The sites of action of allopurinol in Crithidia fasciculata

Pyrazolopyrimidine metabolism in parasitic protozoa

The pyrazolopyrimidines are purine analogs that are cytotoxic toward and metabolized by several genera of parasitic protozoa, including the Leishmania and the Trypanosoma. Examples of pyrazolopyrimidines that are selectively metabolized by these parasites include allopurinol, allopurinol riboside, 4-thiopurinol, 4-thiopurinol riboside, and formycin B. These pathogenic protozoa are capable of efficient conversion of the pyrazolopyrimidines to the nucleotide level. The pyrazolopyrimidine metabolites which are isomers of inosine monophosphate are subsequently aminated and incorporated as the adenylate analog into RNA. Mammalian cells are incapable of these metabolic transformations. The sulfur containing pyrazolopyrimidines, however, are neither aminated nor incorporated into nucleic acid. The selective metabolism of the pyrazolopyrimidines by the intracellular metabolic machinery of the parasites of the Trypanosomatidae family offers a rational approach to the chemotherapy of the diseases caused by these pathogenic hemoflagellates.

The purine metabolism of protozoa

Protozoa possess a wealth of purine-salvage enzymes, many with unique, or unusual, substrate specificities. As a result, many opportunities for the chemotherapist exist. An exemplification is found in the conversion in schistosomes of allopurinol ribonucleoside to the corresponding ribonucleotide followed by further anabolism to the very toxic 4-aminopyrazolo(3,4-d)pyrimidine 1-ribonucleotide. The same organisms convert another inosine analog, formycin B, to the ribonucleotide, but its inhibitory effects appear to be exercised primarily by inhibition of the organism's adenylosuccinate synthase. A substantial segment of the Phylum Protozoa shows no vestigial traces of ability to synthesize purines de novo although thymidylate synthase appears to be present in many. The absence of other tetrahydrofolate catalyzed reactions suggests that these functions were never acquired.

Studies in the nitroimidazole series. I. Synthesis of azomycin A and related compounds

Allopurinol (4-hydroxypyrazolo[3,4-d]pyrimidine) has been shown to inhibit the growth of Leishmania braziliensis in vitro at concentrations which are attainable in human tissues and body fluids. This compound is believed to act by interdicting the de novo synthesis of pyrimidines, probably through the formation of allopurinol ribotide. Its lack of toxicity makes it a potential candidate for animal experimentation and it may serve as a prototype for other agents with similar mechanisms of action.