Selective deuteration of bupropion slows epimerization and reduces metabolism.

Strategically replacing hydrogen with deuterium at sites of metabolism in small molecule drugs can significantly alter clearance and potentially enhance clinical safety. Bupropion is an antidepressant and smoking cessation medication with the potential to cause seizures. We hypothesized that incorporating deuterium at specific sites in bupropion may greatly reduce epimerization, potentially slow metabolism, and reduce the formation of toxic metabolites, namely hydroxybupropion which has been associated with bupropion's toxicity. Four deuterated analogues were synthesized incorporating deuterium at sites of metabolism and epimerization with the aim of altering the metabolic profile of bupropion. Spectroscopic binding and metabolism studies with bupropion and R-or S-d4 and R-or S-d10 analogs were performed with recombinant CYP2B6, human liver microsomes, and human hepatocytes. Results demonstrate that deuterated bupropion analogues exhibited 20-25% decrease in racemization and displayed a significant decrease in the formation of CYP2B6-mediated R,R - or S,S-hydroxybupropion with recombinant protein and human liver microsomes. In primary human hepatocytes, metabolism of deuterated analogs to R,R - and S,S-hydroxybupropion and threo- and erythro-hydrobupropion was significantly less than R/S-d0 bupropion. Selective deuterium substitution at metabolic soft spots in bupropion has the potential to provide a drug with a simplified pharmacokinetic profile, reduced toxicity and improved tolerability in patients.

Altering Metabolic Profiles of Drugs by Precision Deuteration 2: Discovery of a Deuterated Analog of Ivacaftor with Differentiated Pharmacokinetics for Clinical Development.

Ivacaftor is currently used for the treatment of cystic fibrosis as both monotherapy (Kalydeco; Vertex Pharmaceuticals, Boston, MA) and combination therapy with lumacaftor (Orkambi; Vertex Pharmaceuticals). Each therapy targets specific patient populations: Kalydeco treats patients carrying one of nine gating mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) protein, whereas Orkambi treats patients homozygous for the F508del CFTR mutation. In this study, we explored the pharmacological and metabolic effects of precision deuteration chemistry on ivacaftor by synthesizing two novel deuterated ivacaftor analogs, CTP-656 (d9-ivacaftor) and d18-ivacaftor. Ivacaftor is administered twice daily and is extensively converted in humans to major metabolites M1 and M6; therefore, the corresponding deuterated metabolites were also prepared. Both CTP-656 and d18-ivacaftor showed in vitro pharmacologic potency similar to that in ivacaftor, and the deuterated M1 and M6 metabolites showed pharmacology equivalent to that in the corresponding metabolites of ivacaftor, which is consistent with the findings of previous studies of deuterated compounds. However, CTP-656 exhibited markedly enhanced stability when tested in vitro. The deuterium isotope effects for CTP-656 metabolism (DV = 3.8, DV/K = 2.2) were notably large for a cytochrome P450-mediated oxidation. The pharmacokinetic (PK) profile of CTP-656 and d18-ivacaftor were assessed in six healthy volunteers in a single-dose crossover study, which provided the basis for advancing CTP-656 in development. The overall PK profile, including the 15.9-hour half-life for CTP-656, suggests that CTP-656 may be dosed once daily, thereby enhancing patient adherence. Together, these data continue to validate deuterium substitution as a viable approach for creating novel therapeutic agents with properties potentially differentiated from existing drugs.

Altering metabolic profiles of drugs by precision deuteration: reducing mechanism-based inhibition of CYP2D6 by paroxetine.

Selective deuterium substitution as a means of ameliorating clinically relevant pharmacokinetic drug interactions is demonstrated in this study. Carbon-deuterium bonds are more stable than corresponding carbon-hydrogen bonds. Using a precision deuteration platform, the two hydrogen atoms at the methylenedioxy carbon of paroxetine were substituted with deuterium. The new chemical entity, CTP-347 [(3S,4R)-3-((2,2-dideuterobenzo[d][1,3]dioxol-5-yloxy)methyl)-4-(4-fluorophenyl)piperidine], demonstrated similar selectivity for the serotonin receptor, as well as similar neurotransmitter uptake inhibition in an in vitro rat synaptosome model, as unmodified paroxetine. However, human liver microsomes cleared CTP-347 faster than paroxetine as a result of decreased inactivation of CYP2D6. In phase 1 studies, CTP-347 was metabolized more rapidly in humans and exhibited a lower pharmacokinetic accumulation index than paroxetine. These alterations in the metabolism profile resulted in significantly reduced drug-drug interactions between CTP-347 and two other CYP2D6-metabolized drugs: tamoxifen (in vitro) and dextromethorphan (in humans). Our results show that precision deuteration can improve the metabolism profiles of existing pharmacotherapies without affecting their intrinsic pharmacologies.

Identification and structural elucidation of in vitro metabolites of atazanavir by HPLC and tandem mass spectrometry.

