The NCI working group concluded that neither metastasis-directed therapy (MDT) nor systemic hormonal therapy should be required as a control arm in trials for biochemically recurrent (BCR) prostate cancer. This facilitates testing novel non-hormonal agents and reflects that surveillance is often a reasonable standard of care.

The decision to exclude the standard-of-care chemotherapy docetaxel was driven by its variable real-world use, fears it would hinder patient recruitment, and the challenge of timing PARP inhibitor therapy post-chemo. This prioritized a "clean" trial design over including every possible therapy.

Early neoadjuvant trials in the 1990s failed to show clinical benefit because they included many low-risk patients and used less potent hormonal therapies. The PROTEUS trial's success was built on learning from this history by strictly enrolling high-risk patients and using a powerful androgen receptor pathway inhibitor (ARPI).

The practice-changing KEYNOTE-689 trial was open-label, meaning patients knew their treatment. This could introduce bias; patients on the standard care arm may have dropped out ("bailed"), while those on the pembrolizumab arm might have progressed, artificially making the rates of patients reaching surgery appear similar.

The lack of a placebo arm in some adjuvant trials is not necessarily a fatal flaw. One expert view is that it mirrors real-world practice where treatments are known. This perspective places trust in the investigators' ability to assess disease progression accurately without blinding.

The control arm in the EMBARK study was blinded to PSA results, preventing physicians from intervening with standard-of-care AR antagonists at PSA progression. This design likely delayed subsequent effective therapies, making the control arm underperform and potentially exaggerating the overall survival benefit of the experimental arms.

Developers often test novel agents in late-line settings because the control arm is weaker, increasing the statistical chance of success. However, this strategy may doom effective immunotherapies by testing them in biologically hostile, resistant tumors, masking their true potential.

The highly anticipated PROTEUS trial is testing a new drug combination against a control arm of ADT plus placebo for prostatectomy patients. This design is controversial because ADT is not standard care in this setting, raising concerns that a positive result could be driven by a suboptimal control arm.

Even when testing drugs in heavily pre-treated patients, clinical trials incorporate subtle biological selection criteria. For instance, the COMPASS trial excludes patients with visceral metastases, a tactic to enrich for a population more likely to respond and avoid the most aggressive disease subtypes.