Current Pharmaceutical Biotechnology - Volume 12, Issue 4, 2011

Molecular imaging has been recognized as an important area of bio-medical research, mainly because of its ability to visually represent, characterize and quantify biological processes in living subjects. For instance, techniques such as Positron Emission Tomography (PET), Single-photon Emission Computed Tomography (SPECT), Magnetic Resonance Imaging (MRI), and optical imaging (Fluorescence and Bioluminescence) have been extensively used for imaging different biological pathways of cancer cells in living small animals. Molecular imaging uses non-invasive imaging techniques to detect signals that originate from molecules, often in the form of an injected tracers' interaction with a specific cellular target, or a substrate that specifically reacts with enzymes expressed in the target cells, or sometimes from the labeled protein itself, in vivo in living subjects. These techniques are capable of measuring trace amounts of radiolabeled probes or optical signal generated from the probes accumulated in the cells or signal emitted by the enzyme-substrate reactions in vivo, and quantifying the molecular kinetic processes that are under review (or being studied). This technique will then provide a wealth of information, facilitating the drug development process thus enabling therapeutic evaluation and planning. Currently, molecular imaging techniques have been combined with other biochemical assay systems to improve early diagnosis of cancers in the clinic. The importance of molecular imaging techniques is evident in the pharmaceutical industry starting from drug screening to clinical applications, as well as intermediate processes such as in vitro and pre-clinical evaluations. To highlight the importance of molecular imaging in different areas of pharmaceutical research, in addition to its proven role in biomedical research, in this issue we cover some important areas where molecular imaging plays a growing role as a research tool. After the completion of the Human Genome Project (HGP), we now have detailed knowledge of complete nucleotide sequences, as well as the number of functional genes and their importance. However, there are unsolved fundamental questions untouched by HGP. For instance, how do proteins cross talk and execute different biological functions in response to extracellular stimuli? The first few topics of the issue highlight the importance of reporter gene technologies developed for both in vitro and in vivo applications for studying protein-protein interactions. Importantly, reporter genes are now widely used for the indirect tracking of biological events both in vitro and very recently in vivo. As several imaging modalities with different levels of image resolutions and tomographic informations are currently available, the availability of multiple reporters expressed in fusion provides an important advantage. By combining the information derived from each modality, it is now possible to validate the authenticity of the biological event than with just a single reporter. To highlight the importance of fusion reporters, we next discuss the development and application of different fusion reporters. We and others have developed reporter gene-based imaging technologies to study different sub-cellular events such as protein-folding, including protein dimerizations, protein-ligand interactions, and protein- protein interactions that can serve as therapeutic targets in treating different diseases such as cancer. These assays are not only useful for imaging sub-cellular events, but can also be used for screening drugs that modulate these processes and for pre-clinical evaluation of these drugs in living animals. Several of the articles in this issue further highlight the importance of available assays and their applications for studying different sub-cellular events in living animals. Stem cell-based therapeutic approaches have recently been used for treating different diseases. In vivo monitoring of therapeutic stem cells for their movement (cell trafficking), accumulation in the targets sites and differentiation into required cell lineage are important to thoroughly evaluate their role. With PET and high resolution MR-imaging, it is now possible to track stem cells at single cell resolution in small animals and in humans. To highlight the applications of molecular imaging in cell-based therapy, in this issue we have included an extensive review that discusses the systematic method in which imaging can help in the functional evaluation of stem cell therapy. In addition to stem cell-based therapy, it is now common in the use of purified islet cells derived from the organs (Pancreas) of donors to be successfully engrafted and for patients to achieve insulin independent survival in type-1 diabetes. The β-cells' survival after engraftment was challenging and was successful only for limited periods of time. To improve the survival of engrafted β-cells, several approaches including growth factor supplementations have been attempted. Molecular imaging using MRI offers an effective method of tracking the viability of engrafted cells over time. This issue features a research article describing research on monitoring the viability of transplanted islets and insulin-secreting cells in vivo in rats by using a 3T MR scanner. In cancers and other cellular diseases, the cellular receptors not only contribute to the pathophysiology of diseases, but can also serve as ideal targets for treating these diseases. For example, in breast cancer the phenotypic pattern of this disease for the expression of different cellular receptors such as HER2, Estrogen Receptor (ER) and Progesterone Receptor (PR) are crucial for designing therapeutic strategies for treatment. To examine the importance of cellular receptors and their roles, we have manuscripts that review different cellular receptors involved in the development and the treatment of breast cancer. Finally, we have included a research article that for the first time investigated therapeutic response of Gastrointestinal Stromal Tumor (GIST) using dual energy CT. Overall, this issue highlights the importance of molecular imaging in different area of bio-pharmaceuticals, including some promising novel technologies.

