# Immune Checkpoint Inhibitors for Microsatellite Instability High Unresectable Obstructive Colon Cancer: A Report of Two Cases

## TL;DR

This paper reports two successful cases where immune checkpoint inhibitors helped treat advanced colon cancer, avoiding traditional surgery and leading to full recovery.

## Contribution

The study demonstrates that immune checkpoint inhibitors can safely and effectively treat obstructive MSI-H colon cancer, avoiding decompressive procedures.

## Key findings

- Pembrolizumab monotherapy rapidly improved symptoms and allowed for curative resection in two patients with MSI-H obstructive colon cancer.
- Both patients achieved complete or near-complete pathological responses without needing adjuvant chemotherapy.
- ICI treatment facilitated oncologically safe R0 resection, avoiding colostomy and other decompressive procedures.

## Abstract

Patients with obstructive colon cancer (OCC) with distant metastases often present with a poor general condition, including malnutrition, anemia, and systemic inflammation. Traditionally, these patients undergo stoma creation and/or primary tumor resection followed by systemic chemotherapy. However, for patients with DNA mismatch repair-deficient/microsatellite instability high (dMMR/MSI-H) colorectal cancer, the emergence of immune checkpoint inhibitors (ICIs) has revolutionized treatment strategies, with remarkable antitumor effects. We report two cases of successful management of MSI-H OCC, achieving curative resection while avoiding decompressive procedures, including colostomy creation.

Case 1: A 29-year-old man diagnosed with MSI-H obstructive transverse colon cancer (cT4b stomach, N1b, M1c1) was treated with pembrolizumab monotherapy (200 mg/body, every 3 weeks). The colorectal obstructive scoring system score was 2 at pembrolizumab administration. The patient showed rapid improvement in his abdominal symptoms within 3 days and achieved clinical complete response after eight courses. Laparoscopic partial colectomy with D3 lymph node dissection was subsequently performed safely. He was discharged on postoperative day 8 without postoperative complications. Histopathological analysis confirmed pathological complete response, and the patient was recurrence-free 15 months after surgery without adjuvant chemotherapy. Case 2: A 58-year-old man diagnosed with MSI-H obstructive ascending colon cancer (cT4aN3M1a, LYM) was treated with pembrolizumab monotherapy. The colorectal obstructive scoring system score was 1 at pembrolizumab administration. The patient’s abdominal symptoms improved within 5 days, with marked tumor shrinkage after nine courses. Laparoscopic extended right hemi-colectomy with D3 lymph node dissection was subsequently performed safely, and he was discharged on postoperative day 7 without postoperative complications. Histopathological analysis showed major pathological response (less than 10% viable cancer cells in the resected specimen), with no viable tumor cells in the primary lesion. The patient was recurrence-free 1-year post-surgery, without adjuvant chemotherapy.

This report highlights the potential benefits of ICI treatment for dMMR/MSI-H OCC, particularly for rapid relief of obstruction-related symptoms and facilitating oncologically safe R0 resection. In cases of MSI-H OCC, ICIs can be highly effective as an alternative to traditional decompression procedures.