INHIBITION OF CHOLESTEROL-SYNTHESIS BY SQUALENE SYNTHASE INHIBITORS DOES NOT INDUCE MYOTOXICITY IN-VITRO

The cholesterol-lowering HMG CoA reductase inhibitors (HMGRI), pravast atin and lovastatin, have been associated with skeletal myopathy in hu mans and in rats. In a previous in vitro study, HMGRI-induced changes in neonatal rat skeletal muscle cells were characterized by reversible inhibition of protein synthesis and loss of differentiated myotubes a t concentrations markedly lower than those inducing enzyme leakage. My otoxicity was determined to be directly related to inhibition of HMG C oA reductase, since mevalonate, the immediate product of HMG CoA reduc tase metabolism, abrogated the drug-induced changes. Farnesol, geranyl geraniol, and squalene are metabolites of mevalonate. Squalene, formed from farnesol by squalene synthase, is the first metabolite solely co mmitted to cholesterol synthesis. In contrast,-geranylgeraniol, formed by the addition of an isoprene group to farnesol, is the first metabo lite uncommitted to cholesterol synthesis. The objective of the presen t study was to determine the role of inhibition of cholesterol synthes is in HMGRI-induced in vitro myotoxicity. HMGRI-treated neonatal rat s keletal muscle cultures were supplemented with farnesol and geranylger aniol, and in another study, muscle cultures were exposed to two squal ene synthase inhibitors (SSI), BMS-187745 and its prodrug eater, BMS-1 88494. Endpoints evaluated for both studies included protein synthesis ([H-3]leucine incorporation), total cellular protein (a measure of ce ll loss), intra- and extracellular lactate dehydrogenase activity (a m easure of membrane integrity), cholesterol biosynthesis ([C-14]acetate incorporation), and morphology. HMG CoA reductase inhibitor-induced m orphologic changes and inhibition of protein synthesis were significan tly ameliorated by supplementation with farnesol and geranylgeraniol. In contrast to HMGRI-induced in vitro myotoxicity, SSI induced an irre versible, minimal cytotoxicity at close to maximum soluble concentrati ons. These results indicate that depletion of metabolites of geranylge ranyl pyrophosphate, and not inhibition of cholesterol synthesis, is t he primary cause of HMG CoA reductase-induced myotoxicity. (C) 1997 Ac ademic Press.