Tafolecimab Combined With Sintilimab and SOX in the Treatment of pMMR/MSS Gastric Cancer

Background:

Gastric cancer remains a significant health burden globally, particularly in China, where the majority of patients present with advanced disease at diagnosis. While immune checkpoint inhibitors (ICIs) targeting PD-1 have revolutionized treatment for various malignancies, their efficacy in proficient mismatch repair (pMMR) or microsatellite stable (MSS) gastric cancer-which constitutes over 85% of cases-remains limited. Recent Phase III trials (CheckMate 649, ATTRACTION-04, Orient-16) have demonstrated that combining PD-1 inhibitors with chemotherapy improves outcomes in advanced gastric cancer, leading to approved indications. However, the benefit in pMMR/MSS populations is modest, highlighting an urgent need for novel combination strategies to overcome immunotherapy resistance.

Preclinical research published in Nature (Liu et al., 2020) revealed that inhibiting PCSK9 (Proprotein Convertase Subtilisin/Kexin type 9)-a key regulator of cholesterol metabolism-can potentiate immune checkpoint therapy through a novel mechanism independent of its lipid-lowering function. PCSK9 inhibition was shown to increase tumor cell surface expression of MHC class I molecules by preventing their lysosomal degradation, thereby enhancing tumor antigen presentation and promoting cytotoxic T lymphocyte infiltration. This mechanistic insight suggests that combining a PCSK9 inhibitor with PD-1 blockade could synergistically improve antitumor immunity, particularly in immunologically "cold" tumors like pMMR/MSS gastric cancer.

Tafolecimab is the first domestically developed fully humanized PCSK9 monoclonal antibody approved in China for hypercholesterolemia, with a favorable safety profile and extended half-life. Based on this strong preclinical rationale and the established efficacy of PD-1 plus chemotherapy in gastric cancer, this investigator-initiated trial aims to clinically translate the concept of PCSK9 inhibition as an immunomodulatory strategy.

Study Population:

This study will enroll 30 patients with the following key eligibility criteria:

Inclusion: Adults aged 18-75 years with histologically confirmed pMMR/MSS gastric or gastroesophageal junction adenocarcinoma; initially unresectable or advanced disease (including metastatic); ECOG performance status 0-1; measurable disease per RECIST v1.1; adequate organ function.

Exclusion: HER2-positive, EBER-positive, or CLDN18.2-positive tumors; prior systemic anticancer therapy for advanced disease; active autoimmune disease requiring immunosuppression; uncontrolled intercurrent illness; LDL-C \<30 mg/dL; history of PCSK9 inhibitor allergy; prior exposure to anti-PD-1/PD-L1 or PCSK9-targeted therapies.

Study Objectives:

This is a multicenter, prospective, single-arm exploratory trial with a safety run-in phase (first 6 patients monitored for dose-limiting toxicities). The treatment regimen consists of:

Sintilimab (PD-1 inhibitor): 200 mg IV, day 1, every 3 weeks (Q3W) Tafolecimab (PCSK9 inhibitor): 300 mg subcutaneous injection, day 1, Q3W (dose reduction to 150 mg if DLTs occur) SOX chemotherapy: Oxaliplatin 130 mg/m² IV, day 1 + S-1 40 mg/m² orally twice daily, days 1-14, Q3W cycles Treatment continues until disease progression, unacceptable toxicity, or withdrawal of consent.

Primary Endpoint:

Objective Response Rate (ORR) assessed by RECIST v1.1

Secondary Endpoints:

Progression-Free Survival (PFS) Disease Control Rate (DCR) Conversion surgery rate and R0 resection rate Pathological Complete Response (pCR) and Major Pathological Response (MPR) rates in resected patients Overall Survival (OS) Safety and tolerability (incidence of TRAEs, ≥Grade 3 AEs, irAEs per CTCAE v5.0)

Exploratory Endpoints:

Association between tumor biomarkers (including PD-L1 CPS, H. pylori infection status, and tumor PCSK9 expression) and treatment efficacy Multi-omics analyses using paired pre- and post-treatment tumor tissue, peripheral blood, and fecal samples

Sample Size and Duration:

A fixed sample size of 30 patients will be recruited over approximately 12 months, with survival follow-up extending to 36 months. This exploratory study is designed to generate preliminary efficacy and safety signals to inform future larger-scale investigations. The safety run-in design ensures close monitoring for potential additive toxicities, particularly given the novel combination of PCSK9 inhibition with immunotherapy and chemotherapy.