Introduction: A Year of Phase 3 Results Across the Disease Spectrum

ASCO 2026 brought multiple practice-influencing trials in prostate cancer spanning localized high-risk disease, metastatic hormone-sensitive disease, and de-escalation for exceptional responders. The PROTEUS trial establishes perioperative apalutamide plus Androgen Deprivation Therapy (ADT) as a new standard for high-risk localized prostate cancer; important caveats about patient selection and cardiovascular risk apply. TALAPRO-3 demonstrates that PARP inhibitor combinations benefit the  Homologous Recombination Repair (HRR)-altered population in metastatic castration-sensitive disease (Androgen Pathway Modulation Sensitive/Naïve), not just BRCA1/2 mutation carriers. And A-DREAM opens a conversation about treatment break for exceptional responders that the field has not had before.

PROTEUS: Perioperative Apalutamide in High-Risk Localized Prostate Cancer

Trial Design

The PROTEUS phase 3 trial (NCT03767244) enrolled 2,109 patients with high-risk localized prostate cancer. Patients were randomized one to one to apalutamide 240 mg daily plus ADT versus placebo plus ADT, given for twelve months in a perioperative fashion: six months before radical prostatectomy and six months after. Stratification was by cN0/N1 status, Gleason score, and geographic region. The composite primary endpoint was pathological complete response (pCR) or minimal residual disease (MRD) at prostatectomy combined with metastasis-free survival assessed by both conventional imaging and PSMA PET/CT. PSMA PET was added by protocol amendment in April 2022.

Results: Pathological Primary Endpoint

The pCR/MRD endpoint was met decisively. MRD was defined as five millimeters or less in the greatest dimension of the largest tumor lesion in prostate-confined disease, or no tumor identified. Nine-fold improvement: 8.9% pCR/MRD with apalutamide versus 1.0% with placebo (OR 10.17, p<0.0001). The exploratory low residual cancer burden endpoint (defined as 0.25 cm³ or less of residual tumor) was 30.6% versus 11.7%, also significant.

These pathological responses matter clinically: ten percent of patients treated with apalutamide had minimal to no cancer at prostatectomy. The question this raises is whether those patients needed the full six months of adjuvant therapy after surgery. All patients received it per protocol, but the data now invite a future de-escalation question for pathological complete responders.

Importantly, even the best systemic response still left the majority of patients with residual tumor, confirming that local treatment (surgery or radiation) remains essential. Systemic therapy alone is insufficient.

Results: Metastasis-Free Survival

The composite MFS endpoint was also met: HR 0.80 (95% CI 0.67-0.96, p=0.02), a twenty percent reduction in metastasis or death. However, the interpretation here requires nuance. The conventional imaging subset alone did not reach statistical significance (HR 0.84, p=0.15). The overall significance was driven by the PSMA PET component of the composite endpoint. Since PSMA PET is far more sensitive than conventional imaging, it identifies metastases earlier, creating a potential stage migration effect. Whether PSMA PET-based MFS is a validated surrogate for OS is not yet established.

Additional meaningful findings: time to first subsequent therapy was 74.2 versus 41.5 months, nearly three additional treatment-free years with apalutamide. Systemic therapy use in follow-up was reduced (26.7% versus 36.4%) and radiation use was also reduced (13.0% versus 18.4%). Event-free survival was significantly improved: HR 0.71 (95% CI 0.63-0.80), median 57.1 versus 38.4 months.

Safety: The Seven Deaths

Treatment-related adverse events leading to death occurred in seven patients in the apalutamide arm versus one in placebo. The causes were cardiovascular-related: cardiorespiratory arrest (two), cardiac arrest (two), cardiogenic shock (one), and pulmonary embolism (two), leading to a protocol amendment requiring cardiovascular risk assessment at screening and before surgery, with thromboprophylaxis administered per local guidelines for high-risk patients.

