Population characteristics of hepatic dihydropyrimidine dehydrogenase activity, a key metabolic enzyme in 5-fluorouracil chemotherapy

Dihydropyrimidine dehydrogenase is the initial rate-limiting enzyme in catabolism of 5-fluorouracil, one of the most widely used cancer chemotherapeutic agents. Previous studies have shown the clinical importance of determination of dihydropyrimidine dehydrogenase in patients with cancer, particularly in those with deficiency of this enzyme who experience severe 5-fluorouracil-associated toxicity (including death) after 5-fluorouracil treatment. In the present study, dihydropyrimidine dehydrogenase activity was determined in 138 donor liver samples. A normal distribution of human liver dihydropyrimidine dehydrogenase activity was shown. The mean enzyme activity was compared across different groups by age, gender, and race, with no significant differences observed except that the mean enzyme activity in women was slightly higher than that observed in men. With use of a polyclonal antibody generated against human liver dihydropyrimidine dehydrogenase, Western blot analysis showed that dihydropyrimidine dehydrogenase protein in the liver cytosol correlated with the activity of this enzyme.

Absorption, tissue distribution and in vivo stability in rats of a hybrid antisense oligonucleotide following oral administration

In vivo stability and oral bioavailability of an oligodeoxynucleotide phosphorothioate containing segments of 2'-O-methyloligoribonucleotide phosphorothioates at both the 3'- and 5'-ends (hybrid oligonucleotide) were studied. A 25-mer 35S-labeled hybrid oligonucleotide was administered to rats by gavage at a dose of 50 mg/kg body weight. HPLC analysis revealed that this hybrid oligonucleotide was stable in the gastrointestinal tract for up to 6 hr following oral administration. Radioactivity associated with the hybrid oligonucleotide was detectable in portal venous plasma, systemic plasma, various tissues, and urine. Intact hybrid oligonucleotide was detected, by HPLC analysis, in portal venous plasma, systemic plasma, and various tissues. The majority of the radioactivity in urine was associated with degradative products with lower molecular weights, but the intact form was also detected. In summary, the hybrid oligonucleotide was absorbed intact through the gastrointestinal tract, indicating the possibility of oral administration of oligonucleotides, a finding that may be important in the development of antisense oligonucleotides as therapeutic agents.

In vivo stability, disposition and metabolism of a "hybrid" oligonucleotide phosphorothioate in rats

Oligodeoxynucleotide phosphorothioates containing segments of 2'-O-methyloligoribonucleotide phosphorothioates at both 3'- and 5'-ends (hybrid oligonucleotide) have been shown to be potent antisense agents. In the present study, in vivo biostability, disposition, and excretion of a 25-mer hybrid oligonucleotide were determined in rats after i.v. bolus administration of the 35S-labeled oligonucleotide at a dose of 30 mg/kg. The plasma disappearance curve for the hybrid oligonucleotide could be described by a two-compartmental model, with half-lives of 0.34 and 52.02 hr, respectively. The majority of the radioactivity in plasma was associated with the intact hybrid oligonucleotide. Urinary excretion represented the major pathway of elimination, with 21.98 +/- 3.21% (mean +/- SD) of the administered dose excreted within 24 hr and 38.13 +/- 2.99% over 240 hr post-dosing. The majority of the radioactivity in urine was associated with the degradative products with lower molecular weights, but the intact form was also detected by HPLC analysis. Fecal excretion was a minor pathway of elimination with 2.34 +/- 0.13% of the administered dose excreted over 24 hr and 6.74 +/- 0.40% over 240 hr post-dosing. A wide tissue distribution of hybrid oligonucleotide was observed based on radioactivity levels, and analysis by HPLC showed that the majority of the radioactivity in tissues was associated with the intact hybrid oligonucleotide. Further analyses of the experimental data provided a comprehensive pharmacokinetic analysis of hybrid oligonucleotide in each tissue. Compared with a previously examined oligodeoxynucleotide phosphorothioate (GEM 91) that has a similar nucleotide sequence, the hybrid oligonucleotide had a shorter distribution half-life and a longer elimination half-life, based on the quantitation of radioactivity in plasma. Although it had a similar tissue distribution pattern compared with other oligonucleotide phosphorothioates such as GEM 91, the hybrid oligonucleotide was more stable in vivo, which may be important in the development of antisense oligonucleotides as therapeutic agents.

