Pharmacogenomics of MRP transporters (ABCC1-5) and BCRP (ABCG2)

Association between genetic variants of membrane transporters and the risk of high-grade hematologic adverse events in a cohort of Mexican children with B-cell acute lymphoblastic leukemia

Background

Advances in the understanding of the pathobiology of childhood B-cell acute lymphoblastic leukemia (B-ALL) have led towards risk-oriented treatment regimens and markedly improved survival rates. However, treatment-related toxicities remain a major cause of mortality in developing countries. One of the most common adverse effects of chemotherapy in B-ALL is the hematologic toxicity, which may be related to genetic variants in membrane transporters that are critical for drug absorption, distribution, and elimination. In this study we detected genetic variants present in a selected group genes of the ABC and SLC families that are associated with the risk of high-grade hematologic adverse events due to chemotherapy treatment in a group of Mexican children with B-ALL.

Methods

Next generation sequencing (NGS) was used to screen six genes of the ABC and seven genes of the SLC transporter families, in a cohort of 96 children with B-ALL. The grade of hematologic toxicity was classified according to the National Cancer Institute's Common Terminology Criteria for Adverse Events (CTCAE) version 5.0, Subsequently, two groups of patients were formed: the null/low-grade (grades 1 and 2) and the high-grade (grades 3 to 5) adverse events groups. To determine whether there is an association between the genetic variants and high-grade hematologic adverse events, logistic regression analyses were performed using co-dominant, dominant, recessive, overdominant and log-additive inheritance models. Odds ratio (OR) and 95% confidence intervals (95% CI) were calculated.

Results

We found two types of associations among the genetic variants identified as possible predictor factors of hematologic toxicity. One group of variants associated with high-grade toxicity risk: ABCC1 rs129081; ABCC4 rs227409; ABCC5 rs939338, rs1132776, rs3749442, rs4148575, rs4148579 and rs4148580; and another group of protective variants that includes ABCC1 rs212087 and rs212090; SLC22A6 rs4149170, rs4149171 and rs955434.

Conclusion

There are genetic variants in the SLC and ABC transporter families present in Mexican children with B-ALL that can be considered as potential risk markers for hematologic toxicity secondary to chemotherapeutic treatment, as well as other protective variants that may be useful in addition to conventional risk stratification for therapeutic decision making in these highly vulnerable patients.

Interplay of Breast Cancer Resistance Protein (Bcrp/Abcg2), Sex, and Fed State in Oral Pharmacokinetic Variability of Furosemide in Rats

Poor and variable oral bioavailability of furosemide (FUR) presents critical challenges in pharmacotherapy. We investigated the interplay of breast cancer resistance protein (Bcrp)-mediated transport, sex, and fed state on FUR pharmacokinetics (PK) in rats. A crossover PK study of FUR (5 mg/kg, oral) was performed in Sprague-Dawley rats (3 males and 3 females), alone or with a Bcrp inhibitor, novobiocin (NOV) (20 mg/kg, oral), in both fed and fasted states. Co-administration of NOV significantly increased FUR extent (AUC) and rate (C_max) of exposure by more than two-fold, which indicates efficient Bcrp inhibition in the intestine. The female rats showed two-fold higher AUC and C_max, and two-fold lower renal clearance of FUR compared to the male rats. The latter was correlated with higher renal abundance of Bcrp and organic anion transporters (Oats) in the male rats compared to age-matched female rats. These findings suggest that the PK of Bcrp and/or Oat substrates could be sex-dependent in rats. Moreover, allometric scaling of rat PK and toxicological data of Bcrp substrates should consider species and sex differences in Bcrp and Oat abundance in the kidney. Considering that Bcrp is abundant in the intestine of rats and humans, a prospective clinical study is warranted to evaluate the effect of Bcrp inhibition on FUR PK. The potential confounding effect of the Bcrp transporter should be considered when FUR is used as a clinical probe of renal organic anion transporter-mediated drug-drug interactions. Unlike human data, no food-effect was observed on FUR PK in rats.

An association study of ABCG2 rs2231142 on the concentrations of allopurinol and its metabolites

ABCG2 is a gene that codes for the human breast cancer resistance protein (BCRP). It is established that rs2231142 G>T, a single nucleotide polymorphism of the ABCG2 gene, is associated with gout and poor response to allopurinol, a uric acid‐lowering agent used to treat this condition. It has also been suggested that oxypurinol, the primary active metabolite of allopurinol, is a substrate of the BCRP. We thus hypothesized that carrying the rs2231142 variant would be associated with decreased oxypurinol concentrations, which would explain the lower reduction in uric acid. We performed a cross‐sectional study to investigate the association between the ABCG2 rs2231142 variant and oxypurinol, allopurinol, and allopurinol riboside concentrations in 459 participants from the Montreal Heart Institute Hospital Cohort. Age, sex, weight, use of diuretics, and estimated glomerular filtration rate were all significantly associated with oxypurinol plasma concentration. No association was found between rs2231142 and oxypurinol, allopurinol and allopurinol riboside plasma concentrations. Rs2231142 was not significantly associated with daily allopurinol dose in the overall population, but an association was observed in men, with T carriers receiving higher doses. Our results do not support a major role of ABCG2 in the pharmacokinetics of allopurinol or its metabolites. The underlying mechanism of the association between rs2231142 and allopurinol efficacy requires further investigation.

ATP-Binding Cassette Transporters: Snap-on Complexes?

ATP-binding cassette (ABC) transporters are one of the largest families of membrane proteins in prokaryotic organisms. Much is now understood about the structure of these transporters and many reviews have been written on that subject. In contrast, less has been written on the assembly of ABC transporter complexes and this will be a major focus of this book chapter. The complexes are formed from two cytoplasmic subunits that are highly conserved (in terms of their primary and three-dimensional structures) across the whole family. These ATP-binding subunits give rise to the name of the family. They must assemble with two transmembrane subunits that will typically form the permease component of the transporter. The transmembrane subunits have been found to be surprisingly diverse in structure when the whole family is examined, with seven distinct folds identified so far. Hence nucleotide-binding subunits appear to have been bolted on to a variety of transmembrane platforms during evolution, leading to a greater variety in function. Furthermore, many importers within the family utilise a further external substrate-binding component to trap scarce substrates and deliver them to the correct permease components. In this chapter, we will discuss whether assembly of the various ABC transporter subunits occurs with high fidelity within the crowded cellular environment and whether promiscuity in assembly of transmembrane and cytoplasmic components can occur. We also discuss the new AlphaFold protein structure prediction tool which predicts a new type of transmembrane domain fold within the ABC transporters that is associated with cation exporters of bacteria and plants.

