Sirolimus

Sirolimus, also known as rapamycin, is a macrocyclic lactone antibiotic produced by bacteria Streptomyces hygroscopicus, which was isolated from the soil of the Vai Atari region of Rapa Nui (Easter Island). It was first isolated and identified as an antifungal agent with potent anticandida activity; however, after its potent antitumor and immunosuppressive activities were later discovered, it was extensively investigated as an immunosuppressive and antitumour agent. Its primary mechanism of action is the inhibition of the mammalian target of rapamycin (mTOR), which is a serine/threonine-specific protein kinase that regulates cell growth, proliferation, and survival. mTOR is an important therapeutic target for various diseases, as it was shown to regulate longevity and maintain normal glucose homeostasis. Targeting mTOR received more attention especially in cancer, as mTOR signalling pathways are constitutively activated in many types of human cancer.

Sirolimus was first approved by the FDA in 1999 for the prophylaxis of organ rejection in patients aged 13 years and older receiving renal transplants. In November 2000, the drug was recognized by the European Agency as an alternative to calcineurin antagonists for maintenance therapy with corticosteroids. In May 2015, the FDA approved sirolimus for the treatment of patients with lymphangioleiomyomatosis. In November 2021, albumin-bound sirolimus for intravenous injection was approved by the FDA for the treatment of adults with locally advanced unresectable or metastatic malignant perivascular epithelioid cell tumour (PEComa). Sirolimus was also investigated in other cancers such as skin cancer, Kaposi’s Sarcoma, cutaneous T-cell lymphomas, and tuberous sclerosis. The topical formulation of sirolimus, marketed as HYFTOR, was approved by the FDA in April 2022: this marks the first topical treatment approved in the US for facial angiofibroma associated with tuberous sclerosis complex.

Sirolimus is indicated for the prophylaxis of organ rejection in patients aged 13 years or older receiving renal transplants. In patients at low-to moderate-immunologic risk, it is recommended that sirolimus be used initially in a regimen with cyclosporine and corticosteroids; cyclosporine should be withdrawn two to four months after transplantation. In patients at high-immunologic risk (defined as Black recipients and/or repeat renal transplant recipients who lost a previous allograft for immunologic reason and/or patients with high panel-reactive antibodies [PRA; peak PRA level > 80%]), it is recommended that sirolimus be used in combination with cyclosporine and corticosteroids for the first year following transplantation.

It is also used to treat lymphangioleiomyomatosis.

In the US, albumin-bound sirolimus for intravenous injection is indicated for the treatment of adult patients with locally advanced unresectable or metastatic malignant perivascular epithelioid cell tumour (PEComa).

In Europe, it is recommended that sirolimus for the prophylaxis of organ rejection in renal transplants is used in combination with cyclosporin microemulsion and corticosteroids for two to three months. Sirolimus may be continued as maintenance therapy with corticosteroids only if cyclosporin microemulsion can be progressively discontinued.

Topical sirolimus is indicated for the treatment of facial angiofibroma associated with tuberous sclerosis in adults and pediatric patients six years of age and older.