Evaluation of intestinal absorption into the portal system in enterohepatic circulation by measuring the difference in portal-venous blood concentrations of diclofenac

Characterization of P-Glycoprotein Inhibitors for Evaluating the Effect of P-Glycoprotein on the Intestinal Absorption of Drugs

For developing oral drugs, it is necessary to predict the oral absorption of new chemical entities accurately. However, it is difficult because of the involvement of efflux transporters, including P-glycoprotein (P-gp), in their absorption process. In this study, we conducted a comparative analysis on the inhibitory activities of seven P-gp inhibitors (cyclosporin A, GF120918, LY335979, XR9576, WK-X-34, VX-710, and OC144-093) to evaluate the effect of P-gp on drug absorption. GF120918, LY335979, and XR9576 significantly decreased the basal-to-apical transport of paclitaxel, a P-gp substrate, across Caco-2 cell monolayers. GF120918 also inhibited the basal-to-apical transport of mitoxantrone, a breast cancer resistance protein (BCRP) substrate, in Caco-2 cells, whereas LY335979 hardly affected the mitoxantrone transport. In addition, the absorption rate of paclitaxel after oral administration in wild-type mice was significantly increased by pretreatment with LY335979, and it was similar to that in mdr1a/1b knockout mice. Moreover, the absorption rate of topotecan, a BCRP substrate, in wild-type mice pretreated with LY335979 was similar to that in mdr1a/1b knockout mice but significantly lower than that in bcrp knockout mice. These results indicate that LY335979 has a selective inhibitory activity for P-gp, and would be useful for evaluating the contribution of P-gp to drug absorption.

Assessment of contribution of BCRP to intestinal absorption of various drugs using portal-systemic blood concentration difference model in mice

Prediction of the intestinal absorption of new chemical entities (NCEs) is still difficult, in part because drug efflux transporters, including breast cancer resistance protein (BCRP) and P-glycoprotein (P-gp), restrict their intestinal permeability. We have demonstrated that the absorptive quotient (AQ) obtained from the in vitro Caco-2 permeability study would be a valuable parameter for estimating the impact of BCRP on the intestinal absorption of drugs. In this study, in order to assess the correlation between the in vitro AQ for BCRP and in vivo contribution of BCRP on drug absorption, we evaluated the oral absorption of various compounds by portal-systemic blood concentration (P-S) difference method in wild-type (WT), Bcrp(-/-), and Mdr1a/1b(-/-) mice. In addition, we also calculated a rate of BCRP contribution (Rbcrp ). Ciprofloxacin and nitrofurantoin showed the low Rbcrp value (0.05 and 0.15), and their apparent fractions of intestinal absorption in WT mice were 46.5% and 63.7%, respectively. These results suggest that BCRP hardly affects their intestinal absorption in mice. On the other hand, the apparent fraction of intestinal absorption of topotecan and sulfasalazine was significantly lower in WT mice than in Bcrp(-/-) mice. Moreover, their Rbcrp values were 0.42 and 0.79, respectively, indicating the high contribution of BCRP to their oral absorption. Furthermore, in vivo Rbcrp calculated in this study was almost comparable to in vitro AQ obtained from Caco-2 permeability study. This study provides useful concepts in assessing the contribution of BCRP on intestinal absorption in drug discovery and development process.

Impact of P-Glycoprotein on Intestinal Absorption of an Inhibitor of Apoptosis Protein Antagonist in Rats: Mechanisms of Nonlinear Pharmacokinetics and Food Effects

PURPOSE

This study was designed to investigate the effects of P-glycoprotein (P-gp) expressed in the intestine on the nonlinear pharmacokinetics (PK) of T-3256336, an inhibitor of apoptosis protein inhibitor, and food effects on its bioavailability in rats.

METHODS

To investigate the factors that contribute to nonlinear PK of T-3256336 in the intestine and liver, rats double-cannulated in the portal vein and femoral artery (PS rats) were used. F_aF_g (F_a, absorption ratio; F_g, intestinal availability) and hepatic availability (F_h) were simultaneously evaluated based on the difference between the portal and systemic blood area under the concentration-time curve (AUC). Elacridar was used as a P-gp inhibitor to assess the impact of P-gp on the intestinal absorption.

RESULTS

After oral administration of T-3256336 to PS rats at 3 and 30 mg/kg, F_aF_g value increased with dose escalation, whereas F_h value was nearly constant. Moreover, co-administration of elacridar resulted in a 5-fold increase in the F_aF_g value at 3 mg/kg. The AUC value of T-3256336 under fed conditions was 3-fold lower than that under fasted conditions. This food effect on the oral bioavailability (BA) was reduced by concomitant administration of elacridar.

CONCLUSION

P-gp expressed in the intestine would cause nonlinear PK and a food effect on BA of T-3256336 in rats.

