Human jejunal permeability of cyclosporin A: influence of surfactants on P-glycoprotein efflux in Caco-2 cells

Microemulsions as drug delivery systems to improve the solubility and the bioavailability of poorly water-soluble drugs

IMPORTANCE OF THE FIELD

Microemulsions have been studied extensively as potential drug delivery vehicles for poorly water-soluble drugs. An understanding of the physicochemical and biopharmaceutical characteristics of the microemulsions according to administration routes will provide guidance for designing the formulations of microemulsions.

AREAS COVERED IN THIS REVIEW

In this paper, the use and the characteristics of microemulsions as drug delivery vehicles are reviewed. As the formulations of the microemulsion always include a great amount of surfactant and co-surfactant, which may cause hemolysis or histopathological alterations of the tissue, the potential toxicity or the irritancy of microemulsions is also discussed in this paper.

WHAT THE READER WILL GAIN

Developments of microemulsions for poorly water-soluble drugs in recent years are included in this review. Several factors limiting the commercial or clinical use of microemulsions are also discussed.

TAKE HOME MESSAGE

Considering the potential in enhanced drug uptake/permeation and facing the limitations, their unique properties make microemulsions a promising vehicle for poorly water-soluble drugs.

How well can the Caco-2/Madin-Darby canine kidney models predict effective human jejunal permeability?

The study aimed to predict effective human jejunal permeability (P(eff)) using a biophysical model based on parametrized paracellular, aqueous boundary layer, and transcellular permeabilities, and the villus-fold surface area expansion factor (k(VF)). Published human jejunal data (119 P(eff), 53 compounds) were analyzed by a regression procedure incorporating a dual-pore size paracellular model. Transcellular permeability, scaled by k(VF), was equated to that of Caco-2 at pH 6.5. The biophysical model predicted human jejunal permeability data within the experimental uncertainty. This investigation revealed several surprising predictions: (i) many molecules permeate predominantly (but not exclusively) by the paracellular route, (ii) the aqueous boundary layer thickness in the intestinal perfusion experiments is larger than expected, (iii) the mucosal surface area in awake humans is apparently nearly entirely accessible to drug absorption, and (iv) the relative "leakiness" of the human jejunum is not so different from that observed in a number of published Caco-2 studies.

Passive asymmetric transport of hesperetin across isolated rabbit cornea

Hesperetin, an aglycone of the flavanone hesperidin, is a potential candidate for the treatment of diabetic retinopathy and macular edema. The purpose of this investigation was to determine solubility, stability and in vitro permeability characteristics of hesperetin across excised rabbit corneas. Aqueous and pH dependent solubility was determined using standard shake flask method. Solution stability was evaluated as a function of pH (1.2-9) and temperature (25 and 40 degrees C). Permeability of hesperetin was determined across the isolated rabbit cornea utilizing a side-bi-side diffusion apparatus, in the apical to basolateral (A-B) and basolateral to apical (B-A) directions. Hesperetin displayed asymmetrical transcorneal transport with a 2.3-fold higher apparent permeability in the B-A direction compared to the A-B direction. The transport process was observed to be pH dependent. Surprisingly, however, the involvement of efflux transporters or proton-coupled carrier-systems was not evident in this asymmetric transcorneal diffusion process. The passive and pH dependent corneal transport of hesperetin could probably be attributable to corneal ultrastructure, physicochemical characteristics of hesperetin and the role of transport buffer components.

Formulation and evaluation of a protein-loaded solid dispersions by non-destructive methods

