Interest in drugs that covalently modify their target is driven by the desire for enhanced efficacy that can result from the silencing of enzymatic activity until protein resynthesis can occur, along with the potential for increased selectivity by targeting uniquely positioned nucleophilic residues in the protein. However, covalent approaches carry additional risk for toxicities or hypersensitivity reactions that can result from covalent modification of unintended targets. Here we describe methods for measuring the reactivity of covalent reactive groups (CRGs) with a biologically relevant nucleophile, glutathione (GSH), along with kinetic data for a broad array of electrophiles. We also describe a computational method for predicting electrophilic reactivity, which taken together can be applied to the prospective design of thiol-reactive covalent inhibitors.

Tofacitinib is a novel, oral Janus kinase inhibitor. The objectives of this study were to summarize the pharmacokinetics and metabolism of tofacitinib in humans, including clearance mechanisms. Following administration of a single 50-mg 14 C-labeled tofacitinib dose to healthy male subjects, the mean (standard deviation) total percentage of administered radioactive dose recovered was 93.9% (63.6), with 80.1% (63.6) in the urine (28.8% parent), and 13.8% (61.9) in feces (0.9% parent). Tofacitinib was rapidly absorbed, with plasma concentrations and total radioactivity peaking at around 1 hour after oral administration. The mean terminal phase half-life was approximately 3.2 hours for both parent drug and total radioactivity. Most (69.4%) circulating radioactivity in plasma was parent drug, with all metabolites representing less than 10% each of total circulating radioactivity. Hepatic clearance made up around 70% of total clearance, while renal clearance made up the remaining 30%. The predominant metabolic pathways of tofacitinib included oxidation of the pyrrolopyrimidine and piperidine rings, oxidation of the piperidine ring side-chain, N-demethylation and glucuronidation. Cytochrome P450 (P450) profiling indicated that tofacitinib was mainly metabolized by CYP3A4, with a smaller contribution from CYP2C19. This pharmacokinetic characterization of tofacitinib has been consistent with its clinical experience in drug-drug interaction studies.

dinucleotide phosphate, reduced form; ROE, rotating Overhauser effect; V max , maximal velocity; K M , substrate concentration at half-maximal velocity; CL int,app , apparent intrinsic clearance; CL int,app,scaled , scaled apparent intrinsic clearance; f u,mic , fraction unbound in liver microsomes; f CL , fractional clearance; f m , fraction metabolized; f a , fraction of the po dose absorbed from the gastrointestinal lumen; CL renal , renal clearance; Vd ss , steady state distribution volume; C max , maximal plasma concentration; T max , time to reach C max ; AUC, area under the plasma concentration-time curve; t 1/2 , half-life; P app , apparent absorptive permeability; MDCKII-LE, low efflux Madin-Darby canine kidney cells; BCRP, breast cancer resistant protein; MDR1, multidrug resistant gene 1; f u,p , plasma unbound fraction; BPR, blood to plasma ratio; CL hep , in

Sudoxicam and meloxicam are nonsteroidal anti-inflammatory drugs (NSAIDs) from the enol-carboxamide class. While the only structural difference between the two NSAIDs is the presence of a methyl group on the C5-position of the 2-carboxamidothiazole motif in meloxicam, a marked difference in their toxicological profile in humans has been discerned. In clinical trials, sudoxicam was associated with several cases of severe hepatotoxicity that led to its discontinuation, while meloxicam has been in the market for over a decade and is devoid of hepatotoxicity. In an attempt to understand the biochemical basis for the differences in safety profile, an in vitro investigation of the metabolic pathways and covalent binding of the two NSAIDs was conducted in NADPH-supplemented human liver microsomes. Both compounds demonstrated NADPH-dependent covalent binding to human liver microsomes; however, the extent of binding of [(14)C]-meloxicam was approximately 2-fold greater than that of [(14)C]-sudoxicam. While inclusion of glutathione (GSH) in microsomal incubations resulted in a decrease in covalent binding for both NSAIDs, the reduction in binding was more pronounced for meloxicam. Metabolite identification studies on [(14)C]-sudoxicam in NADPH-supplemented human liver microsomes indicated that the primary route of metabolism involved a P450-mediated thiazole ring scission to the corresponding acylthiourea metabolite (S3), a well-established pro-toxin. The mechanism of formation of S3 presumably proceeds via (a) epoxidation of the C4-C5-thiazole ring double bond, (b) epoxide hydrolysis to the corresponding thiazole-4,5-dihydrodiol derivative, which was observed as a stable metabolite (S2), (c) ring opening of the thiazole-4,5-dihydrodiol to an 2-oxoethylidene thiourea intermediate, and (d) hydrolysis of the imine bond within this intermediate to yield S3. In the case of meloxicam, the corresponding acylthiourea metabolite M3 was also observed, but to a lesser extent; the main route of meloxicam metabolism involved hydroxylation of the 5'-methyl group, a finding that is consistent with the known metabolic fate of this NSAID. Inclusion of GSH led to a decrease in the formation of M3 with the concomitant formation of an unusual two-electron reduction product (metabolite M7). The formation of M7 is proposed to arise via reduction of the imine bond in 2-oxopropylidene thiourea, an intermediate in the thiazole ring scission pathway in meloxicam. In conclusion, the results of our analysis suggest that if the covalent binding of the two NSAIDs is important to the overall hepatotoxicity risk, the differences in metabolism (differential preponderance of formation of the acylthiourea relative to total metabolism), differential effects of GSH on covalent binding, and finally differences in daily doses of the two NSAIDs may serve as a plausible explanation for the marked differences in toxicity.

Acyl-CoA:diacylglycerol acyltransferase-1 (DGAT-1) catalyzes the final committed step in the biosynthesis of triglycerides. DGAT-1 knockout mice have been shown to be resistant to diet-induced obesity and have increased insulin sensitivity. Thus, inhibition of DGAT-1 may represent an attractive target for the treatment of obesity or type II diabetes. Herein, we report the discovery and characterization of a potent and selective DGAT-1 inhibitor PF-04620110 (3). Compound 3 inhibits DGAT-1 with an IC50 of 19 nM and shows high selectivity versus a broad panel of off-target pharmacologic end points. In vivo DGAT-1 inhibition has been demonstrated through reduction of plasma triglyceride levels in rodents at doses of ≥0.1 mg/kg following a lipid challenge. On the basis of this pharmacologic and pharmacokinetic profile, compound 3 has been advanced to human clinical studies.