One-Pot Synthesis of Highly Substituted Nicotinic Acid Derivatives Based on a Formylation Strategy of Enamino Keto Esters.

A facile one-pot synthesis of 4-chloro or 4-bromonicotinic acid esters with optional 2- and 2,5-disubstitution on the pyridine ring has been developed from easily accessible enamino keto esters by a formylation followed by in situ intramolecular cyclization strategy under optimized Vilsmeier reaction conditions. The effect of the substituents on the ß-carbon and the nature of the keto functionality were explored in detail to understand the mechanism of pyridine ring formation under the described conditions.

Understanding the clinical pharmacokinetics of a GABAA partial agonist by application of in vitro tools.

1. 4-Oxo-4,5,6,7-tetrahydro-1H-indole-3-carboxylic acid (4-methylaminomethyl-phenyl)-amide (1), developed for general anxiety disorder, was discontinued from clinical development due to unsuitable oral pharmacokinetics. 2. In humans, (1) demonstrated an unacceptable high apparent oral clearance (Cl(p)/F) that also demonstrated a supraproportional dose-exposure relationship. Secondary peaks in the plasma concentration-time profile suggested possible enterohepatic recirculation of (1). A combination of in vitro mechanistic tools was applied to better understand the processes underlying these complex clinical pharmacokinetic profiles of (1). 3. In metabolism experiments, (1) was shown to be a substrate of monoamine oxidase A (MAO-A) as well as being metabolized by cytochrome P450. The former appeared to be a high K(M) process with a high capacity, while the latter showed saturation between 1 and 10 microM, consistent with the supraproportional dose-exposure relationship. 4. In a sandwich-cultured hepatocyte model, (1) was shown to be a substrate for both uptake and efflux into the canicular space, which is consistent with the observation of pharmacokinetics suggestive of enterohepatic recirculation. Finally, in human epithelial colon adenocarcinoma cell line (Caco-2) and Madin-Darby canine kidney cells transwell flux experiments, (1) was shown to have relatively low permeability and a basolateral-to-apical flux ratio consistent with the activity of P-glycoprotein. 5. In combination, a compounding of the contributions of MAO-A, hepatic uptake and efflux transporters, and P-glycoprotein to the disposition of (1) may underlie the low oral exposure, saturable clearance, and aberrant concentration versus time profiles observed for this compound in humans.

Comparison of the bioactivation potential of the antidepressant and hepatotoxin nefazodone with aripiprazole, a structural analog and marketed drug.

In vitro metabolism/bioactivation of structurally related central nervous system agents nefazodone (hepatotoxin) and aripiprazole (nonhepatotoxin) were undertaken in human liver microsomes in an attempt to understand the differences in toxicological profile. NADPH-supplemented microsomal incubations of nefazodone and glutathione generated conjugates derived from addition of thiol to quinonoid intermediates. Inclusion of cyanide afforded cyano conjugates to iminium ions derived from alpha-carbon oxidation of the piperazine ring in nefazodone and downstream metabolites. Although the arylpiperazine motif in aripiprazole did not succumb to bioactivation, the dihydroquinolinone group was bioactivated via an intermediate monohydroxy metabolite to a reactive species, which was trapped by glutathione. Studies with synthetic dehydroaripiprazole metabolite revealed an analogous glutathione conjugate with molecular weight 2 Da lower. Based on the proposed structure of the glutathione conjugate(s), a bioactivation sequence involving aromatic ortho-or para-hydroxylation on the quinolinone followed by oxidation to a quinone-imine was proposed. P4503A4 inactivation studies in microsomes indicated that, unlike nefazodone, aripiprazole was not a time- and concentration-dependent inactivator of the enzyme. Overall, these studies reinforce the notion that not all drugs that are bioactivated in vitro elicit a toxicological response in vivo. A likely explanation for the markedly improved safety profile of aripiprazole (versus nefazodone) despite the accompanying bioactivation liability is the vastly improved pharmacokinetics (enhanced oral bioavailability, longer elimination half-life) due to reduced P4503A4-mediated metabolism/bioactivation, which result in a lower daily dose (5-20 mg/day) compared with nefazodone (200-400 mg/day). This attribute probably reduces the total body burden to reactive metabolite exposure and may not exceed a threshold needed for toxicity.