Intestinal drug absorption and bioavailability: beyond involvement of single transport function

Nanostructured lipid carriers: An emerging platform for improving oral bioavailability of lipophilic drugs

Nowadays exploration of novel lipid-based formulations is akin to a magnet for researchers worldwide for improving the in vivo performance of highly lipophilic drugs. Over the last few years, new compositions of lipids have been developed, and the probable bioavailability enhancement has been investigated. We reviewed the most recent data dealing with backlogs of conventional lipid-based formulations such as physical instability, limited drug loading capacities, drug expulsion during storage along with all the possible hindrances resulting in poor absorption of highly lipophilic drugs such as P-glycoprotein efflux, extensive metabolism by cytochrome P450 etc. In tandem with these aspects, an exclusive formulation approach has been discussed in detail in this paper. Therefore, this review focuses on resolving the concerned ambiguity with successful oral administration of highly lipophilic drugs through designing novel lipidic formulations (nanostructured lipid carriers [NLC]) that constitute a blend of solid and liquid lipids. The article highlights the potential role of such formulation in normalizing the in vivo fate of poorly soluble drugs. Finally, the present manuscript discusses the dominance of NLC over other lipid-based formulations and provides a perspective of how they defeat and overcome the barriers that lead to the poor bioavailability of hydrophobic drugs.

The use of biopharmaceutic classification of drugs in drug discovery and development: current status and future extension

Bioavailability (BA) and bioequivalence (BE) play a central role in pharmaceutical product development and BE studies are presently being conducted for New Drug Applications (NDAs) of new compounds, in supplementary NDAs for new medical indications and product line extensions, in Abbreviated New Drug Applications (ANDAs) of generic products and in applications for scale-up and post-approval changes. The Biopharmaceutics Classification System (BCS) has been developed to provide a scientific approach for classifying drug compounds based on solubility as related to dose and intestinal permeability in combination with the dissolution properties of the oral immediate-release (IR) dosage form. The aim of the BCS is to provide a regulatory tool for replacing certain BE studies by accurate in-vitro dissolution tests. The aim of this review is to present the status of the BCS and discuss its future application in pharmaceutical product development. The future application of the BCS is most likely increasingly important when the present framework gains increased recognition, which will probably be the case if the BCS borders for certain class II and III drugs are extended. The future revision of the BCS guidelines by the regulatory agencies in communication with academic and industrial scientists is exciting and will hopefully result in an increased applicability in drug development. Finally, we emphasize the great use of the BCS as a simple tool in early drug development to determine the rate-limiting step in the oral absorption process, which has facilitated the information between different experts involved in the overall drug development process. This increased awareness of a proper biopharmaceutical characterization of new drugs may in the future result in drug molecules with a sufficiently high permeability, solubility and dissolution rate, and that will automatically increase the importance of the BCS as a regulatory tool over time.

Modulating effect of polyethylene glycol on the intestinal transport and absorption of prednisolone, methylprednisolone and quinidine in rats by in-vitro and in-situ absorption studies

The effects of polyethylene glycol 20000 (PEG 20000) on the intestinal absorption of prednisolone, methylprednisolone and quinidine, three P-glycoprotein (P-gp) substrates, across the isolated rat intestinal membranes were examined by an in-vitro diffusion chamber system. The serosal-to-mucosal (secretory) transport of these P-gp substrates was greater than their mucosal-to-serosal (absorptive) transport, indicating that their net movement across the intestinal membranes was preferentially in the secretory direction. The polarized secretory transport of these drugs was remarkably diminished and their efflux ratios decreased in the presence of PEG 20000. In addition, PEG 20000 did not affect the transport of Lucifer yellow, a non-P-gp substrate. The intestinal membrane toxicity of PEG 20000 was evaluated by measuring the release of alkaline phosphatase (ALP) and protein from the intestinal membranes. The release of ALP and protein was enhanced in the presence of 20 mM sodium deoxycholate (NaDC), a positive control, while these biological parameters did not change in the presence of 0.1–5% (w/v) PEG 20000. These findings indicated that the intestinal membrane damage caused by PEG 20000 was not a main reason for the enhanced absorptive transport of these P-gp substrates in the presence of PEG 20000. Furthermore, the transepithelial electrical resistance (TEER) of rat jejunal membranes in the presence or absence of PEG 20000 was measured by a diffusion chamber method. PEG 20000 (0.1–5.0 % w/v) did not change the TEER values of the rat jejunal membranes, indicating that the increase in the absorptive transport of these P-gp substrates might not be due to the increased transport of these P-gp substrates via a paracellular pathway caused by PEG 20000. Finally, the effect of PEG 20000 on the intestinal absorption of quinidine was examined by an in-situ closed-loop method. The intestinal absorption of quinidine was significantly enhanced in the presence of 0.1-1.0% (w/v) PEG 20000. These findings suggest that PEG 20000 might be a useful excipient to improve the intestinal absorption of quinidine, which is mainly secreted by a P-gp-mediated efflux system in the intestine.

