Transport characteristics of grepafloxacin and levofloxacin in the human intestinal cell line Caco-2

Development and Evaluation of EDTA-Treated Rabbits for Bioavailability Study of Chelating Drugs Using Levofloxacin, Ciprofloxacin, Hemiacetal Ester Prodrugs, and Tetracycline

Laboratory rabbits are fed foods rich with cationic metals, and while fasting cannot empty gastric contents because of their coprophagic habits. This implies that, in rabbits, the oral bioavailability of chelating drugs could be modulated by the slow gastric emptying rates and the interaction (chelation, adsorption) with gastric metals. In the present study, we tried to develop a rabbit model with low amounts of cationic metals in the stomach for preclinical oral bioavailability studies of chelating drugs. The elimination of gastric metals was achieved by preventing food intake and coprophagy and administering a low concentration of EDTA 2Na solution one day before experiments. Control rabbits were fasted but coprophagy was not prevented. The efficacy of rabbits treated with EDTA 2Na was evaluated by comparing the gastric contents, gastric metal contents and gastric pH between EDTA-treated and control rabbits. The treatment with more than 10 mL of 1 mg/mL EDTA 2Na solution decreased the amounts of gastric contents, cationic metals and gastric pH, without causing mucosal damage. The absolute oral bioavailabilities (mean values) of levofloxacin (LFX), ciprofloxacin (CFX) and tetracycline hydrochloride (TC), chelating antibiotics, were significantly higher in EDTA-treated rabbits than those in control rabbits as follows: 119.0 vs. 87.2%, 9.37 vs. 13.7%, and 4.90 vs. 2.59%, respectively. The oral bioavailabilities of these drugs were significantly decreased when Al(OH)₃ was administered concomitantly in both control and EDTA-treated rabbits. In contrast, the absolute oral bioavailabilities of ethoxycarbonyl 1-ethyl hemiacetal ester (EHE) prodrugs of LFX and CFX (LFX-EHE, CFX-EHE), which are non-chelating prodrugs at least in in vitro condition, were comparable between control and EDTA-treated rabbits irrespective of the presence of Al(OH)₃, although some variation was observed among rabbits. The oral bioavailabilities of LFX and CFX from their EHE prodrugs were comparable with LFX and CFX alone, respectively, even in the presence of Al(OH)₃. In conclusion, LFX, CFX and TC exhibited higher oral bioavailabilities in EDTA-treated rabbits than in control rabbits, indicating that the oral bioavailabilities of these chelating drugs are reduced in untreated rabbits. In conclusion, EDTA-treated rabbits were found to exhibit low gastric contents including metals and low gastric pH, without causing mucosal damage. Ester prodrug of CFX was effective in preventing chelate formation with Al(OH)₃ in vitro and in vivo, as well as in the case of ester prodrugs of LFX. EDTA-treated rabbits are expected to provide great advantages in preclinical oral bioavailability studies of various drugs and dosage formulations. However, a marked interspecies difference was still observed in the oral bioavailability of CFX and TC between EDTA-treated rabbits and humans, possibly due to the contribution of adsorptive interaction in rabbits. Further study is necessary to seek out the usefulness of the EDTA-treated rabbit with less gastric contents and metals as an experimental animal.

In Vitro Mechanistic Study of the Distribution of Lascufloxacin into Epithelial Lining Fluid

The present study aimed to clarify the mechanism underlying the high distribution of lascufloxacin in epithelial lining fluid (ELF). Involvement of transporters was examined by transcellular transport across Calu-3 and transporter-overexpressing cells; the binding of lascufloxacin to ELF components was examined by an organic solvent-water partitioning system that employed pulmonary surfactant and phospholipids. Transcellular transport across the transporter-overexpressing cells indicated lascufloxacin to be a substrate of both P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP); therefore, its transport across Calu-3 cells was inhibited by P-gp and BCRP inhibitors. However, permeability and efflux ratios of lascufloxacin were similar to those of the other quinolones with relatively low ELF distribution, indicating the existence of another mechanism for lascufloxacin distribution in ELF. Amongst pulmonary surfactants, which are a primary component of ELF, lascufloxacin preferentially bound to phosphatidylserine (PhS) from several phospholipids, and the binding was significantly greater than that for other quinolones. This binding was saturable with two apparent classes of binding sites and inhibited by some weakly basic drugs, indicating the presence of an ionic bond. In conclusion, the results of this study suggest that the binding of lascufloxacin to PhS in the pulmonary surfactant is the major mechanism of the high distribution of lascufloxacin in the ELF.