In vivo stability and disposition of a self-stabilized oligodeoxynucleotide phosphorothioate in rats

The use of antisense oligonucleotides represents a novel, genetically based therapy. The biostability and pharmacokinetics of a 33-mer self-stabilized oligodeoxynucleotide with significant anti-HIV activity was determined in rats after intravenous administration of [35S]oligodeoxynucleotide. Plasma disappearance of the labeled oligodeoxynucleotide could be described by a two-compartment model, with half-lives of 0.54 and 41.44 h. The oligodeoxynucleotide in plasma remained mainly intact. Urinary excretion represented the major elimination pathway, with approximately 27% of the administered dose excreted within 24 h and 57% over 240 h. The majority of radioactivity in urine was attached to degradative products. Fecal excretion was a minor elimination pathway. A wide tissue distribution of the oligonucleotide was observed, with the majority of radioactivity in most tissues being intact. Compared with other linear oligonucleotide phosphorothioates, the self-stabilized oligonucleotide was more stable in vivo, which may be important in development of antisense oligonucleotides as therapeutic agents.

In vivo stability and disposition of a self-stabilized oligodeoxynucleotide phosphorothioate in rats

The use of antisense oligonucleotides represents a novel, genetically based therapy. The biostability and pharmacokinetics of a 33-mer self-stabilized oligodeoxynucleotide with significant anti-HIV activity was determined in rats after intravenous administration of [35S]oligodeoxynucleotide. Plasma disappearance of the labeled oligodeoxynucleotide could be described by a two-compartment model, with half-lives of 0.54 and 41.44 h. The oligodeoxynucleotide in plasma remained mainly intact. Urinary excretion represented the major elimination pathway, with approximately 27% of the administered dose excreted within 24 h and 57% over 240 h. The majority of radioactivity in urine was attached to degradative products. Fecal excretion was a minor elimination pathway. A wide tissue distribution of the oligonucleotide was observed, with the majority of radioactivity in most tissues being intact. Compared with other linear oligonucleotide phosphorothioates, the self-stabilized oligonucleotide was more stable in vivo, which may be important in development of antisense oligonucleotides as therapeutic agents.

Pharmacokinetics and tissue distribution in rats of an oligodeoxynucleotide phosphorothioate (GEM 91) developed as a therapeutic agent for human immunodeficiency virus type-1

An antisense oligodeoxynucleotide phosphorothioate, namely gene expression modulator 91 (GEM 91), has been demonstrated to have significant anti-human immunodeficiency virus activity in various tissue culture models. The present study was undertaken to determine the pharmacokinetics and tissue distribution of GEM 91 in rats following i.v. bolus administration of 35S-radiolabeled GEM 91. Plasma disappearance curves for GEM 91-derived radioactivity could be described by the sum of two exponentials, with half-lives (mean +/- SEM) of 0.95 (+/- 0.07) and 47.57 (+/- 14.48) hr. Urinary excretion represented the major pathway of elimination of GEM 91, with 26.67 +/- 6.46% (mean +/- SD) of the administered dose excreted within 24 hr and 58.12 +/- 4.36% over 240 hr after GEM 91 administration. Fecal excretion was a minor pathway of elimination of GEM 91 with 1.4 +/- 0.62% (mean +/- SD) of the administered dose excreted over 24 hr and 8.54 +/- 0.64% over 240 hr. A wide tissue distribution of GEM 91 was observed. During the initial 30 min, the highest levels of tissue radioactivity were found in the kidney, liver, spleen, lungs, and heart. Radioactivity was retained over longer time periods in the kidneys, liver, heart, and intestine. Analyses of the extracted radioactivities from plasma, kidney, and liver by gel electrophoresis showed the presence of both intact GEM 91 and degradative products with smaller molecular weights. Radioactivity in urine was found to be degradative metabolites of GEM 91. Based on the experimental data, pharmacokinetic parameters for GEM 91 in each tissue and biological fluids were calculated using computer-based two-compartmental i.v. bolus or absorption models. This study is important not only in providing the basis for future studies of GEM 91 in humans, but also in understanding the pharmacology and toxicology of antisense oligodeoxynucleotide phosphorothioates, in general.

