The effect of ketoconazole on the in vivo intestinal permeability of fexofenadine using a regional perfusion technique

AIMS: To investigate whether the drug-drug interaction between fexofenadine and ketoconazole is localized to efflux transport proteins of the small intestine, and to determine and classify the effective jejunal permeability (Peff) of fexofenadine according to the Biopharmaceutics Classification System (BCS). METHODS: Two separate jejunal perfusion experiments were performed using the Loc-I-Gut technique in eight healthy volunteers. During treatment 1 (T1), we investigated the acute effect of ketoconazole on the Peff and plasma pharmacokinetics of fexofenadine. In treatment 2 (T2) we examined the effect of oral pretreatment with ketoconazole (200 mg daily for 5 days) on the same absorption parameters. Each experiment was divided into two periods of 100 min and the jejunal segment was perfused with 93 micro m fexofenadine during both periods. In period 2 of each treatment, fexofenadine was coadministered with 94 micro m ketoconazole. The concentrations of fexofenadine in intestinal perfusate and plasma were measured by liquid chromatography with mass detection. RESULTS: During T1, the mean (+/- s.d.) Peff of fexofenadine was low according to the BCS (0.11 +/- 0.11 and 0.04 +/- 0.13 x 10(-4) cm s(-1) in periods 1 and 2, respectively), and the coadministration of ketoconazole in period 2 had no significant acute effect on Peff (95% confidence interval (CI) on the difference -0.37, 0.51). After pretreatment with ketoconazole (T2), the jejunal Peff of fexofenadine increased to 0.29 +/- 0.47 and 0.22 +/- 0.31 x 10-4 cm s(-1) in both periods 1 and 2, respectively, but the change was not statistically significant when compared with T1 (95% CI on the difference -0.62, 0.27 for T1 0-100 min vs T2 0-100 min; -0.54, 0.34 for T1 0-100 min vs T2 100-200 min). Fexofenadine plasma AUC from 0-100 mg showed no significant difference after pretreatment with ketoconazole (55 +/- 101 and 51 +/- 33 micro g ml(-1) min(-1) respectively; 95% CI on the difference -108, 115). Total plasma AUC (0-720 min) was 318 +/- 426 and 426 +/- 232 ng ml(-1) min in T1 and T2, respectively (95% CI on the difference -622, 405). CONCLUSIONS: No significant effect of acute coadministration or pretreatment with ketoconazole on the in vivo intestinal absorption of fexofenadine was detected in this study.     

Increased Absorption of Digoxin from the Human Jejunum Due to Inhibition of Intestinal Transporter-Mediated Efflux

BACKGROUND AND OBJECTIVES: The contribution of transport in the small intestine by the apically located efflux pump P-glycoprotein to variable drug absorption in humans is still poorly understood. We therefore investigated whether inhibition of intestinal P-glycoprotein-mediated efflux by quinidine leads to increased absorption of the P-glycoprotein substrate digoxin. METHODS: Using a multilumen perfusion catheter, we investigated the impact of P-glycoprotein inhibition on absorption of two compounds: the P-glycoprotein substrate digoxin and the marker for passive transcellular absorption antipyrine. Two 20cm adjacent jejunal segments were isolated with the multilumen perfusion catheter in seven healthy subjects. Unlabelled and deuterated digoxin and antipyrine, respectively, were simultaneously infused into either of the intestinal segments. One of the segments was additionally perfused with the P-glycoprotein inhibitor quinidine. Intestinal perfusates were collected for 3 hours, and drug concentrations were determined in the intestinal perfusates, plasma and urine. RESULTS: Quinidine did not affect the disposition of antipyrine. In contrast, coadministration of quinidine into one jejunal segment caused a considerable increase in the amount of digoxin absorbed from this segment compared with the absorption from the other quinidine-free segment (22.3 +/- 8.9% vs 55.8 +/- 21.2% of the dose; p < 0.05). Accordingly, the area under the plasma concentration-time curve and the maximum plasma concentration of digoxin were considerably higher when luminal quinidine was coadministered (p < 0.05 and p < 0.001, respectively). Differences in digoxin absorption from the two intestinal segments were also reflected by pronounced differences in urinary digoxin elimination (5.5 +/- 3.3% vs 19.2 +/- 8.1% of the dose; p < 0.01). CONCLUSIONS: P-glycoprotein inhibition in enterocytes increases systemic exposure of orally administered drugs that are P-glycoprotein substrates. These data highlight the importance of the small intestine as an active barrier against xenobiotics.     

