βIII-tubulin expression as a predictor of docetaxel resistance in metastatic castrate-resistant prostate cancer.

Abstract

e15174 Background: Docetaxel is a tubulin-targeting cytotoxic that remains first-line therapy in metastatic castrate-resistant prostate cancer (mCRPC) patients (pts) even though half of pts are reported to be non-responders. A predictive marker to identify those who will benefit from docetaxel-therapy will assist clinical decision making. High βIII-tubulin (TUBB3) expression has previously been reported to correlate with lack of response to taxanes in other cancers. We evaluated TUBB3 expression as a predictor of docetaxel-resistance in mCRPC. Methods: mCRPC pts treated with at least 3 cycles of docetaxel between 1990 and 2011 were identified retrospectively. TUBB3 immunostaining was performed on archival formalin-fixed, paraffin-embedded tissue. Stain intensity was scored from 0 to 3; 2 and 3 were interpreted as positive. Rates of PSA response were compared between pts with positive (+) and negative (-) TUBB3 expression. Two definitions of PSA response were evaluated (any PSA decline and at least 50% decline). Overall survival (OS) distribution between TUBB3+ and TUBB3- pts was estimated by the Kaplan-Meier method. Results: Of 73 pts, 26 (35%) expressed TUBB3. At diagnosis, the mean age was 65.7 years and the median Gleason score was 8. At the time of docetaxel therapy, the mean age was 71.2 years, the median PSA level was 70.9 (range, 0.2-5253) and 76% had ECOG performance status ≤1. The median number of docetaxel cycles was 7 (range, 3-18). The total dose of docetaxel was not different between groups (p=0.705). The median OS was 19.2 mo. TUBB3 expression was not correlated with any clinical or pathological characteristic (age, Gleason score, stage, ECOG, PSA, LDH, alkaline phosphatase, hemoglobin, visceral disease or chemotherapy before docetaxel). 65% of TUBB3+ pts had any PSA decline compared to 89% of pts with TUBB3- (p=0.0267). 52% of TUBB3+ pts had a PSA decline of ≥ 50% compared to 70% of TUBB3- pts (p=0.0144). Median OS for TUBB3+ pts was 16.8 mo compared to 20.4 mo in TUBB3- pts (p=0.039). Conclusions: High TUBB3 expression was associated with shorter OS and lower PSA response rates in mCRPC pts treated with docetaxel. These findings need to be validated prospectively.