Recent Advancements in Nanodiamond Mediated Brain Targeted Drug Delivery and Bioimaging of Brain Ailments: A Holistic Review

BACKGROUND

The brain is a vital and composite organ. By nature, the innate make-up of the brain is such that in anatomical parlance, it is highly protected by the "Blood-Brain Barrier", which is a nexus of capillary endothelial cells, basement membrane, neuroglial membrane and glialpodocytes. The same barrier, which protects and isolates the interstitial fluid of the brain from capillary circulation, also restricts the therapeutic intervention. Many standing pharmaceutical formulations are ineffective in the treatment of inimical brain ailments because of the inability of the API to surpass and subsist inside the Blood Brain Barrier.

OBJECTIVE

This is an integrated review that emphasizes on the recent advancements in brain-targeted drug delivery utilizing nanodiamonds (NDs) as a carrier of therapeutic agents. NDs are a novel nanoparticulate drug delivery system, having carbon moieties as their building blocks and their surface tenability is remarkable. These neoteric carbon-based carriers have exceptional, mechanical, electrical, chemical, optical, and biological properties, which can be further rationally modified and augmented.

CONCLUSION

NDs could be the next"revolution "in the field of nanoscience for the treatment of neurodegenerative disorders, brain tumors, and other pernicious brain ailments. What sets them apart from other nanocarriers is their versatile properties like diverse size range and surface modification potential, which makes them efficient enough to move across certain biological barriers and offer a plethora of brain targeting and bioimaging abilities. Lay Summary: The blood-brain barrier (BBB) poses a major hurdle in the way of treating many serious brain ailments. A range of nanoparticle based drug delivering systems have been formulated, including solid lipid nanoparticles, liposomes, dendrimers, nanogels, polymeric NPs, metallic NPs (gold, platinum, andironoxide) and diamondoids (carbonnanotubes). Despite this development, only a few of these formulations have shown the ability to cross the BBB. Nanodiamonds, because of their small size, shape, and surface characteristics, have a potential in moving beyond the diverse and intricate BBB, and offer a plethora of brain targeting capabilities.

Drug Resistance in Metastatic Breast Cancer: Tumor Targeted Nanomedicine to the Rescue

Breast cancer, specifically metastatic breast, is a leading cause of morbidity and mortality in women. This is mainly due to relapse and reoccurrence of tumor. The primary reason for cancer relapse is the development of multidrug resistance (MDR) hampering the treatment and prognosis. MDR can occur due to a multitude of molecular events, including increased expression of efflux transporters such as P-gp, BCRP, or MRP1; epithelial to mesenchymal transition; and resistance development in breast cancer stem cells. Excessive dose dumping in chemotherapy can cause intrinsic anti-cancer MDR to appear prior to chemotherapy and after the treatment. Hence, novel targeted nanomedicines encapsulating chemotherapeutics and gene therapy products may assist to overcome cancer drug resistance. Targeted nanomedicines offer innovative strategies to overcome the limitations of conventional chemotherapy while permitting enhanced selectivity to cancer cells. Targeted nanotheranostics permit targeted drug release, precise breast cancer diagnosis, and importantly, the ability to overcome MDR. The article discusses various nanomedicines designed to selectively target breast cancer, triple negative breast cancer, and breast cancer stem cells. In addition, the review discusses recent approaches, including combination nanoparticles (NPs), theranostic NPs, and stimuli sensitive or “smart” NPs. Recent innovations in microRNA NPs and personalized medicine NPs are also discussed. Future perspective research for complex targeted and multi-stage responsive nanomedicines for metastatic breast cancer is discussed.

Personalised medicine in hypercholesterolaemia: the role of pharmacogenetics in statin therapy

Statins are the first-line choice in Lipid-lowering therapy to reduce cardiovascular risk. In a continuous attempt to optimise treatment success, there is a need for additional research on genes and related molecular pathways that can determine the efficacy and toxicity of lipid-lowering drugs. Several variations within genes associated with lipid metabolism, including those involved in uptake, distribution and metabolism of statins have been reported. The purpose of this study was to evaluate the effect of genetic variations in the key genes responsible for statins' metabolism and their role in personalised medicine and pharmacogenetic testing (PGx) in patients treated with such drugs. Genetic assessment for specific known SNPs within the most known genes such as ABCG2, SLCO1B1, CYP3A4, and HMGCR, appears likely to predict the efficacy of statin therapy and prevent their side effects but does not necessarily reduce the risk of cardiovascular events. Key Messages Hypercholesterolaemia patients show different response to statin therapy. Several variations within genes associated with statin metabolism have been investigated. Genetic assessment for specific known SNPs within the most known genes may improve the efficacy of statins treatment and prevent their side effects.

Comprehensive Analysis of ABCG2 Genetic Variation in the Polish Population and Its Inter-Population Comparison

ATP-binding cassette sub-family G member 2 (ABCG2), also known as breast cancer resistance protein (BCRP), is one of the key efflux ATP-binding cassette (ABC) transporters of xenobiotics, their metabolites and endogenous compounds such as urate. Some of its genetic variants have been found to influence protein functioning, resulting in serious clinical implications concerning chemotherapy response, as well as gout or blood group phenotype Jr(a-). Previous reports have suggested that the frequencies of certain crucial polymorphisms, such as c.34G>A (p.Val12Met) and c.421C>A (p.Gln141Lys) differ significantly between the Polish population and other Caucasian populations. Thus, to clarify this issue, the present study performs a complete analysis of the genetic variation of ABCG2 coding sequence in the Polish population. The genetic variation in 14 out of 15 coding exons of the ABCG2 gene, as well as their flanking intron sequences, were examined among 190 healthy representatives of the Polish population using scanning with High Resolution Melting (HRM). HRM scanning revealed 17 polymorphisms: eight in the exons (including five missense variants and one point-nonsense mutation) and nine in the intron sequences (eight single nucleotide polymorphisms (SNPs) and one deletion variant). These included variants correlating with the presence of gout and phenotype Jr(a-). Linkage disequilibrium, haplotype blocks and haplotype analyses were also performed. The frequencies of the most common polymorphisms in the Polish population did not differ significantly to those observed for other Caucasian populations, but demonstrated divergence from non-Caucasian populations. We hope that our findings may be helpful for other researchers and clinicians, evaluating the pharmacogenetic role of ABCG2.

