ro-13-6298 and Osteoporosis

A potent and selective inhibitor of the osteoclastic V-H(+)-ATPase, (2Z,4E)-5-(5,6-dichloro-2-indolyl)-2-methoxy-N-(1,2,2,6, 6-pentamethylpiperidin-4-yl)-2,4-pentadienamide (SB 242784), was evaluated in two animal models of bone resorption. SB 242784 completely prevented retinoid-induced hypercalcemia in thyroparathyroidectomized (TPTX) rats when administered orally at 10 mg/kg. SB 242784 was highly efficacious in the prevention of ovariectomy-induced bone loss in the rat when administered orally for 6 months at 10 mg/kg/d and was partially effective at 5 mg/kg/d. Its activity was demonstrated by measurement of bone mineral density (BMD), biochemical markers of bone resorption, and histomorphometry. SB 242784 was at least as effective in preventing bone loss as an optimal dose of estrogen. There were no adverse effects of compound administration and no effects on kidney function or urinary acidity. Selectivity of the inhibitor was further studied using an in situ cytochemical assay for bafilomycin-sensitive V-H(+)-ATPase using sections of osteoclastoma and numerous other tissues. SB 242784 inhibited the osteoclast enzyme at 1,000-fold lower concentrations than enzymes in any of the other tissues evaluated. SB 242784 demonstrates the utility of selective inhibition of the osteoclast V-H(+)-ATPase as a novel approach to the prevention of bone loss in humans.

Topics: Acids; Administration, Oral; Animals; Benzoates; Bone Density; Bone Resorption; Drug Interactions; Enzyme Inhibitors; Estradiol; Female; Femur; Hypercalcemia; Indoles; Lumbosacral Region; Osteoclasts; Osteoporosis; Ovariectomy; Parathyroidectomy; Piperidines; Proton-Translocating ATPases; Rats; Rats, Sprague-Dawley; Retinoids; Spine; Thyroidectomy; Urine; Vacuolar Proton-Translocating ATPases; Vacuoles

The process of bone resorption by osteoclasts involves the dissolution of mineral salts and enzymatic degradation of the mainly collagenous extracellular matrix. Cysteine proteinases, which can efficiently degrade collagen at acidic pH, have been suggested to play an important role in the bone resorptive process. The cysteine proteinase cathepsin L is secreted by osteoclasts, and inhibitors of this enzyme can prevent bone resorption in vitro. The activity of acetyl-leu-leu-norleucinol (ALLN), a selective inhibitor of cathepsin L, was investigated in two models of bone resorption in vivo. In the first study, the ability of ALLN to inhibit bone resorption was investigated in Ro-13-6298 (arotinoid)-treated thyroparathyroidectomized (TPTX) rats. ALLN [100 mg/kg, intraperitoneally (i.p.)] inhibited hypercalcemia by 62.8% acutely (p < 0.001), compared to 94.9% (p < 0.001) inhibition by salmon calcitonin (sCT) (10 IU/kg, subcutaneously). In rats treated for 3 days with ALLN, arotinoid-induced reduction in cortical bone mineral density measured by peripheral quantitative computed tomography (pQCT) was inhibited by 86.4% (p < 0.05) in rats treated with ALLN 100 mg/kg, i.p., and by 82% in rats treated with 50 mg/kg, i.p. (p < 0.05). In a second study, the efficacy of ALLN was tested in a longitudinal study in ovariectomized (ovx) rats. Bone loss, measured by pQCT, was unaffected by treatment with ALLN. The bisphosphonate alendronate, however, inhibited bone loss in this model. These data demonstrate the ability of a cathepsin L inhibitor to inhibit bone resorption in arotinoid-treated TPTX rats, a process which may be dependent on the activity of cathepsin L-like cysteine proteinases. In contrast to its effects in TPTX rats, ALLN had no inhibitory activity on bone resorption in ovx rats. It is possible that in chronic bone resorption in ovx rats, the activity of other enzymes such as cathepsins OC-2 or K allows the process of resorption to continue even when cathepsin L is inhibited by ALLN. Further studies are required to determine why the activity of ALLN varies between different animal models. These data indicate that there may be variations in the effects of drugs in different animal models of bone resorption which should be considered when investigating novel antiresorptive therapies.

Topics: Animals; Benzoates; Bone Resorption; Cathepsin L; Cathepsins; Cysteine Endopeptidases; Cysteine Proteinase Inhibitors; Disease Models, Animal; Endopeptidases; Female; Humans; Hypercalcemia; Leupeptins; Male; Osteoporosis; Ovariectomy; Ovary; Parathyroid Glands; Parathyroidectomy; Rats; Rats, Wistar; Retinoids; Thyroid Gland; Thyroidectomy