Intestinal drug efflux: formulation and food effects

Investigation of regional mechanisms responsible for poor oral absorption in humans of a modified release preparation of the alpha-adrenoreceptor antagonist, 4-amino-6,7-dimethoxy-2-(5-methanesulfonamido-1,2,3,4 tetrahydroisoquinol-2-yl)-5-(2-pyridyl)quinazoline (UK-338,003): the rational use of ex vivo intestine to predict in vivo absorption

Modified release (MR) formulations are used to enhance the safety and compliance of existing drugs by improving their pharmacokinetics. Predicting the likely success of MR formulations is often difficult before clinical studies. A systematic in vitro approach using mouse and human tissues was adopted to rationalize the in vivo pharmacokinetics of 9- and 15-h MR formulations of an alpha-adrenoreceptor antagonist, 4-amino-6,7-dimethoxy-2-(5-methanesulfonamido-1,2,3,4 tetrahydroisoquinol-2-yl)-5-(2-pyridyl)quinazoline (UK-338,003). Immediate release UK-338,003 was well absorbed in humans consistent with moderate Caco-2 cell monolayer permeability. In contrast, 9- and 15-h modified release formulations showed marked reductions in C(max) (47.1 and 68.9%) and AUC(0-72) (32.6 and 54.0%). Colonic intubation resulted in 81.3 and 73.8% reductions in C(max) and AUC(0-72). Mechanistic studies in isolated mouse tissues showed that colonic UK-338,003 permeability (P(app) < 0.5 x 10(-6) cm/s) was at least 40 times lower than that for ileum with marked asymmetry. UK-338,003 was found to be a substrate for P-glycoprotein (PGP) with a weaker interaction for multidrug resistance-associated protein-type transporters in mouse intestine. PGP inhibition dramatically increased colonic UK-338,003 permeability to the levels observed in ileum. Low UK-338,003 apical to basolateral permeability was also observed in ex vivo human distal intestine, but both the asymmetry and increase in permeability after PGP inhibition were significantly lower. In conclusion, the poor absorption of MR UK-338,003 in humans can be explained by a combination of PGP-dependent efflux and low intrinsic permeability in the lower bowel. Regional permeability studies in ex vivo tissues used during drug development can highlight absorption problems in the distal bowel and assess the feasibility of developing successful MR formulations.

Effects of Labrasol and Other Pharmaceutical Excipients on the Intestinal Transport and Absorption of Rhodamine123, a P-Glycoprotein Substrate, in Rats

Effects of Labrasol and other pharmaceutical excipients on the intestinal transport and absorption of rhodamine123, a P-glycoprotein substrate (P-gp) were examined. Intestinal transport and absorption studies were examined by an in vitro diffusion chamber method and an in situ closed loop method. We evaluated the intestinal membrane damage produced by Labrasol by measuring the release of protein and alkaline phosphatase (ALP). Labrasol (0.075-0.1% (v/v)) increased the absorptive transport of rhodamine123 and decreased its secretory transport in the in vitro transport studies. However, Labrasol did not change the transport of Lucifer yellow, a non-P-gp substrate, suggesting that the effect of Labrasol on the transport of drugs was specific for rhodamine123. We observed almost no intestinal membrane damage in the presence of Labrasol. These findings suggest that the increase in the absorptive transport of rhodamine123 in the presence of Labrasol may not be due to its intestinal membrane damage. In the in situ absorption studies, we found that Labrasol (0.1% (v/v)) significantly enhanced the intestinal absorption of rhodamine123 in rats, although the absorption enhancing effect of Labrasol was much less than that of verapamil. These findings suggest that low concentrations of Labrasol might inhibit the function of P-gp in the intestine, thereby increasing intestinal absorption and bioavailability of P-gp substrates including rhodamine123. However, we may also consider the contribution to the enhanced intestinal absorption of rhodamine123 via a passive transport in addition to the inhibitory action of Labrasol for the function of P-gp in the intestine.

