Alteration of the secondary structure of newly synthesized DNA from murine bone marrow cells by 5-fluorouracil

The present study evaluates the effects of 5-fluorouracil (FUra) on the structure of newly synthesized DNA purified from bone marrow cells. DNA synthesis was decreased by 30 and 45% of control in the presence of 19 and 100 microM FUra, respectively. Furthermore at these concentrations of FUra, the DNA strand sizes were smaller as determined by alkaline sucrose gradients. Enzymatic digestion of the DNA demonstrated that most of the FUra (greater than 90%) was localized in the internucleotide linkage and not at the chain terminus. As the concentration of FUra was varied, the percentage of FUra at the chain terminus was unchanged, suggesting that the decrease in chain size as well as inhibition of DNA synthesis was not due to chain termination. DNA that had been synthesized in the presence of FUra was shown to fragment after increasing time as demonstrated by alkaline sucrose gradient analysis. This time-dependent fragmentation was associated with an increased number of strand breaks as determined by neutral and alkaline sucrose gradient analysis. A parallel study demonstrated a time-dependent excision of FUra from DNA over this same time period. In summary, these studies demonstrate an association between the excision of FUra from DNA and the changes in secondary structure of newly synthesized DNA.

The effect of 5-fluorouracil on DNA chain elongation in intact bone marrow cells

The effect of 5-fluorouracil (FUra) on DNA elongation was assessed in intact bone marrow cells that had been pulsed for 1 hr with [3H]-dThd in the absence or presence of FUra, chased in fresh media from 0 to 3 hr, and then analyzed on alkaline sucrose gradients. While DNA from control cells elongated at an average rate of 86 nucleotides per sec over a 3 hr interval, DNA from FUra-treated cells did not elongate and in contrast decreased in size over the same interval. In a parallel study to examine what happens to the FUra that was incorporated into DNA, bone marrow cells were pulsed for 1 hr with 50 microM [3H]-FUra, and then chased in fresh media from 0 to 2 hr. An aliquot of cells from each time point was lysed on an alkaline sucrose gradient to assess the size of [3H]-FUra-containing DNA, while another aliquot of cells from each time point was analyzed for radioactivity remaining in total DNA. The percentage of replicon-size DNA (greater than or equal to 100S) containing radiolabel decreased over the 2 hr chase while the percentage of small molecular weight DNA (greater than or equal to 7.2S) increased over the same interval. These changes in DNA size were accompanied by a decrease in radioactivity in total DNA. These studies suggest that excision of FUra from nascent DNA chains may prevent further elongation of DNA.

Dihydrofluorouracil, a fluorouracil catabolite with antitumor activity in murine and human cells

Dihydrofluorouracil (FUH2), the initial catabolite of 5-fluorouracil (FUra), was examined to determine whether this derivative had antitumor activity or host cell (bone marrow) toxicity. Studies were undertaken with Ehrlich ascites tumor and bone marrow cells isolated from CF-1 mice. Cells were exposed for 1 h either to no drug (control) or to varying concentrations, ranging from 1 to 250 microM, of either FUra, FUH2, or alpha-fluoro-beta-alanine. Cells were then cultured and colony formation was assessed after 10 to 14 days. Ehrlich ascites tumor cells were more sensitive to FUra [50% lethal dose (LD50) = 18 microM] than to FUH2 [LD50 = 50 microM], with no sensitivity to alpha-fluoro-beta-alanine even at 250 microM. Bone marrow cells had a toxicity profile similar to that of FUra (LD50 = 10 microM) but were relatively insensitive to FUH2 (LD50 greater than 250 microM), with no sensitivity to alpha-fluoro-beta-alanine. Subsequent studies examined colony formation of the human breast carcinoma cell line MCF-7 following 1 h exposure to varying concentrations of FUra and FUH2. These cells were less sensitive to both FUra (LD50 approximately 80 microM) and FUH2 (LD50 approximately 350 microM). Initial studies on the mechanism of toxicity of FUH2 demonstrated that this FUra catabolite could produce inhibition of thymidylate synthase activity in Ehrlich ascites tumor cells with a pattern similar to that resulting from exposure to FUra. This is the first study to demonstrate that FUH2 (a quantitatively important catabolite of FUra) is cytotoxic, and it suggests that FUH2 may contribute to the toxicity of FUra in vivo, possibly by being anabolized to FUra.

