Successful desensitization to horse antithymocyte globulin for aplastic anemia: two case reports and literature review.

Background: Horse antithymocyte globulin (hATG) is an important therapeutic option for aplastic anemia (AA). However, hATG carries the risk of fatal anaphylaxis, and skin tests are performed to identify high-risk patients. We report on the successful desensitization of two AA patients with positive skin tests to hATG.

Case presentation: Case 1: A 72-year-old man with a history of successful treatment with rabbit ATG was referred for pancytopenia. Neutrophil, reticulocyte, and platelet counts were 546 /µL, 32,000 /µL, and 19,000 /µL, despite the oral administration of eltrombopag and cyclosporine. Bone marrow biopsy showed hypocellularity, and he was diagnosed with relapsed severe AA. Case 2: A 69-year-old man was referred for anemia and thrombocytopenia, and diagnosed with non-severe AA. Neutrophil, reticulocyte, and platelet counts were 2,044 /µL, 23,000 /µL, and 37,000 /µL. Bone marrow biopsy revealed hypocellularity. Neither patient had a history of allergy, and the skin prick test (SPT) of hATG was negative, but the intradermal test (IDT) was positive. The result of the IDT in case 2 was reproducible. They received hATG desensitization under close monitoring of vital signs in our high-care unit. The protocol consisted of gradually increasing doses of hATG (four intradermal, two subcutaneous, and four intravenous (IV) push) and some premedications prior to administration of the full dose IV drip. They completed the course without developing any systemic allergic reactions.

Conclusions: Despite the risk of anaphylaxis, hATG desensitization can be beneficial in AA patients with a positive skin test, especially when no alternative is available or hATG is preferred.