Effects of controlled-release on the pharmacokinetics and absorption characteristics of a compound undergoing intestinal efflux in humans

OBJECTIVE

The number of active pharmaceutical ingredients (API) undergoing inhibitable and saturable intestinal efflux is considerable. As a consequence, absorption and bioavailability may depend on the intestinal concentration profile of the drug and may vary as a function of dose and release rate of the drug from the dosage form. The impact of controlled versus immediate-release on the absorption of P-glycoprotein substrates is currently unknown. Thus, the main focus of the present study was a comparison of the pharmacokinetics of the P-gp model substrate talinolol following administration of immediate-release (IR) and controlled-release (CR) tablets to healthy human volunteers with a particular focus on the absorption characteristics of the active pharmaceutical ingredients.

METHODS

Talinolol immediate-release (Cordanum), 100mg), one controlled-release (100mg) and two controlled-release tablets (200mg) were administered as single doses to fasting healthy volunteers in a crossover design with a 1 week washout period between treatments. Sufficient blood and urine samples were drawn and analysed using a specific HPLC method with UV detection to describe the resulting plasma and urinary excretion versus time profiles.

RESULTS

The bioavailability of talinolol in term of AUC(0-->infinity) for IR talinolol was approximately twice as high as compared to the administration of the same dose in a controlled-release dosage form. After administration of talinolol IR tablets, the drug was rapidly absorbed and reached maximum concentrations C(max) of 204.5 ng/ml+/-121.8 (means+/-S.D.) 2h after dosing. The terminal half-life of the drug averaged 19.8h following IR administration in comparison to 32 h under CR dosing conditions. Following administration of the IR dosage form, significant secondary peaks were observed in one healthy subject. Secondary peaks were not clearly apparent in the CR plasma profiles.

CONCLUSION

The present study demonstrates a considerable loss of bioavailability of drugs that are substrates of intestinal secretory transporters upon their administration in controlled-release dosage forms.

Preclinical formulations for discovery and toxicology: physicochemical challenges

Formulations play a key role in assessing the biological properties of a molecule during drug discovery. Maximising exposure is the primary objective in early animal experimentation, so that the pharmacokinetics, pharmacodynamics and toxicological signals can be put into context with the biological response to specific targets. Consistency in the exposure is also a key aspect, and effective formulation and drug delivery strategies are important to achieve this. Diversity in the physiology between various animal species, routes of administration and limitations posed by specific pharmacological models make formulation development that much more challenging. Poor physicochemical properties of compounds in the early stages need to be kept under consideration while screening for formulation vehicles. This review captures the various challenges posed at different stages of drug discovery for formulation of a compound to dose in animals. Approaches to formulations for various routes of administration are discussed. Limitations posed by the goals for various animal studies such as early efficacy studies, pharmacokinetic studies and toxicology studies are identified and some strategies are proposed. Physicochemical characterisations that are needed to select formulation vehicles as well as to identify potential issues are suggested.

Flavonoid-mediated inhibition of intestinal ABC transporters may affect the oral bioavailability of drugs, food-borne toxic compounds and bioactive ingredients

The transcellular transport of ingested food ingredients across the intestinal epithelial barrier is an important factor determining bioavailability upon oral intake. This transcellular transport of many chemicals, food ingredients, drugs or toxic compounds over the intestinal epithelium can be highly dependent on the activity of membrane bound ATP binding cassette (ABC) transport proteins, able to export the compounds from the intestinal cells. The present review describes the ABC transporters involved in the efflux of bioactive compounds from the intestinal cells, either to the basolateral blood side, facilitating absorption, or back into the intestinal lumen, reducing bioavailability. The role of the ABC transporters in intestinal transcellular uptake also implies a role for inhibitors of these transporters in modulation of the bioavailability upon oral uptake. The present paper focuses on the role of flavonoids as important modulators or substrates of intestinal ABC transport proteins. Several examples of such an effect of flavonoids are presented. It can be concluded that flavonoid-mediated inhibition of ABC transporters may affect the bioavailability of drugs, bioactive food ingredients and/or food-borne toxic compounds upon oral uptake. All together it appears that the flavonoid-mediated interactions at the level of the intestinal ABC transport proteins may be an important mechanism for unexpected food-drug, food-toxin or food-food interactions. The overview also indicates that future studies should focus on i) in vivo validation of the flavonoid-mediated effects on bioavailability of drugs, toxins and beneficial bioactive food ingredients detected in in vitro models, and on ii) the role of flavonoid phase II metabolism in modulating the activity of the flavonoids to act as ABC transporter inhibitors and/or substrates.

