Background: Aromatase Inhibitor (AI) therapy and androgen deprivation therapy (ADT) significantly accelerate bone loss and increase fracture risk. Vitamin D (VITD) protects against bone loss, but it is unclear whether the recommended daily allowance (RDA; 600 IU/day for ages 51-70) of VITD is sufficient for cancer patients. Data from two RCTs were pooled to examine the safety and efficacy of high-dose VITD versus the RDA of VITD on bone mineral density (BMD). Methods: 164 breast and prostate cancer patients on AIs and ADT, respectively, with low VITD (<32 ng/ml) were randomized to either high-dose VITD (50,000 IU/week; n=99) or placebo (n=65) for 24 weeks. All subjects received 600 IU/day of VITD. Of the 99 subjects assigned to high-dose VITD, 38 breast subjects also received the Exercise for Cancer Patients (EXCAP) program combining walking and resistance training. Serum VITD and calcium were assessed at weeks 0, 6, 12, 18, and 24. BMD was assessed at the hip and spine via DXA at weeks 0 and 24. The effect of high-dose VITD was tested via ANCOVA model adjusted for cancer type, baseline BMD and VITD. Results: High-dose VITD significantly reduced the amount of hip BMD loss versus the RDA of VITD (high-dose VITD: −0.8% vs placebo: −2.6%; p<0.01) over 24 weeks. Hip BMD loss was greater for subjects on ADT (high-dose VITD: −1.5% vs placebo: −4.1%; p=0.03) than subjects on AI therapy (high-dose VITD: −0.2% vs placebo: −1.7%; p=0.02). Among the high-dose VITD group, there was no BMD difference at the total hip between those who received EXCAP exercise vs no EXCAP (p=0.96). The largest differences in BMD were for those with lower baseline VITD levels (<27 ng/ml) for both total hip (high-dose VITD: −0.6% vs placebo: −3.2%; p<0.001) and femoral neck (high-dose VITD: +0.2% vs placebo: −2.4%; p=0.03). No between-group pooled differences were noted for total spine BMD (high-dose VITD: −0.2% vs placebo: −0.1%; p=0.82). High-dose VITD increased serum VITD without negatively affecting serum calcium (Table). Conclusions: High-dose VITD was safe and effective in significantly reducing hip BMD loss, with the largest benefit in those with lower baseline VITD levels. A phase III RCT is needed to confirm these findings. NCI Funding: K07 CA168911/R21 CA175793/UG1 CA189961/T32 CA102618 Clinical trial information: NCT02064946, NCT01419730.

*p<0.05, high-dose VITD vs. placebo