When PSMA PET became the standard of care for detecting prostate cancer metastasis, the PROTEUS trial amended its protocol to include it alongside conventional imaging. The trial's positive Metastasis-Free Survival (MFS) result was driven by this composite endpoint, as the analysis using conventional imaging alone was not statistically significant.

PROTEUS changed its primary endpoint mid-study to include PSMA PET-detected metastasis. While this led to a statistically positive result, it raises concerns that the benefit is merely earlier detection (lead-time bias) rather than a true improvement in curing micrometastatic disease and improving overall survival.

In metastatic hormone-sensitive prostate cancer, many patients receive multiple subsequent therapies, making Overall Survival (OS) a difficult endpoint to achieve. Therefore, a large, meaningful improvement in radiographic progression-free survival (RPFS) is considered a critical and actionable outcome for patients.

The PROTEUS trial used ADT plus surgery as its control, not surgery alone. This design is controversial because many patients choose surgery specifically to avoid systemic therapies like ADT. This complicates the interpretation of results and reflects a disconnect from real-world patient motivations.

The standard practice of using rising PSA to trigger early salvage radiotherapy after surgery is complicated by the PROTEUS protocol's neoadjuvant hormone therapy. This approach muddies the interpretation of biochemical recurrence, making it difficult for clinicians to know when or if to intervene with salvage treatment, potentially leading to worse outcomes.

The PROTEUS trial used two pathologic endpoints. The investigator suggests Residual Cancer Burden (RCB), which measures cellularity, is a more meaningful reflection of response than just residual tumor size. The RCB endpoint showed a much larger treatment effect (30% vs. 11%) compared to the tumor size endpoint (9% vs. 1%).

When looking only at the original endpoint of metastasis-free survival by conventional imaging, the PROTEUS trial was not positive. This result is consistent with the negative ENZERAD trial, which tested a similar drug combination with radiation instead of surgery. This cross-trial consistency suggests adding an ARPI may not provide a substantial benefit in this setting.

In metastatic hormone-sensitive prostate cancer (mHSPC), radiographic progression-free survival (rPFS) is no longer seen as a convincing primary endpoint on its own. Clinicians demand a clear signal for overall survival (OS) improvement, citing historical data where early treatment intensification showed significant OS gains.

Despite meeting its primary endpoint, the PROTEUS trial provides no validated biomarkers to identify which patients actually benefit from the intensified therapy. This lack of a predictive signature means applying the results in the clinic amounts to uniform escalation, likely overtreating many patients for an uncertain benefit, making it difficult to implement.