In Silico Modeling of Non-Linear Drug Absorption for the P-gp Substrate Talinolol and of Consequences for the Resulting Pharmacodynamic Effect

PURPOSE

The aim of the present work was to demonstrate P-glycoprotein's involvement in the non-linear talinolol pharmacokinetics using an advanced compartment and transit model (ACAT) and to compare the results predicted from the model to the finding of a phase I dose escalation study with oral talinolol doses increasing from 25 to 400 mg.

MATERIALS AND METHODS

Besides minimum input parameters for the compound (pKa(s), solubility at one or more pH's, Peff, doses, formulation, diffusivity), physiological and pharmacokinetic properties, transporter data are included in these predictions. The simulations assumed higher expression levels in lower gastrointestinal regions, in particular in the colon, which is in accordance with the results of intestinal rat perfusion studies and intestinal distribution data from rats, catfishes, micropigs and humans reported in the literature. Optimized values for P-glycoprotein (P-gp) Km and Vmax were used for the final simulation results and for a stochastic virtual trial with 12 patients.

RESULTS

Talinolol, a P-gp substrate, exhibits non-linear dose AUC relationship after administration of 25, 50, 100 and 400 mg immediate-release tablets. This dose dependency is due to a decrease of efflux transport caused by saturation of P-gp by talinolol. It was found that oral bioavailability increases after administration of higher doses of talinolol. The predicted bioavailability of the p.o. 25, 50, 100 and 400 mg doses of talinolol was 64, 76, 85, 94%, respectively. Pharmacokinetic parameters (AUC, Cmax) from in silico simulations are within acceptable range comparing with data, observed in vivo. However, the in vitro value of Km for talinolol's interactions with P-gp could not be used in the simulation and still reproduce the observed non-linear dose dependence. For each of the four doses, GastroPlus was used to model pharmacodynamic (PD) response and to optimize the values of CLe, Emax, and EC5o with the effect compartment linked indirectly to the central compartment. For all simulations, EC50 was 114 nM and E0 was 83 bpm.

CONCLUSION

Comparison between the results of the in vivo study and the in silico simulations determined the quality and reliability of the in silico predictions and demonstrate the simulation of dose dependent absorption. In contrast to previous simulation work for the non-linear dose dependence of interaction with intestinal transporters or enterocyte metabolism, optimized Km and Vmax values were required to reproduce the clinically observed non-linear dose dependence. The model developed may be useful in the prediction of absorption of other P-gp substrates including pharmacodynamic consequences.

Assessment of the feasibility of oral controlled release in an exploratory development setting

Controlled release (CR) formulations have generally been considered as follow-ons to conventional immediate release formulations to manage the life cycle of a product. Although significant opportunities exist to use CR as an enabling technology for certain exploratory drug candidates, they have not been fully exploited. However, progress made in assessing CR feasibility based on the physicochemical and biopharmaceutical properties of the drug, together with advances made in understanding the various CR technologies and developing formulations in a fast and efficient manner, have increasingly made it possible to consider CR in an exploratory development setting.

Modulation of MPP+ uptake by tea and some of its components in Caco-2 cells

The entry of most xeno/endobiotics into the organism is limited by their intestinal absorption. The interference of certain foods with the therapeutic efficacy of drugs or with chemical toxicity is becoming evident and growing attention is being given to these subjects. The aim of this work was to study the effect of green tea (GT) and black tea (BT), as well as some of their components, on the transport of organic cation molecules. For this purpose, 3H-MPP+ (radiolabeled 1-methyl-4-phenylpyridinium) was used as a model organic cation and Caco-2 cells were used as an intestinal epithelial model. Our results showed that both GT and BT significantly increased 3H-MPP+ absorption in these cells. Additionally, we studied the effect of epigallocatechin-3-gallate (EGCG), myricetin, caffeine, and theophylline. Whereas EGCG (2 mM) increased, myricetin (50 microM) and caffeine (1 mM) decreased, and theophylline (1 mM) had no effect on the uptake of 3H-MPP+ into Caco-2 cells. When GT was supplemented with caffeine or theophylline, we observed a partial loss of its effect. When BT was supplemented with EGCG, its ability to increase 3H-MPP+ uptake was much more pronounced than that observed with BT alone. In conclusion, this study showed that GT and BT might interfere with the absorption of the model organic cation MPP+ by the intestinal epithelium. Since important compounds are organic cations, the consequences of this interference may have an impact on human health. Although this constitutes only preliminary work and further studies are needed, tea should be included in the growing list of foodstuffs that have the potential to be involved in food-drug interactions.

