Identification of the human liver cytochrome P450 enzymes involved in the in vitro metabolism of a novel 5-lipoxygenase inhibitor.

In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) forms involved in the oxidative metabolism of [14C]ABT-761 and its N-dehydroxylated metabolite, [14C]ABT-438, by human liver microsomes. The two compounds were metabolized by parallel pathways, to form the corresponding methylene bridge hydroxy metabolites. There was no evidence of sulfoxidation and/or ring hydroxylation. Over the ABT-761 and ABT-438 concentration ranges studied (1-300 microM), the rate of NADPH-dependent hydroxylation was linear with respect to substrate concentration ([S]) and did not conform to saturable Michaelis-Menten kinetics. Under these conditions ([S] < KM), the intrinsic clearance (Vmax/KM) of ABT-438 was 10-fold higher than that of ABT-761 (1.7 +/- 0.8 vs. 0.17 +/- 0.06 microl/min/mg, mean +/- SD, N = 3 livers). The hydroxylation of both compounds was shown to be highly correlated (r = 0.83, p < 0.01, N = 11 different human livers) with CYP3A-selective erythromycin N-demethylase activity, and the correlation between ABT-761 hydroxylation and tolbutamide hydroxylase (CYP2C9-selective) activity (r = 0.63, p < 0.05, N = 10) was also statistically significant. Ketoconazole (2.0 microM), a CYP3A-selective inhibitor, inhibited the hydroxylation of both compounds by 53-67%, and sulfaphenazole (CYP2C9-selective) decreased activity by 10-20%. By comparison, alpha-naphthoflavone, a known activator of CYP3A, stimulated the hydroxylation of ABT-761 (8-fold) and ABT-438 (4-fold). In addition, the abundance-normalized rates of cDNA-expressed CYP-dependent metabolism indicated that hydroxylation was largely mediated (66-86%) by CYP3A(4). Therefore, it is concluded that the hydroxylation of ABT-761 and ABT-438 (

Pharmacokinetics of zileuton and its metabolites in patients with renal impairment.

The pharmacokinetics of zileuton and its conjugated metabolites were evaluated in patients with chronic renal impairment. Five healthy volunteers (creatinine clearance > 90 mL/min), five patients with renal failure requiring hemodialysis, six with mild (creatinine clearance, 60-90 mL/min), eight with moderate (creatinine clearance, 30-59 mL/min), and six with severe (creatinine clearance < 30 mL/min) renal impairment participated in the study. Zileuton was well tolerated by all participants including those with severe renal impairment and those receiving hemodialysis. The pharmacokinetics of zileuton were similar in healthy volunteers; in patients with mild, moderate and severe renal impairment; and in patients with renal failure requiring hemodialysis. The mean metabolite/parent-area ratios for the pharmacologically inactive zileuton glucuronides progressively increased with the decline in renal function. A very small percentage of the administered zileuton dose (< 0.5%) was removed by hemodialysis. Therefore, adjustment in the dose regimen of zileuton does not appear to be necessary for patients with various degrees of renal impairment and patients with renal failure requiring hemodialysis.

Lansoprazole pharmacokinetics in subjects with various degrees of kidney function.

The pharmacokinetics of lansoprazole, a new benzimidazole proton pump inhibitor, was evaluated after multiple-dose oral administration to 20 subjects with various degrees of kidney function. Multiple blood samples were obtained after doses 1 and 7 of the once-daily seven-dose regimen, and plasma concentrations of lansoprazole and five metabolites were quantitated with use of HPLC. The free fraction of lansoprazole increased as kidney function declined. A significant, although weak, relationship existed between creatinine clearance (CLCR) and area under the plasma concentration versus time curve (AUC) and terminal disposition half-life (t1/2), calculated with total concentration data. Those individuals with lower CLCR values also had lower total AUC and t1/2 values. However, there was no statistically significant relationship between CLCR and peak plasma concentration or AUC, calculated with unbound concentration data. No adjustment of lansoprazole dose is recommended on the basis of impaired kidney function.

Hepatic peroxisomal and drug metabolizing activity in CD-1 mice after oral treatment with a novel 5-lipoxygenase inhibitor.

