1-Hydroxy-3-(methylpentylamino)-propylidene-1,1-bisphosphonic acid as a potent inhibitor of squalene synthase.

Squalene synthase, the first committed enzyme for sterol synthesis, converts farnesyl pyrophosphate to squalene with presqualene pyrophosphate as an intermediate. It was discovered that BM 21.0955 (1-hydroxy-3-(methylpentylamino)-propylidene-1,1-bisphosphon ic acid), in development for the treatment of bone disorders, inhibited rat liver microsomal squalene synthase (K(i) = nmol/l). BM 21.0955 also inhibited sterol biosynthesis from mevalonate (IC50 = 42 nmol/l), and cholesterol biosynthesis in J774 cells (IC50 = mumol/l). Structural modifications on this molecule to make it more lipophilic may result in a new class of cholesterol-lowering agents.

RG 12561 (dalvastatin): a novel synthetic inhibitor of HMG-CoA reductase and cholesterol-lowering agent.

RG 12561 (dalvastatin) is a prodrug which converts to its open hydroxyacid form in the body. The Na salt of RG 12561 (RG 12561-Na) is a potent inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in the cholesterol biosynthetic pathway. It competitively inhibits rat liver HMG-CoA reductase with an IC50 value of 3.4 nmol/l. In the same assay, the IC50 values for other potent HMG-CoA reductase inhibitors, lovastatin-Na and pravastatin, were 2.3 and 8.9 nmol/l, respectively. In Hep G2 liver cells, RG 12561-Na, lovastatin-Na and pravastatin inhibited cholesterol biosynthesis from radiolabeled octanoate with IC50 values of 4 and 5 nmol/l and 1.1 mumol/l, respectively. In a rat ex vivo assay, orally administered RG 12561, lovastatin and pravastatin inhibited cholesterol biosynthesis in liver slices with ED50 values of 0.9, 0.5 and 12 mg/kg, respectively. In cholestyramine-fed hamsters, RG 12561 (0.1% in food for 18 days) reduced LDL cholesterol, whereas HDL was slightly increased. The reductions in the LDL/HDL ratio for RG 12561, RG 12561-Na, lovastatin and lovastatin-Na were 35, 76, 88 and 88%, respectively. At a higher dose, RG 12561 (0.4% in food) reduced serum cholesterol, LDL and LDL/HDL by 84, 97 and 91%, respectively. In WHHL rabbits, RG 12561 and lovastatin (5 mg/kg, b.i.d., 12 days) reduced serum cholesterol by 17 and 16%, respectively. These results demonstrate that RG 12561 is a potent cholesterol-lowering agent.

Bisphosphonates used for the treatment of bone disorders inhibit squalene synthase and cholesterol biosynthesis.

Some bisphosphonates used for the treatment of bone disorders are also potent inhibitors of squalene synthase, a critical enzyme for sterol biosynthesis. Among seven drugs tested, YM 175 (cycloheptylaminomethylene-1,1-bisphosphonic acid) was the most potent inhibitor of rat liver microsomal squalene synthase (Ki = 57 nM) and sterol biosynthesis from [14C]mevalonate in rat liver homogenate (IC50 = 17 nM). EB 1053 (3-(1-pyrolidino)-1-hydroxypropylidene-1,1-bisphosphonic acid) and PHPBP (3-(1-piperidino)-1-hydroxypropylidene-1,1-bisphosphonic acid) were less potent inhibitors in both these assays. Pamidronate and alendronate were poor inhibitors of squalene synthase (IC50 > 10 microM) but were potent inhibitors of sterol biosynthesis from mevalonate (IC50 = 420 and 168 nM, respectively), suggesting that the latter two agents may have inhibited other enzymes involved in the synthesis of farnesyl pyrophosphate from mevalonate. Etidronate and clodronate were inactive in both these assays. YM 175 also inhibited sterol biosynthesis in mouse macrophage J774 cells (IC50 = 64 microM) and in rats, when administered acutely, it inhibited cholesterol biosynthesis in the liver (ED50 = 30 mg/kg, s.c.). Structural modifications on YM 175 to enhance cell permeability may result in a new class of cholesterol-lowering agents.