Intestinal absorption of drugs mediated by drug transporters: mechanisms and regulation

The absorption of drugs from the gastrointestinal tract is one of the important determinants for oral bioavailability. Development of in vitro experimental techniques such as isolated membrane vesicles and cell culture systems has allowed us to elucidate the transport mechanisms of various drugs across the plasma membrane. Recent introduction of molecular biological techniques resulted in the successful identification of drug transporters responsible for the intestinal absorption of a wide variety of drugs. Each transporter exhibits its own substrate specificity, though it usually shows broad substrate specificity. In this review, we first summarize the recent advances in the characterization of drug transporters in the small intestine, classified into peptide transporters, organic cation transporters and organic anion transporters. In particular, peptide transporter (PEPT1) is the best-characterized drug transporter in the small intestine, and therefore its utilization to improve the oral absorption of poorly absorbed drugs is briefly described. In addition, regulation of the activity and expression levels of drug transporters seems to be an important aspect, because alterations in the functional characteristics and/or expression levels of drug transporters in the small intestine could be responsible for the intra- and interindividual variability of oral bioavailability of drugs. As an example, regulation of the activity and expression of PEPT1 is summarized.

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Regional Difference in P-glycoprotein Punction in Rat Intestine

It has been reported that inhibition of the P-glycoprotein (P-gp) results in the improved absorption of P-gp substrate in the intestinal tract. In fact, the increased permeability of P-gp substrate across the intestinal epithelium was observed following inhibition of P-gp in in vitro experiments. To develop the formulation containing P-gp inhibitor and P-gp substrate for practical use, it is necessary to know whether the results obtained in the in vitro experiments are reproducible at whole body level. It is also important to find out the regional difference of the P-gp activity in the intestinal tract. In this study, we examined whether verapamil, a specific inhibitor of P-gp, improves the absorption of rhodamine123 (Rho123), a substrate of P-gp, from the jejunum, ileum, and colon of rats using the in situ loop method. The water content in the loop decreased during the experiment, resulting in a significant change of the Rho123 concentration in the loop. Thus, to accurately determine the absorption rate of Rho123, it was necessary to measure the water movement. It was found that there was a regional difference in the water movement, i.e., greatest in colon, followed by ileum. Verapamil did not change the water movement in any intestinal regions. When the concentration of Rho123 in the loop was corrected by water movement, the Rho123 clearance was in the order of ileum (1.15 microL/min/cm), colon (0.83 microL/min/cm) and jejunum (0.47 microL/min/cm). In the presence of verapamil, the Rho123 clearance was significantly increased at jejunum and ileum but not in colon (ileum: 2.08 microL/min/cm, colon: 1.14 microL/min/cm, jejunum: 1.28 microL/min/cm). These results suggest that P-gp inhibits the drug absorption in jejunum and ileum. From these results, it is possible to evaluate the role of P-gp and its regional difference in the in situ experiments. In particular, the inhibition of P-gp results in an increase in absorption of the P-gp substrate limited to jejunum and ileum.

Effects of dietary supplements on induction and inhibition of cytochrome P450s protein expression in rats

Recent surveys show that 18% of adults in the United States use prescription drugs concurrently with herbal or vitamin dietary products. Despite possible dietary supplement-drug interactions through modulation of cytochrome P450s (CYPs), dietary supplements have not been studied at a screening scale to assess their effects on CYPs. In this study, 116 herbal dietary supplements, commercially available for nutrient supply and weight management, were administered to rats to test whether they modulate the expressions of CYP1A2, 2C11, 2D1, 2E1 and 3A1 proteins. Seventy-five percent of the 116 dietary supplements modulated at least one CYP, while 25% had no effect. CYP2C11 protein expression was the most inhibited by supplements (51%), whereas CYP1A2, 2D1, 2E1 and 3A1 were the least inhibited (12-18%). CYP1A2 was the most induced, modulated by 21% supplements, while CYP2E1 and 3A1 were moderately induced (7-8%). CYP2C11 and 2D1 were not induced by any supplement tested in this study. Thus, this study suggests that dietary supplement-drug interactions may occur through modulation of CYPs in humans when they are taken simultaneously.