Alteration of fexofenadine disposition in the rat isolated perfused liver following injection of bacterial lipopolysaccharide

1. The aim of the present study was to examine the effect of bacterial lipopolysaccharide (LPS) on the disposition of an organic anion transporting polypeptide and P-glycoprotein substrate in the rat isolated perfused liver. 2. Male Sprague-Dawley rats were divided into four groups. Three of the groups received 1, 2.5 or 5 mg/kg, i.p., Escherichia coli LPS in sterile saline. The fourth group received an equivalent volume of sterile saline i.p. Twenty-four hours after treatment, rats were anaesthetized and the liver isolated and perfused with fexofenadine at an initial concentration of 2000 ng/mL in a recirculating system. Perfusate and bile samples were collected for 60 min and the liver was collected at the end of the perfusion. Fexofenadine concentrations were determined by HPLC. Fexofenadine pharmacokinetic parameters, the final liver : perfusate (L : P) and bile : liver (B : L) concentration ratios were determined. 3. Injection of LPS changed the hepatic disposition of fexofenadine. The changes were most marked in the 5 mg/kg LPS group. Notably, clearance from the perfusate (CL) and into the bile (CLB; 5.9 +/- 0.6 and 1.24 +/- 0.20 mL/min, respectively), L : P (44 +/- 11) and B : L (17 +/- 2) were all reduced (P < 0.05) in this group compared with control (CL 10.0 +/- 1.1 mL/min; CLB 2.7 +/- 0.5 mL/min; L : P 87 +/- 14; and B : L 30 +/- 4). 4. In conclusion CL and CLB were reduced following treatment with LPS in a manner consistent with downregulation of both canalicular and sinusoidal transport.     

Verapamil decreases glucuronidase activity in the gut

The present investigation addressed the role of verapamil for oral pharmacokinetics of morphine-6-beta-glucuronide (M6G). Male Sprague-Dawley rats received 62.5 mg kg(-1) M6G-dihydrate orally w/wo pre-treatment with 70 mg kg(-1) verapamil. Intravenous M6G (3.9 mg kg(-1) ) and oral morphine (52.7 mg kg(-1) morphine-hydrochloride) were also employed. Oral bioavailability of M6G and the fraction of M6G deglucuronidated to morphine were estimated from areas under the plasma-concentration vs. time curves (AUC) of morphine and its glucuronides. As initial results pointed towards inhibition of glucuronidases by verapamil, its capability to specifically inhibit E. coli and/or rat intestinal beta-glucuronidase was assessed using altered cleavage of the model substrate 4-methylumbelliferyl-beta-D-glucuronide (MUG). Oral bioavailability of M6G was 2.1%; 13% of oral M6G was deglucuronidated to morphine. Co-administration of verapamil did not increase the AUC of M6G. AUCs of morphine and morphine-3-glucuronide were smaller in the verapamil group than in controls. Verapamil co-administration decreased the fraction of M6G deglucuronidated to morphine to 4.6%. In vitro experiments provided evidence that verapamil inhibits beta-glucuronidase from E. coli with an IC(50) of 30 microM, whereas no inhibition of the rat beta-glucuronidase from small intestine was seen. In conclusion, verapamil decreased intestinal deglucuronidation of M6G by inhibiting E. coli beta-glucuronidase. This indicates that verapamil is not suited as P-gp inhibitor in experiments involving glucuronides. An increase in the intestinal absorption of M6G due to P-gp-inhibition was not observed at the verapamil dose studied.     

The effect of amiloride on the in vivo effective permeability of amoxicillin in human jejunum: experience from a regional perfusion technique.

The purpose of this human intestinal perfusion study (in vivo) was twofold. Firstly, we aimed to determine the effective in vivo jejunal permeability (P(eff)) of amoxicillin and to classify it according to the Biopharmaceutics Classification System (BCS). Secondly, we investigated the acute effect of amiloride on the jejunal P(eff) of amoxicillin. Amoxicillin, a beta-lactam antibiotic, has been reported to be absorbed across the intestinal mucosa by both passive diffusion and active transport. A regional single-pass perfusion of the jejunum was performed using a Loc-I-Gut perfusion tube in 14 healthy volunteers. Each perfusion lasted for 200 min and was divided into two periods of 100 min each. The concentration of amoxicillin entering the jejunal segment was 300 mg/l in both periods, and amiloride, an inhibitor of the Na+/H+ exchanger, was added at 25 mg/l in period 2. The concentrations of amoxicillin and amiloride in the outlet jejunal perfusate were measured with two different HPLC-methods. Antipyrine and [14C]PEG 4000 were added as internal standards to the perfusion solution. Amiloride had no significant effect on the jejunal P(eff) of amoxicillin. The human in vivo jejunal P(eff) for amoxicillin was 0.34+/-0.11 x 10(-4) and 0.46+/-0.12 x 10(-4) cm/s in periods 1 and 2, respectively. The high jejunal P(eff) for amiloride was 1.63+/-0.51 x 10(-4) cm/s which predicts an intestinal absorption of more than 90%. Following the BCS amoxicillin was classified as a low P(eff) drug, and amiloride had no acute effect on the in vivo jejunal P(eff) of amoxicillin.     