Influence of OATP1B1 and BCRP polymorphisms on the pharmacokinetics and pharmacodynamics of rosuvastatin in elderly and young Korean subjects

A lack of information regarding whether genetic polymorphisms of SLCO1B1 and ABCG2 affect the pharmacokinetics (PKs)/pharmacodynamics (PDs) of rosuvastatin in elderly subjects prevents optimal individualized pharmacotherapy of rosuvastatin in clinical settings. This study aimed to investigate the effect of age and genetic polymorphisms and possible differences in genetic effects on the PKs/PDs of rosuvastatin between elderly and young subjects. Two separate clinical studies designed as open-label, one-sequence studies with multiple-dose administration for elderly (n = 20) and young (n = 32) subjects were conducted. All subjects received 20 mg of rosuvastatin once daily for 21 days. The exposure to rosuvastatin, characterized by the area under the time curve (AUC), increased by 23% in the elderly subjects compared with that of young subjects, which was not significant. When compared to the subjects with breast cancer resistance protein (BCRP) normal function, the exposure to rosuvastatin increased by 44% in young subjects (p = 0.0021) with BCRP intermediate function (IF) and by 35% and 59% (p > 0.05 for both) in elderly subjects with BCRP IF and low function, respectively. SLCO1B1 521T > C was also partially associated with a higher AUC of rosuvastatin in young subjects and a less pronounced increasing trend in elderly subjects (p > 0.05 for both). The lipid-lowering effect of rosuvastatin was less pronounced in the elderly subjects than in the young subjects, and genetic polymorphisms of neither SLCO1B1 nor ABCG2 significantly affected the PDs of rosuvastatin. The ABCG2 421C > A polymorphism was associated with the PKs of rosuvastatin and was identified as a more important determinant than the SLCO1B1 521T > C polymorphism in both elderly and young subjects.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

Maternal western-style diet enhances the effects of chemically-induced mammary tumors in female rat offspring through transcriptome changes

Previous studies have shown that early life intake of high-fat diet or western-style diet (WD) enhances the development of mammary tumors in adult female rats. Thus, we hypothesized that maternal WD throughout pregnancy and the lactation period could speed up the development of MNU-induced mammary tumors and alter their gene expression. For this, the present study investigated the gene expression profile of chemically-induced mammary tumors in female rat offspring from dams fed a WD or a control diet. Pregnant female Sprague-Dawley rats received a WD (high-fat, low-fiber and oligoelements) or a control diet from gestational day 12 until post-natal day (PND) 21. At PND 21, female offspring received a single dose of N-Methyl-N-Nitrosourea (MNU, 50 mg/kg body weight) and were fed a control diet for 13 weeks. Tumor incidence, multiplicity, and latency were recorded and mammary gland samples were collected for histopathology and gene expression analysis. Tumor multiplicity and histological grade were significantly higher and tumor latency was lower in WD offspring compared to control offspring. Transcriptome profiling identified 57 differentially expressed genes in tumors from WD offspring as compared to control offspring. There was also an increase in mRNA expression of genes such as Emp3, Ccl7, Ets1, Abcc5, and Cyr61, indicative of more aggressive disease detected in tumors from WD offspring. Thus, maternal WD diet increased MNU-induced mammary carcinogenesis in adult female offspring through transcriptome changes that resulted in a more aggressive disease.

Development and Uses of Offline and Web-Searchable Metabolism Databases - The Case of Benzo[a]pyrene

BACKGROUND

The present work describes development of offline and web-searchable metabolism databases for drugs, other chemicals, and physiological compounds using human and model species, prompted by the large amount of data published after year 1990. The intent was to provide a rapid and accurate approach to published data to be applied both in science and to assist therapy.

METHODS

Searches for the data were done using the Pub Med database, accessing the Medline database of references and abstracts. In addition, data presented at scientific conferences (e.g., ISSX conferences) are included covering the publishing period beginning with the year 1976.

RESULTS

Application of the data is illustrated by the properties of benzo[a]pyrene (B[a]P) and its metabolites. Analysis show higher activity of P450 1A1 for activation of the (-)- isomer of trans-B[a]P-7,8-diol, while P4501B1 exerts higher activity for the (+)- isomer. P450 1A2 showed equally low activity in the metabolic activation of both isomers.

CONCLUSION

The information collected in the databases is applicable in prediction of metabolic drug-drug and/or drug-chemical interactions in clinical and environmental studies. The data on the metabolism of searched compound (exemplified by benzo[a]pyrene and its metabolites) also indicate toxicological properties of the products of specific reactions. The offline and web-searchable databases had wide range of applications (e.g. computer assisted drug design and development, optimization of clinical therapy, toxicological applications) and adjustment in everyday life styles.

Bile Acids Increase Doxorubicin Sensitivity in ABCC1-expressing Tumour Cells

Tumour cells possess or acquire various mechanisms to circumvent the cytotoxic effects of chemotherapy drugs. One such mechanism involves the overexpression of ABC transporters that facilitate the extrusion of a variety of structurally distinct chemotherapy drugs from the cytoplasm into the extracellular space. While specific ABC transporter inhibitors have been developed, many affect other ABC transporters, particularly at elevated concentrations. It is also unclear whether they show clear efficacy for combatting drug resistance in cancer patients with minimal host toxicity. In this study, we demonstrate the ability of two bile acids [β-cholanic acid (urso-cholanic acid) and deoxycholic acid] to specifically inhibit ABCC1-mediated drug transport, augmenting doxorubicin accumulation in breast and lung tumour cells selected for doxorubicin resistance through overexpression of the ABCC1 (but not ABCB1) drug transporter. The bile acids could also restore uptake and sensitivity to doxorubicin in human endothelial kidney cells genetically engineered to overexpress the ABCC1 drug transporter. These observations suggest a previously unreported role for bile acids as ABCC1 inhibitors or regulators. Given its additional properties of minimal clinical toxicity in humans and its ability to inhibit aldo-keto reductases involved in anthracycline resistance and anthracycline-induced cardiotoxicity, β-cholanic acid merits further in vivo and clinical investigation.