Atazanavir (marketed as Reyataz®) is an important member of the human immunodeficiency virus protease inhibitor class. LC-UV-MS(n) experiments were designed to identify metabolites of atazanavir after incubations in human hepatocytes. Five major (M1-M5) and seven minor (M7-M12) metabolites were identified. The most abundant metabolite, M1, was formed by a mono-oxidation on the t-butyl group at the non-prime side. The second most abundant metabolite, M2, was also a mono-oxidation product, which has not yet been definitively identified. Metabolites, M3 and M4, were structural isomers, which were apparently formed by oxidative carbamate hydrolysis. The structure of M5 comprises the non-prime side of atazanavir which contains a pyridinyl-benzyl group. Metabolite M6a was formed by the cleavage of the pyridinyl-benzyl side chain, as evidenced by the formation of the corresponding metabolic product, the pyridinyl-benzoic acid (M6b). Mono-oxidation also occurred on the pyridinyl-benzyl group to produce the low abundance metabolite M8. Oxidation of the terminal methyl groups produced M9 and M10, respectively, which have low chemical stability. Trace-level metabolites of di-oxidations, M11 and M12, were also detected, but the complexity of the molecule precluded identification of the second oxidation site. To our knowledge, metabolites M6b and M8 have not been reported.

Revealing the metabolic sites of atazanavir in human by parallel administrations of D-atazanavir analogs.

Atazanavir (Reyataz(®)) is an important member of the HIV protease inhibitor class. Because of the complexity of its chemical structure, metabolite identification and structural elucidation face serious challenges. So far, only seven non-conjugated metabolites in human plasma have been reported, and their structural elucidation is not complete, especially for the major metabolites produced by oxidations. To probe the exact sites of metabolism and to elucidate the relationship among in vivo metabolites of atazanavir, we designed and performed two sets of experiments. The first set of experiments was to determine atazanavir metabolites in human plasma by LC-MS, from which more than a dozen metabolites were discovered, including seven new ones that have not been reported. The second set involved deuterium labeling on potential metabolic sites to generate D-atazanavir analogs. D-atazanavir analogs were dosed to human in parallel with atazanavir. Metabolites of D-atazanavir were identified by the same LC-MS method, and the results were compared with those of atazanavir. A metabolite structure can be readily elucidated by comparing the results of the analogs and the pathway by which the metabolite is formed can be proposed with confidence. Experimental results demonstrated that oxidation is the most common metabolic pathway of atazanavir, resulting in the formation of six metabolites of monooxidation (M1, M2, M7, M8, M13, and M14) and four of dioxidation (M15, M16, M17, and M18). The second metabolic pathway is hydrolysis, and the third is N-dealkylation. Metabolites produced by hydrolysis include M3, M4, and M19. Metabolites formed by N-dealkylation are M5, M6a, and M6b.

Preclinical profile of VX-950, a potent, selective, and orally bioavailable inhibitor of hepatitis C virus NS3-4A serine protease.

VX-950 is a potent, selective, peptidomimetic inhibitor of the hepatitis C virus (HCV) NS3-4A serine protease, and it demonstrated excellent antiviral activity both in genotype 1b HCV replicon cells (50% inhibitory concentration [IC50] = 354 nM) and in human fetal hepatocytes infected with genotype 1a HCV-positive patient sera (IC50 = 280 nM). VX-950 forms a covalent but reversible complex with the genotype 1a HCV NS3-4A protease in a slow-on, slow-off process with a steady-state inhibition constant (K(i)*) of 7 nM. Dissociation of the covalent enzyme-inhibitor complex of VX-950 and genotype 1a HCV protease has a half-life of almost an hour. A >4-log10 reduction in the HCV RNA levels was observed after a 2-week incubation of replicon cells with VX-950, with no rebound of viral RNA observed after withdrawal of the inhibitor. In several animal species, VX-950 exhibits a favorable pharmacokinetic profile with high exposure in the liver. In a recently developed HCV protease mouse model, VX-950 showed excellent inhibition of HCV NS3-4A protease activity in the liver. Therefore, the overall preclinical profile of VX-950 supports its candidacy as a novel oral therapy against hepatitis C.

Inhibitors of hepatitis C virus NS3.4A protease. Part 3: P2 proline variants.

We recently described the identification of an optimized alpha-ketoamide warhead for our series of HCV NS3.4A inhibitors. We report herein a series of HCV protease inhibitors incorporating 3-alkyl-substituted prolines in P(2). These compounds show exceptional enzymatic and cellular potency given their relatively small size. The marked enhancement of activity of these 3-substituted proline derivatives relative to previously reported 4-hydroxyproline derivatives constitutes additional evidence for the importance of the S(2) binding pocket as the defining pharmacophore for inhibition of the NS3.4A enzyme.

Inhibitors of hepatitis C virus NS3.4A protease 2. Warhead SAR and optimization.

The alpha-ketoamide warhead (e.g., 15) was found to be a practical replacement for aliphatic aldehydes in a series of HCV NS3.4A protease inhibitors. Structure-activity relationships and prime side optimization are discussed.

Inhibitors of hepatitis C virus NS3.4A protease 1. Non-charged tetrapeptide variants.

Tetrapeptide-based peptidomimetic compounds have been shown to effectively inhibit the hepatitis C virus NS3.4A protease without the need of a charged functionality. An aldehyde is used as a prototype reversible electrophilic warhead. The SAR of the P1 and P2 inhibitor positions is discussed.