaction has been well explored. Since two decades, the research focus on deuterated bioactive molecules has been gaining attention for investigating the therapeutic potential of deuterium replacement in a chemical structure. Recently, Food Drug Administration (FDA) approved the first deuterium-labeled drug "deutetrabenazine", and notified the deuterated drugs as new chemical entities (NCEs). Henceforth, the deuterium substitution driven structure activity relationship, preclinical pharmacokinetics, and toxicity studies were much initiated. Deuteration of a bioactive molecule often results in improved therapeutic efficacy due to the altered pharmacokinetic profile. This review provides a conceptual framework on the importance of deuterium atom in chemical structure of a drug, and its biological value in improved physiochemical properties, pharmacokinetics, biological target interaction, diagnosis, and toxicity. In addition, this review concisely updated the recent deuteration methods, chemical stability, challenges in drug development, deuterium-based imaging in diagnosis, and selected synthetic scheme of deuterated molecules.

One-pot H/D exchange and low-coordinated iron electrocatalyzed deuteration of nitriles in D2O to α,β-deuterio aryl ethylamines

Developing a step-economical approach for efficient synthesis of α,β-deuterio aryl ethylamines (α,β-DAEAs) with high deuterium ratios using an easy-to-handle deuterated source under ambient conditions is highly desirable. Here we report a room-temperature one-pot two-step transformation of aryl acetonitriles to α,β-DAEAs with up to 92% isolated yield and 99% α,β-deuterium ratios using D₂O as a deuterium source. The process involves a fast α-C - H/C - D exchange and tandem electroreductive deuteration of C ≡ N over an in situ formed low-coordinated Fe nanoparticle cathode. The moderate adsorptions of nitriles/imine intermediates and the promoted formation of active hydrogen (H*) on unsaturated Fe sites facilitate the electroreduction process. In situ Raman confirms co-adsorption of aryl rings and the C ≡ N group on the Fe surface. A proposed H*-addition pathway is confirmed by the detected hydrogen and carbon radicals. Wide substrate scope, parallel synthesis of multiple α,β-DAEAs, and successful preparation of α,β-deuterated Melatonin and Komavine highlight the potential.

Chemical exchange of labile protons by deuterium enables selective detection of pharmaceuticals in solid formulations

Chemically assisted swapping of labile protons by deuterons is presented for amino acids, polysaccharides, pharmaceutical compounds, and their solid formulations. Solid-state packing interactions in these compounds are elucidated by ¹H-²H isotope correlation NMR spectroscopy (iCOSY). A minuscule concentration of dopamine, 5 wt% or ∼100 μg, in a solid formulation can be detected by ²H NMR at 28.2 T (¹H, 1200 MHz) in under a minute.

Hydrogen/Deuterium Transfer from Anisole to Methoxy Radicals: A Theoretical Study of a Deuterium-Labeled Drug Model

Recently, deuterium-labeled drugs, such as deutetrabenazine, have attracted considerable attention. Consequently, understanding the reaction mechanisms of deuterium-labeled drugs is crucial, both fundamentally and for real applications. To understand the mechanisms of H- and D-transfer reactions, in this study, we used deuterated anisole as a deutetrabenazine model and computationally considered the nuclear quantum effects of protons, deuterons, and electrons. We demonstrated that geometrical differences exist in the partially and fully deuterated methoxy groups and hydrogen-bonded structures of intermediates and transition states due to the H/D isotope effect. The observed geometrical features and electronic structures are ascribable to the different nuclear quantum effects of protons and deuterons. Primary and secondary kinetic isotope effects (KIEs) were calculated for H- and D-transfer reactions from deuterated and undeuterated anisole, with the calculated primary KIEs in good agreement with the corresponding experimental data. These results reveal that the nuclear quantum effects of protons and deuterons need to be considered when analyzing the reaction mechanisms of H- and D-transfer reactions and that a theoretical approach that directly includes nuclear quantum effects is a powerful tool for the analysis of H/D isotope effects in H- and D-transfer reactions.

