In a male patient with locally advanced rectal cancer that is microsatellite instability‑high (MSI‑H), should neoadjuvant immunotherapy with a PD‑1 inhibitor be used, and what regimen is recommended?

Immunotherapy for MSI-H Locally Advanced Rectal Cancer

Immunotherapy with a PD-1 inhibitor is strongly recommended as the initial treatment approach for this patient with MSI-H locally advanced rectal cancer, and can potentially eliminate the need for chemoradiotherapy and surgery. 1

Primary Recommendation

The 2024 ASCO guideline provides a strong recommendation for immunotherapy in MSI-H or dMMR rectal cancers despite low-quality evidence 1. This represents a paradigm shift from traditional total neoadjuvant therapy (TNT) approaches. The NCCN 2024 guidelines similarly endorse immunotherapy for this molecular subtype 2.

Specific Regimen

Single-agent PD-1 inhibitor monotherapy is the preferred approach:

• Dostarlimab is the most extensively studied agent, administered at 500 mg every 3 weeks for 6 months (approximately 12 doses) 3
• Alternative PD-1 inhibitors include sintilimab (200 mg every 3 weeks for 6 cycles) 4 or nivolumab 5

The landmark dostarlimab study demonstrated 100% clinical complete response (cCR) in all 12 initial patients, with extended follow-up showing sustained cCR in all 42 patients who completed treatment, with no progression or recurrence and no patient requiring chemoradiotherapy or surgery 3, 2. This represents unprecedented efficacy in solid tumor oncology.

Why Immunotherapy Over Standard TNT

MSI-H/dMMR tumors have distinct biology that makes them:

• Highly sensitive to PD-1 blockade due to high tumor mutational burden and neoantigen expression
• Less responsive to fluorouracil-based chemotherapy historically 1
• Potentially radioresistant, making standard chemoradiotherapy less effective 6

The complete response rates with immunotherapy (75-100% cCR, 47.6-100% pCR) far exceed those achieved with conventional chemoradiotherapy in this molecular subtype 6.

Treatment Algorithm

1. Confirm MSI-H/dMMR status using both immunohistochemistry (IHC) for MMR proteins AND molecular testing (PCR or NGS) - dual confirmation is critical as discordant results can occur 4

2. Exclude contraindications to immunotherapy (active autoimmune disease, need for immunosuppression, etc.)

3. Initiate PD-1 monotherapy - dostarlimab 500 mg IV every 3 weeks for 6 months

4. Assess response at 8 ± 4 weeks after completion using:

   • Digital rectal examination (no palpable tumor)
   • Flexible sigmoidoscopy/rectoscopy (no residual tumor, no erythematous ulcer)
   • MRI (substantial downsizing, no residual tumor or fibrosis only)
   • Do NOT perform biopsy if above criteria met 1

5. If cCR achieved: Consider nonoperative management (watch-and-wait) with intensive surveillance

6. If incomplete response: Proceed to surgery (total mesorectal excision)

Nonoperative Management Considerations

For patients achieving cCR, organ preservation through watch-and-wait is increasingly accepted 2. This approach offers:

• Avoidance of surgical morbidity and permanent ostomy
• Preservation of bowel, sexual, and urinary function
• Particularly valuable for distal tumors requiring abdominoperineal resection

Critical surveillance protocol (based on OPRA trial) 2:

• Years 1-2: Digital rectal exam, flexible sigmoidoscopy, and CEA every 4 months; MRI every 6 months
• Years 3-5: Exams every 6 months; MRI yearly
• 94% of regrowth occurs within first 2 years, 99% within 3 years 2

Important Caveats

This approach requires:

• Experienced multidisciplinary team at a high-volume center
• Patient understanding of rigorous surveillance requirements
• Patient acceptance of small risk of tumor regrowth (though salvage surgery remains effective)
• Confirmation that patient can adhere to intensive follow-up schedule

If immunotherapy is contraindicated, standard TNT options (chemoradiotherapy with or without systemic chemotherapy) remain appropriate, though dMMR tumors have shown sensitivity to chemoradiotherapy despite reduced efficacy of fluorouracil-based chemotherapy 1.

Combination Strategies

While dual checkpoint blockade (PD-1 + CTLA-4) shows promise in metastatic disease 7, single-agent PD-1 inhibition remains the evidence-supported standard for locally advanced disease 6. Combination with chemotherapy or radiotherapy may increase toxicity without clear benefit in this highly immunotherapy-sensitive population 6.

Safety Profile

Grade 3-4 immune-related adverse events occur in approximately 12-16% of patients receiving PD-1 monotherapy 4, 6, which is manageable and compares favorably to the toxicity profile of chemoradiotherapy. No grade 3+ adverse events were reported in the original dostarlimab cohort 3.