Challenges in oral drug delivery of antiretrovirals and the innovative strategies to overcome them

Development of novel drug delivery systems (DDS) represents a promising opportunity to overcome the various bottlenecks associated with the chronic antiretroviral (ARV) therapy of the human immunodeficiency virus (HIV) infection. Oral drug delivery is the most convenient and simplest route of drug administration that involves the swallowing of a pharmaceutical compound with the intention of releasing it into the gastrointestinal tract. In oral delivery, drugs can be formulated in such a way that they are protected from digestive enzymes, acids, etc. and released in different regions of the small intestine and/or the colon. Not surprisingly, with the exception of the subcutaneous enfuvirtide, all the marketed ARVs are administered orally. However, conventional (marketed) and innovative (under investigation) oral delivery systems must overcome numerous challenges, including the acidic gastric environment, and the poor aqueous solubility and physicochemical instability of many of the approved ARVs. In addition, the mucus barrier can prevent penetration and subsequent absorption of the released drug, a phenomenon that leads to lower oral bioavailability and therapeutic concentration in plasma. Moreover, the frequent administration of the cocktail (ARVs are administered at least once a day) favors treatment interruption. To improve the oral performance of ARVs, the design and development of more efficient oral drug delivery systems are called for. The present review highlights various innovative research strategies adopted to overcome the limitations of the present treatment regimens and to enhance the efficacy of the oral ARV therapy in HIV.

Ciprofloxacin blocked enterohepatic circulation of diclofenac and alleviated NSAID-induced enteropathy in rats partly by inhibiting intestinal β-glucuronidase activity

AIM

Diclofenac is a non-steroidal anti-inflammatory drug (NSAID), which may cause serious intestinal adverse reactions (enteropathy). In this study we investigated whether co-administration of ciprofloxacin affected the pharmacokinetics of diclofenac and diclofenac-induced enteropathy in rats.

METHODS

The pharmacokinetics of diclofenac was assessed in rats after receiving diclofenac (10 mg/kg, ig, or 5 mg/kg, iv), with or without ciprofloxacin (20 mg/kg, ig) co-administered. After receiving 6 oral doses or 15 intravenous doses of diclofenac, the rats were sacrificed, and small intestine was removed to examine diclofenac-induced enteropathy. β-Glucuronidase activity in intestinal content, bovine liver and E coli was evaluated.

RESULTS

Following oral or intravenous administration, the pharmacokinetic profile of diclofenac displayed typical enterohepatic circulation, and co-administration of ciprofloxacin abolished the enterohepatic circulation, resulted in significant reduction in the plasma content of diclofenac. In control rats, β-glucuronidase activity in small intestinal content was region-dependent: proximal intestine<distal intestine<ileal valve. Administration of ciprofloxac caused significant reduction of β-glucuronidase activity in distal small intestine, and particularly in ileal valve. Furthermore, ciprofloxacin (10-2000 μmol/L) dose-dependently inhibited β-glucuronidase activity in distal small intestine content or E coli incubated in vitro, but did not affect that in proximal small intestine content or bovine liver incubated in vitro. After receiving 6 oral doses or 15 intravenous doses of diclofenac, typical enteropathy was developed with severe enteropathy occurred in distal small intestine. Co-administration of ciprofloxacin significantly alleviated diclofenac-induced enteropathy.

CONCLUSION

Co-administration of ciprofloxacin attenuated enterohepatic circulation of diclofenac and alleviated diclofenac-induced enteropathy in rats, partly via the inhibition of intestinal β-glucuronidase activity.

Absorption and cleavage of enalapril, a carboxyl ester prodrug, in the rat intestine: in vitro, in situ intestinal perfusion and portal vein cannulation models

In recent years prodrug strategy has been used extensively to improve the pharmacokinetic properties of compounds exhibiting poor bioavailability. Mechanistic understanding of the absorption and the role of intestine and liver in the activation of oral prodrugs is crucial. Enalapril, a carboxyl ester prodrug, is reported to be metabolized by human carboxylesterase-1 (CES1) but not by carboxylesterase-2 (CES2) to its active metabolite enalaprilat. Further, it has been reported that the small intestines of both rat and human contain mainly CES2. The objective of this work was to understand whether enalapril remains unchanged as it is absorbed through the intestine into the portal circulation. This was evaluated using different intestinal preparations, an in situ intestinal perfusion experiment and a portal vein cannulated rat model. No turnover of enalapril was seen with commercial rat intestinal S9 and microsomes, but reasonable turnover was observed with freshly prepared rat intestinal and mucosal homogenate and S9. In the intestinal perfusion study, both enalapril and enalaprilat were observed in the mesenteric plasma with the data suggesting 32% hydrolysis of enalapril in the intestine. In the portal vein cannulated rat, about 51% of enalapril absorbed into intestine was converted to enalaprilat. Overall, it was demonstrated that even though enalapril has been shown to be a specific substrate for CES1, it is converted to enalaprilat to a significant extent in the intestine. Such experimental techniques can be applied by other scientific groups who are working on prodrugs to determine the region and extent of activation. Copyright © 2015 John Wiley & Sons, Ltd.