The purpose of this investigation was to develop solid dispersion (SD) formulation of cyclosporine (CyA) using polyethylene glycol (PEG-6000) to enhance its dissolution rate followed by nondestructive method for the prediction of both drug and carrier. SD formulations were prepared by varying the ratio of CyA and PEG-6000 by solvent evaporation technique and characterized by dissolution, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD), near infrared (NIR) and near infrared chemical imaging (NIR-CI). Dissolution data revealed enhanced dissolution of CyA when compared with pure CyA. DSC results showed that the crystallinity of PEG-6000 has decreased as indicated by decrease in the enthalpy of fusion and melting peak in the formulations. FTIR data demonstrated no chemical interaction between drug and carrier. The surface morphology of SD formulations was similar to PEG-6000 particle. NIR-CI disclosed homogeneity of SD matrix as indicated by symmetrical histograms with smaller values of skewness. Similar to NIR, a multivariate peak evaluation with principal component analysis and partial least square (PLS) were carried out with PXRD spectral data. PLS models with both techniques showed good correlation coefficient and smaller value of root mean square of errors. The accuracy of model for predicting CyA and PEG-6000 in NIR and PXRD data were 5.22%, 5.35%, 5.27%, and 2.10%, respectively. In summary, chemometric applications of non-destructive method sensors provided a valuable means of characterization and estimation of drug and carrier in the novel formulations.

Segmental dependent transport of low permeability compounds along the small intestine due to P-glycoprotein: the role of efflux transport in the oral absorption of BCS class III drugs

The purpose of this study was to investigate the role of P-gp efflux in the in vivo intestinal absorption process of BCS class III P-gp substrates, i.e. high-solubility low-permeability drugs. The in vivo permeability of two H (2)-antagonists, cimetidine and famotidine, was determined by the single-pass intestinal perfusion model in different regions of the rat small intestine, in the presence or absence of the P-gp inhibitor verapamil. The apical to basolateral (AP-BL) and the BL-AP transport of the compounds in the presence or absence of various efflux transporters inhibitors (verapamil, erythromycin, quinidine, MK-571 and fumitremorgin C) was investigated across Caco-2 cell monolayers. P-gp expression levels in the different intestinal segments were confirmed by immunoblotting. Cimetidine and famotidine exhibited segmental dependent permeability through the gut wall, with decreased P(eff) in the distal ileum in comparison to the proximal regions of the intestine. Coperfusion of verapamil with the drugs significantly increased the permeability in the ileum, while no significant change in the jejunal permeability was observed. Both drugs exhibited significantly greater BL-AP than AP-BL Caco-2 permeability, indicative of net mucosal secretion. Concentration dependent decrease of this secretion was obtained by the P-gp inhibitors verapamil, erythromycin and quinidine, while no effect was evident by the MRP2 inhibitor MK-571 and the BCRP inhibitor FTC, indicating that P-gp is the transporter mediates the intestinal efflux of cimetidine and famotidine. P-gp levels throughout the intestine were inversely related to the in vivo permeability of the drugs from the different segments. The data demonstrate that for these high-solubility low-permeability P-gp substrates, P-gp limits in vivo intestinal absorption in the distal segments of the small intestine; however P-gp plays a minimal role in the proximal intestinal segments due to significant lower P-gp expression levels in this region.

Formulation development and bioavailability evaluation of a self-nanoemulsified drug delivery system of oleanolic acid

This study aims to formulate and evaluate bioavailability of a self-nanoemulsified drug delivery system (SNEDDS) of a poorly water-soluble herbal active component oleanolic acid (OA) for oral delivery. Solubility of OA under different systems was determined for excipient selection purpose. Four formulations, where OA was fixed at the concentration of 20 mg/g, were prepared utilizing Sefsol 218 as oil phase, Cremophor EL and Labrasol as primary surfactants, and Transcutol P as cosurfactant. Pseudo-ternary phase diagrams were constructed to identify self-emulsification regions for the rational design of SNEDDS formulations. Sefsol 218 was found to provide the highest solubility among all medium-chained oils screened. Efficient self-emulsification was observed for the systems composing of Cremophor EL and Labrasol. The surfactant to cosurfactant ratio greatly affected the droplet size of the nanoemulsion. Based on the outcomes in dissolution profiles, stability data, and particle size profiles, three optimized formulations were selected: Sefsol 218/Cremophor EL/Labrasol (50:25:25, w/w), Sefsol 218/Cremophor EL/Labrasol/Transcutol P (50:20:20:10, w/w), and Sefsol 218/Cremophor EL/Labrasol/Transcutol P (50:17.5:17.5:15, w/w). Based on the conventional dissolution method, a remarkable increase in dissolution was observed for the SNEDDS when compared with the commercial tablet. The oral absorption of OA from SNEDDS showed a 2.4-fold increase in relative bioavailability compared with that of the tablet (p < 0.05), and an increased mean retention time of OA in rat plasma was also observed compared with that of the tablet (p < 0.01). These results suggest the potential use of SNEDDS to improve dissolution and oral bioavailability for poorly water-soluble triterpenoids such as OA.

