Unlike in breast or bladder cancer, the significant improvement in pathologic response in the PROTEUS trial did not translate to a similarly impressive benefit in metastasis-free survival. This disconnect suggests that eliminating tumor in the prostate specimen is not a reliable surrogate for long-term outcomes in this disease setting.

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.

While PSMA PET scans are more sensitive, they create a clinical dilemma because pivotal trials defining treatment efficacy were based on conventional imaging (CT/bone scans). This forces oncologists to either re-image patients with older technology to match trial criteria or make treatment decisions based on PET data that lacks a clear evidence-based framework for response assessment.

The patient population in pivotal trials like EMBARK, defined as non-metastatic by conventional imaging, is being re-evaluated. A UCLA study showed that over 80% of a similar patient group would have been positive on a PSMA PET scan, suggesting the "M0" classification is largely an artifact of older imaging technology and that these patients likely have micrometastatic disease.

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.

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.

The introduction of highly sensitive PSMA PET scans means established endpoints like Metastasis-Free Survival (MFS) may no longer be valid. A metastasis detected by PET likely has a different, better prognosis than one found with older imaging, requiring new validation for this key endpoint.

Though EMBARK trial patients were negative on conventional imaging, an analysis suggests over 80% had PSMA PET-detectable disease. This reframes the landmark study, suggesting its findings may apply more to treating low-volume metastatic disease intermittently rather than purely biochemical recurrence.

Event-free survival rates at four years were concerningly low in both arms of the PROTEUS trial. This suggests the trial may have enrolled a population with a very high baseline risk of recurrence, potentially including patients with pre-existing micrometastatic disease, which could limit the study's applicability to a broader high-risk population.