Prediction of metabolic clearance of diclofenac in adjuvant-induced arthritis rats using a substrate depletion assay

1. The purpose of this study was to evaluate drug clearance measured by the metabolic intrinsic clearance (CL(int)) in a substrate depletion assay in comparison with the in vivo clearance (CL(tot)) observed in adjuvant-induced arthritis (AA) rats. 2. After intravenous administration of diclofenac as a model drug, CL(tot) was 2.8-fold higher in AA rats than in control rats. In two different substrate depletion assays with liver microsomes for glucuronidation and hydroxylation, the CL(int) values for glucuronidation was significantly decreased in AA rats to 60% of the value in control rats, whereas the CL(int) values for hydroxylation were similar. The unbound fraction of diclofenac in plasma (f(u, plasma)) was significantly higher (2.8-fold) in AA rats than in control rats. 3. Hepatic clearance predicted from the CL(int) values for both biotransformation pathways and f(u, plasma) was higher in AA rats than in control rats, with good consistency between predicted and observed values. The same results were obtained for experiments using hepatocytes. 4. The plasma protein-binding activities, rather than metabolic clearance, in both types of rats would be a determining factor in the pharmacokinetic behaviour differences between control and AA rats. 5. In summary, substrate depletion assays with liver microsomes and hepatocytes in combination with protein binding assessment can help to predict changes in pharmacokinetics under AA conditions.

Pharmacokinetics of diclofenac in rats intoxicated with CCL4, and in the regenerating liver

The pharmacokinetics of an intravenous and oral diclofenac dose of 3.2 mg/kg was studied in male Wistar rats under control conditions, 1 and 3 days after liver damage and regeneration induced by an oral injection of CCl(4). One day after CCl(4) administration, indicators of necrosis (alanine aminotransferase), cholestasis (gamma-glutamyl transpeptidase) and regeneration (alpha-fetoprotein) were significantly increased; these effects were reversed after 3 days. In nonintoxicated rats, t(1/2) was 43.83 +/- 4.95 min, V(d) was 0.37 +/- 0.04 l/kg, Cl was 129.21 +/- 9.20 ml/min kg, AUC(i.v.) was 25.62 +/- 1.45 microg/min ml, and AUC(p.o.) was 20.21 +/- 1.03. One day after intoxication, when the liver was damaged and regenerating, the metabolism was decreased: diclofenac t(1/2) was increased to 258.21 +/- 30.80 min but V(d) did not change significantly, therefore Cl was reduced to 32.81 +/- 3.38 ml/min kg. By day 3 after intoxication, liver function, regeneration and pharmacokinetics returned to normal. The results show that liver damage and regeneration increases the bioavailability by decreasing elimination. The present observations suggest that reduction of the pharmacokinetic parameters may lead to drug accumulation in the regenerating-damaged liver with an attendant possible increase in toxic effects. The results in rats, also suggest that once hepatic injury is finished and regeneration is complete, diclofenac can be administered normally.

Population pharmacokinetic modelling of the enterohepatic recirculation of diclofenac and rofecoxib in rats

BACKGROUND AND PURPOSE

Enterohepatic recirculation (EHC) is a common pharmacokinetic phenomenon that has been poorly modelled in animals. The presence of EHC leads to the appearance of multiple peaks in the concentration-time profile and increased exposure, which may have implications for drug effect and extrapolation across species. The aim of this investigation was to develop a population pharmacokinetic model for diclofenac and rofecoxib that describes EHC and to assess its consequence for the pharmacodynamics of both drugs.

EXPERIMENTAL APPROACH

The pharmacokinetics of diclofenac and rofecoxib was characterized in male rats following intravenous, intraperitoneal and oral administration. Blood samples were collected at pre-defined time points after dosing to determine plasma concentrations over time. A parametric approach using nonlinear mixed effects modelling was applied to describe EHC, whilst simulations were used to evaluate its impact on PGE(2) inhibition.

KEY RESULTS

For diclofenac, EHC was described by a compartmental model with periodic transfer rate and metabolite formation rate. For rofecoxib, EHC modelling required a conversion compartment with first-order recycling rate and lag time. Based on model predictions, EHC causes an increase of 95% in the systemic exposure to diclofenac and of 15% in the exposure to rofecoxib. In addition, EHC prolongs the inhibition of PGE(2) and increases the duration of the anti-inflammatory effect (24 h for rofecoxib 10 mg kg(-1)) without affecting maximum inhibition.

CONCLUSIONS AND IMPLICATIONS

Our findings show the relevance of exploring EHC in a quantitative manner to accurately interpret pharmacodynamic findings in vivo, in particular when scaling across species.