Identifying 24 h variation in the pharmacokinetics of levofloxacin: a population pharmacokinetic approach

AIM

The objective of this study was to investigate whether the pharmacokinetics of orally administered levofloxacin show 24 h variation. Levofloxacin was used as a model compound for solubility and permeability independent absorption and passive renal elimination.

METHODS

In this single centre, crossover, open label study, 12 healthy subjects received an oral dose of 1000 mg levofloxacin at six different time points equally divided over the 24 h period. Population pharmacokinetic modelling was used to identify potential 24 h variation in the pharmacokinetic parameters of this drug.

RESULTS

The pharmacokinetics of levofloxacin could be described by a one compartment model with first order clearance and a transit compartment to describe drug absorption. The fit of the model was significantly improved when the absorption rate constant was described as a cosine function with a fixed period of 24 h, a relative amplitude of 47% and a peak around 08.00 h in the morning. Despite this variation in absorption rate constant, simulations of a once daily dosing regimen showed that tmax , Cmax and the area under the curve at steady-state were not affected by the time of drug administration.

CONCLUSION

The finding that the absorption rate constant showed considerable 24 h variation may be relevant for drugs with similar physicochemical properties as levofloxacin that have a narrower therapeutic index. Levofloxacin, however, can be dosed without taking into account the time of day, at least in terms of its pharmacokinetics.

Upregulation of P-glycoprotein expression by ophthalmic drugs in different corneal in-vitro models

OBJECTIVES

The purpose of this study was to analyse P-glycoprotein (P-gp) expression in different human in-vitro cornea models (HCE-T epithelial model and Hemicornea construct) after stimulation with P-gp substrates (rhodamine 123, levofloxacin and acebutolol).

METHODS

The influence of P-gp substrates on mRNA expression was analysed using reverse transcriptase polymerase chain reaction (PCR) and real-time PCR. The effect of stimulation on the transporter functionality was estimated with a digoxin efflux assay. The Caco-2 cell line was used as positive control.

KEY FINDINGS

The reverse transcriptase PCR results showed an increase in band intensity compared with the control medium for all substrates. The real-time PCR for the Caco-2 and HCE-T epithelial model yielded a similar outcome, in which all tested substrates upregulated P-gp. In contrast, the Hemicornea construct showed no significant increase in the mRNA expression after stimulation. Both in-vitro models possessed similar drug transport profiles after stimulation. A significantly increased efflux of digoxin was measured after 24 and 72 h of stimulation with levofloxacin and acebutolol.

CONCLUSIONS

The expression and functionality of the P-gp in corneal tissue can be influenced through time exposure with specific substrates. However, the exact mechanism still requires further elucidation.

Stereoselective and multiple carrier-mediated transport of cetirizine across Caco-2 cell monolayers with potential drug interaction

The aim of this study was to explore potential transport mechanisms of cetirizine enantiomers across Caco-2 cells. Cetirizine displayed polarized transport at concentrations ranging from 4.0 to 80.0 microM, with the permeability in the secretory direction being 1.4- to 4.0-fold higher than that in the absorptive direction. Cetirizine enantiomers were transported distinctively different from each other. In the presence of inhibitors of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP), the absorptive transport was enhanced and secretory efflux was diminished. When verapamil, indomethacin, or probenecid were present, the difference in the absorptive permeability of R-cetirizine and S-cetirizine substantially intensified, whereas quinidine could eliminate. R-cetirizine significantly increased the efflux ratio of rhodamine-123 and doxorubicin in a fashion indicative of the upregulation of P-gp and MRP activities. However, S-cetirizine played a role of an inhibitor for P-gp and MRP. Ranitidine modified the absorption of cetirizine enantiomers, suggesting that the potential drug-drug interaction would significantly change the cetirizine pharmacokinetics. In conclusion, the results indicated that there are several efflux transporters including P-gp and MRP participating the absorption and efflux of cetirizine, which showed enantioselectivity in the transmembrane process. In addition, both P-gp and MRP functions could be modulated by cetirizine in chiral discriminative ways.