The effects of zileuton, a 5-lipoxygenase inhibitor, on hepatic peroxisomal enzyme activity as well as hepatic drug metabolizing activity in male and female CD-1 mice were assessed after oral administration of the drug (50, 150, or 450 mg/kg/day) for 14 days. The effects were compared to those in mice receiving clofibrate (CLOF;462 mg/kg/day, po) or sodium phenobarbital (PB; 50 mg/kg/day, po). Zileuton pretreatment caused hepatomegaly and elevated liver peroxisomal KCN-insensitive palmitoyl CoA oxidase activity in a dose-dependent manner. However, these changes were marginal (< or = 121% increase), when compared to those elicited by CLOF (approximately 370% increase). In both sexes, zileuton pretreatment also caused a dose-dependent increase in the levels of liver microsomal cytochrome P450 2B and cytochrome P450 4A (CYP4A) proteins, and their associated monoxygenase activity. In the case of CYP4A, the induction of lauric acid 12-hydroxylase activity by zileuton was more pronounced in female (maximal 851% increase) than in male mice (maximal 111% increase). Based on the dose normalized response observed in CD-1 mice, zileuton can be considered a relatively weak inducer of peroxisome enzyme activities (cf.CLOF) and a moderate inducer of cytochromes P450. Moreover, zileuton exhibits characteristics of both a PB- and a CLOF-type hepatic enzyme inducer, especially in the female mice.

Identification of the human liver cytochrome P450 enzymes involved in the metabolism of zileuton (ABT-077) and its N-dehydroxylated metabolite, Abbott-66193.

In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) forms involved in the oxidative metabolism of [14C]zileuton (ABT-077) and its N-dehydroxylated metabolite, [14C]Abbott-66193, by human liver microsomes. The two compounds were metabolized by parallel pathways to form the corresponding ring-hydroxylated and diastereomer sulfoxide metabolites. Results suggested that whereas the metabolism of zileuton and Abbott-66193 were mediated by the same CYP forms, the CYP forms responsible for hydroxylation (CYP1A2 and CYP2C9/10) were distinct from those involved in sulfoxidation (CYP3A > CYP2C9/10). Sulfoxidation (zileuton, Km = 0.82 +/- 0.40 mM, Vmax = 39.1 +/- 21.8 pmol/min/mg; Abbott-66193, Km = 0.23 +/- 0.06 mM, Vmax = 507 +/- 215 pmol/min/mg; mean +/- SD, N=3) was highly correlated with the CYP3A-specific erythromycin N-demethylase activity (r=0794-0.856; p<0.01, N=11) in human microsomes and was inhibited (32-67%) by ketoconazole and troleandomycin. In addition, purified recombinant human CYP3A4/rat NADPH-P450 reductase fusion protein catalyzed only the sulfoxidation of zileuton and Abbott-66193; no hydroxylated metabolites were detected. On the other hand, hydroxylation of the two compounds (zileuton, Km = 0.34 +/- 0.25 mM, Vmax = 17.8 +/- 5.58 pmol/min/mg; Abbott-66193,Km = 0.39 +/- 0.14 mM, Vmax = 1061 +/- 220 pmol/min/mg) was significantly correlated with 7-ethoxyresorufin O-deethylase (CYP1A2; r=0.652-0.762; p<0.01, N=11) and tolbutamide methyl hydroxylase (CYP2C9/10; r=0.863-0.935; p<0.01, N=10) activity in human liver microsome, and was inhibited (26-51%) by well-known CYP1A2 inhibitors (furafylline and alpha-naphthoflavone). Furthermore, microsomes from human B-lymphoblastoid cells expressing CYP1A2 catalyzed only the hydroxylation of zileuton and Abbott-66193; sulfoxide were not formed. Abbott-66193 was a better substrate for CYP2C9/10, when compared with zileuton: 1) the effect of sulfaphenazole on hydroxylation in human liver microsomes was more pronounced for Abbott-66193 than zileuton (56% vs. 9% inhibition); and 2) the rate of Abbott-66193 hydroxylation by purified CYP2C9 was almost 30-fold greater than that of zilueton.

Pharmacokinetics of lansoprazole in hemodialysis patients.