Application of a Statistical Method to the Absorption of a New Model Drug from Micellar and Lipid Formulations—Evaluation of Qualitative Excipient Effects

The scope of the present article is to study formulation parameters of micellar and of lipid delivery systems on the exposure of a new drug compound A in Wistar rats. A statistical analysis is to be performed a posteriori from a data set of all rat studies that were conducted during the preclinical development of the drug. Several formulations were evaluated mainly in view of sufficient exposure in toxicological studies. Because of the low solubility and high lipophilicity of compound A, the preclinical formulation development focused on micellar solutions and different kinds of lipid drug delivery systems. Candidate formulations were first tested for their dilution in artificial intestinal fluids before they were evaluated in the rat. A partial least square model was applied to the entire pharmacokinetic data set, and the type of delivery system, as well as excipients, were investigated in view of effects on the area under the plasma level curve. The results showed that self-emulsifying systems and in particular self-microemulsifying drug delivery systems were most effective in pushing the exposure of compound A. Another significant factor was the dose. A data subset showed nonlinearity in the pharmacokinetics with respect to the dose. However, the most important findings of the multivariate data analysis were overall effects of excipients on the exposure. These effects are considered as a sum of several influences so that the underlying mechanism is essentially complex and is not fully understood. Cremophor and lecithin exhibited a positive effect, whereas TPGS containing systems reached only below average exposure. No significant effect was observed with polysorbate 80 or Solutol HS. The model indicated the favorable use of a cosurfactant, in particular Capmul MCM. Similarly the use of a cosolvent showed a positive coefficient and ethanol was here best in class. No marked effects were observed for the oil selection, but a tendency toward below average exposure was displayed when long-chain triglycerides were in the formulation. The a posteriori analysis of the pharmacokinetic data using multivariate statistical models was very helpful to clarify effects of drug delivery systems as well as of general effects of excipients. Guidance was provided for the formulator, but further studies are needed to better understand the complex effects on a mechanistic level.

Lipopolysaccharide increases cell surface P-glycoprotein that exhibits diminished activity in intestinal epithelial cells

Increasingly, it is recognized that commensal microflora regulate epithelial cell processes through the dynamic interaction of pathogen-associated molecular patterns and host pattern recognition receptors such as Toll-like receptor 4 (TLR4). We therefore investigated the effects of bacterial lipopolysaccharide (LPS) on intestinal P-glycoprotein (P-gp) expression and function. Human SW480 (P-gp+/TLR4+) and Caco-2 (P-gp+/TLR4-) cells were treated with medium control or LPS (100 ng/ml) for 24 h prior to study. P-gp function was assessed by measuring the intracellular concentration of rhodamine 123 (Rh123). To confirm P-gp-specific effects, breast cancer resistance protein (BCRP/ABCG2) and multidrug resistance-associated protein 2 (MRP-2/ABCC2) were also analyzed. Treatment of SW480 cells with LPS led to diminished P-gp activity, which could be prevented with polymyxin B (control: 207+/-16 versus LPS: 402+/-22 versus LPS+polymyxin B: 238+/-26 pmoles Rh123/mg protein, p<0.05 control versus LPS). These effects could be blocked by using polymyxin B and were not seen in the P-gp+/TLR4--Caco-2 cell line (control: 771+/-28 versus LPS: 775+/-59 pmoles Rh123/mg protein). Total cellular levels of P-gp did not change in LPS-treated SW480 cells; however, a significant increase in cell surface P-gp was detected. No change in activity, total protein, or apically located MRP-2 was detected following LPS treatment. Sequence analysis confirmed wild-type status of SW480 cells. These data suggest that activation of TLR4 in intestinal epithelial cells leads to an increase in plasma membrane P-gp that demonstrates a diminished capacity to transport substrate.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

