Modulation of Oral Drug Bioavailability: From Preclinical Mechanism to Therapeutic Application

Roles of Hepatic Drug Transporters in Drug Disposition and Liver Toxicity

Hepatic drug transporters are mainly distributed in parenchymal liver cells (hepatocytes), contributing to drug's liver disposition and elimination. According to their functions, hepatic transporters can be roughly divided into influx and efflux transporters, translocating specific molecules from blood into hepatic cytosol and mediating the excretion of drugs and metabolites from hepatic cytosol to blood or bile, respectively. The function of hepatic transport systems can be affected by interspecies differences and inter-individual variability (polymorphism). In addition, some drugs and disease can redistribute transporters from the cell surface to the intracellular compartments, leading to the changes in the expression and function of transporters. Hepatic drug transporters have been associated with the hepatic toxicity of drugs. Gene polymorphism of transporters and altered transporter expressions and functions due to diseases are found to be susceptible factors for drug-induced liver injury (DILI). In this chapter, the localization of hepatic drug transporters, their regulatory factors, physiological roles, and their roles in drug's liver disposition and DILI are reviewed.

National trends in use and costs of oral anticancer drugs in France: An 8-year population-based study

PURPOSE

During the last decade, many oral anticancer drugs (OAcDs) have been marketed, providing interesting but potentially costly pharmaceutical alternatives to intravenous treatments. This study aims to provide updated information on their use and costs.

METHODS

A cross-sectional yearly repeated study was conducted from 2006 to 2014 using the representative sample of the French national health care insurance system claims database (EGB). OAcD use was described for each year, among prevalent (ie, patients with at least 1 OAcD reimbursement) and incident users (ie, patients with no OAcD reimbursement within the prior year) and according to their pharmacological classes (Hormone Therapy [HT], Cytotoxic Therapy [CT], Targeted Therapy [TT], and others). Demographic characteristics were described for both users; comorbidities and direct medical costs were described for incident users only.

RESULTS

The yearly prevalence and incidence of OAcD use, mainly represented by HT, remained stable from 2006 to 2014 (1.2%; 0.4%). Compared with users of other OAcD classes, the proportion of TT users substantially increased over the 8-year study period (+9.3%), and TT incident users had more severe comorbidities at treatment initiation. The health expenditures were the most important in TT users with median monthly medical direct costs varying from 2995€ to 4968€ per patient between 2006 and 2014.

CONCLUSION

With the development of new OAcDs, the TTs use reaches a wider population of patients but is responsible for increasing health expenditures.

New oral targeted therapies for metastatic breast cancer disrupt the traditional patients' management-A healthcare providers' view

Although a cure still cannot be expected for metastatic breast cancer, thanks to progressive advances in treatments, life expectancy has been increasing over the past 15 years. This study aims to present the impact on the organisation of patients' management of newly released oral targeted therapies dedicated to metastatic breast cancer and the obstacles to their diffusion. Our work is based on the analysis of 40 semi-structured interviews, conducted with oncology healthcare professionals in three regions of France (2015-2016). It shows three main results. First, the prescription of an oral targeted therapy requires greater collaboration between healthcare professionals than traditional intravenous oncology drugs, which may be challenging. Second, there remain many barriers to the dissemination of oral targeted therapies. Third, taking an oral targeted therapy keeps the patient away from the hospital facility and asks for a strong therapeutic alliance. The management of oral targeted therapies is time-consuming for medical oncologists and disrupts the traditional care pathway. The multiplication of actors involved in patients' management reinforces the slowdown in the deployment and acceptance of therapeutic innovations. More players equal a higher risk of slowdown. Questioning and re-designing hospital organisation and management modalities towards this type of care are critical.

Inventory of oral anticancer agents: Pharmaceutical formulation aspects with focus on the solid dispersion technique

Dissolution from the pharmaceutical formulation is a prerequisite for complete and consistent absorption of any orally administered drug, including anticancer agents (oncolytics). Poor dissolution of an oncolytic can result in low oral bioavailability, high variability in blood concentrations and with that suboptimal or even failing therapy. This review discusses pharmaceutical formulation aspects and absorption pharmacokinetics of currently licensed orally administered oncolytics. In nearly half of orally dosed oncolytics poor dissolution is likely to play a major role in low and unpredictable absorption. Dissolution-limited drug absorption can be improved with a solid dispersion which is a formulation method that induces super-saturated drug dissolution and with that it enhances in vivo absorption. This review discusses formulation principles with focus on the solid dispersion technology and how it works to enhance drug absorption. There are currently three licensed orally dosed oncolytics formulated as a solid dispersion (everolimus, vemurafenib and regorafenib) and these formulations result in remarkably improved dissolution and absorption compared to what can be achieved with conventional formulations of the respective oncolytics. Because of the successful implementation of these three solid dispersion formulations, we encourage the application of this formulation method for poorly soluble oral oncolytics.

Validation of a liquid chromatographic method for the pharmaceutical quality control of products containing elacridar

Many anticancer drugs have an impaired bioavailability and poor brain penetration because they are substrates to drug efflux pumps such as P-glycoprotein and Breast Cancer Resistance Protein. Elacridar is a strong inhibitor of these two drug efflux pumps and therefore has great potential to improve oral absorption and brain penetration of many anticancer drugs. Currently, a clinical formulation of elacridar is unavailable and therefore the pharmaceutical development of a drug product is highly warranted. This also necessitates the availability of an analytical method for its quality control. A reverse-phase high-performance liquid chromatographic method with ultraviolet detection was developed for the pharmaceutical quality control of products containing elacridar as the active pharmaceutical ingredient. The analytical method was validated for linearity, accuracy, precision, selectivity, carry-over, stability of stock and reference solutions, stability of the final extract, stability-indicating capability and impurity testing. We found that elacridar is unstable in aqueous solutions that are exposed to light because a hydroxylation product of elacridar is formed. Therefore, sample solutions with elacridar must be protected from light.

