Antineoplastic effect and toxicity of 1,25-dihydroxy-16-ene-23-yne- vitamin D₃ in athymic mice with Y-79 human retinoblastoma tumors

Objectives: To evaluate the in vivo efficacy and toxicity of the 1,25- dihydroxy-16-ene-23-yne-vitamin D₃ (16,23-D₃) analogue in athymic nude mice injected with Y-79 human retinoblastoma cells and to compare the efficacy and toxicity of this compound with those of 1,25-dihydroxycholecalciferol (D₃, calcitriol). Methods: Thirty athymic nude mice (4-6 weeks old) were injected subcutaneously with 1 x 10⁷ Y-79 human retinoblastoma cells suspended in a 1:1 mixture of Iscove culture medium supplemented with 20% fetal bovine serum and basement membrane matrix suspension. Five days after tumor injection, the mice were randomized to 3 groups of 10 mice each. The first group served as a control group and received intraperitoneal injections of 0.25 mL of mineral oil (vehicle) 5 times a week. The second group received intraperitoneal injections of 0.05 μg of calcitriol in 0.25 mL of mineral oil intraperitoneally 5 times a week. The third group received intraperitoneal injections of 0.5 μg of 16,23-D₃ in 0.25 mL of mineral oil 5 times a week. Injections were continued for 5 weeks, during which tumor size and mouse weight were individually measured. Toxicity was assessed by clinical measures such as lethargy, weight loss, and death. The mice were then killed and the size, volume, and weight of each tumor were determined. Also, in representative animals in each group, kidneys were evaluated for calcification and serum calcium concentration was measured. Results: All experimental and control animals developed tumors subcutaneously. The 16,23- D₃-treated mice had significantly smaller average tumor size (1.55 cm³) than the control mice (3.45 cm³) (P = .02), less gam in average body weight from the beginning of treatment (2.4 g vs 5.5 g) (P = .06), and a 40% mortality. The calcitriol-treated mice did not have significantly smaller average tumor size (1.26 cm³) than the 16,23-D₃-treated mice (P = .35), had significant body weight loss compared with the control animals (calcitriol- treated mice lost 4.03 g) (P= .001), and had a mortality of 90% by the completion of the experiment. Histologically, there was no difference in the degree of tumor necrosis and calcification between control and experimental mice. Serum calcium concentrations were equivalent between the control (2.15 mmol/L [8.6 mg/dL]) and experimental groups (calcitriol, 1.88 mmol/L [7.5 mg/dL] [P = .97]; 16,23-D₃, 2.15 mmol/L [8.6 mg/dL] [P = .42]). Mild bilateral renal tubular calcification occurred in 3 of 4 mice in the calcitriol-treated group and in 2 of 4 mice in the 16,23-D₃-treated group. Conclusions: The growth of subcutaneous Y-79 human retinoblastoma cells in athymic nude mice is significantly reduced by treatment with intraperitoneal injections of 16,23-D₃. The antineoplastic effect of calcitriol is not statistically significantly different but is associated with significantly more toxicity. 1,25-Dihydroxy-16-ene-23-yne-vitamin D₃ may be a useful chemotherapeutic adjunct in the treatment of retinoblastoma.