Enhanced 9-(2-phosphonylmethoxyethyl)adenine secretion by a specific, indomethacin-sensitive efflux pump in a mutant 9-(2-phosphonylmethoxyethyl)adenine-resistant human erythroleukemia K562 cell line

Human immunodeficiency virus protease inhibitors interact with ATP binding cassette transporter 4/multidrug resistance protein 4: a basis for unanticipated enhanced cytotoxicity

Human immunodeficiency virus (HIV) pharmacotherapy, by combining different drug classes such as nucleoside analogs and HIV protease inhibitors (PIs), has increased HIV-patient life expectancy. Consequently, among these patients, an increase in non-HIV-associated cancers has produced a patient cohort requiring both HIV and cancer chemotherapy. We hypothesized that multidrug resistance protein 4/ATP binding cassette transporter 4 (MRP4/ABCC4), a widely expressed transporter of nucleoside-based antiviral medications as well as cancer therapeutics might interact with PIs. Among the PIs evaluated (nelfinavir, ritonavir, amprenavir, saquinavir, and indinavir), only nelfinavir both effectively stimulated MRP4 ATPase activity and inhibited substrate-stimulated ATPase activity. Saos2 and human embryonic kidney 293 cells engineered to overexpress MRP4 were then used to assess transport and cytotoxicity. MRP4 expression reduced intracellular accumulation of nelfinavir and consequently conferred survival advantage to nelfinavir cytotoxicity. Nelfinavir blocked Mrp4-mediated export, which is consistent with its ability to increase the sensitivity of MRP4-expressing cells to methotrexate. In contrast, targeted inactivation of Abcc4/Mrp4 in mouse cells specifically enhanced nelfinavir and 9-(2-phosphonylmethoxyethyl) adenine cytotoxicity. These results suggest that nelfinavir is both an inhibitor and substrate of MRP4. Because nelfinavir is a new MRP4/ABCC4 substrate, we developed a MRP4/ABCC4 pharmacophore model, which showed that the nelfinavir binding site is shared with chemotherapeutic substrates such as adefovir and methotrexate. Our studies reveal, for the first time, that nelfinavir, a potent and cytotoxic PI, is both a substrate and inhibitor of MRP4. These findings suggest that HIV-infected cancer patients receiving nelfinavir might experience both enhanced antitumor efficacy and unexpected adverse toxicity given the role of MRP4/ABCC4 in exporting nucleoside-based antiretroviral medications and cancer chemotherapeutics.

Expression of multidrug resistance associated protein 5 (MRP5) on cornea and its role in drug efflux

PURPOSE

The purpose of this manuscript is to investigate the presence of nucleoside/nucleotide efflux transporter in cornea and to evaluate the role in ocular drug efflux.

METHODS

RT-PCR, immunoprecipitation followed by Western blot analysis and immunostaining were employed to establish molecular presence of multidrug resistance associated protein 5 (MRP5) on cornea. Corneal efflux by MRP5 was studied with bis(POM)-PMEA and acyclovir using rabbit and human corneal epithelial cells along with MRP5 over expressing cells (MDCKII-MRP5). Ex vivo studies using excised rabbit cornea and in vivo ocular microdialysis in male New Zealand white rabbits were used to further evaluate the role of MRP5 in conferring ocular drug resistance.

RESULTS

RT-PCR confirms the expression of MRP5 in both rabbit and human corneal epithelial cells along with MDCKII-MRP5 cells. Immunoprecipitation followed by Western blot analysis using a rat (M511-54) monoclonal antibody that reacts with human epitope confirms the expression of MRP5 protein in human corneal epithelial cells and MDCKII-MRP5 cells. Immunostaining performed on human cornea indicates the localization of this efflux pump on both epithelium and endothelium. Efflux studies reveal that depletion of ATP decreased PMEA efflux significantly. MRP5 inhibitors also diminished PMEA and acyclovir efflux. However, depletion of glutathione did not alter efflux. MDR1 and MRP2 did not contribute to PMEA efflux. However, MRP2 is involved in acyclovir efflux while MDR1 do not participate in this process. TLC/autoradiography suggested the conversion of bis(POM)-PMEA to PMEA in rabbit and human corneal epithelial cells. Two well known antiglaucoma drugs, bimatoprost and latanoprost were rapidly effluxed by MRP5. Ex vivo study on intact rabbit corneas demonstrated accumulation of PMEA in cornea in the presence of ATP-depleting medium. In vivo ocular pharmacokinetics also revealed a significant increase in maximum aqueous humor concentration (C(max)) and area under the aqueous humor time curve (AUC) of acyclovir in the presence of MK-571, a specific MRP inhibitor.

