Mary F. Hebert

Differentiation of absorption and first‐pass gut and hepatic metabolism in humans: Studies with cyclosporine

The low and variable bioavailability of cyclosporine has been attributed to poor absorption. However, recent studies have suggested that intestinal first‐pass metabolism exerts a significant effect on bioavailability. We describe theory and methods to differentiate the contribution from oral absorption and intestinal and hepatic metabolism to overall cyclosporine bioavailability. Analysis of data from previous studies in our laboratories shows that in the absence of intestinal metabolism, cyclosporine absorption from its presently available dosage form averages at least 65% ± 12% in healthy volunteers and 77% ± 19% in kidney transplant patients. Analysis also suggests that the extraction ratio for cyclosporine in the gut is approximately twice the hepatic extraction and that cyclosporine absorption does not present a problem, with an average of 86% of the drug absorbed intact from its commercially available product in healthy volunteers. The boundary condition analysis described should have broad application in the differentiation of factors responsible for poor bioavailability.

Tacrolimus oral bioavailability doubles with coadministration of ketoconazole.

To quantitate the effect of ketoconazole, an azole antifungal agent and potent inhibitor of CYP3A4 and P‐glycoprotein, on the bioavailability of tacrolimus, a substrate of the CYP3A system and of P‐glycoprotein.

EFFECT OF CYP3A5 POLYMORPHISM ON TACROLIMUS METABOLIC CLEARANCE IN VITRO

Previous investigations of solid organ transplant patients treated with tacrolimus showed that individuals carrying a CYP3A5*1 allele have lower dose-adjusted trough blood concentrations compared with homozygous CYP3A5*3 individuals. The objective of this investigation was to quantify the contribution of CYP3A5 to the hepatic and renal metabolic clearance of tacrolimus. Four primary tacrolimus metabolites, 13-O-desmethyl tacrolimus (13-DMT) (major), 15-O-desmethyl tacrolimus, 31-O-desmethyl tacrolimus (31-DMT), and 12-hydroxy tacrolimus (12-HT), were generated by human liver microsomes and heterologously expressed CYP3A4 and CYP3A5. The unbound tacrolimus concentration was low (4–15%) under all incubation conditions. For CYP3A4 and CYP3A5, Vmax was 8.0 and 17.0 nmol/min/nmol enzyme and Km,u was 0.21 and 0.21 μM, respectively. The intrinsic clearance of CYP3A5 was twice that of CYP3A4. The formation rates of 13-DMT, 31-DMT, and 12-HT were ≥1.7-fold higher, on average, in human liver microsomes with a CYP3A5*1/*3 genotype compared with those with a homozygous CYP3A5*3/*3 genotype. Tacrolimus disappearance clearances were 15.9 ± 9.8 ml/min/mg protein and 6.1 ± 3.6 ml/min/mg protein, respectively, for the two genotypes. In vitro to in vivo scaling using both liver microsomes and recombinant enzymes yielded higher predicted in vivo tacrolimus clearances for patients with a CYP3A5*1/*3 genotype compared with those with a CYP3A5*3/*3 genotype. In addition, formation of 13-DMT was 13.5-fold higher in human kidney microsomes with a CYP3A5*1/*3 genotype compared with those with a CYP3A5*3/*3 genotype. These data suggest that CYP3A5 contributes significantly to the metabolic clearance of tacrolimus in the liver and kidney.

Pharmacokinetics of tacrolimus in liver transplant patients

To characterize the pharmacokinetics of the immunosuppressive agent tacrolimus (FK 506) in liver transplant patients.

Pharmacokinetics of Micafungin in Healthy Volunteers, Volunteers With Moderate Liver Disease, and Volunteers With Renal Dysfunction

Micafungin is an antifungal agent metabolized by arylsulfatase with secondary metabolism by catechol‐O‐methyltransferase. The objectives of this study were to estimate the pharmacokinetic parameters and plasma protein binding of micafungin in volunteers with moderate hepatic dysfunction (n = 8), volunteers with creatinine clearance <30 mL/min (n = 9), and matched controls (n = 8 and n = 9, respectively). Single‐dose micafungin pharmacokinetics were estimated using noncompartmental techniques. There was a statistically lower area under the observed micafungin concentration‐time curve (AUC) from time 0 to infinity for subjects with moderate hepatic dysfunction as compared to control subjects (97.5 ± 19 μg•h/mL vs 125.9 ± 26.4 μg•h/mL, P = .03), although there was no difference in micafungin weight‐adjusted clearance (10.9 ± 1.7 mL/h/kg vs 9.8 ± 1.8 mL/h/kg, P = .2). The difference in area under the concentration‐time curve may be explained by the differences in body weight between subjects and controls. Renal dysfunction did not alter micafungin pharmacokinetics.

Are We Optimizing Gestational Diabetes Treatment With Glyburide? The Pharmacologic Basis for Better Clinical Practice

Glyburides pharmacokinetics (PK) and pharmacodynamics have not been studied in women with gestational diabetes mellitus (GDM). The objective of this study was to assess steady‐state PK of glyburide, as well as insulin sensitivity, β‐cell responsivity, and overall disposition indices after a mixed‐meal tolerance test (MMTT) in women with GDM (n = 40), nonpregnant women with type 2 diabetes mellitus (T2DM) (n = 26), and healthy pregnant women (n = 40, MMTT only). At equivalent doses, glyburide plasma concentrations were ~50% lower in pregnant women than in nonpregnant subjects. The average umbilical cord/maternal plasma glyburide concentration ratio at the time of delivery was 0.7 ± 0.4. Insulin sensitivity was approximately fivefold lower in women with GDM as compared with healthy pregnant women. Despite comparable β‐cell responsivity indices, the average β‐cell function corrected for insulin resistance was more than 3.5‐fold lower in women with glyburide‐treated GDM than in healthy pregnant women. Women with GDM in whom glyburide treatment has failed may benefit from alternative medication or dosage escalation; however, fetal safety should be kept in mind.

