Highlights From the American Association of Pharmaceutical Scientists/ International Transporter Consortium Joint Workshop on Drug Transporters in Absorption, Distribution, Metabolism, and Excretion: From the Bench to the Bedside - Clinical Pharmacology Considerations

Problems associated with effective pharmacotherapy of the elderly patients (geriatrics): A review

The worlds older population is growing dramatically. At the same time, ensuring an appropriate high standard of living for the elderly by reducing of morbidity and disability of geriatric patients is one of the main objectives of the modern healthcare system. However, changes associated with body aging necessitate application of novel approaches to the correction of pharmacotherapy and usage of specialized dosage forms. Such medicinal products provide both an appropriate therapeutic effect and facilitate their use. Presented review considers several features of pharmacotherapy of geriatric patients.

ASCT1 and ASCT2: Brother and Sister?

The SLC1 family includes seven members divided into two groups, namely, EAATs and ASCTs, that share similar 3D architecture; the first one includes high-affinity glutamate transporters, and the second one includes SLC1A4 and SLC1A5, known as ASCT1 and ASCT2, respectively, responsible for the traffic of neutral amino acids across the cell plasma membrane. The physiological role of ASCT1 and ASCT2 has been investigated over the years, revealing different properties in terms of substrate specificities, affinities, and regulation by physiological effectors and posttranslational modifications. Furthermore, ASCT1 and ASCT2 are involved in pathological conditions, such as neurodegenerative disorders and cancer. This has driven research in the pharmaceutical field aimed to find drugs able to target the two proteins.This review focuses on structural, functional, and regulatory aspects of ASCT1 and ASCT2, highlighting similarities and differences.

Considerations for Determining Direct Versus Indirect Functional Effects of Solubilizing Excipients on Drug Transporters for Enhancing Bioavailability

Excipients used in drug formulations at clinically safe levels have been considered to be pharmacologically inert; however, numerous studies have suggested that many solubilizing agents may modulate drug transporter activities and intestinal absorption. Here, the reported interactions between various solubilizing excipients and drug transporters are evaluated to consider various potential underlying mechanisms. This forms the basis for debate in the field in regard to whether or not the effects are based on "direct" interactions or "indirect" consequences arising from the role of the excipients. For example, an increase in apparent drug solubility can give rise to saturation of transporters according to Michaelis-Menten kinetics. This is also drawing the attention of regulatory agencies as they seek to understand the role of formulation additives. The continued application of excipients as a tool in solubility enhancement is crucial in the drug development process, creating a need for additional data to verify the proposed mechanism behind these changes. A literature review is provided here with some guidance on other factors that should be considered to delineate the effects that arise from direct physiological interactions or indirect effects. The results of such studies may aid the rational design of bioavailability-enhancing formulations.

Transport of Bupropion and its Metabolites by the Model CHO and HEK293 Cell Lines

BACKGROUND

Bupropion (BUP) is widely used as an antidepressant and smoking cessation aid. There are three major pharmacologically active metabolites of BUP, Erythrohydrobupropion (EB), Hydroxybupropion (OHB) and Threohydrobupropion (TB). At present, the mechanisms underlying the overall disposition and systemic clearance of BUP and its metabolites have not been well understood, and the role of transporters has not been studied.

OBJECTIVE

The goal of this study was to investigate whether BUP and its active metabolites are substrates of the major hepatic uptake and efflux transporters.

METHOD

CHO or HEK293 cell lines or plasma membrane vesicles that overexpress OATP1B1, OATP1B3, OATP2B1, OATP4A1, OCT1, BCRP, MRP2 or P-gp were used in cellular or vesicle uptake and inhibition assays. Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) was used to quantify transport activity.

RESULTS

BUP and its major active metabolites were actively transported into the CHO or HEK293 cells overexpressing OATP1B1, OATP1B3 or OATP2B1; however, such cellular active uptake could not be inhibited at all by prototypical inhibitors of any of the OATP transporters. These compounds were not transported by OCT1, BCRP, MRP2 or P-gp either. These results suggest that the major known hepatic transporters likely play a minor role in the overall disposition and systemic clearance of BUP and its active metabolites in humans. We also demonstrated that BUP and its metabolites were not transported by OATP4A1, an uptake transporter on the apical membrane of placental syncytiotrophoblasts, suggesting that OATP4A1 is not responsible for the transfer of BUP and its metabolites from the maternal blood to the fetal compartment across the placental barrier in pregnant women.

CONCLUSION

BUP and metabolites are not substrates of the major hepatic transporters tested and thus these hepatic transporters likely do not play a role in the overall disposition of the drug. Our results also suggest that caution should be taken when using the model CHO and HEK293 cell lines to evaluate potential roles of transporters in drug disposition.

Transporter and Lysosomal Mediated (Multi)drug Resistance to Tyrosine Kinase Inhibitors and Potential Strategies to Overcome Resistance

Tyrosine kinase inhibitors are a class of chemotherapeutic drugs that target specific protein kinases. These tyrosine kinase inhibitors constitute a relatively new class of drugs which target for instance Bcr-Abl, Epidermal Growth Factor Receptor (EGFR) and Vascular Endothelial Growth Factor Receptor (VEGFR). Despite some initial successes, the overall therapeutic benefit of tyrosine kinase inhibitors in the clinic has been mixed. Next to mutations in the target, multidrug resistance is a major obstacle for which still no clinically effective strategies have been developed. Major mechanisms of multidrug resistance are mediated by drug efflux transporter proteins. Moreover, there is accumulating evidence that multidrug resistance can also be caused by lysosomal sequestration of drugs, effectively trapping tyrosine kinase inhibitors and preventing them from reaching their target. Lysosomal drug sequestration seems to work together with ATP-binding cassette transporters, increasing the capacity of lysosomes to mediate sequestration. Both membrane efflux transporter proteins and lysosomes present potential therapeutic targets that could reverse multidrug resistance and increase drug efficacy in combination therapy. This review describes both mechanisms and discusses a number of proposed strategies to circumvent or reverse tyrosine kinase inhibitor-related multidrug resistance.

Drug Transporters: Advances and Opportunities

Drug transporter research conducted over the last several decades has led to a greatly advanced understanding of the mechanisms underlying the principles of drug absorption and disposition. Although many transporters remain poorly characterized, there is ample evidence that the drug transporter field will ultimately provide vital support to routine patient management, and will play a key role in the discovery, development, and evaluation of innovative, cutting-edge therapies.