Effect of ring size in R-(+)-pulegone-mediated hepatotoxicity: Studies on the metabolism of R-(+)-4-methyl-2-(1-methylethylidene)-cyclopentanone and DL-camphorone in rats

R-(+)-Pulegone, a monoterpene ketone, is a potent hepatotoxin. The present study was designed to evaluate whether the reduction of the ring sire in R- (+)-pulegone would affect its mode of metabolism and its hepatotoxic potent ial. Metabolic fate of R-(+)-4-methyl-2-(1-methylethylidene)-cyclopentanone (I) and 5-methyl-2-(1-methylethylidene)-cyclopentanone (DL-camphorone; II) were examined in rats, Compounds I and II were administered orally (250 mg /kg of b.wt./day) to rats for 5 to 7 days. The following metabolites were i solated and identified from the urine of rats dosed with I: 3-methyl-5-(1-m ethylethylidene)-cyclopent-2-enone (Ie), Z-4-methyl-2-(1-hydroxymethylethyl idene)-cyclopentanone (Ib), E-4-methyl-1-(1-hydroxymethylethylidene)-cyclop entanone (Ia), 3-hydroxy-4-methyl-2-(1-methylethylidene)-cyclopentanone (If ), 4-hydroxy-4-methyl-2-(1-methylethylidene)-cyclopentanone (Ic), and E-4-m ethyl-2-(1-carboxyethylidene)-cyclopentanone (Id), Phenobarbital (PB)-induc ed rat liver microsomes in the presence of NADPH transformed compound I int o metabolites, which were identified as Ia, Ib, Ic, Ie, and If. The followi ng urinary metabolites were isolated and identified from compound II: 5-hyd roxy-5-methyl-2-(1-methylethylidene)-cyclopentanone (IIc), 5-hydroxy-5-meth yl-2-(1-methylethyl)-cyclopentanone (IIg), Z-5-methyl-2-(1-hydroxymethyleth ylidene)-cyclopentanone (IIb), 5-methyl-2-(1-hydroxymethylethyl)-cyclopenta none (IIf), E-5-methyl-2-(1-hydroxymethylethylidene)-cyclopentanone (IIa), E-5-methyl-5-(1-carboxyethylidene)-cyclopentanone (IId), and 5-methyl-2-(1- carboxyethyl)-cyclopentanone (Ile), PB-induced rat liver microsomes in the presence of NADPH were shown to transform compound II to IIa, IIb, and IIc. Studies carried out in vitro demonstrated that hydroxylation at the tertia ry carbon atom or oxidation of the isopropylidene methyl groups in II can b e specifically blocked through structural modifications as seen in compound s 2,2-dimethyl-5-(1-methylethylidene)cyclopentanone (III) and 5-methyl-2-(1 -ethyl-1-propylidene)-cyclopentanone (IV). Similar observation was also mad e when isopropylidene methyl groups in R-(+)-pulegone were replaced by ethy l groups. Intraperitoneal administration of a single dose (250 mg/kg) of I and II to rats did not elicit hepatotoxicity as judged by serum alanine ami notransaminase levels and liver microsomal drug metabolizing enzyme activit ies.