OBJECTIVE: To evaluate the effect of BXL628, a vitamin D3 analog, on prostate volume in patients with benign prostatic hyperplasia (BPH). METHODS: We conducted a phase II, double blind, randomized, placebo controlled, clinical study. Patients eligible were aged>or=50 years, had a diagnosis of BPH and a prostate volume>or=40 ml. Eligible patients were randomized and given either BXL628 150 mcg daily or placebo for 12 weeks. All randomized patients underwent at baseline and at the end of study pelvic MRI to measure prostatic volume, uroflowmetry (Qmax), American Urological Association Symptom Index (AUASI), serum PSA, testosterone, dihydrotestosterone and luteizing hormone. RESULTS: A total of 119 patients were randomized: 57 patients to BXL628 and 62 to placebo. The percentage change of prostate volume at 12 week was -2.90 in the BXL628 group vs. +4.32 in the placebo group (p-value<0.0001). The estimated difference between treatments (BXL628 minus placebo) was -7.22% (95% confidence limit -9.27 to -5.18). Considering Qmax, mean change vs. baseline was -0.30 in BXL628 vs. +1.50 in the placebo group: this finding was not statistically significant. The mean change of the AUASI total score at final visit vs. baseline was -1.77 in the BXL628 group vs. -3.45 in the placebo group (p=not significant). CONCLUSION: BXL628 was able to arrest prostate growth within 12 weeks in men aged>or=50 years with prostatic volume>or=40 ml. Its unprecedented mechanism of action may offer a new opportunity for the treatment of BPH.