Evidence from rat hepatocytes of an unrecognized pathway of 5-fluorouracil metabolism with the formation of a glucuronide derivative

Isolated rat hepatocytes in suspension were exposed to [3H]-5-fluorouracil for intervals over 2 h, following which the cells were removed from the media and sonicated, and the cytoplasm was sampled. High-performance liquid chromatography was used to separate 5-fluorouracil (FUra) from its known anabolites and catabolites, with subsequent quantitation of these metabolites by measurement of radioactivity. As the extracellular concentration of FUra was increased above 30 microM, the intracellular levels of FUra increased, with detection of a new peak of radioactivity distinct from any of the known anabolites or catabolites. This new metabolite, "G," increased in concentration as the extracellular concentration of FUra was raised above 1 mM. Inhibition of FUra catabolism by 2 mM thymine resulted in a further increase in intracellular FUra (approaching the extracellular FUra concentration) and was accompanied by a further increase in the intracellular concentration of "G," demonstrating that "G" was not formed via the catabolic pathway. The increase in intracellular FUra and "G" was not accompanied by an increase in intracellular anabolites, suggesting that "G" was formed via a novel metabolic pathway. "G" was retained within the hepatocytes, although it was not bound to intracellular macromolecules. "G" was converted to FUra in the presence of beta-D-glucuronidase; this reaction was inhibited with the addition of saccharo-1,4-beta-lactone, a specific inhibitor of the beta-D-glucuronidase. This data, together with evidence from hepatocyte homogenates in which formation of "G" was shown to be dependent on the concentration of uridine-5'-diphosphoglucuronic acid, demonstrates that "G" is a glucuronide of FUra. The formation of "G" suggests that FUra is metabolized via a previously unrecognized metabolic pathway.

Modulation of 5-fluorouracil catabolism in isolated rat hepatocytes with enhancement of 5-fluorouracil glucuronide formation

The catabolism of 5-fluorouracil (FUra), which accounts for 90% of the elimination of this antimetabolite in vivo, has recently been characterized in freshly isolated rat hepatocytes in suspension using a highly specific high-performance liquid chromatographic methodology. The present study evaluates the effect of thymine and uracil, which are thought to be catabolized by the same enzymes as FUra, on the metabolism and transmembrane distribution of FUra in isolated rat hepatocytes. Following simulataneous exposure of cells for 5 min to 30 microM [6-3H]FUra and increasing concentrations of either thymine or uracil, dihydrofluorouracil (FUH2) levels decreased in a concentration-dependent manner, and the concentration determined for 50% inhibition of FUra catabolism was 8.0 +/- 0.3 (S.D.) and 67.8 +/- 15.6 microM for thymine and uracil, respectively. Analysis of intracellular and extracellular 3H from 1 min to 2 hr after simultaneous incubation of the hepatocytes with 30 microM FUra and thymine (or uracil) in a 1:7 molar ratio resulted in a decrease of intracellular and extracellular FUH2 and alpha-fluoro-beta-alanine (FBAL), while alpha-fluoro-beta-ureidopropionic acid (FUPA) was enhanced. Unmetabolized FUra (not detected in the absence of thymine or uracil) was detected intracellularly in the presence of thymine or uracil and was accompanied by the appearance of a novel metabolite, preliminarily identified as a glucuronide of the FUra base which reached intracellular levels of 44 +/- 9.76 and 27.45 +/- 1.35 microM in the presence of thymine or uracil, respectively, within 1 hr. This metabolite, which penetrates the cell membrane only slowly, accounted for approximately 60% of the intracellular 3H in the presence of 300 microM FUra and 2 mM thymine, whereas FUra catabolism was inhibited by more than 99% under these conditions. The formation of FUra anabolites was insignificant in the presence of thymine and uracil, and incorporation of FUra into RNA was not enhanced. The lack of anabolism of FUra in isolated hepatocytes exposed to either high initial concentrations of FUra or high intracellular FUra concentrations resulting from modulation (inhibition) of FUra catabolism is consistent with the clinical observation of minimal hepatotoxicity with FUra, despite exposure of the liver to high blood levels. These studies indicate that thymine is a more potent modulator of FUra catabolism in hepatocytes than is uracil. Further studies are needed to clarify the biological importance of the glucuronide of the base FUra which accumulates intracellularly as the concentration of FUra increases within the hepatocytes.