The pharmacokinetics of the new benzimidazole proton pump inhibitor lansoprazole and five of its metabolites were assessed after single oral dose administration to five hemodialysis patients. Patients were studied on dialysis and nondialysis days. Multiple blood and dialysate samples were collected after dosing and were assayed for lansoprazole and metabolite content via high-performance liquid chromatography. The degree of lansoprazole plasma protein binding was lower in hemodialysis patients than in subjects with normal renal function or patients with renal impairment not requiring dialytic therapy, although this tended to moderate when assessed immediately after dialysis. Examination of venous plasma concentration, paired arterial-venous concentration, and dialysate data revealed that lansoprazole and its metabolites were poorly dialyzable. No dosage adjustment of lansoprazole is necessary in hemodialysis patients nor is supplementation after hemodialysis sessions necessary.

In vitro plasma protein binding of zileuton and its N-dehydroxylated metabolite.

An ultrafiltration technique or equilibrium dialysis has been used to study the in vitro human plasma protein binding of racemic zileuton, its individual enantiomers, and its pharmacologically inactive metabolite N-dehydroxyzileuton. The plasma protein binding of zileuton and N-dehydroxyzileuton over the concentration range of 0.1 to 100 mg/L averaged 93.1 +/- 0.22 and 92.0 +/- 0.12%, respectively. However, there appeared to be a stereoselective effect, with the R(+) enantiomer of zileuton demonstrating greater binding to plasma proteins than the S(-) enantiomer (96 vs 88%, respectively). Zileuton was bound to both human serum albumin (40 g/L) and alpha 1-acid glycoprotein (1 g/L), although binding affinity to albumin was approximately 3-fold greater. Displacement interactions of zileuton with warfarin, salicylate, theophylline, naproxen, ibuprofen, prednisone, and terfenadine were minimal. The blood to plasma concentration ratio for zileuton and N-dehydroxyzileuton ranged from 0.65 to 0.68, indicating that these compounds were mainly distributed in the plasma. Thus, zileuton is approximately 93% bound to plasma proteins at expected therapeutic concentrations in vitro, and this figure is largely unaffected by several commonly prescribed agents with which the drug may be coadministered.

The pharmacokinetics of zileuton in healthy young and elderly volunteers.

The effects of age and gender on the single and multiple dose pharmacokinetics of zileuton have been examined in a phase I nonblinded study. A total of 27 healthy volunteers were evaluable, 9 in the young group (age range 20 to 40 years; 5 males and 4 females) and 18 in the elderly group (range 65 to 81 years; 9 males and 9 females). A single oral dose of zileuton 600mg was given to all volunteers on day 1 of the study and at 6-hour intervals from days 3 to 7. Analysis of variance showed slight but significant decreases in the mean apparent clearance of total and free drug in the healthy elderly population after a single zileuton dose, but no significant age-related differences after multiple 6-hourly doses. Similarly, zileuton peak and trough plasma concentrations, and values for half-life, volume of distribution and protein binding were not significantly affected by age after either a single dose or multiple administration. Moreover, gender effects on the pharmacokinetics were also absent after correction for bodyweight differences. From the results of the present study, it is concluded that there is no pharmacokinetic basis for alteration of zileuton dosage schedules in elderly patients.

The effect of mild or moderate hepatic impairment (cirrhosis) on the pharmacokinetics of zileuton.

The pharmacokinetics of zileuton and its R(+) and S(-) glucuronide metabolites were determined after single and multiple (400mg every 8 hours) oral dose administration in healthy subjects (n = 5) and patients with mild or moderate hepatic impairment (cirrhosis; n = 8). The clearance of total zileuton (unbound plus bound to plasma proteins) in patients with hepatic impairment (approximately 350 ml/min) was approximately half than in healthy subjects (approximately 670 ml/min), with similar values in patients with mild or moderate cirrhosis. However, the clearance of unbound zileuton in patients with moderate hepatic impairment was nearly half that in patients with mild hepatic impairment, and one quarter that in healthy subjects. On the basis of these findings, it may be necessary to reduce the dose in patients with impaired hepatic function to maintain levels similar to those in healthy subjects.

The in vitro hepatic metabolism of ABT-418, a cholinergic channel activator, in rats, dogs, cynomolgus monkeys, and humans.