Prodrugs: design and clinical applications

Prodrugs are bioreversible derivatives of drug molecules that undergo an enzymatic and/or chemical transformation in vivo to release the active parent drug, which can then exert the desired pharmacological effect. In both drug discovery and development, prodrugs have become an established tool for improving physicochemical, biopharmaceutical or pharmacokinetic properties of pharmacologically active agents. About 5-7% of drugs approved worldwide can be classified as prodrugs, and the implementation of a prodrug approach in the early stages of drug discovery is a growing trend. To illustrate the applicability of the prodrug strategy, this article describes the most common functional groups that are amenable to prodrug design, and highlights examples of prodrugs that are either launched or are undergoing human trials.

Biopharmaceutical challenges associated with drugs with low aqueous solubility--the potential impact of lipid-based formulations

The percentage of new chemical entities synthesised with low aqueous solubility and high therapeutic efficacy is growing, this presents major challenges for the drug delivery scientists. The role of physicochemical properties in identification of suitable drug candidates for oral lipid-based delivery systems is discussed. A knowledge of the interplay of physicochemical and biopharmaceutical drug properties with the physiological environment of the gastro-intestinal tract (GIT), as a prerequisite to successful formulation design, is reviewed. The importance of excipient selection with an emphasis on bioactive excipients is stressed. The need for more examples of in vitro-in vivo correlations as a means of maximizing the development potential and commercial future for lipid-based formulations, and, promoting confidence within the industry for these delivery systems is highlighted.

Novel SLC22A16 polymorphisms and influence on doxorubicin pharmacokinetics in Asian breast cancer patients

OBJECTIVE

To identify novel polymorphisms in the solute carrier SLC22A16 gene and determine their influence on the pharmacokinetics of doxorubicin and doxorubicinol in Asian breast cancer patients.

METHODS

SLC22A16 coding regions were screened in a total of 400 healthy subjects belonging to three distinct Asian ethnic groups (Chinese [n = 100], Malays [n = 100] and Indians [n = 100]) and in the Caucasian population (n = 100). Pharmacokinetic parameters of doxorubicin and doxorubicinol were estimated in Asian breast cancer patients undergoing adjuvant chemotherapy to investigate genotype-phenotype correlations.

RESULTS

Four novel polymorphisms (c.146A>G [exon 2], c.312T>C, c.755T>C [exon 4] and c.1226T>C [exon 5]) were identified. The genotypic frequency of the homozygous c.146GG polymorphism was approximately twofold higher in the healthy Chinese (13%) & Malay (18%) populations compared with the Indian (7%) and Caucasian (9%) populations. The genotypic frequency of the c.1226T>C polymorphism was observed to be significantly higher among the Caucasian (11%) and Indian (8%) study subjects compared with the Chinese (1%) and Malay (1%) ethnic groups (p < 0.005 in each case). Breast cancer patients harboring the 146GG genotype showed a trend towards higher exposure levels to doxorubicin (AUC(0 negative infinity)/dose/body surface area [BSA] [hm(-5)]: 21.6; range: 18.8-27.7) compared with patients with either the reference genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 17.4; range: 8.2-26.3, p = 0.066) or heterozygotes (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 15.4; range: 6.2-38.0, p = 0.055). The exposure levels of doxorubicinol were also higher in patients harboring the variant 146GG genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 13.3; range: 8.8-21.7) when compared with patients harboring the reference genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]): 9.8; range: 6.1-24.3, p = 0.137) or heterozygotes (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 8.98; range: 3.7-20.6, p = 0.047).