Glycine and taurine conjugation of bile acids by a single enzyme. Molecular cloning and expression of human liver bile acid CoA:amino acid N-acyltransferase

In order to establish whether a single enzyme in human liver was capable of conjugating bile acids with both glycine and taurine, a cDNA encoding human liver bile acid-CoA:amino acid N-acyltransferase (hBAT) has been isolated and characterized. A specific immunoaffinity-purified rabbit anti-hBAT polyclonal antibody was used to screen a lambda Zap XR human liver cDNA library resulting in the isolation of two unique clones. hBAT8 and hBAT9 (1669 and 1491 base pairs in length, respectively) were isolated following screening of 4 x 10(5) clones of the cDNA library. Restriction mapping and sequence analysis demonstrated that the cDNAs were identical except hBAT8 contained an additional 178 bases of 5' sequence; hBAT8 was completely sequenced, characterized, and used for all subsequent studies. hBAT8 consisted of a 184-nucleotide 5'-nontranslated region, an open reading frame of 1,254 bases predicting a protein of 418 amino acids with a molecular mass of 46,296 Da, and a 3'-nontranslated region of 209 nucleotides followed by a poly(A)+ tail. The identity of the cDNA was confirmed by the following findings: 1) the open reading frame began with an ATG codon and was followed by a nucleotide sequence which, when translated, corresponded exactly to the first 17 NH2-terminal amino acids of purified human liver BAT; 2) cytosol of Escherichia coli XL1-Blue cells transfected with hBAT8 subcloned into an expression vector, pKK233-2, demonstrated significant enzymatic activity for the conjugation of both taurine and glycine with cholic acid; 3) bacterial expression of hBAT8 generated a protein that comigrated with hBAT from human liver during SDS-polyacrylamide gel electrophoresis and cross-reacted with a specific polyclonal rabbit anti-hBAT antibody during immunoblot analysis; 4) kinetic characteristics of the expressed enzyme were very similar to those reported for purified liver BAT. These data demonstrate that a single cDNA is present in human liver which codes for a protein capable of catalyzing the conjugation of cholic acid with both glycine and taurine.

Dihydropyrimidine dehydrogenase activity in human peripheral blood mononuclear cells and liver: population characteristics, newly identified deficient patients, and clinical implication in 5-fluorouracil chemotherapy

Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the catabolism of 5-fluorouracil (FUra), one of the most widely used anticancer drugs. Previous studies from our laboratory demonstrated the clinical importance of DPD in cancer patients (G. D. Heggie, J-P. Sommadossi, D. S. Cross, W. J. Huster, and R. B. Diasio. Cancer Res., 47: 2203-2206, 1987; B. E. Harris, R. Song, S-j. Soong, and R. B. Diasio. Cancer Res., 50: 197-201, 1990), particularly in those with DPD deficiency who experience severe FUra toxicity (including death) following FUra treatment [R. B. Diasio, T. L. Beavers, and J. T. Carpenter. J. Clin. Invest., 81: 47-51, 1988; B. E. Harris, J. T. Carpenter, and R. B. Diasio. Cancer (Phila.), 68: 499-501, 1991]. We now suggest that measurement of DPD activity may be useful in routine screening of cancer patients prior to FUra treatment. In this paper, we describe the following serial studies: (a) we developed a sensitive, accurate, and precise DPD assay and a storage method to stabilize DPD activity, permitting large scale DPD screening in cancer patients; (b) we demonstrated a normal distribution (Gaussian distribution) of human DPD activity from peripheral blood mononuclear cells (PBM-DPD) in a population study. Baselines for PBM-DPD with fresh and frozen samples were 0.425 +/- 0.124 (SD) and 0.189 +/- 0.064 nmol/min/mg protein, respectively. The 95% and 99% distribution ranges for both fresh and frozen samples were also determined, providing criteria for detection of DPD-deficient patients; (c) we identified nine new patients with profound or partial DPD deficiency; (d) we determined a baseline for human liver DPD activity, which was shown to be 0.360 +/- 0.182 nmol/min/mg protein (frozen samples); (e) we did a preliminary evaluation of liver DPD from deficient patients. Low liver DPD activity in two deficient patients correlated with low PBM-DPD activity. Using a polyclonal antibody raised against human liver DPD in our laboratory (Z. Lu, R. Zhang, and R. B. Diasio. J. Biol. Chem., 267: 17102-17109, 1992), Western blot analysis demonstrated decreased DPD protein in the liver cytosol from DPD-deficient patients compared to normal subjects. These results may be useful in improving the effectiveness and/or lessening the toxicity of FUra chemotherapy.

