ASCO GU 2019: Assessment of Circulating Tumor Cell Number as a Transitional Surrogate Endpoint for Survival in Phase II Trials for Metastatic Castrate-Resistant Prostate Cancer

San Francisco, CA ( In this presentation, Howard Sher, MD discussed data supporting the use of circulating tumor cells (CTCs) as an appropriate clinical endpoint in phase II trials in patients with metastatic castrate-resistant prostate cancer (mCRPC).  Systemic therapies for CRPC continue to evolve, with the majority of drug approvals based upon time to clinical events that often occur late (ie skeletal-related events, symptoms, metastasis-free survival, death/overall survival). Dr. Sher stated that with the exception of pain control, no drug approvals have been based on favorable changes in disease manifestation.

Changes in CTC numbers have the potential to fulfill a need as a clinical biomarker.  Previous studies have demonstrated that patients with prostate, breast, colorectal cancers may tend a poor prognosis, but only a single CTC assay currently has active FDA approval (CellSearchTM, approved in 2004). 

Association of CTCs and PSA response endpoints with survival has studied using individual patient data from 5 Phase III trials across mCRPC.  Moreover, CTC0 (defined as the detection of >1 CTCs pre-treatment and 0 post-treatment) and PSA50 (defined as a >50% decline from baseline) were explored in this study as well.  

Greater than 6000 patients were enrolled.  Comparing responder to non-responder survival curves, CTC0 had greater separation than PSA50 (0.35 vs 0.29 survival probability).  These trends persisted at the 6, 12, 15 and 18-month landmark.  Utilizing a k-month survival probability endpoint, CTC0 explained >60% of the variation in survival and as found to be more informative than PSA50.  Furthermore, neither CTC0 or PSA 50 or CTC0 and PSA50 added predictive power over CTC0 alone. 

Dr. Sher stated that these biomarkers may be able to improve the management of patients and improve clinical decision making.  The goal of which is that CTC may be a future clinical endpoint which assesses clinical benefit in phase III clinical trials.  However, more studies are needed at this time. 

Presented by: Howard I. Sher, MD, FASCO, Medical Oncologist, Co-Chair, Center for Mechanism-Based Therapy; Head of the Biomarker Development Initiative; D. Wayne Calloway Chair in Urologic OncologyMemorial Sloan Kettering Cancer Center

Written by: David B. Cahn, DO, MBS, @dbcahn, Fox Chase Cancer Center at the 2019 American Society of Clinical Oncology Genitourinary Cancers Symposium, (ASCO GU) #GU19, February 14-16, 2019 - San Francisco, CA

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