CONCLUSION

Among the four novel SLC22A16 polymorphisms identified, the c.146A>G and c.1226T>C polymorphisms exhibited interethnic variations in allele and genotype frequencies. This exploratory study suggests that the c.146A>G variation could contribute to the variations in the pharmacokinetics of doxorubicin and doxorubicinol in Asian cancer patients. Further in vitro studies are required to determine the functional impact of these novel polymorphisms on doxorubicin pharmacokinetics in cancer patients.

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.

Transporters and their impact on drug disposition

OBJECTIVE

To review the recent advances in knowledge about human transporters and their effect on drug disposition.

DATA SOURCES

A MEDLINE search (1996-March 2005) was performed to identify pertinent literature on human transporters and their impact on drug disposition. Additional articles were identified from a manual search of the references of retrieved articles.

STUDY SELECTION AND DATA EXTRACTION

Based on the identified studies, data were extracted on the impact of transporters on drug absorption, distribution, and elimination.

DATA SYNTHESIS

The pharmacokinetic disposition of drugs is known to be influenced by metabolic enzymes, kidney function, and transporters. Recent research on human transporters has greatly advanced our understanding of their diversity and importance in drug disposition. In particular, members of the multidrug resistance family of transporters (MDR, MRP) are present in organs and tissues throughout the body and are known to significantly affect the absorption, distribution, and elimination of commonly prescribed drugs. A growing number of studies now demonstrate that alterations in transporter function as a result of drug interactions or genetic polymorphisms may explain a significant portion of the variability in treatment response for certain drugs.

CONCLUSIONS

Human transporters contribute significantly to the pharmacokinetic disposition of drugs. Knowledge of substrates, inducers, and inhibitors of these transporters is necessary to ensure optimal patient outcomes.

Intestinal and blood-brain drug transport: beyond involvement of a single transport function

It is considered that an increased understanding of drug transport in the intestine, liver, kidney and blood-brain barrier is vital to the successful development of new and effective drugs. Hence, there is a need for models that generate even more accurate prediction values than those currently applied. Future improvements in the accuracy of the discovery based predictions will probably need a more in vivo focus rather than a strictly reductionist focus on single cell models or enzymes/transporters. It is also generally believed that these transporters affect the local concentration time profiles of drugs and their metabolites in various tissues, such as liver, brain and kidney.:

Characterization of jejunal absorption and apical efflux of ropivacaine, lidocaine and bupivacaine in the rat using in situ and in vitro absorption models

The purpose of this study was to characterize the rat jejunal passive transport and the possible active efflux of three local anaesthetics, ropivacaine, lidocaine and bupivacaine using two different absorption models, the in situ single-pass intestinal perfusion and the in vitro Ussing chamber model, as well as P-glycoprotein (Pgp)-mediated calcein transport inhibition in Caco-2 cells. Concentration and pH dependence, efflux inhibition by verapamil and digoxin and bi-directional permeability studies were performed to investigate the potential involvement of efflux carriers in the intestinal absorption of the local anaesthetics. In the jejunal perfusion the permeability of these agents appeared to be high, predicting complete intestinal absorption (>90%). There was no effect of the Pgp inhibitors on net absorption for any of the local anaesthetics in the two absorption models. However, in the Ussing chamber at an equal pH of 7.4 at mucosal and serosal sides, the observed jejunal permeability ratios (S-M)/(M-S), of 2.3, 1.8 and 3.0 for ropivacaine, lidocaine and bupivacaine, respectively, indicated at least some involvement of carrier-mediated intestinal secretion. This idea was supported in the calcein AM Pgp transport assay in which two of the tested local anaesthetic agents affected cellular calcein retention. As anticipated for these agents, the mucosal pH conditions were shown to largely affect the gut permeability. The jejunal permeabilities of the local anaesthetics as measured in the two absorption models fitted well in a model comparison that incorporated the permeabilities of six other structurally unrelated drugs. In conclusion, the jejunal permeability of ropivacaine, lidocaine and bupivacaine was high and although evidence was obtained for carrier-mediated intestinal efflux this process appeared not to have a significant influence on the rate and extent of in vivo intestinal absorption. Rather, passive diffusion of these agents seems to be the major mechanism for the intestinal absorption.