Auto-immune gastritis induced by pembrolizumab, an anti-PD-1, in a melanoma patient

We report a case of a 67-years-old woman presenting a severe acute lymphocytic gastritis induced by pembrolizumab, an immune check point inhibitor (ICI). This gastritis was her third auto-immune adverse event after 5 years of treatment with pembrolizumab, it was metabolically active at the PET Scan and confirmed by analysis of the gastric biopsies. Pembrolizumab treatment cessation and high doses of corticosteroids completely normalized the stomach clinically, endoscopically and histologically. This patient was in complete remission of her metastatic melanoma. Therefore, pembrolizumab therapy was not restarted and the patient is still in remission 6 months later. This strategy is supported by recent publications describing a relapse rate inferior to 10% in patients in complete remission after 2 years of immunotherapy. Particularities of this case are: rareness of this adverse event, late onset after introduction of pembrolizumab, evocative PET scan image, specific endoscopic aspect and histology. In addition, the favorable oncologic evolution of the patient after treatment cessation confirms the prolonged remission after immunotherapy.