Prediction of Human Bioavailability from Human Oral Administration Data and Animal Pharmacokinetic Data Without Data from Intravenous Administration of Drugs in Humans

Purpose  To predict the absolute oral bioavailabilities (BAs) of drugs in humans without using pharmacokinetic data from intravenous administration in humans. Methods  The distribution volume of the terminal phase () in humans was predicted by three methods using animal pharmacokinetic data. Then, total body clearance (CL_tot) was calculated by multiplying the elimination rate constant and , and the BA was calculated as a ratio between CL_tot and oral clearance. The predicted and observed values were compared for 67 drugs for which pharmacokinetic data after intravenous administration in humans were available. Results  For , predicted values within twice the observed value were obtained for 72.1% of drugs by both methods Ia and Ib, respectively, in which only rat pharmacokinetic data were used. The corresponding percentage was 75.0% for method II, in which pharmacokinetic data from animals other than rats were used. For BA, predicted values within 1.3 times the observed values were obtained for 66.7% and 57.4% of drugs by methods Ia and Ib, respectively, and 75.0% by method II. Conclusions  Using the present methods, it is possible to predict BA from human oral administration data combined with animal pharmacokinetic data to a certain level without using intravenous injection data. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The bioavailability of bromazepam, omeprazole and paracetamol given by nasogastric feeding tube

Aims

To characterize and compare the pharmacokinetic profiles of bromazepam, omeprazole and paracetamol when administered by the oral and nasogastric routes to the same healthy cohort of volunteers.

Methods

In a prospective, monocentric, randomized crossover study, eight healthy volunteers received the three drugs by the oral (OR) and nasogastric routes (NT). Sequential plasma samples were analyzed by high-performance liquid chromatography–UV, pharmacokinetic parameters (C_max, ${\text{AUC}}_{0 - \infty } Aims  To characterize and compare the pharmacokinetic profiles of bromazepam, omeprazole and paracetamol when administered by the oral and nasogastric routes to the same healthy cohort of volunteers. Methods  In a prospective, monocentric, randomized crossover study, eight healthy volunteers received the three drugs by the oral (OR) and nasogastric routes (NT). Sequential plasma samples were analyzed by high-performance liquid chromatography–UV, pharmacokinetic parameters (C_max, , t_?, k_e, t_max) were compared statistically, and C_max, and t_max were analyzed for bioequivalence. Results  A statistically significant difference was seen in the of bromazepam, with nasogastric administration decreasing availability by about 25%: AUC_OR = 2501 ng mL⁻¹ h; AUC_NT = 1855 ng mL⁻¹ h (p < 0.05); ratio (geometric mean) = 0.74 [90% confidence interval (CI) 0.64–0.87]. However, this does not appear to be clinically relevant given the usual dosage range and the drug’s half-life (approx. 30 h). A large interindividual variability in omeprazole parameters prevented any statistical conclusion from being drawn in terms of both modes of administration despite their similar average profile: AUC_OR = 579 ng mL⁻¹ h; AUC_NT = 587 ng mL⁻¹ h (p > 0.05); ratio (geometric mean) = 1.01 (90% CI 0.64–1.61). An extended study with a larger number of subjects may possibly provide clearer answers. The narrow 90% confidence limits of paracetamol indicate bioequivalence: AUC_OR = 37 μg mL⁻¹ h; AUC_NT = 41 μg mL⁻¹ h(p > 0.05); ratio (geometric mean) = 1.12 (90% CI 0.98–1.28). Conclusion  The results of this study show that the nasogastric route of administration does not appear to cause marked, clinically unsuitable alterations in the bioavailability of the tested drugs.     

