5075 Background: mPC pts experiencing a short period of sensitivity to initial androgen deprivation therapy (ADT) have an unfavourable prognosis and lower probability of response to 2nd-line endocrine therapy suggestive of androgen receptor (AR) pathway-independent mechanisms of resistance. Recent studies have shown that docetaxel may kill PC cells through AR targeting. We thus investigated whether docetaxel may have efficacy in pts with early CR. Methods: Two independent prospective databases (Institut Gustave Roussy and Institut Jean Godinot) of mCRPC pts were analyzed. Early CR was defined as progression within the first year of ADT. The primary endpoint was time to progression (TTP) on docetaxel. Secondary criteria were clinical benefit (decrease in 2 points on a 0-to-10-pain-scale or in dose or type of pain medication), PSA response (decrease > 50% in PSA serum level) and overall survival (OS). Pts with early CR were compared to pts with sensitivity to initial ADT > 1 year (t-test for quantitative data, χ-2 test or Fischer’s exact test for qualitative data, Logrank test for TTP and OS). Results: 188 pts were selected for analysis. A higher frequency of unfavourable prognostic factors in pts with early CR as compared with others pts was observed with a median PSA serum level of 81 ng/dl (4-8000) vs 35 ng/dl (4-4260) (p=5.10-5), presence of visceral metastasis in 11% vs 2% of pts (p=0.01) and Gleason score ≥ 8 in 63% vs 41% of pts respectively (p=0.001). Clinical benefit and PSA response on docetaxel were observed in 84% vs 89% of pts (p=0.45) and 67% vs 81% of pts (p=0.10) respectively. Median TTP was 6.1 months (95%CI=6-7.5) vs 7.8 months (95%CI=7-8.3) (p=0.04). Median OS was 22.4 months (95%CI=17.1–28.6) vs 36.1 months (95%CI=26.3–42.4) (p=0.002). Conclusions: mPC pts experiencing early CR have more unfavourable baseline prognostic factors, shorter TTP on docetaxel and shorter OS as compared with pts experiencing time to CR > 1 year. However, similar clinical and biological benefits from docetaxel chemotherapy were observed in this subset of pts. Since they have lower probability of response to second-line endocrine therapy, docetaxel appears as the best option after progression on initial ADT.