Implementation of a pharmaceutical care programme for patients receiving new molecular-targeted agents in a clinical trial unit

A pharmaceutical care programme was implemented at our hospital in early 2013. The main objectives were to analyse and describe the pharmaceutical interventions made, to calculate adherence, interventions and to evaluate patient satisfaction with the care programme. We performed a single-centre descriptive and prospective intervention in cancer patients who received oral chemotherapy as part of a clinical trial in 2013. Eighty-three patients were included. Median age was 58 years (range, 31-80) and 42 patients (50.6%) were men. We recorded 23 interventions, 13 of which were associated with drug interactions. The mean percentage of adherence was 98.9%. The interview with the pharmacist was considered to be very important by 84.6% of the respondents. A total of 92.3% said that they would like to speak to the pharmacist at subsequent visits. The doubts detected during the visits enable us to conclude that the information patients receive with respect to their study medication is usually incomplete. An integrated pharmaceutical care programme for cancer patients participating in clinical trials with oral cytostatic drugs was successful in terms of adherence and patient satisfaction and makes it possible to guarantee the safety and effectiveness of treatment on an individual basis.

The implications of out-of-pocket cost of cancer treatment in the USA: a critical appraisal of the literature

Advances in cancer diagnosis and treatment have led to increased societal costs and out-of-pocket patient cost. We reviewed the literature on the impact of out-of-pocket cancer care costs on the patient experience, and described efforts made to address these costs. A critical appraisal of articles published in the USA from 2004 to 2014 was performed. The literature revealed that even insured patients receiving anticancer therapy are vulnerable to financial distress, which can impel patients to borrow money, deplete their savings, or engage in cost-coping strategies including nonadherence to prescribed treatment. Additional research is required to define financial distress risk factors, patient-physician communication of the costs of cancer care, and supportive care models for patients and survivors with substantial financial burdens.

Planar bioadhesive microdevices: a new technology for oral drug delivery

The oral route is the most convenient and least expensive route of drug administration. Yet, it is accompanied by many physiological barriers to drug uptake including low stomach pH, intestinal enzymes and transporters, mucosal barriers, and high intestinal fluid shear. While many drug delivery systems have been developed for oral drug administration, the physiological components of the gastro intestinal tract remain formidable barriers to drug uptake. Recently, microfabrication techniques have been applied to create micron-scale devices for oral drug delivery with a high degree of control over microdevice size, shape, chemical composition, drug release profile, and targeting ability. With precise control over device properties, microdevices can be fabricated with characteristics that provide increased adhesion for prolonged drug exposure, unidirectional release which serves to avoid luminal drug loss and enhance drug permeation, and protection of a drug payload from the harsh environment of the intestinal tract. Here we review the recent developments in microdevice technology and discuss the potential of these devices to overcome unsolved challenges in oral drug delivery.

In vitro effects of standardized extract of Bacopa monniera and its five individual active constituents on human P-glycoprotein activity

1. For centuries Bacopa monniera (BM) has been used as an herbal drug for the treatment of various mental ailments. A chemically standardized alcoholic extract of BM is clinically available over the counter herbal remedy for memory enhancement in children and adults. Consumption of herbal preparations has been reported to alter the function of membrane transporters, especially P-glycoprotein (P-gp), ATP-dependent drug efflux transporter responsible for the development of herb-drug interactions. 2. In the present study, we evaluated the in vitro effect of BM extract and its five individual active constituents (namely, bacopaside I, bacopaside II and bacopasaponin C, bacoside A and bacoside A3) on P-gp function using luminescent P-gp ATPase assay and Rh123 transport assay across human MDR1 gene transfected LLC-GA5-COL150 cell line. 3. It was observed that BM extract and its five individual constituents inhibited both basal activity as well as verapamil-stimulated ATPase activity, suggesting their affinity towards P-gp. Further, BM and its five active constituents inhibited the rhodamine 123 (Rh123) transport across LLC-GA5-COL150 cell monolayer with bacopaside II being the most potent inhibitor of P-gp, which decreased P-gp efflux ratio of Rh123 by fourfold in comparison to control. 4. Our finding may prove beneficial in predicting the potential herb-drug interactions of BM on concomitant medication with P-gp substrate drugs in clinical settings.

Site of reactivity models predict molecular reactivity of diverse chemicals with glutathione

Drug toxicity is often caused by electrophilic reactive metabolites that covalently bind to proteins. Consequently, the quantitative strength of a molecule's reactivity with glutathione (GSH) is a frequently used indicator of its toxicity. Through cysteine, GSH (and proteins) scavenges reactive molecules to form conjugates in the body. GSH conjugates to specific atoms in reactive molecules: their sites of reactivity. The value of knowing a molecule's sites of reactivity is unexplored in the literature. This study tests the value of site of reactivity data that identifies the atoms within 1213 reactive molecules that conjugate to GSH and builds models to predict molecular reactivity with glutathione. An algorithm originally written to model sites of cytochrome P450 metabolism (called XenoSite) finds clear patterns in molecular structure that identify sites of reactivity within reactive molecules with 90.8% accuracy and separate reactive and unreactive molecules with 80.6% accuracy. Furthermore, the model output strongly correlates with quantitative GSH reactivity data in chemically diverse, external data sets. Site of reactivity data is nearly unstudied in the literature prior to our efforts, yet it contains a strong signal for reactivity that can be utilized to more accurately predict molecule reactivity and, eventually, toxicity.

Pharmacokinetics and antitumor efficacy of paclitaxel-cyclodextrin complexes loaded in mucus-penetrating nanoparticles for oral administration

AIM

The authors report a novel approach for enhancing the oral absorption of paclitaxel (PTX) by encapsulation in poly(anhydride) nanoparticles (NPs) containing cyclodextrins and poly(ethylene glycol).

MATERIALS & METHODS

Formulations were prepared using the solvent displacement method. Subsequently, pharmacokinetics and organ distribution assays were evaluated after oral administration into C57BL/6J mice. In addition, antitumor efficacy studies were performed in a subcutaneous tumor model of Lewis lung carcinoma.

RESULTS

PTX-loaded NPs displayed sizes between 190-300 nm. Oral NPs achieved drug plasma levels for at least 24 h, with an oral bioavailability of 55-80%. Organ distribution studies revealed that PTX, orally administered in NPs, underwent a similar distribution to intravenous Taxol(®) (Bristol-Myers Squibb, NJ, USA). For in vivo antitumor assays, oral strategy maintained a slower tumor growth than intravenous Taxol.

CONCLUSION

PTX orally administered in poly(anhydride) NPs, combined with cyclodextrins and poly(ethylene glycol), displayed sustained plasma levels and significant antitumor effect in a syngenic tumor model of carcinoma in mice.