Stereoselective transport and uptake of propranolol across human intestinal Caco-2 cell monolayers

The transport and uptake of individual propranolol (PPL) enantiomers were studied in human intestinal Caco-2 cell monolayers, and a reversed-phase HPLC-UV assay was used for quantitative analysis. S-PPL and R-PPL across Caco-2 cell monolayers was determined in the concentrations range of 10-500 microM in both apical (AP) to basolateral (BL) and BL to AP directions. S-PPL exhibited greater permeability than R-PPL in the AP to BL direction, whereas in the BL to AP direction S-enantiomer transported less than R-enantiomer. Uptake of R-PPL was significantly higher than that of S-PPL either from AP side or from BL side. The statistically significant differences in uptake were observed at the concentrations range from 10 to 50 microM. Furthermore, the apparent Michaelis constant (K(m)) and maximal velocity (V(max)) also showed significant difference between the two enantiomers. Moreover, the AP to BL transport of PPL enantiomer was markedly decreased by lowering the pH of the apical side but it did not affect the stereoselectivity of PPL across Caco-2 cell monolayers. The transport and uptake of PPL in the BL to AP direction was not influenced by several protein inhibitors. The results suggest that PPL enantiomers showed stereoselective transport and uptake across the Caco-2 cell monolayers. A special transport mechanism capable of directing the PPL enantiomers might be present in the Caco-2 monolayers.

Transport characteristics of rutin deca (H-) sulfonate sodium across Caco-2 cell monolayers

The aim of this study was to explore potential transport mechanisms of rutin deca (H-) sulfonate sodium (RDS) across Caco-2 cell monolayers. As an in-vitro model of human intestinal epithelial membrane, Caco-2 cells were utilized to evaluate the transepithelial transport characteristics of this hydrophilic macromolecular compound. Bi-directional transport study of RDS demonstrated that the apparent permeability (Papp) in the secretory direction was 1.4∼4.5-fold greater than the corresponding absorptive Papp at concentrations in the range 50.0∼2000 μm. The transport of RDS was shown to be concentration, temperature and pH dependent. In the presence of ciclosporin and verapamil, potent inhibitors of P-glycoprotein (P-gp)/MRP2, the absorptive transport was enhanced and secretory efflux was diminished. RDS significantly reduced the efflux ratio of the P-gp substrate rhodamine-123 in a fashion indicative of P-gp activity suppression, while rhodamine-123 competitively inhibited the polarized transport of the compound. In conclusion, the results indicated that RDS was likely a substrate of P-gp. Several efflux transporters, including P-gp, participated in the absorption and efflux of RDS and they might play significant roles in limiting the oral absorption of the compound. These observations offered important information for the pharmacokinetics of RDS.

Relative contributions of active mediated transport and passive diffusion of fluoroquinolones with various lipophilicities in a Calu-3 lung epithelial cell model

The transport characteristics of six fluoroquinolones (FQs) with various lipophilicities were compared in a Calu-3 cell model. For each FQ, an active polarized transport was observed in the direction of the apical side. However, the apparent permeability of FQs resulted from active transport and passive diffusion that were highly variable between compounds and mainly governed by lipophilicity. Therefore, active transport was predominant for compounds with relatively low lipophilicity but minor for FQs with higher lipophilicity.

Temperature-dependent specific transport of levofloxacin in human intestinal epithelial LS180 cells

It was reported previously that specific levofloxacin uptake in Caco-2 cells was inhibited by nicotine, enalapril, L-carnitine and fexofenadine. The aim of the present study was to characterize the cellular uptake of levofloxacin using another human intestinal cell line, LS180. Levofloxacin uptake in LS180 cells was temperature-dependent and optimal at neutral pH, but was Na(+)-independent. The rank order of inhibitory effects of the four compounds on [(14)C] levofloxacin uptake in LS180 cells was nicotine>enalapril>L-carnitine>fexofenadine, which is consistent with that in Caco-2 cells. The mRNA levels of OATP1A2, 1B1, 1B3 and 2B1 in LS180 cells were markedly different from those in Caco-2 cells, and OATP substrates/inhibitors had no systematic effect on the levofloxacin uptake. The mRNA levels of OCTN1 and 2 in LS180 cells were similar to those in Caco-2 cells. However, the inhibitory effect of nicotine on L-[(3)H]carnitine uptake was much less potent than that of unlabeled L-carnitine. These results indicate that the specific uptake system for levofloxacin in LS180 cells is identical/similar to that in Caco-2 cells, but that OATPs and OCTNs contribute little to levofloxacin uptake in the human intestinal epithelial cells.

Fluoroquinolone efflux mediated by ABC transporters

Quinolones and fluoroquinolones are broad spectrum bactericidal drugs, which are widely used in both human and veterinary medicine. These drugs can quite easily enter cells and are often used to treat intracellular pathogens. Some fluoroquinolones have been reported to undergo efflux, which could explain their low bioavailability. There is a growing need to understand resistance mechanisms to quinolones, involving for instance mutations or the action of efflux pumps. Several members of the ATP-binding cassette (ABC) drug efflux transporter family (MDR, MRP, ABCG2) significantly affect the pharmacokinetic disposition of quinolones. Active secretory mechanisms common to all fluoroquinolones have been suggested, as well as competition between fluoroquinolones at transporter sites. For grepafloxacin and its metabolites, MRP2 has been demonstrated to mediate biliary excretion. However, MDR1 is responsible for grepafloxacin intestinal secretion. Recently it has been shown that ciprofloxacin and enrofloxacin are efficiently transported ABCG2 substrates which are actively secreted into milk. It appears that multiple ABC transporters contribute to the overall secretion of fluoroquinolones. The objective of this work is to review the recent advances in insights into ABC transporters and their effects on fluoroquinolone disposition and resistance including data on drug secretion into milk.