Effect of erythromycin on the absorption of fexofenadine in the jejunum, ileum and colon determined using local intubation in healthy volunteers

OBJECTIVE: To investigate the in vivo intestinal absorption mechanism(s) and systemic availability of fexofenadine in the jejunum, ileum and colon in humans. METHOD: A single dose of fexofenadine hydrochloride (60 mg as solution) was applied under fasting conditions, either alone or directly after a solution of erythromycin lactobionate (corresponding to a dose of 250 mg erythromycin), to the jejunum, ileum and colon in 6 healthy volunteers (3 male and 3 female) using a regional intubation dosing technology (Bioperm AB, Lund, Sweden). A total of 36 fexofenadine administrations were performed. The administration of fexofenadine to the specified location either alone or in combination with erythromycin was conducted in a randomized manner on 2 consecutive days with a 5-day washout period between doses. RESULTS: The plasma AUC for fexofenadine (mean +/- SEM) was higher (2.7-to 2.3-fold, p < 0.001) after application to the jejunum (1090 +/- 134 h x ng/ml) than to the ileum (404 +/- 102 h x ng/ml) or colon (476 +/- 212 h x ng/ml). No significant differences were found between application to the ileum and colon. The administration of erythromycin affected the absorption rate after jejunal application with a prolonged tmax from a median of 40 min (range 10-90 min) to a median of 3 hours (range 10-180 min) (p = 0.009). A change in tmax was not observed with application to the ileum and colon. The concomitant administration of erythromycin in the jejunum tended to increase the plasma AUC of fexofenadine from 1090 +/- 134 to 1750 +/- 305 h x ng/ml (p = 0.069). CONCLUSIONS: The systemic availability of fexofenadine was significantly higher after jejunal administration in accordance with a low permeability compound. The effects of erythromycin suggest that absorption of fexofenadine involves an uptake transport in addition to passive diffusion in the jejunum and predominantly passive diffusion in the ileum and colon.     

Fexofenadine: new preparation. Terfenadine, without cardiotoxicity

(1) Fexofenadine, a non anticholinergic non sedative antihistamine, is available in France for oral treatment of allergic rhinitis and chronic urticaria. (2) Fexofenadine is actually an active metabolite of terfenadine, a drug taken off the market because of its cardiotoxicity. (3) In seasonal allergic rhinitis a comparative trial showed that the effect of fexofenadine (120 mg/day in a single intake) was moderate and not different from that of cetirizine. (4) In chronic urticaria a dose-finding study showed that the optimal oral dose of fexofenadine was 180 mg/day. The lack of comparative trials means that the efficacy of fexofenadine in relation to other antihistamines is not known. (5) Fexofenadine seems to be well tolerated. Animal studies and limited clinical experience have failed to detect any cardiotoxicity.     

Fexofenadine: a review of its use in the management of seasonal allergic rhinitis and chronic idiopathic urticaria

Fexofenadine, the active metabolite of terfenadine, is a selective histamine H1 receptor antagonist that does not cross the blood brain barrier and appears to display some anti-inflammatory properties. Fexofenadine is rapidly absorbed (onset of relief < or = 2 hours) and has a long duration of action, making it suitable for once daily administration. Clinical trials (< or = 2 weeks' duration) have shown fexofenadine 60 mg twice daily and 120 mg once daily to be as effective as loratadine 10 mg once daily, and fexofenadine 120 mg once daily to be as effective as cetirizine 10 mg once daily in the overall reduction of symptoms of seasonal allergic rhinitis. When given in combination, fexofenadine and extended release pseudoephedrine had complementary activity. Fexofenadine was effective in relieving the symptoms of sneezing, rhinorrhoea, itchy nose palate or throat, and itchy, watery, red eyes in patients with seasonal allergic rhinitis. There were often small improvements in nasal congestion that were further improved by pseudoephedrine. Fexofenadine produced greater improvements in quality of life than loratadine to an extent considered to be clinically meaningful, and enhanced patients' quality of life when added to pseudoephedrine treatment. Although no comparative data with other H1 antagonists exist, fexofenadine 180 mg once daily was effective in reducing the symptoms of chronic idiopathic urticaria for up to 6 weeks. Fexofenadine was well tolerated in clinical trials in adults and adolescents and the adverse event profile was similar to placebo in all studies. The most frequently reported adverse event during fexofenadine treatment was headache, which occurred with a similar incidence to that seen in placebo recipients. Fexofenadine does not inhibit cardiac K+ channels and is not associated with prolongation of the corrected QT interval. When given alone or in combination with erythromycin or ketoconazole, it was not associated with any adverse cardiac events in clinical trials. As it does not cross the blood brain barrier, fexofenadine is free of the sedative effects associated with first generation antihistamines, even at dosages of up to 240 mg/day. CONCLUSIONS: fexofenadine is clinically effective in the treatment of seasonal allergic rhinitis and chronic idiopathic urticaria for which it is a suitable option for first-line therapy. Comparative data suggest that fexofenadine is as effective as loratadine or cetirizine in the treatment of seasonal allergic rhinitis. In those with excessive nasal congestion the combination of fexofenadine plus pseudoephedrine may be useful. In clinical trials fexofenadine is not associated with adverse cardiac or cognitive/psychomotor effects.     