Effects of allicin on CYP2C19 and CYP3A4 activity in healthy volunteers with different <Emphasis Type="Italic">CYP2C19</Emphasis> genotypes

Objective  To investigate the interaction between allicin and omeprazole and to observe the effects of allicin on CYP2C19 and CYP3A4 activity in healthy Chinese male volunteers with different CYP2C19 genotypes. Methods  Eighteen subjects (six CYP2C19*1/CYP2C19*1, four CYP2C19*1/CYP2C19*2, two CYP2C19*1/ CYP2C19*3, and six CYP2C19*2/ CYP2C19*2) were enrolled in a two-phase randomized crossover trial. In each phase, all subjects received placebo or a 180 mg allicin capsule once daily for 14 consecutive days. The pharmacokinetics of omeprazole (20 mg orally on day 15) was determined for up to 12 h following administration by high-performance liquid chromatography. Results  In carriers of the CYP2C19*1/CYP2C19*1 and CYP2C19*1/CYP2C19*2 or *3 genotype, allicin treatment increased the peak plasma concentration (C_max) of omeprazole by 49.7 ± 7.2 (p < 0.001) and 54.2 ± 9.2% (p < 0.001), and increased the area under the plasma time–concentration curve ( ) of omeprazole by 48.1 ± 9.0 (p = 0.001) and 73.6 ± 26.7% (p < 0.001), respectively. The ratio of of 5-hydroxyomeprazole to omeprazole (a marker for CYP2C19 activity) decreased significantly (p < 0.001 and p = 0.001, respectively). However, no pharmacokinetic parameters were significantly changed by allicin in CYP2C19*2/CYP2C19*2. The C_max and of omeprazole sulfone were unchanged in all three genotypes. Conclusions  Allicin reduced the metabolism of omeprazole by inhibiting CYP2C19 activity in individuals with the CYP2C19*1/CYP2C19*1 and CYP2C19*1/CYP2C19*2 or *3 genotypes, but not in those with the CYP2C19*2/ CYP2C19*2 genotype. Allicin did not significantly affect the activity of CYP3A4 in all subjects.     

Estimation of the area under the concentration-time curve of racemic lansoprazole by using limited plasma concentration of lansoprazole enantiomers

Objective The purpose of this study was to identify the common time point that gives plasma concentrations of lansoprazole enantiomers that adequately reflect the AUC of racemic lansoprazole. Methods A randomized, double-blind, placebo-controlled, crossover study in three phases was conducted at intervals of 2 weeks. Eighteen healthy Japanese volunteers, including three CYP2C19 genotype groups, took a single 60-mg oral dose of lansoprazole after 6 days of pretreatment, with either clarithromycin (800 mg/day), fluvoxamine (50 mg/day), or a placebo. Multiple linear regression analysis was used to identify the most informative sampling times of (R)- and (S)-lansoprazole, using one to three samples to estimate the AUC_0−∞ of racemic lansoprazole. Results The best R ² in each prediction formula for the AUC of racemic lansoprazole using one, two, and three sampling points of (R)- and (S)-lansoprazole based on the data sets from all three pretreatment groups (n = 54) were 0.897, 0.930, and 0.929, respectively. The best prediction formula for the AUC of racemic lansoprazole, using the fewest sampling points of (R)- and (S)-lansoprazole, was (P < 0.001), where C_3h is the plasma concentration 3 h after administration, G1 = 1 for the homozygous extensive metabolizer (EM) and 0 for the other genotypes, G2 = 1 for the heterozygous EM and 0 for the other genotypes. Conclusions C_3h monitoring of (R)- and (S)-lansoprazole is a useful time point to estimate the AUC of racemic lansoprazole. This method of plasma concentration monitoring at a few time points within 3 h might be more suitable for AUC estimation than CYP2C19 genotyping, particularly when lansoprazole is co-administered with CYP inhibitors.     

Effect of silymarin on the pharmacokinetics of losartan and its active metabolite E-3174 in healthy Chinese volunteers