Comparison of dihydropyrimidine dehydrogenase from human, rat, pig and cow liver. Biochemical and immunological properties

Dihydropyrimidine dehydrogenase (DPD, EC 1.3.1.2) is the initial and rate-limiting enzyme in the catabolic pathway of pyrimidines and has an important role in cancer chemotherapy with fluoropyrimidine drugs. Recently, we purified and characterized this enzyme from human liver and raised a rabbit polyclonal antibody against the purified human enzyme (Lu et al., J Biol Chem 267: 17102-17109, 1992). In the present study, using this purification procedure, DPD was purified to homogeneity from three other mammalian species, i.e. pig, rat, and cow. Comparison of the biochemical properties of these purified enzymes was conducted. Molecular masses of DPD from human, pig, rat, and cow liver were: 210, 204, 210, and 216 kDa, respectively. DPD from all four species appeared to be composed of two subunits. The apparent pI values were 4.6, 4.8, 4.85, and 5.25, respectively. Kinetic studies with uracil, thymine, 5-fluorouracil, and NADPH were carried out with the purified DPD preparation, suggesting species differences in kinetic parameters. Amino acid composition of these purified enzymes also demonstrated slight species differences. In the present study, a rabbit polyclonal antibody against rat liver DPD was raised. Using polyclonal antibodies against human and rat liver DPD, immunoblotting demonstrated cross-reactivity among the four species. In summary, purification and comparison of DPD from different mammalian species will provide a basis for further biochemical and molecular studies of this enzyme.

Phase I trial of low-dose, prolonged continuous infusion fluorouracil plus interferon-alfa: evidence for enhanced fluorouracil toxicity without pharmacokinetic perturbation

PURPOSE

To determine the maximum-tolerable dose (MTD) of fluorouracil (5-FU) administered as a low-dose, prolonged continuous intravenous infusion (PCI) plus interferon-alfa (IFN-alpha) that would permit treatment for at least 28 consecutive days, and to determine the effect of IFN-alpha on 5-FU pharmacokinetics.

PATIENTS AND METHODS

Twenty-six assessable patients with advanced cancer received low-dose PCI 5-FU (150, 200, 250, and 300 mg/m2/d) plus IFN-alpha, 5 x 10(6) IU/m2 administered subcutaneously (SC) at hour 48 of the 5-FU infusion, then thrice weekly thereafter in cohorts of at least three patients. Treatment continued until treatment-limiting toxicity (TLT) developed, such as mucositis, diarrhea, or fatigue. Escalation to the next 5-FU dose level occurred if none of three or zero to two of six patients developed TLT before day 28. Quantitation of plasma 5-FU concentration by high-performance liquid chromatography was performed in 15 patients. Data were standardized using the Cosinor method and compared before and after IFN-alpha administration using the paired t test.

RESULTS

The mean number of days of continuous 5-FU therapy for patients receiving 150, 200, 250, and 300 mg/m2/d of 5-FU plus IFN alfa-2a (IFN-alpha 2a) was 75, 54, 37, and 22 days, respectively. The MTD of PCI 5-FU by our criteria that could be combined with IFN-alpha was 250 mg/m2/d. Comparison of the standardized pharmacokinetic data showed no significant effect of IFN-alpha on plasma 5-FU concentration, and no alteration of the normal circadian variation in plasma 5-FU concentration that was evident before IFN-alpha administration. Objective response occurred in patients with adenocarcinoma of the pancreas (n = 3), kidney (n = 2), and lung (n = 1).