The metabolism of the cholinergic channel activator [3H]ABT-418 was studied in 9,000g supernatant (S-9) fractions and precision-cut tissue slices prepared from rat, dog, monkey, and human livers. In rat S-9 fractions and tissue slices, the lactam and trans N'-oxide were detected as major metabolites. The lactam was also the major metabolite in monkey and human S-9 fractions and tissue slices, although the rate of formation was greater in monkey (Vmax' of 428 vs. 103 pmol/min/mg S-9 protein). Trans N'-oxide was not detected in either species, but low levels of the cis N'-oxide were detected in tissue slice preparations from two human subjects. In contrast, trans ABT-418 N'-oxide was identified as a major metabolite in dog S-9 fractions (Vmax' of 266 pmol metabolite formed/min/mg S-9 protein) and tissue slices. Although identified as a minor metabolite in dog S-9 fractions, the lactam metabolite was shown to account for a sizeable proportion of the total radioactivity in the corresponding tissue slice preparations (22% of the total radioactivity at 12 hr); the rank order of lactam formation by the precision-cut liver slices was monkey > human > rat > or = dog. Evidence that N'-oxidation and C-oxidation (to lactam) of ABT-418 was mediated by liver microsomal flavin-containing mono-oxygenase (FMO) and cytochromes P-450 (CYPs), respectively, was obtained with the inhibitors thiobenzamide and clotrimazole. The involvement of cytosolic aldehyde oxidase (AO) was suggested by a significant correlation (r2 = 0.998, p < 0.01) between the observed rate of lactam formation and AO (N1-methylnicotinamide oxidase) activity in rat, dog, monkey, and human S-9 fractions; inhibition of lactam formation by the AO substrate N1-methylnicotinamide; and the lack of lactam formation in the absence of cytosol. Data indicate that the species-related differences in the hepatic metabolism of ABT-418 may be dependent on the relative levels and/or activity of FMO, CYP, and AO. In this regard, ABT-418 is very similar to nicotine. However, unlike nicotine, the N-demethylation of parent drug and the further products of lactam metabolism was not detected.

Measurement of liver microsomal cytochrome p450 (CYP2D6) activity using [O-methyl-14C]dextromethorphan.

The activity of human liver microsomal cytochrome P4502D6 (CYP2D6) is readily estimated by following the O-demethylation of [O-methyl-14C]dextromethorphan. The basis of the assay is the quantitative measurement of [14C]formaldehyde (0.05-4.0 microM) after addition of NaOH to the microsomal incubates and extraction with methylene chloride. The assay is relatively simple, sensitive (limit of detection is approximately 5.0 pmol HCHO/h/mg microsomal protein) and does not require the use of HPLC or an internal standard. Formation of radiolabeled formaldehyde in human liver microsomes is linear for 20 min, up to a final protein concentration of 1.0 mg/ml. Furthermore, the O-demethylase activity in a panel of microsomes prepared from a series of human livers was significantly correlated with the immunochemically determined levels of CYP2D6 protein (r = 0.925, p < 0.001), and was inhibited (> 89%) by quinidine and lobeline. In addition, [O-methyl-14C]-dextromethorphan O-demethylation was exclusively catalyzed by cDNA-expressed CYP2D6 in microsomes prepared from human B-lymphoblast cells. The method is suitable for rapid screening of compounds as potential CYP2D6 cosubstrates and/or inhibitors.

The properties of A-69412: a small hydrophilic 5-lipoxygenase inhibitor.

A compound which inhibits leukotriene biosynthesis could be clinically useful in treating several allergic and inflammatory diseases. One site for such inhibition is at the enzyme 5-lipoxygenase. Most inhibitors of this enzyme thus far described are poorly bioavailable. A-69412 is a small, relatively hydrophilic compound of the N-hydroxyurea class, which exhibits minimal plasma protein binding (6-12%). The compound was found to be a potent long-acting inhibitor of leukotriene formation in vivo in the rat (oral ED50 = 5 mg/kg) and ex vivo in several species. In addition, the compound exhibits excellent bioavailability in dogs and monkeys with a relatively long elimination half-life in both the species (6 and 3 h, respectively). The biochemical activity and pharmacological profile of A-69412 indicates its potential utility in asthma and ulcerative colitis, and possibly other inflammatory and allergic conditions.

Metabolism of 14C-estazolam in dogs and humans.