The process of drug discovery and development requires enormous resources and time, with increasing cost for new drug development. Molecular imaging techniques have tremendous potential for improving the efficiency of drug screening, assessing the pharmacokinetics of new drugs, and evaluating drug effects. Appropriate application of molecular imaging to drug discovery and development can markedly reduce costs and the time required for new drug development. This review focuses on the contributions of molecular imaging for drug discovery and development, particularly drug screening, pharmacokinetic, preclinical and clinical drug evaluation, and for personalized and lesionalized medicine.

Stem-cell technology is a major area within cell therapy that promises significant therapeutic outcome. The plasticity and self-renewal capabilities of stem cells make them valuable tools for potential application in regenerative medicine and tissue replacement following injury or disease. Here, we discuss the different types of stem cells currently used in research, preclinical and early clinical development, their potential therapeutic and diagnostic applications, and current barriers to translating basic research into clinical therapies. Biomedical imaging is increasingly being used to monitor the fate of transplanted stem cells, including their survival, proliferation, differentiation and homing to targeted organs and tissues. We discuss different imaging modalities currently utilized to track stem calls, the advantages and challenges, and future implications in clinical applications.

Transplantation of pancreatic islets is a promising strategy for restoring insulin secretion in diabetes mellitus. To monitor transplanted islets, a method to evaluate the distribution in a non-invasive manner in vivo is needed. INS-1E, a stable differentiated insulin secreting cell line, and rodent islets were used to monitor cell transplantation by MRI. For labeling INS-1E cells in vitro, increasing concentrations of Resovist® in culture medium were tested. For MR imaging in a clinical 3T scanner, we placed a layer of labeled INS-1E cells between two layers of 4% gelatin. Viability assay was performed. Cell function was evaluated by static incubation assay to assess insulin secretion. For in vivo imaging, iron labeled rodent islets were transplanted into the liver of streptozotocin induced diabetic rats and visualized by MRI. Blood sugar values were controlled and liver tissue was removed for histological analysis. SPIO labeled INS-1E cells did not show altered viability or reduced glucose stimulated insulin secretion in vitro. Double staining of labeled and unlabeled INS-1E cells showed no difference in the staining pattern. Labeling of rodent islets with SPIOs does not reduce their secretory activity or alter their viability. We visualized SPIO-labeled INS-1E cells and rat islets in vitro using a clinical 3T scanner. Diabetic rats transplanted with SPIO-labeled islets became normoglycemic. MR imaging successfully verified the distribution of labeled transplanted cells in vivo. Labeling INS-1E cells and rat islets with SPIOs does not alter their viability, while enabling MR imaging of labeled cells in vitro and within the living organism.

Gene-directed enzyme prodrug therapy (GDEPT) is one of the promising alternatives to conventional chemotherapy. Suicide gene therapy based anticancer strategy involves selective introduction of a foreign gene into tumor cells to produce a foreign enzyme that can activate an inert prodrug to its cytotoxic form and cause tumor cell death. In this review, we present three most promising suicide gene/prodrug combinations (1) herpes simplex virus thymidine kinase (HSV1-TK) with ganciclovir (GCV), (2) cytosine deaminase (CD) from bacteria or yeast with 5-fluorocytodine (5- FC) and (3) bacterial nitroreductase (NTR) with 5-(azaridin-1-yl)-2,4-dinitrobenzamide (CB1954) and discuss how molecular imaging may improve therapy strategies. Current advances in noninvasive imaging technologies can measure vector dose, tumor selectivity, transgene expression and biodistribution of therapeutic gene with the aid of reporter genes and imageable probes from live animal. In this review we will discuss various imaging modalities - Optical, Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT), and highlight some of the approaches that can advance prodrug cancer therapy from bench to clinic.

