Purpose: Epidemiologic studies have shown that reduced levels of vitamin D represent a major risk factor for prostate cancer. In this report, we have examined the efficacy of 1α,25-dihydroxyvitamin D₃ (1,25 D₃) as a chemopreventive agent using Nkx3.1; Pten mutant mice, which recapitulate stages of prostate carcinogenesis from prostate intraepithelial neoplasia (PIN) to adenocarcinoma.

Experimental Design: 1,25 D₃ (or vehicle) was delivered continuously to Nkx3.1; Pten mutant or control mice for a 4-month period beginning before (precancerous cohort) or after (cancerous cohort) these mice developed PIN. At the conclusion of the study, the mice were analyzed for the occurrence of PIN and/or cancer phenotypes by histologic analyses and immunostaining using known markers of cancer progression in these mice.

Results: We found that sustained delivery of 1,25 D₃ to the Nkx3.1; Pten mutant mice resulted in a significant reduction in the formation of PIN while having no apparent effect on the control mice. Furthermore, 1,25 D₃ was maximally effective when delivered before, rather than subsequent to, the initial occurrence of PIN. We further show that this 1,25 D₃–mediated inhibition of PIN was coincident with up-regulation of vitamin D receptor expression in the prostatic epithelium of the mutant mice, as well as in CASP prostate epithelial cell lines developed from these mice, while having no effect on androgen receptor expression or androgen receptor signaling.

Conclusion: Our findings show the value of chemoprevention studies using Nkx3.1; Pten mutant mice, particularly for evaluating the efficacy and underlying mechanisms of potential agents and to gain insights about the optimal timing of their delivery. In particular, our study predicts that vitamin D may have differential effects during early-stage versus late-stage disease and that it is more likely to be beneficial if delivered either before the overt manifestation of clinically detectable disease or during the earliest disease stages, rather than in advanced disease. Thus, our findings support the assessment of vitamin D analogues for chemoprevention in clinical trials targeting patients with early-stage disease and also establish molecular markers that can be used in such trials to determine biological activity and to optimize further clinical trials.