The medicinal chemistry of multidrug resistance (MDR) reversing drugs

Multidrug resistance (MDR) is a kind of resistance of cancer cells to multiple classes of chemotherapic drugs that can be structurally and mechanistically unrelated. Classical MDR regards altered membrane transport that results in lower cell concentrations of cytotoxic drug and is related to the over expression of a variety of proteins that act as ATP-dependent extrusion pumps. P-glycoprotein (Pgp) and multidrug resistance protein (MRP1) are the most important and widely studied members of the family that belongs to the ABC superfamily of transporters. It is apparent that, besides their role in cancer cell resistance, these proteins have multiple physiological functions as well, since they are expressed also in many important non-tumoural tissues and are largely present in prokaryotic organisms. A number of drugs have been identified which are able to reverse the effects of Pgp, MRPI and sister proteins, on multidrug resistance. The first MDR modulators discovered and studied in clinical trials were endowed with definite pharmacological actions so that the doses required to overcome MDR were associated with unacceptably high side effects. As a consequence, much attention has been focused on developing more potent and selective modulators with proper potency, selectivity and pharmacokinetics that can be used at lower doses. Several novel MDR reversing agents (also known as chemosensitisers) are currently undergoing clinical evaluation for the treatment of resistant tumours. This review is concerned with the medicinal chemistry of MDR reversers, with particular attention to the drugs that are presently in development.     

The functions and structure of ABC transporters: implications for the design of new inhibitors of Pgp and MRP1 to control multidrug resistance (MDR)

Multidrug resistance (MDR) is a kind of acquired resistance of microorganisms and cancer cells to chemotherapic drugs that are characterized by different chemical structure and different mechanism of action. Classic MDR is the consequence of the over-expression of a variety of proteins that extrude the chemotherapic from the cell, lowering its concentration below the effective one. The ABC (ATP Binding Cassette) is a ubiquitous and important family of such transporter proteins. Members of this super family are present in mammals as well as in prokaryotic organisms and use ATP as the energy source to activate the extrusion process. P-glycoprotein (Pgp) and Multidrug Resistance Proteins (MRP1 and sister proteins) are the most important and widely studied members of ABC super family. Our knowledge about the structures and functions of transporter proteins has definitely improved in recent years, following the resolution of the structure of bacterial pumps which opened the way to the building of homology models for the more complex Pgp and MRP. It can be anticipated that these results will have a strong impact on the design of more potent and safer MDR reverters. A huge number of small molecules, many of natural origin, are able to reverse multidrug resistance by inhibiting the functions of Pgp, MRP1 and sister proteins and their action has been considered a possible way to reverse MDR. However, while a few compounds have reached clinical trials, none of them has, so far, been cleared for therapeutic use. Two main reasons are at the base of this difficulty: i) MDR is a complex phenomenon that may arise from several different biochemical mechanisms, with the consequence that inhibition of transporter proteins may be insufficient to reverse it; ii) the physiological role of Pgp and sister proteins requires more potent modulators with proper selectivity and pharmacokinetic in order to avoid unwanted side effects. This paper first reviews the most recent discoveries on the structures and functions of the ABC super family, in particular Pgp and MRP. Then, the medicinal chemistry of MDR reverters, in light of these findings, is discussed and the molecules that are presently in development are reviewed.     