Classification analysis of P-glycoprotein substrate specificity

Prediction of P-glycoprotein substrate specificity (S(PGP)) can be viewed as a constituent part of a compound's "pharmaceutical profiling" in drug design. This task is difficult to achieve due to several factors that raised many contradictory opinions: (i) the disparity between the S(PGP) values obtained in different assays, (ii) the confusion between Pgp substrates and inhibitors, (iii) the confusion between lipophilicity and amphiphilicity of Pgp substrates, and (iv) the dilemma of describing class-specific relationships when Pgp has no binding sites of high ligand specificity. In this work, we compiled S(PGP) data for 1000 compounds. All data were represented in a binary format, assigning S(PGP) = 1 for substrates and S(PGP) = 0 for non-substrates. Each value was ranked according to the reliability of experimental assay. Two data sets were considered. Set 1 included 220 compounds with S(PGP) from polarized transport across MDR1 transfected cell monolayers. Set 2 included the entire list of 1000 compounds, with S(PGP) values of generally lower reliability. Both sets were analysed using a stepwise classification structure-activity relationship (C-SAR) method, leading to derivation of simple rules for crude estimation of S(PGP) values. The obtained rules are based on the following factors: (i) compound's size expressed through molar weight or volume, (ii) H-accepting given by the Abraham's beta (that can be crudely approximated by the sum of O and N atoms), and (iii) ionization given by the acid and base pKa values. Very roughly, S(PGP) can be estimated by the "rule of fours". Compounds with (N + O) > or = 8, MW > 400 and acid pKa > 4 are likely to be Pgp substrates, whereas compounds with (N + O) < or = 4, MW < 400 and base pKa < 8 are likely to be non-substrates. The obtained results support the view that Pgp functioning can be compared to a complex "mini-pharmacokinetic" system with fuzzy specificity. This system can be described by a probabilistic version of Abraham's solvation equation, suggesting a certain similarity between Pgp transport and chromatographic retention. The chromatographic model does not work in the case of "marginal" compounds with properties close to the "global" physicochemical cut-offs. In the latter case various class-specific rules must be considered. These can be associated with the "amphiphilicity" and "biological similarity" of compounds. The definition of class-specific effects entails construction of the knowledge base that can be very useful in ADME profiling of new drugs.

Modulation of Intestinal P-Glycoprotein Function by Cremophor EL and Other Surfactants by an In Vitro Diffusion Chamber Method Using the Isolated Rat Intestinal Membranes