A mathematical model of the kinetics of 5-fluorouracil and its catabolites in freshly isolated rat hepatocytes

A mathematical model for the kinetics of 5-fluorouracil (FUra) catabolism in liver cells is proposed. It is based on published data for the metabolism of FUra by isolated rat hepatocytes. The model relies on biochemical knowledge of the catabolic pathway. The key-steps are: the cellular uptake and the conversion of the unchanged drug to dihydrofluorouracil (FUH2) and subsequently to alpha-fluoro-beta-alanine (FBAL); the cellular fluxes of the 2 catabolites, FUH2 and FBAL. Water is partitioned between the extracellular and intracellular spaces. The first step is described by Michaelis-Menten kinetics and the other processes by first-order kinetics. Satisfactory fitting of the model validates these simplifications and provides values for the parameters describing the process. The model indicates that the kinetics of FUra disappearance are non linear, the Vmax of the first step being between 3.1 and 5.0 microM/min and the Km between 12 and 37 microM; the rate limiting step is the degradation of FUH2 (the major intracellular catabolite) with a rate constant of 0.1 to 0.02 min-1; the FUH2 transmembrane exchange is active; the exchange of the final catabolite FBAL is by diffusion.

5-Fluorouracil incorporation into DNA of CF-1 mouse bone marrow cells as a possible mechanism of toxicity

Isolated CF-1 mouse bone marrow cells were exposed for 1 hr to 5-fluorouracil (FUra) at concentrations from 1.8 to 50 microM and then washed and suspended in a soft agar growth medium to assess the effect on toxicity (measured as reduction in colony growth compared to control). These data were used to determine specific toxic concentrations ranging from 25 to 90% lethal doses. Subsequent studies examined in parallel the effect of these toxic concentrations of FUra on the possible sites of toxicity including: (a) inhibition of thymidylate synthetase activity using a modified 3H release assay; (b) incorporation of FUra into RNA (FUra-RNA); and (c) incorporation of FUra into DNA (FUra-DNA). Thymidylate synthetase activity was slightly decreased (75% of control) after 1-hr exposure to a 50% lethal dose and was not significantly further reduced as the FUra concentration was increased to an 85% lethal dose. Furthermore, subsequent exposure of FUra-treated cells to a nontoxic thymidine dose (5 microM) failed to reverse toxicity. FUra-RNA increased during 1-hr exposure to increasing concentrations of FUra (25 to 90% lethal doses). Although initially suggesting a relationship between the level of FUra-RNA and toxicity, subsequent studies in cells exposed to FUra in the presence of uridine demonstrated a significantly decreased toxicity while, at the same time, a minimal decrease of FUra in RNA. In contrast, FUra-DNA was significantly decreased in the presence of uridine and correlated with decreased toxicity. In additional subsequent studies, an apparent decrease in subsequent DNA synthesis was observed (measured by 32P or [3H]thymidine incorporation into DNA) as the level of FUra-DNA increased. In conclusion, FUra is demonstrated to be incorporated into DNA of isolated CF-1 mouse bone marrow cells, and the level of FUra-DNA appears to be closely associated with toxicity and inhibition of further DNA synthesis. The parallel studies of thymidylate synthetase activity and FUra-RNA suggest that FUra-DNA may be an unrecognized mechanism of FUra toxicity in these cells.