Effects of itraconazole and diltiazem on the pharmacokinetics of fexofenadine, a substrate of P-glycoprotein

AIMS: Fexofenadine is a substrate of several drug transporters including P-glycoprotein. Our objective was to evaluate the possible effects of two P-glycoprotein inhibitors, itraconazole and diltiazem, on the pharmacokinetics of fexofenadine, a putative probe of P-glycoprotein activity in vivo, and compare the inhibitory effect between the two in healthy volunteers. METHODS: In a randomized three-phase crossover study, eight healthy volunteers were given oral doses of 100 mg itraconazole twice daily, 100 mg diltiazem twice daily or a placebo capsule twice daily (control) for 5 days. On the morning of day 5 each subject was given 120 mg fexofenadine, and plasma concentrations and urinary excretion of fexofenadine were measured up to 48 h after dosing. RESULTS: Itraconazole pretreatment significantly increased mean (+/-SD) peak plasma concentration (Cmax) of fexofenadine from 699 (+/-366) ng ml-1 to 1346 (+/-561) ng ml-1 (95% CI of differences 253, 1040; P<0.005) and the area under the plasma concentration-time curve [AUC0,infinity] from 4133 (+/-1776) ng ml-1 h to 11287 (+/-4552) ng ml-1 h (95% CI 3731, 10575; P<0.0001). Elimination half-life and renal clearance in the itraconazole phase were not altered significantly compared with those in the control phase. In contrast, diltiazem pretreatment did not affect Cmax (704+/-316 ng ml-1, 95% CI -145, 155), AUC0, infinity (4433+/-1565 ng ml-1 h, 95% CI -1353, 754), or other pharmacokinetic parameters of fexofenadine. CONCLUSIONS: Although some drug transporters other than P-glycoprotein are thought to play an important role in fexofenadine pharmacokinetics, itraconazole pretreatment increased fexofenadine exposure, probably due to the reduced first-pass effect by inhibiting the P-glycoprotein activity. As diltiazem pretreatment did not alter fexofenadine pharmacokinetics, therapeutic doses of diltiazem are unlikely to affect the P-glycoprotein activity in vivo.     

Transport Characteristics of Fexofenadine in the Caco-2 Cell Model

PURPOSE: To investigate the membrane transport mechanisms of fexofenadine in the Caco-2 model. METHODS: Transport studies were performed in Caco-2 cell monolayers 21-25 days after seeding. The apparent permeability (Papp) of fexofenadine was determined in the concentration range 10-1000 microM in the basolateral-to-apical (b-a) and 50-1000 microM in the apical-to-basolateral (a-b) direction. The concentration-dependent effects of various inhibitors of P-glycoprotein (P-gp) (GF120918, ketoconazole, verapamil, erythromycin), multidrug resistant associated protein (MRP) (indomethacin, probenecid), and organic anion transporting polypeptide (OATP) (rifamycin SV) on the bidirectional transport of 150 microM fexofenadine were also examined. RESULTS: Fexofenadine displayed polarized transport, with the Pappb-a being 28- to 85-fold higher than the Papp(a-b). The Papp(a-b) was independent of the concentration applied, whereas Pappb-a decreased with increasing concentration (Vmax = 5.21 nmol cm(-2)s(-1) and K(M) = 150 microM), suggesting saturation of an apical efflux transporter. All four P-gp inhibitors had a strong, concentration-dependent effect on the Papp of fexofenadine in both directions, with GF 120918 being the most specific among them. The IC50 of verapamil was 8.44 microM on the P-gp-mediated secretion of fexofenadine. The inhibitors of OATP or MRP appeared not to affect the Papp(a-b) of fexofenadine in the Caco-2 model. CONCLUSIONS: This study clearly indicates that P-gp was the main transport protein of fexofenadine in the Caco-2 model. Even though P-gp was completely inhibited, fexofenadine was predicted to have a low fraction dose absorbed in humans due to poor intestinal permeability, and low passive diffusion seems to be the major absorption mechanism.     

