Drug Breakdown: SIROLIMUS

Abstract
In this column, Sharon Rees aims to refresh knowledge and interest in some of the commonly used drugs in a series of posts on X. This month she is talking about #sirolimus
Day 1: A streptomyces soil isolate from Easter Island (correct name Rapa Nui) found in the 1960s, the generic name for #rapamycin is #sirolimus. It is a macrolide compound with immune suppressive, antitumour & antifungal properties
Day 2: #sirolimus tablet or oral solution are licensed for the prevention of kidney organ rejection in adults. 6 mg loading dose day 1, then 2 mg x1/day maintenance. Used as ongoing treatment combined with ciclosporin & corticosteroids initially, then continued alone after 2–3 months & ciclosporin wash out
Day 3: #sirolimus kinetics; bioavailability is better for tablet compared to the oral solution. Metabolism via phase 2 demethylation & hydroxylation in gut/liver >inactive metabolites. Faecal elimination. Elimination mean t½ 62hrs. Substrate for CYP3A4 & P-gp, so potential for drug drug interactions. #sirolimus has a narrow therapeutic index; requires therapeutic monitoring with dose adjusted according to trough concentration. Safety monitoring periodically throughout treatment
Day 4: #sirolimus is a potent inhibitor of T & B lymphocytes; it forms a complex with intracellular protein FKB12. This blocks the protein kinase TOR (target of rapamycin) & halts cell division. Differs from cousin drug #tacrolimus which ↓ T-cell activation by blocking calcineurin
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