Successful Treatment of pMMR MSS IVB Colorectal Cancer Using Anti-VEGF and Anti-PD-1 Therapy in Combination of Gut Microbiota Transplantation: A Case Report

Immune checkpoint inhibitors (ICI) have shown great promise in treating advanced or metastatic colorectal cancer (mCRC), especially for CRC patients with deficient mismatch repair (dMMR) and high microsatellite instability (MSI-H). For the remainder of CRC patients presenting with proficient mismatch repair (pMMR) and microsatellite stable (MSS) or low microsatellite instability (MSI-L), ICI showed a low-level response. This study describes a 57-year-old Chinese man diagnosed with pMMR MSS IVb CRC with liver metastasis. Primarily, the patient was administered two consecutive treatments, one composed of an anti-EGFR and modified FOLFOX6 and the other composed of an anti-VEGF and FOLFOXIRI. Due to severe chemotherapy side effects, the patient discontinued treatment and decided to take a third investigational treatment, where an anti-PD-1 and an anti-VEGF were given in combination with fecal microbiota transplantation (FMT) capsules. The patient achieved a partial response (PR), and the tumor size decreased to the extent amenable to surgical resection. After surgery, the patient achieved a pathological complete response (pCR). Patients with pMMR MSS or MSI-L hardly benefit from anti-PD-1 immunotherapy. This study indicated that, to a limited extent, FMT might improve the response to ICI for pMMR MSS CRC patients.