ABC-transporter blockage mediated by xanthotoxin and bergapten is the major pathway for chemosensitization of multidrug-resistant cancer cells

Furanocoumarins derived from herbal and citrus extracts can act as antibacterial, antioxidant, immunomodulator, apoptotic, and selective anticancer agents, prompting a biological investigation to determine and predict their clinical therapeutic significance. Here, the cell cytotoxic effects of bergapten and xanthotoxin were analyzed alone and in combination with standard chemotherapeutics on three multidrug resistant cells and their nonresistant parental counterparts. The furanocoumarins modulatory effects on MDR1, BCRP, and MRP pump expression and function were investigated. Although quantitative real time PCR demonstrated that the MDR transcript level changes in a time dependent manner, flow cytometric analyses using fluorescent-labeled antibodies have indicated that bergapten and xanthotoxin had no significant effect on the protein levels. FACS analyses indicated that these prominent anticancer agents significantly blocked MDR1, BCRP, and MRP transporter function. Maximum furanocoumarin-mediated pump activity blockage in the MDR-resistant cells was quantified as 87% of normal and consequently, chemotherapeutic accumulation increased up to 2.7-fold and cytotoxicity tension increased 104-fold. MDR1 efflux kinetics also revealed that the maximum velocity and the pump affinity to daunorubicin were uncompetitively decreased. We conclude that bergapten and xanthotoxin are cytotoxic agents capable of preventing daunorubicin, mitoxantrone, and cisplatin binding to ABC-transporters and subsequently inhibiting their efflux out of cells and they may be a potential combination therapy for malignant cancers.

Dubin-Johnson syndrome and intrahepatic cholestasis of pregnancy in a Sri Lankan family: a case report

Background

Dubin–Johnson syndrome and intrahepatic cholestasis of pregnancy are rare chronic liver disorders. Dubin–Johnson syndrome may manifest as conjugated hyperbilirubinemia, darkly pigmented liver, presence of abnormal pigment in the parenchyma of hepatocytes and abnormal distribution of the coproporphyrin isomers I and III in the urine. Intrahepatic cholestatic jaundice of pregnancy presents as pruritus, abnormal liver biochemistry and increased serum bile acids.

Case presentation

A Sri Lankan girl presented with recurrent episodes of jaundice. She had conjugated hyperbilirubinaemia with diffuse, coarse brown pigments in the hepatocytes. Urine coproporphyrin examination suggested Dubin–Johnson syndrome. Genetic studies confirmed missense homozygous variant p.Trp709Arg in the ATP-binding cassette sub-family C member 2 gene ABCC2 that encodes the Multidrug resistance-associated protein 2 that causes Dubin–Johnson syndrome. The gene study of the mother revealed the same missense variant in ABCC2/MRP2 but with a heterozygous status, and in addition a homozygous missense variant p.Val444Ala in the ATP-binding cassette, sub-family B member 11 gene ABCB11 that encodes the bile salt export pump.

Conclusion

Dubin–Johnson syndrome should be considered when the common causes for conjugated hyperbilirubinaemia have been excluded, and patient has an increased percentage of direct bilirubin relative to total bilirubin concentration. Its early diagnosis prevents repeated hospital admissions and investigations. Knowledge of a well known homozygous variant in ABCB11 gene could help in the management of pregnancy.

Pharmacogenomics of Drug Metabolizing Enzymes and Transporters: Relevance to Precision Medicine

The interindividual genetic variations in drug metabolizing enzymes and transporters influence the efficacy and toxicity of numerous drugs. As a fundamental element in precision medicine, pharmacogenomics, the study of responses of individuals to medication based on their genomic information, enables the evaluation of some specific genetic variants responsible for an individual’s particular drug response. In this article, we review the contributions of genetic polymorphisms to major individual variations in drug pharmacotherapy, focusing specifically on the pharmacogenomics of phase-I drug metabolizing enzymes and transporters. Substantial frequency differences in key variants of drug metabolizing enzymes and transporters, as well as their possible functional consequences, have also been discussed across geographic regions. The current effort illustrates the common presence of variability in drug responses among individuals and across all geographic regions. This information will aid health-care professionals in prescribing the most appropriate treatment aimed at achieving the best possible beneficial outcomes while avoiding unwanted effects for a particular patient.

Selective speciation improves efficacy and lowers toxicity of platinum anticancer and vanadium antidiabetic drugs

Improving efficacy and lowering resistance to metal-based drugs can be addressed by consideration of the coordination complex speciation and key reactions important to vanadium antidiabetic drugs or platinum anticancer drugs under biological conditions. The methods of analyses vary depending on the specific metal ion chemistry. The vanadium compounds interconvert readily, whereas the reactions of the platinum compounds are much slower and thus much easier to study. However, the vanadium species are readily differentiated due to vanadium complexes differing in color. For both vanadium and platinum systems, understanding the processes as the compounds, Lipoplatin and Satraplatin, enter cells is needed to better combat the disease; there are many cellular metabolites, which may affect processing and thus the efficacy of the drugs. Examples of two formulations of platinum compounds illustrate how changing the chemistry of the platinum will result in less toxic and better tolerated drugs. The consequence of the much lower toxicity of the drug, can be readily realized because cisplatin administration requires hospital stay whereas Lipoplatin can be done in an outpatient manner. Similarly, the properties of Satraplatin allow for development of an oral drug. These forms of platinum demonstrate that the direct consequence of more selective speciation is lower side effects and cheaper administration of the anticancer agent. Therefore we urge that as the community goes forward in development of new drugs, control of speciation chemistry will be considered as one of the key strategies in the future development of anticancer drugs.

A pharmacogenetic investigation of intravenous furosemide in decompensated heart failure: a meta-analysis of three clinical trials

We conducted a meta-analysis of pharmacogenomic substudies of three randomized trials conducted in patients with decompensated heart failure (HF) that were led by National Heart Lung and Blood Institute (NHLBI)-funded HF Network to test the hypothesis that candidate genes modulate net fluid loss and weight change in patients with decompensated HF treated with a furosemide-based diuretic regimen. Although none of the genetic variants previously shown to modulate the effects of loop diuretics in healthy individuals were associated with net fluid loss after 72 h of treatment, a set of rare variants in the APOL1 gene, which codes for apolipoprotein L1 (P=0.0005 in the random effects model), was associated with this end point. Moreover, a common variant in the multidrug resistance protein-4 coding gene (ABCC4, rs17268282) was associated with weight loss with furosemide use (P=0.0001). Our results suggest that both common and rare genetic variants modulate the response to a furosemide-based diuretic regimen in patients with decompensated HF.

