Toxicity and dose-response studies of 1α-hydroxyvitamin D2 in a retinoblastoma xenograft model

Background: Although calcitriol (1,25-dihydroxycholecalciferol) and vitamin D-2 inhibit retinoblastoma growth in the athymic (nude) mouse xenograft (Y-79 cell line) model of retinoblastoma, they can cause severe toxicity. Objective: To examine the toxicity of and dose-dependent response for the inhibition of tumor growth for 1alpha-hydroxyvitamin D-2 (1alpha-OH-D-2), an analogue with reduced systemic toxicity, in the athymic Y-79 mouse model. Methods: Mice were randomized into treatment and control groups for 5-week toxicity and dose-response studies. Treatment was via oral gavage 5 times per week. Dose-response studies measured tumor inhibition and drug serum levels. Tumor size and body weight were measured weekly together with various criteria for toxicity. Animals were euthanized at the end of the treatment period. Tumors and kidneys were harvested, and serum was analyzed for calcium and drug levels. Results: Doses of 0.1 to 1.2 mug/d were selected on the basis of toxicity studies for the dose-response trial. Tumor weight and volume in the 0.2-mug and 0.3-mug doses were significantly lower than in controls. Mortality rates and kidney calcification in mice treated with doses of 0.1 to 0.3 mug were lower than those observed in studies of calcitriol and vitamin D-2. Conclusion: A vitamin D analogue, 1alpha-OH-D-2, inhibits tumor growth in this xenograft model of retinoblastoma with less toxicity than calcitriol and vitamin D-2.