Lipids and lipid-based formulations: optimizing the oral delivery of lipophilic drugs

Highly potent, but poorly water-soluble, drug candidates are common outcomes of contemporary drug discovery programmes and present a number of challenges to drug development - most notably, the issue of reduced systemic exposure after oral administration. However, it is increasingly apparent that formulations containing natural and/or synthetic lipids present a viable means for enhancing the oral bioavailability of some poorly water-soluble, highly lipophilic drugs. This Review details the mechanisms by which lipids and lipidic excipients affect the oral absorption of lipophilic drugs and provides a perspective on the possible future applications of lipid-based delivery systems. Particular emphasis has been placed on the capacity of lipids to enhance drug solubilization in the intestinal milieu, recruit intestinal lymphatic drug transport (and thereby reduce first-pass drug metabolism) and alter enterocyte-based drug transport and disposition.

Impact of Cremophor-EL and Polysorbate-80 on Digoxin Permeability across Rat Jejunum: Delineation of Thermodynamic and Transporter Related Events Using the Reciprocal Permeability Approach

The effect of Cremophor-EL (Cr-EL) and polysorbate-80 (PS-80) on the transepithelial permeability of digoxin (DIG) has been evaluated using the reciprocal permeability approach to delineate thermodynamic and transporter related events. Permeability data were corrected for solubilization using the micellar association constant (Ka) obtained from Papp data generated in the presence of the nonspecific ATPase inhibitor sodium orthovanadate. In the presence of mucosal Cr-EL, a concentration dependent decrease in serosal-mucosal (S-M) and increase in M-S transport was observed. Whilst serosal Cr-EL resulted in a reduction in S-M DIG transport, no impact on M-S transport was apparent. For PS-80, the presence of either serosal or mucosal surfactant led to a decrease in secretory (S-M) DIG transport, however no effect on absorptive transport was evident. The data confirm the potential P-gp inhibitory effects of Cr-EL, but suggest that in contrast to Cr-EL, PS-80 is not a potent inhibitor of P-gp and is incapable of increasing absorptive drug transport, at least in excised rat intestinal tissue and at the concentrations tested. The data are also consistent with the involvement of additional transporters (both apical and basolateral) in the intestinal permeability of DIG, although more definitive data is required to confirm this possibility.

Development of supersaturatable self-emulsifying drug delivery system formulations for improving the oral absorption of poorly soluble drugs

The supersaturatable self-emulsifying drug delivery system (S-SEDDS) represents a new thermodynamically stable formulation approach wherein it is designed to contain a reduced amount of a surfactant and a water-soluble cellulosic polymer (or other polymers) to prevent precipitation of the drug by generating and maintaining a supersaturated state in vivo. The S-SEDDS formulations can result in enhanced oral absorption as compared with the related self-emulsifying drug delivery systems (SEDDS) formulation and the reduced surfactant levels may minimise gastrointestinal surfactant side effects.

pH-dependent functional activity of P-glycoprotein in limiting intestinal absorption of protic drugs: Kinetic analysis of quinidine efflux in situ

