Merck & Co has announced that in the KEYNOTE-859 trial (NCT03675737), Keytruda (pembrolizumab), in combination with chemotherapy in front-line HER2-negative patients with gastric and gastroesophageal junction (GEJ) adenocarcinoma, prolongs overall survival (OS) when compared to chemotherapy alone (median OS 12.9 months vs 11.5 months in the control arm, hazard ratio [HR]: 0.78). Progression-free survival (PFS) and overall response rate (ORR) were also increased in the experimental populations, in data presented at the European Society for Medical Oncology (ESMO) virtual plenary in February 2023 (PFS was 6.9 months vs. 5.6 months in the control arm, ORR was 51.3% vs. 42.0% in the control arm). In addition, BeiGene’s PD-1 inhibitor tislelizumab has recently been approved in China for the treatment of patients with HER2-negative, PD-L1-positive metastatic gastric or GEJ adenocarcinomas. But with Bristol Myers Squibb’s (BMS) Opdivo (nivolumab) launching in 2021 in this setting, will Merck & Co be able to attract physicians back to its blockbuster PD-1 inhibitor in the US? And how might BeiGene’s tislelizumab fare should a US approval be forthcoming?
PD-1 inhibitors entered the gastric and GEJ adenocarcinoma market relatively recently. In the US, however, Opdivo is now approved in the adjuvant setting and in the first line in HER2-negative patients, while Keytruda is used primarily in the first line in combination with trastuzumab and chemotherapy in HER2-positive patients. The HER2-positive market is a much larger and therefore lucrative population, meaning Opdivo’s earlier launch here confers a significant advantage. Additionally, Merck & Co may face an uphill struggle attracting physicians back to Keytruda.
In January 2022, just months after receiving an accelerated approval for the treatment of gastric cancer patients in the third line, Merck & Co chose to withdraw Keytruda from this setting following discouraging results from two trials. The company has indicated that the results of KEYNOTE-859 are not at odds with previous data; indeed, the addition of Keytruda to chemotherapy in KEYNOTE-062 did prolong survival, although this was not significantly different from chemotherapy alone.
The approval of tislelizumab in China for the treatment of patients with HER2-negative, PD-L1-positive metastatic gastric or GEJ adenocarcinomas is based on interim analysis of data from the RATIONALE-305 trial (NCT03777657), which saw the addition of tislelizumab to chemotherapy significantly prolong OS (17.2 months vs 12.6 months in the control arm, HR: 0.74). The trial featured sites across the US and Europe, and it is anticipated that BeiGene, in collaboration with Novartis, which has commercialisation rights in North America, Europe and Japan, will push for a US approval in the second half of this year.
An approval for tislelizumab in the US would certainly see competition in this market heat up in 2023. But with little to separate these therapies in terms of efficacy, physicians’ familiarity with Opdivo is expected to see this drug remain the dominant PD-1 inhibitor in the gastric and GEJ adenocarcinoma market.
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