Identification of influx transporter for the quinolone antibacterial agent levofloxacin

Quinolone antibacterial agents exhibit high intestinal absorption, selective tissue distribution, and renal and biliary excretion. Several ATP-binding cassette transporters are involved in efflux transport of these agents, but no influx transporters have yet been molecularly identified. In the present study, we aimed to identify the influx transporter(s) of quinolone antibiotics using levofloxacin as a model compound. Several candidate transporter genes were selected based on differential expression of mRNAs among Caco-2 cell subclones that exhibited differential uptake activities for levofloxacin. Based on a functional analysis of each transporter gene for which a good correlation was found between expression level and levofloxacin transport activity in the Caco-2 subclones, organic anion transporting polypeptide 1A2 (OATP1A2 (OATP-A), SLCO1A2) was concluded to transport levofloxacin. When OATP1A2 was expressed in Xenopus oocytes, levofloxacin transport was essentially pH-independent and was not stereoselective. OATP1A2-mediated uptake of levofloxacin showed a K(m) value of 136 microM. Apparent uptake of levofloxacin by Caco-2 cells showed high- and low-affinity components with K(m) values of 0.489 and 14.6 mM, respectively. Accordingly, plural transporters are functional for the transport of levofloxacin in Caco-2 cells, and OATP1A2 is likely to function as a high-affinity transporter. The inhibitory effects and the expression of transport activity of other quinolone antibacterial agents suggested that OATP1A2 commonly transports all the agents tested. In conclusion, this is the first identification of an influx transporter for fluoroquinolones, and the results suggest that active influx transport at least partially explains the high membrane permeability of the quinolone agents in various tissues.

Intestinal transport and metabolism of acrylamide

There has been an intensive debate whether dietary exposure to acrylamide could increase the risk of human cancer since the first description of the presence of acrylamide in food in 2002. As the intestinal mechanisms of acrylamide absorption are poorly investigated we studied the transport of acrylamide in differentiated Caco-2 cells and its effects on biotransformation enzymes (CYP2E1 and glutathione S-transferase) and glutathione levels. We found that the apparent permeability of [1-(14)C] acrylamide from the basal to the apical compartment was approximately 20% higher compared to that in the opposite direction. No differences were detected for apical-basal transport against a basal gradient. Transport rates from the apical to the basal chamber at 4 degrees C were about 50% lower than at 37 degrees C. Concentration dependent transport from apical to basal was linear. Predominantly, basal to apical transport was decreased when energy metabolism of the cells was inhibited by application of sodium azide and 2-deoxy-d-glucose. Finally, more acrylamide was transported at luminal pH 6 compared to pH 7.4 from basal to the apical direction. Increasing levels of acrylamide showed no effects on the activity of glutathione S-transferase but resulted in a depletion of total glutathione concentrations. In conclusion transport of acrylamide in the intestine is mediated primarily by passive processes possibly combined with a modest energy- and pH-dependent active secretory component. Depletion of cellular glutathione levels may be one potential mechanism for acrylamide (geno)toxicity.

Functional studies on the activity of efflux transporters in an ex vivo model with chicken splenocytes and evaluation of selected fluoroquinolones in this model

The efflux proteins P-glycoprotein (P-gp), BCRP and members of the MRP-family (MRPs) are increasingly recognized as determinants of the absorption, tissue distribution and excretion of numerous drugs. A widely applied in vitro screening method, to assess the effect of these efflux transporters in transmembrane transport of drugs is based on the use of peripheral blood mononuclear cells (PBMC), in which the efflux of fluorescent dye Rhodamine 123 (Rh-123) can be easily measured. In avian species, the isolation of PBMCs is compromised by the presence of thrombocytes having approximately the same size. As an alternative, we validated the use of isolated splenocytes to assess Rhodamine 123 transport in the presence and absence of specific inhibitors for P-gp, MRPs and BCRP. Rh-123 efflux was concentration-dependent with the percentage of efflux that decreased with increasing concentrations. P-gp inhibitors, PSC833 and GF120918, significantly inhibit Rh-123 efflux, whereas inhibitors for MRPs and BCRP, MK571 and Ko-143, respectively, have a limited inhibitory effect. However, the effect of GF120918 was more pronounced as compared to PSC833, suggesting an additional role for BCRP next to P-gp in Rh-123 efflux. Moreover, fluoroquinolones were selected to test the applicability of the described model. None of these fluoroquinolones significantly inhibit P-gp function at concentrations up to 50 microM, with exception of danofloxacin and danofloxacin mesylate that were found to reduce Rh-123 efflux by approximately 15%.