Effects of various surfactants on the transport of rhodamine123, a P-glycoprotein (P-gp) substrate, across the isolated rat intestinal membranes were examined by an in vitro diffusion chamber system. The jejunal serosal-to-mucosal transport (Jsm) of rhodamine123 was more than threefold greater than its mucosal-to-serosal transport (Jms), suggesting that the net movement of rhodamine123 across the rat jejunum was preferentially secretory direction. There exists a regional difference in the intestinal transport of rhodamine123 and the secretory directed transport was remarkably observed in the jejunum. The Jsm/Jms ratio of rhodamine123 decreased in the presence of 0.3 mM verapamil and 10 mM sodium azide (NaN3) + 1 mM sodium fluoride (NaF), confirming that rhodamine123 might be secreted from the intestinal tissue into the lumen by a P-gp-mediated efflux system. Nonionic surfactants [0.1% Cremophor EL, Tween 80 and n-dodecyl-beta-D-maltopyranoside (LM)] reduced the Jsm/Jms ratio of rhodamine123, whereas its ratio was not influenced in the presence of 0.1% cationic surfactant (hexadecyltrimethylammonium bromide, C16TAB) and anionic surfactant (sodium dodecyl sulfate, SDS). Therefore, these findings suggested that charge of surfactants was possibly related to the action of these surfactants on the intestinal absorption of P-gp substrates. On the other hand, the transfer of rhodamine123 was not affected by the addition of Cremophor EL to the serosal side. Because the c.m.c. of Cremophor EL is 0.0095 w/v%, interactions between rhodamine123 and the micellar form of Cremophor EL may decrease the P-gp-mediated efflux of rhodamine123 at higher concentrations. In the kinetic analysis, the Vmax value (nmol/min/g wet tissue) of rhodamine123 decreased, although the Km value (mM) was constant in the presence of Cremophor EL. Therefore, Cremophor EL inhibited the efflux transport of rhodamine123 in a noncompetitive manner. Cremophor EL did not affect the transport of [14C]Gly-Sar and [3H]3-O-methyl-D-glucose, suggesting that the action of Cremophor EL might be P-gp specific. These findings indicated that nonionic surfactants including Cremophor EL and Tween 80 may be useful pharmaceutical excipients for inhibiting the function of P-gp, thereby increasing the intestinal absorption of various drugs, which are secreted by a P-gp-mediated efflux system in the intestine.

The use of a dendrimer-propranolol prodrug to bypass efflux transporters and enhance oral bioavailability

The aim of the study was to determine the effects on the transport of propranolol across monolayers of the human colon adenocarcinoma cell line, Caco-2, of forming a prodrug by conjugating to generation 3 (G3) and lauroyl-G3 PAMAM dendrimers. Propranolol is a poorly soluble drug and known substrate of the P-glycoprotein (P-gp) efflux transporter. Propranolol-G3 dendrimer conjugates were synthesised by surface attachment of two, four or six propranolol molecules. The apical (A) to basolateral (B) apparent permeability coefficient, P(app), of propranolol was increased and its B-->A P(app) decreased following conjugation to G3 dendrimers. Conjugation of propranolol to lauroyl-G3 dendrimers further increased its A-->B P(app). Our findings show that the A-->B P(app) of propranolol conjugates was reduced in the presence of the endocytosis inhibitor colchicine and was lower at 4 degrees C than at 37 degrees C, suggesting that the enhancement mechanism involves endocytosis-mediated transepithelial transport. The A-->B P(app) of conjugated propranolol was not altered in the presence of the P-gp inhibitor cyclosporin A suggesting that conjugation of drug to dendrimer allows the bypassing of the efflux transporter. The results suggest that dendrimer-drug prodrugs may be used to increase drug solubility and bypass drug efflux transporters, therefore increasing drug bioavailability.

Pectin-based systems for colon-specific drug delivery via oral route

Pectin-derived matrices are now being examined and tested for controlled drug delivery. Pectin is intact in the upper gastrointestinal tract and degraded by colonic microflora. The composition of this microflora remains relatively consistent across a diverse human population. Thus, pectin-derived drug carriers provide promising potential for colon-specific drug delivery. This paper reviews recent developments in pectin-derived formulations. Subjects reviewed include gelation of pectin, calcium cross-linked pectinate, composites of pectin and other polymers, technologies to fabricate pectin into useful drug delivery vehicles, and methods to evaluate release kinetics of incorporated drugs. This article discusses advantages, limitations, and possible future developments in pectin-based formulations with particular emphasis on the field of colon-specific drug delivery.

Relationships between the hydrophilic-lipophilic balance values of pharmaceutical excipients and their multidrug resistance modulating effect in Caco-2 cells and rat intestines