This finding cannot be dismissed. The cardiovascular risk associated with apalutamide, particularly in the context of ADT, is meaningful and must be incorporated into patient selection discussions. Patients with significant cardiovascular comorbidities, metabolic syndrome, or poor cardiac reserve require careful individualized assessment before initiating perioperative apalutamide.

Rash was the most common treatment-emergent adverse event of special interest: 33.0% versus 15.3% any grade, 8.6% versus 2.2% grade 3, manageable with dose modification.

Contextualizing PROTEUS

How does PROTEUS fit with prior perioperative intensification trials? ENZARAD (NCT02446444) (ADT + radiation therapy +/- enzalutamide) was negative (HR 0.88, p=0.34); PROTEUS was positive despite a similar HR structure. The comparison is complicated by different patient populations and treatment modalities. STAMPEDE (NCT00268476), which added abiraterone to radiation and ADT, enrolled a higher-risk population (92% T3-T4 disease, 39% lymph node positive). PROTEUS enrolled a population more similar to ENZARAD: PSA of 14 ng/mL, twelve percent cN1, thirty-five percent T3-T4 disease.

One hypothesis: the difference in outcomes between ENZARAD and PROTEUS may not be drug-related (apalutamide versus enzalutamide are functionally equivalent second-generation AR inhibitors). The difference may lie in the composite endpoint design and the inclusion of PSMA PET-based MFS, which is more sensitive and may have provided statistical traction that conventional imaging alone could not.

Overall survival data are immature. The OS HR was 1.08 (below the prespecified threshold for harm, not a benefit signal) and the trial was not powered for OS at this follow-up. A substudy comparing perioperative apalutamide+ADTplus RP to RP alone is ongoing.

Clinical bottom line: PROTEUS sets a new standard for perioperative ADT plus apalutamide in high-risk localized prostate cancer. But not all high-risk patients need this treatment. Individual cardiovascular risk, comorbidities, and tumor characteristics should guide the decision. The seven deaths in the trial are a meaningful signal that demands careful patient selection and pre-treatment cardiovascular assessment.

TALAPRO-3: PARP Inhibition Benefits the Full HRR-Altered Population

TALAPRO-3 (NCT04821622) enrolled men with metastatic castration-sensitive prostate cancer (mCSPC) who had HRR alterations, not limited to BRCA1/2 but spanning the broader HRR gene panel. Patients were randomized to ADT plus enzalutamide plus talazoparib versus ADT plus enzalutamide plus placebo. The primary endpoint was radiographic PFS. Thirty-five percent of patients had BRCA1/2 mutations; sixty-five percent had other HRR alterations.

Primary endpoint: Fifty-two percent reduction in risk of progression or death with talazoparib (median rPFS not reached versus 45.8 months; HR 0.481, 95% CI 0.357–0.647; p<0.0001).

The subgroup that changed the conversation: In prior PARP inhibitor trials in prostate cancer, the overall benefit was often driven entirely by BRCA1/2 carriers, leaving the broader HRR-altered population with uncertain benefit. TALAPRO-3 disrupts this pattern.

In the non-BRCA1/2 HRR subgroup: still significant, with a forty-three percent risk reduction. In ATM-altered patients (a population where PARP inhibitor benefit has historically been inconsistent): fifty-seven percent risk reduction. In CDK12-mutant patients (a population where PARP inhibitors have generally not shown benefit): seventy-two percent risk reduction. These are the largest ATM and CDK12 cohorts studied in a PARP inhibitor trial, and they showed clinically meaningful benefit.

Why TALAPRO-3 shows benefit in ATM- and CDK12-altered patient population where other trials have not, remains an open question. Differences in patient selection, the backbone of enzalutamide, or the specific talazoparib mechanism may contribute.

Safety: Fatigue and anemia were the most common treatment-related events. Grade 3-4 anemia was notable at fifty-one percent, with forty percent of patients receiving RBC transfusions (the trial mandated transfusion for grade 3 anemia). Overall grade 3-4 adverse events 79% versus 41%; permanent discontinuation 19% versus 10%. Three MDS cases and two AML cases occurred in the talazoparib arm versus one MDS case in placebo; in patients living for many years on effective therapy, this secondary malignancy risk requires ongoing consideration.