CONCLUSION

IFN-alpha substantially enhanced the gastrointestinal toxicity of low-dose PCI 5-FU without affecting 5-FU pharmacokinetics, contrary to previous reports using alternative 5-FU schedules in which IFN-alpha-related enhancement of 5-FU toxicity was attributable to reduced 5-FU clearance. Our findings suggest that under certain conditions, mechanisms other than altered 5-FU pharmacokinetics may be responsible for the ability of IFN-alpha to enhance the toxic effects of 5-FU.

Relationship between circadian-dependent toxicity of 5-fluorodeoxyuridine and circadian rhythms of pyrimidine enzymes: possible relevance to fluoropyrimidine chemotherapy

Previous studies in experimental animals and patients have suggested a circadian variation in host toxicity following administration of 5-fluorodeoxyuridine (FdUrd) although the biochemical mechanisms are not fully understood. Thymidine kinase (TK; EC 2.7.1.21), the initial enzyme in the thymidine-phosphorylation pathway, is the first enzyme in the anabolism of FdUrd. Dihydropyrimidine dehydrogenase (DPD; EC 1.3.1.2), is the rate-limiting enzyme in the pyrimidine catabolic pathway and has been shown to be the key enzyme in FdUrd catabolism. The present study examined the relationship between the suggested circadian variation in FdUrd toxicity and potential circadian variations in the activity of these enzymes. Initial studies in Sprague-Dawley rats confirmed that the time of FdUrd administration affected death rate and other drug-related toxicities including loss of body weight, diarrhea, and bone marrow suppression, with the least toxicity and highest survival rate being observed in rats receiving FdUrd at 12:00 noon and 4:00 p.m. and the greatest toxicity and lowest survival rate at 12:00 midnight and 4:00 a.m. Statistical analysis revealed a circadian pattern in FdUrd toxicity (Cosinor analysis, P < 0.001). In subsequent studies with the same species, we simultaneously measured TK and DPD activities in several tissues at various times over 24 h. Under standardized light conditions (lights on, 6:00 a.m. to 6:00 p.m.; lights off, 6:00 p.m. to 6:00 a.m.), with sampling at 4-h intervals (4:00 and 8:00 a.m.; 12:00 noon; 4:00 and 8:00 p.m., and 12:00 midnight), a circadian variation in TK activity was observed (P < 0.0001, Cosinor analysis) in bone marrow, intestinal mucosa, liver, and spleen. In the same group of animals, a circadian pattern of DPD activity in liver and bone marrow was also observed (Cosinor analysis, P < 0.0001) that was inverse compared to the circadian variation in TK activity (Pearson correlation analysis, P < 0.05). Further statistical analysis indicated that the observed circadian variation in FdUrd toxicity was correlated with the circadian variation of TK activity and inversely correlated with DPD activity (Pearson correlation analysis, P < 0.05). Based on the above data, we conclude that the circadian pattern of TK and DPD activity may explain the observed circadian variation in toxicity as the time of FdUrd administration is varied. These results may be useful in the design of improved chemotherapeutic regimens using time-modified administration of FdUrd.

A mathematical model of the kinetics and tissue distribution of 2-fluoro-beta-alanine, the major catabolite of 5-fluorouracil

2-Fluoro-beta-alanine (FBAL) is the major metabolite of 5-fluorouracil (FUra), one of the most widely used anticancer drugs. It has been suggested previously that FBAL and/or its metabolites may have a role in the hepatotoxicity, neurotoxicity and cardiotoxicity resulting from FUra chemotherapy. Studies in patients and experimental animals have demonstrated that FBAL has a prolonged elimination compared with the parent drug, FUra. In the present manuscript, a mathematical model is developed for the kinetics and tissue distribution of FBAL. This model is based on recently published data from a study of the pharmacokinetics and disposition of FBAL in rats (Zhang et al., Drug Metab Dispos 20: 113-119, 1992). Satisfactory agreement was achieved between predicted and measured values, permitting an accurate evaluation of the kinetic and distribution parameters for FBAL. This model indicates that: (1) FBAL accumulates in several tissues including brain, heart, spleen, and enterohepatic system; and (2) enterohepatic circulation of FBAL and its bile acid conjugates has an important role in FBAL kinetics and distribution as demonstrated by a model in which enterohepatic circulation parameters were deleted.