Purpose  To investigate the effects of silymarin on the pharmacokinetics of losartan and its active metabolite E-3174 and its relationship with CYP2C9 genotypes. Methods  Twelve healthy adult men of known CYP2C9 genotype (six CYP2C9*1/*1 and six CYP2C9*1/*3) were recruited in a two-phase randomized crossover design study. The pharmacokinetics of losartan and E-3174 were measured before and after a 14-day treatment with 140 mg of silymarin three times daily. Results  The area under the plasma concentration–time curve (AUC) of losartan increased significantly following a 14-day silymarin treatment in subjects with the CYP2C9*1/*1 genotype, but not in those with the CYP2C9*1/*3 genotype. The AUC of E-3174 decreased significantly with a silymarin pretreatment in both CYP2C9*1/*1 and the CYP2C9*1/*3 subjects. The metabolic ratio of losartan (ratio of of E-3174 to of losartan) decreased significantly after a 14-day treatment with silymarin in individuals with the CYP2C9*1/*1 genotype (p < 0.05), but not in those with the CYP2C9*1/*3 genotype (p = 0.065). Conclusion  Silymarin inhibits the metabolism of losartan to E-3174, with the magnitude of the interaction differing in individuals with different CYP2C9 genotypes.     

Structure in Dehydrated Trehalose Dihydrate—Evaluation of the Concept of Partial Crystallinity

Purpose (i) To use trehalose as a model compound to evaluate the concept of crystallinity in pharmaceuticals. (ii) To understand the structural nature of dehydrated trehalose dihydrate.Materials and Methods Trehalose dihydrate was dehydrated isothermally at several temperatures below 100°C and the anhydrous product was characterized by XRD, DSC and water vapor sorption.Results XRD and DSC suggested that the dehydration product was a partially crystalline α-polymorphic form of anhydrous trehalose . An increase in the temperature of dehydration resulted in a decrease in lattice order. In agreement with earlier findings, the ordered regions in the dehydrated lattice converted to the dihydrate at much lower RH values than amorphous trehalose. However, the lattice order in the dehydrated product dictated the RH at which this conversion was initiated—the higher the lattice order the lower this RH. The structural nature of these samples can be explained based on the one-state model of crystallinity. In dehydrated trehalose, there is a continuum in lattice order ranging from highly crystalline to a completely disordered (i.e., amorphous) state.Conclusion The extent of lattice order in anhydrous trehalose was dictated by the kinetics of water removal from trehalose dihydrate. The partially crystalline nature of anhydrous trehalose produced by dehydration could be described on a continuous scale of lattice order based on the one-state model of crystallinity.     

Voriconazole and fluconazole increase the exposure to oral diazepam

Objective We assessed the effect of voriconazole and fluconazole on the pharmacokinetics and pharmacodynamics of diazepam. Methods Twelve healthy volunteers took 5 mg of oral diazepam in a randomised order on three study sessions: without pretreatment, after oral voriconazole 400 mg twice daily on the first day and 200 mg twice daily on the second day, or after oral fluconazole 400 mg on the first day and 200 mg on the second day. Plasma concentrations of diazepam and N-desmethyldiazepam were determined for up to 48 h. Pharmacodynamic variables were measured for 12 h. Results In the voriconazole phase, the area under the plasma concentration time curve of diazepam was increased (geometric mean ratio) 2.2-fold (p < 0.05; 90% confidence interval [CI] 1.56 to 2.82). This was associated with the prolongation of the mean elimination half-life (t_1/2) from 31 h to 61 h (p < 0.01) after voriconazole. In the fluconazole phase, the of diazepam was increased 2.5-fold (p < 0.01; 90% CI 1.94 to 3.40), and the t_1/2 was prolonged from 31 h to 73 h (p < 0.001). The peak plasma concentration of diazepam was practically unchanged by voriconazole and fluconazole. The pharmacodynamics of diazepam were changed only modestly. Conclusion Both voriconazole and fluconazole considerably increase the exposure to diazepam. Recurrent administration of diazepam increases the risk of clinically significant interactions during voriconazole or fluconazole treatment, because the elimination of diazepam is impaired significantly.     