Breast cancer, a leading cause of cancer death in women, is strongly correlated with the up- and downregulation of hormone and growth factor receptors. Therefore, improving our understanding of such receptor status in different stages of breast cancer will help in the development of novel diagnostic and therapeutic solutions. In particular, molecular imaging technology in association with advanced molecular and cell biology techniques could reveal in detail dynamic molecular events in cells, allowing the study of crucial molecular pathological changes occurring in cancer and other diseases. Molecular imaging techniques such as PET, SPECT, MRI, and the combinatorial techniques have made tremendous strides in elucidating the role of cellular receptors, helping to monitor the course of breast cancer development and the therapeutic efficacy of novel drugs. Optical imaging of cellular receptors is emerging as a powerful tool given the advancement of fluorescent and bioluminescent proteins. Estrogen receptor, progesterone receptor, and HER2/neu have been adopted clinically to detect different types of breast cancer and to test novel treatment strategies; however, other cellular receptors may also be involved in breast cancer subtyping and could emerge as treatment prospects. This review will focus on the recent developments of imaging various cellular receptors pertaining to the growth and development of breast cancer.

Breast cancer is increasing at an alarming rate in women around the world, where medical biology is confronted by this disease on two crucial fronts. The first step is the early accurate diagnosis, which is very critical and the second step involves the appropriate clinical management. The current trend in molecular imaging of breast cancer provides not only an excellent tool in diagnosing the disease but also useful in validating the potentiality of pharmaceutical interventions. This review addresses the effectiveness of targeted imaging technologies to resolve the molecular characterisation and pathological evidences in breast cancer. The focus is on the present practices in breast cancer imaging, specifically targeting cellular machineries such as hormone receptors and angiogenic factors.

Development and marketing of new drugs require stringent validation that are expensive and time consuming. Non-invasive multimodality molecular imaging using reporter genes holds great potential to expedite these processes at reduced cost. New generations of smarter molecular imaging strategies such as Split reporter, Bioluminescence resonance energy transfer, Multimodality fusion reporter technologies will further assist to streamline and shorten the drug discovery and developmental process. This review illustrates the importance and potential of molecular imaging using multimodality reporter genes in drug development at preclinical phases.

Purpose: Advanced gastrointestinal stromal tumors (GISTs) are treated with tyrosine kinase inhibitors, which also have antiangiogenic properties. Dual-energy CT (DECT) allows to acquire semi-quantitative iodine images which might correlate with blood pool and tumor vascularity. In this feasibility-study, we correlated lesional iodine uptake estimations in correlation to tumor size changes under targeted therapy as first step in the evaluation of dedicated DECT based strategies for monitoring molecular therapies in GIST. Patients and Methods: 48 tumor lesions in 18 patients with metastasized histologically proven GIST under tyrosine kinase inhibitor (TKI) therapy were analyzed. Patients were examined with a dual-source CT in dual-energy mode (Voltage tube A: 80 kV, tube B: 140 kV). Using the dual-energy software virtual unenhanced, selective iodine (overlay) and mixed CT numbers (similar to CT numbers at 120 kV) of lesions were calculated. The largest diameter of each lesion on cross-sectional axial images was measured. The mean difference of overlay CT numbers in the baseline and follow-up examinations was calculated and this marker of lesional iodine uptake was compared to lesional size changes under molecular therapy. Results: Utilizing the cut-off value 15 HU of change in overlay, DECT allowed to identify lesions with a stable, increased or decreased lesional iodine uptake with corresponding typical lesion size change patterns after 3 months of targeted therapy: 30 lesions had no significant change of overlay CT numbers (OL) (mean: -2.4 HU) or lesion size (mean: +1.5%). A strong decline of the OL (mean: - 24 HU) in 13 lesions was combined with a pronounced growth (mean: + 26%). 5 lesions showed a strong increase of the absolute OL (mean: + 23 HU) associated with a moderate increase in size (+ 8%). Conclusion: Determination of the overlay CT number with DECT enables to stratify metastases with stable, increasing or decreasing iodine uptake over time with -in our collective- typical lesion size change patterns. Investigation of a larger patient cohort, comparison to histology, alternate imaging biomarkers and correlatrion to long-term response will further clarify the significance of these findings for monitoring targeted therapies in GIST.