Oral anticancer drugs: mechanisms of low bioavailability and strategies for improvement

The use of oral anticancer drugs has increased during the last decade, because of patient preference, lower costs, proven efficacy, lack of infusion-related inconveniences, and the opportunity to develop chronic treatment regimens. Oral administration of anticancer drugs is, however, often hampered by limited bioavailability of the drug, which is associated with a wide variability. Since most anticancer drugs have a narrow therapeutic window and are dosed at or close to the maximum tolerated dose, a wide variability in the bioavailability can have a negative impact on treatment outcome. This review discusses mechanisms of low bioavailability of oral anticancer drugs and strategies for improvement. The extent of oral bioavailability depends on many factors, including release of the drug from the pharmaceutical dosage form, a drug's stability in the gastrointestinal tract, factors affecting dissolution, the rate of passage through the gut wall, and the pre-systemic metabolism in the gut wall and liver. These factors are divided into pharmaceutical limitations, physiological endogenous limitations, and patient-specific limitations. There are several strategies to reduce or overcome these limitations. First, pharmaceutical adjustment of the formulation or the physicochemical characteristics of the drug can improve the dissolution rate and absorption. Second, pharmacological interventions by combining the drug with inhibitors of transporter proteins and/or pre-systemic metabolizing enzymes can overcome the physiological endogenous limitations. Third, chemical modification of a drug by synthesis of a derivative, salt form, or prodrug could enhance the bioavailability by improving the absorption and bypassing physiological endogenous limitations. Although the bioavailability can be enhanced by various strategies, the development of novel oral products with low solubility or cell membrane permeability remains cumbersome and is often unsuccessful. The main reasons are unacceptable variation in the bioavailability and high investment costs. Furthermore, novel oral anticancer drugs are frequently associated with toxic effects including unacceptable gastrointestinal adverse effects. Therefore, compliance is often suboptimal, which may negatively influence treatment outcome.

Reversal of multidrug resistance by the inhibition of ATP-binding cassette pumps employing "Generally Recognized As Safe" (GRAS) nanopharmaceuticals: A review

Pumps of the ATP-binding cassette superfamily (ABCs) regulate the access of drugs to the intracellular space. In this context, the overexpression of ABCs is a well-known mechanism of multidrug resistance (MDR) in cancer and infectious diseases (e.g., viral hepatitis and the human immunodeficiency virus) and is associated with therapeutic failure. Since their discovery, ABCs have emerged as attractive therapeutic targets and the search of compounds that inhibit their genetic expression and/or their functional activity has gained growing interest. Different generations of pharmacological ABC inhibitors have been explored over the last four decades to address resistance in cancer, though clinical results have been somehow disappointing. "Generally Recognized As Safe" (GRAS) is a U.S. Food and Drug Administration designation for substances that are accepted as safe for addition in food. Far from being "inert", some amphiphilic excipients used in the production of pharmaceutical products have been shown to inhibit the activity of ABCs in MDR tumors, emerging as a clinically translatable approach to overcome resistance. The present article initially overviews the classification, structure and function of the different ABCs, with emphasis on those pumps related to drug resistance. Then, the different attempts to capitalize on the activity of GRAS nanopharmaceuticals as ABC inhibitors are discussed.

Phase II study of single agent oral vinorelbine as first-line treatment in patients with HER-2 negative metastatic breast cancer

PURPOSE

Previous studies indicated that oral chemotherapy is convenient and preferred by many patients. We hereby report the efficacy and safety of oral vinorelbine as first-line chemotherapy for metastatic breast cancer (MBC).

METHODS

Thirty-one patients with HER-2 negative MBC were enrolled between January 2007 and December 2010 in a prospective phase II trial. Patients were treated every 3 weeks with oral vinorelbine 60 mg/m² Days 1 and 8 for the 1st cycle and thereafter 80 mg/m² Days 1 and 8 every 3 weeks. Treatment was administered until disease progression or unexpected adverse event or patient refusal to continue. Primary endpoint was objective response rate (ORR); secondary endpoints were time-to-progression (TTP), overall survival (OS) and safety. Follow-up results until October 2012 are reported.

RESULTS

Median age was 42 years (range 33-75). 26 (84 %) patients had 2 or more metastatic sites. A median of 6 cycles were administered (range 2-20). ORR was achieved in 9 (29 %) patients including 1 complete and 8 partial responses. 12 (39 %) patients had stable disease, resulting in a disease control rate of 68 %. Median TTP was 5.2 months [95 % CI 2.8-7.5]. Median OS was 16 months [95 % CI 11.3-20.7]. 3 (10 %) patients developed Grade 3-4 neutropenia. No events of febrile neutropenia, cardiac, renal toxicities or alopecia were recorded. Grade 3 thrombocytopenia and nausea-vomiting were reported in 2 (6 %) and 5 (16 %) patients, respectively.

CONCLUSION

Results show a good efficacy and tolerance profile of oral vinorelbine as first-line chemotherapy for HER-2 negative MBC patients.

An evaluation of the anti-tumor efficacy of oleanolic acid-loaded PEGylated liposomes

The effective delivery of oleanolic acid (OA) to the target site has several benefits in therapy for different pathologies. However, the delivery of OA is challenging due to its poor aqueous solubility. The study aims to evaluate the tumor inhibition effect of the PEGylated OA nanoliposome on the U14 cervical carcinoma cell line. In our previous study, OA was successfully encapsulated into PEGylated liposome with the modified ethanol injection method. Oral administration of PEGylated OA liposome was demonstrated to be more efficient in inhibiting xenograft tumors. The results of organ index indicated that PEG liposome exhibited higher anti-tumor activity and lower cytotoxicity. It was also found that OA and OA liposomes induced tumor cell apoptosis detected by flow cytometry. Furthermore, effects of OA on the morphology of tumor and other tissues were observed by hematoxylin and eosin staining. The histopathology sections did not show pathological changes in kidney or liver in tested mice. In contrast, there was a significant difference in tumor tissues between treatment groups and the negative control group. These observations imply that PEGylated liposomes seem to have advantages for cancer therapy in terms of effective delivery of OA.