Behavioral evaluation of mice deficient in GABAB(1) receptor isoforms in tests of unconditioned anxiety

Rationale Emerging data support a role for GABA_B receptors in anxiety. GABA_B receptors are comprised of a heterodimeric complex of GABA_B1 and GABA_B2 receptor subunits. The predominant neuronal GABA_B1 receptor isoforms are GABA_B(1a) and GABA_B(1b). Recent findings indicate specific roles for these isoforms in conditioned fear responses, although their influence on behavior in tests of unconditioned anxiety is unknown. Objective The aim of this study was to examine the role of the GABA_B(1) isoforms in unconditioned anxiety. Materials and methods Mice deficient in the GABA_B(1a) or GABA_B(1b) receptor isoforms were examined in a battery of anxiety tests. Results In most tests, genotype did not significantly affect anxious behavior, including the elevated plus maze, marble burying, and stress-induced hypothermia tests. Corticosterone and adrenocorticotropic hormone levels were similarly unaffected by genotype. Female, but not male, and mice showed increased anxiety relative to wild-type controls in the elevated zero maze. In the staircase test, male mice defecated more than male mice, although no other test parameter was influenced by genotype. In the light–dark box, female mice spent less time in the light compartment compared to the females, whereas male mice made fewer light–dark transitions than males. Conclusions Specific roles for either GABA_B(1) isoform in unconditioned anxiety were not explicit. This differs from their contribution in conditioned anxiety and from the anxious phenotype of GABA_B1 and GABA_B2 subunit knockout mice. The findings suggest that the GABA_B(1) isoforms have specific relevance for anxiety with a cognitive component, rather than for innate anxiety per se.     

Pharmacological profile of NOP receptors coupled with calcium signaling via the chimeric protein Gαqi5

In this study, the Gα_qi5 protein was used to force the human nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor to signal through the Ca²⁺ pathway in CHO cells. [Ca²⁺]_i levels were monitored using the fluorometer FlexStation II and the Ca²⁺ dye Fluo 4 AM. Concentration response curves were generated with a panel of full and partial agonists, while NOP antagonists were assessed in inhibition-response curves. The following rank order of potency of antagonists was measured: naloxone, which is superimposable to literature findings. The rank order of potency of full and partial agonists is also similar to that obtained in previous studies with the exception of a panel of ligands (UFP-112, Ro 64-6198, ZP120, UFP-113) whose potency was relatively low in the Gα_qi5–NOP receptor calcium assay. Interestingly, these NOP ligands are characterized by slow kinetic of interaction with the NOP receptor, as demonstrated by bioassay experiments. These results demonstrated that the FlexStation II–Gα_qi5–NOP receptor calcium assay represents an adequate and useful screening for NOP receptor ligands, particularly for antagonists.     

β-Relaxation of Insulin Molecule in Lyophilized Formulations Containing Trehalose or Dextran as a Determinant of Chemical Reactivity

Purpose The purpose of this study was to elucidate whether the degradation rate of insulin in lyophilized formulations is determined by matrix mobility, as reflected in glass transition temperature (T_g), or by β-relaxation, as reflected in rotating-frame spin-lattice relaxation time . Methods The storage stability of insulin lyophilized with dextran was investigated at various relative humidities (RH; 12–60%) and temperatures (40–90°C) and was compared with previously reported data for insulin lyophilized with trehalose. Insulin degradation was monitored by reverse-phase high-performance liquid chromatography. Furthermore, the of the insulin carbonyl carbon in the lyophilized insulin–dextran and insulin–trehalose systems was measured at 25°C by ¹³C solid-state NMR, and the effect of trehalose and dextran on was compared at various humidities. Results The degradation rate of insulin lyophilized with dextran was not significantly affected by the T_g of the matrix, even at low humidity (12% RH), in contrast to that of insulin lyophilized with trehalose. The insulin–dextran system exhibited a substantially greater degradation rate than the insulin–trehalose system at a given temperature below the T_g. The difference in degradation rate between the insulin–dextran and insulin–trehalose systems observed at 12% RH was eliminated at 43% RH. In addition, the of the insulin carbonyl carbon at low humidity (12% RH) was prolonged by the addition of trehalose, but not by the addition of dextran. This difference was eliminated at 23% RH, at which point the solid remained in the glassy state. These findings suggest that the β-relaxation of insulin is inhibited by trehalose at low humidity, presumably as a result of insulin–trehalose interaction, and thus becomes a rate determinant. In contrast, dextran, whose ability to interact with insulin is thought to be less than that of trehalose, did not inhibit the β-relaxation of insulin, and thus, the chemical activational barrier (activation energy) rather than β-relaxation becomes the major rate determinant. Conclusions β-Relaxation rather than matrix mobility seems to be more important in determining the stability of insulin in the glassy state in lyophilized formulations containing trehalose and dextran.     