Bioluminescence resonance energy transfer (BRET) assay is a comparatively new cell-based assay technology that is assuming more prominent roles in the field of studying protein-protein interactions, protein dimerization and signal transduction. In the last few years BRET related research has gained significant momentum in terms of adding versatility in the assay format as well as a variety of new applications where it has been suitably used. Beyond the scope of quantitative measurement of protein-protein interactions and protein dimerization, molecular imaging applications based on BRET assays have broaden its scope as a great tool for high-throughput screening (HTS) of pharmacologically important compounds. This article will highlight the landmarks in BRET research, with those which have significant contributions towards making it an attractive single format assay that shuttles between in vitro and in vivo measurements.

In 1976, Drs. Juliano and Ling reported that overexpression of a membrane protein in colchicine-resistant Chinese hamster ovary cells resulted in acquired resistance to many structurally unrelated antineoplastic drugs [i. e., multidrug resistance (MDR)]. The membrane protein overexpressed was later determined to be a member of the ATP-binding cassette (ABC) transporter superfamily. Based on DNA sequencing, there are at least 48 genes that code for ABC proteins in the human genome. The ABC domains of these proteins hydrolyze ATP, providing energy for transport of various substrates across the membrane against a high concentration gradient (In-to-out). The ABC transporters can be divided into seven sub-groups based on their sequence homology: ABCA (12 members), ABCB (11 members), ABCC (12 members), ABCD (4 members), ABCE (1 member), ABCF (3 members) and ABCG (5 members). The ABC genes are essential for many cellular processes, (notably protecting cells against xenobiotics and toxic endogenous molecules). The mutations in at least 13 of these genes cause or contribute to human genetic disorders. This highlighted issue of Current Pharmaceutical Biotechnology contains several reviews that summarize the role of the ABC transporters in cancer chemotherapy as well as in protecting healthy tissue from the xenobiotics and other toxic compounds. In this issue, A.K. Tiwari et al. discussed recent research regarding the involvement of ABC transporters in multidrug resistance in cancer chemotherapy. The review by R. W. Robey et al. focused on clinical interventions with ABCG2, an important ABC transporter that affects drug bioavailability, distribution, and renal excretion, as well as the development of resistance to specific antineoplastic drugs. Two in-depth articles on the latest important developments in utilizing therapeutic strategies to overcome ABC transporter-mediated drug resistance are reviewed by C-P. Wu et al. (Natural product modulators of ABC drug transporters) and E. Stolarczyk et al. (Targeting phosphorylation to regulate ABC transporter function). In addition to conferring MDR, the major physiological functions of ABC transporters include (but not limited to) the transport of cholesterol and lipids, bile salts, and peptides for antigen presentation. Many ABC transporters in the liver play an important role in controlling the chronic inflammation caused by bile acid toxicity, and this is discussed in the article of V. Ling's group. A review by P. Krishnamurthy and J.D. Schuetz emphasizes the role of ABC transporters, especially ABCB6 and ABCG2, in porphyrin metabolism and cell survival. This is interesting as ABCB6 is a recently discovered novel mitochondrial porphyrin uptake pump. Another important pharmacological/physiological function of some ABC transporters is to provide protection by effluxing xenobiotics (including drugs used to treat diseases) from tissues including brain, kidney, testes, and developing human fetus. A.M.S. Hartz and B. Bauer summarize the recent findings on ABC transporter regulation in the CNS especially at the blood-brain barrier and discuss the role of ABC transporters in CNS diseases, including brain cancer, seizures, epilepsy, and Alzheimer's disease. Another article from Z. Ni and Q. Mao provides a thorough discussion of the protective roles of ABC transporters in the placenta that attenuates toxicity in the developing fetus. It is very exciting that J.P. Gillet and M.M. Gottesman discussed how novel biotechnological approaches can be used to develop a molecular diagnostic assay to determine the expression levels of ABC transporters. It is critical to have this kind of assay (in the form of a kit) in the market for the investigation of ABC transporters and MDR in the clinic. Finally, T. Ishikawa's group presents their new biotechnologic assay (SmartAmp2 method) to detect single nucleotide polymorphisms (SNPs) in human ABC transporter genes. It is essential to validate ABC transporter genetic polymorphisms and develop new biotechnologies to rapidly detect clinically important variants of these pumps to advance the concept of individualized medical treatment. Unfortunately, due to a strict time-table for the publication of this issue, several review articles related to this topic could not be included, such as those discussing ABCC6 and Pseudoxanthoma elasticum (PXE), ABCC7 and cystic fibrosis, ABCG5/G8 and plant sterol transport. We hope that these articles will be published in a future issue of this journal. In this special issue, we have reviewed the functions of ABC transporters, emphasizing biochemical mechanisms and genetic defects. The topics covered by experts in this field will illustrate the importance of ABC transporters in pharmacology, physiology and human diseases. I would like to express my appreciation to all the authors for their contributions. I thank Dean R. Mangione, College of Pharmacy and Allied Health Professions, St. John's University, Chairman, L. Trombetta (Department of Pharmaceutical Sciences, St. John's University) for their encouragement and support. I would like to extend my thanks to my colleague Dr. C.R. Ashby, Jr. my postdoctoral fellows, visiting scholars and graduate students for their critical reading of this editorial and the format of the articles in this issue.