Double-blind multiple-dose crossover study of the antiemetic effect of intramuscular levonantradol compared to prochlorperazine

Twenty cancer patients who received chemotherapy were entered into a double-blind crossover design antiemetic study comparing 1 mg levonantradol, an investigational synthetic cannabinoid, to 10 mg prochlorperazine. Sixteen patients completed the crossover. For each antiemetic course, four doses of each study medication were given intramuscularly 2 hours before chemotherapy and then 2, 6, and 10 hours after chemotherapy administration. There were no statistical differences in patients' responses to levonantradol and prochlorperazine. The frequency of side effects was greater with levonantradol than with prochlorperazine. The most common side effect of levonantradol were somnolence, dry mouth, dizziness, tachycardia, postural hypotension, and blurred vision, while those for prochlorperazine were somnolence, dry mouth, and tachycardia.

5-fluorocytosine susceptibility of pathogenic fungi in the presence of allopurinol: potential for improving the therapeutic index of 5-fluorocytosine

The minimal inhibitory concentration of 5-fluorocytosine in 18 pathogenic fungal isolates was not altered by either allopurinol (100 microM) or oxypurinol (100 microM). Since allopurinol at this level clinically has been demonstrated to interfere with 5-fluorouracil anabolism, thereby reducing toxicity owing to 5-fluorouracil, allopurinol may be useful in counteracting the 5-fluorouracil-induced myelotoxicity observed in patients being treated with 5-fluorocytosine without interfering with the antifungal activity of 5-fluorocytosine.

Decreased immunosuppression associated with antitumor activity of 5-deoxy-5-fluorouridine compared to 5-fluorouracil and 5-fluorouridine

5-Fluorouracil (5-FUra), 5-deoxy-5-fluorouridine (5'dFUrd), and 5-fluorouridine were compared for their relative antitumor activity, their capacity to inhibit leukocyte exudation and macrophage (macrophage) killing of tumor cells in vivo and in vitro, and their ability to induce leukopenia and monocytopenia. 5'dFUrd was less toxic than 5-FUra and exhibited anti-Ehrlich ascites activity over a wider range of drug doses. Inflammatory exudates induced by thioglycollate or pyran were inhibited up to 91% by prior 5-FUra injection but were inhibited not more than 62% by 5'dFUrd. Pyran-induced macrophage inhibition of Ehrlich ascites proliferation in vivo was diminished up to 5-fold by 5-FUra but was never diminished more than 2-fold by 5'dFUrd, while neither agent suppressed in vitro macrophage cytotoxicity of in vivo pyran-activated macrophage. At high doses, 5-FUra reduced white blood cell counts 73%, in contrast to the 8% reduction caused by 5'dFUrd, while at their optimal anti-Ehrlich ascites doses, 5-FUra and 5'dFUrd both lowered white blood cell counts by only 20%. However, 5-FUra caused a severe monocytopenia not seen in animals given injections of comparable doses of 5'dFUrd. Therefore, 5-FUra appeared to inhibit the inflammatory response and antitumor activity by inhibiting the influx of immature macrophage into the peritoneal cavity, not by inhibiting the function of mature effector cells.

Improved measurement of thymidylate synthetase activity by a modified tritium-release assay

Accurate quantitation of thymidylate synthetase activity using a tritium-release assay is dependent upon measurement of only that tritium released from deoxy[5-3H]uridine monophosphate ([3H]dUMP) during the biosynthesis of thymidylate. Removal of remaining [3H]dUMP on completion of the assay by charcoal adsorption and correction for the nonenzymatic release of tritium are necessary. Although over 99% of [3H]dUMP is removed immediately following addition of charcoal, these studies demonstrate that sufficient [3H]dUMP can remain to prevent accurate measurement of low levels of thymidylate synthetase activity. By delaying measurement of radioactivity for at least 24 h following addition of charcoal, this problem is minimized. To account for nonenzymatic release of tritium, a blank containing enzyme extract with omission of +/- ,L-5,10-methylenetetrahydrofolate is demonstrated to be more effective than the commonly used blank in which water is substituted for enzyme extract. In samples containing 5-fluoro-2'-deoxyuridine monophosphate (FdUMP), a potent inhibitor of thymidylate synthetase activity, an alternative blank containing a high concentration of FdUMP (approximately 1 mM) is useful in demonstrating a theoretical maximal or complete inhibition of thymidylate synthetase activity.

