rapamycin chemical structureBackground Information

  1. Rapamycin, also known as sirolimus, is an immunosuppressant drug used to prevent rejection in organ transplantation; it is especially useful in kidney transplants.
  2. It prevents activation of T cells and B cells by inhibiting their response to interleukin-2 (IL-2).
  3. A macrolide, sirolimus was discovered by Brazilian researchers as a product of the bacterium Streptomyces hygroscopicus in a soil sample from Easter Island an island also known as Rapa Nui.
  4. It was approved by the FDA in September 1999 and is marketed under the trade name Rapamune by Pfizer (formerly by Wyeth).
  5. Sirolimus was originally developed as an antifungal agent. However, this use was abandoned when it was discovered to have potent immunosuppressive and antiproliferative properties.
  6. It has since been shown to prolong the life of mice and might also be useful in the treatment of certain cancers.

 

Mechanism of action

  1. Unlike the similarly named tacrolimus, sirolimus is not a calcineurin inhibitor, but it has a similar suppressive effect on the immune system.
  2. Sirolimus inhibits the response to interleukin-2 (IL-2), and thereby blocks activation of T and B cells.
  3. In contrast, tacrolimus inhibits the secretion of IL-2.
  4. The mode of action of sirolimus is to bind the cytosolic protein FK-binding protein 12 (FKBP12) in a manner similar to tacrolimus.
  5. Unlike the tacrolimus-FKBP12 complex which inhibits calcineurin (PP2B), the sirolimus-FKBP12 complex inhibits the mammalian target of rapamycin (mTOR, rapamycin being an older name for sirolimus) pathway by directly binding the mTOR Complex1 (mTORC1).
  6. mTOR has also been called FRAP (FKBP-rapamycin associated protein), RAFT (rapamycin and FKBP target), RAPT1, or SEP.
  7. The earlier names FRAP and RAFT were coined to reflect the fact that sirolimus must bind FKBP12 first, and only the FKBP12-sirolimus complex can bind mTOR. However, mTOR is now the widely accepted name, since Tor was first discovered via genetic and molecular studies of sirolimus-resistant mutants of Saccharomyces cerevisiae that identified FKBP12, Tor1, and Tor2 as the targets of sirolimus and provided robust support that the FKBP12-sirolimus complex binds to and inhibits Tor1 and Tor2.

 

Cancer

  1. The antiproliferative effects of sirolimus may have a role in treating cancer.
  2. Sirolimus was shown to inhibit the progression of dermal Kaposi's sarcoma in patients with renal transplants.
  3. Other mTOR inhibitors, such as temsirolimus (CCI-779) oreverolimus (RAD001), are being tested for use in cancers such as glioblastoma multiforme and mantle cell lymphoma. However, these drugs have a higher rate of fatal adverse events in cancer patients than control drugs.
  4. A combination therapy of doxorubicin and sirolimus has been shown to drive AKT-positive lymphomas into remission in mice.
  5. Akt signalling promotes cell survival in Akt-positive lymphomas and acts to prevent the cytotoxic effects ofchemotherapy drugs, such as doxorubicin or cyclophosphamide.
  6. Sirolimus blocks Akt signalling and the cells lose their resistance to the chemotherapy.
  7. Bcl-2-positive lymphomas were completely resistant to the therapy; eIF4E expressing lymphomas are not sensitive to sirolimus.

Source: Vézina C, Kudelski A, Sehgal SN, Pritchard

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