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Differentiation syndrome during ivosidenib treatment with immunohistochemistry showing isocitrate dehydrogenase R132H mutation

We report a case of differentiation syndrome in a patient receiving the IDH1 inhibitor ivosidenib, with skin biopsy showing isocitrate dehydrogenase (IDH) R132H‐mutated leukemia cutis. A 72‐year‐old man with IDH1‐mutated acute myeloid leukemia (AML), status‐post allogeneic cell transplantation, on ivosidenib for 6 months, was admitted for culture‐negative neutropenic fever, pink and purpuric plaques and patches on the legs, abdomen and back, edema, hypotension, and shortness of breath. Skin biopsy revealed an infiltrate of atypical, immature, myeloperoxidase‐positive mononuclear cells compatible with leukemia cutis or Sweet syndrome. Although dermal edema and interstitial neutrophilic infiltrate with karyorrhexis characteristic of Sweet syndrome were not seen, the atypical cells lacked expression of CD117 and CD34, which were expressed in the original leukemia. Additional immunohistochemical staining of suspected blasts was strongly positive for IDH1 R132H, suggesting a diagnosis of leukemia cutis. As the immunophenotype of blasts in skin infiltrates can significantly differ from the immunophenotype seen in blood and bone marrow, this case shows that mutation‐specific antibodies such as anti‐IDH1 R132H may be useful to help distinguish malignant from non‐malignant infiltrates in the skin. Furthermore, differentiation syndrome may show histopathologic features of leukemia cutis on skin biopsy.