Ono Pharmaceutical Co., Ltd. presented Phase 2 data showing that ONO-4578, an oral EP4 antagonist, improved progression-free survival when added to Opdivo and chemotherapy in previously untreated HER2-negative unresectable advanced or recurrent gastric cancer and gastroesophageal junction cancer. The data position ONO-4578 as a tumour microenvironment-targeting immunotherapy enhancer as Ono Pharmaceutical and Deciphera Pharmaceuticals prepare for a possible global Phase 3 programme.

Why does ONO-4578 matter in first-line HER2-negative gastric and gastroesophageal junction cancer?

The importance of ONO-4578 begins with the treatment setting. First-line HER2-negative advanced gastric cancer and gastroesophageal junction cancer remain commercially significant and clinically difficult, even after the arrival of immune checkpoint inhibitors in combination with chemotherapy. Opdivo and chemotherapy have already changed expectations in parts of this market, but many patients still progress early, and benefit can vary widely by biomarker status.

ONO-4578 is designed to address that problem from a different angle. Rather than directly targeting the tumour cell like a traditional targeted therapy, the oral EP4 antagonist is intended to interfere with prostaglandin E2 mediated immune suppression. The development thesis is that blocking EP4 signaling may help restore antitumour immunity and make PD-1 blockade work better when combined with chemotherapy.

That is why the Phase 2 result is more interesting than a standard add-on study. Ono Pharmaceutical is testing whether modulation of the tumour microenvironment can improve the backbone of modern gastric cancer treatment. If the effect is real and reproducible, ONO-4578 could become part of a broader move toward immunotherapy enhancement rather than simply another checkpoint inhibitor combination. The unresolved question is whether the signal is strong enough, durable enough and safe enough to justify adding a fourth component to an already complex first-line regimen.

What does the Phase 2 progression-free survival signal reveal about clinical relevance?

The central data point is progression-free survival. ONO-4578 plus Opdivo and chemotherapy produced a median progression-free survival of 9.0 months, compared with 6.9 months for placebo plus Opdivo and chemotherapy. The hazard ratio of 0.67 suggests a clinically meaningful reduction in the risk of progression or death in the overall study population.

That improvement matters because first-line gastric cancer treatment decisions are shaped heavily by disease control. Patients with unresectable advanced or recurrent gastric cancer often have symptomatic disease, nutritional compromise and limited tolerance for repeated lines of therapy. Extending the time before progression can help maintain treatment continuity and preserve clinical options, especially when the regimen does not appear to introduce new safety signals.

However, the Phase 2 nature of the evidence matters. A 2.1-month median progression-free survival gain is promising, but not automatically practice-changing. Regulators, payers and clinicians will want to see whether the benefit translates into longer overall survival, consistent subgroup performance and manageable toxicity in a larger global trial. The data support further development, but they do not yet establish ONO-4578 as a new standard.

Why does the PD-L1 CPS ≥1 subgroup make the ONO-4578 story more compelling?

The PD-L1 CPS ≥1 subgroup is where the ONO-4578 signal becomes more strategically interesting. In that population, median progression-free survival reached 9.9 months with ONO-4578 plus Opdivo and chemotherapy, compared with 5.7 months in the placebo combination group. Median overall survival was not reached in the ONO-4578 arm, compared with 12.7 months in the control arm, while objective response rate was also higher.

This matters because gastric cancer immunotherapy already depends heavily on biomarker interpretation. PD-L1 expression has influenced how clinicians think about checkpoint inhibitor benefit, even though real-world treatment decisions can vary by region, reimbursement system and evolving guideline language. A stronger ONO-4578 signal in PD-L1 CPS ≥1 patients suggests that EP4 blockade may be especially relevant where baseline immunotherapy responsiveness is present but still incomplete.

The risk is that subgroup strength can complicate rather than simplify development. If the overall population benefit is moderate but the PD-L1-positive population benefit is stronger, Ono Pharmaceutical and Deciphera Pharmaceuticals must decide whether the global Phase 3 strategy should focus broadly or prioritize a biomarker-defined group. A narrower population may improve the chance of a cleaner result, but it could reduce commercial reach. A broader population may expand the opportunity, but it could dilute the treatment effect.