[Pharmacokinetic study of cancer chemotherapy]

We reported that the rate of conversion of lactone to carboxylate forms of irinotecan (CPT-11) and its metabolites plays a major role in the biliary excretion of these compounds. Sulfobromophthalein partially inhibited the secretion of SN-38-glucronide into the gastrointestinal lumen, whereas little change was seen in that of active metabolite SN-38. Co-administration of sulphobromophthalein with CPT-11 might lower the late-onset gastrointestinal toxicity observed during treatment with CPT-11 without lowering anticancer activity. In the ileum, the level of transport in the direction form the serosal layer to mucosal layer was significantly greater than that in the direction form the mucosal layer to serosal layer, whereas a significant difference was not observed in the jejunum. This secretory transport required metabolic energy was diminished by sulfobromophthalein. A specific transport system plays a major role in the secretion of SN-38 and that this secretory transport system predominantly exists in the ileum. Uptake of SN-38 was significantly reduced at 4 degrees C. Baicalin inhibited the uptake of SN-38. A specific transport system mediates the uptake of SN-38 across the apical membrane in Caco-2 cells. Inhibition of this transporter would be a useful means for reducing late-onset diarrhea.

Mechanism of the inhibitory effect of zwitterionic drugs (levofloxacin and grepafloxacin) on carnitine transporter (OCTN2) in Caco-2 cells

L-Carnitine plays an important role in lipid metabolism by facilitating the transport of long-chain fatty acids across the mitochondrial inner membrane followed by fatty acid beta-oxidation. It is known that L-carnitine exists as a zwitterion and that member of the OCTN family play an important role in its transport. The aims of this study were to characterize L-carnitine transport in the intestine by using Caco-2 cells and to elucidate the effects of levofloxacin (LVFX) and grepafloxacin (GPFX), which are zwitterionic drugs, on L-carnitine uptake. Kinetic analysis showed that the half-saturation Na+ concentration, Hill coefficient and Km value of L-carnitine uptake in Caco-2 cells were 10.3 +/- 4.5 mM, 1.09 and 8.0 +/- 1.0 microM, respectively, suggesting that OCTN2 mainly transports L-carnitine. LVFX and GPFX have two pKa values and the existence ratio of their zwitterionic forms is higher under a neutral condition than under an acidic condition. Experiments on the inhibitory effect of LVFX and GPFX on L-carnitine uptake showed that LVFX and GPFX inhibited L-carnitine uptake more strongly at pH 7.4 than at pH 5.5. It was concluded that the zwitterionic form of drugs plays an important role in inhibition of OCTN2 function.

Pharmacokinetic analysis of transcellular transport of quinidine across monolayers of human intestinal epithelial Caco-2 cells

To investigate the mechanism responsible for the intestinal absorption of a lipophilic organic cation, quinidine, we performed a pharmacokinetic analysis of transcellular transport across Caco-2 cell monolayers grown on a porous membrane. Basolateral-to-apical transport of the drug was almost constant in the concentration range of 100 nM-100 microM. Transcellular transport was greater in the apical-to-basolateral direction than in the opposite direction. Apical-to-basolateral transport was greater at a concentration of 100 microM than 100 nM. The calculated influx clearance value of the apical membrane was much greater than the other influx/efflux clearance values of cell membranes, and was 5.6-fold the influx clearance value of the basolateral membrane at the drug concentration of 100 microM. We also investigated the uptake of quinidine at the apical membrane of Caco-2 cells grown on plastic dishes. The uptake was markedly increased by alkalization of the apical medium at 37 degrees C, and was decreased at low temperature (4 degrees C). In addition, it was inhibited by diphenhydramine and levofloxacin, but not by carvedilol, rifamycin SV, or L-carnitine. These findings indicated that the influx at the apical membrane was the direction-determining step in the transcellular transport of quinidine across Caco-2 cell monolayers, and that some specific transport system was involved in this influx.