Advanced pharmacokinetic models based on organ clearance, circulatory, and fractal concepts

Three advanced models of pharmacokinetics are described. In the first class are physiologically based pharmacokinetic models based on in vitro data on transport and metabolism. The information is translated as transporter and enzyme activities and their attendant heterogeneities into liver and intestine models. Second are circulatory models based on transit time distribution and plasma concentration time curves. The third are fractal models for nonhomogeneous systems and non-Fickian processes are presented. The usefulness of these pharmacokinetic models, with examples, is compared.

GLUCURONIDATION AS A MAJOR METABOLIC CLEARANCE PATHWAY OF 14C-LABELED MURAGLITAZAR IN HUMANS: METABOLIC PROFILES IN SUBJECTS WITH OR WITHOUT BILE COLLECTION

The metabolism and disposition of 14C-labeled muraglitazar (Pargluva), a novel dual alpha/gamma peroxisome proliferator-activated receptor activator, was investigated in eight healthy male subjects with and without bile collection (groups 1 and 2) after a single 20-mg oral dose. Bile samples were collected for 3 to 8 h after dosing from group 2 subjects in addition to the urine and feces collection. In plasma, the parent compound was the major component, and circulating metabolites, including several glucuronide conjugates, were minor components at all time points. The exposure to parent drug (Cmax and area under the plasma concentration versus time curve) in subjects with bile collection was generally lower than that in subjects without bile collection. The major portion of the radioactive dose was recovered in feces (91% for group 1 and 51% for group 2). In addition, 40% of the dose was recovered in the bile from group 2 subjects. In this 3- to 8-h bile, the glucuronide of muraglitazar (M13, 15% of dose) and the glucuronides of its oxidative metabolites (M17a,b,c, M18a,b,c, and M20, together, 16% of dose) accounted for approximately 80% of the biliary radioactivity; muraglitazar and its O-demethylated metabolite (M15) each accounted for approximately 4% of the dose. In contrast, fecal samples only contained muraglitazar and its oxidative metabolites, suggesting hydrolysis of biliary glucuronides in the intestine before fecal excretion. Thus, the subjects with and without bile collection showed different metabolic profiles of muraglitazar after oral administration, and glucuronidation was not observed as a major pathway of metabolic clearance from subjects with the conventional urine and fecal collection, but was found as a major elimination pathway from subjects with bile collection.

[High throughput screening of pharmacokinetics and metabolism in drug discovery (I)--establishment of assessment system for absorption to compounds with a wide diversity of physical properties]

The application of combinatorial chemistry and high-throughput screening to biological targets has led to efficient identification of lead compounds in wide therapeutic areas. However, the physicochemical properties of some lead compounds are lipophilic with low water soluble. Since these parameters determine in vivo absorption, we established robust screening methods for solubility and Caco-2 membrane permeability which are applicable to our screening strategy based on the structure-pharmacokinetic parameter relationship (SPR). Of test compounds with different core structures, turbidimetric solubility and apparent solubility as determined by HPLC-UV analysis after dilution of aqueous media from DMSO stock solution was overestimated in comparison with the corresponding thermodynamic solubility obtained using a traditional shake-flask method. A new powder-dissolution method providing thermodynamic solubility similar to that in the traditional method was developed using 96-well plates for equilibrium dialysis. The throughput of the method was the almost the same as that using the apparent solubility method. In a conventional Caco-2 assay, membrane permeability (P(app)) of some lipophilic compounds was underestimated due to low solubility in the apical site and adhesion to the device, resulting in a poor relationship between the in vivo absorption fraction and the P(app) values. The addition of 0.1% Gelucire 44/14 into the apical site and 4% bovine serum albumin into the basolateral site improved the relationship. These newly developed methods are therefore useful to optimize lead compounds with less water solubility and high lipophilicity on the basis of SPR.

Effect of drug release rate on therapeutic outcomes: formulation dependence of gastrointestinal toxicity of diclofenac in the rat

- The use of the non-steroidal anti-inflammatory drug, diclofenac, is associated with occasional serious side effects in the gastrointestinal (GI) tract. We examined the effect of altering the site of release of diclofenac sodium on GI tract side effects. Dissolution and pharmacokinetic studies were carried out to substantiate the sustained-release nature of crushed sustained release tablet. Adult male Sprague-Dawley rats were administered diclofenac 10 mg/kg orally as either immediate-release or sustained-release preparations. Upper and lower GI permeability, as a surrogate marker of toxicity, were measured using sucrose and 51Cr-EDTA, respectively. Immediate- and sustained-release preparations similarly increased upper GI permeability. The induced toxicity in the lower GI tract, however, caused by the sustained-release formulation lasted longer than that of the immediate release formulation. Since both immediate- and sustained-release preparations of diclofenac increased sucrose permeability, the upper GI damage caused by diclofenac may be attributable mainly to a systemic mechanism. The prolonged lower GI toxicity following the sustained-release preparation may be related to a greater residence time therein.