Emerging nanodelivery strategies of RNAi molecules for colon cancer therapy: preclinical developments

Although local colonic delivery is achievable through several strategies, colon cancer is still considered one of the leading causes of death worldwide. Failure of chemotherapeutics to exhibit efficient anticancer activity might be attributed to the development of multidrug resistance (MDR) mechanisms including the overexpression of certain oncogenes such as MDR1/P-gp. One of the major reasons for the shortcoming of P-gp inhibitors in clinic is the nonspecific distribution of them to nontarget organs, which leads to reduced elimination and increased toxicity of its substrates including anticancer agents. Numerous studies have demonstrated the effectiveness of gene-silencing approaches in reversing the P-gp-mediated MDR. However, none have reached clinical trials yet. Several drug-delivery systems have been investigated primarily to address P-gp and the observed improved anticancer efficacy suggests that nanomedicine provides new opportunities to overcome MDR in cancer. In this review, novel therapeutic strategies for colon cancer therapy will be discussed in the context of P-gp inhibition by low-molecular-weight agents and RNAi molecules.

Molecular buckets: cyclodextrins for oral cancer therapy

The oral route is preferred by patients for drug administration due to its convenience, resulting in improved compliance. Unfortunately, for a number of drugs (e.g., anticancer drugs), this route of administration remains a challenge. Oral chemotherapy may be an attractive option and especially appropriate for chronic treatment of cancer. However, this route of administration is particularly complicated for the administration of anticancer drugs ascribed to Class IV of the Biopharmaceutical Classification System. This group of compounds is characterized by low aqueous solubility and low intestinal permeability. This review focuses on the use of cyclodextrins alone or in combination with bioadhesive nanoparticles for oral delivery of drugs. The state-of-the-art technology and challenges in this area is also discussed.

In vivo evaluation of oral anti-tumoral effect of 3,4-dihydroquinazoline derivative on solid tumor

An extension of our previously reported 3,4-dihydroquinazoline derivative is investigated. Oral anti-tumoral activity of 3,4-dihydroquinazoline derivative (KYS05090) as potent and selective T-type calcium channel blocker was in vivo evaluated against A549 xenograft in BALB/c(nu/nu) nude mice. The rate of tumor volume increment in mouse model with KYS05090-treated group was remarkably slower than that of control group. With respect to tumor weight, it exhibited 60% and 67% tumor growth inhibition through oral administration of 1 and 5mg/kg of bodyweight, respectively, compared to control and was more potent than paclitaxel (53%). In addition, KYS05090 (10 and 50mg/kg, po) was found to have a marked analgesic effect in acetic acid-induced writhing test, whereas it did not show any effect on hot plate test.

Future Perspectives for Brain Pharmacotherapies: Implications of Drug Transport Processes at the Blood-brain Barrier

Years ago, ischemic stroke was regarded as a model disease for the development of neuroprotective therapies by the pharmacological industry. Results were disappointing. There are still no treatments available allowing the rescue of brain tissue once a stroke has occurred. Study failure is not only a problem in the stroke field. In other neurodegenerative conditions and in non-degenerative brain disorders, progress in drug development was also rather scarce until recently. An important factor in drug failure is the blood-brain barrier, which expresses active transporters that eliminate drugs from the brain. These transporters exhibit strong variations between different animals, which make it difficult to predict brain concentrations of drugs over species barriers. This paper claims that more detailed knowledge about: (1) the biology of blood-brain barrier transporters; (2) their regulation in brain disease, (3) the affinity of transporters to candidate drugs; and (4) the accumulation of drugs in brain tissue is needed for the overall success of clinical trials to be improved. An alternative strategy could be the use of disease-modifying treatments that do not have to enter the brain to exert their function. As such, restorative and anti-inflammatory strategies acting at the blood-brain interface might gain therapeutic potential in the future.

Impact of abcc2 [multidrug resistance-associated protein (MRP) 2], abcc3 (MRP3), and abcg2 (breast cancer resistance protein) on the oral pharmacokinetics of methotrexate and its main metabolite 7-hydroxymethotrexate

The ATP-binding cassette (ABC) transporters ABCC2 [multidrug resistance-associated protein (MRP) 2], ABCC3 (MRP3), and ABCG2 (breast cancer resistance protein) are involved in the efflux of potentially toxic compounds from the body. We have shown before that ABCC2, ABCC3, and ABCG2 together influence the pharmacokinetics of the anticancer and antirheumatic drug methotrexate (MTX) and its toxic metabolite 7-hydroxymethotrexate (7OH-MTX) after intravenous MTX administration. We now have used Abcc2;Abcc3;Abcg2(-/-) and corresponding single and double knockout mice to investigate the relative impact of these transporters on MTX and 7OH-MTX pharmacokinetics after oral MTX administration (50 mg/kg). The plasma areas under the curve (AUC(plasma)) in Abcg2(-/-) and Abcc2;Abcg2(-/-) mice were 1.7- and 3.0-fold higher than those in wild-type mice, respectively, suggesting additive effects of Abcc2 and Abcg2 on oral MTX pharmacokinetics. However, the AUC(plasma) in Abcc2;Abcc3;Abcg2(-/-) mice was not different from that in wild-type mice, indicating that Abcc3 protein is necessary for increased MTX plasma concentrations in the absence of Abcc2 and/or Abcg2. Furthermore, 2 h after administration, MTX liver levels were increased in Abcg2-deficient strains and MTX kidney levels were 2.2-fold increased in Abcc2;Abcg2(-/-) mice compared with those in wild-type mice. The absence of Abcc2 and/or Abcg2 also led to significantly increased liver and kidney levels of 7OH-MTX. Our results suggest that inhibition of ABCG2 and/or ABCC2, genetic polymorphisms or mutations reducing expression or activity of these proteins may increase the oral availability of MTX. Such conditions may also present risk factors for increased MTX-related toxicity in patients treated with oral MTX.

Toward a new age of cellular pharmacokinetics in drug discovery

Pharmacokinetics, pharmacology, and toxicology are the major determinants of the success or failure of candidates during drug development. Because inappropriate pharmacokinetics often leads to inefficacy, even toxicity, pharmacokinetics studies have been regarded as crucial components in drug preclinical and clinical research. However, new data increasingly reveal that drug concentrations in plasma or tissues cannot totally explain the efficacy of drug on the target organ. For most drugs that interact with targets localized in cells, intracellular penetration, accumulation, distribution, and elimination are important parameters governing the efficacy in the target cells. So, there is a pressing need to clarify the cellular pharmacokinetics and thus evaluate the efficacy of drugs in the target cells. This review provides a general overview regarding current knowledge about cellular pharmacokinetics in some specific cells and also summarizes the factors that can influence cellular pharmacokinetics. It concludes by discussing potential strategies for optimizing cellular pharmacokinetics and advocating that global cellular pharmacokinetics studies be conducted in future research toward improving drug efficacy.

