Efficacy of HX008 in high microsatellite instability/mismatch repair–defificient (MSI-H/dMMR) solid tumors: Results from a multicenter phase II open-label study.

2572 Background: The subsequent treatment choices are limited for the patients with advanced solid tumors who had failed the standard therapies. PD-1 blockade monotherapy demonstrated robust antitumor activity in patients with MSI-H/dMMR. The aim of this study is to identify the efficacy and safety of HX008, an anti-PD-1 monoclonal antibody, in patients with advanced MSI-H/dMMR solid tumors. Methods: Eligible patients were age ≥18 years with histologically/cytologically confirmed advanced MSI-H/dMMR solid tumors, who have failed at least one line of standard systemic therapy. MSI-H/dMMR status was assessed centrally. Patients received HX008 200 mg once every 3 weeks until disease progression, unacceptable toxicity, or patient withdrawal. Radiologic imaging was performed 9 weeks after the first treatment, then every 6 weeks for the first year of therapy, and every 12 weeks thereafter. The primary end point was objective response rate (ORR) per RECIST1.1. Results: One hundred patients were enrolled from October 2018 to December 2020, with a median age of 53 (range 20-74) years. All of the patients were ≥ second-line patients. The most common cancer types were colorectal cancer (N=74) and gastric cancer (N=10). Median follow-up is 8.97 (range 0.03-25.53) months at the time of data cutoff. Among 86 patients who had reached the initial response evaluation, there were 8 CR, 33 PR, 24 SD, 17 PD and 4 NE. ORR was 47.67% (95%CI 36.79%-58.73%), and DCR was 75.58% (95%CI 65.13%-84.20%). ORR and DCR for the 66 colorectal cancer patients were 50% (95%CI 37.43-62.57%) and 75.76% (95%CI 63.64-85.46%). Median PFS was not reached (95%CI 6.18-NR) for all enrolled patients, while the 6-month and 12-month PFS rates were 62.66% (95%CI 50.98%-72.31%) and 52.70% (95%CI 39.96%-63.94%), respectively. Median OS was not reached. Treatment-related adverse events occurred in 77 patients (77%). Twelve patients (12%) had grade 3 or 4 treatment-related adverse events and there were no grade 5 treatment-related adverse events. The grade 3 or 4 treatment-related adverse events with incidence >1% included anemia (2%) and leukopenia (2%). Immune-related adverse events were observed in 15 patients (15%), including hypothyroidism in 9 patients (all were grade 1-2), and hepatitis, hyperglycemia, myocarditis, creatin kinase/creatin kinase MB increased, hypopigmentation of the vulva, rash, each in 1 patient. Conclusions: HX008 as a ≥second-line therapy showed promising efficacy and a manageable safety profile in patients with MSI-H/dMMR advanced solid tumors. Clinical trial information: NCT03704246.