Beneficial effects of a novel thyromimetic on lipoprotein metabolism

Although L-triiodothyronine (L-T-3) lowers cholesterol, this hormone is not used to treat hypercholesterolemia because of its cardiotoxic effects. Thyromimetics, such as the novel compound CGS 23425, that mimic the beneficial but lack the detrimental effects of T-3, may be useful in the treatment of hypercholesterolemia. To show that CGS 23425 has no cardiotoxicity, atrial contractility and force were both measured and found to be unchanged in rats treated with up to 10 mg/kg drug. The lipid lowering actions of this drug resulted in a 44% decrease in low-density lipoprotein (LDL) cholesterol in hypercholesterolemic rats treated with 10 mu g/kg of the compound. Normal rats required a higher dose of 1000 mu g/kg to elicit a similar 50% reduction in LDL cholesterol, Both CGS 23425 or T-3 (10 nM) increased the specific binding of I-125-labeled LDL to Hep G(2) cells and increased LDL receptor number by 44 and 49%, respectively. These data indicate that CGS 23425 enhances hepatic clearance of serum LDL cholesterol. Normal and fat-fed animals treated with the drug showed a dose-dependent increase in apolipoprotein Al, a protein that promotes the efflux of cholesterol from peripheral tissues. Transient transfection of a rat apolipoprotein Al promoter-chloramphenicol acetyltransferase construct, in human hepatoma cells, showed a dose-dependent increase in chloramphenicol acety/transferase activity with EC50 values of 2 x 10(-12) M and 10(-10) M for thyroid hormone receptors beta 1 and alpha 1, respectively, with maximal responses at 10(-7) M. These data indicate that CGS 23425 is a thyromimetic that increases apolipoprotein Al expression via thyroid hormone receptor. In summary, CGS 23425 ameliorates hypercholesterolemia by increasing apolipoprotein A1 and the clearance of LDL cholesterol. Therefore, a compound like CGS 23425 may be useful for the prevention and reversal of atherosclerosis.