Studies on the Mode of Action of 5-Fluorocytosine in <i>Aspergillus</i> Species

Abstract

The mode of action of 5-fluorocytosine (5-FC) was studied in three isolates of pathogenic Aspergillus with varying degrees of susceptibility to the drug. Distribution studies showed that susceptibility or resistance to 5-FC was not dependent on uptake of the drug. While only a small percentage of the total 5-FC taken up was found in the RNA fraction of the cells, most remained in the acid-soluble intracellular pool. 5-FC, 5-fluoro-uracil (5-FU) and 5-fluorodeoxyuridine monophosphate (5-FdUMP) were among metabolites identified in the pool. In addition, fluoroorotic acid appeared to be a major constituent of the metabolites derived from 5-FC. The aspergilli also were capable of utilizing cytosine as a nitrogen source and this is suggested as a possible mechanism of resistance. A dual mode of action for 5-FC in the aspergilli is proposed. This consists of, first, incorporation of 5-FU into RNA and, second, inhibition of DNA synthesis by production of 5-FdUMP.

Noninvasive and quantitative 19F nuclear magnetic resonance study of flucytosine metabolism in Candida strains

“…Investigation of the intrafungal pathway of the drug is an important key to the understanding of the mechanisms of drug action and drug resistance. Previous studies of the FC mode of action have involved invasive techniques requiring extraction procedures and, most often, the use of labeled drugs (5,15,19,20).We used '9F nuclear magnetic resonance (NMR) to study the mode of action of FC. This noninvasive technique allows a direct analysis of heterogeneous biological samples without prior extraction and does not require a labeled drug, as the observed probe, the '9F nucleus, is an inherent part of the parent drug and its metabolites.…”

“…Investigation of the intrafungal pathway of the drug is an important key to the understanding of the mechanisms of drug action and drug resistance. Previous studies of the FC mode of action have involved invasive techniques requiring extraction procedures and, most often, the use of labeled drugs (5,15,19,20).…”

Study of the metabolism of flucytosine in Aspergillus species by 19F nuclear magnetic resonance spectroscopy

“…Moreover, less is known about the metabolic pathways of 5FC in aspergilli than in yeasts, and an intriguing pathway leading to the formation of SFO has been described in Aspergillus sp. (14). The present study was therefore designed to investigate the metabolism of 5FC in two species of Aspergillus (A. fumigatus and A. niger) by '9F NMR.…”

“…Although the metabolic fate of 5FC has been extensively investigated in yeasts (15 and references cited therein), much less is known of it in filamentous fungi of the Aspergillus type (2,8,10, 13,14). Among the studies done, one in particular has held our attention as it proposed an unexpected pathway for 5FC metabolism, with 5-fluoroorotic acid (SFO) being the principal metabolite (14).…”

“…Although the metabolic fate of 5FC has been extensively investigated in yeasts (15 and references cited therein), much less is known of it in filamentous fungi of the Aspergillus type (2,8,10,13,14). Among the studies done, one in particular has held our attention as it proposed an unexpected pathway for 5FC metabolism, with 5-fluoroorotic acid (SFO) being the principal metabolite (14). We thought that fluorine-19 nuclear magnetic resonance ('9F NMR) spectroscopy, already used to study the metabolic transformation of 5FC in Candida yeasts (3,12), could lead to better knowledge of the metabolism of this antifungal agent and confirm or invalidate the results obtained by the authors cited above.…”