The combination of fluconazole 1200 mg/d and flucytosine 100 mg/kg/d was associated with a markedly more rapid rate of clearance of infection compared to fluconazole alone. Indeed, accepting the limitations of comparisons between trials, the EFA of this combination (−0.28 log CFU/ml/d) is the closest an oral antifungal regimen has come to the fungicidal activity of AmB (−0.31 log CFU/ml/d for AmB 0.7 mg/kg monotherapy in Thailand [15
]). This study was not powered for clinical endpoints, however there was a trend toward decreased early mortality in favor of the combination arm. Although this study was too small to conclude a clinical benefit, given the association of EFA with survival in this and in prior analyses [16
] the improvement in fungicidal activity with combination therapy also suggests a survival advantage.
Flucytosine provided an additive effect when combined with fluconazole in murine models of cryptococcal infection [26
] although not in a study in rabbits [29
]. One mouse study evaluated a range of fluconazole and 5FC doses with AmB and determined the optimal regimen combined high doses of fluconazole with lower doses of 5FC [28
]. A clinical trial combining fluconazole 400 mg/day with high dose 5FC (150 mg/kg/day) for 10 weeks produced a relatively short median time to CSF sterilization of 23 days, but there was no control arm, and side effects by 10 weeks were frequent [18
]. Two subsequent trials suggested an additive effect when fluconazole was combined with 5FC. In one, the dose of fluconazole was low (200 mg/day) [8
]. In the second, 5FC 100 mg/kg/d was given for 4 weeks with increasing doses of fluconazole and did have an additive benefit (higher percent of patients alive with CSF culture negative at 10 weeks) that was most pronounced with fluconazole 800 – 1200 mg/day [19
]. As suggested by this latter trial and the earlier mouse data, we used high dose fluconazole and historically low dose 5FC to unequivocally demonstrate the microbiological efficacy of combining 5FC with fluconazole.
Neutropenia, although more frequent in the combination arm compared to fluconazole alone, was not a clinically significant problem in most instances and rarely limited treatment in this study, suggesting 5FC could be used with benefit in resource limited settings. However, our study alone was too small to adequately address toxicity issues. An earlier study using 5FC 100 mg/kg/d (but for 4 weeks instead of 2) found grade IV neutropenia occurring in 18% of patients, without evidence of increased infection [19
]. In an ongoing study in Cape Town using 2 weeks of 5FC with AmB, 4.5% of 66 patients developed grade IV neutropenia (authors’ unpublished data). In Thailand, the same 5FC regimen was not associated with significant neutropenia, and no patients discontinued 5FC before 2 weeks. In that setting, bioavailability of oral 5FC was 50% that of intravenous 5FC, and the serum levels with oral 5FC were well below those associated with bone marrow toxicity [30
]. Analysis of 5FC and 5-fluorouracil (5FU) levels in this trial will determine whether bioavailability or gut conversion of 5FC to 5FU [31
] is different for patients in Africa. It will be important to continue to monitor the tolerability of 5FC in Africa, as more data will determine the optimal hematological monitoring needed for safe expanded use of 5FC in this setting. Any potential for toxicity must be balanced against the advantage of 5FC in antifungal efficacy and possibly survival.
Analysis of serum and CSF fluconazole levels in this trial will clarify the effect of rifampicin on the bioavailability of fluconazole. The association of concomitant rifampicin with reduced rates of fungal clearance implies a clinically significant reduction in fluconazole levels and suggests that, even at relatively high doses, fluconazole should be increased when given with rifampicin.
Donation of fluconazole has allowed countries like Malawi to increase the fluconazole dose recommended in cryptococcal treatment guidelines to 800 mg/day. However, the rate of clearance of infection is unacceptably slow and mortality unacceptably high at this dose [12
]. Even with fluconazole 1200 mg/day – both in this study and in Uganda [12
]– the rate of fungal clearance, although more rapid, was well below that achieved with AmB. Although it is impossible to accurately compare the EFA slopes between trials in different patient populations, the combination of fluconazole 1200 mg/day and 5FC studied in this trial is the most rapidly fungicidal oral regimen to date, and therefore may be the optimal regimen in the absence of AmB.
In many parts of the developing world, limited resources, personnel, and clinical and laboratory facilities prevent use of the standard 2-week induction course of AmB [32
]. Until these problems are resolved, our results argue that wider access to flucytosine should be a priority in resource limited settings. Flucytosine is a simple molecule that is off-patent and was once registered in South Africa. Further studies are needed to evaluate regimens that incorporate, in addition, a short course of AmB, which should require minimal additional monitoring, and be easier to implement compared with standard 2 week AmB courses.