several steps; at first, it is slowly and almost completely converted to 6-mercaptopurine (6-mp) by reductive cleavage of the thioether (–s–). this is mediated by glutathione and similar compounds in the intestinal wall, the liver, and on red blood cells, without the aid of enzymes. 6-mp is metabolized analogously to natural purines, giving thioguanosine triphosphate (tgtp) and thiodeoxyguanosine triphosphate (tdgtp) via thioinosine monophosphate (timp) and several further intermediates. on a second path, the sulfur atom of 6-mp and timp is methylated. the end products of azathioprine metabolism are thiouric acid (38%) and various methylated and hydroxylated purines, which are excreted via the urine. mechanism of action azathioprine inhibits purine synthesis. purines are needed to produce dna and rna. by inhibiting purine synthesis, less dna and rna are produced for the synthesis of white blood cells, thus causing immunosuppression. azathioprine is converted within tissues to 6-mp, some of which is converted, in turn, to 6-thioguanine by the addition of an amino group. both 6-mp and 6-thioguanine are conjugated with ribose, and then phosphorylated to form the nucleotides thioinosinic acid and thioguanylic acid, respectively. these nucleotides masquerade, respectively, as inosinic acid and guanylic acid; the former is the starting point for purine nucleotide biosynthesis, while the latter is one of the building blocks of dna and rna. the nucleotides are incorporated into newly synthesized (but nonfunctional) dna, halting replication. the nucleotides act to inhibit glutamine-phosphoribosyl pyrophosphate amidotransferase (gpat), one of the enzymes involved in purine biosynthesis, one of the earlier steps in the synthesis of dna and rna. they achieve gpat inhibition through a form of negative feedback called product inhibition. because actively replicating cells (such as cancer cells and the t cells and b cells of the immune system) are most active in synthesizing purine, making new dna, these cells are most strongly affected. a portion of the nucleotides is additionally phosphorylated to the triphosphate forms. these bind to gtp-binding protein rac1, blocking synthesis of the protein bcl-xl, thus sending activated t cells and mononuclear cells into apoptosis (programmed cell death). increased apoptosis of mononuclear cells is seen in inflammatory bowel disease patients treated with azathioprine. chemistry azathioprine is a thiopurine linked to a second heterocycle (an imidazole derivative) via a thioether. it is a pale yellow solid with a slightly bitter taste and a melting point of 238–245 °c. it is practically insoluble in water and only slightly soluble in lipophilic solvents such as chloroform, ethanol, and diethylether. it dissolves in alkaline aqueous solutions, where it hydrolyzes to 6-mercaptopurine. azathioprine is synthesized from 5-chloro-1-methyl-4-nitro-1h-imidazole and 6-mercaptopurine in dimethyl sulfoxide. the synthesis of the former starts with an amide from methylamine and diethyl oxalate, which is then cyclized and chlorinated with phosphorus pentachloride; the nitro group is introduced with nitric and sulfuric acid. history azathioprine was synthesized by george herbert hitchings and gertrude elion in 1957 (named bw 57-322) to produce 6-mp in a metabolically active, but masked form, and at first used as a chemotherapy drug. robert schwartz investigated the effect of 6-mp on the immune response in 1958 and discovered that it profoundly suppresses the formation of antibodies when given to rabbits together with antigens. following the work done by sir peter medawar and gertrude elion in discovering the immunological basis of rejection of transplanted tissues and organs, and schwartz's researches on 6-mp, sir roy calne, the british pioneer in transplantation, introduced 6-mp as an experimental immunosuppressant for kidney and heart transplants. when calne asked elion for related compounds to investigate, she suggested azathioprine, which was subsequently found out to be superior (as effective and less toxic to the bone marrow) by calne. in april 1962, with regimens consisting of azathioprine and prednisone, the transplantation of kidneys to unrelated recipients (allotransplantation) was successful for the first time. for many years, this kind of dual therapy with azathioprine and glucocorticoids was the standard antirejection regimen, until ciclosporin was introduced into clinical practice (by calne as well) in 1978. ciclosporin has now replaced some of the azathioprine use due to a longer survival time, especially in heart-related transplantations. moreover, despite being considerably more expensive, mycophenolate mofetil is also increasingly being used in place of azathioprine in organ transplantation, as it is associated with less bone marrow suppression, fewer opportunistic infections, and a lower incidence of acute rejection.