Concentrations Of Flucytosine Research Articles

Pharmacokinetics and Pharmacodynamics of Antifungals in Children and their Clinical Implications

Invasive fungal infections are a significant cause of morbidity and mortality in children. Successful management of these systemic infections requires identification of the causative pathogen, appropriate antifungal selection, and optimisation of its pharmacokinetic and pharmacodynamic properties to maximise its antifungal activity and minimise toxicity and the emergence of resistance. This review highlights salient scientific advancements in paediatric antifungal pharmacotherapies and focuses on pharmacokinetic and pharmacodynamic studies that underpin current clinical decision making. Four classes of drugs are widely used in the treatment of invasive fungal infections in children, including the polyenes, triazoles, pyrimidine analogues and echinocandins. Several lipidic formulations of the polyene amphotericin B have substantially reduced the toxicity associated with the traditional amphotericin B formulation. Monotherapy with the pyrimidine analogue flucytosine rapidly promotes the emergence of resistance and cannot be recommended. However, when used in combination with other antifungal agents, therapeutic drug monitoring of flucytosine has been shown to reduce high peak flucytosine concentrations, which are strongly associated with toxicity. The triazoles feature large inter-individual pharmacokinetic variability, although this pattern is less pronounced with fluconazole. In clinical trials, posaconazole was associated with fewer adverse effects than other members of the triazole family, though both posaconazole and itraconazole display erratic absorption that is influenced by gastric pH and the gastric emptying rate. Limited data suggest that the clinical response to therapy may be improved with higher plasma posaconazole and itraconazole concentrations. For voriconazole, pharmacokinetic studies among children have revealed that children require twice the recommended adult dose to achieve comparable blood concentrations. Voriconazole clearance is also affected by the cytochrome P450 (CYP) 2C19 genotype and hepatic impairment. Therapeutic drug monitoring is recommended as voriconazole pharmacokinetics are highly variable and small dose increases can result in marked changes in plasma concentrations. For the echinocandins, the primary source of pharmacokinetic variability stems from an age-dependent decrease in clearance with increasing age. Consequently, young children require larger doses per kilogram of body weight than older children and adults. Routine therapeutic drug monitoring for the echinocandins is not recommended. The effectiveness of many systemic antifungal agents has been correlated with pharmacodynamic targets in in vitro and in murine models of invasive candidiasis and aspergillosis. Further study is needed to translate these findings into optimal dosing regimens for children and to understand how these agents interact when multiple antifungal agents are used in combination.

A prospective descriptive study of cryptococcal meningitis in HIV uninfected patients in Vietnam - high prevalence of Cryptococcus neoformans var grubii in the absence of underlying disease

BackgroundMost cases of cryptococcal meningitis occur in patients with HIV infection: the course and outcome of disease in the apparently immunocompetent is much more poorly understood. We describe a cohort of HIV uninfected Vietnamese patients with cryptococcal meningitis in whom underlying disease is uncommon, and relate presenting features of patients and the characteristics of the infecting species to outcome.MethodsA prospective descriptive study of HIV negative patients with cryptococcal meningitis based at the Hospital for Tropical Diseases, Ho Chi Minh City. All patients had comprehensive clinical assessment at baseline, were cared for by a dedicated study team, and were followed up for 2 years. Clinical presentation was compared by infecting isolate and outcome.Results57 patients were studied. Cryptococcus neoformans var grubii molecular type VN1 caused 70% of infections; C. gattii accounted for the rest. Most patients did not have underlying disease (81%), and the rate of underlying disease did not differ by infecting species. 11 patients died while in-patients (19.3%). Independent predictors of death were age ≥ 60 years and a history of convulsions (odds ratios and 95% confidence intervals 8.7 (1 - 76), and 16.1 (1.6 - 161) respectively). Residual visual impairment was common, affecting 25 of 46 survivors (54.3%). Infecting species did not influence clinical phenotype or outcome. The minimum inhibitory concentrations of flucytosine and amphotericin B were significantly higher for C. neoformans var grubii compared with C. gattii (p < 0.001 and p = 0.01 respectively).ConclusionIn HIV uninfected individuals in Vietnam, cryptococcal meningitis occurs predominantly in people with no clear predisposing factor and is most commonly due to C. neoformans var grubii. The rates of mortality and visual loss are high and independent of infecting species. There are detectable differences in susceptibility to commonly used antifungal drugs between species, but the clinical significance of this is not clear.

Molecular Mechanism of Flucytosine Resistance in Candida lusitaniae : Contribution of the FCY2 , FCY1 , and FUR1 Genes to 5-Fluorouracil and Fluconazole Cross-Resistance

Inactivation of the FCY2 (cytosine permease), FCY1 (cytosine deaminase), and FUR1 (uracil phosphoribosyltransferase) genes in Candida lusitaniae produced two patterns of resistance to flucytosine. Mutant fur1 demonstrated resistance to 5-fluorouracil, whereas mutants fcy1 and fcy2 demonstrated fluconazole resistance in the presence of subinhibitory flucytosine concentrations.