P-glycoprotein as a drug target in the treatment of multidrug resistant cancer

Multidrug resistance (MDR) is a major obstacle to successful cancer chemotherapy. One important mechanism of MDR involves the multidrug transporter, P-glycoprotein (Pgp), which confers upon cancer cells the ability to resist lethal doses of certain cytotoxic drugs by pumping the drugs out of the cells and thus reducing their cytotoxicity. Pgp belongs to the ATP-binding cassette (ABC) family of transporter molecules which require hydrolysis of ATP to run the transport mechanism. The substrates of Pgp may be endogenous (steroid hormones, cytokines) or exogenous (cytostatic drugs). A number of studies have demonstrated a negative correlation between Pgp expression levels and chemosensitivity or survival in a range of human malignancies. In principle, Pgp mediated drug resistance can be circumvented by treatment regimens that either exclude Pgp substrate drugs or include Pgp inhibitory agents. Experimental studies have demonstrated that certain structural modifications of anthracyclines confer the ability to escape Pgp transport. The therapeutic benefit of Pgp inhibitors as chemosensitizers is currently being explored in phase III clinical trials, and the first promising results have already been reported. Another therapeutic option for Pgp inhibitors has recently evolved as several Pgp inhibitors, many of which are generally low-toxic substances, by themselves constrain proliferation and cause cell death by apoptosis in certain MDR cancer cell lines. The dual effect of Pgp inhibitors, targeting MDR cancer cells selectively, may translate into improved efficacy of cancer chemotherapy and perhaps new and less toxic drug treatment strategies in human MDR cancer.     

ABC multidrug transporters: target for modulation of drug pharmacokinetics and drug-drug interactions

Nine proteins of the ABC superfamily (P-glycoprotein, 7 MRPs and BCRP) are involved in multidrug transport. Being localised at the surface of endothelial or epithelial cells, they expel drugs back to the external medium (if located at the apical side [P-glycoprotein, BCRP, MRP2, MRP4 in the kidney]) or to the blood (if located at the basolateral side [MRP1, MRP3, MRP4, MRP5]), modulating thereby their absorption, distribution, and elimination. In the CNS, most transporters are oriented to expel drugs to the blood. Transporters also cooperate with Phase I/Phase II metabolism enzymes by eliminating drug metabolites. Their major features are (i) their capacity to recognize drugs belonging to unrelated pharmacological classes, and (ii) their redundancy, a single molecule being possibly substrate for different transporters. This ensures an efficient protection of the body against invasion by xenobiotics. Competition for transport is now characterized as a mechanism of interaction between co-administered drugs, one molecule limiting the transport of the other, potentially affecting bioavailability, distribution, and/or elimination. Again, this mechanism reinforces drug interactions mediated by cytochrome P450 inhibition, as many substrates of P-glycoprotein and CYP3A4 are common. Induction of the expression of genes coding for MDR transporters is another mechanism of drug interaction, which could affect all drug substrates of the up-regulated transporter. Overexpression of MDR transporters confers resistance to anticancer agents and other therapies. All together, these data justify why studying drug active transport should be part of the evaluation of new drugs, as recently recommended by the FDA.     

The ABC transporters MDR1 and MRP2: multiple functions in disposition of xenobiotics and drug resistance

ATP-binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically relevant agents. We review the function of the human multidrug resistance protein MDR1, (P-glycoprotein, ABCB1) and the multidrug resistance protein MRP2 (ABCC2). We focus on four topics namely 1) structure and physiological functions of these transporters, 2) substrates e.g., drugs, xenotoxins, and environmental toxicants including their conjugates, 3) drug-drug interactions, and the role of chemosensitizers which may be able to reverse drug resistance, and 4) pharmacologically and toxicologically relevant genetic polymorphisms in transport proteins and their clinical implications.     

Structure, function and regulation of P-glycoprotein and its clinical relevance in drug disposition

