Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Abstract

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Fig. (1)

De novo and salvage pathways to uridine 5′-monophosphate, the precursor of other pyrimidine nucleotides. P,T,C indicate that the pathway or step is utilised in Plasmodium, Toxoplasma and Cryptosporidium respectively. Cryptosporidium is also able to salvage thymidine via a thymidine kinase activity to form dTMP (see also Fig. (2)) and to salvage cytidine via the bifunctional UPRT-UK enzyme to form CMP (not shown). For abbreviations of enzymes (boxed), see text.

Fig. (2)

De novo and salvage pathways to reduced folate, and (dashed boxed) the thymidylate cycle. Polyglutamated forms of folate (known to be the preferred substrates in other organisms, and experimentally demonstrated in P. falciparum [49, 50]) are omitted for clarity. The triphosphate group is removed from 7,8-dihydroneopterin before the DHNA step, but it is not yet known whether this is by a non-enzymatic loss of pyrophosphate and subsequent removal of the final phosphate by a non-specific phosphatase activity, or whether the entire step is enzymatically catalysed. Genes encoding a DHNA activity are as yet unidentified in Plasmodium and Toxoplasma. The oxidation state of salvaged folate is unspecified, although the major form in the human host is 5-methyltetrahydrofolate. Experimental evidence for folate salvage in P. falciparum and Toxoplasma has been established, but is lacking for Cryptosporidium, despite its evident necessity (see text). The targets of current antifolate drugs in clinical use against malaria and toxoplasmosis are DHPS and DHFR. P,T,C indicate that the pathway or step is utilised in Plasmodium, Toxoplasma and Cryptosporidium respectively. For abbreviations of enzymes (boxed), see text.

Fig. (3)

Primary purine salvage pathways. P,T,C indicate that the pathway or step is utilised in Plasmodium, Toxoplasma and Cryptosporidium respectively. In Toxoplasma, the only parasite of the three capable of all of the reactions depicted, the predominant route to the purine nucleoside monophosphates is via AdoK. Numbered enzymes are as follows: 1, AD; 2, AdoD; 3, PNP; 4, AMP deaminase; 5, adenoylsuccinate synthase + adenoylsuccinate lyase; 6, IMPDH; 7, GMP synthetase. Note also the ability of P. falciparum PNP to process 5′-methylthioinosine arising from the polyamine biosynthetic pathway (see text). Figure adapted from [196]. For abbreviations of enzymes, see text.