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

Pharmacokinetics of flucytosine in a critically ill patient on continuous venovenous hemodiafiltration

PURPOSE

To describe the pharmacokinetics of flucytosine in a critically ill patient undergoing continuous venovenous hemodiafiltration (CVVHDF) treated for cryptococcal meningitis.

SUMMARY

A 20-year-old female weighing 93.4 kg with a body mass index of 34.3 kg/m2 with a past medical history of systemic lupus erythematous with diffuse proliferative lupus nephritis (class IV) was admitted to the hospital after several months of worsening dyspnea, fatigue, myalgia, vomiting, and diarrhea. The patient developed worsening renal function and volume overload requiring CVVHDF on hospital day 7. She was diagnosed with cryptococcal meningitis on hospital day 8, and flucytosine 2,500 mg enterally every 12 hours and liposomal amphotericin B 500 mg intravenously every 24 hours were initiated. Flucytosine serum concentrations were collected on day 4 of therapy, and pharmacokinetics were performed on 2 sequential levels. Pharmacokinetic calculations displayed an elimination rate constant of 0.0338 h-1, a volume of distribution between 0.42 and 0.43 L/kg, a half-life of 20.5 hours, and a total drug clearance between 1.32 and 1.36 L/h while on CVVHDF. The nonsequential levels displayed good correlation, and no further monitoring or dosage adjustment was required. The patient completed therapy, with clinical resolution of her infection, and no toxicities due to flucytosine were noted.

CONCLUSION

Flucytosine dosed at 25 mg/kg of actual body weight every 12 hours during CVVHDF conferred therapeutic levels with no appreciable toxicities. Because of its narrow therapeutic index and risk of toxicity, additional pharmacokinetic studies are needed to determine optimal drug dosing of this medication in patients requiring renal replacement therapy.

Simultaneous determination of flucytosine and fluorouracil in human plasma by high-performance liquid chromatography

A validated, sensitive and precise reversed-phase high-performance liquid chromatographic method for the simultaneous determination of 5-flucytosine (5-FC) and 5-fluorouracil (5-FU) in human plasma is described. Two compounds, 5-methylcytosine (5-MC) and 5-chlorouracil (5-CU), were used as internal standards for the determination of 5-FC and 5-FU, respectively. Plasma samples were deproteinized with trichloroacetic acid and chromatographed on an octylsilica column, maintained at 30 degrees C during elution, using a 0.04 M phosphate buffer, pH 7.0, as eleunt. Spectrophotometric diode array detection was used at 266 nm. 5-FC, 5-FU, 5-MC and 5-CU were found to have retention times of 4.8, 5.8, 7.7 and 11.0 min respectively. Recoveries of 91-120% with reproducibility and repeatability coefficients of variation of 0.8-6% were obtained. Mean correlation coefficients of 0.99989 and 0.9995 were found for the linear calibration curves (n = 2) of 5-FC (4.816-192.6 mg/l) and 5-FU (0.05368-5.368 mg/l), respectively. The limits of quantitation were 0.3 mg/l for 5-FC and 0.05 mg/l for 5-FU.

Comparison of high-performance liquid chromatography and bioassay for the determination of 5-fluorocytosine in serum

An HPLC method using a reverse phase system, an isocratic mobile phase and simple protein precipitation and a plate diffusion bioassay using an amphotericin B resistant strain of Candida pseudotropicalis for the measurement of 5-fluorocytosine in serum were compared. Both methods permit a determination of 5-fluorocytosine in sera also in the presence of amphothericin B. The correlation between bioassay and HPLC runs was found to be r = 0.96. Both methods are useful for monitoring the serum level of 5-fluorocytosine in clinical routine.