1. P-glycoprotein (P-gp/MDR1), one of the most clinically important transmembrane transporters in humans, is encoded by the ABCB1/MDR1 gene. Recent insights into the structural features of P-gp/MDR1 enable a re-evaluation of the biochemical evidence on the binding and transport of drugs by P-gp/MDR1. 2. P-gp/MDR1 is found in various human tissues in addition to being expressed in tumours cells. It is located on the apical surface of intestinal epithelial cells, bile canaliculi, renal tubular cells, and placenta and the luminal surface of capillary endothelial cells in the brain and testes. 3. P-gp/MDR1 confers a multi-drug resistance (MDR) phenotype to cancer cells that have developed resistance to chemotherapy drugs. P-gp/MDR1 activity is also of great clinical importance in non-cancer-related drug therapy due to its wide-ranging effects on the absorption and excretion of a variety of drugs. 4. P-gp/MDR1 excretes xenobiotics such as cytotoxic compounds into the gastrointestinal tract, bile and urine. It also participates in the function of the blood-brain barrier. 5. One of the most interesting characteristics of P-gp/MDR1 is that its many substrates vary greatly in their structure and functionality, ranging from small molecules such as organic cations, carbohydrates, amino acids and some antibiotics to macromolecules such as polysaccharides and proteins. 6. Quite a number of single nucleotide polymorphisms have been found for the MDR1 gene. These single nucleotide polymorphisms are associated with altered oral bioavailability of P-gp/MDR1 substrates, drug resistance, and a susceptibility to some human diseases. 7. Altered P-gp/MDR1 activity due to induction and/or inhibition can cause drug-drug interactions with altered drug pharmacokinetics and response. 8. Further studies are warranted to explore the physiological function and pharmacological role of P-gp/MDR1.     

Structure,function and regulation of P-glycoprotein and its clinical relevance in drug disposition

1. P-glycoprotein (P-gp/MDR1), one of the most clinically important transmembrane transporters in humans, is encoded by the ABCB1/MDR1 gene. Recent insights into the structural features of P-gp/MDR1 enable a re-evaluation of the biochemical evidence on the binding and transport of drugs by P-gp/MDR1.

2. P-gp/MDR1 is found in various human tissues in addition to being expressed in tumours cells. It is located on the apical surface of intestinal epithelial cells, bile canaliculi, renal tubular cells, and placenta and the luminal surface of capillary endothelial cells in the brain and testes.

3. P-gp/MDR1 confers a multi-drug resistance (MDR) phenotype to cancer cells that have developed resistance to chemotherapy drugs. P-gp/MDR1 activity is also of great clinical importance in non-cancer-related drug therapy due to its wide-ranging effects on the absorption and excretion of a variety of drugs.

4. P-gp/MDR1 excretes xenobiotics such as cytotoxic compounds into the gastrointestinal tract, bile and urine. It also participates in the function of the blood–brain barrier.

5. One of the most interesting characteristics of P-gp/MDR1 is that its many substrates vary greatly in their structure and functionality, ranging from small molecules such as organic cations, carbohydrates, amino acids and some antibiotics to macromolecules such as polysaccharides and proteins.

6. Quite a number of single nucleotide polymorphisms have been found for the MDR1 gene. These single nucleotide polymorphisms are associated with altered oral bioavailability of P-gp/MDR1 substrates, drug resistance, and a susceptibility to some human diseases.

7. Altered P-gp/MDR1 activity due to induction and/or inhibition can cause drug–drug interactions with altered drug pharmacokinetics and response.

8. Further studies are warranted to explore the physiological function and pharmacological role of P-gp/MDR1.     

Efflux transporter mRNA expression profiles in differentiating JEG-3 human choriocarcinoma cells as a placental transport model

The kinetics of drug transport across the trophoblast layer is determined by several factors. Human choriocarcinoma cell lines like BeWo and JEG-3 have been used as models of the trophoblast layer to examine the placental transport of drugs. Previously, the drugs examined in these models have been readily transported across the trophoblast layer via cellular gap junctions. These backgrounds enabled us to establish the differentiating JEG-3 cell (DJEG) layer model, which suppresses paracellular drug transport, as an evaluation system of placental drug transport. The efflux transporters on the trophoblast layer assume the meaningful role of protecting the fetus from xenobiotic substances. In order to clarify the usefulness of our DJEG placental drug transport model, this study examined the mRNA expression profiles of the efflux transporters MRPs, MDR1, and BCRP in JEG-3 cells and compared them with those of BeWo cells and their known placental expression. We suggest that the mRNA of efflux transporters MRP 1-8 and BCRP are expressed widely in JEG-3 cells; however, expression levels of MDR1 mRNA were undetectable. It was also indicated that polymorphisms of BCRP C421A in both the BeWo and JEG-3 cells are of the wild-type. We demonstrated the efflux transporters' expression profiles, as well as those of the BeWo cells, was demonstrated in the DJEG placental drug transport evaluating model as well as the BeWo cells, in the DJEG placental drug transport evaluation model. Based on these findings, we hope that the DJEG model will be adequate for use in evaluating placental drug transport in relation to the transporter proteins.     

