PROTEUS Final Analysis at ASCO 2026: Perioperative Apalutamide Delivers And The Localized High-Risk Paradigm Shifts

Much-anticipated results from the PROTEUS trial were shared at the plenary session at this year’s American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago. The oncology community had been waiting years for these results, which indicate that perioperative apalutamide plus ADT can keep high-risk prostate cancer from returning for longer.

The results show perioperative apalutamide plus ADT produced a statistically significant improvement in both dual primary endpoints — pathological complete response or minimal residual disease, and metastasis-free survival — compared to placebo plus ADT in patients with high-risk localized or locally advanced prostate cancer undergoing radical prostatectomy.

The New England Journal of Medicine, which published the results simultaneously, ran an accompanying editorial calling it “a watershed moment” for the perioperative treatment of prostate cancer. For a disease setting where roughly half of surgical patients recur, and where intensified perioperative strategies have been studied but never definitively validated in a phase 3 randomized trial of this scale, these results land with real weight.

At ASCO, SurvivorNet sat down with Dr. Russell Pachynski, a medical oncologist at Washington University in St. Louis, who called PROTEUS “a landmark study,” but raised a few questions clinicians need to consider before moving forward.

Trial Design

PROTEUS (NCT03767244) is a global, double-blind, randomized phase 3 study enrolling patients with high-risk localized or locally advanced prostate cancer (HR LPC/LAPC) who were candidates for radical prostatectomy with pelvic lymph node dissection. Stratification variables included Gleason score (7 vs. ≥8) and nodal status (N0 vs. N1).

Patients were randomized 1:1 to apalutamide 240 mg/day plus ADT or placebo plus ADT for six months before and six months after RP, a true perioperative window, not purely neoadjuvant or adjuvant. Treatment was interrupted two weeks before planned surgery and resumed four weeks post-RP, contingent on resolution of clinically significant AEs.

Enrollment spanned more than 200 sites across 18 countries, with 2,109 patients enrolled, making PROTEUS one of the largest perioperative trials ever conducted in localized prostate cancer.

The dual primary endpoints were:

• Pathological complete response (pCR): defined as no residual cancer in the prostatectomy specimen
• Metastasis-free survival (MFS), both assessed by blinded independent central review.

MFS was evaluated both by conventional imaging and by PSMA PET, the latter reflecting the evolution of staging technology during the trial’s long enrollment window.

Results

Pathological Complete Response or Minimal Residual Disease

The percentage of patients achieving pCR or minimal residual disease was 8.9% in the apalutamide group versus 1.0% in the placebo group: an odds ratio of 10.17 (95% CI, 5.27 to 19.64; p<0.001).

• Complete tumor eradication (stage ypT0) was achieved in 5.1% of apalutamide patients versus 0.4% in the placebo group.
• At surgery, positive surgical margins were present in 20.9% of the apalutamide group versus 42.7% of the placebo group; a clinically meaningful difference in operative findings.

Metastasis-Free Survival

• The probability of metastasis-free survival at 5 years was 78.2% (95% CI, 75.3–80.8) in the apalutamide group versus 73.5% (95% CI, 70.4–76.3) in the placebo group, hazard ratio for distant metastasis or death 0.80 (95% CI, 0.67–0.96; p=0.02).

It is worth noting that the between-group difference in MFS as assessed by conventional imaging alone was not statistically significant (HR 0.84; 95% CI, 0.67–1.07); the significant MFS result was driven substantially by PSMA PET detection of distant metastases. This is not a flaw in the trial, it reflects current clinical practice, but it is a nuance worth understanding when interpreting the data.

Key Secondary Endpoints

The secondary endpoint cascade was uniformly positive:

• Event-free survival: 57.1 months (apalutamide) vs. 38.4 months (placebo); HR 0.71 (95% CI, 0.63–0.80; p<0.001)
• Time to first subsequent local or systemic therapy: 74.2 months vs. 41.5 months; HR 0.65 (95% CI, 0.57–0.73; p<0.001)
• Time to distant metastasis on conventional imaging or PSMA PET: HR 0.68 (95% CI, 0.55–0.83; p<0.001)
• Freedom from disease at 4 years: 21.9% vs. 18.3% (OR 1.25; p=0.04)
• Low residual cancer burden at surgery: 30.6% vs. 11.7% (OR 3.36)
• Time to castration-resistant prostate cancer: HR 0.59 (95% CI, 0.46–0.76)

Overall Survival

Overall survival was evaluated as an exploratory safety measure. At a median follow-up of five years with an overall mortality of 8.5%, the hazard ratio for death was 1.08 — below the prespecified threshold for an unacceptable level of detriment, but not a demonstration of OS benefit. The NEJM editorial was direct on this point: if a survival benefit is eventually confirmed with additional follow-up, “that would be a game changer in the field.” It has not been confirmed yet, and the trial was not powered to detect it at this stage.

Safety

Grade 3 or 4 adverse events occurred in 39.6% of patients in the apalutamide group versus 31.0% in the placebo group, with the difference driven primarily by rash — 21.2% versus 10.0%.