The third-generation P-glycoprotein inhibitor tariquidar may overcome bacterial multidrug resistance by increasing intracellular drug concentration

OBJECTIVES

The use of efflux pump inhibitors may be a powerful strategy to overcome transporter-mediated bacterial multidrug resistance. In the present study, we set out to investigate the potency of tariquidar, a third-generation P-glycoprotein inhibitor in clinical development, for overcoming bacterial resistance towards ciprofloxacin.

METHODS

Staphylococcus aureus 29213 (SA29213) and S. aureus 1199B (SA1199B), which overexpresses the multidrug transporter NorA, as well as Pseudomonas aeruginosa 27853 and Stenotrophomonas maltophilia BAA-85, which expresses SmeDEF, were exposed to ciprofloxacin in the presence and absence of tariquidar or, for comparative reasons, elacridar. Activity of both P-glycoprotein inhibitors was evaluated by determination of MICs and time-kill curves, and by quantification of uptake of ciprofloxacin into bacterial cells.

RESULTS

Activity of tariquidar and elacridar was comparable for S. aureus strains, and both dose-dependently increased susceptibility towards ciprofloxacin. Highest effects were observed for SA1199B, where the addition of tariquidar resulted in a 10-fold reduction of the ciprofloxacin MIC, while no effect was observed for P. aeruginosa. For S. maltophilia, elacridar but not tariquidar improved susceptibility. Uptake of [14C]ciprofloxacin and modification of susceptibility showed significant correlations (r=0.89, P<0.0001). Tariquidar had no intrinsic activity against any strain tested.

CONCLUSIONS

We conclude that tariquidar has potent inhibitory effect against certain bacterial efflux pumps in vitro. Their high activity at clinically achievable concentrations might yield this class of drugs promising for future applications in infectious diseases.

Anticancer oral therapy: emerging related issues

The use of oral anticancer drugs has shown a steady increase. Most patients prefer anticancer oral therapy to intravenous treatment primarily for the convenience of a home-based therapy, although they require that the efficacy of oral therapy must be equivalent and toxicity not superior than those expected with the intravenous treatment. A better patient compliance, drug tolerability, convenience and possible better efficacy for oral therapy as compared to intravenous emerge as the major reasons to use oral anticancer agents among oncologists. Inter- and intra-individual pharmacokinetic variations in the bioavailability of oral anticancer drugs may be more relevant than for intravenous agents. Compliance is particularly important for oral therapy because it determines the dose-intensity of the treatment and ultimately treatment efficacy and toxicity. Patient stands as the most important determinant of compliance. Possible measures for an active and safe administration of oral therapy include a careful preliminary medical evaluation and selection of patients based on possible barriers to an adequate compliance, pharmacologic issues, patient-focused education, an improvement of the accessibility to healthcare service, as well as the development of home-care nursing symptom-focused interventions. Current evidences show similar quality of life profile between oral and intravenous treatments, although anticancer oral therapy seems to be more convenient in terms of administration and reduced time lost for work or other activities. Regarding cost-effectiveness, current evidences are in favor of oral therapy, mainly due to reduced need of visits and/or day in hospital for the administration of the drug and/or the management of adverse events.

Targeting drug transporters - combining in silico and in vitro approaches to predict in vivo

Transporter proteins are expressed throughout the human body in different vital organs. They play an important role to various extents in determining absorption, distribution, metabolism, excretion, and toxicity (ADME/Tox) properties of therapeutic molecules. Over the past decade, numerous drug transporters have been cloned and considerable progress has been made toward understanding the molecular characteristics of individual transporters. In this chapter several in vitro and in silico techniques are described with applications to understand transporter behavior. These include employing new techniques to rapidly identify novel ligands for transporters. Ultimately these methods should lead to a greater overall appreciation of the role of transporters in vivo.

Tyrosine kinase inhibitor enhances the bioavailability of oral irinotecan in pediatric patients with refractory solid tumors

PURPOSE

To assess the maximum-tolerated dosages (MTDs), and dose-limiting toxicities (DLTs) of the epidermal growth factor receptor inhibitor gefitinib and of intravenous (IV) irinotecan when administered together in children with refractory solid tumors. To assess the effect of gefitinib on the pharmacokinetics of IV irinotecan and on the bioavailability of a single oral dose of irinotecan.

PATIENTS AND METHODS

IV irinotecan (15 or 20 mg/m(2)) was given daily for 5 days of 2 consecutive weeks. Oral gefitinib (150 or 112.5 mg/m(2)) was concomitantly given daily for 12 or 21 days. A single oral dose of irinotecan was given on day 9 of course 2 to allow pharmacokinetic analysis.

RESULTS

The study enrolled 29 patients with recurrent solid tumors. The 21-day regimen of oral gefitinib with irinotecan was not tolerated. Diarrhea was the most common DLT. The MTD of the combination regimen was 15 mg/m(2)/d of IV irinotecan for 5 days of 2 consecutive weeks and 112.5 mg/m(2)/d of gefitinib given for 12 days. Gefitinib increased the bioavailability of oral irinotecan by four-fold over that observed in historical controls (median, 0.09 v 0.42; P < .000001), reducing the apparent clearance (an inverse measure of exposure) of irinotecan and SN-38 by 37% and 38%, respectively (P < .0001). A partial response was observed in a patient with refractory Ewing sarcoma.

CONCLUSION

IV irinotecan given with 12 days of oral gefitinib is well tolerated in children. We observed one partial response. Gefitinib significantly enhances the bioavailability of oral irinotecan. This combination warrants further investigation, particularly with orally administered irinotecan.

Functionally Overlapping Roles of Abcg2 (Bcrp1) and Abcc2 (Mrp2) in the Elimination of Methotrexate and Its Main Toxic Metabolite 7-Hydroxymethotrexate In vivo

Purpose: ABCC2 (MRP2) and ABCG2 (BCRP) transport various endogenous and exogenous compounds, including many anticancer drugs, into bile, feces, and urine. We investigated the possibly overlapping roles of Abcg2 and Abcc2 in the elimination of the anticancer drug methotrexate (MTX) and its toxic metabolite 7-hydroxymethotrexate (7OH-MTX).