Linker length in podophyllotoxin-acridine conjugates determines potency in vivo and in vitro as well as specificity against MDR cell lines

We have synthesized two podophyllotoxin-acridine conjugates-pACR6 and pACR8. In these compounds an 9-acridinyl moiety is beta linked to the C4 carbon of the four ring system in 4'-demethylepipodophyllotoxin (epiDPT) via eighter an N-6-aminohexanylamide linker (pACR6) or via an N-8-aminooctanylamide linker containing two more carbon atoms (pACR8). The acridine-linker moiety occupies the position where different glucoside moieties, dispensable for activity, are normally linked to epiDPT in the well known epipodophyllotoxins VP-16 and VM-26. As with VP-16 and VM-26, pACR6 and pACR8 show evidence of being topoisomerase II poisons as they stimulate topoisomerase II mediated DNA cleavage in vitro and induce DNA damage in vivo. This in vivo DNA damage, as well as pACR6/pACR8 mediated cytotoxicity, is antagonized by the catalytic topoisomerase II inhibitors ICRF-187 and aclarubicin, demonstrating that topoisomerase II is a functional biological target for these drugs. Despite their structural similarities, pACR6 was more potent than pACR8 in stimulating topoisomerase II mediated DNA cleavage in vitro as well as DNA damage in vivo and pACR6 was accordingly more cytotoxic towards various human and murine cell lines than pACR8. Further, marked cross-resistance to pACR6 was seen among a panel of multidrug-resistant (MDR) cell lines over-expressing the MDR1 (multidrug resistance protein 1) ABC drug transporter, while these cell lines remained sensitive towards pACR8. pACR8 was also capable of circumventing drug resistance among at-MDR (altered topoisomerase II MDR) cell lines not over-expressing drug transporters, while pACR6 was not. Two resistant cell lines, OC-NYH/pACR6 and OC-NYH/pACR8, were developed by exposure of small cell lung cancer (SCLC) OC-NYH cells to gradually increasing concentrations of pACR6 and pACR8, respectively. Here, OC-NYH/pACR6 cells were found to over-express MDR1 and, accordingly, displayed active transport of 3H-labeled vincristine, while OC-NYH/pACR8 cells did not, further suggesting that pACR6, but not pACR8, is a substrate for MDR1. Our results show that the spatial orientation of podophyllotoxin and acridine moieties in hybrid molecules determine target interaction as well as substrate specificity in active drug transport.     

Energy dependent transport of xenobiotics and its relevance to multidrug resistance

Transport mechanisms for the exclusion of toxic xenobiotics and their metabolites from cellular environment are crucial for living organisms. Accumulation of these toxins may affect a number of regulatory and other functions, ultimately leading to cell death. This trafficking of toxins and their metabolites is an energy dependent, primary active process, involving the hydrolysis of nucleotide triphosphates (ATP or GTP), while transferring substrate molecules across the cell membrane, against a concentration gradient of the substrate. Therefore, specific membrane associated proteins, known as efflux pumps, are required to remove these undesirable molecules from the cellular environment. These transport proteins have diverse structural characteristics with molecular weights ranging from 28 kDa to 190 kDa and a broad substrate specificity ranging from anionic to weakly cationic compounds. While these transport mechanisms constitute an important part of the cellular defense machinery, they also pose a formidable threat to the efficacy of chemotherapy against pathogenic bacteria and cancer cells. In cancer cells, the over expression of these proteins may confer a multidrug resistance (MDR) phenotype. This problem of MDR in cancer cells has so far been attributed to the two major families of efflux pumps, P-glycoprotein (Pgp) and multidrug resistance associated proteins (MRP), which belong to the ATP-binding cassette (ABC) super family. However, the existence of these pumps has not been able to explain all types of acquired MDR. Therefore, the importance of transport mechanisms other than these ABC-transporters cannot be ruled out. One such transporter is DNP-SG ATPase, whose identity has recently been established with RLIP76, a Ral binding GTPase activating protein known to be involved in the Ras-Rho-Ral mediated signaling mechanism. In the present article, we review the comparative functional, structural, and molecular characteristics of some transporters and discuss their role in xenobiotic transport and multidrug resistance.     