Implications for Clinical Pharmacodynamic Studies of the Statistical Characterization of an In Vitro Antiproliferation Assay

Modeling of nonlinear pharmacodynamic (PD) relationships necessitates the utilization of a weighting function in order to compensate for the heteroscedasticity. The structure of the variance was studied for concentration–effect data generated in an in vitro 96-well plate cell growth inhibition assay, where data are numerous (480 data points per experiment) and replication is easy. From the five candidate models that were considered, the power function , where is the sample mean and is the sample variance, was shown to be the most appropriate to describe the nonuniformity of the variance along the range of measured effect for 253 sets of data. The Hill model was fit to the concentration–effect data with weighted nonlinear regression, where the weights were equal to the reciprocal of the predicted variance. The examination of the distribution of the 253 sets of parameters of the PD model showed that IC₅₀ was lognormally distributed whereas the distribution of was normal. The characterization of the appropriate variance function and concentration–effect function in a simple in vitro experimental setting with a large number of experiments, with each experiment including a large number of data points, will be useful for guiding similar in vitro concentration–effect studies where data are plentiful and for guiding PD modeling in complex clinical settings in which extensive data for model characterization is impossible to obtain.     

The Effects of Milling on the Surface Properties of Form I Paracetamol Crystals

Purpose The effects of milling and particle size on surface energies of form I paracetamol crystals are reported.Methods Paracetamol crystals (75–850 μm) were obtained by cooling methanol and acetone saturated solutions. Additionally, macroscopic (>2 cm) single crystals were grown by slow solvent evaporation from saturated solutions, ball milled and sieved into different particle size fractions. Surface properties were characterised using Inverse Gas Chromatography and compared with those calculated from sessile drop contact angle measurements on macroscopic single crystals.Results Dispersive surface energies, for milled samples increased by 20% with decreasing particle size. With decreasing particle size acceptor numbers, K _A values were constant but donor numbers, K _B decreased. For unmilled materials K _B was comparable to K _A but with a significantly lower of only 33 mJ/m². Milling resulted in fracture along the crystal's lowest attachment energy plane (010), exposing facets of different surface chemistry to that of the native external facets. θ for the (010) fracture plane confirmed a higher compared to external facets such as (011) of single crystals.Conclusions Milling exposes a hydrophobic surface for paracetamol form I crystals which becomes increasingly more dominant with decreasing particle size.     

Synthesis and characterization of coordination compounds of silver and tryptophan

It is shown using the isomolar series method that the synthesis of coordination compounds of silver and tryptophan both by direct interaction of AgNO₃ with tryptophan and via the formation of an intermediate silver hydroxide yields target compounds with silver:tryptophan molar ratios 1:2, 1:1, and 2:1. It is established by pH-titration that synthesis of coordination compounds of silver and tryptophan via the intermediate silver oxide forms four complexes with logarithms of stability constants log , , , , and . Translated from Khimiko-Farmatsevticheskii Zhurnal, Vol. 42, No. 12, pp. 36–39, December, 2008.     

Chloroquine pharmacokinetics in pregnant and nonpregnant women with vivax malaria

Purpose  We compared the pharmacokinetics of chloroquine in pregnant and nonpregnant women treated for Plasmodium vivax malaria. Methods  Twelve pregnant women and 15 nonpregnant women of child-bearing age with acute P. vivax malaria were treated with 25 mg chloroquine base/kg over 3 days on the northwestern border of Thailand. Blood concentrations of chloroquine and desethylchloroquine were measured using hydrophilic interaction liquid chromatography coupled with fluorescence detection. Twenty-five women completed the pharmacokinetic study. Results  Although increasing gestational age was associated with reduced chloroquine , there was no significant difference overall in the pharmacokinetics of chloroquine between pregnant and nonpregnant women. Fever was associated with lower chloroquine values. Desethylchloroquine area under the curve (AUC) values were not significantly affected by pregnancy. Conclusions  Pregnancy did not significantly affect blood concentrations of chloroquine or its metabolite, desethylchloroquine, in women with P. vivax malaria.     