Experimental Design: We generated and characterized Abcc2;Abcg2-/- mice, and used these to determine the overlapping roles of Abcc2 and Abcg2 in the elimination of MTX and 7OH-MTX after i.v. administration of 50 mg/kg MTX.

Results: Compared with wild-type, the plasma areas under the curve (AUC) for MTX were 1.6-fold and 2.0-fold higher in Abcg2-/- and Abcc2-/- mice, respectively, and 3.3-fold increased in Abcc2;Abcg2-/- mice. The biliary excretion of MTX was 23-fold reduced in Abcc2;Abcg2-/- mice, and the MTX levels in the small intestine were dramatically decreased. Plasma levels of 7OH-MTX were not significantly altered in Abcg2-/- mice, but the areas under the curve were 6.2-fold and even 12.4-fold increased in Abcc2-/- and Abcc2;Abcg2-/- mice, respectively. This indicates that Abcc2 compensates for Abcg2 deficiency but that Abcg2 can only partly compensate for Abcc2 absence. Furthermore, 21-fold decreased biliary 7OH-MTX excretion in Abcc2;Abcg2-/- mice and substantial 7OH-MTX accumulation in the liver and kidney were seen. We additionally found that in the absence of Abcc2, Abcg2 mediated substantial urinary excretion of MTX and 7OH-MTX.

Conclusions: Abcc2 and Abcg2 together are major determinants of MTX and 7OH-MTX pharmacokinetics. Variations in ABCC2 and/or ABCG2 activity due to polymorphisms or coadministered inhibitors may therefore substantially affect the therapeutic efficacy and toxicity in patients treated with MTX.

Global Lung Oncology Branch trial 3 (GLOB3): final results of a randomised multinational phase III study alternating oral and i.v. vinorelbine plus cisplatin versus docetaxel plus cisplatin as first-line treatment of advanced non-small-cell lung cancer

BACKGROUND

The study compared the efficacy of a first-line treatment with day 1 i.v. vinorelbine (NVBiv) and day 8 oral vinorelbine (NVBo) versus docetaxel (DCT) in a cisplatin-based combination in advanced non-small-cell lung cancer, in terms of time to treatment failure (TTF), overall response, progression-free survival (PFS), overall survival (OS), tolerance and quality of life (QoL).

METHODS

Patients were randomly assigned to receive cisplatin 80 mg/m2 with NVBiv 30 mg/m2 on day 1 and NVBo 80 mg/m2 on day 8 every 3 weeks, after a first cycle of NVBiv 25 mg/m2 on day 1 and NVBo 60 mg/m2 on day 8 (arm A) or cisplatin 75 mg/m2 and DCT 75 mg/m2 on day 1 every 3 weeks (arm B), for a maximum of six cycles in both arms.

RESULTS

From 2 February 2004 to 1 January 2006, 390 patients were entered in a randomised study and 381 were treated. The patient characteristics are as follows (arms A/B): metastatic (%) 80.5/84.8; patients with three or more organs involved (%) 45.3/40.8; median age 59.4/62.1 years; male 139/146; squamous (%) 34.2/33.5; adenocarcinoma (%) 41.6/39.3; median TTF (arms A/B in months) [95% confidence interval (CI)]: 3.2 (3.0-4.2), 4.1 (3.4-4.5) (P = 0.19); overall response (arms A/B) (95% CI): 27.4% (21.2% to 34.2%), 27.2% (21.0% to 34.2%); median PFS (arms A/B in months) (95% CI): 4.9 (4.4-5.9), 5.1 (4.3-6.1) (P = 0.99) and median OS (arms A/B in months) (95% CI): 9.9 (8.4-11.6), 9.8 (8.8-11.5) (P = 0.58). The median survival for squamous histology was 8.87/9.82 months and for adenocarcinoma 11.73/11.60 months for arms A and B, respectively. Main haematological toxicity was grade 3-4 neutropenia: 24.4% (arm A) and 28.8% (arm B). QoL as measured by the Lung Cancer Symptom Scale was similar in both arms.

CONCLUSIONS

Both arms provided similar efficacy in terms of response, time-related parameters and QoL, with an acceptable tolerance profile. In the current Global Lung Oncology Branch trial 3, NVBo was shown to be effective as a substitute for the i.v. formulation. This can relieve the burden of the i.v. injection on day 8 and can optimise the hospital's resources and improve patient convenience.

Different renal toxicity profiles in the association of cyclosporine and tacrolimus with sirolimus in rats

BACKGROUND

The association of calcineurin inhibitors (CNIs) with mTOR inhibitors (mTORi) is still a problem in clinical practice and there is substantial interest in better understanding the impact of these associations on kidney toxicity. We aimed to analyse the functional and histological profiles of damage and to define the contribution of inflammatory and pro-fibrotic mediators in the association of cyclosporine (CsA) and/or tacrolimus (Tac) with sirolimus (SRL).

METHODS

A well-defined model of nephrotoxicity in salt-depleted male rats was used. Monotherapy groups were distributed as a non-treated control group with saline solution (n = 12), the Tac group (n = 16) (tacrolimus 6 mg/kg/day) and the CsA group (n = 13) (CsA 15 mg/kg/day). The groups with different associations were scattered as the Tac + SRL group (n = 14) (tacrolimus 6 mg/kg/day and rapamycin 3 mg/kg/day) and the CsA + SRL group (n = 7) (CsA 15 mg/kg/day and rapamycin 3 mg/kg/day). Groups were divided into 30 and 70 days of follow-up, but the CsA + SRL group was only studied for 30 days because animals became sick.