The purpose of this investigation is to evaluate the quantitative contribution of pH-dependent passive permeability on the functional activity of P-glycoprotein (P-gp) in limiting intestinal absorption of weakly basic drugs, in order to include this effect in prediction models. pH-dependent octanol/buffer partition coefficient, artificial membrane permeability and in situ rat intestinal permeability of quinidine were determined in the physiological pH range of gastrointestinal tract. In situ permeability, as a function of luminal pH, was also determined in the presence of P-gp inhibitor, verapamil (500 microM). Octanol/buffer partition coefficient, transport across artificial membrane, and rat in situ permeability showed high pH-dependency. Absorption quotient (AQ), calculated from in situ permeability to express the functional activity of P-gp, declined with increase in luminal pH or increase in luminal quinidine concentration because of the increased passive permeability or saturation of P-gp. AQ was 0.57 +/- 0.02 and 0.41 +/- 0.05, while passive permeability was 0.32 +/- 0.01 x 10(-4) cm/sec and 0.43 +/- 0.02 x 10(-4) cm/sec, in jejunum and ileum, respectively, at pH 7.4. Further, apparent Michaelis-Menten constants (K(M), J(P-gp,max)) for the quinidine efflux in jejunum indicated that efflux activity was more at luminal pH 4.5 over pH 7.4. K(M) values for jejunum quinidine efflux at pH 4.5 and pH 7.4 were determined to be 77.63 +/- 10.90 and 22.86 +/- 5.22 microM, with J(P-gp,max) values of 1.47 +/- 0.08 and 0.62 +/- 0.04 nM/cm2/sec, respectively. AQ vs passive permeability showed significant relationship indicating dependency of P-gp-mediated efflux on pH-dependent passive permeability, which is dictated by ionization status for a protic or ampholytic drug. In conclusion, an orally administered drug is absorbed from various segments of intestine, which inherit difference in luminal pH, transcellular permeability and P-gp expression. In situ data suggests that pH-dependency and regional variability in passive permeability of protic substrates significantly influence their P-gp-mediated efflux and may have implications on predictions of the in vivo drug absorption.

Human bioavailability of propranolol from a matrix-in-cylinder system with a HPMC-Gelucire® core

The bioavailability of propranolol from a matrix-in-cylinder system for sustained drug delivery, consisting of a hot-melt extruded ethylcellulose pipe surrounding a drug-containing HPMC-Gelucire 44/14 core, was determined. An oral dose of 80 mg propranolol hydrochloride was administered to healthy volunteers (n = 10) in a randomized cross-over study design either as a commercial pellet formulation (Inderal retard mitis) or as a matrix-in-cylinder system. The influence of concomitant food intake on drug release from the matrix-in-cylinder system was also studied. During the first 10 h after administration, the matrix-in-cylinder system resulted in similar plasma levels as the reference formulation Inderal. The concomitant intake of a high-fat, high-calorie breakfast did not cause dose-dumping. Between 10 h and 24 h after administration of the matrix-in-cylinder system, a remarkable increase of the propranolol plasma levels was noticed (compared to Inderal). This effect was even more pronounced under fed conditions. The matrix-in-cylinder system had a relative bioavailability of 156% (fasted conditions) and 222% (fed conditions) compared to the marketed reference product. In order to elucidate the origin of this increased bioavailability, Caco-2 experiments and dog lymph studies were performed. However, none of these experiments was able to provide a conclusive answer.

Prediction of in vivo intestinal absorption enhancement on P-glycoprotein inhibition, from rat in situ permeability

The purpose of this study is to determine the functional role of P-glycoprotein (P-gp) in intestinal absorption of drugs and to quantitatively predict the in vivo absorption enhancement on P-gp inhibition. In situ single-pass rat ileum permeability and aqueous solubility were measured for a set of 16 compounds. Permeability studies were also carried out in the presence of P-gp inhibitor to estimate the permeability enhancement on P-gp inhibition. A significant correlation was obtained between rat ileum permeability and the literature human intestinal absorption (HIA), F(a,human) (r = 0.891; p < 0.01). Compounds with permeability >0.2 x 10(-4) cm/s are completely absorbed; however, few practically insoluble compounds were overestimated with this relationship. Inhibition of P-gp increased the permeability (p < 0.05) of three moderately and three highly permeable compounds. Efflux inhibition ratio (EIR), the ratio of permeability due to P-gp-mediated efflux activity and passive permeability only, for these compounds was in the order of digoxin > paclitaxel > fexofenadine > quinidine > verapamil > cyclosporine. Integration of EIR with permeability versus F(a,human) predicted that modulation of P-gp has no significant effect on the absorption of highly permeable compounds (quinidine, verapamil, and cyclosporine A), while for moderately permeable compounds (digoxin, paclitaxel, and fexofenadine), P-gp profoundly influences the intestinal permeability. The in situ permeability in rat ileum may be used to predict the in vivo P-gp function and its quantitative contribution to intestinal drug absorption. Integration of the functional activity of P-gp with the characteristics of BCS may explain drug interactions and explore the possible pharmacokinetic advantage on P-gp inhibition.

