Purity >98%. HQL-79 is a selective inhibitor of hematopoietic prostaglandin D (PGD) synthase. Structurally, it is a synthetic tetrazole compound originally prepared as a possible antihistamine. Subsequent evaluation in models of allergy and asthma demonstrates the HQL-79 is orally available, and inhibits the synthesis of PGD2 . HQL-79 is thus a likely lead compound for the preparation of potent, selective hematopoietic PGD synthase inhibitors.