Role of ABC transporters in the chemoresistance of human gliomas

Malignant gliomas are frequently chemoresistant and this resistance seems to depend on at least two mechanisms. First, the poor penetration of many anticancer drugs across the blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB) and blood-tumor barrier (BTB), due to their interaction with several ATP-binding cassette (ABC) drug efflux transporters that are overexpressed by the endothelial or epithelial cells of these barriers. Second, resistance may involve the tumor cells themselves. Although ABC drug efflux transporters in tumor cells confer multidrug resistance (MDR) on several other solid tumors, their role in gliomas is unclear. This review focuses on astrocytes and summarizes the current state of knowledge about the expression, distribution and function of ABC transporters in normal and tumor astroglial cells. The recognition of anticancer drugs by ABC transporters in astroglial cells and their participation in the multidrug resistance phenotype of human gliomas is discussed.     

The (patho)physiological functions of the MRP family

The identification of certain members of the large superfamily of ATP binding cassette transport proteins such as MDR1 -P-glycoprotein and the multidrug resistance protein MRP1 as ATP-dependent drug efflux pumps has been a major contribution in our understanding of the multidrug resistance phenotype of cancer cells. Importantly, both transport proteins that exhibit only low structural homology have a very different substrate specificity but confer resistance to a similar spectrum of natural product chemotherapeutic drugs. In contrast to the drug transporter MDR1, MRP1 mainly transports anionic Phase II-conjugates. In addition MRP1-mediated drug resistance is highly dependent on high intracellular glutathione levels which may be linked to the apparent physiological involvement of MRP1 in glutathione-related cellular processes. This review summarizes the current knowledge about functional aspects of MRP1 and its five recently cloned homologues MRP2–MRP6 and discusses their substrate specificities and cellular localization with emphasis on drug resistance.     

丹参酮逆转结肠癌细胞多药耐药性应用于治疗的潜能（英文）

Multidrug resistance(MDR)develops during chemotherapy in nearly all colorectal cancerpatients.It is envisaged that reversal of MDR plays a pivotal role in the success of chemotherapy.This study investigated thepotential pharmacological action in reversing MDR in colon cancer cells by the two most potent tanshinones,namely cryptotanshinone and dihydrotanshinone.They targeted two common MDR mechanisms,including overexpression of P-glycoprotein(P-gp)and suppression of apoptosis.Using a bi-directional transport assay,the two tanshinones decreased P-gp-mediated digoxin effluxin Caco-2 cells.They also potentiated the cytotoxicities of doxorubicin and irinotecan in P-gp overexpressing SW620Ad300 cells via increased intracellular accumulation of both anti-cancer drugs,as a result of down-regulation of P-gp mRNA and protein levels as well as inhibition of P-gp ATPase activity.In addition,the level of apoptosis was also found to be relatively suppressed in SW620Ad300 cells as compared with the parental SW620 cells.Interestingly,although cryptotanshinone and dihydrotanshinone induced less apoptosis in SW620Ad300 cells as compared to their parental cells,they produced more autophagic cell death in these MDR cells.In this regard,the drug resistant SW620Ad300 cells were more prone to cell death in response to the anti-cancer action of thetwo tanshinones.Furthermore,the cytotoxic action of the two tanshinones was shown to be p53-independent,further demonstrated theirunique anti-cancer activities in overcoming drug resistance due to the reduction of p53 expression together with a decrease of apoptosis in colon cancer cells.Taken together,the current findings indicate a great potential for cryptotanshinone and dihydrotanshinone against MDR colon cancer cells,in spite of P-gp overexpression and suppression of apoptosis.They are promising candidates to be developed as therapeutic agents and/or as an adjuvant therapy for colorectal cancer,especially for patients with MDR cancer types.     