14C-Estazolam (2 mg) administered orally to dogs and human subjects was rapidly and completely absorbed with peak plasma levels occurring within one hour. In humans, plasma levels peaked at 103 +/- 18 ng/ml and declined monoexponentially with a half-life of 14 h. The mean concn. of estazolam in dog plasma at 0.5 h was 186 ng/ml. Six metabolites were found in dog plasma at 0.5 and 8 h, whereas only two metabolites were detected in human plasma up to 18 h. Metabolites common to both species were 1-oxo-estazolam (I) and 4-hydroxy-estazolam (IV). Major metabolites in dog and human plasma were free and conjugated 4-hydroxy-estazolam; the concn. were higher in dogs. After five days, 79% and 87% of the administered radioactivity was excreted in dog and human urine, respectively. Faecal excretion accounted for 19% of the dose in dog and 4% in man. Eleven metabolites were found in the 0-72 h urine of dogs and humans; less than 4% dose was excreted unchanged. Four metabolites were identified as: 1-oxo-estazolam (I), 4'-hydroxy-estazolam (II), 4-hydroxy-estazolam (IV) and the benzophenone (VII), as free metabolites and glucuronides. The major metabolite in dog urine was 4-hydroxy-estazolam (20% of the dose), while the predominant metabolite in human urine (17%) has not been identified, but is likely to be a metabolite of 4-hydroxy-estazolam. The metabolism of estazolam is similar in dog and man.

The hepatotoxicity of valproic acid and its metabolites in rats. I. Toxicologic, biochemical and histopathologic studies.

Valproic acid (VPA), its unsaturated metabolites and pent-4-enoate (4-PA) were studied for potential hepatotoxicity in rats. 4-PA, 4-en-VPA and 2,4-dien-VPA were potent inducers of microvesicular steatosis in young rats. Microvesicular steatosis induced by the 4-en-VPA was accompanied by ultrastructural changes characterized by myeloid bodies, lipid vacuoles and mitochondrial abnormalities. Myeloid bodies and lipid vacuoles were seen to a lesser extent in 2,4-dien-VPA and 4-PA-treated rats. VPA failed to induce discernible liver lesions in young rats even at near lethal doses of 700 mg per kg per day. The drug did, however, induce hepatic lipid accumulation in mature rats and in young rats dosed concomitantly with phenobarbital. beta-oxidation inhibition and several other biochemical alterations were observed in rats dosed with VPA, its unsaturated metabolites and 4-PA. It was suggested that beta-oxidation inhibition observed in both VPA and en-metabolite-treated rats occurred by different mechanisms. VPA inhibits by a transient sequestering of CoA while the CoA esters of some en-VPA-metabolites, particularly 4-en-VPA, inhibit specific enzyme(s) in the beta-oxidation sequences.

The hepatotoxicity of valproic acid and its metabolites in rats. II. Intermediary and valproic acid metabolism.

The role of metabolites in valproic acid (VPA)-associated hepatotoxicity was studied in rats. The most steatogenic mono-unsaturated metabolite, 4-en-VPA, caused the greatest changes in indicators of beta-oxidation inhibition (dicarboxylic aciduria, beta-hydroxybutyrate reduction); however, the biochemical effects were much less pronounced than those reported for hypoglycin. Steatosis in VPA-treated rats occurred only at nearly lethal doses. Phenobarbital induction was confirmed as a predisposing factor; however, it appeared not to greatly enhance production of 4-en-VPA or its recognized metabolites, which collectively comprised only 0.5% of the dose. Elevated oxo-VPA metabolites in serum and 2-propylglutarate in liver were associated with toxicity. Among the newly discovered minor metabolites with possible biologic effects were diols (suggesting epoxide precursors) and a series of dienes and trienes. The rarity of severe human hepatotoxicity indicates that, normally, beta-oxidation inhibition is compensated, and cellular defense mechanisms prevail over reactive metabolites. This requires adequate nutrition; on the other hand, severe glycogen depletion may promote toxicity by compromising glucuronidation, the major clearance route. Other literature comments are also supported: (i) caution is indicated for patients with various unusual congenital disorders (e.g., organic acidurias or other mitochondrial defects), and (ii) monotherapy obviates both the predisposition to toxicity and the requirement of large doses to produce therapeutic levels.

Metabolism of [14C]Cefmenoxime in normal subjects after intramuscular administration.