Kinetic modelling of the intestinal transport of sarafloxacin. Studies in situ in rat and in vitro in Caco-2 cells

The absorption kinetics of sarafloxacin, as a model of fluoroquinolone structure, were studied in the rat small intestine and in Caco-2 cells. The objective of the study was to investigate the mechanistic basis of the drug's intestinal transport in comparison with other members of the fluoroquinolone family and to apply a mathematical modelling approach to the transport process. In the rat small intestine, sarafloxacin showed dual mechanisms of intestinal absorption with a passive diffusional component and an absorptive carrier-mediated component. The characteristics of the animal study design made it suitable for population analysis, thus allowing the accurate estimation of transport parameters and their inter and intra-individual variances. The transport system in the rat model was ATP-dependent, as sodium azide was able to decrease the absorption rate constant in a concentration-dependent fashion. The inhibition mechanism of sodium azide was modelled based on its ATP depletion capacity. The rationale of this approach was to consider the inhibitor-carrier interaction as a concentration- dependent response. This interaction was accurately described by a non-competitive mechanism. In Caco-2 cells, sarafloxacin showed a concentration dependent permeability in both directions apical to basal, and basal to apical. The permeability values and ratios of permeability values at different concentrations suggested the presence of two carriers (absorption and efflux carriers). The passive diffusion component in both systems was compared to that predicted by the absorption-partition correlation, previously established for two series of fluoroquinolones. The discrepancy between the experimental and predicted value suggested the presence of an efflux mechanism similar to that already described for other fluoroquinolones. The differences and similarities of the in situ and the in vitro results are discussed as well as the usefulness of the modelling approach.

Biliary secretion of moxifloxacin in obstructive cholangitis and the non-obstructed biliary tract

BACKGROUND

Biliary secretion of antibiotic agents into the bile is considerably compromised by biliary obstruction, a precondition of bacterial cholangitis. Moxifloxacin may be advantageous according to secretion and antimicrobial spectrum.

AIM

To establish the secretion of moxifloxacin into obstructed and non-obstructed bile.

METHODS

Biliary excretion of moxifloxacin was determined in plasma and bile of 10 patients with biliary obstruction and cholangitis and 10 patients without biliary obstruction 30 min after administration of 400 mg of moxifloxacin intravenously.

RESULTS

The plasma concentration of moxifloxacin was similar in both groups (4.45 +/- 1.58 microg/mL; 4.33 +/- 1.23 microg/mL). The concentration of moxifloxacin in the bile was significantly lower in patients with biliary obstruction than without (4.63 +/- 3.94 microg/mL; range 0.71-14.40; vs. 16.90 +/- 13.77 microg/mL; range 1.79-42.50; P = 0.043). Although significantly different, the penetration index was extensively high in those without biliary obstruction (4.41 +/- 4.40; range 0.35-14.45) but still sufficient in those patients with obstructive cholangitis (1.02 +/- 0.74; range 0.29-2.83; P = 0.035).

CONCLUSION

These findings are suggestive of an active secretion mechanism for moxifloxacin into the obstructed bile, producing a biliary concentration sufficiently above the minimal inhibitory concentrations for most of the expected bacteria.

Accumulation and oriented transport of ampicillin in Caco-2 cells from its pivaloyloxymethylester prodrug, pivampicillin

Pivampicillin (PIVA), an acyloxymethylester of ampicillin, is thought to enhance the oral bioavailability of ampicillin because of its greater lipophilicity compared to that of ampicillin. The fate of PIVA in intestinal cells and the exact location of its conversion into ampicillin have, however, never been unambiguously established. Polarized Caco-2 cells have been used to examine the handling of PIVA and the release of ampicillin from PIVA by the intestinal epithelium. Experiments were limited to 3 h. Cells incubated with PIVA (apical pole) showed a fast accumulation of ampicillin and transport toward the basolateral medium, whereas PIVA itself was only poorly accumulated and transported. Cells incubated with free ampicillin accumulated and transported only minimal amounts of this drug. Release of ampicillin from cells incubated with PIVA was unaffected by PEPT1 and OCTN2 inhibitors but was sharply decreased after ATP depletion or addition of bis(4-nitrophenyl)-phosphate (BNPP; an esterase inhibitor). PIVA incubated with Caco-2 lysates released free ampicillin, and this release was inhibited by BNPP. Efflux studies showed that the ampicillin that accumulated in cells after incubation with PIVA was preferentially transported out of the cells through the basolateral pole. This efflux was decreased by multidrug resistance-associated protein (MRP) inhibitors (probenecid, MK-571) and by ATP depletion. A phthalimidomethylester of ampicillin that resists cellular esterases failed to cause any significant release (cell lysate) or transport (polarized Caco-2 cells) of ampicillin. These results show that when PIVA is given to Caco-2 cells from their apical pole, ampicillin is released intracellularly and that ampicillin is thereafter preferentially effluxed into the basolateral medium through an MRP-like transporter.

