Current Topics in Medicinal Chemistry - Volume 9, Issue 2, 2009

Drug bioavailability depends on several pharmacokinetic aspects such as passive diffusion and active transport through several physiological barriers, in particular membranes of the gastrointestinal tract and the blood brain barrier. These two barriers represent the first line of defense of the organism through which the passive and active transports as well as the metabolism modulate drug absorption and disposition. Several pharmacokinetic parameters such as LogP, LogD and Ka address the passive diffusion of xenobiotics but these three features alone can not explain the physiological absorption and transport of drugs. Drug delivery is a combination with other factors such as metabolism and active transport. In this issue, the roles of the most studied proteins involved in the active transport, such as ATP Binding Cassette (ABC) transporters, have been deepened. The physiological involvement of ABC transporters in Blood Brain Barrier (BBB) and in GastroIntestinal (GI) tract has been illustrated focusing in particular on the role of these transporters in drugs influx and efflux (Colabufo et al.; Dobson et al.; Nicolazzo et al.). The methodologies to overcome the restrictive and selective physiological lines of defense of the organism (BBB and GI) have been outlined and the recent technological progresses in this field have been reported (Hammarlund-Udenaes et al.; Trapani et al.). Since ABC transporters are also implicated in MultiDrug Resistance (MDR), the involvement of some pumps, such as P-gp, BCRP and MRPs in chemotherapeutic agents active transport has been reported (Tucci et al.). In addition, it has been recently demonstrated that ABC transporters are involved in genetic pathologies such as cystic fibrosis, in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and in pathologies related to the trafficking of endogenous proteins, lipids, aminoacids etc (Rapposelli et al.; Azzariti et al.). Many thanks to all the authors for the prompt submission of their manuscripts and for their comprehensive contributions.

Pharmacokinetic limitations affect drug bioavailability determining the loss or the reduction of the pharmacological effects. The Gastro Intestinal tract (GI) and the Blood Brain Barrier (BBB) are the most important restrictive and selective physiological lines of defense of the organism. Although several parameters such as LogP, LogD and Ka have been extensively employed for determining drug bioavailability, the active transports, present in these biological barriers, play an important role for dosing and limiting cell drugs concentration. In particular, ATP Binding Cassette (ABC) transporters are involved in the active transport both in GI and BBB. Their strategic activity and biochemical and pharmacological role are herein treated.

With the discovery of novel therapeutic targets within the central nervous system (CNS), there has been a significant effort to synthesize a multitude of drug molecules with increasing potency and selectivity. However, the impact of the blood-brain barrier (BBB) in limiting effective concentrations of drug candidate from reaching the brain parenchyma is often ignored, resulting in a lack of efficacy when administered to animal models or humans. Intercellular drug transport across the BBB is negligible due to the impermeable tight junctions formed by interconnecting endothelial cells. Furthermore, drug permeability via the transcellular route cannot be assumed for all molecules due to the high expression of drug efflux transport proteins, which effectively extrude compounds from the brain endothelial cell back into the cerebral vasculature. In addition to the extensively-studied P-glycoprotein (P-gp, ABCB1), the brain endothelial cells also express multidrug resistance associated proteins (MRP, ABCC) and breast cancer resistance protein (BCRP, ABCG2), amongst other efflux transporters. While more research has focussed on the impact of P-gp and MRP on drug transport across the BBB, the role of ABCG2 in limiting exposure of drug molecules to the CNS is now becoming more clearly understood. The purpose of this review, therefore, is to summarise the findings of the various studies assessing the expression profile of ABCG2 at the BBB, to provide an overview on the current research being undertaken to identify specific ABCG2 inhibitors with therapeutic benefit, and to critically assess the functional role of ABCG2 on drug transport across the BBB.

In the area of lead optimization for potential CNS-active drugs in medicinal chemistry, there is a great need for experimental methodologies that can generate data relevant to estimates of free (unbound) drug exposure within the CNS. The methods chosen have to be efficient and have to measure a pharmacologically relevant entity. The lack of methods for generating such data is probably linked with the lack of successful lead optimization strategies within CNS drug discovery. This article evaluates available methods for estimating drug delivery to the brain, and discusses the relevance of the methods from the perspective of CNS exposure to free drug. It is suggested that the extent of drug delivery is the most important investigative parameter, since permeability (rate of transfer) can vary within a relatively wide range and still allow effects within the CNS. Following this suggestion would shift the focus from the current way of thinking and could lead to the development of less lipophilic compounds than are currently being investigated. It is concluded that an extensive collection of quality data on brain drug delivery, transporter affinities and in vivo behavior is urgently required so as to be able to build relevant predictive in vitro and in silico models for the future. These models need to be much more focused on the asymmetry of active transport across the BBB than on permeability data.