RESULTS

Rats with the CsA + SRL association were the only ones which showed a significant reduction in body weight, impairment of renal function and severe and diffuse tubular vacuolization and tubular atrophy following a striped distribution, and scarce areas of the kidney were still preserved. The Tac + SRL association did not produce renal function impairment, and mild histological damage including enhanced periglomerular tubular atrophy was observed. This local damage affected the distal convoluted tubule involving macula densa and juxtaglomerular apparatus. Pro-inflammatory mediators paralleled functional and structural data. ED-1 and TNF-alpha were noticeably higher in the CsA + SRL than in the Tac + SRL association. Only in the CsA + SRL association an important increase in alpha-SMA+ cells was seen, mainly found in the areas with tubular atrophy. TGF-beta1 was also markedly enhanced in the CsA + SRL association whilst monotherapy or Tac + SRL groups at 30 days TGF-beta1 did not show any changes. However, at 70 days of treatment TGF-beta1 was significantly increased in the Tac + SRL group. Animals receiving SRL showed a decrease in renal vascular endothelial growth factor (VEGF) expression. This growth factor was significantly down-regulated in both CNI associations than in SRL monotherapy. P-glycoprotein (Pgp) was overexpressed in CsA and CsA + SRL therapy whilst Tac and TAC + SRL showed a middle increase Pgp expression but higher than the control and SRL group.

CONCLUSION

We conclude that the association of SRL with high doses of CsA or Tac produces a different functional, histological, inflammatory and pro-fibrogenic pattern. Thus, the addition of SRL to high doses of CsA leads to severe renal injury. Combination with high doses of Tac is clearly less deleterious in the short term. However, there is a low grade of pro-fibrotic inflammatory expression when this association is prolonged.

Influence of CYP3A4 inhibition on the steady-state pharmacokinetics of imatinib

PURPOSE

To evaluate the effects of ritonavir, a potent inhibitor of CYP3A4, on the steady-state pharmacokinetics of imatinib.

EXPERIMENTAL DESIGN

Imatinib pharmacokinetics were evaluated in cancer patients receiving the drug for at least 2 months, after which ritonavir (600 mg) was administered daily for 3 days. Samples were obtained on the day before ritonavir (day 1) and on the third day (day 4). The in vitro metabolism of imatinib with or without ritonavir and the effect of imatinib on 1-OH-midazolam formation rate, a probe for CYP3A4 activity, were evaluated with human CYP3A4 and pooled liver microsomes.

RESULTS

In 11 evaluable patients, the geometric mean (95% confidence interval) area under the curve of imatinib on days 1 and 4 were 42.6 (33.0-54.9) microg.h/mL and 41.2 (32.1-53.1) microg.h/mL, respectively (P = 0.65). A population analysis done in NONMEM with a time-dependent covariate confirmed that ritonavir did not influence the clearance or bioavailability of imatinib. In vitro, imatinib was metabolized to the active metabolite CGP74588 by CYP3A4 and CYP3A5 and, to a lesser extent, by CYP2D6. Ritonavir (1 micromol/L) completely inhibited CYP3A4-mediated metabolism of imatinib to CGP74588 but inhibited metabolism in microsomes by only 50%. Imatinib significantly inhibited CYP3A4 activity in vitro.

CONCLUSION

At steady state, imatinib is insensitive to potent CYP3A4 inhibition and relies on alternate elimination pathways. For agents with complex elimination pathways that involve autoinhibition, interaction studies that are done after a single dose may not be applicable when drugs are administered chronically.

Substrate-dependent effects of human ABCB1 coding polymorphisms

One of the many obstacles to effective drug treatment is the efflux transporter P-glycoprotein (P-gp), which can restrict the plasma and intracellular concentrations of numerous xenobiotics. Variable drug response to P-gp substrates suggests that genetic differences in ABCB1 may affect P-gp transport. The current study examined how ABCB1 variants alter the P-gp-mediated transport of probe substrates in vitro. Nonsynonymous ABCB1 variants and haplotypes with an allele frequency >/=2% were transiently expressed in HEK293T cells, and the transport of calcein acetoxymethyl ester and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY-FL)-paclitaxel was measured in the absence or presence of the P-gp inhibitor cyclosporin A. The A893S, A893T, and V1251I variants and the N21D/1236C>T/A893S/3435C>T haplotype altered intracellular accumulation compared with reference P-gp in a substrate-dependent manner. It is interesting that certain variants showed altered sensitivity to cyclosporin A inhibition that was also substrate-specific. These functional data demonstrate that nonsynonymous polymorphisms in ABCB1 may selectively alter P-gp transport and drug-drug interactions in a substrate- and inhibitor-dependent manner.

Effective oral chemotherapy for breast cancer: pillars of strength

Traditionally, anticancer therapy has been dominated by intravenous drug therapy. However, oral agents provide an attractive approach to chemotherapy and use of oral treatments is increasing. We discuss the benefits and challenges of oral chemotherapy from the perspectives of patients, healthcare providers and healthcare funders. Important issues include patient preference, efficacy, compliance, bioavailability, reimbursement, use in special patient populations, financial and staff time savings and flexibility of dosing. We review data for traditional oral agents (e.g. cyclophosphamide, methotrexate), newer oral chemotherapies (e.g. capecitabine), oral formulations of traditionally intravenous agents (e.g. vinorelbine, idarubicin) and new biologic agents under evaluation in breast cancer (e.g. tyrosine kinase inhibitors). Lastly, we review studies of all-oral combination regimens. The wealth of data available and the increasing use of oral agents in breast cancer suggest that many of the concerns and perceptions about oral therapy, including efficacy and bioavailability, have been overcome, and that oral therapy will play a major role in breast cancer management in the future in both the metastatic and adjuvant settings.

ABCG2: determining its relevance in clinical drug resistance

Multidrug resistance is a major obstacle to successful cancer treatment. One mechanism by which cells can become resistant to chemotherapy is the expression of ABC transporters that use the energy of ATP hydrolysis to transport a wide variety of substrates across the cell membrane. There are three human ABC transporters primarily associated with the multidrug resistance phenomenon, namely Pgp, MRP1, and ABCG2. All three have broad and, to a certain extent, overlapping substrate specificities, transporting the major drugs currently used in cancer chemotherapy. ABCG2 is the most recently described of the three major multidrug-resistance pumps, and its substrates include mitoxantrone, topotecan, irinotecan, flavopiridol, and methotrexate. Despite several studies reporting ABCG2 expression in normal and malignant tissues, no trials have thus far addressed the role of ABCG2 in clinical drug resistance. This gives us an opportunity to critically review the disappointing results of past clinical trials targeting Pgp and to propose strategies for ABCG2. We need to know in which tumor types ABCG2 contributes to the resistance phenotype. We also need to develop standardized assays to detect ABCG2 expression in vivo and to carefully select the chemotherapeutic agents and clinical trial designs. This review focuses on our current knowledge about normal tissue distribution, tumor expression profiles, and substrates and inhibitors of ABCG2, together with lessons learned from clinical trials with Pgp inhibitors. Implications of SNPs in the ABCG2 gene affecting the pharmacokinetics of substrate drugs, including many non-chemotherapy agents and ABCG2 expression in the SP population of stem cells are also discussed.