Interaction of estrogen therapy and thyroid hormone replacement in postmenopausal women

Based on the use of estrogen therapy/hormone therapy (ET/HT) in postmenopausal women and the prevalence of hypothyroidism in this population, it is estimated that approximately 5% of all postmenopausal women receive treatment with both ET/HT and thyroid hormone replacement. Hormone therapy generally refers to the combined use of estrogens and progestins, the latter administered on a continuous or intermittent basis. HT is indicated for the treatment of postmenopausal women with intact uteri, whereas ET is used in women who have had hysterectomies. Because of its hepatic first-pass effect, oral estrogen therapy, the most commonly used modality of ET/HT, raises the circulating levels of thyroxine-binding globulin (TBG), thereby increasing the bound fraction and decreasing the free (bioactive) fraction of circulating thyroxine (T(4)). As a consequence, oral ET/HT may increase the T(4) dosage requirements of women being treated for primary hypothyroidism as well as alter the pituitary-thyroid axis in euthyroid women. This paper reviews the potential interaction between ET/HT and thyroid hormone replacement based on the prevalence of their concomitant use, mechanistic aspects of the interaction, and recent clinical studies of the effects of oral ET in euthyroid and hypothyroid women. Other agents known to interact with thyroid hormone replacement, including soy supplements, are also reviewed. Because transdermal ET does not affect TBG levels and would not be expected to alter thyroid function, it may be a preferable modality for postmenopausal women who require concomitant treatment with ET/HT and T(4).

The antinociceptive effect of iontophoretic direct application of diclofenac to arthritic knee-joints of rats

This study compared the antinociceptive effect produced by cathodic iontophoresis of sodium diclofenac close to an arthritic knee-joint in rats with that of systemic application. Arthritic nocifensive incapacitation was induced by LPS (1 microg) injection into a knee-joint previously (72 h) primed with carrageenan (300 microg). Diclofenac (0.1, 0.25 and 0.5 mg/kg) given intraperitoneally 1 h after LPS injection caused dose-dependent inhibition of incapacitation. Diclofenac iontophoresis was performed by varying either the current density (0.1, 0.2, and 0.3 mA/cm2) or the duration of application (4, 10, 20 and 30 min) of a polyvinylpirrolydone-hydroxymethylcellulose gel containing 1% sodium diclofenac. A clear, current density-dependent effect was observed for 0.1, 0.2 and 0.3 mA/cm2 (10 min period), which was similar to the effect observed for the intraperitoneal application of 0.1, 0.25 and 0.5 mg/kg doses. Combining different application periods with different current densities, in a manner that resulted in the same total current (1.6 mA*min) application, did not produce similar therapeutic effects, but the antinociceptive effect was directly proportional to the current density. The ipsilateral iontophoresis (0.25 mA/cm2 x 10 min or 0.5 mA/cm2 x 4 min) of diclofenac produced an effect significantly greater than the same contralateral application (p<0.05). In conclusion, our results suggest that the therapeutic effect depends on the current density but not on the application time, and also that the iontophoretic, direct application to the inflamed knee-joint enhances the therapeutic effect probably as a result of the direct delivery of the drug.

A recirculatory model with enterohepatic circulation by measuring portal and systemic blood concentration difference

The present study describes a recirculatory model for the evaluation of pharmacokinetic characteristics of drugs possessing enterohepatic circulation (EHC). The advantage of the model is to separately define the extent and rate of absorption for the dosage and EHC after oral administration. Cephradine was used as a model drug and was intravenously or orally administered to rats. Portal and systemic bloods were simultaneously collected in order to estimate various local moments after defining the global moments obtained by non-compartment analysis. For the zero-order moments, bioavailability (BA), the hepatic recovery ratio (Fh), the sum of the local absorption ratio for the dosage and recirculatory local absorption ratio for EHC (F(a)po), and the recirculatory local absorption ratio for EHC (F(a)ehc) after oral administration were estimated to be 95.6, 77.9, 172, and 71.5%, respectively. These data indicate that a complete absorption and substantial EHC contribute high oral exposure of cephradine. For the first-order moments, the sum of the mean local absorption times for the dosage and EHC (t(a)po) and the mean transit time for a single pass of EHC (tc) were 2.50 and 0.117 hr, suggesting a rapid EHC of cephradine compared with the absorption from the dosage. With this model, the absorption rate-time profiles for the dosage and EHC were separately simulated by using a program of nonlinear least squares (MULTI) with fast inverse Laplace transform (FILT). The cumulative biliary excretion ratio (Fbile) calculated by the model was in good agreement with the experimental value obtained in the bile ductcannulated rats. These results suggest that the model proposed in this study would be useful for evaluating the extent and rate of ECH along with absorption from the dosage after oral administration of drugs.