MDR1 gene polymorphisms and disposition of the P-glycoprotein substrate fexofenadine

AIMS: The C3435T polymorphism in the human MDR1 gene is associated with lower intestinal P-glycoprotein expression, reduced protein function in peripheral blood cells and higher plasma concentrations of the P-glycoprotein substrate digoxin. Using fexofenadine, a known P-glycoprotein substrate, the hypothesis was tested whether this polymorphism also affects the disposition of other drugs in humans. METHODS: Ten Caucasian subjects homozygous for the wild-type allele at position 3435 (CC) and 10 individuals homozygous for T at position 3435 participated in this study. A single oral dose of 180 mg fexofenadine HCl was administered. Plasma and urine concentrations of fexofenadine were measured up to 72 h using a sensitive LC/MS method. In addition, P-glycoprotein function was assessed using efflux of the P-glycoprotein substrate rhodamine 123 from CD56+ cells. Results Fexofenadine plasma concentrations varied considerably among the study population. However, fexofenadine disposition was not significantly different between the CC and TT groups (e.g. AUC(0,infinity) CC vs TT: 3567.1+/-1535.5 vs 3910.1+/-1894.8 ng ml-1 h, NS; 95% CI on the difference -1364.9, 2050.9). In contrast, P-glycoprotein function was significantly decreased in CD56+ cells of the TT compared with the CC group (rhodamine fluorescence CC vs TT: 45.6+/-7.2% vs 61.1+/-12.3%, P<0.05; 95% CI on the difference 5.6, 25.5). Conclusions In spite of MDR1 genotype-dependent differences in P-glycoprotein function in peripheral blood cells, there was no association of the C3435T polymorphism with the disposition of the P-glycoprotein substrate fexofenadine in this German Caucasian study population. These data indicate that other mechanisms including uptake transporter function are likely to play a role in fexofenadine disposition.     

Influence of nifedipine,nitrendipine and verapamil at low concentration on antipyrine metabolism examined by extracorporeal rat liver perfusion

An increase in calcium ion concentration in the cytoplasm due to the influence of various toxic agents causes disturbances in the structure and function of hepatocytes, leading to their damage and even death. Calcium ions enter the cell mostly through calcium channels, therefore, it has been suggested that calcium channel inhibitors (CCI) could protect hepatocytes from the action of toxic substances. The present study investigated the effect of the selected CCI (nifedipine, nitrendipine and verapamil) on liver function, measured by the efficiency of oxidation reaction, in this case by determination of the rate of antipyrine metabolism. The experiment was carried out using the method of extracorporeal liver perfusion (ELP). None of the studied CCI applied at a concentration of 50 micromol/l increased the rate of antipyrine metabolism over the whole period of ELP. However, supplementation of perfusion fluid with nifedipine, nitrendipine or verapamil at a concentration of 20 micromol/l considerably improved metabolic liver efficiency during the second hour of perfusion, i.e. at the time, when large number of hepatocytes started to perish, which could indicate protective action of the tested CCI. However, the CCI-induced acceleration of antipyrine metabolism was not a result of their influence on calcium channels, since these drugs block calcium channels, when given at the concentrations as high as 100-400 micromol/l. Moreover, it seems that facilitation of antipyrine metabolism during ELP was not due to their action on microsomal enzymes because CCI were administered at very low concentrations, besides, they are metabolic inhibitors, and not inducers. The present experiment suggests that low concentrations of CCI can exert hepatoprotective effect. However, confirmation of this conclusion requires further studies using other experimental methods.     