Drug entry into cells was previously believed to be via diffusion through the lipid bilayer of the cell membrane, with the contribution to uptake by transporter proteins being of only marginal importance. Now, however, drug uptake is understood to be mainly transporter-mediated. This suggests that uptake transporters may be a major determinant of idiosyncratic drug response and a site at which drug-drug interactions occur. Accurately modelling drug pharmacokinetics is a problem of Systems Biology and requires knowledge of both the transporters with which a drug interacts and where those transporters are expressed in the body. Current physiology-based pharmacokinetic models mostly attempt to model drug disposition from the biophysical properties of the drug, drug uptake by diffusion being thereby an implicit assumption. It is clear that the incorporation of transporter proteins and their drug interactions into such models will greatly improve them. We discuss methods by which tissue localisations and transporter interactions can be determined. We propose a yeast-based transporter expression system for the initial screening of drugs for their cognate transporters. Finally, the central importance of computational modelling of transporter substrate preferences by structure-activity relationships is discussed.

This paper provides a mini-review of some recent approaches for the treatment of brain pathologies examining both medicinal chemistry and pharmaceutical technology contributions. Medicinal chemistry-based strategies are essentially aimed at the chemical modification of low molecular weight drugs in order to increase their lipophilicity or the design of appropriate prodrugs, although this review will focus primarily on the use of prodrugs and not analog development. Recently, interest has been focused on the design and evaluation of prodrugs that are capable of exploiting one or more of the various endogenous transport systems at the level of the blood brain barrier (BBB). The technological strategies are essentially non-invasive methods of drug delivery to malignancies of the central nervous system (CNS) and are based on the use of nanosystems (colloidal carriers) such as liposomes, polymeric nanoparticles, solid lipid nanoparticles, polymeric micelles and dendrimers. The biodistribution of these nanocarriers can be manipulated by modifying their surface physico-chemical properties or by coating them with surfactants and polyethylene-glycols (PEGs). Liposomes, surfactant coated polymeric nanoparticles, and solid lipid nanoparticles are promising systems for delivery of drugs to tumors of the CNS. This mini-review discusses issues concerning the scope and limitations of both the medicinal chemistry and technological approaches. Based on the current findings, it can be concluded that crossing of the BBB and drug delivery to CNS is extremely complex and requires a multidisciplinary approach such as a close collaboration and common efforts among researchers of several scientific areas, particularly medicinal chemists, biologists and pharmaceutical technologists.

Multi-drug resistance (MDR) frequently contributes to the failure of chemotherapeutic treatments in cancer patients. Mechanisms underlying the development of MDR have been extensively studied and are considered multifactorial. Among them, the ATP-Binding Cassette (ABC) family of proteins plays a pivotal role. Processes of cellular distribution and subcellular localization of MDR-ABC proteins are not yet well explored and to enlighten these topics could be crucial to understand cellular drug uptake and retention. In this review, we analysed literature data concerning i) intracellular trafficking of MDR-ABC proteins (BCRP, P-gp and MRP1) and ii) mechanisms altering their cellular localization and trafficking. Moreover, we describe single nucleotide polymorphisms (SNP) that have been reported for some multidrug resistance (MDR) transporters, such as BCRP and P-gp, emphasizing their ability to affect the expression, function and localization of the transporters, with implications on drug resistance phenotypes.

This paper describes an overview of recent insights concerning some socially relevant neurodegenerative diseases such as Alzheimer's (AD), Parkinson's (PD), Huntington's (HD) and Creutzfeldt-Jakob's (CJD) diseases, amyotrophic lateral sclerosis (ALS) and epilepsy. For each pathological state, the direct and/or indirect involvement of Pglycoprotein (P-gp) efflux transport is underlined. On this basis, P-gp still represents an innovative target which can offer new tools for the development of more effective and preventive therapeutic strategies for neurodegenerative disorders. For each of them, therefore, a possible use of drugs affecting P-gp transport activity has been suggested.

Drug resistance is a major drawback for cancer chemotherapy protocols and previous studies have demonstrated the overexpression of the P-glycoprotein (P-gp) as mechanism by which myeloma cells develop multidrug resistance (MDR). However, other molecules may apparently promote MDR in multiple myeloma (MM). They include both lung resistance-related protein (LRP) and p53 activation. The inhibition of P-gp in MM patients treated with melphalan (PAM) has been associated to increased toxicity, whereas defective apoptosis due to down-modulation of the NF-kB is a feature of MDR+ myeloma cells. On the contrary, clinical trials with proteasome inhibitors have been successfully carried out to overcome MDR despite their toxicity profile. Recently, sigma receptors (σR)S, namely σR1 and σR2, have been found to be overexpressed in breast cancer cells. In addition, their levels correlate with both P-gp upregulation and MDR development. By contrast, selective inhibitors of σRS as PB28, disrupt the P-gp signals and restore the apoptosis machinery in malignant cells. We have reviewed the major pathogenetic events promoting MDR in MM and focused on the σRS as potential mechanism driving this function. We demonstrate that MDR+ myeloma cells overexpress the σR2 and that the treatment with PB28 induces P-gp down-modulation through the activation of the caspases enrolled in both extrinsic and intrinsic apoptotic pathways. Thus, σR2 inhibitors may be tentatively proposed for the treatment of PAM-resistant MM patients.

Full text available