Determination of diclofenac in rat bile and its interaction with cyclosporin A using on-line microdialysis coupled to liquid chromatography

Diclofenac is a potent inhibitor of prostaglandin synthesis, as well as an established antipyretic and analgesic agent. To determine diclofenac in rat bile and investigate its hepatobiliary excretion, a procedure using rapid and sensitive high-performance liquid chromatography coupled to microdialysis sampling system was developed. A shunt linear microdialysis probe was inserted into the common bile duct between the liver and the duodenum for continuous sampling of the drug from bile fluids following intravenous administration of diclofenac (1 mg/kg). Separation and quantitation of diclofenac in the bile dialysates were achieved using a microbore reversed-phase C18 column (150x1.0 mm I.D.; particle size 5 microm) maintained at ambient temperature. Samples were eluted with a mobile phase containing 100 mM sodium dihydrogenphosphate (pH 3.1)-acetonitrile (30:70, v/v), and the flow-rate of the mobile phase was 0.05 ml/min. The UV detector wavelength was set at 280 nm. The concentration-response relationship from the present method indicated linearity (r2>0.995) over a concentration range of 5-5000 ng/ml for diclofenac. Intra-assay and inter-assay precision and accuracy of diclofenac fell well within the predefined limits of acceptability (< or = 15%). The diclofenac in rat bile appeared to have a slow elimination phase, with a peak concentration at 20 min following diclofenac administration. The results demonstrated that diclofenac might be secreted into bile in unconjugated form by a canalicular bile acid transporter, and then go through hepatobiliary excretion. These results may provide good clinical evidence showing the value of diclofenac for the treatment of biliary colic. The elimination half-life of diclofenac in the biliary elimination was prolonged by treatment with cyclosporin A, indicating that the drug-drug interaction might affect the hepatobiliary excretion of diclofenac.

A recirculatory model for local absorption and disposition of ciprofloxacin by measuring portal and systemic blood concentration difference

A recirculatory model for the portal-systemic blood concentration difference (P-S difference) was developed to separately evaluate the rate and extent of absorption from the gastrointestinal tract into the portal system and disposition of a drug in the body. To apply this model to pharmacokinetic analysis, ciprofloxacin was selected as a model drug possessing a moderate intestinal absorption, and renal and hepatic elimination. The portal and systemic blood samples were simultaneously taken from rats at appropriate time points after intravenous and oral administration of ciprofloxacin at a dose of 5 mg/kg. After intravenous administration, little or no difference in the concentrations between the portal and systemic blood was observed, whereas after oral administration the concentrations of ciprofloxacin in the portal blood were consistently higher than those in the systemic blood over the time studied. This difference observed after oral administration is attributed to the absorption of ciprofloxacin from the gastrointestinal tract into the portal system. On the basis of the moment analysis deduced from the recirculatory model, the portal blood flow rate (Q(p)), the local absorption ratio from the gastrointestinal tract into the portal system (F(a)), the hepatic recovery ratio (F(h)), and bioavailability (BA) were then estimated. The obtained Q(p) of 2.81 L/h/kg, F(a) of 32.6, F(h) of 68.1, and BA of 22.2% were found to be in good agreement with the reported values. Furthermore, the mean local absorption time from the gastrointestinal tract into the portal system (t(a)) calculated by a nonlinear least-squares program [MULTI (FILT)] was almost identical with that by the global moments. These results suggest that the model proposed in this study would be useful for evaluating both in vivo absorption and disposition of drugs.

Introduction of recirculatory analysis into portal and systemic concentration difference method

Recirculatory analysis was introduced into the portal and systemic concentration difference method with double dosing (PS-DD method), which is an evaluation system for the local intestinal and hepatic first-pass effect. 5-Fluorouracil (5-FU) and cephalexin (CEX) were selected as model drugs. A new recirculatory system was constructed to predict the time courses of a drug concentration in the systemic and portal bloods. Bioavailability (F), local absorption ratio (Fa), hepatic recovery ratio (FH), and local mean absorption time (ta) estimated by recirculatory analysis were close to those calculated by moment analysis with numerical integration. Using recirculatory analysis, the sampling period was considerably shortened and the sampling number was also reduced, which demonstrates that recirculatory analysis is useful in PS-DD method.

Evaluation of absorbability of centpropazine in rats: in-situ and in-vivo appraoches

Intestinal absorption of centpropazine was studied in rats by both in-situ (closed-loop method) and in-vivo (portal-venous difference) approaches. The drug was found to be well absorbed from solution in in-situ studies. However, the results obtained in-vivo suggested that very low amounts of drug reach the portal circulation after oral dosing. This could imply extensive binding to the mucosa or metabolism in the intestinal wall. The presence of higher amounts of metabolites in the portal vein compared with the inferior vena cava samples signal their formation in the gastrointestinal tract or enterohepatic recirculation. These findings will be useful in incorporating suitable structural and formulation modifications for enhancing the bioavailability of centpropazine and its analogues.

