Lipid-lowering activity of atorvastatin and lovastatin in rodent species: triglyceride-lowering in rats correlates with efficacy in LDL animal models

Since inhibitors of HMG-CoA reductase lower plasma triglycerides rather than cholesterol in rats, we compared the triglyceride-lowering activity of lovastatin in rats to that of atorvastatin, a more potent synthetic inhibitor, prior to evaluating these drugs in established animal models in which low density lipoproteins (LDL) rather than high density lipoproteins (HDL) are the major transporters of plasma cholesterol. Atorvastatin was more efficacious than lovastatin in normal, chow-fed rats, and more potent in rats with endogenous hypertriglyceridemia (sucrose-fed). In hypertriglyceridemic rats plasma apoB concentrations decreased only with atorvastatin (30 mg/kg), and VLDL-triglyceride secretion (Triton method) was also decreased more by atorvastatin. The inactive enantiomer of atorvastatin did not lower plasma triglycerides. Thus, triglyceride-lowering was dependent upon inhibition of HMG-CoA reductase. Liver unesterified cholesterol and cholesteryl esters (mg/g) were increased by both drugs in normal rats but remained unchanged in hypertriglyceridemic rats. In normal, chow-fed guinea pigs atorvastatin was a more potent cholesterol-lowering drug, and unlike lovastatin, lowered plasma triglycerides and VLDL-cholesterol. In casein-fed rabbits with endogenous hypercholesterolemia and in chow-fed rabbits atorvastatin lowered LDL-cholesterol more potently than lovastatin, but in chow-fed rabbits neither drug had an effect on the in vivo rate of VLDL-lipid secretion, suggesting that efficacy was due to inhibition of direct LDL production and/or enhanced LDL clearance. We conclude that normal rats can be used as a preclinical tool to assess the efficacy of HMG-CoA reductase inhibitors since triglyceride-lowering correlates with cholesterol-lowering in LDL animal models. In this regard atorvastatin is a more potent hypolipidemic agent than lovastatin in animals. A common but not sole mechanism for these drugs may be direct inhibition of the hepatic production of the major apoB-containing lipoprotein in a given species, e.g. VLDL in rats and LDL in guinea pigs and rabbits.