The Effect of 5-Fluorocytosine on the Blastospores and Hyphae of Candida Albicans

Abstract

5-FLUOROCYTOSINE (5FC) has an important place in the treatment of systemic mycoses in man. Its mode of action was initially explained by an extensive incorporation of the deaminated form, 5-fluorouracil, into fungal RNA, with this leading to a disturbance of the internal amino-acid pool (Polak, 1974). Recently, the inhibition of RNA and DNA syntheses has been reported by Polak and Wain, (1977; and by Diasio, Bennett and Myers (1978). The inhibition of DNA synthesis in Escherichia coli by trimethoprim or by 5-fluorodeoxyuridine is rapidly bactericidal (Cohen et al., 1958 ;Angehrn and Then, 1973) but there is no evidence of 5FC having a rapid fungicidal action on Candida albicans (Scholer and Polak, 1973).Primary resistance to 5FC has been reported by Speller and Davies (1973) and is considered to involve several mechanisms in the pathway of RNA synthesis (Polak and Scholer, 1975), which mechanisms may also affect DNA synthesis. The dimorphism of C. albicans affects the development of resistance to 5FC (Davies and Savage, 1974) and the hyphal form may be associated with the invasive ability of this opportunist fungus (Montes and Wilborn, 1968 ;Cawson and Rajasingham, 1972; Wain, Price and Cawson, 1976~).Because the inhibition of DNA synthesis by 5FC was accompanied by an increase in blastospore cell volume and hyphal length, the effect of the drug on the protein and carbohydrate metabolism of the fungus in its hyphal and blastospore forms has been investigated and studies with radioactive precursors have been undertaken to elucidate the inhibition of DNA and RNA syntheses. MATERIALS AND METHODS Microorganisms

Mechanisms of action of 5-fluorocytosine

“…FURTP is incorporated into fungal RNA in place of uridylic acid (11,15), which alters amino acylation of tRNA in vitro (4) and is believed to cause unbalanced internal pool and disturbed synthesis of proteins and carbohydrates (6,13,14). There is a statistical correlation between incorporation of the fluorinated metabolite into RNA and the antifungal activity of 5-FC, although there are considerable individual variations in incorporation in mutants with similar 5-FC susceptibility (1,7,10).…”

Mode of action of 5-fluorocytosine and 5-fluorouracil in dematiaceous fungi

“…After the formation of 5FU two different pathways have to be considered: incorporation of fluorouridylic acid into RNA and formation of fluorodeoxyuridine rnonophosphate, inhibiting thymidylate synthetase activity and consequently DNA synthesis [7][8][9][10][11].…”

“…After each incubation period the quotient of the number of viable particles and the number of particles found initially was determined. Fungicidal activity is indicated by a quotient smaller than I. intervals cells were collected by centrifugation, and the internal pool was extracted by treatment with 10~ perchloric acid in the cold (4~C), and the RNA was extracted by treatment with 10~o perchtoric acid at 70°C [8,9]. Radioactivity was measured in a Beckman scintillation counter and was expressed as ng pyrimidine per mg isolated RNA.…”

“…After various time intervals samples were taken and the incorporation of radiolabelled phosphate determined according to the method of Polak & Wain [7,8].…”

“…5FC or 5FU was added at 0, 8, and 24h. DNA, RNA, carbohydrate and protein were measured as previously described [8,9].…”