Vitamin D analogs, a new treatment for retinoblastoma: The first Ellsworth Lecture

Purpose: This lecture honors the memory of Dr. Robert M. Ellsworth, an important figure in the development of current treatments of retinoblastoma (RB), and reviews our studies of vitamin D analogs as treatments for retinoblastoma in two experimental mouse models. We identified vitamin D receptors in retinoblastoma and examined the effectiveness and mechanism of action of these analogs. Methods: Reverse-transcriptase polymerase chain reaction (RT-PCR) amplification was used to detect vitamin D receptor mRNAs in human and mouse retinoblastomas. The effectiveness and toxicity of vitamin D₂, calcitriol, and synthetic analogs were studied in the athymic/Y-79 xenograft and transgenic mouse models of RB. Dosing was 5X/week for five weeks. Dose-response studies focused on tumor inhibition; toxicity studies investigated survival and serum calcium. The mechanism of action of vitamin D was investigated using terminal transferase dUTP labeling 3′-overhang ligation to measure apoptosis; immunohistochemistry measured p53-dependent gene expression and cell proliferation. Result: Vitamin D receptor mRNAs were detectable in Y-79 RB cells, LHβ-Tag tumors, and human RB specimens using RT-PCR. Calcitriol inhibited cell growthin vitro. Calcitriol and vitamin D₂ inhibited in vivo growth in xenograft and transgenic models, but therapeutic levels were toxic due to hypercalcemia. Two analogs, 16,23-D₃ and 1α-OH-D₂, inhibited tumors in animal models of RB with reduced toxicity.The mechanism of action appears related to increased p53-related gene expression resulting in increased apoptosis. Conclusion: 16,23-D₃ and 1α-OH-D₂ are effective in tumor reduction in two mouse models of RB with low toxicity. These results warrant initiating phase 1 and phase 2 clinical studies in children.