Effectiveness of vitamin D analogues in treating large tumors and during prolonged use in murine retinoblastoma models

Objective: To investigate the effectiveness of the vitamin D analogues 1,25-(OH)₂-16-ene-23-yne vitamin D₃ (16,23-D₃) and 1α-hydroxyvitamin D₂ (1α-OH-D₂) in inhibiting retinoblastoma growth in large tumors in a xenograft model and with prolonged use in a transgenic model. Methods: For the large-tumor study, the xenograft athymic mouse/human retinoblastoma cell (Y-79) model was used. Subcutaneous tumors were allowed to grow to an average volume of 1600 mm ³. Systemic treatment with 1 of the vitamin D analogues or with vehicle (control groups) was carried out for 5 weeks. For the long-term study, transgenic β-luteinizing hormone-large T antigen (LHβ-Tag) mice were systemically treated with 1 of the 2 compounds or vehicle (control groups) for up to 15 weeks. Tumor size and signs of toxicity were assessed. Results: in the large-tumor study, tumor volume ratios for the 1α-OH-D₂ and 16,23-D₃ groups were significantly lower than those for controls (P<.002). No significant differences in tumor volume were seen between the 1α-OH-D₂ and 16,23-D₃ groups (P=15). In the long-term study, the lot-OH-D2 group showed significantly smaller tumor size compared with its control (P<.001). No significant difference was seen between the 16,23-D₃ group and its control. Some toxic effects related to hypercalcemia were seen in both studies. Conclusions: In athymic mice in the large-tumor study, both 1α-OH-D₂ and 16,23-D ₃ were effective in inhibiting tumor growth compared with controls. In the long-term study, 1α-OH-D₂ inhibited tumor growth but 16,23-D₃ did not. Effective doses of both compounds caused hypercalcemia and a significant increase in mortality. Clinical Relevance: Use of 1α-OH-D₂ inhibited tumor growth in large tumors and with long-term treatment compared with controls. Because of hypercalcemia-related toxic effects seen in the present experiments, in clinical trials, serum calcium levels should be carefully monitored. This analogue may require use with drugs that lower serum calcium levels or use of relatively lower doses or skipped doses. The ideal alternative solution would be to identify vitamin D analogues that retain the anti-neoplastic action without the calcemic activity.