Effect of MRP2 and MRP3 Polymorphisms on Anastrozole Glucuronidation and MRP2 and MRP3 Gene Expression in Normal Liver Samples

Anastrozole is an aromatase inhibitor (AI) used as adjuvant therapy for breast cancer. Anastrozole is subject to direct glucuronidation catalyzed by UDP-glucuronosyltransferase1A4 (UGT1A4). Interindividual variability in anastrozole glucuronidation may be affected by UGT1A4 SNPs. Interplay between drug metabolizing genes such as UGT1A4 and transporter genes may also be affected by genetic variability. Thus, we hypothesize that genetic variability in MRPs could influence anastrozole glucuronidation. The correlation between UGT1A4 and MRP2 or MRP3 transporter gene expressions and the correlation between MRP2 or MRP3 mRNA and anastrozole glucuronidation were analyzed in normal human liver samples. MRP2 and MRP3 mRNA levels were significantly correlated with UGT1A4 mRNA, with anastrozole glucuronidation and with each other (p<0.05). The data also demonstrated that MRP2 SNPs are positively correlated with MRP2 mRNA expression, while there was no association between MRP3 SNPs from this study and MRP3 expression. Significant correlations (p<0.05) between certain MRP2 SNPs (3972C>T, 2366C>T and -24C>T) and anastrozole glucuronidation were observed. There were no observed correlations between MRP3 SNPs and anastrozole glucuronidation. MRP2 polymorphisms have been identified as playing a role in the disposition of other drugs, and the data presented here indicate for the first time that MRP2 SNPs could influence anastrozole metabolism and contribute to interindividual variation in treatment responses.

Marked Alteration of Rosuvastatin Pharmacokinetics in Healthy Chinese with ABCG2 34G>A and 421C>A Homozygote or Compound Heterozygote

Rosuvastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor used to lower blood low-density lipoprotein cholesterol, is a substrate of the membrane ABCG2 exporter. ABCG2 variants have been shown to alter rosuvastatin disposition. The objective of this study is to determine the impact of ABCG2 34/421 compound haplotypes on rosuvastatin pharmacokinetics in healthy Chinese volunteer subjects. Eight hundred healthy Chinese males were genotyped by polymerase chain reaction-pyrosequencing for ABCG2 34G>A, ABCG2 421C>A, SLCO1B1 521T>C, and CYP2C9*3 variants. Sixty-two male subjects with wild-type SLCO1B1 c.521TT and CYP2C9*3 were recruited for this pharmacokinetic study of rosuvastatin. A single oral dose of 10 mg rosuvastatin was administrated to each subject, and blood samples were collected before and at various time points after drug administration. Plasma concentration of rosuvastatin was determined by high-performance liquid chromatography-tandem mass spectrometry, and pharmacokinetic analysis was carried out using the WinNonlin program. In Chinese males, high allele frequency of ABCG2 c.34G>A (0.275) and c.421C>A (0.282) was observed, resulting in a considerable portion (23.3%) of subjects being ABCG2 34/421 compound heterozygotes. Compared with subjects with ABCG2 wild-type (c.34GG/421CC), plasma rosuvastatin Cmax and area under the curve, AUC0-∞, were significantly higher, while the apparent oral clearance, CL/F, was significantly lower in subjects with c.34AA, c.421AA, and c.34GA/421CA genotypes. Both t1/2 and Tmax were similar among subjects with different genotypes. A high frequency of ABCG2 c.34G>A and c.421C>A variants was present in Chinese males, and the disposition of rosuvastatin was significantly affected by both variants. These data suggest that it is advisable to genotype these variants when prescribing rosuvastatin to Chinese subjects, leading to a precise dose for each individual.

Rosuvastatin pharmacokinetics and pharmacogenetics in Caucasian and Asian subjects residing in the United States

PURPOSE

Systemic exposure to rosuvastatin in Asian subjects living in Japan or Singapore is approximately twice that observed in Caucasian subjects in Western countries or in Singapore. This study was conducted to determine whether pharmacokinetic differences exist among the most populous Asian subgroups and Caucasian subjects in the USA.

METHOD

Rosuvastatin pharmacokinetics was studied in Chinese, Filipino, Asian-Indian, Korean, Vietnamese, Japanese and Caucasian subjects residing in California. Plasma concentrations of rosuvastatin and metabolites after a single 20-mg dose were determined by mass spectrometric detection. The influence of polymorphisms in SLCO1B1 (T521>C [Val174Ala] and A388>G [Asn130Asp]) and in ABCG2 (C421>A [Gln141Lys]) on exposure to rosuvastatin was also assessed.

RESULTS

The average rosuvastatin area under the curve from time zero to time of last quantifiable concentration was between 64 and 84 % higher, and maximum drug concentration was between 70 and 98 % higher in East Asian subgroups compared with Caucasians. Data for Asian-Indians was intermediate to these two ethnic groups at 26 and 29 %, respectively. Similar increases in exposure to N-desmethyl rosuvastatin and rosuvastatin lactone were observed. Rosuvastatin exposure was higher in subjects carrying the SLCO1B1 521C allele compared with that in non-carriers of this allele. Similarly, exposure was higher in subjects carrying the ABCG2 421A allele compared with that in non-carriers.

CONCLUSION

Plasma exposure to rosuvastatin and its metabolites was significantly higher in Asian populations residing in the USA compared with Caucasian subjects living in the same environment. This study suggests that polymorphisms in the SLCO1B1 and ABCG2 genes contribute to the variability in rosuvastatin exposure.

Repositioning of Tyrosine Kinase Inhibitors as Antagonists of ATP-Binding Cassette Transporters in Anticancer Drug Resistance

The phenomenon of multidrug resistance (MDR) has attenuated the efficacy of anticancer drugs and the possibility of successful cancer chemotherapy. ATP-binding cassette (ABC) transporters play an essential role in mediating MDR in cancer cells by increasing efflux of drugs from cancer cells, hence reducing the intracellular accumulation of chemotherapeutic drugs. Interestingly, small-molecule tyrosine kinase inhibitors (TKIs), such as AST1306, lapatinib, linsitinib, masitinib, motesanib, nilotinib, telatinib and WHI-P154, have been found to have the capability to overcome anticancer drug resistance by inhibiting ABC transporters in recent years. This review will focus on some of the latest and clinical developments with ABC transporters, TKIs and anticancer drug resistance.