Revisiting the ABCs of multidrug resistance in cancer chemotherapy

The adenosine tri-phosphate 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 clinical 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 role 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.     

Protein-mediated transbilayer movement of lipids in eukaryotes and prokaryotes: the relevance of ABC transporters

Lipid distribution across cellular membranes is regulated by specific membrane proteins controlling transbilayer movement of lipids. Flippases facilitate flip-flop of lipids and allow them to equilibrate between the two membrane leaflets independent of ATP. Distinct P-Type-ATPases transport specific lipids unidirectionally across the membrane at the expense of ATP. A group of ATP-dependent lipid transporters, the ATP-binding cassette (ABC) transporter family, was identified in studies originally related to multidrug resistance (MDR) in cancer cells. Meanwhile, lipid transport activity has been shown for full and half size ABC proteins in eukaryotic and prokaryotic cells. This activity may not only modify the organisation of lipids in membranes, but could also be of significant consequence for cell homeostasis. The various types of lipid movement mediating proteins and their cellular localisation in eukaryotes and prokaryotes are reviewed.     

Vascular and parenchymal mechanisms in multiple drug resistance: a lesson from human epilepsy

Long term treatment with antiepileptic drugs (AEDs) is the standard therapeutic approach to eradicate seizures. However, a small but significant number of patients fail AED treatment. Intrinsic drug resistance may depend on two main and not necessarily mutually exclusive mechanisms: 1) Loss of pharmacological target (e.g., GABAA receptors); 2) poor penetration of the drug into the central nervous system (CNS). The latter is due to the action of multiple drug resistance proteins capable of active CNS extrusion of drugs. These include MDR1 (P-glycoprotein, PgP), the multidrug resistance related proteins MRP1-5, and lung-resistance protein (LRP). Overexpression of MDR1 occurs in human epileptic brain. It has therefore been proposed that MDR1/PgP may contribute to multiple drug resistance in epilepsy. In addition to MDR1/PgP, other genes such as MRP2, MRP5, and human cisplatin resistance-associated protein are also overexpressed in drug-resistant epilepsy. In normal brain tissue MDR1/PgP is expressed almost exclusively by endothelial cells (EC), while in epileptic cortex both EC and perivascular astrocytes express MDR1/PgP. The underlying causes for tissue differences may be genomic (i.e., at the DNA level), or MDR1/PgP could be induced by seizures, previous drug treatment, or a combination of the above. We will present evidence showing that expression of multiple drug resistance genes in epilepsy is a complex phenomenon and that glial cells are involved. This second line of defense for xenobiotics may have profound implications for the pharmacokinetic properties of antiepileptic drugs and their capacity to reach neuronal targets.     

Bilayer-forming synthetic lipids: drugs or carriers?

Since their introduction as bilayer-forming synthetic compounds in the eighties, dioctadecyldimethylammonium (DODA) and dihexadecylphosphate (DHP) salts have found many uses in strategic, applied areas. In particular, DODA chloride or bromide vesicles interacted with negatively charged prokaryotic or eukaryotic cells, yielding adsorption isotherms of high affinity for the cell surface, causing cell adhesion and flocculation, changing the cell surface charge from negative to positive, and causing loss of cell viability over DODA concentration ranges that depended on the cell type being tested. This work reviews data on DODA effects on cell viability (bacteria, fungus and cultured mammalian cells) to propose DODA salts as effective anti-microbial agents that exhibit differential cytotoxicity in vitro and, therefore, deserve to be investigated as potential drugs. The full utility of these inexpensive synthetic bilayers and bilayer fragments able to act as drugs themselves and, simultaneously, as drug, gene or vaccine carriers remains hitherto unexplored.     

