Genotoxicity of 2-(3-Chlorobenzyloxy)-6-(piperazinyl)pyrazine, a Novel 5-Hydroxytryptamine2c Receptor Agonist for the Treatment of Obesity: Role of Metabolic Activation

2-(3-Chlorobenzyloxy)-6-(piperazin-1-yl)pyrazine (3) is a potent and selective 5-HT(2C) agonist that exhibits dose-dependent inhibition of food intake and reduction in body weight in rats, making it an attractive candidate for treatment of obesity. However, examination of the genotoxicity potential of 3 in the Salmonella Ames assay using tester strains TA98, TA100, TA1535, and TA1537 revealed a metabolism (rat S9/NADPH)- and dose-dependent increase of reverse mutations in strains TA100 and TA1537. The increase in reverse mutations was attenuated upon coincubation with methoxylamine and glutathione. The irreversible and concentration-dependent incorporation of radioactivity in calf thymus DNA after incubations with [14C]3 in the presence of rat S9/NADPH suggested that 3 was bioactivated to a reactive intermediate that covalently bound DNA. In vitro metabolism studies on 3 with rat S9/NADPH in the presence of methoxylamine and cyanide led to the detection of amine and cyano conjugates of 3. The mass spectrum of the amine conjugate was consistent with condensation of amine with an aldehyde metabolite derived from hydroxylation of the secondary piperazine nitrogen-alpha-carbon bond. The mass spectrum of the cyano conjugate suggested a bioactivation pathway involving N-hydroxylation of the secondary piperazine nitrogen followed by two-electron oxidation to generate an electrophilic nitrone, which reacted with cyanide. The 3-chlorobenzyl motif in 3 was also bioactivated via initial aromatic ring hydroxylation followed by elimination to a quinone-methide species that reacted with glutathione or with the secondary piperazine ring nitrogen in 3 and its monohydroxylated metabolite(s). The metabolism studies described herein provide a mechanistic basis for the mutagenicity of 3.

Dual-action hypoglycemic and hypocholesterolemic agents that inhibit glycogen phosphorylase and lanosterol demethylase

Diabetic dyslipidemia requires simultaneous treatment with hypoglycemic agents and lipid-modulating drugs. We recently described glycogen phosphorylase inhibitors that reduce glycogenolysis in cells and lower plasma glucose in ob/ob mice (J. Med. Chem., 41: 2934, 1998). In evaluating the series prototype, CP-320626, in dogs, up to 90% reduction in plasma cholesterol was noted after 2 week treatment. Cholesterol reductions were also noted in ob/ob mice and in rats. In HepG2 cells, CP-320626 acutely and dose-dependently inhibited cholesterolgenesis without affecting fatty acid synthesis. Inhibition occurred together with a dose-dependent increase in the cholesterol precursor, lanosterol, suggesting that cholesterolgenesis inhibition was due to lanosterol 14alpha-demethylase (CYP51) inhibition. In ob/ob mice, acute treatment with CP-320626 resulted in a decrease in hepatic cholesterolgenesis with concomitant lanosterol accumulation, further implicating CYP51 inhibition as the mechanism of cholesterol lowering in these animals. CP-320626 and analogs directly inhibited rhCYP51, and this inhibition was highly correlated with HepG2 cell cholesterolgenesis inhibition (R2 = 0.77). These observations indicate that CP-320626 inhibits cholesterolgenesis via direct inhibition of CYP51, and that this is the mechanism whereby CP-320626 lowers plasma cholesterol in experimental animals. Dual-action glycogenolysis and cholesterolgenesis inhibitors therefore have the potential to favorably affect both the hyperglycemia and the dyslipidemia of type 2 diabetes.

Isozyme-nonselective N-substituted bipiperidylcarboxamide acetyl-CoA carboxylase inhibitors reduce tissue malonyl-CoA concentrations, inhibit fatty acid synthesis, and increase fatty acid oxidation in cultured cells and in experimental animals

Inhibition of acetyl-CoA carboxylase (ACC), with its resultant inhibition of fatty acid synthesis and stimulation of fatty acid oxidation, has the potential to favorably affect the multitude of cardiovascular risk factors associated with the metabolic syndrome. To achieve maximal effectiveness, an ACC inhibitor should inhibit both the lipogenic tissue isozyme (ACC1) and the oxidative tissue isozyme (ACC2). Herein, we describe the biochemical and acute physiological properties of CP-610431, an isozyme-nonselective ACC inhibitor identified through high throughput inhibition screening, and CP-640186, an analog with improved metabolic stability. CP-610431 inhibited ACC1 and ACC2 with IC50s of approximately 50 nm. Inhibition was reversible, uncompetitive with respect to ATP, and non-competitive with respect to bicarbonate, acetyl-CoA, and citrate, indicating interaction with the enzymatic carboxyl transfer reaction. CP-610431 also inhibited fatty acid synthesis, triglyceride (TG) synthesis, TG secretion, and apolipoprotein B secretion in HepG2 cells (ACC1) with EC50s of 1.6, 1.8, 3.0, and 5.7 microm, without affecting either cholesterol synthesis or apolipoprotein CIII secretion. CP-640186, also inhibited both isozymes with IC50sof approximately 55 nm but was 2-3 times more potent than CP-610431 in inhibiting HepG2 cell fatty acid and TG synthesis. CP-640186 also stimulated fatty acid oxidation in C2C12 cells (ACC2) and in rat epitrochlearis muscle strips with EC50s of 57 nm and 1.3 microm. In rats, CP-640186 lowered hepatic, soleus muscle, quadriceps muscle, and cardiac muscle malonyl-CoA with ED50s of 55, 6, 15, and 8 mg/kg. Consequently, CP-640186 inhibited fatty acid synthesis in rats, CD1 mice, and ob/ob mice with ED50s of 13, 11, and 4 mg/kg, and stimulated rat whole body fatty acid oxidation with an ED50 of approximately 30 mg/kg. Taken together, These observations indicate that isozyme-nonselective ACC inhibition has the potential to favorably affect risk factors associated with the metabolic syndrome.

Pharmacokinetic screening of o-naphthoquinone 5-lipoxygenase inhibitors

The o-naphthoquinone derivative, CGS 8515 (I), is a potent inhibitor (IC50, 0.1 microM) of 5-lipoxygenase, but its therapeutic potential is compromised by a short plasma half-life (22 min) and extremely poor oral bioavailability (less than 2%). Poor biopharmaceutical properties of CGS 8515 were attributed to poor aqueous solubility and rapid in vivo hydrolysis of its methyl ester function to an inactive metabolite (IC50, 100 microM). An active amide analogue (II) was synthesized to prevent rapid hydrolysis. While analogue II appeared to be stable in vivo, its plasma half-life was also short (10 min), possibly because of rapid tissue distribution rather than metabolic elimination. Therefore, three potent analogues with increased aqueous solubilities were synthesized and compared with respect to their pharmacokinetic properties. The analogue with the highest aqueous solubility (V) demonstrated a plasma concentration vs time profile with the largest area under the curve (AUC) and the smallest distribution (alpha) phase of all the analogues studied. The percentage AUC of the terminal phase (beta) for three analogs paralleled their aqueous solubilities. The oral bioavailability of V was improved to 27%, compared to 2% for the parent compound, CGS 8515.