Regulatory implications: AMPLITUDE (NCT04497844) established niraparib plus abiraterone and ADT for BRCA2-mutant mCSPC, and received an FDA label specifically for BRCA2. TALAPRO-3's broader HRR benefit data will likely seek an HRR-wide label. This reinforces the imperative: every patient presenting with metastatic prostate cancer needs comprehensive germline and somatic HRR testing, not just BRCA2 but the full panel, because there are now frontline treatment implications for the broader HRR group.

A-DREAM: Treatment De-escalation in Exceptional Responders

The A-DREAM (NCT05241860) Alliance phase 2 trial addressed a question many patients raise but few studies have examined: for patients who respond exceptionally well to ADT plus an AR pathway inhibitor in the metastatic hormone-sensitive setting, can therapy be safely stopped?

The trial enrolled seventy-nine patients with mCSPC who had been on ADT for eighteen to twenty-four months and an ARPI for at least twelve months. All stopped both ADT and ARPI and were followed until PSA rose to five ng/mL or met Prostate Cancer Working Group 3 criteria or had symptomatic progression. The primary endpoint was eighteen-month treatment-free survival with testosterone recovery to at least 150 ng/dL.

Results: Forty-one percent met the primary endpoint: treatment-free at eighteen months with testosterone recovery to 150 ng/dL. Overall testosterone recovery occurred in sixty-seven percent of patients. Fifty-eight percent remained disease-free at eighteen months. The treatment-free interval was longer than anticipated at 24.5 months, with 38.5% still treatment-free at twenty-six months.

Patients less likely to maintain treatment-free interval were those with high-volume disease per CHAARTED criteria (notably, thirty five percent of enrolled patients had high-volume disease). Patients who had received prior radiation to metastatic sites were less likely to need to restart. Among twenty-nine patients who did restart, four progressed, suggesting that reinitiation regains disease control in the vast majority.

There were four deaths in the trial, one from prostate cancer.

Clinical context: PSA response at six to twelve months of mCSPC treatment is prognostic. Deep responders (PSA 0.2 ng/mL or below) have median OS of five to seven years; poor responders have approximately three years. A-DREAM is not ready for standard practice adoption but provides real numbers to share with patients who express the desire to stop treatment, an informed conversation that previously had no data to anchor it.

Ongoing trials exploring adjacent questions include TRIPLE-SWITCH (NCT06592924), which is evaluating whether adding docetaxel improves OS for patients with PSA above 0.2 ng/mL after ADT plus ARPI.

TRIPLE-SWITCH: An Enrolling Trial Worth Knowing About

TRIPLE-SWITCH PR.26 is a joint SWOG/CTG randomized trial enrolling patients with mCSPC on ADT plus an ARPI whose PSA remains above 0.2 ng/mL at six to twelve months, the high-risk incomplete-responder population. Patients are randomized to continue current treatment or add docetaxel, with OS as the primary endpoint. The trial is open at multiple US and Canadian sites.

TRIPLE-SWITCH (CCTG PR.26/SWOG; NCT06592924) is a Phase III international randomized trial designed to address a critical unmet need: what is the optimal strategy for patients with mCSPC who fail to achieve a deep PSA response on ADT plus an ARPI? These incomplete responders, defined as PSA above 0.2 ng/mL at six to twelve months of ADT and at least four months of ARPi treatment, represent a high-risk population with significantly worse long-term outcomes compared to deep responders. Despite this, no prospective randomized data have guided intensification strategies for this group.

TRIPLE-SWITCH trial randomizes patients to either continue ADT plus the ARPI they are currently receiving (standard arm) or add docetaxel to ADT and ARPi (experimental arm), with OS as the primary endpoint. The design leverages PSA kinetics as a real-time biomarker to risk-stratify patients and assign treatment accordingly, an approach that is straightforward to implement in clinical practice and avoids the expense and toxicity of triplet therapy for all mCSPC patients upfront.

