Robert B. Diasio

Evidence for Conversion of 5-Fluorocytosine to 5-Fluorouracil in Humans: Possible Factor in 5-Fluorocytosine Clinical Toxicity

A gas chromatographic-mass spectrometric method for detecting 5-fluorouracil (5-FU) in serum at concentrations as low as 10 ng/ml was used to determine to what extent 5-FU was present in the serum of patients taking oral 5-fluorocytosine (5-FC). Preliminary studies in two patients and two healthy volunteers given an initial 2-g oral dose of 5-FC demonstrated sustained serum 5-FU levels (>100 ng/ml) during the 5 h after ingestion of drug. Pharmaceutical preparations of 5-FC used in these studies were shown to be insignificantly contaminated with 5-FU (<0.03%), suggesting in vivo conversion of 5-FC to 5-FU had occurred. Serum samples from seven patients with cryptococcal meningitis treated with amphotericin B and 5-FC were examined for 5-FU. Five of these patients had experienced hematological or other toxicity attributed to 5-FC at some time during the course of therapy. Of 41 serum samples, 20 were observed to have 5-FU levels greater than 1,000 ng/ml in the range observed with cancer chemotherapeutic doses of 5-FU known to be associated with hematological toxicity. It is concluded that conversion of 5-FC to 5-FU occurs in humans and furthermore that 5-FU may account for some of the toxicity observed with 5-FC.

Rapid determination of serum 5-fluorocytosine levels by high-performance liquid chromatography.

A method for the rapid, quantitative determination of 5-fluorocytosine (5-FC) in serum by high-performance liquid chromatography (HPLC) has been developed. After initially ultrafiltrating the serum, a portion was injected onto a cation exchange column. 5-FC was separated by using an ammonium-phosphate buffer as the mobile phase and detected by ultraviolet absorption at 254 nm. Quantitation of 5-FC was based on the linear relationship between peak area in the chromatograms and known concentrations of 5-FC in a set of serum standards (prepared by adding from 10 to 200 micrograms of 5-FC to 1-ml aliquots of pooled human serum). This method was compared with the standard microbiological method for 5-FC. Advantages of the HPLC method include: Determination of 5-FC levels within 30 min; lack of interference from other antimicrobial drugs, particularly amphotericin B; more accurate determination of true 5-FC level, particularly at concentrations of less than 25 micrograms/ml or greater than 100 micrograms/ml; and ease with which the assay may be automated for routine use.