Ability of Vitamin D Analogs to Induce Regression in Long Term Treatment of LHß-Tag Mice

Abstract

Abstract: : Purpose: Two recently developed vitamin D analogues, 1α-hydroxyvitamin-D₂ (1α-OH-D₂) and 1,25-dihydroxy-16-ene-23-yne-vitamin D₃ (16,23-D₃) cause less toxicity than previously studied vitamin D compounds and have been shown to inhibit tumor growth in the athymic/Y-79 mouse model and a transgenic (LHß-Tag) model of the disease. It is not known whether with long-term treatment this tumor remains sensitive to these compounds or becomes refractory. This study examines the long-term ability of both vitamin D analogs to inhibit the growth of intraocular retinoblastoma in the transgenic mouse model. Methods: LHß-Tag transgenic 8-10 week old mice (N=120) were randomized to one of four treatment groups 0.5 µg/day of 16,23-D₃ (IP injection; 40 mice/group), 0.2 µg/day of 1α-OH-D₂ (oral gavage; 40 mice/group), mineral oil (IP injection control group; 20 mice/group), and coconut oil (control gavage group; 20 mice/group). One-third of the animals in each group were euthanized at 5, 10 and 15 weeks of treatment. At euthanization, the eyes were enucleated and blood was drawn for determination of serum calcium. The liver, lungs and kidneys were harvested. Three representative sections of each eye were stained with H&E, examined microscopically, and tumors were measured using Optimus software. Serum calcium level, kidney calcification, and liver and lung metastases were also examined. Results: At ten weeks the average tumor size of the 1α-OH-D₂ treated group was 656,798 µm² and was statistically different from the corresponding coconut oil control group 3,145,541 µm² (p = 0.03). Average tumor size of the 16,23-D₃ group (675,182 µm²) was not statistically different from the mineral oil control group (1,231,585 µm²) (p = 0.21). Conclusion: After ten weeks of vitamin D analogue therapy there was a statistically significant decrease in tumor size in the 1α-OH-D₂ group when compared to the control group. At the same time point, 16,23-D₃ was not significantly statistically different from its control group. 1α-OH-D₂ shows similar effectiveness with long-term therapy as that previously published with 5-week therapy.

Keywords: retinoblastoma