Inhibition of cholesterol synthesis and hepatic 3-hydroxy-3-methylglutaryl--CoA reductase in rats by simvastatin and pravastatin.

In this communication we attempt to provide one possible explanation for the observed differences regarding kinetics and distribution between simvastatin and pravastatin. Rats treated with simvastatin or pravastatin exhibited a reduction in the incorporation of [2-(14)C] acetate into liver cholesterol and displayed lower plasma mevalonate levels as compared to control animals. Moreover, both the total and dephosphorylated 3-hydroxy-3-methylglutaryl--CoA (HMG-CoA) reductase (EC 1.1.1.34) activities, particularly 1 h after treatment, were greatly reduced in liver microsomes obtained from simvastatin-treated as compared to control rats. During the same time frame, these parameters were actually elevated with pravastatin treatment. It is known that HMG-CoA reductase synthesis and activity increase following their competitive inhibition. Our results suggest that pravastatin, at 1 h following treatment, was no longer bound to the enzyme; however, it had entered the liver because its inhibitory effect on cholesterol synthesis was manifest at early times after administration. These data provide a plausible rationale for the earlier observation that activity of simvastatin persists longer in plasma than does that of pravastatin.