Effect of pentobarbital anaesthesia on intestinal absorption and hepatic first-pass metabolism of oxacillin in rats, evaluated by portal-systemic concentration difference

The effects of anaesthesia on intestinal drug absorption and hepatic first-pass metabolism in rats were investigated by observing the difference in the drug concentration between portal and systemic bloods. Oxacillin and pentobarbital were selected as a model drug and as an anaesthetic, respectively. Rats were divided into a conscious control group and an anaesthetized group. All rats were cannulated simultaneously in the portal vein and in the femoral artery, and oxacillin was orally administered after its intra-arterial injection (double dosing). For the anaesthetized group, pentobarbital was intrasubcutaneously administered twice, first before intra-arterial injection and again before oral administration of oxacillin. The arterial blood alone was sampled from the cannula in the femoral artery before oral administration, whereas the arterial and portal bloods were simultaneously sampled from both cannulated sites after oral administration. Oxacillin concentrations in plasma were assayed by HPLC. The anaesthesia increased the absolute bioavailability (F), the mean absorption time (MAT) and the hepatic recovery ratio (F(H)), but caused little change in the local absorption ratio into the portal system (Fa) and the total clearance (CL). The hepatic clearance (CL(H)) was significantly decreased, resulting in an apparent small change in CL-CL(H) which is considered to be renal clearance. By this method, it was shown directly that an increase in F due to pentobarbital anaesthesia was attributable to the significant increase in F(H). It is expected that the method is useful not only to evaluate the effect of anaesthesia on the first-pass effect, but also to assess the effect of co-administration of drugs on first-pass metabolism.

Possible mechanism by which the carbapenem antibiotic panipenem decreases the concentration of valproic acid in plasma in rats

There is evidence indicating that the carbapenem antibiotic panipenem decreases plasma concentrations of valproic acid (VPA) in epileptic patients during VPA therapy. The mechanism for panipenem-induced changes in the pharmacokinetics of VPA was investigated in rats with and without bile duct cannulation. The effect of panipenem on the pharmacokinetics of diclofenac, which undergoes extensive enterohepatic recirculation, was also examined. VPA (50 mg/kg of body weight) or diclofenac (10 mg/kg of body weight) was administered intravenously under the steady-state plasma panipenem concentration of 4 microgram/ml, which had been achieved by a constant infusion rate. Panipenem decreased the plasma VPA concentrations in rats without bile duct cannulation but did not change the volume of the initial space and protein binding of VPA. However, panipenem had no effect on the plasma VPA concentrations and the biliary excretion of VPA in rats with bile duct cannulation. The secondary increase in plasma diclofenac concentration observed in the absence of panipenem was diminished in the presence of panipenem. These findings suggest that panipenem decreases plasma VPA concentrations by suppressing its enterohepatic recirculation, probably due to a panipenem-induced decrease in the numbers of enteric bacteria.

Short-period double-dosing for simultaneous evaluation of intestinal absorption and hepatic disposition in a single conscious rat using cephalexin as test drug

A new method has been developed for simultaneous evaluation of local absorption from the intestine into the portal system and local disposition through the liver, and for assessment of the bioavailability of a drug in a single conscious rat. The method is based on the difference between plasma concentrations in portal and systemic blood (PS method). Because cephalexin is known to be absorbed completely from the intestine and not to be eliminated through the liver, it was used as test drug to confirm the validity of the new method. The portal vein and the femoral artery of a rat were simultaneously cannulated and blood samples were obtained from both sites. Two methods of administration, single-dosing and double-dosing, were investigated and the efficacy of double-dosing (DD) was demonstrated. Rats received an intra-arterial (group A) or oral (group B) dose in single-dosing, whereas rats used for double-dosing received an oral dose 3 h after an intra-arterial dose (group C). After administration of cephalexin, the portal and arterial plasma concentrations were determined by HPLC. Groups A and B were monitored for 4 h and group C for 8 h. The portal-blood flow rate was measured by means of an electromagnetic flow-meter. Global and local moments were calculated by trapezoidal integration with extrapolation to infinite time. On the basis of the PS method, the local absorption ratio (Fa) and the mean local absorption time (t(a)) were estimated to be 0.975 +/- 0.104 and 2.19 +/- 0.51 h, respectively, in group B. By comparing the averaged moments between groups A and B, the extent of bioavailability (F), the mean absorption time (MAT) and the hepatic recovery ratio (FH) were calculated to be 1.01, 1.92 h and 1.04, respectively. The mean hepatic transit time (tH) was negligible. In group C, Fa = 0.936 +/- 0.107, tH = 1.55 +/- 0.32 h, F = 1.08 +/- 0.07, MAT = 1.55 +/- 0.40 h and F(H) = 1.17 +/- 0.14 h, the mean values being close to those from groups A and B. In conclusion, the PS method with short-period double-dosing (PS-DD method) can offer an effective means of evaluating the local absorption kinetics of drugs, because F, MAT and F(H) are obtained from a single conscious rat, and consequently the standard deviations of the quantities can be quickly estimated.