The adenosine triphosphate binding cassette (ABC) transporters are one of the largest transmembrane gene families in humans. The ABC transporters are present in a number of tissues, providing protection against xenobiotics and certain endogenous molecules. Unfortunately, their presence produces suboptimal chemotherapeutic outcomes in cancer patient tumor cells. It is well established that they actively efflux antineoplastic agents from cancer cells, producing the multidrug resistance (MDR) phenotype. The inadequate response to chemotherapy and subsequent poor prognosis in cancer patients can be in part the result of the overexpression of ABC transporters. In fact, one of the targeted approaches for overcoming MDR in cancer cells is that directed towards blocking or inhibiting ABC transporters. Indeed, for almost three decades, research has been conducted to overcome MDR through pharmacological inhibition of ABC transporters with limited clinical success. Therefore, contemporary strategies to identify or to synthesize selective “resensitizers” of ABC transporters with limited nonspecific toxicity have been undertaken. Innovative approaches en route to understanding specific biochemical roles of ABC transporters in MDR and tumorigenesis will prove essential to direct our knowledge towards more effective targeted therapies. This review briefly discusses the current knowledge regarding the clinical involvement of ABC transporters in MDR to antineoplastic drugs and highlights approaches undertaken so far to overcome ABC transporter-mediated MDR in cancer.

ABCG2, or breast cancer resistance protein (BCRP), is an ATP-binding cassette half-transporter that has been shown to transport a wide range of substrates including chemotherapeutics, antivirals, antibiotics and flavonoids. Given its wide range of substrates, much work has been dedicated to developing ABCG2 as a clinical target. But where can we intervene clinically and how can we avoid the mistakes made in past clinical trials targeting P-glycoprotein? This review will summarize the normal tissue distribution, cancer tissue expression, substrates and inhibitors of ABCG2, and highlight the challenges presented in exploiting ABCG2 in the clinic. We discuss the possibility of inhibiting ABCG2, so as to increase oral bioavailability or increase drug penetration into sanctuary sites, especially the central nervous system; and at the other end of the spectrum, the possibility of improving ABCG2 function, in the case of gout caused by a single nucleotide polymphism. Together, these aspects of ABCG2/BCRP make the protein a target of continuing interest for oncologists, biologists, and pharmacologists.