Selective activation of 5'-deoxy-5-fluorouridine by tumor cells as a basis for an improved therapeutic index

The intracellular metabolism of a new fluoropyrimidine, 5'-deoxy-5-fluorouridine (5'-dFUrd), was compared with the metabolism of 5-fluorouracil (FUra), 5-fluorouridine (FUrd), and 5-fluoro-2'-deoxyuridine (FdUrd) in freshly isolated bone marrow cells and Ehrlich ascites tumor cells. Following exposure to tumor cells, all four fluoropyrimidines were metabolized to identical products (i.e., FUra, 5-fluorouridine 5'-monophosphate, 5-fluorouridine 5'-diphosphate, 5-fluorouridine 5'-triphosphate, and 5-fluoro-2'-deoxyuridine 5'-monophosphate), all produced an incorporation of FUra into RNA (FUd greater than FUra greater than FdUrd greater than 5'-dFUrd), and all completely inhibited thymidylate synthetase activity by 1 hr. However, in bone marrow cells, very different patterns were observed. 5'-dFUrd accumulated in the cells, but there were no measurable metabolism, no incorporation of FUra into RNA, and no inhibition of thymidylate synthetase activity. In contrast, both FUra and FUrd were metabolized and produced an incorporation of FUra into RNA (2.7 pmol FUra per micrograms RNA and 4.8 pmol FUra per micrograms RNA at 2 hr, respectively) in bone marrow. Only a minor inhibition of thymidylate synthetase activity was detected. FdUrd also was metabolized by bone marrow cells, produced a low level of FUra incorporation into RNA (0.23 pmol FUra per micrograms RNA at 2 hr), and produced a complete inhibition of thymidylate synthetase activity. Since 5'-dFUrd is not directly cytotoxic itself, its superior therapeutic index compared to other fluoropyrimidines may largely reflect the selective activation of 5'-dFUrd by sensitive tumor cells as opposed to bone marrow cells, which can activate FUra, FUrd, and FdUrd.

Oral levonantradol in the treatment of chemotherapy-induced emesis: preliminary observations

Levonantradol, a new synthetic cannabinoid, was examined for antiemetic effectiveness in 27 patients with refractory chemotherapy-induced emesis. Thirty-one courses of levonantradol were administered orally beginning 2 hours before chemotherapy and continuing every 4 hours for a minimum of 12 to 24 hours at one of three different dose levels. At the 0.5-mg dose, 14 patients were evaluable with seven partial response (50 per cent) and one complete response (7 per cent). At the 1.0-mg dose, 11 patients were evaluable with five partial responses (45 per cent) and three complete responses (27 per cent). Only one patient has thus far been treated at the 1.5-mg dose with no response noted. Side effects observed included somnolence (90 per cent), dry mouth (83 per cent), dizziness (67 per cent), decreased concentration (40 per cent), dysphoria (33 per cent), and altered perception (30 per cent). Euphoria ("high") was infrequent (9 per cent). No relationship between dose (0.5 and 1.0 mg) and side effects was observed. There was a suggestion of improved antiemetic efficacy at the 1.0-mg dose. Although this study is preliminary, it appears that levonantradol is a relatively well-tolerated oral antiemetic that deserves further evaluation.

