Drug interaction of (S)-warfarin, and not (R)-warfarin, with itraconazole in a hematopoietic stem cell transplant recipient.

BACKGROUND: Itraconazole is a potent inhibitor of CYP3A4 and P-glycoprotein, but not CYP2C9. Herein, we report a case study in which the plasma concentration of the CYP2C9 substrate (S)-warfarin, and not the CYP3A4 substrate (R)-warfarin, increased with itraconazole coadministration. CASE: A 67-y-old man received an allogenic bone marrow transplant for acute lymphoid leukemia. He was taking oral itraconazole (200mg/day) and was started on a warfarin dose of 2.0mg/day. The plasma concentrations of (S)- and (R)-warfarin 3 days after starting warfarin administration were 216 and 556 ng/mL, respectively (INR 0.98), and after 10 days, the concentrations were 763 and 545 ng/mL, respectively (INR 2.43). On day 11 after withdrawal of itraconazole, the concentrations of (S)- and (R)-warfarin were 341 and 605ng/mL, respectively (INR 1.38). The concentration of (R)-warfarin was not affected by itraconazole; however, the final (S)-warfarin concentration had increased 7.3-fold. The (S)-warfarin/(S)-7-hydroxywarfarin ratio decreased to 2.45 from 8.40 after discontinuation of itraconazole. The permeability of warfarin enantiomers across Caco-2 cells was not influenced by itraconazole and showed no difference between enantiomers. CONCLUSIONS: Careful INR monitoring is necessary for warfarin co-administration with itraconazole. Further examination is necessary to elucidate mechanisms of the interaction between warfarin and itraconazole.

Monitoring of mycophenolic acid predose concentrations in the maintenance phase more than one year after renal transplantation.

BACKGROUND: Routine therapeutic drug monitoring of mycophenolic acid (MPA) is generally performed using the area under the concentration-time curve from 0 to 12 hours (AUC0-12) with recommended values between 30 and 60 µg·h/mL. OBJECTIVE: The aim of this study was to examine whether the monitoring of the MPA predose concentration (C0) in patients who are stable for >1 year after renal transplantation was practical and to determine factors that cause MPA C0 variability among patients. METHODS: Eighty-six Japanese patients who had undergone renal transplantation and were taking tacrolimus and who had their MPA C0 analyzed >6 times by high-performance liquid chromatography for >1 year posttransplantation were enrolled. RESULTS: Recipients with MPA AUC0-12 levels<30 µg·h/mL on day 28 and 1 year after transplantation had an MPA C0 of <2.0 µg/mL, with a sensitivity of 90.9% and a specificity of 70.7%. There was no significant difference in the mean dose-adjusted MPA C0>1 year after transplantation between subjects with either the UGT (1A1, 1A9, and 2B7) or drug transporter (SLCO1B3, ABCC2, and ABCG2) genotypes. However, in a multiple regression analysis, the dose-adjusted mean MPA C0>1 year after transplantation was significantly associated with age (P=0.0035), creatinine clearance (P=0.0001), and the dose-adjusted MPA AUC0-12 at 1 year (P=0.0147). CONCLUSIONS: To keep the MPA AUC0-12>30 µg·h/mL, the plasma threshold for maintaining the MPA C0 with tacrolimus should be set >2.0 µg/mL as determined by high-performance liquid chromatography. For patients who are stable for >1 year after transplantation, continued monitoring of the MPA C0 using the same samples used to monitor the tacrolimus C0 and the additional assessment of the MPA AUC0-12 at the 1-year time point seem to be a viable option. If a change of the mycophenolate mofetil dose seems necessary based on the routine MPA C0 information, the determination of MPA AUC0-12 using a limited sampling strategy is recommended.

Simultaneous determination of warfarin and 7-hydroxywarfarin enantiomers by high-performance liquid chromatography with ultraviolet detection.

Warfarin is administered clinically as a racemic mixture of two enantiomers, (R)-warfarin and (S)-warfarin. (S)-Warfarin has more potent anticoagulant activity than (R)-warfarin and is metabolized mainly to (S)-7-hydroxywarfarin by CYP2C9. A simple, rapid, and sensitive high-performance liquid chromatography method with ultraviolet detection was developed for the simultaneous quantitative determination of the (R)- and (S)-enantiomers of warfarin and 7-hydroxywarfarin in human plasma. Analytes and the internal standard (p-chlorowarfarin) were separated using a mobile phase of 0.5% KH2PO4 (pH 3.5)-methanol (41:59, v/v) on a Chiral CD-Ph column at a flow rate of 0.5 mL/min and were detected at a ultraviolet absorbance of 305 nm. Analysis required 200 µL of plasma and involved a simple and rapid solid-phase extraction with an Oasis HLB cartridge, which gave high recovery (greater than 91.8%) and good selectivity for all analytes. The lower limit of quantification for the (R)- and (S)-enantiomers of warfarin and 7-hydroxywarfarin was 2.5 ng/mL for each analyte. Inter- and intraday coefficients of variation for all analytes were less than 14.2% and accuracies were within 6.6% over the linear range. Our results indicate that this method is applicable to the simultaneous monitoring of the enantiomers of warfarin and 7-hydroxywarfarin in human plasma. The S/R-enantiomer ratio of warfarin and the (S)-warfarin/(S)-7-hydroxywarfarin ratio 3 hours after administration in 67 CYP2C9*1/*1 patients ranged from 0.24 to 0.75 and from 1.83 to 19.02, respectively, whereas these ratios in a CYP2C9*3/*3 patient were 1.12 and 17.02, respectively.

Involvement of carboxylesterase 1 and 2 in the hydrolysis of mycophenolate mofetil.

Mycophenolate mofetil (MMF) is the ester prodrug of the immunosuppressant agent mycophenolic acid (MPA) and is rapidly activated by esterases after oral administration. However, the role of isoenzymes in MMF hydrolysis remains unclear. Although human plasma, erythrocytes, and whole blood contain MMF hydrolytic activities, the mean half-lives of MMF in vitro were 15.1, 1.58, and 3.20 h, respectively. Thus, blood esterases seemed to contribute little to the rapid MMF disappearance in vivo. In vitro analyses showed that human intestinal microsomes exposed to 5 and 10 µM MMF exhibited hydrolytic activities of 2.38 and 4.62 nmol/(min · mg protein), respectively. Human liver microsomes exhibited hydrolytic activities of 14.0 and 26.1 nmol/(min · mg protein), respectively, approximately 6-fold higher than those observed for intestinal microsomes. MMF hydrolytic activities in human liver cytosols were 1.40 and 3.04 nmol/(min · mg protein), respectively. Because hepatic cytosols generally contain 5-fold more protein than microsomes, MMF hydrolysis in human liver cytosols corresponded to approximately 50% of that observed in microsomes. Fractions obtained by 9000g centrifugation of supernatants from COS-1 cells expressing human carboxylesterase (CES) 1 or 2 exhibited MMF hydrolytic activity, with CES1-containing fractions showing higher catalytic efficiency than CES2-containing fractions. The CES inhibitor bis-p-nitrophenylphosphate inhibited MMF hydrolysis in human liver microsomes and cytosols with IC(50) values of 0.51 and 0.36 µM, respectively. In conclusion, both intestinal and hepatic CESs and in particular CES1 may be involved in MMF hydrolysis and play important roles in MMF bioactivation. Hepatic CES1 activity levels may help explain the between-subject variability observed for MMF usage.