How does EP4 blockade fit into the next wave of immuno-oncology development?

EP4 blockade fits into a broader industry effort to improve immune checkpoint inhibitor outcomes by changing the tumour microenvironment. PD-1 and PD-L1 inhibitors transformed oncology, but their benefit is uneven across tumour types and patient groups. Many cancers either fail to respond or develop resistance because the tumour microenvironment suppresses immune activity through multiple pathways.

Prostaglandin E2 signaling through EP4 is one of those pathways. By blocking EP4, ONO-4578 is intended to reduce immunosuppressive signaling and allow immune cells to act more effectively against cancer. In theory, this could make checkpoint inhibition more powerful without requiring an entirely new immune checkpoint target.

The challenge is that tumour microenvironment therapies have often looked elegant mechanistically but inconsistent clinically. Cancer immunity is not controlled by one pathway, and suppressive signals can overlap. A drug can affect the intended biology and still produce only modest clinical benefit if other resistance mechanisms dominate. ONO-4578 now has a more credible clinical signal, but global Phase 3 evidence will be needed to prove that EP4 blockade is not just biologically interesting, but therapeutically decisive.

Why does the combination with Opdivo and chemotherapy matter commercially?

The combination strategy is commercially important because it builds on an established treatment backbone rather than asking clinicians to adopt an entirely unfamiliar regimen. Opdivo, also known as nivolumab, is already part of the modern gastric cancer immunotherapy landscape. Chemotherapy remains foundational in first-line disease. Adding ONO-4578 to this backbone gives Ono Pharmaceutical a development route that aligns with current practice patterns.

This approach has obvious advantages. It may reduce physician education barriers and make the drug easier to position if Phase 3 results are positive. It also gives ONO-4578 a clear role as an immunotherapy enhancer, which could be commercially attractive if the oral agent meaningfully improves outcomes without adding difficult toxicity.

The downside is treatment complexity. First-line advanced gastric cancer therapy is already burdensome for many patients. Adding another drug means another layer of adherence, adverse-event monitoring, reimbursement assessment and drug interaction review. ONO-4578 is oral, which may help convenience, but oral oncology drugs introduce their own adherence and access challenges. The commercial question is not only whether ONO-4578 works. It is whether the incremental benefit justifies the added layer of therapy.

What does the safety profile suggest and what remains unresolved?

The Phase 2 update indicated that no new safety signals were identified. That is important because any add-on to immunotherapy and chemotherapy must clear a high tolerability bar. Advanced gastric cancer patients may already face gastrointestinal symptoms, weight loss, fatigue, anemia and chemotherapy-related adverse events. A new therapy that adds meaningful toxicity could struggle even if efficacy improves.

The absence of new safety signals supports continued development and may strengthen the rationale for a global Phase 3 trial. It suggests that ONO-4578 can be integrated into a standard treatment backbone without creating an obvious safety barrier at this stage. That is particularly important for an oral immune-modulating agent, where chronic exposure and combination safety will be closely watched.

Still, safety questions are not fully resolved. Larger and more geographically diverse studies may reveal less common adverse events, cumulative toxicity or population-specific tolerability differences. The safety profile will also need to be interpreted alongside efficacy. A modest benefit with minimal toxicity may still be attractive, while the same benefit with meaningful added burden may not be enough. Phase 3 will decide that balance.

How could a global Phase 3 trial shape the future of ONO-4578?

A global Phase 3 trial is the real test of ONO-4578’s value. The Phase 2 study was conducted in Japan, South Korea and Taiwan, which provides important regional evidence but does not fully answer how the regimen will perform across broader international populations, different chemotherapy patterns and diverse clinical practice settings. Global development will need to prove that the signal is reproducible outside the initial trial geography.

Trial design will be critical. Ono Pharmaceutical and Deciphera Pharmaceuticals must choose whether to enrich for PD-L1 CPS ≥1 patients, include all HER2-negative patients, or use a stratified strategy that preserves both scientific clarity and commercial flexibility. They must also select endpoints that satisfy regulators and clinicians. Progression-free survival may support efficacy, but overall survival and quality-of-life evidence could be decisive in determining adoption.

