Figure 3. Purine bases (adenine, guanine and hypoxanthine) are synthesized from simple precursors. Nucleosides are then formed by the addition of sugar moieties (deoxyribose or ribose) and subsequently converted into nucleotides by the addition of phosphate (mono-, di- and triphosphate). Nucleotides take part in cellular metabolism and are the building blocks in the synthesis of RNA and DNA. Structural analogues of the natural substances can specifically block the different metabolic steps. Some examples are given in the figure.
Already in 1948 Elion and Hitchings discovered a substance, diaminopurine, an adenine antagonist, which inhibited the growth of L. casei (Figure 3). It was also found to have an effect on experimentally induced leukemia. Clinical trials in patients were initially promising but had to be interrupted due to toxic side effects. Stimulated by this finding Elion and Hitchings continued their research which soon resulted in two new chemotherapeutic drugs, thioguanine (1950) and 6-mercaptopurine (1951). In collaboration with the Sloan-Kettering Institute 6-mercaptopurine was tried in leukemic patients who were resistant to methotrexate. About one third of the patients responded with complete remission (1953). The finding was soon confirmed, and 6-mercaptopurine (as well as thioguanine) are still used in the treatment of leukemia (Table II).
Elion and Hitchings tried to improve the therapeutic properties of 6-mercaptopurine by using sulphur-substituted compounds. The result was azathioprine (1957) which replaced mercaptopurine as an inhibitor of the immune response. For a long time azathioprine was the only drug available to prevent rejection of transplanted organs. It is still used for that purpose but also for the treatment of autoimmune diseases.
Attempts were also made to prolong the effect of 6-mercaptopurine by blocking its metabolism by xanthine oxidase which is involved in the endogenous production of uric acid (Figure 3). In 1963 this research resulted in another new drug, allopurinol, which blocks the formation of uric acid and therefore is used in the treatment of primary and secondary gout.
Hitchings and collaborators also developed pyrimethamine (1950) and trimethoprim (1956) which were found to be effective in the treatment of malaria and bacterial infections, respectively. Both drugs have a strong affinity to the enzyme dihydrofolate reductase, but pyrimethamine is 2000 times more toxic to the enzyme system in the malaria parasite than in the host. Trimethoprim has 100 000 times higher affinity to the bacterial compared to the human enzyme. An important discovery was that the chemotherapeutic effects of these two compounds were markedly enhanced by sulphonamides, drugs which inhibit the synthesis of folic acid. This pharmacotherapeutic principle is used in the combination drugs trimethoprim-sulfa and pyrimethamine-sulfa which are used in the treatment of bacterial infections and malaria, respectively.
A more recent application of Elion’s and Hitchings’ research philosophy is acyclovir, a drug used in the treatment of infections with herpes virus. Already in the 1950s they had shown that antipurines had to be transformed into nucleotides in order to become active in the cell. The herpes virus carries information which leads to the production of a new enzyme which transforms nucleosides to nucleotides (thymidine kinase) in the infected cell. This enzyme has considerably less substrate specificity than the cell’s normal enzyme. Therefore, acyclovir is transformed into its corresponding nucleotide which is the active antimetabolite and the growth of the virus is inhibited (Figure 3).
Acyclovir was described by Elion and coworkers in 1977 and is a modern example of the realization of the basic ideas from 1950. An even more recent application of these ideas is the development of azidothymidine (AZT) which was described in 1985 by other scientists from the same institute, and which is the hitherto best documented drug in the treatment of AIDS. It can be added that trimethoprim-sulfa is used in the treatment of Pneumocystis carinii, a relatively common complication to AIDS.
The clinical use of the drugs discovered by Elion and Hitchings is summarized in Table II.
G. Gahrton, B. Lundh: Blodsjukdomar. Lärobok i hematologi. Natur och Kultur, Stockholm, 1983.
Läkemedelsboken 1987/88, Apoteksbolaget, Stockholm, sid, 87-88, 150-151, 249, 625.
J.H. Shelley: Creativity in Drug Research. Trends in Pharmacological Sciences. 1983, vol. 4.
L. Stryer: Biochemistry, 3rd edition. W.H. Freeman and Company, San Francisco, 1988, chapter 25, 601-625.
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