Stimulating effects of the antihistamine fexofenadine: testing the dopamine transporter hypothesis

Rationale First- and second-generation antihistamines are known to produce different degrees of sedation. However, a few studies have shown that the H1-antagonist fexofenadine produces mild stimulating effects. One hypothesis suggests that this is due to fexofenadine producing an increase in dopamine levels by blocking the dopamine transporter.Objective In this study, it was investigated whether a high dose of fexofenadine blocks the dopamine transporter in the striatum. In addition, the effect of fexofenadine on cognitive performance and motor impulsivity was investigated.Methods Sixteen healthy subjects were given either placebo or fexofenadine 360 mg. The binding potential of N-w-fluoropropyl-2β-carbomethoxy-3β-[4-iodophenyl] nortropane ([¹²³I]FP-CIT) was measured using single-photon emission computed tomography (SPECT). Cognitive performance was measured in 40 subjects (20 placebo, 20 fexofenadine) using a digit symbol substitution test (DSST) and a stop signal task. In addition, subjective and physiological effects of fexofenadine were observed.Results The SPECT data demonstrated that there was no difference in the binding potential of FP-CIT at the dopamine transporter in the striatum between the placebo- and fexofenadine-treated subjects. The behavioral results showed that fexofenadine improved performance on the DSST at T _max of the drug. Fexofenadine did not affect motor impulsivity, subjective experience, or physiological measures.Conclusion No evidence was provided to support the hypothesis that fexofenadine stimulates performance by blocking the dopamine transporter. The behavioral data suggest that a high dose of fexofenadine can stimulate performance in cognitive tasks.     

Dose proportionality and comparison of single and multiple dose pharmacokinetics of fexofenadine (MDL 16 455) and its enantiomers in healthy male volunteers

The pharmacokinetics and dose proportionality of fexofenadine, a new non-sedating antihistamine, and its enantiomers were characterized after single and multiple-dose administration of its hydrochloride salt. A total of 24 healthy male volunteers (31±8 years) received oral doses of 20, 60, 120 and 240 mg fexofenadine HCl in a randomized, complete four-period cross-over design. Subjects received a single oral dose on day 1, and multiple oral doses every 12 h on day 3 through the morning on day 7. Treatments were separated by a 14-day washout period. Serial blood and urine samples were collected for up to 48 h following the first and last doses of fexofenadine HCl. Fexofenadine and its R(+) and S(−) enantiomers were analysed in plasma and urine by validated HPLC methods. Fexofenadine pharmacokinetics were linear across the 20–120 mg dose range, but a small disproportionate increase in area under the plasma concentration–time curve (AUC) (<25%) was observed following the 240 mg dose. Single-dose pharmacokinetics of fexofenadine were predictive of steady-state pharmacokinetics. Urinary elimination of fexofenadine played a minor role (10%) in the disposition of this drug. A 63:37 steady-state ratio of R(+) and S(−) fexofenadine was observed in plasma. This ratio was essentially constant across time and dose. R(+) and S(−) fexofenadine were eliminated into urine in equal rates and quantities. All doses of fexofenadine HCl were well tolerated after single and multiple-dose administration. © 1998 John Wiley & Sons, Ltd.     

The effect of ketoconazole on the jejunal permeability and CYP3A metabolism of (R/S)-verapamil in humans

AIMS: The purpose of this human intestinal perfusion study was to investigate the effect of ketoconazole on the jejunal permeability and first-pass metabolism of (R)- and (S)-verapamil in humans. METHODS: A regional single-pass perfusion of the jejunum was performed using a Loc-I-Gut(R) perfusion tube in six healthy volunteers. Each perfusion lasted for 200 min and was divided into two periods of 100 min each. The inlet concentration of (R/S)-verapamil was 120 mg l-1 in both periods, and ketoconazole was added at 40 mg l-1 in period 2. (R/S)-verapamil was also administered as a short intravenous infusion of 5 mg, over a period of 10 min. The appearance ratios of the CYP3A formed metabolites (R)- and (S)-norverapamil were also estimated in the outlet jejunal perfusate. RESULTS: The effective jejunal permeability (Peff) of both (R)- and (S)-verapamil was unaffected by the addition of ketoconazole in period 2 suggesting that ketoconazole had no effect on the P-glycoprotein mediated efflux. However, the appearance ratio of both (R)- and (S)-norverapamil in the outlet jejunal perfusate decreased in the presence of ketoconazole. The rate of absorption into plasma of (R)- and (S)-verapamil increased despite the low dose of ketoconazole added, indicating an inhibition of the gut wall metabolism of (R/S)-verapamil by ketoconazole. CONCLUSIONS: Ketoconazole did not affect the jejunal Peff of (R/S)-verapamil, but it did increase the overall transport into the systemic circulation (bioavailability), probably by inhibition of the gut wall metabolism of verapamil. This might be due to ketoconazole being less potent as an inhibitor of P-glycoprotein than of CYP3A4 in vivo in humans.     

Accuracy and clinical utility of simplified tests of antipyrine metabolism.