The metabolism of cefmenoxime (SCE-1365) was studied in four healthy male volunteers after intramuscular administration of a single 500-mg dose of the 14C-labeled drug. Plasma levels of total radioactivity and cefmenoxime peaked at 0.5 and 1.0 h, corresponding to 16.5 micrograms eq/ml and 15.8 micrograms/ml, respectively. Thereafter, parent drug levels declined rapidly, with a terminal elimination half-life of ca. 1.5 h. No significant differences were noted between total radioactivity and parent drug levels up to 2 h after drug administration. After 3 h, low but persistent levels of radioactivity were significantly greater than parent drug levels, indicating metabolism or degradation of cefmenoxime. The terminal elimination half-life of total radioactivity was estimated to be ca. 40 h. The radioactive plasma metabolite(s) remaining at the end of the 5-day study represented only 1% of the administered dose. Urinary excretion was the major route of elimination of cefmenoxime, accounting for ca. 86% of the dose in 12 h. Analysis of cefmenoxime in urine by total radioactivity, high-pressure liquid chromatography, and a microbiological assay showed that 80 to 92% of the excreted dose was parent drug. Radioactivity was also excreted into the feces via the bile and represented ca. 11% of the dose after 5 days. Although extensive degradation of cefmenoxime was found in fecal samples, it was proposed that this may be due to the metabolic activity of the intestinal flora rather than in vivo biotransformation in the liver. This study supports the concept that cefmenoxime undergoes minimal metabolism in humans and is excreted largely as unchanged drug.

Aspects of the metabolism of valproic acid.

The metabolic routes of valproic acid (VPA) were studied by i.p. administration of the mono-unsaturated and hydroxylated metabolites to rats. Conjugation with glucuronic acid was a major metabolic route for VPA and its metabolites. Conjugation with glycine was a minor route for VPA, but was of more importance with the unsaturated metabolites. The hydroxylated metabolites, which were further oxidized to oxo-derivatives and subsequently to the dicarboxylic acids, were not metabolically dehydrated to form unsaturated metabolites. Multiple metabolic pathways, including dehydrogenation, isomerization, hydration, hydroxylation, reduction and epoxidation were inferred from the metabolites obtained after dosage of the unsaturated metabolites. Six dien-VPA metabolites were detected in VPA-treated rats, four of which are present in patients. It was concluded that 3-en-VPA and 4-en-VPA pathways, originating through dehydrogenation, are distinct from the omega- and omega-1-hydroxylation pathways. Enzyme induction from co-administration of phenobarbital caused enhancement of the minor omega-1-oxidation pathway, yet the largest effect on clearance came from increases in glucuronidation. Mitochondrial processes were unaffected, resulting in decreased contribution to the total clearance.

Riboflavin nutritional status and absorption in oral contraceptive users and nonusers.

Riboflavin nutritional status, based on erythrocyte glutathione reductase (EGR) determinations, and the absorption of the vitamin after 10-mg oral doses were assessed in a group of oral contraceptive users (group OC, n = 18) and in a group of nonusers (group NOC, n = 17). Before administration of test doses of riboflavin, mean EGR levels of 4.06 (group OC) and 4.44 (group NOC) mumoles glutathione reduced per minute per gram of hemoglobin were not significantly different. After oral administration of riboflavin, a significant increase in EGR levels was observed within each group although no significant differences were detected between groups. Mean EGR levels continued to be slightly lower for group OC compared to group NOC. Oral absorption of riboflavin based on urinary excretion results was not significantly different between the groups. These findings indicate OC use does not significantly affect riboflavin nutritional status in a subject population having adequate nutrition of moderate to high socioeconomic status. Significant reductions in EGR levels which have been previously reported for OC users appear to be associated with a low socioeconomic status.

PIP: Riboflavin nutritional status and its oral absorption was investigated in 2 groups of women -- a group of 18 oral contraceptive (OC) users and a control group of 17 women. The riboflavin nutrition status was based on erythrocyte glutathione reductase (EGR) determinations. Prior to administration of test doses of riboflavin, mean EGR levels of 4.96 (OC group) and 4.44 (control group) umoles glutathione reduced per minute per gram of hemoglbin were not significantly different. After oral administration of riboflavin, there was a significant increase in EGR levels within each group but no significant differences between groups. Mean EGR levels continued to be slightly lower for the OC group compared to the control group. Oral absorption of riboflavin based on urinary excretion results was not significantly different between the groups. The findings indicate that OC use does not significantly affect riboflavin nutritional status in a subject population of moderate to high socioeconomic status that has adequate nutrition. The significant reductions in EGR levels which have been previously reported for OC users seem to be associated with a low socioeconomic status.