Preclinical pharmacokinetics and bioavailability of noscapine, a tubulin-binding anticancer agent

BACKGROUND

Noscapine, a naturally occurring antitussive phthalideisoquinoline alkaloid, is a tubulin-binding agent currently in Phase I/II clinical trials for anticancer therapy. Unlike currently available antimitotics such as taxanes and vincas, noscapine is water-soluble, well tolerated, and shows no detectable toxicity.

OBJECTIVE

The goal was to develop a simple, sensitive, quantitative, selective, and less time-consuming high-performance liquid chromatography (HPLC) method for determination of noscapine and to study its pharmacokinetics in mice models.

METHOD

Noscapine was extracted from mice plasma using the protein-precipitation method and detected using a reversed-phase C8 column with mobile phase consisting of 35% acetonitrile and 65% ammonium acetate buffer (pH 4.5) at 232 nm wavelength. Pharmacokinetic studies of noscapine were performed in mice following intravenous bolus at 10 mg/kg and oral administrations at 75, 150, and 300 mg/kg.

RESULTS

The standard curves for noscapine estimation were linear between 390 and 50,000 ng/ml (lower limit of quantification was 390 ng/ml) and the recovery was approximately 80%. Following 10 mg/kg intravenous dose, mean plasma concentrations of 7.88 microg/ml were achieved at 5 min in mice and declined with undetectable levels at 4 h. The mean total body clearance was 4.78 l/h. The mean volume of distribution (V (d)) was 5.05 l. Non-compartmental analysis yielded the mean area under the plasma concentration-time curve (AUC) for noscapine as 53.42, 64.08, and 198.35 h microg/ml reaching maximum plasma concentrations (C (max)) of 12.74, 23.24, and 46.73 microg/ml at a t (max) of 1.12, 1.50, and 0.46 h at the linearly increasing dose levels.

CONCLUSION

A rapid and simple HPLC/UV method for the quantification of noscapine in plasma has been developed to study pharmacokinetics of noscapine at tumor-suppressive doses in the mouse. Since orally available anticancer drugs are rare, therefore, noscapine, an innocuous agent, having a mean oral bioavailability of 31.5% over the studied dose range merits its further advancement in humans for anticancer therapy.

Chemical modification and formulation approaches to elevated drug transport across cell membranes

Drug delivery across cellular barriers, such as intestinal, nasal, buccal, alveolar, vaginal, ocular and blood-brain, is a challenging task. Multiple physiological mechanisms, such as cellular organisation, efflux, and chemical and enzymatic degradation, as well as physicochemical properties of the drug molecule itself, determine the penetration of xenobiotics across epithelial cell layers. Limited intestinal absorption of many novel and highly potent lead compounds has stimulated an intense search for strategies that can effectively enhance permeation across these biological barriers. This review discusses some of the approaches that have been, and are currently being, investigated for transepithelial drug delivery. Transdermal drug delivery requires a separate discussion on its own and is thus outside the scope of this review article.

In vitro activity of novel dual action MDR anthranilamide modulators with inhibitory activity at CYP-450

Synthesis and in vitro cytotoxicity assays of new anthranilamide MDR modulators have been performed to assess their inhibition potency of the P-glycoprotein (P-gp) transporter. The aromatic spacer group between nitrogen atoms (N1 and N2) in the known inhibitor XR9576 was replaced with a flexible alkyl chain of 2 to 6 carbon atoms in length. 6,7-Dimethoxy-1,2,3,4-tetrahydroisoquinoline and their open-chain N-methylhomoveratrylamine counterparts were shown to be potent P-gp inhibitors. The maximal inhibition was obtained when using an ethyl or propyl spacer. Several compounds were more potent than verapamil and intrinsically less cytotoxic than XR9576. In addition, in vitro metabolism studies of 23a with a subset of human CYP-450 isoforms revealed that, unlike XR9576, 23a inhibited CYP3A4, an enzyme that colocalizes with P-gp in the intestine and contributes to tumor cell chemoresistance by enhancing the biodisposition of anticancer drugs such as paclitaxel toward metabolism. In this context, 22a might be a suitable candidate for further drug development.

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.

Functional role of P-glycoprotein in limiting peroral drug absorption: optimizing drug delivery

P-glycoprotein (P-gp) associated multi-drug resistance is one of the major challenges in the chemotherapy of various cancers. On the other hand, it is now widely recognized that P-gp influences drug transport across various biological membranes. To this end, there is an increasing trend to optimize pharmacokinetics and drug delivery right from the initial stages of drug discovery by exploring all the possible mechanisms involved in 'deliverability'. Recent advances in molecular biology techniques and biochemical characterization methodologies have helped in identification of various transporters involved in absorption or secretion of drugs. P-gp, an efflux pump expressed along the gastrointestinal tract, limits the permeability of many drugs and thus affects their peroral absorption and bioavailability. A fundamental insight and thorough understanding of P-gp and its functional role in limiting drug absorption is critical to improve predictability of dynamic absorption models and aid in selection of new candidates for development, and also widen the scope of peroral delivery for 'challenging' molecules.

The use of isotopes in the determination of absolute bioavailability of drugs in humans

Absolute bioavailability studies in humans are not routinely performed as part of the drug registration process. They tend to be reasonably demanding, not least because toxicology data are required to support intravenous administration of a drug. Moreover, the classical crossover design of an absolute bioavailability study can suffer from artefacts caused by concentration-dependent pharmacokinetics. Many of the problems associated with absolute bioavailability studies can be alleviated using isotopically labelled drugs. Stable isotopes have been used in the performance of absolute bioavailability studies in humans for > 30 years. More recently, the advantages of using radiolabelled drugs have been expanded by using the ultrasensitive technology of accelerator mass spectrometry. Isotopic labelling not only allows for the accurate and efficient determination of absolute bioavailability, but can also provide information on first-pass effects and other pharmacokinetic parameters.