Kiyomi Ito

Effects of Intestinal CYP3A4 and P-Glycoprotein on Oral Drug Absorption—Theoretical Approach

AbstractPurpose. To evaluate the effects of gut metabolism and efflux on drug absorption by simulation studies using a pharmacokinetic model involving diffusion in epithelial cells. Methods. A pharmacokinetic model for drug absorption was constructed including metabolism by CYP3A4 inside the epithelial cells, P-gp-mediated efflux into the lumen, intracellular diffusion from the luminal side to the basal side, and subsequent permeation through the basal membrane. Partial differential equations were solved to yield an equation for the fraction absorbed from gut to the blood. Effects of inhibition of CYP3A4 and/or P-gp on the fraction absorbed were simulated for a hypothetical substrate for both CYP3A4 and P-gp. Results. The fraction absorbed after oral administration was shown to increase following inhibition of P-gp. This increase was more marked when the efflux clearance of the drug was greater than the sum of the metabolic and absorption clearances and when the intracellular diffusion constant was small. Furthermore, it was demonstrated that the fraction absorbed was synergistically elevated by simultaneous inhibition of both CYP3A4 and P-gp. Conclusions. The analysis using our present diffusion model is expected to allow the prediction of in vivo intestinal drug absorption and related drug interactions from in vitro studies using human intestinal microsomes, gut epithelial cells, CYP3A4-expressed Caco-2 cells, etc.

Quantitative Prediction of In Vivo Drug-Drug Interactions from In Vitro Data Based on Physiological Pharmacokinetics: Use of Maximum Unbound Concentration of Inhibitor at the Inlet to the Liver

AbstractPurpose. To assess the degree to which the maximum unboundconcentration of inhibitor at the inlet to the liver (Iinlet,u,max), used in theprediction of drug-drug interactions, overestimates the unboundconcentration in the liver. Methods. The estimated value of Iinlet,u,max was compared with theunbound concentrations in systemic blood, liver, and inlet to the liver,obtained in a simulation study based on a physiological flow model.As an example, a tolbutamide/sulfaphenazole interaction was predictedtaking the plasma concentration profile of the inhibitor intoconsideration. Results. The value of Iinlet,u,max differed from the concentration in eachcompartment, depending on the intrinsic metabolic clearance in theliver, first-order absorption rate constant, non-hepatia clearance andliver-to-blood concentration ratio (Kp) of the inhibitor. The AUC oftolbutamide was predicted to increase 4-fold when co-administeredwith sulfaphenazole, which agreed well with in vivo observations andwas comparable with the predictions based on a fixed value of Iinlet,u,max.The blood concentration of tolbutamide was predicted to increase whenit was co-administered with as little as 1/100 of the clinical doseof sulfaphenazole. Conclusions. Although Iinlet,u,max overestimated the unboundconcentration in the liver, the tolbutamide/sulfaphenazole interaction couldbe successfully predicted by using a fixed value of Iinlet,u,max, indicatingthat the unbound concentration of sulfaphenazole in the liver after itsclinical dose is by far larger than the concentration to inhibitCYP2C9-mediated metabolism and that care should be taken when it isco-administered with drugs that are substrates of CYP2C9.

Prediction of in vivo interaction between triazolam and erythromycin based on in vitro studies using human liver microsomes and recombinant human CYP3A4.

AbstractPurpose. To quantitatively predict the in vivo interaction betweentriazolam and erythromycin, which involves mechanism-basedinhibition of CYP3A4, from in vitro studies using human liver microsomes(HLM) and recombinant human CYP3A4 (REC). Methods. HLM or REC was preincubated with erythromycin in thepresence of NADPH and then triazolam was added. α- and 4-hydroxy(OH) triazolam were quantified after a 3 min incubation and the kineticparameters for enzyme inactivation (kinact and K′app) were obtained.Drug-drug interaction in vivo was predicted based on aphysiologically-based pharmacokinetic (PBPK) model, using triazolam anderythromycin pharmacokinetic parameters obtained from the literature and kineticparameters for the enzyme inactivation obtained in the in vitro studies. Results. Whichever enzyme was used, triazolam metabolism was notinhibited without preincubation, even if the erythromycin concentrationwas increased. The degree of inhibition depended on preincubationtime and erythromycin concentration. The values obtained for kinactand K′app were 0.062 min−1 and 15.9 μM (α-OH, HLM), 0.055 min−1and 17.4 μM (4-OH, HLM), 0.173 min−1 and 19.1 μM (α-OH, REC),and 0.097 min−1 and 18.9 μM (4-OH, REC). Based on the kineticparameters obtained using HLM and REC, the AUCpo of triazolamwas predicted to increase 2.0- and 2.6-fold, respectively, followingoral administration of erythromycin (333 mg t.i.d. for 3 days), whichagreed well with the reported data. Conclusions. In vivo interaction between triazolam and erythromycinwas successfully predicted from in vitro data based on a PBPK modelinvolving a mechanism-based inhibition of CYP3A4.