Determination of the stereoselectivity of chiral drug transport across Caco-2 cell monolayers

This study aimed to determine the transport characteristics of chiral drug enantiomers across Caco-2 cell monolayers as a model of human intestinal epithelial membrane. Esmolol was chosen as a model drug, and the study focused on the transepithelial transport of esmolol enantiomers in this in vitro model system. Separation and quantitation of (S)- and (R)-esmolol were performed by RP-HPLC with the use of GITC as a precolumn derivatizing agent. Bidirectional transport studies of 5.0-400.0 micromol/l esmolol demonstrated that the two enantiomers were transported mainly by a passive, transcellular mechanism. At concentrations of 5.0-100.0 micromol/l, enantioselective permeability of esmolol was observed. In the absorptive transport, Papp of (S)-esmolol was smaller than (R)-esmolol and vice versa for secretory transport. The enantioselectivity disappeared when the drug concentration was increased to 200.0 micromol/l. In conclusion, the transport characteristics of (S)- and (R)-esmolol were distinctly different. An enantioselective carrier-mediated mechanism in addition to passive diffusion was involved in the transport process of esmolol across Caco-2 cell monolayers.

Uptake of irinotecan metabolite SN-38 by the human intestinal cell line Caco-2

PURPOSE

The aim of this study was to investigate the transport mechanisms of transporters that contribute to the intestinal uptake of 7-ethyl-10-hydroxycamptothecin (SN-38).

METHODS

Human intestinal epithelial Caco-2 cells were used to investigate the mechanistic basis of transepithelial uptake of SN-38. We investigated the characteristics of SN-38 uptake into Caco-2 cells. The effects of baicalin and sulfobromophthalein (BSP) on the uptake of SN-38 by Caco-2 cells were examined.

RESULTS

Uptake of SN-38 was significantly reduced at 4 degrees C. Baicalin inhibited the uptake of SN-38 in a concentration-dependent manner. BSP significantly reduced the uptake of SN-38. However, probenecid, pravastatin and grepafloxacin did not affect the uptake of SN-38.

CONCLUSIONS

The results suggest that a specific transport system mediates the uptake of SN-38 across the apical membrane in Caco-2 cells.

Cotransport of Macrolide and Fluoroquinolones, a Beneficial Interaction Reversing P-glycoprotein Efflux

The purpose of this study was to determine the interactions of erythromycin and various fluoroquinolones with P-glycoprotein (P-gp) and in turn assess their effects on transport kinetics across a model cell monolayer. MDCKII-MDRI cells were selected as a model monolayer to evaluate the effects of various fluoroquinolones, ie, norfloxacin, lomefloxacin, ofloxacin, enoxacin, grepafloxacin, levofloxacin, and sparfloxacin on the P-gp-mediated efflux of H-cyclosporine (CsA) and C-erythromycin. IC50 values associated with grepafloxacin-, levofloxacin-, and sparfloxacin-mediated inhibition of P-gp were calculated across Caco-2 cells with erythromycin as the model P-gp substrate. Transport of erythromycin was then studied with P-gp saturable concentrations of fluoroquinolones. Western blot analysis was performed on Caco-2 cells to confirm P-gp expression. Only grepafloxacin elevated the uptake of H-CsA across the MDCKII-MDRI cell monolayer, whereas norfloxacin, lomefloxacin, ofloxacin, and enoxacin did not exert any effect on H-CsA uptake. Inhibition studies indicate that grepafloxacin, levofloxacin, and sparfloxacin are potent inhibitors of P-gp-mediated efflux of C-erythromycin in the MDCKII-MDRI cell monolayer. Similar studies were conducted across Caco-2 cells and IC50 values were calculated. Inhibitory potency of sparfloxacin (IC50 = 607.6 microM) exceeded that of levofloxacin (IC50 = 1644 microM) and grepafloxacin (IC50 = 2266 microM). Permeability ratio (BL-AP/AP-BL) of C-erythromycin was found to be 8.67, which was reduced to 1.18, 1.83, and 1.39 in the presence of grepafloxacin (1 mmol/L), levofloxacin (5 mmol/L), and sparfloxacin (1 mmol/L), respectively. Log partition coefficient of grepafloxacin (1.58), levofloxacin (0.553), and sparfloxacin (0.45) were correlated with the inhibition of P-gp. Western blot analysis indicated the expression of P-gp in Caco-2 cells. Fluoroquinolones like grepafloxacin, levofloxacin, and especially sparfloxacin significantly inhibit the efflux of erythromycin, which can modulate oral absorption and disposition of macrolide drugs when administered concomitantly.