Pharmacogenetics research on chemotherapy resistance in colorectal cancer over the last 20 years

During the past two decades the first sequencing of the human genome was performed showing its high degree of inter-individual differentiation, as a result of large international research projects (Human Genome Project, the 1000 Genomes Project International HapMap Project, and Programs for Genomic Applications NHLBI-PGA). This period was also a time of intensive development of molecular biology techniques and enormous knowledge growth in the biology of cancer. For clinical use in the treatment of patients with colorectal cancer (CRC), in addition to fluoropyrimidines, another two new cytostatic drugs were allowed: irinotecan and oxaliplatin. Intensive research into new treatment regimens and a new generation of drugs used in targeted therapy has also been conducted. The last 20 years was a time of numerous in vitro and in vivo studies on the molecular basis of drug resistance. One of the most important factors limiting the effectiveness of chemotherapy is the primary and secondary resistance of cancer cells. Understanding the genetic factors and mechanisms that contribute to the lack of or low sensitivity of tumour tissue to cytostatics is a key element in the currently developing trend of personalized medicine. Scientists hope to increase the percentage of positive treatment response in CRC patients due to practical applications of pharmacogenetics/pharmacogenomics. Over the past 20 years the clinical usability of different predictive markers has been tested among which only a few have been confirmed to have high application potential. This review is a synthetic presentation of drug resistance in the context of CRC patient chemotherapy. The multifactorial nature and volume of the issues involved do not allow the author to present a comprehensive study on this subject in one review.

Phase 0 and phase III transport in various organs: combined concept of phases in xenobiotic transport and metabolism

The historical phasing concept of drug metabolism and elimination was introduced to comprise the two phases of metabolism: phase I metabolism for oxidations, reductions and hydrolyses, and phase II metabolism for synthesis. With this concept, biological membrane barriers obstructing the accessibility of metabolism sites in the cells for drugs were not considered. The concept of two phases was extended to a concept of four phases when drug transporters were detected that guided drugs and drug metabolites in and out of the cells. In particular, water soluble or charged drugs are virtually not able to overcome the phospholipid membrane barrier. Drug transporters belong to two main clusters of transporter families: the solute carrier (SLC) families and the ATP binding cassette (ABC) carriers. The ABC transporters comprise seven families with about 20 carriers involved in drug transport. All of them operate as pumps at the expense of ATP splitting. Embedded in the former phase concept, the term "phase III" was introduced by Ishikawa in 1992 for drug export by ABC efflux pumps. SLC comprise 52 families, from which many carriers are drug uptake transporters. Later on, this uptake process was referred to as the "phase 0 transport" of drugs. Transporters for xenobiotics in man and animal are most expressed in liver, but they are also present in extra-hepatic tissues such as in the kidney, the adrenal gland and lung. This review deals with the function of drug carriers in various organs and their impact on drug metabolism and elimination.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Clinical and pharmacogenetic predictors of circulating atorvastatin and rosuvastatin concentrations in routine clinical care

BACKGROUND

A barrier to statin therapy is myopathy associated with elevated systemic drug exposure. Our objective was to examine the association between clinical and pharmacogenetic variables and statin concentrations in patients.

METHODS AND RESULTS

In total, 299 patients taking atorvastatin or rosuvastatin were prospectively recruited at an outpatient referral center. The contribution of clinical variables and transporter gene polymorphisms to statin concentration was assessed using multiple linear regression. We observed 45-fold variation in statin concentration among patients taking the same dose. After adjustment for sex, age, body mass index, ethnicity, dose, and time from last dose, SLCO1B1 c.521T>C (P<0.001) and ABCG2 c.421C>A (P<0.01) were important to rosuvastatin concentration (adjusted R(2)=0.56 for the final model). Atorvastatin concentration was associated with SLCO1B1 c.388A>G (P<0.01) and c.521T>C (P<0.05) and 4β-hydroxycholesterol, a CYP3A activity marker (adjusted R(2)=0.47). A second cohort of 579 patients from primary and specialty care databases were retrospectively genotyped. In this cohort, genotypes associated with statin concentration were not differently distributed among dosing groups, implying providers had not yet optimized each patient's risk-benefit ratio. Nearly 50% of patients in routine practice taking the highest doses were predicted to have statin concentrations greater than the 90th percentile.

CONCLUSIONS

Interindividual variability in statin exposure in patients is associated with uptake and efflux transporter polymorphisms. An algorithm incorporating genomic and clinical variables to avoid high atorvastatin and rosuvastatin levels is described; further study will determine whether this approach reduces incidence of statin myopathy.

Effect of the ATP-binding cassette transporter ABCG2 on pharmacokinetics: experimental findings and clinical implications

INTRODUCTION

The ATP-binding cassette transporter ABCG2 can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. Thereby, ABCG2 limits oral drug bioavailability, mediates hepatobiliary and renal excretion and participates functionally in the blood-brain barrier.

AREAS COVERED

The paper provides a review of the clinical evidence of the role of ABCG2 in the bioavailability and brain disposition of drugs. It also sheds light on the value of experimental/preclinical data in predicting the role of ABCG2 in pharmacokinetics in humans.

EXPERT OPINION

Experimental studies indicate that ABCG2 may limit the oral bioavailability and brain penetration of many drugs. ABCG2 has also been recognized as an important determinant of the disposition of some drugs in humans. For example, loss-of-function variants of ABCG2 affect the pharmacokinetics and pharmacodynamics of rosuvastatin in a clinically significant manner. Moreover, clinically relevant pharmacokinetic drug-drug interactions have been attributed to ABCG2 inhibition. However, examples from human studies are still rare compared with the overwhelming evidence from experimental studies. The large degree of functional redundancy of ABCG2 with other transporters such as P-glycoprotein may explain the rare occurrence of ABCG2-dependent drug-drug interactions in humans. Providing clinicians with consolidated information on the clinically relevant interactions of drugs with ABCG2 remains a matter of future exploration.

Systemic exposure to atorvastatin between Asian and Caucasian subjects: a combined analysis of 22 studies

The aim of the present study was to determine whether there is a differing pattern of systemic exposure to atorvastatin in Asian versus Caucasian subjects by comparison of data obtained from completed pharmacokinetic studies. Pharmacokinetic data were analyzed from completed single-dose (10-80 mg) studies in Asian and Caucasian subjects. Dose normalized area under the concentration-time curve (AUC) and maximum observed concentration (Cmax) (AUC(dn) and Cmax(dn)) were obtained by dividing each value by the administered dose. Dose-per-bodyweight normalized AUC and Cmax (AUC(dn,wt) and Cmax,(dn,wt)) were obtained by dividing each value by the administered dose per unit bodyweight. Mean difference and 90% confidence intervals for Asian versus Caucasian comparisons were calculated for atorvastatin pharmacokinetic values based on the t statistic and expressed as ratios using Caucasians as the reference. Data were analyzed from 310 Asians and 579 Caucasians from 22 studies. AUC(dn) (Asian = 2.35, Caucasian = 2.06 [ng·hr·mL(-1)]/mg) and Cmax(dn) (Asian = 0.39, Caucasian = 0.40 Cmax(dn,wt)) and the equivalent dose-per-bodyweight normalized values for atorvastatin (AUC(dn,wt): Asian = 157.5, Caucasian = 156.4 [ng·hr·mL(-1)]/[mg·kg(-1)]; Cmax(dn,wt): Asian = 26.2, Caucasian = 30.3 [ng·mL(-1)]/[mg·kg(-1)]) were similar in both ethnic groups. Mean differences and 90% confidence interval for the differences fell within the limits (0.8-1.25) except for Cmax(dn,wt), for which the lower limit was slightly below 80%. No differences were noted in the systemic exposure to atorvastatin between Asian and Caucasian subjects. These data therefore demonstrate that dosing considerations in the current labels for atorvastatin are similar for Asian compared with Caucasian subjects.

