Phase1b Trial Investigates Atezolizumab and Radium-223 in mCRPC

Michael J. Morris, MD, clinical director of Genitourinary Medical Oncology Service and Prostate Cancer Section head of the Division of Solid Tumor Oncology at Memorial Sloan Kettering Cancer Center, discusses the safety and clinical activity of atezolizumab (Tecentriq) plus radium-223 dichloride (Xofigo) in patients with second-line metastatic castration-resistant prostate cancer (mCRPC).

Morris says the most important part of the phase 1b trial results were the immunotherapy-related adverse effects (AEs) observed, some of which were significant. There were 2 grade 5 events related to atezolizumab. The AEs observed were toxicities normally seen with this type of treatment, such as autoimmune hepatitis and autoimmune myositis. There were some patients with count reductions and neutropenia, which may have been caused by radium-223.

In terms of a countervailing clinical benefit, they did not see anything unexpected with either radium-223 or atezolizumab independently, according to Morris. Radium-223 does not independently significantly decline prostate-specific antigen levels, and there was no improvement observed in that way with the addition of atezolizumab. There were 3 patients who had significant and durable radiographic responses: 1 patient’s response lasted 192 days, another’s for 275 days, and the last for 337 days.

However, these responses were out of 44 patients total. Morris explains that they didn't believe they could identify who those patients were that responded, which has frequently been a problem in terms of immunotherapy and prostate cancer. They felt the toxicity to clinical benefit ratio did not warrant pushing the combination forward.

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