The effects of a series of pharmaceutical excipients, including Span 80, Brij 30, Tween 20, Tween 80, Myrj 52, and sodium lauryl sulfate (with increasing hydrophilic-lipophilic balance (HLB) values) on the intracellular accumulation, transport kinetics, and intestinal absorption of epirubicin were investigated in both the human colon adenocarcinoma (Caco-2) cell line and the everted gut sacs of rat jejunum and ileum. The possible use of these excipients as multidrug resistance (MDR) reversing agents also was examined. Epirubicin uptake experiments using a flow cytometer showed that these selected excipients markedly enhanced the intracellular accumulation of epirubicin in Caco-2 cells in a dose-dependent manner. The optimal effect on the epirubicin uptake was characteristic of excipients with intermediate HLB values ranging from 10 to 17. Moreover, the optimal net efficacy was observed for excipients with polyoxyethylene chains and intermediate chain length of fatty acid and fatty alcohol (monolaurate for Tween 20, monooleate for Tween 80, monostearate for Myrj 52, and lauryl alcohol for Brij 30). These excipients significantly increased apical to basolateral absorption and substantially reduced basolateral to apical efflux of epirubicin across Caco-2 monolayers. Furthermore, the addition of Tween 20, Tween 80, Myrj 52, and Brij 30 markedly enhanced mucosal to serosal absorption of epirubicin in the rat jejunum and ileum. This study suggests that inhibition of intestinal P-glycoprotein (P-gp), multidrug resistance associated protein family (MRPs), or other transporter proteins by pharmaceutical excipients may improve oral absorption of drugs in MDR spectrum. The optimal HLB values of surfactant systems with suitable hydrocarbon chains and polar groups are an important factor in designing promising epirubicin formulations for reversing MDR. In conclusion, therapeutic efficacy of epirubicin may be enhanced by the use of such low toxicity excipients as absorption enhancers and MDR modulators in formulations. This provides a potential strategy for improving bioavailability in the optimization of formulations for drugs performing intestinal absorption and secretion.

Different Dissolution Media Lead to Different Crystal Structures of Talinolol with Impact on Its Dissolution and Solubility

During the performance of dissolution tests with immediate and controlled-release talinolol tablets it was detected that the type of the buffer used as dissolution medium had a strong influence on the solubility and the dissolution behavior of the drug. It was proven that talinolol appeared in different crystal structures with strongly differing solubilities when pure water, acetate, or phosphate buffers were employed as dissolution media. The resulting crystal structures were characterized by means of light microscopy, differential scanning calorimetry, and X-ray powder diffraction. All methods were adjuvant to detect changes in talinolol crystal structures. The different solubility and dissolution properties of the talinolol salts or modifications may be viewed as a source for its incomplete and variable bioavailability. Furthermore, the food effect, described in the literature, that leads to a decrease in talinolol absorption, could be due to changes in the composition of gastrointestinal fluids leading to different talinolol crystal structures. Furthermore, it was detected that the addition of sodium chloride increases talinolol solubility and accelerates its dissolution from controlled-release tablets at concentrations between 0% and 1.25%, while an addition of sodium dodecylsulfate (SDS) as surfactant only had a solubility-improving effect at concentrations > 0.75%. At lower concentrations SDS decreased the solubility of talinolol and notably decelerated its release from controlled-release tablets.

The effect of food components on the absorption of P-gp substrates: a review

P-glycoprotein (P-gp), a well characterized efflux mechanism which is functionally expressed in the intestinal epithelium, constitutes, along with intestinal metabolism, an important part of the biochemical barrier function of the intestinal mucosa. This efflux carrier may be responsible for limiting the bioavailability of several drugs after oral intake. Recently, increasing attention is being paid to the interaction of dietary components with the intestinal absorption of drugs. This review focuses on the modulating capacity of food components on the intestinal absorption of P-gp substrates. The possible P-gp inhibitory effects of several dietary constituents are discussed. In addition, this review will also focus on the effect of several bioflavonoids on the P-gp-mediated efflux of drugs. As the role of P-gp (and other efflux carriers, including multidrug resistance-associated proteins and breast cancer resistance protein) in limiting the bioavailability of drugs becomes more clear, more research is required firstly to identify the effect of dietary compounds on these efflux carriers and secondly to reveal the clinical relevance of this interaction.