High-performance liquid chromatography (h.p.l.c.) was used to measure plasma antipyrine concentrations in sixteen healthy subjects; five males, six females taking oral contraceptive steroids (OCS) and five age-matched females not taking OCS. Following oral administration of the drug, antipyrine clearance could be determined with similar precision and accuracy from plasma concentrations at two selected times (4 h and 24 h) ('two-sample antipyrine clearance test') as from six samples taken over two to three elimination half-lives (t1/2) of the drug ('conventional antipyrine clearance test'). Provided times for plasma sampling were modified appropriately, the two-sample antipyrine clearance test also gave reliable results in four patients taking phenytoin (sampling times 4 h and 8 h) who had significantly enhanced antipyrine clearance, and in nine patients with severe liver disease (using 4 h and 48 h) in whom antipyrine clearance was impaired. Urinary excretion of antipyrine metabolites (from 0-24 h) was determined in the above groups. Antipyrine systemic clearance correlated best with the percentage of administered dose excreted as 4-hydroxyantipyrine (r = 0.66, P less than 0.001) but also with 3-hydroxymethylantipyrine (r = 0.54, P less than 0.001) and with total metabolites excreted (r = 0.60, P less than 0.001). Total antipyrine metabolites excreted in urine in 24 h were significantly different from controls only in patients with liver disease (14 +/- 7.6% of administered dose vs 62 +/- 14%, P less than 0.001). The relative proportion of antipyrine metabolites did not appear to be altered when hepatic mixed function oxidation was induced by phenytoin or inhibited by OCS or by severe liver disease.     

Intestinal absorption and presystemic elimination of the prokinetic agent, EM574, in the rabbit.

The purpose of this study was to characterize the pharmacokinetics and dose proportionality of the prokinetic macrolide, EM574, in rabbits following intravenous dosing, and to determine the intestinal absorption and intestinal and hepatic first-pass elimination of EM574 in rabbits. Two doses (0.05 and 0.25 mg/kg) of EM574 were given to rabbits intravenously in a crossover study. In a separate gut perfusion study, rabbit duodenal or jejunal segments were perfused with EM574 solution at 0.2 mL/min for 130 min. Plasma levels of EM574 were determined by a validated LC-MS/MS assay, and concentrations in perfusate were determined by HPLC with UV detection. The absorptive clearance (PeA) of EM574 was calculated from the steady-state rate of disappearance from the gut lumen during perfusion. The cumulative amount (A(app)) of drug appearing in the systemic circulation was calculated by deconvolution, where the input response was the plasma concentration-time profile during intestinal perfusion and the unit impulse response was the mean profile following intravenous bolus dosing to sham-operated rabbits in a separate experiment. F(g)F(h) was calculated from the ratio of A(app) to the total amount disappeared from gut lumen during perfusion. Hepatic first-pass elimination was measured by intraportal venous infusion. EM574 exhibits linear kinetics over the dose range studied. CL, V(ss), and terminal half-life (mean +/- SD) of EM574 were 68.6 +/- 15.5 mL/min/kg, 13.4 +/- 3.0 L/kg, and 2.7 +/- 0.8 h, respectively. EM574 is expected to be absorbed completely from the rabbit small intestine based on its high jejunal PeA values (8.1 +/- 2.2, and 5.5 +/- 1.5 microL/min/cm following low and high dose perfusion, respectively). The first-pass extraction of EM574 was substantial and dose independent. Mean F(g) and F(h) were 0.14 and 0.20, respectively, suggesting that the intestinal and hepatic first-pass elimination of EM574 were comparable. Deconvolution was successfully applied in the determination of gut wall and hepatic first-pass elimination of EM574.     

Differential effects of fexofenadine on arachidonic acid metabolism in cultured human monocytes