The scientific rationale is compelling. Preclinical and clinical data suggest that early AR pathway resistance, reflected by an incomplete PSA nadir, may indicate a tumor phenotype more responsive to taxane-based chemotherapy than continued AR signaling suppression. Whether adding docetaxel to hormonal backbone improves survival in this setting is the central hypothesis TRIPLE-SWITCH is designed to test.

The trial is open at multiple US and Canadian sites, including several SWOG and CCTG member institutions. Enrollment is ongoing. For patients with mCSPC who have been on ADT for six to twelve months and ARPi for at least 4 months and have not achieved a PSA below 0.2 ng/mL, TRIPLE-SWITCH enrollment should be considered and discussed.

For Patients

If you have been diagnosed with high-risk localized prostate cancer and are being considered for surgery, a clinical trial or treatment approach combining apalutamide with hormone therapy before and after surgery has shown it can significantly reduce the amount of cancer remaining at surgery and delay disease recurrence. Ask your oncologist whether this perioperative approach is appropriate for your specific situation and risk profile, including your cardiovascular health.

If you have metastatic prostate cancer and have not yet been tested for HRR gene mutations (including BRCA1, BRCA2, ATM,  and related genes), ask for both germline (blood or saliva) and somatic (tumor or blood) testing; this information affects which treatments are most beneficial from the start of therapy. If you are an exceptional responder to hormone therapy and have been on treatment for a year and a half or more with very low PSA, ask your oncologist about emerging data on treatment-free intervals and whether a clinical trial exploring this approach might be right for you.

Key Takeaways

  • PROTEUS (perioperative apalutamide + ADT in high-risk localized PCa): pCR/MRD 8.9% vs 1.0% (OR 10.17); MFS HR 0.80; time to subsequent therapy 74.2 vs 41.5 months; EFS HR 0.71; 7 treatment-related deaths vs 1 (cardiovascular); OS immature; not all high-risk patients are candidates.

  • PSMA PET-based MFS drove composite endpoint significance; conventional imaging subset alone was not significant; MFS by PSMA PET is not yet a validated OS surrogate.

  • TALAPRO-3 (talazoparib + enzalutamide+ADT in HRR-altered mCSPC): 52% reduction in risk of progression; benefit in non-BRCA1/2 HRR (43% risk reduction): ATM (57%), and CDK12 (72%), the largest signal in these populations to date; grade 3-4 anemia 51%; MDS/AML risk present.

  • All mCSPC patients need comprehensive germline and somatic HRR panel testing (not just BRCA1/2) given frontline PARP inhibitor implications.

  • A-DREAM: Treatment-free at 18 months with testosterone recovery to 150 ng/dL in 41% of mCSPC exceptional responders after stopping ADT plus ARPI; median treatment-free interval is 24.5 months; not yet standard practice; provides data for informed patient conversations.

  • TRIPLE-SWITCH (PR.26): Phase III trial randomizing PSA-incomplete-responders on ADT+ARPI (PSA >0.2 ng/mL at 6-12 months) to continue ARPI+ADT vs add docetaxel to ARPI+ADT; OS primary endpoint; enrolling at multiple US/Canadian sites.

References

  1. Taplin ME, et al. PROTEUS: perioperative apalutamide plus ADT in high-risk localized prostate cancer. New England Journal of Medicine, 2026.

  2. Antonarakis ES. NEJM Editorial on PROTEUS. New England Journal of Medicine, 2026.

  3. Chi KN, et al. TALAPRO-3: talazoparib plus enzalutamide in HRR-altered mCSPC. ASCO 2026.

  4. Attard G, et al. AMPLITUDE: niraparib plus abiraterone in BRCA2-mutant mCSPC. Nature Medicine, 2025.

  5. A-DREAM: ADT plus ARPI interruption in mCSPC exceptional responders. ASCO 2026.

  6. Sokolova A. "Best of ASCO 2026: Prostate Cancer." Presented at the 2026 Best of Oncology, Seattle, WA, June 2026.