Pharmacogenetics of drug transporters in the enterohepatic circulation

This article summarizes the impact of the pharmacogenetics of drug transporters expressed in the enterohepatic circulation on the pharmacokinetics and pharmacodynamics of drugs. The role of pharmacogenetics in the function of drug transporter proteins in vitro is now well established and evidence is rapidly accumulating from in vivo pharmacokinetic studies, which suggests that genetic variants of drug transporter proteins can translate into clinically relevant phenotypes. However, a large amount of conflicting information on the clinical relevance of drug transporter proteins has so far precluded the emergence of a clear picture regarding the role of drug transporter pharmacogenetics in medical practice. This is very well exemplified by the case of P-glycoprotein (MDR1, ABCB1). The challenge is now to develop pharmacogenetic models with sufficient predictive power to allow for translation into drug therapy. This will require a combination of pharmacogenetics of drug transporters, drug metabolism and pharmacodynamics of the respective drugs.

Multidrug resistance protein (MRP) 4 attenuates benzo[a]pyrene-mediated DNA-adduct formation in human bronchoalveolar H358 cells

Multi-drug resistance protein (MRP) 4, an ATP-binding cassette (ABC) transporter, has broad substrate specificity. It facilitates the transport of bile salt conjugates, conjugated steroids, nucleoside analogs, eicosanoids, and cardiovascular drugs. Recent studies in liver carcinoma cells and hepatocytes showed that MRP4 expression is regulated by the aryl hydrocarbon receptor (AhR) and nuclear factor E2-related factor 2 (Nrf2). The AhR has particular importance in the lung and is most commonly associated with the up-regulation of cytochrome P-450 (CYP)-mediated metabolism of benzo[a]pyrene (B[a]P) to reactive intermediates. Treatment of H358, human bronchoalveolar, cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or (-)-benzo[a]pyrene-7,8-dihydro-7,8-diol (B[a]P-7,8-dihydrodiol), the proximate carcinogen of B[a]P, revealed that MRP4 expression was increased compared to control. This suggested that MRP4 expression might contribute to the paradoxical decrease in (+)-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene-2'-deoxyguanosine ((+)-anti-trans-B[a]PDE-dGuo) DNA-adducts observed in TCDD-treated H358 cells. We have now found that decreased MRP4 expression induced by a short hairpin RNA (shRNA), or chemical inhibition with probenecid, increased (+)-anti-trans-B[a]PDE-dGuo formation in cells treated with (-)-B[a]P-7,8-dihydrodiol, but not the ultimate carcinogen (+)-anti-trans-B[a]PDE. Thus, up-regulation of MRP4 increased cellular efflux of (-)-B[a]P-7,8-dihydrodiol, which attenuated DNA-adduct formation. This is the first report identifying a specific MRP efflux transporter that decreases DNA damage arising from an environmental carcinogen.

Functional analysis of nonsynonymous single nucleotide polymorphisms of multidrug resistance-associated protein 2 (ABCC2)

BACKGROUND

Multidrug resistance-associated protein 2 (MRP2; ABCC2) mediates the biliary excretion of glutathione, glucuronide, and sulfate conjugates of endobiotics and xenobiotics. Single nucleotide polymorphisms (SNPs) of MRP2 contribute to interindividual variability in drug disposition and ultimately in drug response.

OBJECTIVES

To characterize the transport function of human wild-type (WT) MRP2 and four SNP variants, S789F, A1450T, V417I, and T1477M.

METHODS

The four SNP variants were expressed in Sf9 cells using recombinant baculovirus infection. The kinetic parameters [Km, (μmol/l); V(max), (pmol/mg/min); the Hill coefficient] of ATP-dependent transport of leukotriene C(4) (LTC(4)), estradiol-3-glucuronide (E(2)3G), estradiol-17β-glucuronide (E(2)17G), and tauroursodeoxycholic acid (TUDC) were determined in Sf9-derived plasma membrane vesicles. Transport activity was normalized for expression level.

RESULTS

The V(max) for transport activity was decreased for all substrates for S789F, and for all substrates except E(2)17G for A1450T. V417I showed decreased apparent affinity for LTC(4), E(2)3G, and E(2)17G, whereas transport was similar between wild-type (WT) and T1477M, except for a modest increase in TUDC transport. Examination of substrate-stimulated MRP2-dependent ATPase activity of S789F and A1450T, SNPs located in MRP2 nucleotide-binding domains (NBDs), demonstrated significantly decreased ATPase activity and only modestly decreased affinity for ATP compared with WT.

CONCLUSION

SNPs in the NBDs (S789F in the D-loop of NBD1, or A1450T near the ABC signature motif of NBD2) variably decreased the transport of all substrates. V417I in membrane spanning domain 1 selectively decreased the apparent affinity for the glutathione and glucuronide conjugated substrates, whereas the T1477M SNP in the carboxyl terminus altered only TUDC transport.

Drug transporters in drug efficacy and toxicity

Drug transporters are now widely acknowledged as important determinants governing drug absorption, excretion, and, in many cases, extent of drug entry into target organs. There is also a greater appreciation that altered drug transporter function, whether due to genetic polymorphisms, drug-drug interactions, or environmental factors such as dietary constituents, can result in unexpected toxicity. Such effects are in part due to the interplay between various uptake and efflux transporters with overlapping functional capabilities that can manifest as marked interindividual variability in drug disposition in vivo. Here we review transporters of the solute carrier (SLC) and ATP-binding cassette (ABC) superfamilies considered to be of major importance in drug therapy and outline how understanding the expression, function, and genetic variation in such drug transporters will result in better strategies for optimal drug design and tissue targeting as well as reduce the risk for drug-drug interactions and adverse drug responses.