The relief of nasal congestion with the antihistamine fexofenadine in seasonal allergic rhinitis is thought to be due to its additional anti-inflammatory properties. The objective of this study was to evaluate the in vitro effects of fexofenadine on stimulated arachidonic acid metabolism. Human monocytes, isolated from blood and donated by 5 healthy volunteers, were either incubated for 20 h with 10 microg/ml lipopolysaccharide, with and without fexofenadine (10(-8)-10(-3) mol/l, n = 8-19), or were incubated for 20 h, with and without fexofenadine, and then stimulated with 0.5 mg/ml zymosan for 2 h. Leukotriene B4 (LTB4), LTC4, LTD4 and LTE4, prostaglandin E2 (PGE2) and F2alpha (PGF2alpha) production was determined by enzyme immunoassay. Zymosan-stimulated production of LTC4, LTD4 and LTE4 was significantly inhibited by clinically relevant concentrations of fexofenadine HCl: 10(-7) mol/l (22% inhibition vs. control, p = 0.008) and 10(-6) mol/l (24% inhibition vs. control, p = 0.020). Higher concentrations of fexofenadine (10(-4) and 10(-3) mol/l) inhibited LTB(4) generation. Lipopolysaccharide-stimulated production of PGE2 was significantly inhibited by fexofenadine HCl 10(-6) mol/l (26% inhibition, p = 0.035) and 10(-5) mol/l (40% inhibition, p = 0.001). Higher concentrations of fexofenadine HCl (10(-4) and 10(-3) mol/l) significantly inhibited PGF2alpha production by 50% (p = 0.026) and 63% (p = 0.001), respectively. Fexofenadine, at both clinically relevant and higher concentrations, inhibits LTC4, LTD4, LTE4 and PGE2 in cultured human monocytes. These additional anti-inflammatory properties may underlie the relief of nasal congestion observed in clinical studies.     

Spline functions in convolutional modeling of verapamil bioavailability and bioequivalence. II: study in healthy volunteers.

The pharmacokinetics of a new verapamil retard tablet formulation have been investigated in a randomized cross-over bioequivalence study on 12 healthy subjects. The drug was given orally at a single new or standard retard tablet dose of 240mg and at a single intravenous dose of 5mg. Plasma verapamil concentrations were determined by HPLC. New retard tablets produced peak plasma verapamil concentrations of 81.34+/-5.69microg/l, time to peak plasma concentrations of 4.91+/-0.89h and an AUC (0-24h) of 1291+/-103.4h x microg/l, with a terminal phase half-life of 55.1+/-14.9h. After intravenous administration verapamil exhibited biphasic elimination kinetics with a terminal plasma half-life of 2.36+/-0.42h and systemic clearance of 34.32+/-5.81 l/h. Bioavailability of the new peroral retard formulation ranged from 19.49+/-4.41% to 67.69+/-11.70%. Absorption rates and amounts were evaluated by means of the spline-convolutional method. Input rates for the new verapamil retard formulation ranged from 0.77+/-0.20mg/h to 5.57+/-1.58mg/h. The cumulative amount of verapamil input was 39.17+/-9.71% for the new retard tablets. All pharmacokinetic parameters for the new verapamil retard tablet formulation, were in reasonable agreement with the data obtained on already registered verapamil retard formulations, indicating their bioequivalence.     

The site of inversion of R(-)-ibuprofen: studies using rat in-situ isolated perfused intestine/liver preparations.

The site of metabolic inversion of R(-)-ibuprofen to the pharmacologically active S(+)-enantiomer has been investigated using an array of in-situ rat perfused organ preparations allowing vascular perfusion (55-60 min) of the separate or combined intestine and liver. After addition of R(-)-ibuprofen (20 mg kg-1 body weight) to the closed (static) lumen of isolated 25 cm lengths of duodenum, jejunum or ileum, and single-pass vascular perfusion, both isomers were measured in the lumen and in vascular perfusate plasma (mean plasma AUC values (+/- s.d., micrograms mL-1 min, n = 5) R(-)-ibuprofen: 1669 +/- 115 (duodenum), 1687 +/- 203 (jejunum), 2061 +/- 188 (ileum); S(+)-ibuprofen: 23 +/- 6 (duodenum), 14 +/- 5 (jejunum), 26 +/- 1 (ileum]. Addition of the same dose of S(+)-ibuprofen to the jejunum (n = 5) resulted in AUC values of 1864 +/- 238 for S(+)-ibuprofen and 6 +/- 3 for R(-)-ibuprofen. After addition of R(-)-ibuprofen (30 micrograms mL-1) to the recirculating vascular perfusate (100 mL) of the entire small intestine (n = 6) AUC values were 1647 +/- 34 for R(-)-ibuprofen and 13 +/- 3 for S(-)-ibuprofen. The same dose of R(-)-ibuprofen to combined intestine/liver (n = 6) and liver only preparations (n = 6) gave AUC values of 1011 +/- 25 and 1021 +/- 49 for R(-)-ibuprofen and 220 +/- 28 and 238 +/- 22 for S(+)-ibuprofen, respectively. In all experiments, except those involving perfusion of the combined intestine/liver and the liver, the concentrations of the isomer opposite to that administered could be accounted for solely by the level of enantiomeric impurity (1.3% for R(-)-ibuprofen and 0.6% for S(+)-ibuprofen). We conclude that inversion of R(-)-ibuprofen to the S(+) antipode occurs in the liver but does not occur on either mucosal or serosal sides of the small intestine of the rat.