Differential Involvement of Multidrug Resistance-Associated Protein 1 and P-Glycoprotein in Tissue Distribution and Excretion of Grepafloxacin in Mice

The involvement of multidrug resistance-associated protein 1 (Mrp1) and P-glycoprotein (mdr1) in the tissue distribution and excretion of grepafloxacin (GPFX), a fluoroquinolone antibiotic, was investigated using gene-deficient mice [mdr1a(-/-), mdr1a/1b(-/-), and mrp1(-/-)]. The plasma concentration-time profile of GPFX in mrp1(-/-) was nearly identical to that in mrp1(+/+), whereas that in mdr1a/1b(-/-) was higher than that in mdr1a/1b(+/+). The urinary clearance of GPFX in mdr1a/1b(-/-) was lower than that in mdr1a/1b(+/+), suggesting that the urinary excretion of GPFX is at least partially mediated by mdr1. The tissue-to-plasma concentration ratios during the beta-phase (K(p beta),) was significantly higher in the heart, trachea, kidney, spleen, and brown fat of mrp1(-/-) than those in mrp1(+/+). In MRP1-transfected LLC-PK1 cells, the efflux of GPFX after preloading into the cells was higher than that observed in the parent cell lines. These results suggest that GPFX is a substrate of MRP1 and that its distribution to these tissues might be limited by Mrp1. On the other hand, a higher K(p beta), and of GPFX in mdr1a(-/-) mdr1a/1b(-/-) compared with mdr1a/1b(+/+) was observed only in the brain. GPFX was efficiently distributed to the lung parenchyma cells and pulmonary airspaces, including the epithelial lining fluid and macrophages that are the pharmacological target of GPFX, although the contribution of Mdr1 and Mrp1 to such distribution seems to be minor. Thus, the present findings reveal that the disposition of GPFX is at least in part governed by these two ABC transporters and that both Mrp1 and Mdr1 are involved in the limited distribution of GPFX to the distinct tissues, including pharmacological and/or toxicological targets by an active efflux mechanism.

Permeability classification of representative fluoroquinolones by a cell culture method

This study was undertaken to categorize representative fluoroquinolone drug substance permeability based on the methods outlined in the Food and Drug Administration's biopharmaceutic classification system (BCS) Guidance for Industry. The permeability of ciprofloxacin, levofloxacin, lomefloxacin, and ofloxacin was measured in an in vitro Caco-2 assay with previously demonstrated method suitability. The permeability class and efflux potential were ascertained by comparing test drug results with standard compounds (metoprolol, atenolol, labetalol, and rhodamine-123). All 4 quinolones drugs demonstrated concentration-dependent permeability, indicating active drug transport. In comparing absorptive versus secretive in vitro transport, the tested fluoroquinolones were found to be subject to efflux in varying degrees (ciprofloxacin > lomefloxacin > rhodamine 123 > levofloxacin > ofloxacin). Based on comparison to labetalol, the high permeability internal standard, ciprofloxacin was classified as a low permeability drug, whereas lomefloxacin, levofloxacin, and ofloxacin were classified as high permeability drugs. The in vitro permeability results matched human in vivo data based on absolute bioavailabilities. This laboratory exercise demonstrated the applicability of an in vitro permeability method for classifying drugs as outlined in the BCS Guidance.

Experimental demonstration of the unstirred water layer effect on drug transport in Caco-2 cells

We previously demonstrated that P-glycoprotein and MRP2 contribute to the secretory transport of grepafloxacin in the small intestine. Although inhibitors of these secretory transporters increased absorptive transport of grepafloxacin, secretory transport was not altered in Caco-2 cells, as determined by a conventional Transwell method. Because the value of the permeability coefficient of grepafloxacin is high, permeation through the unstirred water layer (UL) might be the rate-limiting step. To examine the possibility that the UL effect may mask the involvement of membrane transporters in the transport of drug with high permeability in Caco-2 cells, transport experiments were performed by agitating the experimental solution to decrease the thickness of the UL, and by lowering the temperature to decrease permeation via active transporters. Under these conditions, the UL effect was not rate limiting, and the inhibitory effects of transporter modulators were reflected in the apparent permeability as a decrease in secretory transport as well as an increase in absorptive transport. In conclusion, it was demonstrated that the UL can be the rate-limiting factor for transport of drugs with high membrane permeability in Caco-2 cells. When the UL affects the apparent permeability in an experimental apparatus in vitro, careful analysis is required to evaluate the contributions of transporters from the apparent permeability of drugs.