Multidrug resistance caused by the overexpression of ABC drug transporters is a major obstacle in clinical cancer chemotherapy. For several years, it appeared that direct inhibition of ABC transporters would be the cheapest and most efficient way to combat this problem. Unfortunately, progress in finding a potent, selective inhibitor to modulate ABC transporters and restore drug sensitivity in multidrug-resistant cancer cells has been slow and challenging. Candidate drugs should ideally be selective, potent and relatively non-toxic. Many researchers in recent years have turned their attention to utilizing natural products as the building blocks for the development of the next generation of inhibitors, especially after the disappointing results obtained from inhibitors of the first three generations at the clinical trial stage. The first step is to discover natural substances (distinct from the first three generation inhibitors) that are potent, selective and relatively non-toxic in order to be used clinically. Here, we present a brief overview of the prospect of using natural products to modulate the function of ABC drug transporters clinically and their impact on human physiology and pharmacology.

ATP-binding cassette (ABC) transporters are multispanning membrane proteins that utilize ATP to move a broad range of substrates across cellular membranes. ABC transporters are involved in a number of human disorders and diseases [1]. Overexpression of a subset of the transporters has been closely linked to multidrug resistance in both bacteria and viruses and in cancer. A poorly understood and important aspect of ABC transporter biology is the role of phosphorylation as a mechanism to regulate transporter function. In this review, we summarize the current literature addressing the role of phosphorylation in regulating ABC transporter function. A comprehensive list of all the phosphorylation sites that have been identified for the human ABC transporters is presented, and we discuss the role of individual kinases in regulating transporter function. We address the potential pitfalls and difficulties associated with identifying phosphorylation sites and the corresponding kinase(s), and we discuss novel techniques that may circumvent these problems. We conclude by providing a brief perspective on studying ABC transporter phosphorylation.

The biliary secretion of bile acids is critical for multiple liver functions including digesting fatty nutrients and driving bile flow. When this process is impaired, accumulating bile acids cause inflammatory liver injury. Multiple ABC transporters in the liver are key players to safeguard the hepatocyte and avoid toxicity due to bile acid over-accumulation. BSEP provides for efficient secretion of bile acids across the canalicular membrane against a steep concentration gradient. MDR3/Mdr2 and ABCG5/G8 secrete phosphatidylcholine and cholesterol, respectively, in coordination with BSEPmediated bile acid secretion to mask the detergent/toxic effects of bile acids in the bile ductular space. Several lines of evidence indicate that when these critical steps are compromised, bile acid toxicity in vivo leads to inflammatory liver injury and liver cancer. In bsep-/- mice, liver cancer is rare. These mice display greatly increased expression of alternative bile acid transporters, such as Mdr1a/1b, Mrp3 and Mrp4. We believe these alternative transport systems provide an additional safeguard to avoid bile acid overload in liver. Such backup systems appear to be under-utilized in humans, as defects in BSEP and MDR3 lead to severe, often fatal childhood diseases. It is possible, therefore, that targeting ABC transporters and modulating the toxicity of the bile acid pool could be vital interventions to alleviate chronic inflammation and reduce the incidence of liver cancer in high-risk populations. The combination of an alternative ABC transporter with a novel substrate may prove an effective chemo-preventive or therapeutic strategy.

The porphyrins (such as heme) are essential molecules within cells and have multiple roles in essential cellular processes such as: the mitochondrial electron transport chain, free-radical detoxification, and metabolism. The porphyrins need energy to traverse biological membranes. Our understanding of ABC transporters role in regulating intracellular porphyrin homeostasis is only now beginning to be understood. Two important contributors are members of the ABC transporter gene family: ABCB6 and ABCG2. ABCB6 is the first ABC transporter located in the outer mitochondrial membrane and oriented to facilitate porphyrin import. Consequently, ABCB6 can regulate and appropriately orchestrate porphyrin synthesis. This leads to an ability to regulate the amount of heme associated with heme requiring proteins. This ability can facilitate a cells protective response to an array of toxic insults. ABCG2 also binds and transports porphyrins, however its location at the plasma membrane provides a mechanism to remove excess porphyrins. Because ABCG2 is upregulated by hypoxia this provides a mechanism to export porphyrins, rebalance porphyrins and protect cells from porphyrin overaccumulation. Such a mechanism would be important to hypoxic cells which exhibit an increase in porphyrin synthesis under hypoxic conditions. Finally, we propose that these two transporters (ABCB6 and ABCG2) are coordinately regulated to modulate porphyrin concentrations under normal physiological and pathological conditions.

