Binding of oxamniquine to the DNA of schistosomes

Abstract

Hycanthone-sensitive and hycanthone-resistant schistosomes (which are also sensitive and resistant to oxamniquine) were exposed in vitro to tritium-labelled oxamniquine. The initial uptake of the drug into the schistosomes was essentially the same for the 2 strains. The homogenate of worms incubated with tritiated oxamniquine was fractionated and a purified DNA fraction was obtained by ethanol precipitation, RNAase and protease digestion, repeated phenolchloroform extractions, CsC1 gradient centrifugation and extensive dialysis. The DNA fraction from sensitive worms contained radioactive oxamniquine at a level corresponding to about 1 drug molecule per 50,000 base pairs, while the DNA from resistant worms contained essentially no drug. The results support the hypothesis that oxamniquine, like hycanthone, exerts its activity by alkylating macromolecules of sensitive schistosomes. The possibility is discussed that oxamniquine may lack the mutagenic properties of hycanthone because it is not an intercalating agent.

Enzymatic basis for the lack of oxamniquine activity in Schistosoma haematobium infections

“…We have previously shown that sensitive schistosomes possess an enzymatic activity capable of converting HC and OXA to active esters, which, upon dissociation, yield alkylating moieties that form covalent bonds with parasite macromolecules, including DNA (Pica-Mattoccia et al 1989. The assay we developed actually measures the amount of radioactive drug that becomes covalently bound to macromolecules in the presence of the schistosome enzyme.…”

“…It has been shown that both compounds cause an irreversible inhibition of nucleic acid synthesis in sensitive schistosomes but not in resistant ones (Pica-Mattoccia et al 1981;Pica-Mattoccia and Cioli 1985). Both compounds form covalent bonds with the macromolecules of sensitive parasites (Pica-Mattoccia et al 1988, 1989) as a result of metabolic activation dependent on a schistosome enzyme that is absent in resistant parasites and in mammalian hosts .…”

The schistosome enzyme that activates oxamniquine has the characteristics of a sulfotransferase

“…The evidence originates from genetic crosses between OXA-sensitive and OXAresistant S. mansoni showing that resistance is a recessive trait, thus implying the existence of a parasite factor that is required for drug activity . In addition, a soluble extract from sensitive worms (but not from resistant worms) was capable of converting OXA into an alkylating agent that could bind covalently to the DNA and other macromolecules of the parasite (Pica-Mattoccia et al 1989. Extracts from S. japonicum or S. haematobium (which are not sensitive to OXA), as well as extracts from mammalian tissues, failed to activate OXA.…”

Current and Future Antischistosomal Drugs

“…It is remarkable that such a success has been accorded to a compound that is the direct chemical descendant of a previous antischistosomal drug, hycanthone, that was dismissed and abandoned with the ignominious accusation of being a potent mutagen, a potential carcinogen and a suspected teratogen. A look at Fig 1, will convince of the obvious structural similarities between the two drugs (thick lines) and will also show that hycanthone has a 3-ring planar structure typical of DNA-intercalating agents (like acridine dyes), whereas oxamniquine has a simpler structure and has been proven to be devoid of intercalating activity (Pica-Mattoccia et al, 1989). Thus, while hycanthone has become a model for intercalating drugs that produce frameshift mutations (Waring, 1970), oxamniquine has passed all laboratory tests of mutagenicity and -most important-all test of clinical safety during three decades of administration to millions of patients.…”

“…This occurred in the case of sensitive schistosomes, whereas oxamniquine resistant worms failed to bind any significant amount of tritiated drug. Also interestingly, no oxamniquine-DNA binding occurred in the species that are intrinsically resistant to the drug, like S. haematobium and S. japonicum (Pica-Mattoccia et al, 1989).…”