The influence of dose and <Emphasis Type="Italic">N</Emphasis>-acetyltransferase-2 (NAT2) genotype and phenotype on the pharmacokinetics and pharmacodynamics of isoniazid

Objective This study evaluated the pharmacokinetics of isoniazid (INH) associated with optimal early bactericidal activity (EBA), defined as 90% of the maximum EBA (EBA₉₀) and the influence of N-acetyltransferase-2 (NAT2) subtype on the ability of pulmonary tuberculosis (PTB) patients to reach the identified pharmacokinetic values after INH doses ranging from 0.2 to 10–12 mg/kg body weight. Methods INH serum concentrations and NAT2 subtype were determined during four studies of PTB patients in three of whom the EBA of INH was determined. The relationship of EBA to area under the curve (AUC) and 2-h serum concentrations was examined by exponential regression and fitted curves estimated the and 2-h serum concentrations at which EBA₉₀ was reached. Results EBA₉₀ was reached at an of 10.52 μg/ml per hour and 2-h serum concentrations of 2.19 μg/ml. An of 10.52 μg/ml per hour was reached by all 66 patients receiving a 10–12 mg/kg INH dose and all 21 receiving 6 mg/kg, except 1 of 10 (10%) homozygous fast (FF) acetylators; however, at 5 mg/kg, 4 of 12 (33%) FF and 26 of 27 (96%) heterozygous fast (FS), but all 21 homozygous slow (SS) acetylators did so; and 1 of 3 (33%) FF, 2 of 6 (33%) FS, but all 4 SS acetylators at dose 3 mg/kg. An INH 2-h serum concentration of 2.19 μg/ml was reached by all 66 patients receiving 10–12 mg/kg and all 21 receiving 6 mg/kg, except for 2 (20%) FF acetylators at a dose of 5 mg/kg; however, only 3 (25%) of 12 FF acetylators, but 26 (96%) of 27 FS acetylators, and all 21 SS acetylators reached this concentration; and at a dose of 3 mg/kg, 1 (33%) of 3 FF acetylators, 2 (33%) of 6 FF, but all 4 SS acetylators. Conclusions At a 6 mg/kg dose, all except a minority of FF NAT2 acetylators, achieve an INH and 2-h INH serum concentrations associated with EBA₉₀, as did all 4 SS acetylators receiving 3 mg/kg. Any dose reduction below 6 mg/kg body weight will tend to disadvantage a significant proportion of faster acetylators, but, conversely, SS acetylators require only a 3 mg/kg dose to achieve a satisfactory exposure to INH.     

Accumulation of Succinimide in a Recombinant Monoclonal Antibody in Mildly Acidic Buffers Under Elevated Temperatures

Purpose The purpose of this paper was to identify the location of a succinimide and determine the rate of its formation and hydrolysis in a recombinant human monoclonal IgG2 antibody aged in mildly acidic buffers at elevated temperatures. Materials and Methods Cation exchange (CEX) HPLC separated multiple Main Peaks and high levels (up to 50%) of basic variants, the identification of which was an analytical challenge and required several complementary techniques. The relative abundance of the CEX basic variants was used to quantify the percentage of succinimide and to study the rates of its formation and hydrolysis. Results Mass decrease by approximately 18 Da for intact antibodies from the CEX basic fractions suggested succinimide formation from aspartic acid as the major modification. Reversed-phase HPLC/MS of the reduced and trypsin-digested samples detected an isoaspartate 30 (isoD30) in the light chain peptide A25-R37. Direct evidence that isoD30 was from succinimide was obtained by performing succinimide hydrolysis in followed by tryptic digestion in . Conclusions Succinimide formation increased as pH became more acidic, whereas its hydrolysis was faster as pH became neutral and alkaline. Succinimide hydrolysis in a denatured sample was estimated to have completed in less than 2 h, but approximately three days for a similar pH but without denaturant. These observations suggest that protein conformation affects succinimide hydrolysis.     

