This phase II clinical study was a multicenter, open-label, non-randomized, 2-cohort, phase 2 trial that conducted across 11 hospitals in China. Eligible patients in cohort 1 were ≥18 and≤75 years of age with histologically confirmed diagnosis of advanced G/GEJ cancer; refractory to, or intolerant of first-line chemotherapy (platinum-containing and/or fluorouracil-based regimen); at least one measurable lesion diagnosed according to the Response Evaluation Criteria In Solid Tumors (RECIST) V.1.1 at baseline; an Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1; a life expectancy ≥3 months and adequate organ function. Key exclusion criteria were: active or prior documented autoimmune or inflammatory disease; prior treatment with an agent directed against PD-1, or PD-L1 or CTLA-4; history or current interstitial lung disease or pulmonary fibrosis; symptomatic central nervous system metastases; exposure to any investigational agent within 4 weeks of the first dose of study drug; the adverse reactions of previous treatments failed to recover to National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE; V.5.0) grade score ≤1.

Written informed consent was obtained from all patients before enrolment.

All subjects received HX008 combined with irinotecan. HX008 200 mg was administrated intravenously over 60 min every 3 weeks up to 2 years; irinotecan 160 mg/m² was administered intravenously over 60–120 min every 2 weeks. For each patient during the overlapping cycle of HX008 and irinotecan, HX008 was administrated first, followed sequentially by the irinotecan infusion. Treatment was discontinued on disease progression, unaccepted toxicity, physician decision or patient withdrawal of consent, whichever came first. Clinically stable patients with the first radiographic evidence of progressive disease (PD) could remain on treatment at the investigator’s discretion until confirmed PD. Treatment interruptions were permitted for the management of treatment-related AEs.

The primary endpoint was objective response rate (ORR) per RECIST V.1.1, which was defined as the percentage of patients with a confirmed complete (CR) or partial response (PR). Secondary endpoints included disease control rate (DCR), duration of response (DOR), PFS, OS and safety. DCR was defined as the percentage of patients with a confirmed CR or PR, or stable disease (SD). DOR was defined as time from initial radiographically CR or PR until disease progression. PFS was defined as the time from the date of registration to disease progression or death from any cause, whichever occurred first. OS was defined as the time from the date of registration to death from any cause.

Tumor imaging was performed by CT scans or MRI at baseline, and every 6 weeks thereafter through the first year, then every 12 weeks. RECIST V.1.1 and immune related Response Criteria (iRECIST) were used for tumor Evaluation. Adverse events (AEs) were recorded during the study period from the initiation of treatment to 30 days after the last dose or the start date of subsequent antitumor therapy followed the last dose, whichever came first. Immune-related AEs should be recorded up to 90 days after the last administration of HX008. AEs were coded using the Medical Dictionary for Regulatory Activities (V.20.0) and graded according to the CTCAE V.5.0.

For patients with available tumor samples, PD-L1 tumor expression was assessed by immunohistochemistry in archival or newly collected tumor samples at the central laboratory (SAB028, Shuwen Biotech). PD-L1 expression was measured using the CPS, defined as the number of PD-L1-positive cells (tumor cells, lymphocytes, macrophages) as a proportion of the total number of tumor cells multiplied by 100. Tumors was considered to have positive PD-L1 expression when CPS ≥1.

In this trial, 55 patients were enrolled to obtain a planned sample size of 50 patients if 10% of enrolled patients would not be treated. This provided approximately 80% power, at a one-sided nominal α=0.05, to detect an ORR of 22% or greater with HX008 plus irinotecan compared with a fixed control rate of 10% (based on historical data) using the exact binomial test. Efficacy endpoints were analyzed in the full analysis set (FAS), defined as patients who received at least one dose of study treatment and for whom any efficacy data on treatment were available. Safety endpoints were assessed in patients who received at least one dose of study treatment. ORR and DCR were assessed based on the point estimate and 95% CI using the Clopper-Pearson exact method. The subjects without mitigation data in FAS would be counted as no remission. PFS, DOR and OS were estimated by Kaplan-Meier method. Statistical analysis was performed using SAS V.9.4 (SAS Institute) statistical software.

