Objectives: Calcitriol has been reported to have antitumor efficacy in several cancers. In this study, we hypothesized that calcitriol may potentially function as a cryosensitizer that can enhance cryoablation, and we investigated several molecular marker changes in a murine model of prostate cancer. Methods: Murine prostate tumors (RM-9) were grown in male C57BL/6J mice subcutaneously with neoadjuvant intratumoral injection of calcitriol followed by cryoablation. The microenvironmental changes after cryoablation alone and in combination with calcitriol were analyzed in a comparative fashion using immunohistochemistry and Western blot analyses. Results: Both cryoablation and the combination group could suppress tumor growth after treatment compared with the control. At final pathologic assessment, a larger necrotic area was seen in the combination group (P = .026). Although microvessel density (CD31) and the area of hypoxia (pimonidazole) was not different between the control and combination groups, cell proliferation (Ki-67) significantly decreased in the combination treatment (P = .035). In Western blot analyses, several markers for apoptosis were expressed significantly higher with the combination treatment. Conclusions: The synergistic effect of calcitriol with cryoablation was demonstrated because of enhanced antitumor efficacy by increasing necrosis and apoptosis and reduced cell proliferation. This study suggests that calcitriol is a potentially applicable reagent as a freeze sensitizer to cryoablation.
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