Mutagenic Studies of Folic Acid Antagonists

Abstract

Compounds that compete with folic acid (folic acid antagonists [FAAs]) become limited in their usefulness in the treatment of leukemia, malaria, and bacterial infections by the rapid development of resistance. Assays of the plasma levels of certain of these FAAs led to the observation, in about 25% of the determinations, that a higher density of growth of Streptococcus faecium var. durans (ATCC 8043) was obtained at an FAA concentration just below the completely inhibitory level than at one-half this concentration. This and other considerations suggested that FAAs may act not only as selective agents for resistant organisms but also as mutagens. Seven FAAs including amethopterin, pyrimethamine, trimethoprim, chlorguanide triazine, an experimental quinazoline, WR-158,122, and two experimental triazines, WR-99,210 and WR-38,839, were tested for mutagenicity in the Salmonella reversion assay developed by Ames et al. (1975). All were found to be negative for strains TA1535, TA1537, TA1538, TA98, and TA100, both with and without microsomal activation. These compounds were then tested as mutagens for three traits in the folic acid-requiring S. faecium . FAAs were shown to cause mutations to folic acid independence, rifampin resistance, and FAA resistance. It is postulated that the FAAs induce mutations by causing thymine deprivation in the folic acid-requiring host.

Antimicrobials as promoters of genetic variation

“…Exposure to b-lactam antibiotics also induces the dinB gene and mutagenesis via a SOS-independent pathway [27 ]. On the contrary, the mechanism of the trimethoprim-induced mutation has been attributed to nucleotide pool imbalance [32] as DNA-polymerases replicate with reduced fidelity when faced with nucleotide pool alterations, increasing mutation rate. Moreover, sublethal doses of streptomycin, which interfere with ribosome function and inhibit translation, result in inaccurate translation [42,43] and mistranslation of DNA repair and replication proteins, creating transient mutator states [44].…”

“…Many antibiotics can increase the mutation rate in different ways, including oxidative damage [25 ], SOS response [26 ,27 ,28,29,30,31], nucleotide-pool misbalancing [32], and general stress responses (for a review see [33 ]). …”

Relative sensitivity of fish and mammalian cells to the antibiotic, trimethoprim: cytotoxic and genotoxic responses as determined by neutral red retention, Comet and micronucleus assays

“…In this context, available information pertaining to genotoxic potential of TRIMP is not conclusive. Using a range of bacterial and mammalian systems or assays, both positive and negative responses have been reported in the scientific literature (Stevenson et al 1973;Genther et al 1977;Galloway et al 1998;Rasool et al 1987;Abou-Eisha et al 1999;Abou-Eisha 2006). In addition, despite the fact that genotoxic potency of chemicals is closely tied to their toxicity, which could be determined by different endpoints, there has been very limited information relating to the toxic potential of TRIMP at cellular or sub-cellular levels.…”

Genetic effects of deoxyribonucleotide pool imbalances

“…Certain chemicals may appear negative in such tests, not because the particular compounds are truly nonmutagenic, but because they produce secondary effects that interfere with the detection of any induced mutants. For example, the folate antagonist methotrexate has been found to be negative in conventional bacterial plate assays for mutagenicity [Herbold and Buselmaier, 1976;Benedict et al, 1977;Genther et al, 1977;Matheson et al, 1978;Seino et al, 19781. In growing cells, antifolates cause requirements for a purine source, various amino acids, and thymidine through inhibition of one-carbon metabolism [Cohen and Barner, 1956;Blakley, 19691.…”