Mode of action of 5-fluorocytosine (5-FC) and mechanisms of resistance to the drug are discussed on the basis of experiments performed with Candida albicans ATCC 26790 and with 50 selected clinical isolates of C. albicans belonging to serological type A or B and representing various degrees and models of 5-FC resistance (sensitivity). Incorporation of 5-fluorouridylic acid into RNA appeared as a prerequisite to antifungal activity, although at a given incorporation rate, growth inhibition varied considerably from one strain to the other. The amino acid pool was unbalanced, and there was evidence for disturbance of protein synthesis. These dysfunctions of RNA probably account for growth inhibition and cell death, whereas up to the present, there was no proof of formation of 5-fluorodeoxyuridylic acid nor of subsequent inhibition of thymidylate synthetase. Incorporation of fluorinated pyrimidine into RNA was lower in normally sensitive type B strains than in normally sensitive ones of type A, whereas the frequency of 5-FC-resistant mutants was the same. The two serological types did not differ in the activity of cytosine permease nor in that of cytosine deaminase. Among 29 clinical isolates with 5-FC resistance (or impaired sensitivity) no instance of cytosine permease deficiency was found. Two isolates (belonging to the serological type A) were deficient in cytosine deaminase, whereas the majority was probably deficient in uridine monophosphate pyrophosphorylase or had a surplus of de novo synthesis of pyrimidines. Relative 5-FC resistance was more common than complete resistance.

Combination pairs of the major systematic antimycotic drugs, amphotericin B (AmphB), 5-fluorocytosine (5-FC) and ketoconazole (Ktz) were administered to mice with experimental candidiasis, cryptococcosis and aspergillosis at a variety of combination ratios. The 3 mycoses were produced with 3 strains each of Candida albicans, Cryptococcusneoformans, and Aspergillus jumigatus, respectively, which were preselected to represent 3 different degrees of 5-FC sensitivity (‘normally sensitive’, ‘moderately resistant’, and ‘definitely resistant’). The life-prolonging effect of the combinations was compared with the effect of each partner administered alone at the same and at the double dosage. Using the U test of Mann and Whitney and setting limits which on the whole were more rigorous than those of the isobole methods commonly applied to the study of drug interactions, the effects of the concentrations were classified as ‘synergistic’, ‘additive’, ‘indifferent’ or ‘antagonistic’. The combination AmphB plus 5-FC was definitely synergistic or definitely additive in all 3 candidiasis models, the most pronounced synergism occurring in the infection with the ‘definitely 5-FC-resistant’ C. albicans strain; in cryptococcosis produced by any of the 3 C. neoformans strains the effect was definitely additive, but only slightly additive or indifferent in the 3 aspergillosis models. The combination AmphBplus Ktz was slightly synergistic in candidiasis produced by one C. albicans strain, but definitely antagonistic in this mycosis produced by the remaining 2 strains of the same species; the combination was definitely additive or, even, slightly synergistic in the 3 cryptococcus models, but, again, antagonistic in aspergillosis produced by all 3 strains of A. fumigatus. 5-FC plus Ktz was additive or indifferent in the 3 candidiasis models, but throughout indifferent in cryptococcosis and aspergillosis.

Candida albicans isolates from 402 patients with no prior history of treatment with 5-fluorocytosine were collected at five medical centers from different areas of the United States. Isolates could be separated into four groups based on their minimum inhibitory concentrations (MICs) to 5-fluorocytosine. Group I isolates (60%) had MICs less than or equal to 12.5 micrograms/ml after 7 days, whereas groups II (22%), III (14%), and IV (4%) demonstrated MICs greater than 12.5 micrograms/ml on days 7, 2, and 1, respectively. Serotypes A and B accounted for 50.7 and 49.3%, respectively, of the 398 isolates typed. Serotype B was less prevalent in group I (26%), but predominated in the more resistant groups, groups II (85%), III (86%), and IV (53%). The common practice of identifying as "resistant" those isolates with MICs greater than 12.5 micrograms/ml after 48 h of incubation would yield a resistance rate in the United States of 11.5 to 15.5% in four centers and 35% in the fifth. Although serotype B and small agar disk diffusion zone sizes correlated with poor 5-fluorocytosine susceptibility, their ability to predict tube dilution MICs was limited. The true predictive value of such tests awaits correlation with in vivo studies.

The in vitro susceptibility of Candida albicans isolates to flucytosine was compared to therapeutic effect in experimental murine candidiasis (candidosis). Four groups of 10 isolates were chosen, based upon their broth dilution minimal inhibitory concentrations (MICs), from a group of 402 isolates from patients without prior flucytosine therapy. Group I MICs were less than 12.5 micrograms/ml after seven days, whereas group II, III, and IV MICs exceeded 12.5 micrograms/ml on days 7, 2, and 1, respectively. Pilot experiments selected challenge inocula of similar virulence. Mice were infected intravenously and given various flucytosine doses. Significant prolongation of survival correlated with MICs and with agar disk-diffusion zone diameters (P less than 0.05). In vivo response to therapy was more favorable for group I isolates compared with group IV isolates (P less than 0.01). The present study demonstrates in this animal model that in vitro susceptibility does correlate with in vivo response to therapy, although exceptions occur with individual isolates.

The fungistatic activity of fluorinated pyrimidines, particularly 5-fluorocytosine (5-FC), and the reversal power of bases and nucleosides of the pyrimidines and purine classes were studied, chiefly in Candida albicans. 5-FC was the most potent antifungal agent, followed by 5-fluorouridine. Cytosine, uridine and adenine were the strongest antagonists of 5-FC activity. A direct correlation, independent of the chemical nature of the antagonists, was found between radiolabelled 5-FC taken up in the yeast cells and inhibition of growth. Except for the anatagonistic effect of some purines, particularly adenine, both antifungal and reversal patterns were compatible with the suggested pathway and mechanism of action of 5-FC (formation of 5-fluorouridylic acid after intracellular deamination of the drug to 5-fluorouracil).