A New Insight into an Alternative Therapeutic Approach to Restore Redox Homeostasis and Functional Mitochondria in Neurodegenerative Diseases

Neurodegenerative diseases are accompanied by oxidative stress and mitochondrial dysfunction, leading to a progressive loss of neuronal cells, formation of protein aggregates, and a decrease in cognitive or motor functions. Mitochondrial dysfunction occurs at the early stage of neurodegenerative diseases. Protein aggregates containing oxidatively damaged biomolecules and other misfolded proteins and neuroinflammation have been identified in animal models and patients with neurodegenerative diseases. A variety of neurodegenerative diseases commonly exhibits decreased activity of antioxidant enzymes, lower amounts of antioxidants, and altered cellular signalling. Although several molecules have been approved clinically, there is no known cure for neurodegenerative diseases, though some drugs are focused on improving mitochondrial function. Mitochondrial dysfunction is caused by oxidative damage and impaired cellular signalling, including that of peroxisome proliferator-activated receptor gamma coactivator 1α. Mitochondrial function can also be modulated by mitochondrial biogenesis and the mitochondrial fusion/fission cycle. Mitochondrial biogenesis is regulated mainly by sirtuin 1, NAD⁺, AMP-activated protein kinase, mammalian target of rapamycin, and peroxisome proliferator-activated receptor γ. Altered mitochondrial dynamics, such as increased fission proteins and decreased fusion products, are shown in neurodegenerative diseases. Due to the restrictions of a target-based approach, a phenotype-based approach has been performed to find novel proteins or pathways. Alternatively, plasma membrane redox enzymes improve mitochondrial function without the further production of reactive oxygen species. In addition, inducers of antioxidant response elements can be useful to induce a series of detoxifying enzymes. Thus, redox homeostasis and metabolic regulation can be important therapeutic targets for delaying the progression of neurodegenerative diseases.

Tandem H/D Exchange-SET Reductive Deuteration Strategy for the Synthesis of α,β-Deuterated Amines Using D2O

α,β-Deuterated amines are crucial for the development of deuterated drugs. We intend to introduce the novel tandem H/D exchange-single electron transfer (SET) reductive deuteration strategy with high pot- and reagent-economy by the synthesis of α,β-deuterated amine using nitrile as the precursor. The H/D exchange of the -CH₂CN group was achieved by D₂O/Et₃N, which were also the required reagents in the tandem SmI₂-mediated SET reductive deuteration of the α-deuterated nitrile. The potential application of this method was further showcased by the synthesis of bevantolol-d₄.

A general, versatile and divergent synthesis of selectively deuterated amines

Deuterated organic molecules are of utmost importance in many areas of science and have been recently intensively investigated in medicinal chemistry due to their enhanced metabolic stability. The development of efficient and broadly applicable methods for the selective incorporation of deuterium atoms into organic molecules from readily available starting materials and reagents is therefore of extreme importance. Such methods however often lack generality and selectivity, notably in the nitrogen series. With nitrogen-containing molecules being indeed ubiquitous in medicinal chemistry, there is a strong need for efficient methods enabling the selective synthesis of deuterated amines. In this perspective, we report herein a general, versatile, divergent and metal-free synthesis of amines selectively deuterated at their α and/or β positions. Upon simple treatment of readily available ynamides with a mixture of triflic acid and triethylsilane, either deuterated or not, a range of amines can be smoothly obtained with high levels of deuterium incorporation by a unique sequence involving a domino keteniminium/iminium activation.

A general, versatile, divergent and metal-free synthesis of amines selectively deuterated at their α and/or β positions and relying on a simple treatment of ynamides with triflic acid and triethylsilane, either deuterated or not, is reported.

Deciphering Spinal Endogenous Dopaminergic Mechanisms That Modulate Micturition Reflexes in Rats with Spinal Cord Injury