Between October 2018 and September 2019, a total of 58 patients with advanced G/GEJ cancer were enrolled from 11 centers in China. All patients received at least one dose of HX008 combined with irinotecan. Baseline characteristics are listed in table 1. Median (range) age was 60 (26–70) years. Most patients were male (72.4%) and had an ECOG performance status score of 1 (58.6%). Forty patients were advanced GC and 18 patients were advanced gastroesophageal junction cancer. PD-L1 expression was detected in 22 patients with available tumor samples, with 14 patients had PD-L1 positive tumors.

By the data cut-off date of May 11 2020, the median follow-up was 10.5 months (range 7.4–18.9). Six patients (10.3%) continued to receive study treatment. The most common reason for treatment discontinuation was disease progression (39 patients (67.2%)) (online supplemental figure 1). The median duration of exposure to study treatment was 3.5 months (range 0.03–16.3), and the median number of HX008 and irinotecan were 5 (range 1–22) and 7 (range 1–32), respectively.

Tumor evaluations were listed in table 2. Confirmed PR was observed in 16 patients, for a confirmed ORR of 27.6% (95% CI 16.1% to 39.1%); 19 patients experienced SD, leading to a DCR of 60.3% (95% CI 46.4% to 73.0%). ORR and DCR in evaluable patients were 30.2% (95% CI 18.3% to 44.3%) and 66.0% (95% CI 51.7% to 78.5%), respectively. Six of the responses (37.5%) were ongoing at the time of data cut-off. The median DOR was 8.0 months (range 1.5–12.5) and the median time to response was 1.4 months (range 1.3–7.8). Among patients with ≥1 evaluable postbaseline imaging assessment, 32 (60.4%) experienced reduction in measurable tumor size (figure 1A), and decrease in tumor burden was maintained over several assessments (figure 1B).

By data cut-off, 39 patients had disease progression and 20 patients died. Median PFS was 4.2 months (95% CI 2.2 to 5.5), and the estimated 6-month PFS rate was 28.3% (95% CI 16.5% to 41.3%) (table 2 and figure 2A). Median OS was not reached (NR) (95% CI 8.7 to NR) and the estimated 12 month OS rate was 64.2% (95% CI 49.4% to 75.7%) (figure 2B).

PD-L1 expression could be evaluated in 22 patients at baseline, and 14 patients (63.6%) with PD-L1 CPS of 1 or higher. Among patients with evaluable patients, ORR was 38.5% (5/13) with CPS≥1 and 37.5% (3/8) with CPS＜1, DCR was 84.6% (11/13) and 75% (6/8), respectively. Median PFS was 4.3 months with CPS ≥1 and 5.0 months with CPS＜1, respectively. Median OS was NR with CPS ≥1 and 8.7 months with CPS＜1, respectively.

Treatment-related AEs (TRAEs) of any grade occurred in all patients (table 3). The most common TRAEs of any grade were leukopenia (81.0%), neutropenia (77.6%), nausea (65.5%), vomit (60.3%), decreased appetite (53.4%), fatigue (50.0%), anemia (48.3%), diarrhea (44.8%), decreased lymphocyte count (24.1%) and weight loss (22.4%). Grade 3 or 4 TRAEs occurred in 37 patients (63.8%); the most frequent (>5%) ones included neutropenia (32.8%), leukopenia (31.0%), anemia (17.2%), decreased appetite (8.6%), vomit (6.9%), nausea (6.9%) and fatigue (5.2%). Serious AEs (SAEs) were reported in 10 patients (17.2%), with the most common being neutropenia (6.9%), anemia (5.2%), decreased appetite (5.2%), diarrhea (3.4%) and vomit (3.4%) (table 3). The most common immune-related AEs were grade 1 or 2, including fatigue (29.3%), proteinuria (15.5%), hypothyroidism (13.8%), diarrhea (12.1%), rash (10.3%) (table 4).

Nine patients (15.5%) had AEs that lead to permanent HX008 and/or irinotecan discontinuation, including neutropenia (5.2%), vomit (5.2%), leukopenia (3.4%), fatigue (3.4%) and anemia (1.7%). Dose reduction of irinotecan was performed in 23 (39.7%) out of the 58 patients. The most frequent (>5%) reasons of dose reduction of irinotecan were neutropenia (20.7%), leukopenia (10.3%), nausea (10.3%), vomit (10.3%), decreased appetite (10.3%) and fatigue (8.6%). There were no treatment-related deaths.