UnCHAARTED territory: The role of docetaxel rechallenge following chemohormonal therapy for metastatic castrate-sensitive prostate cancer.

117 Background: Since docetaxel has been advanced to the metastatic castrate-sensitive prostate cancer (mCSPC) setting, there is a lack of evidence guiding its re-introduction upon castrate-resistant (CR) progression. We sought to identify clinical characteristics and outcomes of patients subjected to docetaxel rechallenge (DR) following prior docetaxel exposure in the mCSPC realm. Methods: Patients rechallenged with docetaxel following treatment in the mCSPC setting were identified from three academic centres in Ontario, Canada. Retrospective chart reviews were performed to identify clinical, treatment and outcome variables. Results: Of the 45 patients with DR initiated between 06/2015 and 07/2020, the median age was 65, 60% had a Gleason score of ≥8, and 64% had an ECOG of ≤1. 56% had bone only metastasis, 4% lymph node only metastasis, 29% bone and lymph node metastasis, and 11% had visceral metastasis. In the mCSPC setting, 98% of patients received 6 cycles of docetaxel with 13% requiring dose delays. Of 43 informative patients, all had a PSA response to chemohormonal therapy. 91% achieved at least a 50% PSA response (PSA50), of which 40% had a 50-89% PSA reduction and 51% had a ≥90% PSA reduction. 29% of patients obtained a PSA nadir of < 0.2 ng/mL. 16% had CR progression in < 6 months, 56% in 6-12 months, and 28% in > 12 months. DR was initiated after a median of 20.8 months (range 6.0-40.4) following the last dose of docetaxel for mCSPC, and was given as first line treatment for CR disease to 7%, second line to 51%, third line to 40%, and fourth line or beyond to 2% of patients. 69% of patients had received an androgen-receptor axis targeted therapy prior to DR, 18% radium 223, and 7% had received a trial drug. Notably, no patients had received cabazitaxel prior to DR. The median number of cycles of docetaxel received at rechallenge was 5 (range 1-11) with 18% of patients requiring treatment delays. 64% of patients stopped treatment due to progression, 16% due to side effects, 7% at the patient’s request, 7% due to completion of the planned number of cycles, and 6% due to death or other causes. Among 44 informative patients, 23% achieved at least a PSA50, with 18% having a 50-90% PSA reduction, and 5% having a ≥90% PSA reduction. The median time to progression (biochemical, radiographic, or death) was 2.3 months (95%CI 1.7-4.4) and the median overall survival was 11.0 months (95%CI 8.5-14.3). Conclusions: DR following exposure to docetaxel in the mCSPC setting resulted in a PSA50 in only around one quarter of patients. Both the median time to progression and overall survival were found to be short. With future investigations, we hope to identify clinical variables that will help predict which patients might benefit most from DR.