Engineering polysaccharide-based polymeric micelles to enhance permeability of cyclosporin A across Caco-2 cells

PURPOSE

To assess and compare the effectiveness of two types of polysaccharide-based micelles as delivery vehicles for poorly water soluble drugs by monitoring their permeability across Caco-2 cell monolayers.

METHODS

Dextran (DEX) and hydroxypropylcellulose (HPC) were hydrophobically modified (HM) by grafting polyoxyethylene cetyl ether (POE-C16, 15 mol% and 5.4 mol%, respectively). The onset of micellization and mean diameter of polymeric micelles formed by HM-DEX and HM-HPC were determined by fluorescence spectroscopy and dynamic light scattering, respectively. Cyclosporin A (CsA)-loaded polymeric micelles were prepared by a dialysis procedure, and the amount of incorporated CsA was assayed by high performance liquid chromatography (HPLC). The stability of micelles in simulated gastric and intestinal fluids was studied as a function of contact time, and their cytotoxicity toward Caco-2 cells was evaluated using the MTT colorimetric assay. The bidirectional transport across Caco-2 cell monolayers of CsA entrapped in HM-DEX and HM-HPC micelles and of the polymers themselves was evaluated in the presence and absence of P-glycoprotein inhibitor.

RESULTS

The amount of CsA incorporated in HM-HPC and HM-DEX micelles reached 5.5 and 8.5% w/w, respectively (entrapment efficiency of 22% or more). The polymeric micelles exhibited high stability in gastric and intestinal fluids and no significant cytotoxicity toward Caco-2 cells. The apical to basal permeability of CsA across Caco-2 cells increased significantly when loaded in polymeric micelles compared to free CsA.

CONCLUSIONS

Polysaccharide-based polymeric micelles are promising carriers for the oral delivery of poorly water soluble drugs. In vitro tests indicate that, overall, HM-HPC micelles are more effective compared to HM-DEX micelles.

pH-dependent functional activity of P-glycoprotein in limiting intestinal absorption of protic drugs 1. Simultaneous determination of quinidine and permeability markers in rat in situ perfusion samples

A simple, specific and sensitive reversed-phase high performance liquid chromatographic (RP-HPLC) method with UV absorbance detection was developed and validated for simultaneous determination of quinidine, verapamil and passive permeability markers, in samples obtained from rat intestinal in situ single-pass perfusion studies. Chromatography was carried out on C18 column with mobile phase comprising of acetate buffer (pH 5.0) and methanol in the ratio of 40:60 (v/v) pumped at a flow rate of 0.6 ml/min and UV detection was employed at 230 and 275 nm. The average retention times for hydrochlorthiazide, frusemide, quinidine, propranolol, and verapamil were 4.9, 5.8, 6.9, 8.9 and 11.3 min, respectively. The calibration curves were linear (R(2)>0.9995) in the selected range for each analyte. The method is specific and sensitive with limit of quantification as 25 ng/ml for quinidine and verapamil. The intra- and inter-day accuracy and precision were found to be good for all the five analytes. The method was found to be reliable in permeability determination and to estimate pH-dependent P-glycoprotein (P-gp)-mediated efflux transport of quinidine. Weak bases quinidine, propranolol and verapamil showed pH-dependent permeability, where quinidine permeability increased by 3.6-fold when the luminal pH was changed from pH 4.5-7.4. Inhibition of P-gp by verapamil (200 microM) indicated that about 68% and only 35% of passive transport of quinidine was attenuated by P-gp-mediated efflux at pH 4.5 and 7.4, respectively. In conclusion, low passive transport rates of weakly basic P-gp substrates at lower pH, may lead to more accessibility of these molecules to P-gp within enterocytes thus resulting in pH-dependent functional activity of P-gp as protic drugs moves along the gastrointestinal tract.