Inhibitory effect of fruit extracts on P-glycoprotein-related efflux carriers: an in-vitro screening

In this study, standardized food extracts were screened for their possible inhibitory effect on the P-glycoprotein (P-gp)-mediated efflux of 3H-ciclosporin A (CsA) using the in-vitro Caco-2 model. CsA is commonly used as a substrate for P-gp-related efflux carriers and is characterized by a polarity in transport, the absorptive transport being much lowerthan the secretorytransport (polarityfactor: PF approximately 7). Of the 68 tested, nine extracts showed a decreased efflux of CsA (< 75% of the reference value) and were retained for further experiments on the bidirectional transport of CsA across Caco-2 monolayers. Results of these experiments showed that strawberry, orange, apricot and mint extract exert an inhibitory effect on intestinal P-gp-related functionality (PF < 4.2). The effect of apricot extract was also studied on the bidirectional transport of talinolol, a specific P-gp substrate; inclusion of 1%, v/v, in the apical compartment of Caco-2 monolayers resulted in a significantly reduced polarity in the transport of talinolol (PF reference = 15.5; PF in the presence of apricot extract = 2.5). This study suggests that co-administration of fruit extracts might be a conceptually safe and useful strategy to enhance the intestinal absorption of P-gp substrates. More research is necessary to characterize the impact of this inhibition on P-gp-related efflux mechanisms in other absorption models (in-vitro and in-vivo) and to identify the compounds that are responsible for this inhibitory effect.

A mechanistic approach to understanding the factors affecting drug absorption: a review of fundamentals

This article provides an overview of the patient-specific and drug-specific variables that can affect drug absorption following oral product administration. The oral absorption of any chemical entity reflects a complex spectrum of events. Factors influencing product bioavailability include drug solubility, permeability, and the rate of in vivo dissolution. In this regard, the Biopharmaceutics Classification System has proven to be an important tool for predicting compounds likely to be associated with bioavailability problems. It also helps in identifying those factors that may alter the rate and extent of drug absorption. Product bioavailability can also be markedly influenced by patient attributes such as the integrity of the gastrointestinal tract, physiological status, site of drug absorption, membrane transporters, presystemic drug metabolism (intrinsic variables), and extrinsic variables such as the effect of food or concomitant medication. Through an awareness of a drug's physicochemical properties and the physiological processes affecting drug absorption, the skilled pharmaceutical scientist can develop formulations that will maximize product availability. By appreciating the potential impact of patient physiological status, phenotype, age, gender, and lifestyle, dosing regimens can be tailored to better meet the needs of the individual patient.

Pharmacogenomics of drug transporters: the next drug delivery challenge

Scientifically, the third millennium begins with a major triumph--the publishing of the human genomic map, which is destined to have a momentous impact on the quality of life in our time. Disease prevention, individualized medicine, and genotyped-based medicine will soon become a reality. Pharmacogenetics, the forerunner of pharmacogenomics, began in the 1950s with a series of observations relating drug response to various genetic factors. It took almost two more decades for scientists to discover that cytochrome p450 2D6 was responsible for the metabolism of many drugs. This landmark discovery helped focus attention on how gene expression could impact the response to drugs. The stage was set for a revolution in therapeutics some 30 years later as the Human Genome Project crossed the finishing line triumphantly. A parallel development in drug delivery that may also benefit from the fruits of the Human Genome Project is the growing acceptance/awareness of drug transporters as a gateway to epithelial drug transport. This presentation addresses an area in need of attention: the possible impact of genetic polymorphism of drug transporters in pharmacokinetics and the challenge it poses in drug delivery.

In vitro assessment of oral lipid based formulations

In recent years there has been an increase in interest in the utility of lipid based delivery systems, at least in part as a result of the effective development of lipid based products such as Sandimmun Neoral (cyclosporin), Norvir (ritonavir) and Fortovase (saquinavir). The development pathway for lipid based formulations, however, is still largely empirical, and in vitro models that are predictive of oral bioavailability enhancement are lacking. The use of modified dissolution media, reflecting the bile salt and phospholipid levels in the intestine, has met with some success in terms of the ability to predict the bioavailability of poorly water soluble drugs and the potential bioavailability enhancing effects of food. These approaches, however, do not have the flexibility or complexity to deal with the interactions inherent in the digestion, dispersion and solubilisation of a lipid based formulation and the coincident dissolution profile of a co-administered drug. In this review, the utility of modified dissolution media to predict the impact of food on the absorption of poorly water soluble, lipophilic drugs, is explored. These dissolution based systems are subsequently contrasted with the use of lipid digestion models which have found increasing application in assessment of the interaction of digestible dose forms with the gastrointestinal milieu.