Immune checkpoint inhibitor–associated diarrhea (suspected immune-mediated colitis)
Persistent diarrhea that increases to ~4–5 bowel movements on a given day after starting pembrolizumab requires urgent evaluation for immune checkpoint inhibitor (ICI)–associated colitis and treatment escalation based on severity. Infectious etiologies should be excluded promptly before attributing symptoms to ICI toxicity. [1]
Diarrhea severity classification used for ICI management
CTCAE-based risk stratification treats diarrhea/colitis as at least grade 2 when diarrhea is more than four bowel movements daily above baseline. [1] If the patient has dominant abdominal pain, fever, or bleeding, complication risk increases and expedited assessment for serious colitis is indicated. [1]
Immediate pembrolizumab management
Pembrolizumab should be withheld for moderate (CTCAE grade 2) colitis. [2] Permanent discontinuation is reserved for life-threatening (CTAE grade 4) colitis. [2]
Initial clinical evaluation and diagnostic workup
Infectious causes of diarrhea should be excluded before treating suspected ICI colitis. [1] Stool studies should be obtained for infectious pathogens, including Clostridioides difficile, before or alongside initiation of immune-directed therapy when clinically feasible. [3]
Supportive measures and symptomatic therapy
Oral antidiarrheals may be used for symptom control only after evaluation has started and other serious etiologies have been considered; immune-directed therapy should be prioritized when ICI colitis is suspected at grade 2 or higher. [1]
Immunosuppressive therapy escalation
ICI colitis typically responds to high-dose systemic glucocorticoids given as prednisone-equivalent 0.5–2 mg/kg daily with a taper over 4–6 weeks. [1] Endoscopic confirmation of diagnosis and severity should be considered before initiation of high-dose systemic glucocorticoids. [1] If the patient fails to improve with initial immunosuppressive therapy in patients with grade ≥3 diarrhea or colitis, repeat endoscopy with infectious workup (including C. difficile and CMV) is recommended. [3]
Management of steroid-refractory disease
For glucocorticoid-refractory ICI colitis, infliximab and vedolizumab are reasonable second-line options. [1]
Common pitfalls to avoid
Lactoferrin or calprotectin may support risk stratification, but management should not delay treatment for worsening grade 2 or higher symptoms while infectious etiologies and severity are being assessed. [1] Budenoside is ineffective as prophylaxis for ICI colitis and is not used as the primary treatment for typical ICI colitis. [1]
Treatment goals and response monitoring
The treatment goal is clinical improvement with a subsequent corticosteroid taper over 4–6 weeks. [1] Rapid progression of ICI colitis can occur over days, so prompt initiation of appropriate therapy and close monitoring for escalation are required. [1]
Practical decision framework for the presented scenario
If diarrhea meets CTCAE grade 2 criteria (more than four bowel movements daily above baseline), pembrolizumab should be withheld and workup should include infectious testing. [1][2] Systemic corticosteroids should be started when ICI colitis is suspected and severity is clinically significant, using prednisone-equivalent 0.5–2 mg/kg daily with taper once improved. [1] Urgent escalation to gastroenterology/oncology-directed ICI colitis pathways is warranted if symptoms progress toward grade 3 or if abdominal pain, fever, or bleeding develops. [1]