The most common adverse events overall were:

• Hot flush (63.4% vs. 56.5%)
• Urinary incontinence (50.2% vs. 50.9%)
• Erectile dysfunction (41.6% vs. 41.4%)
• Fatigue (27.7% vs. 26.8%)
• Arthralgia (22.6% vs. 19.4%)

Urinary incontinence and erectile dysfunction were predominantly grade 1 or 2 and were similar between groups, reflecting the shared surgical and ADT burden. Discontinuation due to adverse events occurred in 7.4% of the apalutamide group versus 2.7% of the placebo group.

One finding that deserves attention in the context of patient selection: among patients 75 years of age or older, mortality was 15.2% in the apalutamide group versus 12.8% in the placebo group. The authors acknowledge this finding underscores the importance of careful patient selection, particularly for older patients being considered for both systemic therapy and surgery.

There is also a practical burden, Dr. Pachynski adds.

“This is a treatment of a year, whereas the current standard of care is no treatment at all around surgery,” he explains. “You’re just recovering from the surgery, and your quality of life once you’ve recovered is going to be great. This is now a year of treatment where your quality of life is reduced.” He also raised financial toxicity as a real and underappreciated concern: “Depending on your coverage, you may be paying three to four hundred dollars a month for your treatment.”

Mental health concerns should also be addressed, Dr. Pachynski says.

“I’ve had patients go on ADT and apalutamide and get suicidally depressed,” he says. “So imagine you would have had a surgery… yes, you’re high risk, but if cancer came back, we probably would have caught it and we have treatments. These are conversations that have to be thoughtfully had with patients.”

Questions Still Remain

The accompanying editorial with the study, authored by Dr. Emmanuel Antonarakis of the University of Minnesota, named PROTEUS “a watershed moment” but also raised several unresolved questions that the field will need to grapple with.

The 9% pCR rate is the most provocative. Despite six months of maximal androgen pathway modulation, only 9% of patients in the apalutamide group had a major pathological response. For context, pCR rates with neoadjuvant systemic therapy are approximately 50% in localized urothelial bladder cancer and approximately 25% in resectable non-small cell lung cancer. The prostate cancer number is meaningful, it is nine times higher than placebo, but it also signals that most tumors are not being eradicated before surgery, which raises the question of which patients are deriving the most benefit and which may not be responding at all.

The editorial specifically flagged genomic and transcriptomic predictors of response as a critical unanswered question — noting that SPOP-mutant prostate cancers have shown sensitivity to neoadjuvant apalutamide, while TP53 and PTEN-mutant tumors have shown relative insensitivity. A recent salvage radiotherapy trial (NRG GU006/BALANCE) also suggested that benefit from apalutamide may be concentrated in tumors with a luminal B transcriptional signature, not luminal A or basal. These are not settled data, but they signal that biomarker-driven patient selection is the next frontier for this approach.

The control arm is also worth scrutiny. PROTEUS compared perioperative ADT plus apalutamide against perioperative ADT plus placebo, not against surgery alone, which is the actual current standard of care. A complementary PROTEUS substudy comparing ADT plus apalutamide directly to prostatectomy alone is ongoing, but those data are not yet available.

Dr. Pachynski echoes this tension independently, noting that the broader oncology trend of moving effective drugs earlier in treatment introduces a sequencing problem that hasn’t been fully solved.

“Now you’re going to have patients who have seen these drugs very early, and there are going to be some of these cancers that have become resistant. And now what do you do later? Because that’s where these drugs have been initially developed,” he says. He identified this as an open question across multiple drug classes, not a reason to avoid perioperative intensification, but a reason to think carefully about the downstream implications for individual patients.

Clinical Implications

The immediate implication is regulatory. These data are expected to support an FDA submission for apalutamide in the perioperative localized high-risk prostate cancer setting. Apalutamide currently carries indications for nmCRPC (SPARTAN) and mCSPC (TITAN). PROTEUS adds a third, and arguably earlier, setting to that arc. The NEJM editorial stated plainly that perioperative ADT plus apalutamide “will most likely be rapidly adopted as a new standard of care for patients who select surgical management for high-risk localized prostate cancer.”

Patient selection matters more than ever. PROTEUS enrolled a high-risk population: 95.8% Gleason ≥8, and that selection was deliberate.

“Those would be the select patients where you sit down and say, even though there will be some toxicity and you might have to pay out of pocket, and it’s a year — we think it’s worth it because your prostate cancer looks more aggressive,” Dr. Pachynski says. For lower-risk patients, this conversation does not apply. For patients at the margins of high-risk criteria, it requires individualized judgment, ideally incorporating modern staging tools.

PROTEUS enrolled patients largely staged by conventional imaging, but the protocol was amended during enrollment to include PSMA PET at specific timepoints. In 2026, PSMA PET is standard pre-operative staging at most major centers. The men entering surgery now may be a more accurately staged, and potentially lower-metastatic-burden, population than those enrolled in PROTEUS — which could mean even greater benefit from perioperative intensification in prospective use, or alternatively, a more selected population in whom benefit concentrates.

Finally, the neoadjuvant versus adjuvant question remains open. PROTEUS is a perioperative trial: both phases were required per protocol. Whether the neoadjuvant component is essential for the MFS benefit, or whether the adjuvant phase alone is sufficient, cannot be answered by this dataset. For patients with significant comorbidities or surgical urgency, this is a clinically relevant question that awaits further study.

These are the conversations that will follow from ASCO 2026 across tumor boards and GU oncology practices. For now, the data have arrived. The field has its answer.