Radiographic paradoxical response in metastatic castrate-resistant prostate cancer (mCRPC) managed with new generation anti-androgens: a retrospective analysis.

Background: Prostatic specific antigen (PSA) has well-recognized limitations as a marker for treatment response and disease progression. Post hoc analysis of the PREVAIL trial reported 24.5% of chemotherapy naïve metastatic castration-resistant prostate cancer (mCRPC) patients on enzalutamide had radiographic progression on conventional imaging with nonrising PSA. In this study, we sought to study the discordance of imaging with PSA kinetics in mCRPC patients on second generation anti-androgens (SGA) post-chemotherapy using combined conventional imaging, and new generation imaging in the form of C-11 choline positron emission tomography/computed tomography (C[11] choline PET/CT) scan.

Methods: We retrospectively reviewed the medical records of 123 patients with mCRPC treated with SGA (Abiraterone or Enzalutamide) after docetaxel between 2016 and 2019. Patients underwent PSA testing, and C[11] choline PET/CT scan at baseline level before starting treatment with SGA, then every 3-6 months as part of their follow up evaluation. Loss of response to SGA was defined by increase in corrected maximum standardized uptake value (SUVmax) of pretreatment lesions on C-11 Choline PET/CT, and/or development of new lesions. Suspicious new lesions were confirmed by biopsy and/or conventional imaging.

Results: We identified 123 mCRPC patients who received SGA (Abiraterone, n = 106; Enzalutamide, n = 17) after docetaxel. Median duration of therapy was 13.9 months (interquartile range: 8.75-21.14). Approximately 43% (n = 53) of subjects in this study exhibited an increase in choline avidity while on SGA. Of this group, 60.4% of patients experienced a parallel rise in PSA (Group-A), whereas 39.6% displayed a paradoxical response (PR) (Group-B), defined as increased choline avidity combined with stable or down-trending PSA. Median PSA at time of increase in choline avidity was 3.1 ng/ml for Group-A, and 1.3 ng/ml for Group-B (p = 0.0176). Median SUVmax was similar in both groups (4.9 for Group-A, 4.6 for Group-B; p = 0.6072). The median time for increase in choline avidity was 9.5 versus 3.9 months for Group-A versus Group-B, respectively (Log-Rank = 0.0063).

Conclusion: Nearly 40% of mCRPC patients placed on SGA post docetaxel chemotherapy will exhibit paradoxical responses to therapy, therefore, warranting close follow up with imaging. C-11 choline PET/CT imaging is a useful tool that can help in early predication of disease progression or treatment failure.