Spinal neuronal mechanisms regulate recovered involuntary micturition after spinal cord injury (SCI). It was recently discovered that dopamine (DA) is synthesized in the rat injured spinal cord and is involved in lower urinary tract (LUT) activity. To fully understand the role of spinal DAergic machinery in micturition, we examined urodynamic responses in female rats during pharmacological modulation of the DA pathway. Three to four weeks after complete thoracic SCI, the DA precursor L-DOPA administered intravenously during bladder cystometrogram (CMG) and external urethral sphincter (EUS) electromyography (EMG) reduced bladder overactivity and increased the duration of EUS bursting, leading to remarkably improved voiding efficiency. Apomorphine (APO), a non-selective DA receptor (DR) agonist, or quinpirole, a selective DR₂ agonist, induced similar responses, whereas a specific DR₂ antagonist remoxipride alone had only minimal effects. Meanwhile, administration of SCH 23390, a DR₁ antagonist, reduced voiding efficiency by increasing tonic EUS activity and shortening the EUS bursting period. Unexpectedly, SKF 38393, a selective DR₁ agonist, increased EUS tonic activity, implying a complicated role of DR₁ in LUT function. In metabolic cage assays, subcutaneous administration of quinpirole decreased spontaneous voiding frequency and increased voiding volume; L-DOPA and APO were inactive possibly because of slow entry into the CNS. Collectively, tonically active DR₁ in SCI rats inhibit urine storage and enhance voiding by differentially modulating EUS tonic and bursting patterns, respectively, while pharmacologic activation of DR₂, which are normally silent, improves voiding by enhancing EUS bursting. Thus, enhancing DA signaling achieves better detrusor-sphincter coordination to facilitate micturition function in SCI rats.

Topical application of dopaminergic compounds can inhibit deprivation myopia in chicks

PURPOSE

Animal models have demonstrated a link between dysregulation of the retinal dopamine system and the development of experimental myopia (short-sightedness). However, pharmacological investigations of dopamine in animal models rely heavily on intravitreal or systemic administration, which have several limitations for longer-term experiments. We therefore investigated whether administration of dopamine as a topical eye drop can inhibit the development of form-deprivation myopia (FDM) in chicks. We also examined whether chemical modification of dopamine through deuterium substitution, which might enhance stability and bioavailability, can increase dopamine's effectiveness against FDM when given topically.

METHODS

Dopamine or deuterated dopamine (Dopamine-1,1,2,2-d₄ hydrochloride) was administered as a daily intravitreal injection or as daily topical eye drops to chicks developing FDM over an ascending dose range (min. n = 6 per group). Axial length and refraction were measured following 4 days of treatment.

RESULTS

Both intravitreal (ED₅₀ = 0.002μmoles) and topical application (ED₅₀ = 6.10μmoles) of dopamine inhibited the development of FDM in a dose-dependent manner. Intravitreal injections, however, elicited a significantly higher level of protection relative to topical eye drops (p < 0.01). Deuterated dopamine inhibited FDM to a similar extent as unmodified dopamine when administered as intravitreal injections (p = 0.897) or topical eye drops (p = 0.921).

CONCLUSIONS

Both intravitreal and topical application of dopamine inhibit the development of FDM in a dose-dependent manner, indicating that topical administration may be an effective avenue for longer-term dopamine experiments. Deuterium substitution does not alter the protection afforded by dopamine against FDM when given as either an intravitreal injection or topical eye drop.

The Triple Combination Phentermine Plus 5-HTP/Carbidopa Leads to Greater Weight Loss, With Fewer Psychomotor Side Effects Than Each Drug Alone

Obesity has become a serious public health problem. Although diet, surgery, and exercise are the primary treatments for obesity, these activities are often supplemented using appetite suppressants. A previous study reported that obesity specialists frequently prescribed a new drug combination for its treatment that includes phentermine (Phen; dopaminergic appetite suppressant), a serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP; an appetite suppressant that increases the 5-HT concentration), and carbidopa (CB; peripheral blocker of conversion of 5-HTP to 5-HT). Despite its widespread use, there is neither a preclinical study confirming the drug efficacy nor studies of its effects on the brain. To fill this gap, in rats for seven consecutive days, we administered Phen intraperitoneally at different doses either alone or in combination with a fixed dose of 5-HTP/CB. In a different group, we infused drugs via an intraperitoneal catheter while extracellular-recordings were performed in the nucleus accumbens shell (NAcSh), a brain region with dopamine-releasing effects that is involved in the action of appetite suppressants. We found that the triple-drug combination leads to greater weight-loss than each drug alone. Moreover, and as the treatment progresses, the triple drug combination partially reversed psychomotor side-effects induced by Phen. Electrophysiological results revealed that Phen alone evoked a net inhibitory imbalance in NAcSh population activity that correlated with the onset of psychomotor effects. In addition, and unlike the greater weight loss, the addition of 5-HTP/CB did not alter the Phen-evoked inhibitory imbalance in NAcSh responses. Subsequent experiments shed light on the underlying mechanism. That is the majority of NAcSh neurons modulated by 5-HTP/CB were suppressed by Phen. Notably, and despite acting via a different mechanism of action (DA for Phen vs. 5-HT for 5-HTP/CB), both drugs recruited largely overlapping NAcSh neuronal ensembles. These data suggest that the neural correlates of the greater weight loss could be located outside the NAcSh, in other brain circuits. Furthermore, we conclude that Phen + 5-HTP/CB is a potential treatment for overweight and obesity.