Examining sex-related differences in enteric itraconazole metabolism in healthy adults using grapefruit juice

Objective To explore whether sex-related differences in intestinal itraconazole metabolism exist in healthy adults using grapefruit juice (GFJ) as a selective enteric cytochrome P450 3A4 (CYP3A4) inhibitor. Methods Twenty (ten female) subjects received 240 mL bottled water or single-strength GFJ from a frozen concentrate three times daily for 2 days. On day 3, the subjects received an itraconazole oral solution 200 mg with 240 mL of beverage followed 2 h later by 240 mL of the same beverage. Serial blood sampling for itraconazole and hydroxyitraconazole serum concentrations was performed over a 72-h period. After a 20-day washout, the subjects crossed over and repeated the study. Results Among the female subjects, GFJ reduced itraconazole weight-adjusted apparent oral clearance (Cl/F) (19%, p = 0.006) and increased (30%, p = 0.01), but produced no significant change in hydroxyitraconazole pharmacokinetics. In males, GFJ produced no significant change in either itraconazole, or hydroxyitraconazole pharmacokinetics. Grapefruit juice also significantly reduced the metabolite:parent ratio (12%, p = 0.047), in females, but not males. Itraconazole weight-adjusted oral Cl/F was significantly higher in females than males when itraconazole was administered with water (56%, p = 0.009), and although the extent to which GFJ altered itraconazole weight-adjusted oral CL/F was greater in females, it did not differ significantly between the sexes (p = 0.085). Results The influence of GFJ on the presystemic metabolism of itraconazole was greater in females than males. Repeated ingestion of GFJ significantly reduced itraconazole weight-adjusted oral CL/F and significantly increased exposure in females, but it produced no significant change among males. Although itraconazole weight-adjusted oral Cl/F was much higher in females than in males, the extent to which GFJ altered itraconazole weight-adjusted oral CL/F did not differ significantly between the sexes. This work was presented in part at the American College of Clinical Pharmacy Annual Meeting, October 25, 1999, Kansas City, Missouri and at the 40th Interscience Conference of Antimicrobial Agents and Chemotherapy, September 18, 2000, Toronto Canada.     

Tolfenamic acid is a potent CYP1A2 inhibitor in vitro but does not interact in vivo: correction for protein binding is needed for data interpretation

Objective Our aim was to correlate the in vitro and in vivo CYP1A2 inhibition potential of tolfenamic acid, an NSAID highly (99.7%) bound to plasma proteins, to study the significance of protein binding of inhibitor in metabolic drug interactions. Methods The effect of tolfenamic acid on CYP1A2 (phenacetin O-deethylation) was studied using human liver microsomes, with and without albumin (0–10 mg/ml). In a randomized, crossover study, 10 volunteers took 200 mg tolfenamic acid or placebo t.i.d. for 3 days. On day 2, a caffeine test was performed. On day 3, each ingested 4 mg of the CYP1A2 substrate tizanidine. Plasma tizanidine, its metabolites (M) and tolfenamic acid, and pharmacodynamic variables were measured. Results Tolfenamic acid strongly inhibited phenacetin-O-deethylation in vitro (IC₅₀ 1.8 μM without albumin). Albumin decreased its inhibitory effect in a concentration-dependent manner; the IC₅₀ exceeded 100 μM with 10 mg/ml of albumin. Tolfenamic acid had no effect on the area under the concentration-time curve , peak concentration, time of peak concentration or half-life of tizanidine or M-3; only the of secondary metabolite M-4 was slightly decreased (13%, P = 0.004). The caffeine test and the pharmacodynamic effects of tizanidine were unchanged. Conclusions Tolfenamic acid potently inhibits CYP1A2 in vitro when studied without albumin, but not in vivo. This apparent discrepancy is due to the high protein binding of tolfenamic acid. To avoid overestimation of the interaction potential, the inhibitory effect of highly albumin-bound compounds should also be studied in vitro with albumin, or their exact unbound plasma concentration should be used in predictions.