Drug delivery to the small intestine

Oral delivery of drugs to the small intestine is an important topic in the research and development of more effective oral dose forms. This review highlights several important developments in this area. An overriding theme in drug delivery to the small intestine is how to increase the efficiency (ie, how to increase bioavailability) of absorption. The role of P-glycoprotein and intestinal transporters is discussed in this regard. These systems are normally studied under defined in vitro conditions; recent data suggest that this approach, though useful, may not fully represent the in vivo situation. Recent advances and issues in the characterization and prediction of drug absorption from the small intestine are reviewed. These efforts, if successful, will shorten development timelines by eliminating compounds with poor absorption characteristics early in the process. Nanoparticulate delivery systems and those prepared by microfabrication technology are being used to improve bioavailability of poorly absorbed drugs. A relatively new technique (electroporation) has been proposed to enhance oral delivery of macromolecules, still an unrealized objective in drug delivery.

Self-emulsifying drug delivery systems (SEDDS) for improved oral delivery of lipophilic drugs

The oral delivery of hydrophobic drugs presents a major challenge because of the low aqueous solubility of such compounds. Self-emulsifying drug delivery systems (SEDDS), which are isotropic mixtures of oils, surfactants, solvents and co-solvents/surfactants, can be used for the design of formulations in order to improve the oral absorption of highly lipophilic drug compounds. SEDDS can be orally administered in soft or hard gelatin capsules and form fine relatively stable oil-in-water (o/w) emulsions upon aqueous dilution owing to the gentle agitation of the gastrointestinal fluids. The efficiency of oral absorption of the drug compound from the SEDDS depends on many formulation-related parameters, such as surfactant concentration, oil/surfactant ratio, polarity of the emulsion, droplet size and charge, all of which in essence determine the self-emulsification ability. Thus, only very specific pharmaceutical excipient combinations will lead to efficient self-emulsifying systems. Although many studies have been carried out, there are few drug products on the pharmaceutical market formulated as SEDDS confirming the difficulty of formulating hydrophobic drug compounds into such formulations. At present, there are four drug products, Sandimmune and Sandimmun Neoral (cyclosporin A), Norvir (ritonavir), and Fortovase (saquinavir) on the pharmaceutical market, the active compounds of which have been formulated into specific SEDDS. Significant improvement in the oral bioavailability of these drug compounds has been demonstrated for each case. The fact that almost 40% of the new drug compounds are hydrophobic in nature implies that studies with SEDDS will continue, and more drug compounds formulated as SEDDS will reach the pharmaceutical market in the future.

Development of a supersaturable SEDDS (S‐SEDDS) formulation of paclitaxel with improved oral bioavailability

A new, supersaturable self-emulsifying drug delivery system (S-SEDDS) of paclitaxel was developed employing hydroxypropyl methylcellulose (HPMC) as a precipitation inhibitor with a conventional SEDDS formulation. In vitro dilution of the S-SEDDS formulation results in formation of a microemulsion, followed by slow crystallization of paclitaxel on standing. This result indicates that the system is supersaturated with respect to crystalline paclitaxel, and the supersaturated state is prolonged by HPMC in the formulation. In the absence of HPMC the SEDDS formulation undergoes rapid precipitation, yielding a low paclitaxel solution concentration. A pharmacokinetic study was conducted in male Sprague-Dawley rats to assess exposure after an oral paclitaxel dose of 10 mg/kg in the SEDDS formulations with (S-SEDDS) and without HPMC. The paclitaxel S-SEDDS formulation shows approximately 10-fold higher maximum concentration (C(max)) and five-fold higher oral bioavailability (F approximately 9.5%) compared with that of the orally dosed Taxol formulation (F approximately 2.0%) and the SEDDS formulation without HPMC (F approximately 1%). Coadministration of cyclosporin A (CsA), an inhibitor of P-glycoprotein and CYP 3A4 enzyme, at a dose of 5 mg/kg with the S-SEDDS formulation further increased the oral bioavailability (F approximately 22.6%). This assessment demonstrates that the systemic exposure of paclitaxel following oral administration can be substantially improved via the S-SEDDS approach.