The Phase 3 programme will also need to reflect the evolving treatment landscape. Gastric cancer therapy is not static. Other immunotherapy combinations, targeted therapies, antibody-drug conjugates and biomarker-led strategies continue to advance. ONO-4578 must prove its value in the context of where first-line care is going, not just where it was when Phase 2 began.

Why does Deciphera’s role matter for Ono Pharmaceutical’s global oncology ambitions?

Deciphera Pharmaceuticals gives Ono Pharmaceutical a stronger global development and commercialization bridge. Ono Pharmaceutical has deep experience with Opdivo through its broader immuno-oncology history, but advancing ONO-4578 globally requires operational scale, regulatory planning and commercial strategy beyond a regional Phase 2 success. Deciphera’s integration into Ono Pharmaceutical’s oncology strategy could help move the EP4 antagonist into a more international development frame.

This matters because gastric cancer has distinct regional patterns. East Asia has high disease burden and deep clinical expertise in gastric cancer, while Western markets often differ in diagnosis timing, treatment pathways and patient characteristics. A global Phase 3 programme must be credible across these settings. The ability to coordinate development across regions could determine whether ONO-4578 becomes a global oncology asset or remains a geographically constrained opportunity.

The risk is execution. Global Phase 3 trials are expensive, slow and exposed to competitive changes. Even strong Phase 2 data can lose momentum if trial enrollment lags, endpoint assumptions are too optimistic or the treatment landscape shifts before readout. Ono Pharmaceutical and Deciphera Pharmaceuticals now have a meaningful opportunity, but they must convert it into disciplined late-stage execution.

What could ONO-4578 mean for the broader gastric cancer treatment landscape?

If ONO-4578 succeeds in Phase 3, it could strengthen the argument that gastric cancer treatment will increasingly involve immune-environment modulation rather than checkpoint inhibition alone. That would be a notable shift because many current debates in gastric cancer focus on which patients benefit most from PD-1 blockade and how to sequence targeted agents. EP4 inhibition would add another layer to that conversation.

The most likely early role would be as part of first-line therapy for HER2-negative unresectable advanced or recurrent gastric or gastroesophageal junction cancer, especially if the PD-L1 CPS ≥1 signal remains strong. Over time, the mechanism could invite testing in other gastrointestinal cancers, including colorectal cancer, where immune resistance remains a major obstacle. That broader potential is attractive, but it should not be assumed until disease-specific data support it.

The main limitation is that gastric cancer is biologically heterogeneous. Tumours differ by PD-L1 expression, microsatellite instability status, Epstein-Barr virus association, genomic profile, geography and prior patient health. A therapy that works well in one subgroup may have weaker relevance in another. ONO-4578’s future will depend on whether the biomarker story becomes sharper as development advances.

What should clinicians, regulators and industry observers watch next?

The first thing to watch is the design of the global Phase 3 trial. The population, biomarker strategy, chemotherapy backbone, geographic footprint and primary endpoint will reveal how confident Ono Pharmaceutical and Deciphera Pharmaceuticals are in the Phase 2 signal. A PD-L1 enriched design would signal precision. A broader design would signal commercial ambition. Each path has trade-offs.

Clinicians will watch whether the benefit is clinically meaningful enough to justify adding ONO-4578 to Opdivo and chemotherapy. They will also look for safety consistency, treatment adherence, duration of response and whether patients remain fit enough to continue therapy. In everyday oncology practice, the best regimen is not only the one with the strongest statistical result. It is the one that can be delivered safely and reliably to real patients.

Regulators and payers will watch overall survival, subgroup consistency and value. A progression-free survival benefit can be persuasive, but reimbursement decisions may depend on whether survival direction, quality of life and safety support the overall case. ONO-4578 has moved from an interesting immune-modulating concept to a serious late-stage candidate. The next question is whether it can survive the much harder test of global confirmation.

  ASCO 2026, EP4 antagonist, gastric cancer, gastroesophageal junction cancer, HER2-negative gastric cancer, nivolumab, Ono Pharmaceutical, ONO-4578, Opdivo, PGE2, Phase 2 trial