Monitoring of the deuterated and nondeuterated forms of levodopa and five metabolites in plasma and urine by LC-MS/MS

To compare pharmacokinetics, metabolism and excretion of levodopa and a triply deuterated form, which is being developed as an improved treatment for Parkinson's disease, methods were needed for quantification of the deuterated and nondeuterated forms of levodopa and five metabolites in human plasma and urine. Results: The natural heavy isotopes in the nondeuterated compounds caused an absolute contribution of up to 100% in the response of the deuterated compounds. Similarly, heavy isotopes in the deuterated analytes contributed to the response of the internal standards, but this did not affect the reliability of the results. Conclusion: Deuterated and nondeuterated analytes can be quantified together by LC-MS/MS, but overestimation of the concentrations of the deuterated molecules may be unavoidable and a careful interpretation of the concentration data is essential.

Pharmacokinetics, metabolism and safety of deuterated L-DOPA (SD-1077)/carbidopa compared to L-DOPA/carbidopa following single oral dose administration in healthy subjects

AIMS

SD-1077, a selectively deuterated precursor of dopamine (DA) structurally related to L-3,4-dihydroxyphenylalanine (L-DOPA), is under development for treatment of motor symptoms of Parkinson's disease. Preclinical models have shown slower metabolism of central deuterated DA. The present study investigated the peripheral pharmacokinetics (PK), metabolism and safety of SD-1077.

METHODS

Plasma and urine PK of drug and metabolites and safety after a single oral 150 mg SD-1077 dose were compared to 150 mg L-DOPA, each in combination with 37.5 mg carbidopa (CD) in a double-blind, two-period, crossover study in healthy volunteers (n = 16).

RESULTS

Geometric least squares mean ratios (GMRs) and 90% confidence intervals (90% CI) of SD-1077 vs. L-DOPA for C_max , AUC_0-t , and AUC_0-inf were 88.4 (75.9-103.1), 89.5 (84.1-95.3), and 89.6 (84.2-95.4), respectively. Systemic exposure to DA was significantly higher after SD-1077/CD compared to that after L-DOPA/CD, with GMRs (90% CI) of 1.8 (1.45-2.24; P = 0.0005) and 2.06 (1.68-2.52; P < 0.0001) for C_max and AUC_0-t and a concomitant reduction in the ratio of 3,4-dihydroxyphenylacetic acid/DA confirming slower metabolic breakdown of DA by monoamine oxidase (MAO). There were increases in systemic exposures to metabolites of catechol O-methyltransferase (COMT) reaction, 3-methoxytyramine (3-MT) and 3-O-methyldopa (3-OMD) with GMRs (90% CI) for SD-1077/CD to L-DOPA/CD for 3-MT exposure of 1.33 (1.14-1.56; P = 0.0077) and 1.66 (1.42-1.93; P < 0.0001) for C_max and AUC_0-t , respectively and GMRs (90% CI) for 3-OMD of 1.19 (1.15, 1.23; P < 0.0001) and 1.31 (1.27, 1.36; P < 0.0001) for C_max and AUC_0-t . SD-1077/CD exhibited comparable tolerability and safety to L-DOPA/CD.

CONCLUSIONS

SD-1077/CD demonstrated the potential to prolong exposure to central DA at comparable peripheral PK and safety to the reference L-DOPA/CD combination. A single dose of SD-1077 is safe for further clinical development in Parkinson's disease patients.