Keytruda leads in prostate cancer, but better response analysis is needed

Yesterday’s publication of results from the Keynote-199 study in the Journal of Clinical Oncology will remind investors that prostate cancer is emerging as another tumour type amenable to checkpoint blockade. Keynote-199, a study of Merck & Co's Keytruda, segmented subjects by PD-L1 expression and bone metastasis, but remission rates were poor across the board. The few subjects who responded did so with unexpectedly long duration, a not atypical story in immunotherapy, and the best responses apparently came in those with DNA repair defects. It is interesting that of the immuno-oncology players only Merck has made a significant push into this cancer type, boasting in February of having started three pivotal Keytruda studies in various combinations, including with Parp inhibition (Esmo 2019 – A place for Parps in prostate cancer, September 30, 2019). However, only academic trials appear to focus specifically on prostate cancer driven by DNA repair defects. Bristol-Myers Squibb’s Opdivo trial Checkmate-7DX, somewhat analogous to Merck’s Keynote-921, and Roche’s Tecentriq study Imbassador-250 are the only other phase III tests under way. Other PD-(L)1-blocking agents are being studied in prostate cancer too, but these are largely investigator-sponsored efforts.