Effects of Pregnancy on CYP3A and P‐glycoprotein Activities as Measured by Disposition of Midazolam and Digoxin: A University of Washington Specialized Center of Research Study

The objectives of the study were to evaluate the effects of pregnancy on CYP3A and P‐glycoprotein (P‐gp) activities, as measured by disposition of midazolam and digoxin, respectively. Thirteen women received digoxin (0.25 mg p.o.) and midazolam (2 mg p.o.) in random order, separated by 1–2 weeks at 28–32 weeks gestation, and the same order was repeated at 6–10 weeks postpartum. Plasma and urine concentrations were determined by liquid chromatography–mass spectrometry and analyzed by noncompartmental methods. Midazolam CL/Funbound (593 ± 237 l/min vs. 345 ± 103 l/min; P = 0.007), digoxin CLRenal, unbound (272 ± 45 ml/min vs. 183 ± 37 ml/min; P < 0.002) and digoxin CLsecretion, unbound (109 ± 34 ml/min vs. 58 ± 22 ml/min; P < 0.002) were higher during pregnancy than postpartum. These data are consistent with increased hepatic and/or intestinal CYP3A and renal P‐gp activities during pregnancy.

Effects of Rifampin on Tacrolimus Pharmacokinetics in Healthy Volunteers

Tacrolimus is a marketed immunosuppressant used in liver and kidney transplantation. It is subject to extensive metabolism by CYP3A4 and is a substrate for P‐glycoprotein‐mediated transport. A pharmacokinetic interaction with rifampin, an antituberculosis agent and potent inducer of CYP3A4 and P‐glycoprotein, and tacrolimus was evaluated in six healthy male volunteers. Tacrolimus was administered at doses of 0.1 mg/kg orally and 0.025 mg/kg/4 hours intravenously. The pharmacokinetics of tacrolimus were obtained from serial blood samples collected over 96 hours, after single oral and intravenous administration prior to and during an 18‐day concomitant rifampin dosing phase. Coadministration of rifampin significantly increased tacrolimus clearance (36.0 ± 8.1 ml/hr/kg vs. 52.8 ± 9.6 ml/hr/kg; p = 0.03) and decreased tacrolimus bioavailability (14.4% ± 5.7% vs. 7.0% ± 2.7%; p = 0.03). Rifampin appears to induce both intestinal and hepatic metabolism of tacrolimus, most likely through induction of CYP3A and P‐glycoprotein in the liver and small bowel.

Pharmacokinetics of Metformin during Pregnancy

Our objective was to evaluate the pharmacokinetics of metformin during pregnancy. Serial blood and urine samples were collected over one steady-state dosing interval in women treated with metformin during early to late pregnancy (n = 35) and postpartum (n = 16). Maternal and umbilical cord blood samples were obtained at delivery from 12 women. Metformin concentrations were also determined in breast milk samples obtained over one dosing interval in 6 women. Metformin renal clearance increased significantly in mid (723 ± 243 ml/min, P < 0.01) and late pregnancy (625 ± 130 ml/min, P < 0.01) compared with postpartum (477 ± 132 ml/min). These changes reflected significant increases in creatinine clearance (240 ± 70 ml/min, P < 0.01 and 207 ± 56 ml/min, P < 0.05 versus 165 ± 44 ml/min) and in metformin net secretion clearance (480 ± 190 ml/min, P < 0.01 and 419 ± 78 ml/min, P < 0.01 versus 313 ± 98 ml/min) in mid and late pregnancy versus postpartum, respectively. Metformin concentrations at the time of delivery in umbilical cord plasma ranged between nondetectable (<5 ng/ml) and 1263 ng/ml. The daily infant intake of metformin through breast milk was 0.13 to 0.28 mg, and the relative infant dose was <0.5% of the mother’s weight-adjusted dose. Our results indicate that metformin pharmacokinetics are affected by pregnancy-related changes in renal filtration and net tubular transport and can be roughly estimated by the use of creatinine clearance. At the time of delivery, the fetus is exposed to metformin concentrations from negligible to as high as maternal concentrations. In contrast, infant exposure to metformin through the breast milk is low.

Effects of St. John's Wort (Hypericum perforatum) on Tacrolimus Pharmacokinetics in Healthy Volunteers

Tacrolimus is an immunosuppressant approved for the prevention of rejection following transplantation and is a substrate for CYP3A and P‐glycoprotein. A pharmacokinetic interaction between St. Johns wort (antidepressant herbal product and inducer of CYP3A and P‐glycoprotein) and tacrolimus was evaluated in 10 healthy volunteers. The pharmacokinetics of tacrolimus were obtained from serial blood samples collected following single oral doses (0.1 mg/kg) prior to and during an 18‐day concomitant St. Johns wort dosing phase (300 mg orally three times daily). Coadministration of St. Johns wort significantly decreased tacrolimus AUC (306.9 μg•h/L ±175.8 μg•h/L vs. 198.7 μg•h/L ± 139.6 μg•h/L; p = 0.004) and increased apparent oral clearance (349.0 mL/h/kg ± 126.0 mL/h/kg vs. 586.4 mL/h/kg ± 274.9 mL/h/kg; p = 0.01) and apparent oral volume of distribution at steady state (11.5 L/kg ± 4.3 L/kg vs. 17.6 L/kg ± 9.6 L/kg; p = 0.04). St. Johns wort appears to induce tacrolimus metabolism, most likely through induction of CYP3A and P‐glycoprotein.