The CTC cut point of ≥ 5 excludes many patients from response assessment. COU-AA-301 was a phase 3 trial of abiraterone acetate + prednisone vs prednisone alone in mCRPC patients, demonstrating a significant survival benefit for abiraterone (Clinical trial information: NCT00638690).1
The aim of this study was to explore CTC response defined as the presence of any CTC>0 cells at baseline and undetectable post baseline. Additionally, the authors planned to see if there is a correlation between CTC response and measurable disease response in relation to overall survival (OS). One of the secondary objectives of this trial was to examine CTCs alone and in combination with other biomarkers as a potential surrogate for clinical benefit. CTCs were determined at baseline and at 4, 8, and 12 weeks. Radiographic response was first assessed at week 12. Measurable disease was defined as the presence of >=1 measurable lesion; visceral or extra-nodal lesions needed to be >=10 mm and lymph nodes >= 20 mm.
739/972 patients had baseline CTC > 0, and 53% (517/972) had CTC of 5 or more. 48% (573/972) had measurable disease. Overall, 21.2%, 21.8%, and 19.1% of patients with detectable CTC at baseline achieved CTC=0 status at week 4, 8, and 12, respectively. In contrast, 14.3%, 14.3% and 13.5% with a baseline CTC of 5 or more had a CTC of 4 or lower at week 4, 8, and 12, respectively. CTC responders in general had better OS than CTC non-responders. Among patients with measurable disease and CTC>0 or CTC of 5 or more at baseline objective response (RECIST) at 12 weeks was associated with a survival benefit.
In summary, mCRPC patients with baseline CTC > 0, a CTC response on treatment (CTC = 0) was associated with longer survival and could be considered a response criterion. Additional analysis is required to fully characterize the relationship between CTC = 0 and objective response by RECIST in patients with measurable disease.
Presented By: Howard I. Scher, MD, Memorial Sloan Kettering Cancer Center, New York, NY
Written By: Hanan Goldberg, MD, Urologic Oncology Fellow (SUO), University of Toronto, Princess Margaret Cancer Centre, Toronto, Ontario, Canada
at the 2017 ASCO Annual Meeting - June 2 - 6, 2017 - Chicago, Illinois, USA
1. Berruti A, Pia A, Terzolo M. Abiraterone and increased survival in metastatic prostate cancer. The New England journal of medicine 2011; 365(8): 766; author reply 7-8.