In the present review we provide a summary of ATP-binding cassette (ABC) transporters in the central nervous system (CNS). Our review is focused on transporters of the ABC A, B, C, D, and G families that have been detected in the cells of the neurovascular unit/blood-brain barrier including brain capillary endothelial cells, pericytes, astrocytes, and neurons, as well as in other brain cells, such as microglia, oligodendrocytes, and choroid plexus epithelial cells. In this review, we provide an overview, organized by ABC family, of transporter expression, localization, and function. We summarize recent findings on ABC transporter regulation in the CNS and address the role of ABC transporters in CNS diseases including brain cancer, seizures/epilepsy, and Alzheimer's disease. Finally, we discuss new therapeutic strategies focused on ABC transporters in CNS disease.

Pregnant women are often complicated with diseases including viral or bacterial infections, epilepsy, hypertension, or pregnancy-induced conditions such as depression and gestational diabetes that require treatment with medication. In addition, substance abuse during pregnancy remains a major public health problem. Many drugs used by pregnant women are off label without the necessary dose, efficacy, and safety data required for rational dosing regimens of these drugs. Thus, a major concern arising from the widespread use of drugs by pregnant women is the transfer of drugs across the placental barrier, leading to potential toxicity to the developing fetus. Knowledge regarding the ATP-binding cassette (ABC) efflux transporters, which play an important role in drug transfer across the placental barrier, is absolutely critical for optimizing the therapeutic strategy to treat the mother while protecting the fetus during pregnancy. Such transporters include P-glycoprotein (P-gp, gene symbol ABCB1), the breast cancer resistance protein (BCRP, gene symbol ABCG2), and the multidrug resistance proteins (MRPs, gene symbol ABCCs). In this review, we summarize the current knowledge with respect to developmental expression and regulation, membrane localization, functional significance, and genetic polymorphisms of these ABC transporters in the placenta and their relevance to fetal drug exposure and toxicity.

ATP-Binding Cassette (ABC) transporters are important mediators of multidrug resistance (MDR) in patients with cancer. Although their role in MDR has been extensively studied in vitro, their value in predicting response to chemotherapy has yet to be fully determined. Establishing a molecular diagnostic assay dedicated to the quantitation of ABC transporter genes is therefore critical to investigate their involvement in clinical MDR. In this article, we provide an overview of the methodologies that have been applied to analyze the mRNA expression levels of ABC transporters, by describing the technology, its pros and cons, and the experimental protocols that have been followed. We also discuss recent studies performed in our laboratory that assess the ability of the currently available high-throughput gene expression profiling platforms to discriminate between highly homologous genes. This work led to the conclusion that high-throughput TaqMan-based qRT-PCR platforms provide standardized clinical assays for the molecular detection of ABC transporters and other families of highly homologous MDR-linked genes encoding, for example, the uptake transporters (solute carriers- SLCs) and the phase I and II metabolism enzymes.

Genetic polymorphisms and mutations in drug metabolizing enzymes, transporters, receptors, and other drug targets (e.g., toxicity targets) are linked to inter-individual differences in the efficacy and toxicity of many medications as well as risk of genetic diseases. Validation of clinically important genetic polymorphisms and the development of new technologies to rapidly detect clinically important variants are critical issues for advancing personalized medicine. A key requirement for the advancing personalized medicine resides in the ability of rapidly and conveniently testing patients' genetic polymorphisms and/or mutations. We have recently developed a rapid and cost-effective method, named Smart Amplification Process 2 (SmartAmp2), which enables us to detect genetic polymorphisms or mutations in target genes within 30 to 45 min under isothermal conditions without DNA isolation and PCR amplification. Detection of mutations or single nucleotide polymorphisms (SNPs) in human ABC transporter genes is becoming more important, since their functional impairments are reportedly associated with inherited diseases. Thus, certain genetic polymorphisms of ABC transporters are considered important biomarkers for diagnosis of inherited diseases and/or risk of drug-induced adverse reactions. In this review article, we will present the new technology of the SmartAmp2 method and its clinical applications for detection of SNPs in human ABC transporter genes, i.e., ABCC4 and ABCC11.