An in vitro Study on the Active Conversion of Flucytosine to Fluorouracil by Microorganisms in the Human Intestinal Microflora

Background: Investigation of the rate of active conversion of flucytosine to fluorouracil by microorganisms in the intestinal microflora. Methods: Active conversion of flucytosine was investigated using viable and nonviable Escherichia coli at different flucytosine concentrations. Additionally, flucytosine conversion was studied in fecal specimens from 3 neutropenic patients at the start of the antimicrobial/antifungal prophylaxis (C/A regimen) and 1 week later. Results: Flucytosine levels decreased by an average of 72, 71 and 72% flucytosine after incubation for 48 h of 10¹⁰ viable E. coli/ml suspension in broth containing 13, 130 and 1,300 mg/l flucytosine, respectively. The decreasing flucytosine levels corresponded approximately to an identical increase in fluorouracil levels. Also, a 44% decrease of flucytosine levels occurred when nonviable E. coli were used, indicating that bacterial viability is not necessary for this conversion. When fecal specimens of 2 patients were investigated prior to the C/A regimen, significant flucytosine conversion occurred, whereas this conversion was not observed in the corresponding fecal specimens after 1 week of C/A regimen. Conclusion: These in vitro experiments showed that extensive flucytosine conversion can occur in the human intestinal microflora by E. coli. Consequently, fluorouracil exposure and fluorouracil-related toxicity may occur in the flucytosine-treated patient.

Flucytosine-fluconazole cross-resistance in purine-cytosine permease-deficient Candida lusitaniae clinical isolates: indirect evidence of a fluconazole uptake transporter.

An unusual interaction between flucytosine and fluconazole was observed when a collection of 60 Candida lusitaniae clinical isolates was screened for cross-resistance. Among eight isolates resistant to flucytosine (MIC >/= 128 micro g/ml) and susceptible to fluconazole (0.5 < MIC < 2 micro g/ml), four became flucytosine-fluconazole cross resistant when both antifungals were used simultaneously. Fluconazole resistance occurred only in the presence of high flucytosine concentrations, and the higher the fluconazole concentration used, the greater the flucytosine concentration necessary to trigger the cross-resistance. When the flucytosine- and fluconazole-resistant cells were grown in the presence of fluconazole alone, the cells reversed to fluconazole susceptibility. Genetic analyses of the progeny from crosses between resistant and sensitive isolates showed that resistance to flucytosine was derived from a recessive mutation in a single gene, whereas cross-resistance to fluconazole seemed to vary like a quantitative trait. We further demonstrated that the four clinical isolates were susceptible to 5-fluorouracil and that cytosine deaminase activity was unaffected. Kinetic transport studies with [(14)C]flucytosine showed that flucytosine resistance was due to a defect in the purine-cytosine permease. Our hypothesis was that extracellular flucytosine would subsequently behave as a competitive inhibitor of fluconazole uptake transport. Finally, in vitro selection of spontaneous and induced mutants indicated that such a cross-resistance mechanism could also affect other Candida species, including C. albicans, C. tropicalis, and C. glabrata. This is the first report of a putative fluconazole uptake transporter in Candida species and of a possible resistance mechanism associated with a deficiency in the uptake of this drug.

In vitro pharmacodynamic characteristics of flucytosine determined by time-kill methods☆

Two Candida albicans isolates, three non- albicans Candida isolates ( Candida glabrata, Candida krusei, and Candida tropicalis), and one Cryptococcus neoformans isolate were evaluated by time-kill methods to characterize the relationship of flucytosine concentrations to antifungal activity and the duration of the post-antifungal effect (PAE). Against Candida and Cryptococcus isolates, flucytosine at concentrations > 1 × MIC exhibited fungistatic (≤99% reduction in CFU) activity over a 24-h time-period. The rate and extent of fungistatic activity of flucytosine against all isolates was generally not increased when 5-FC concentrations exceeded 4 × MIC. A notable PAE was detected for flucytosine against both Candida and Cryptococcus species that persisted 2 to 4 h. These in vitro data suggest that flucytosine is predominately a concentration-independent fungistatic agent at clinically achieved serum concentrations. This pharmacodynamic characteristic coupled with the persistent PAE and the relatively long half-life of flucytosine in humans (>5 h), suggests lower daily dosing may possible without loss of antifungal efficacy.

Therapeutic drug monitoring of systemic antifungal therapy.

The use of systemic antifungal therapy has significantly increased in recent years. Individualization of antifungal therapy through the use of serum or plasma concentrations has been suggested, although no specific recommendations have been developed. The important criteria for therapeutic drug monitoring and which of these criteria are satisfied by systemic antifungal agents are presented in this review. No one antifungal is ideally suited for application of therapeutic drug monitoring, but, under certain circumstances, obtaining serum or plasma concentrations can be justified. In patients who are susceptible to flucytosine toxicity, serum flucytosine concentrations should be monitored in an effort to avoid untoward side-effects. In contrast, therapeutic drug monitoring of amphotericin B is not recommended in the clinical setting. Demonstrating that ketoconazole and itraconazole are reaching the systemic circulation by obtaining serum concentrations may be clinically useful due to the large variability in their absorption and issues of patient compliance which may be seen with these agents. The bioavailability of fluconazole is much less varied although validation of compliance is a situation where obtaining serum concentrations may provide additional information.