Circadian rhythm of rat spleen cytoplasmic thymidine kinase

The activity of thymidine kinase (TK, EC 2.7.1.21), the first enzyme of the thymidine phosphorylation pathway, was measured at various times over a 24-hr period in the spleens of Sprague-Dawley rats that had been housed under standardized conditions of light and dark for at least 4 weeks before the study. Spleen cytoplasmic TK activity was assayed with [2-14C]thymidine as substrate. Under "normal" light conditions (lights on 6:00 a.m.-6:00 p.m. and lights off 6:00 p.m.-6:00 a.m.), a circadian variation of TK activity was observed (P < 0.0001), Cosinor analysis) with peak activity (1.98 nmol product/hr/mg protein) at 1:00 a.m. (19 hr after light onset, HALO) and trough activity (0.40 nmol product/hr/mg protein) at 1:00 p.m. (7 HALO). Maximum enzyme activity exceeded minimum activity by approximately 5-fold. Reversing the light-dark cycle resulted in a corresponding shift in TK activity. Under these "reverse" conditions (lights on 6:00 p.m.-6:00 a.m. and lights off 6:00 a.m.-6:00 p.m.), a circadian variation in TK activity was also observed (P < 0.0001, Cosinor analysis) with peak activity (1.14 nmol product/hr/mg protein) at 12:00 noon (18 HALO) and trough activity (0.32 nmol/hr/mg protein) at 12:00 a.m. (6 HALO). Maximum enzyme activity exceeded minimum activity by approximately 4-fold. In summary, this study demonstrated for the first time that TK activity varies over a 24-hr period in association with the light-dark cycle.

Identification of bile acid coenzyme A synthetase in rat kidney

Bile acid CoA synthetase has been discovered in rat kidney. Incubation of kidney microsomes with [14C]chenodeoxycholic acid and CoA produced a single peak with the high performance liquid chromatography (HPLC) retention time of CDC-CoA. This peak, when incubated with purified bile acid CoA: amino acid N-acyltransferase (BAT) from human liver and either taurine or glycine, led to the formation of CDC-taurine or CDC-glycine, respectively. Kinetic analysis revealed apparent Kms for CDC and CoA of 2.5 microM and 2.6 microM, respectively. This activity appeared specific for bile acids as it was not inhibited by benzoic acid or salicylic acid, known substrates for other rat kidney CoA synthetases. This demonstrates that the kidney has the potential for bile acid metabolism and may have a role in bile acid physiology.

Purification and characterization of dihydropyrimidine dehydrogenase from human liver

Although dihydropyrimidine dehydrogenase has been purified to varying degrees from several species, very little is known about the human enzyme. The importance of this enzyme has recently been shown with cancer chemotherapy, particularly in patients with genetic deficiency of this enzyme. In the present study, this enzyme was purified 7800-fold to homogeneity from human liver by introducing several novel methods including chromatofocusing, HPLC gel filtration, reversed-phase HPLC for the enzyme assay. Purified human enzyme has a molecular mass of 210 +/- 5 kDa and appears to be composed of two subunits. The apparent pI is pH 4.6 (+/- 0.2). The human enzyme contains approximately four flavin nucleotide molecules (two each of FAD and FMN) and 33 iron atoms per molecule of enzyme. Kinetic studies with uracil, thymine, 5-fluorouracil, and NADPH were carried out. Amino acid composition and the N-terminal amino acid sequence of this enzyme were reported. A rabbit polyclonal antibody was raised and shown to be specific for the human liver enzyme. In conclusion, in the present manuscript, we report not only a novel procedure for purification of dihydropyrimidine dehydrogenase from human liver but also new data on its properties compared to other species, which will provide a basis for further biochemical and molecular studies of this enzyme.

