Azathioprine - Mechanism of Action

Mechanism of Action

The purine molecule is the framework for two of the four bases that occur in DNA, adenine and guanine. Consequently, blocking the synthesis of purine also hinders DNA synthesis and thus inhibits the proliferation of cells, especially fast-growing cells without a method of nucleotide salvage ("recycling"), such as lymphocytes. Two types of lymphocytes, T cells and B cells, are particularly affected by the inhibition of purine synthesis.

Azathioprine's active metabolite methyl-thioinosine monophosphate (MeTIMP) is a purine synthesis inhibitor that works by blocking the enzyme amidophosphoribosyltransferase, possibly among others. Additional mechanisms, mediated by other metabolites, have been discovered several decades after the drug's introduction, mainly in the 1990s and 2000s: Thioguanosine triphosphate (TGTP) is incorporated into RNA, compromising its functionality. It also interacts with the GTP-binding protein Rac1, blocking upregulation of the protein Bcl-xL and thus sending activated T cells into apoptosis (a kind of programmed cell death). The closely related thio-deoxyguanosine triphosphate (TdGTP) is built into DNA. Thioinosinic acid impedes later steps of DNA synthesis via enzymes such as adenylosuccinate synthase and IMP dehydrogenase. Moreover, azathioprine blocks the downstream effects of CD28 costimulation, a process required for T cell activation. In vivo data indicate inflammatory bowel disease patients treated with azathioprine have more mononuclear cells that have undergone apoptosis than untreated controls, indicating this mechanism may be responsible for the in vivo response to the drug in this disease.