Gastric cancer has moved decisively into a biomarker-directed era, with HER2, PD-L1 and CLDN18.2 now all established before first-line therapy is chosen. PD-1 inhibitors, led by Opdivo and Keytruda, remain the backbone of first-line HER2-negative treatment, but now operate under a narrowed label following the 2024 ODAC vote restricting use to PD-L1 CPS ?1, opening space for Astellas' Vyloy to establish CLDN18.2 as a validated, biomarker-independent target. Two new modalities have just reshaped the landscape further, with AstraZeneca's Imfinzi as the first perioperative immunotherapy, and Jazz Pharmaceuticals' Ziihera as the first bispecific to challenge trastuzumab's decade-long hold on first-line HER2-positive disease. Valued at roughly $2.66 billion in 2025 across the US, Japan and 5EU, with the US contributing close to two-thirds of that revenue, the market is being reshaped less by any single drug than by a wholesale shift from chemotherapy-anchored regimens toward layered, biomarker-selected combinations.

That reshaping continues over the next decade, but is followed by a period of consolidation rather than sustained growth. Revenue is forecast to climb to a peak of roughly $5.21 billion by 2031, as Imfinzi's perioperative launch matures, Ziihera completes its displacement of the HER2-positive standard, and a second wave of CLDN18.2 agents, including sonesitatug vedotin and givastomig, further erodes PD-1 dominance in first-line HER2-negative disease, still the market's largest segment. A concentrated wave of loss of exclusivity across Opdivo, Keytruda, Imfinzi, Vyloy and Enhertu, landing almost entirely between 2028 and 2033, then pulls the market back to roughly $3.92 billion by 2035, a 25% contraction from peak. By that point, half of all revenue depends on four assets not yet approved today, and the relapsed/refractory setting remains the area of greatest unmet need despite the wave of new mechanisms that has otherwise transformed earlier lines of therapy.