Metabolism and biological activity of 5'-deoxy-5-fluorouridine, a novel fluoropyrimidine

5'-Deoxy-5-fluorouridine (5'-dFUrd; Roche 21-9738) is a recently synthesized antineoplastic agent with therapeutic potential. The sensitivity of Ehrlich ascites tumor cells in CF-1 mice to 5'-dFUrd, as well as to 5-fluorouridine, was established. 5'-dFUrd was a more effective antitumor agent and was less toxic over a wide dosage range (50 to 400 mg/kg) than the other agents tested as measured by: (a) the ability to prevent gross development of inoculated tumor; (b) 45-day survival; and (c) weight change over the treatment period. With use of these sensitive tumor cells, the intracellular metabolism of 5'-dFUrd in vitro was investigated. Utilizing liquid chromatographic methodology for separation of acid-soluble metabolites, the only detectable metabolic products of 5'-dFUrd were FUra, 5-fluorouridine 5'-monophosphate, and 5-fluorouridine 5'-triphosphate. Novel metabolites of 5'-dFUrd were not detectable in the acid-soluble fraction or in plasma isolated from mice given [14C]5'-dFUrd. The formation of FUra appears to result from the action of nucleoside phosphorylase. 5'-dFUrd was shown to have a Km of 0.633 mM for this enzyme isolated from Ehrlich ascites tumor cells, an affinity similar to that for 5-fluoro-2'-deoxyuridine (Km, 0.278 mM) but much lower than that for 5-fluorouridine (Km, 0.049 mM). Incorporation of radiolabeled drug into the acid-insoluble fraction (representing greater than 95% incorporation into RNA) was also significant. 5-Fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) was not detectable as an acid-soluble metabolite. However, significant inhibition of thymidylate synthetase activity was detectable by 20 min in cells incubated with 30 microM 5'-dFUrd, suggesting that FdUMP was produced. The production of both 5-fluorouridine 5'-triphosphate and FdUMP appears dependent on the initial expansion of the FUra pool. This correlates with the inability of 5'-dFUrd to form nucleotide directly due to the absence of a 5'-hydroxyl group. It is concluded that the antineoplastic activity of 5'-dFUrd may be dependent on its enzymatic conversion of FUra. The basis for the possible increase in therapeutic index compared with other fluoropyrimidines may involve the rate and duration of the production of the biologically active nucleotides 5-fluorouridine 5'-triphosphate and FdUMP.

Adenocarcinoma of the pancreas associated with hypoglycemia: case report and review of the literature

The occurrence of profound hypoglycemia in a patient with metastatic adenocarcinoma of the pancreas is reported. In contrast to the four previously reported cases, no suggestion of excess insulin production was found. Metabolic studies in this patient suggest both increased peripheral glucose utilization and decreased hepatic glucose production as contributing factors which promoted the hypoglycemia.

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.

Serum copper and zinc measurement in patients with osteogenic sarcoma

Serum copper level (SCL), serum zinc level (SZL), and SCL/SZL ratio were measured in 18 patients with biopsy-proven osteogenic sarcoma. Measurements were made on sequentially collected serum samples beginning prior to the institution of therapy and continuing periodically until documented relapse. All patients were treated by curative resection and adjuvant therapy consisting of high dose methotrexate (with leucovorin rescue) with or without BCG immunotherapy. The SCL, SZI, and SCL/SZL determinations were made using proton-induced x-ray fluorescence spectrometry. SCL was significantly elevated (p less than .0001) in the 18 patients with primary untreated osteogenic sarcoma )173 +/- 30 microgram/dl) compared with a sex and age-matched normal group (115 +/- 16 microgram/dl). A significantly different SZL was not found, however, so that an elevated SCL/SZL ratio in the osteogenic sarcoma patients was primarily due to the altered SCL. SCL and SCL/SZL did not change significantly following curative surgery or become more abnormal in those patients who developed recurrent disease. The SCL and SCL/SZL were noted to be markedly elevated in those patients receiving BCG therapy, raising concern regarding the specificity of these tests as markers of tumor activity. SCL, SZL, and SCL/SZL did not appear useful as markers of tumor activity in patients with osteogenic sarcoma.

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.