Pharmacokinetic optimisation of oral antifungal therapy.

The range of oral antifungal therapy has been expanded recently by the introduction of itraconazole, and terbinafine. These agents have a broader spectrum of activity than griseofulvin and flucytosine, and induce less liver toxicity than ketoconazole. Treatment with these agents may be optimised by application of pharmacokinetic principles. Griseofulvin, ketoconazole and itraconazole should be administered with food to ensure adequate absorption. Maximal absorption of griseofulvin is achieved by administration of the drug as a solid solution in polyethylene glycol. Absorption of azole antifungal agents is impaired by high gastric pH, which is observed in some patients with acquired immunodeficiency syndrome. It is also impaired by frequent vomiting, which commonly occurs in patients with neutropenia. Furthermore, antacids, H2-antagonists and sucralfate interfere with absorption of ketoconazole. The newer oral antifungals are more slowly eliminated and associated with less pronounced drug interactions than ketoconazole. As with ketoconazole, itraconazole and fluconazole influence cyclosporin metabolism. These effects are of clinical relevance and necessitate cyclosporin dosage reduction. However, the cyclosporin dosage reduction required during coadministration of itraconazole and fluconazole (50 to 55%) is less than that required when ketoconazole is concomitantly administered (85%). Monitoring of cyclosporin concentrations during coadministration with these agents is necessary to avoid nephrotoxicity. Drug monitoring is also advisable when phenytoin, carbamazepine or rifampicin (rifampin) are administered concomitantly with azoles, due to a mutual influence on drug metabolism. The antifungal activity of itraconazole is not related exclusively to free drug concentrations. Therefore, the low protein binding of fluconazole does not place this agent at an advantage over itraconazole in the treatment of fungal meningitis. However, terbinafine may be superior to itraconazole for the treatment of tinea unguium, another recalcitrant fungal disease, because terbinafine more rapidly penetrates the nail plate. During repeated use, itraconazole concentrations increase slowly in the nail plate. Steady-state concentrations are reached in the stratum corneum only after several weeks' administration. Following cessation of treatment, terbinafine, itraconazole and ketoconazole concentrations in keratinised tissues decline slowly. This allows a short duration of drug treatment. Some clinical trials suggest that low concentrations of flucytosine, griseofulvin and itraconazole are associated with treatment failure. Flucytosine-induced myelotoxicity also appears to be concentration dependent. This adverse reaction may be caused by fluorouracil (which is produced by metabolism of flucytosine by enterobacillary flora in the gut) rather than by the parent compound.

Performance characteristics of two bioassays and high-performance liquid chromatography for determination of flucytosine in serum

We compared the accuracy and precision of two microbiological methods and one high-pressure liquid chromatography (HPLC) procedure used to measure the concentrations of flucytosine in serum. On the basis of an analysis of six standards, all methods were judged reliable within acceptable limits for clinical use. With the biological methods, a slight loss of linearity was observed in the 75- to 100-micrograms/ml range. Compared with the bioassays, the HPLC method did not present linearity problems and was more precise and accurate in the critical zone of 100 micrograms/ml. On average, results obtained with patient sera containing 50 to 100 micrograms of flucytosine per ml were 10.6% higher with the HPLC method than with the bioassays. Standards for the biological assays may be prepared in serum or water.

Absence of fungistatic antagonism between flucytosine and cytarabine in vitro and in vivo.

Determination of the MIC of flucytosine for 16 wild isolates of Cryptococcus neoformans and Candida spp. in YNBG medium containing 10 mg/l cytosine demonstrated antagonism of fungistatic activity compared with that determined in YNBG alone. Determination of the MIC in YNBG containing 10 mg/l cytarabine showed no change in activity for 14 of the strains, an increase for one and a decrease for another. Determination of serum flucytosine concentrations in a patient receiving cytarabine simultaneously revealed therapeutic levels. Fluctuations in serum flucytosine concentrations were observed in samples taken before, during and after concurrent cytarabine therapy but these may have resulted from unstable renal function rather than in-vivo inactivation of flucytosine by cytarabine. These data do not support significant antagonism of the fungistatic activity of flucytosine by cytarabine.

Ketoconazole and flucytosine alone and in combination against Candida spp. in a neutropenic site in rabbits.

Ketoconazole and flucytosine were administered alone and in combination for ten days to rabbits with four candida isolates growing in subcutaneously implanted semipermeable chambers. The peak concentrations of ketoconazole in serum and in the chamber were 20.3 and 3.8 mg/l, respectively, and the concentrations of flucytosine 47.7 and 37.3 mg/l, respectively. The two drugs combined resulted in better fungistatic activity than either drug alone against all four isolates. Correlation of efficacy in the rabbit model with in-vitro MICs was good for flucytosine, but poor for ketoconazole.

Effect of various concentrations of flucytosine on the accuracy of serum creatinine determinations

Effect of various concentrations of flucytosine on the accuracy of serum creatinine determinations