Moment analysis of intestinal first-pass metabolism by portal-systemic concentration difference in single conscious rat using 5'-deoxy-5-fluorouridine and 5-fluorouracil as model drug system

Intestinal first-pass metabolism was evaluated in a single conscious rat based on a difference in concentrations of parent drug and its metabolite between the portal and systemic bloods (P-S difference method). 5'-Deoxy-5-fluorouridine (5'-DFUR) and 5-fluorouracil (5-FU) were selected as model drug (prodrug of 5-FU) and metabolite pair. The portal vein and the femoral artery of the rat were cannulated so blood samples could be obtained simultaneously from the two sites. 5'-DFUR (100 mg/kg) was administered intraarterially or orally. Concentrations of 5'-DFUR and 5-FU in the portal and arterial samples were assayed by HPLC. The concentration-time profiles of 5'-DFUR and 5-FU were analyzed by local moment analysis. The extent of systemic bioavailability (F) of 5'-DFUR was estimated to be 75.8%. After oral administration, the local absorption ratio (Fa) and the mean local absorption time (ta) of 5'-DFUR were estimated to be 65.8 +/- 7.3% of dose and 74.0 +/- 21.7 min, respectively. The Fa value was close to F, which suggests that the metabolic conversion from 5'-DFUR to 5-FU is not extensive in the liver. The mean absorption time (MAT), calculated to be 76.3 min, almost coincided with ta, which suggests that the mean hepatic transit time is negligible in this experimental scale. The local absorption ratio of metabolite (Fam) was 6.8 +/- 1.7% of orally administered 5'-DFUR, which means that approximately 7% of 5'-DFUR arrived as 5-FU at the portal system. The mean local absorption time (tam) of 5-FU was estimated to be 75.5 min, which is close to that (74.0 min) of 5'-DFUR. Local moment analysis based on P-S difference enabled simultaneous estimation of the local absorption kinetics of a parent compound and the intestinal generation of metabolites by separating the intestinal first-pass metabolism of a drug from the subsequent disposition through the liver and in the systemic circulation.

Local absorption kinetics of levofloxacin from intestinal tract into portal vein in conscious rat using portal-venous concentration difference

PURPOSE

The local absorption kinetics of levofloxacin from the intestinal tract was quantitatively evaluated by simultaneously measuring the portal and venous plasma concentrations in a conscious rat.

METHODS

The venous and upper portal blood vessels were cannulated through the jugular and pyloric veins, respectively. After oral or intravenous administration of levofloxacin, portal and venous concentrations of levofloxacin were simultaneously monitored. The absorption rate from the intestine into the portal system was calculated from the portal-venous difference in the plasma concentration of levofloxacin, considering the distribution of levofloxacin into erythrocytes. Portal blood flow rate was newly measured by an electromagnetic flow meter.

RESULTS

There was little portal-venous difference after an intravenous dose of levofloxacin. In contrast, after oral administration, the plasma concentration in the portal vein was constantly higher than that in the jugular vein, demonstrating that this difference was caused by the intestinal absorption of levofloxacin.

CONCLUSIONS

Approximately 90% levofloxacin was absorbed as the intact form from the intestinal tract into the portal system. By considering the bioavailability of levofloxacin in rat, the hepatic extraction ratio in vivo of levofloxacin was estimated to be 30%. The mean local absorption time (Ta) was 1.44 hr which coincided almost with the mean absorption time (MAT).

Influence of pentobarbitone on in-vivo local disposition of diclofenac in rat liver

Because the liver is the main organ eliminating many drugs from the body and because pentobarbitone and other analogues can inhibit biliary secretion, the influence of pentobarbitone on hepatic local disposition of diclofenac has been investigated. Diclofenac was infused into the portal and femoral veins of non-anaesthetized rats (group A) and rats anaesthetized with pentobarbitone (group B) and the plasma concentration of diclofenac and the total amount of diclofenac excreted in the bile (in both cases intact diclofenac plus its glucuronide) were simultaneously monitored by HPLC at appropriate time intervals. The time-courses of plasma concentration and amount excreted in the bile were evaluated by moment analysis with trapezoidal integration. The hepatic recovery ratio (FH) was calculated by comparing the area under the curve (AUC) of plasma concentration after intravenous infusion with that after intraportal infusion. The mean biliary transit time (tb) was estimated by subtracting the mean residence time (MRT) of the plasma data from the mean biliary residence time (MRTb) of the biliary excretion data. The FH values of diclofenac were 0.664 in group A and 0.643 in group B. The biliary excretion ratio (Fb) of total diclofenac after intravenous administration was 27.0% in group A and 14.1% in group B. The tb values for total diclofenac were estimated to be 0.192 h (intravenous) and 0.159 h (intraportal) in group A, and 0.174 h and 0.238 in group B. Analysis of variance showed that differences among these four tb values were insignificant at the 5% level. The differences in the mean residence time (MRT), total clearance (CL) and distribution volume at steady state (Vss) were insignificant between groups A and B. Whereas total and the hepatic clearance of diclofenac were not affected by pentobarbitone, biliary clearance was extensively reduced. It took a relatively long time for diclofenac to move from the sinusoid into the bile and the time was not affected by pentobarbitone.