Which concentration of the inhibitor should be used to predict in vivo drug interactions from in vitro data

When the metabolism of a drug is competitively or noncompetitively inhibited by another drug, the degree of in vivo interaction can be evaluated from the [I]u/Ki ratio, where [I]u is the unbound concentration around the enzyme and Ki is the inhibition constant of the inhibitor. In the present study, we evaluated the metabolic inhibition potential of drugs known to be inhibitors or substrates of cytochrome P450 by estimating their [I]u/Ki ratio using literature data. The maximum concentration of the inhibitor in the circulating blood ([I]max), its maximum unbound concentration in the circulating blood ([I]max,u), and its maximum unbound concentration at the inlet to the liver ([I]in,max,u) were used as [I]u, and the results were compared with each other. In order to calculate the [I]u/Ki ratios, the pharmacokinetic parameters of each drug were obtained from the literature, together with their reported Ki values determined in in vitro studies using human liver microsomes. For most of the drugs with a calculated [I]in,max,u/Ki ratio less than 0.25, which applied to about half of the drugs investigated, no in vivo interactions had been reported or “no interaction” was reported in clinical studies. In contrast, the [I]max,u/Ki and [I]max/Ki ratio was calculated to be less than 0.25 for about 90% and 65% of the drugs, respectively, and more than a 1.25-fold increase was reported in the area under the concentration-time curve of the co-administered drug for about 30% of such drugs. These findings indicate that the possibility of underestimation of in vivo interactions (possibility of false-negative prediction) is greater when [I]max,u or [I]max values are used compared with using [I]in,max,u values.

Inhibition of in vitro metabolism of simvastatin by itraconazole in humans and prediction of in vivo drug-drug interactions

AbstractPurpose. To evaluate an interaction between simvastatin and itraconazole in in vitro studies and to attempt a quantitative prediction of in vivo interaction in humans. Methods. The inhibitory effect of itraconazole on simvastatin metabolism was evaluated using human liver microsomes and the Ki values were calculated for the unbound drug in the reaction mixture. A physiologically-based pharmacokinetic model was used to predict the maximum in vivo drug-drug interaction. Results. Itraconazole competitively inhibited the metabolism of simvastatin to M-1 and M-2 with Ki values in the nM range. The area under the curve (AUC) of simvastatin after concomitant dosing with itraconazole was predicted to increase ca. 84-101-fold compared with that without administration of itraconazole. Taking into consideration the fact that this method predicts the maximum interaction, this agrees well with the clinical observation of a 19-fold increase. A similar prediction, based on the Ki value without taking into account the drug adsorption to microsomes, led to an underevaluation of the interaction. Conclusions. It was demonstrated that the competitive inhibition of CYP3A4-mediated simvastatin metabolism by itraconazole is the main cause of the drug interaction and that a Ki value corrected for drug adsorption to microsomes is the key factor in quantitatively predicting the maximum in vivo drug interactions.

Assessment of systemic adverse reactions induced by ophthalmic β-adrenergic receptor antagonists

To assess quantitatively the risks of ophthalmic beta-blocking agents for cardiovascular and respiratory adverse reactions, we analyzed the binding kinetics of beta-blocking agents to the beta-1 and beta-2 adrenoceptors. The relationship between the occupancies for beta-1 and beta-2 adrenoceptors and the effects on the exercise pulse rate or the forced expiratory volume in one second (FEV1) after topical administration of carteolol, befunolol, timolol and betaxolol was analyzed using a ternary complex model. The beta-1 and beta-2 receptor occupancies after ophthalmic administration were calculated to be quite high as well as those after oral administration. The maximum occupancies for beta-1 and beta-2 receptors after ordinary ophthalmic administration were 52% and 88% for carteolol, 52% and 61% for befunolol, 62% and 82% for timolol, and 44% and 3% for betaxolol, respectively. Concave relationships were obtained between a decrease in exercise pulse rate and the beta-1 receptor occupancy and between a decrease in FEV1 and beta-2 receptor occupancy, respectively. Nasolacrimal occlusion was estimated to decrease the exercise pulse rate and FEV1 by 65% and 50%, respectively. The beta-1 and beta-2 adrenoceptor occupancies were proved to be the most appropriate indicators for cardiac and pulmonary adverse reactions evoked by ophthalmic beta-blocking agents.

Oral absorption of cephalosporins is quantitatively predicted from in vitro uptake into intestinal brush border membrane vesicles

In order to establish a method to predict oral absorption of drugs, which are absorbed by the oligopeptide transporter (PepT1), fraction absorbed (F) of cephalosporin antibiotics was predicted from in vitro uptake into rat intestinal brush border membrane vesicles (BBMV). Using in vitro uptake data, F values of cephalosporins in humans were predicted using the equation derived from the complete radial mixing (CRM) model, which was proposed by Amidon et al. (Amidon et al., J. Pharm. Sci. 69 (1980) 1369). In the present study, uptake into BBMV was measured at 25 and 4 degrees C in the presence of an H+ -gradient, and the uptake clearance (CLuptake) was calculated. Clearance for the uptake mediated by PepT1 (DeltaCLuptake) was then calculated as CLuptake at 25 degrees C minus that at 4 degrees C. When DeltaCLuptake and F values were analyzed according to the present equation, fairly good correlation between DeltaCLuptake and F was observed. It was further demonstrated that the present method may be able to quantitatively predict F values of cephalosporins by using several cephalosporins as standards.

Comparative study of effects of angiotensin II receptor antagonist, KD3-671, and angiotensin converting enzyme inhibitor, enalaprilat, on cough reflex in guinea pig

SummaryAngiotensin converting enzyme (ACE) inhibitor prevents the inactivation of bradykinin by inhibiting ACE activity, leading to side effects such as dry cough and angioedema. KD3-671 is a novel nonpeptide angiotensin II antagonist which is expected to exhibit persistent hypotensive action without these side effects. In this study, we investigated the relationship between the pharmacokinetics and cough-inducing effect of this drug in guinea-pig, compared with that of an ACE inhibitor, enalaprilat. KD3-671 was not significantly different from the vehicle treatment in the ability to induce coughing, whereas enalaprilat significantly enhanced coughing compared with the vehicle treatment. Thus, as expected from its mechanism of pharmacological action, KD3-671 did not induce coughing. We suggest that the citric acid-induced guinea pig coughing model will be useful in preclinical studies to examine the effect of drug on pulmonary function.