Clinical pharmacokinetics of 3'-azido-3'-deoxythymidine (zidovudine) and catabolites with formation of a toxic catabolite, 3'-amino-3'-deoxythymidine

This study investigated pharmacokinetics and metabolism of 3'-azido-3'-deoxythymidine (zidovudine) in patients after a 1-hour intravenous infusion of 2.5 mg/kg zidovudine with a radiolabeled tracer amount of [5-3H]-zidovudine. In addition to unchanged drug and its 5'-O-glucuronide (zidovudine glucuronide), two novel catabolites of zidovudine were detected as 3'-amino-3'-deoxythymidine (AMT), and its 5'-O-glucuronide (GAMT). The AMT apparent plasma elimination half-life (2.70 +/- 0.7 hours) was longer than that of zidovudine (1.20 +/- 0.30 hours) and zidovudine glucuronide (1.60 +/- 0.5 hours). The zidovudine/AMT plasma peak concentration and area under the concentration-time curve ratios were approximately 8 and 5, respectively. Urinary recovery of radioactivity was essentially complete within 24 hours. AMT glucuronide was not detected in urine or plasma, and only low levels of this catabolite were detected in bile. In contrast, AMT was not detected in bile. The substantial levels of AMT in the plasma of patients after zidovudine administration suggests that this catabolite may affect the pharmacodynamic properties of zidovudine in relation to its activity against human immunodeficiency virus replication and cytotoxicity to host cells.

Differential intestinal deconjugation of taurine and glycine bile acid N-acyl amidates in rats

Mechanisms responsible for the difference in the relative amounts of taurine- and glycine-conjugated bile acid N-acyl amidates (Tau/Gly ratio) are not fully understood. In the present study, the stability of taurine- and glycine-conjugated bile acid N-acyl amidates during intestinal transit and absorption was examined to investigate the contribution of intestinal deconjugation to the Tau/Gly ratio in rat bile. Radiolabeled chenodeoxycholic acid (CDC) and its N-acyl amidates with glycine (CDC-Gly) or taurine (CDC-Tau) were introduced into the lumen of the upper small intestine in the biliary fistula rats, and radioactive metabolites in bile, blood, urine, and tissues were identified and quantitated by high-performance liquid chromatography. Results indicated that 1) extensive deconjugation of CDC-Gly occurs during intestinal absorption; 2) CDC-Tau is recovered in bile largely intact; and 3) newly synthesized CDC-Tau and CDC-Gly are formed in a ratio of less than 2:1 after administration of [14C]-CDC. In summary, the present study demonstrates that resistance of taurine-conjugated bile acid N-acyl amidates to hydrolysis in the intestine, rather than a difference in synthesis of taurine- and glycine-conjugated N-acyl amidates in liver, may account for the high Tau/Gly ratio in rat bile.

Identification of bile acid-CoA: amino acid N-acyltransferase in rat kidney

A novel location of the bile-acid-conjugating enzyme bile acid-CoA:amino acid N-acyltransferase (BAT) has been discovered in the cytosolic fraction of rat kidney. Both taurine and glycine were utilized as substrates. Formation of bile acid N-acyl amidates was verified by h.p.l.c. by comparison with authentic standards and by specific hydrolysis using cholylglycine hydrolase. Immunoblot analysis using a human liver anti-BAT polyclonal antibody indicated that rat kidney BAT has the same molecular mass as rat liver BAT. These findings suggest that the kidney has a role in bile acid metabolism and physiology.

Severe 5-fluorouracil toxicity secondary to dihydropyrimidine dehydrogenase deficiency. A potentially more common pharmacogenetic syndrome

This study describes the inheritance of a defect in pyrimidine catabolism and its association with drug-induced toxicity in a patient receiving 5-fluorouracil (FUra) as adjuvant chemotherapy for breast carcinoma. The study population included the affected patient (proband), nine of her blood relatives, and seven healthy volunteers. The activity of dihydropyrimidine dehydrogenase (DPD), the initial enzyme of pyrimidine (and FUra) catabolism, in peripheral blood mononuclear cells was measured in each subject by a specific radiometric assay using FUra as the substrate. The proband had no detectable DPD activity. When enzyme levels in the proband and relatives were compared with that in controls, an autosomal recessive pattern of inheritance was demonstrated. This is the third patient with severe FUra toxicity secondary to an alteration in pyrimidine catabolism and the second from our clinic population suggesting that the frequency of this genetic defect may be greater than previously thought. Monitoring DPD activity may be important in the management of patients experiencing severe toxicity secondary to FUra chemotherapy.