Subcellular drug distribution: mechanisms and roles in drug efficacy,toxicity, resistance,and targeted delivery

Abstract

After administration, drug molecules usually enter target cells to access their intracellular targets. In eukaryotic cells, these targets are often located in organelles, including the nucleus, endoplasmic reticulum, mitochondria, lysosomes, Golgi apparatus, and peroxisomes. Each organelle type possesses unique biological features. For example, mitochondria possess a negative transmembrane potential, while lysosomes have an intraluminal delta pH. Other properties are common to several organelle types, such as the presence of ATP-binding cassette (ABC) or solute carrier-type (SLC) transporters that sequester or pump out xenobiotic drugs. Studies on subcellular drug distribution are critical to understand the efficacy and toxicity of drugs along with the body’s resistance to them and to potentially offer hints for targeted subcellular drug delivery. This review summarizes the results of studies from 1990 to 2017 that examined the subcellular distribution of small molecular drugs. We hope this review will aid in the understanding of drug distribution within cells.     

Approaches to multidrug resistance reversal

Multidrug resistance (MDR) is characterised by cross-resistance between unrelated anticancer drugs and is associated with the overexpression of a membrane bound high-molecular weight glycoprotein, named P-glycoprotein, which is able to actively expel the drugs out of the cells. In vitro, numerous compounds have demonstrated the ability to inhibit the transport activity of P-glycoprotein, resulting in enhanced intracellular drug accumulation and MDR reversal. Such compounds include drugs of current use in other therapeutic areas, such as verapamil, cyclosporin A, quinidine or tamoxifen. Clinical trials have been performed on these drugs with the aim of reversing drug-resistance, but their toxicity was often too high. Therefore pharmaceutical firms have preferred to evaluate either analogues of these drugs, or compounds specifically designed for resistance reversal. Drugs that have clearly shown a potential for sensitisation of resistant cancers with acceptable toxicity include dexverapamil one of the two enantiomers constituting verapamil, valspodar (PSC-833), an analogue of cyclosporine A, and original compounds, named VX-710 and GF-120918. Positive results have most often been obtained in haematological malignancies (myelomas, lymphomas and acute myeloblastic leukaemias), but sometimes also in solid tumours (breast and ovarian carcinomas). Randomised Phase III studies are ongoing for compounds showing a definite activity in Phase II studies, with the aim of analysing the benefits of the combination of an MDR reverter and conventional chemotherapy, in terms of patients’ survival. However, drug-resistance is a multifactorial phenomenon, with MDR constituting only part of it. In addition, a rigorous clinical evaluation of MDR will have to be performed, which has not always been the case in early trials.     

Stereoselective transport of hydrophilic quaternary drugs by human MDR1 and rat Mdr1b P-glycoproteins

1. The present study was performed to evaluate and compare the ability of human MDR1-, and rat Mdr1b- and Mdr2-P-glycoproteins to transport hydrophilic monoquaternary drugs. Transport studies were performed with plasma membrane vesicles isolated from MDR1-, Mdr1b-, or Mdr2-overexpressing insect cells. 2. As model substrates we used the N-methylated derivatives of the diastereomers quinidine and quinine, the monoquaternary compounds N-methylquinidine and N-methylquinine. Vincristine, an established MDR1 substrate, was used as a reference. 3. We observed ATP-dependent uptake of all drugs studied into MDR1- and Mdr1b-expressing vesicles. Mdr2 was not able to transport these compounds. MDR1- and Mdr1b-mediated transport was saturable, and could be inhibited by various drugs, including PSC-833. 4. For both MDR1 and Mdr1b the V(max)/K(m) ratios (or clearance) of N-methylquinidine were greater than those determined for N-methylquinine. This stereoselective difference was also evident from differential inhibitory studies with the two isomers. 5. Comparison of normalized clearance indicated that human MDR1 was more effective in transporting the tested substrates than rat Mdr1b. 6. In conclusion, our results demonstrate that MDR1 and Mdr1b, but not Mdr2, are able to transport the monoquaternary model drugs; both MDR1 and Mdr1b display stereospecificity for these cations; and indicate human MDR1 is more efficient in transporting these cations than its rat orthologue Mdr1b.