Systematic evaluation of genetic variants in three biological pathways on patient survival in low-stage non-small cell lung cancer

INTRODUCTION

Studies from selected candidate genes suggest that single-nucleotide polymorphisms (SNPs) involved in glutathione metabolism, DNA repair, or inflammatory responses may affect overall survival (OS) in stages I to II or low-stage non-small cell lung cancer (LS-NSCLC); however, results are inconclusive. In this study, we took a systematic pathway-based approach to simultaneously evaluate the impact of genetic variation from these three pathways on OS after LS-NSCLC diagnosis.

METHODS

DNA from 647 patients with LS-NSCLC was genotyped for 480 SNPs (tag-SNPs) tagging 57 genes from the three candidate pathways. Associations of tag-SNPs with OS were assessed at the individual SNP and whole gene levels, adjusting for age, tumor stage, surgery type, and adjuvant therapy. The genotype combinations of the SNPs associated with OS were also estimated.

RESULTS

Among the 412 tag-SNPs that were successfully genotyped and passed quality assessments, 28 showed association with OS (p < 0.05). Two of the 28 were estimated to have less than a 20% chance of being false positives (rs3768490 in GSTM5: p = 1.32 × 10, q = 0.06; rs1729786 in ABCC4: p = 9.25 × 10, q = 0.20). Gene-based analysis suggested that in addition to GSTM5 and ABCC4, variation in two other genes, PTGS2 and GSTA2, was also associated with OS.

CONCLUSIONS

We describe further evidence that variations in genes involved in the glutathione and inflammatory response pathways are associated with OS in patients with LS-NSCLC. Further studies are warranted to verify our findings and elucidate their functional mechanisms and clinical utility leading to improved survival for patients with lung cancer.

Pharmacogenomics of drug metabolizing enzymes and transporters: implications for cancer therapy

The new era of personalized medicine, which integrates the uniqueness of an individual with respect to the pharmacokinetics and pharmacodynamics of a drug, holds promise as a means to provide greater safety and efficacy in drug design and development. Personalized medicine is particularly important in oncology, whereby most clinically used anticancer drugs have a narrow therapeutic window and exhibit a large interindividual pharmacokinetic and pharmacodynamic variability. This variability can be explained, at least in part, by genetic variations in the genes encoding drug metabolizing enzymes, transporters, or drug targets. Understanding of how genetic variations influence drug disposition and action could help in tailoring cancer therapy based on individual's genetic makeup. This review focuses on the pharmacogenomics of drug metabolizing enzymes and drug transporters, with a particular highlight of examples whereby genetic variations in the metabolizing enzymes and transporters influence the pharmacokinetics and/or response of chemotherapeutic agents.

TSdb: a database of transporter substrates linking metabolic pathways and transporter systems on a genome scale via their shared substrates

TSdb ( http://tsdb.cbi.pku.edu.cn ) is the first manually curated central repository that stores formatted information on the substrates of transporters. In total, 37608 transporters with 15075 substrates from 884 organisms were curated from UniProt functional annotation. A unique feature of TSdb is that all the substrates are mapped to identifiers from the KEGG Ligand compound database. Thus, TSdb links current metabolic pathway schema with compound transporter systems via the shared compounds in the pathways. Furthermore, all the transporter substrates in TSdb are classified according to their biochemical properties, biological roles and subcellular localizations. In addition to the functional annotation of transporters, extensive compound annotation that includes inhibitor information from the KEGG Ligand and BRENDA databases has been integrated, making TSdb a useful source for the discovery of potential inhibitory mechanisms linking transporter substrates and metabolic enzymes. User-friendly web interfaces are designed for easy access, query and download of the data. Text and BLAST searches against all transporters in the database are provided. We will regularly update the substrate data with evidence from new publications.

In vitro and in vivo evidence for the importance of breast cancer resistance protein transporters (BCRP/MXR/ABCP/ABCG2)

The breast cancer resistance protein (BCRP/ABCG2) is a member of the G-subfamiliy of the ATP-binding cassette (ABC)-transporter superfamily. This half-transporter is assumed to function as an important mechanism limiting cellular accumulation of various compounds. In context of its tissue distribution with localization in the sinusoidal membrane of hepatocytes, and in the apical membrane of enterocytes ABCG2 is assumed to function as an important mechanism facilitating hepatobiliary excretion and limiting oral bioavailability, respectively. Indeed functional assessment performing mouse studies with genetic deletion or chemical inhibition of the transporter, or performing pharmacogenetic studies in humans support this assumption. Furthermore the efflux function of ABCG2 has been linked to sanctuary blood tissue barriers as described for placenta and the central nervous system. However, in lactating mammary glands ABCG2 increases the transfer of substrates into milk thereby increasing the exposure to potential noxes of a breastfed newborn. With regard to its broad substrate spectrum including various anticancer drugs and environmental carcinogens the function of ABCG2 has been associated with multidrug resistance and tumor development/progression. In terms of cancer biology current research is focusing on the expression and function of ABCG2 in immature stem cells. Recent findings support the notion that the physiological function of ABCG2 is involved in the elimination of uric acid resulting in higher risk for developing gout in male patients harboring genetic variants. Taken together ABCG2 is implicated in various pathophysiological and pharmacological processes.

Spatial expression and functionality of drug transporters in the intact lung: objectives for further research

This commentary provides a background appraising evidence in the intact lung on the spatial expression of drug transporters and, where available, evidence in the intact lung of the impact, or otherwise, that such transporters can have upon pulmonary drug absorption and disposition. Ultimately drug discovery and development scientists will wish to identify in a 'pulmonary' context the effect of disease upon transporter function, the potential for drug transporters to contribute to drug-drug interactions and to inter-individual variation in drug handling and response. The rate and extent of lung epithelial permeation of drugs involve an interplay between the dose and the deposition site of drug within the lung and physiological variables operational at the epithelial-luminal interface. Amongst the latter variables is the potential impact of active transporter processes which may well display regio-selective characteristics along the epithelial tract. In pulmonary tissues the spatial pattern of drug transporter expression is generally poorly defined and the functional significance of transporters within the intact lung is explored in only a limited manner. Active transporters in the lung epithelium may affect airway residence times of drug, modulate access of drug to intracellular targets and to submucosal lung tissue, and potentially influence airway to systemic drug absorption profiles. Transporters in the lung tissue may also have the capacity to mediate uptake of drug from the systemic circulation resulting in drug accumulation in the lung. Transporters have physiological roles and new drug candidates while not necessarily serving as transport substrates may modulate transporter activity and hence physiology. The commentary highlights a series of recommendations for further work in pulmonary drug transporter research.