CONCLUSIONS

Taken together immunolocalization on human cornea, in vitro efflux in human, rabbit corneal and MRP5 over expressing cells, ex vivo and in vivo studies in intact rabbit cornea suggest that MRP5 on cornea can significantly lower the permeability of antiviral and glaucoma drugs. These findings may be valuable in developing formulation strategies to optimize ocular bioavailability of topically administered ocular agents.

The human multidrug resistance protein 4 (MRP4, ABCC4): functional analysis of a highly polymorphic gene

ABCC4 encodes multidrug resistance protein 4 (MRP4), a member of the ATP-binding cassette family of membrane transporters involved in the efflux of endogenous and xenobiotic molecules. The aims of this study were to identify single nucleotide polymorphisms of ABCC4 and to functionally characterize selected nonsynonymous variants. Resequencing was performed in a large ethnically diverse population. Ten nonsynonymous variants were selected for analysis of transport function based on allele frequencies and evolutionary conservation. The reference and variant MRP4 cDNAs were constructed by site-directed mutagenesis and transiently transfected into human embryonic kidney cells (HEK 293T). The function of MRP4 variants was compared by measuring the intracellular accumulation of two antiviral agents, azidothymidine (AZT) and adefovir (PMEA). A total of 98 variants were identified in the coding and flanking intronic regions of ABCC4. Of these, 43 variants are in the coding region, and 22 are nonsynonymous. In a functional screen of ten variants, there was no evidence for a complete loss of function allele. However, two variants (G187W and G487E) showed a significantly reduced function compared to reference with both substrates, as evidenced by higher intracellular accumulation of AZT and PMEA compared to the reference MRP4 (43 and 69% increase in accumulation for G187W compared with the reference MRP4, with AZT and PMEA, respectively). The G187W variant also showed decreased expression following transient transfection of HEK 293T cells. Further studies are required to assess the clinical significance of this altered function and expression and to evaluate substrate specificity of this functional change.

Substrate overlap between Mrp4 and Abcg2/Bcrp affects purine analogue drug cytotoxicity and tissue distribution

The use of probe substrates and combinations of ATP-binding cassette (ABC) transporter knockout (KO) animals may facilitate the identification of common substrates between apparently unrelated ABC transporters. An unexpectedly low concentration of the purine nucleotide analogue, 9-(2-(phosphonomethoxy)ethyl)-adenine (PMEA), and up-regulation of Abcg2 in some tissues of the Mrp4 KO mouse prompted us to evaluate the possibility that Abcg2 might transport purine-derived drugs. Abcg2 transported and conferred resistance to PMEA. Moreover, a specific Abcg2 inhibitor, fumitremorgin C, both increased PMEA accumulation and reversed Abcg2-mediated PMEA resistance. We developed Mrp4 and Abcg2 double KO mice and used both single KOs of Abcg2 and Mrp4 mice to assess the role of these transporters in vivo. Abcg2 contributed to PMEA accumulation in a variety of tissues, but in some tissues, this contribution was only revealed by the concurrent absence of Mrp4. Abcg2 also transported and conferred resistance to additional purine analogues, such as the antineoplastic, 2-chloro-2'-deoxyadenosine (cladribine) and puromycin, a protein synthesis inhibitor that is often used as a dominant selectable marker. Purine analogues interact with ABCG2 by a site distinct from the prazosin binding site as shown by their inability to displace the substrate analogue and photoaffinity tag [(125)I]iodoarylazidoprazosin. These studies show that Abcg2, like Mrp4, transports and confers resistance to purine nucleoside analogues and suggest that these two transporters work in parallel to affect drug cytotoxicity and tissue distribution. This new knowledge will facilitate an understanding of how Abcg2 and Mrp4, separately and in combination, protect against purine analogue host toxicity as well as resistance to chemotherapy.

Human Multidrug Resistance Associated Protein 4 Confers Resistance to Camptothecins

PURPOSE

The multidrug resistance associated protein (MRP) 4 is a member of the adenosine triphosphate (ATP)-binding cassette transporter family. Camptothecins (CPTs) have shown substantial anticancer activity against a broad spectrum of tumors by inhibiting DNA topoisomerase I, but tumor resistance is one of the major reasons for therapeutic failure. P-glycoprotein, breast cancer resistance protein, MRP1, and MRP2 have been implicated in resistance to various CPTs including CPT-11 (irinotecan), SN-38 (the active metabolite of CPT-11), and topotecan. In this study, we explored the resistance profiles and intracellular accumulation of a panel of CPTs including CPT, CPT-11, SN-38, rubitecan, and 10-hydroxy-CPT (10-OH-CPT) in HepG2 cells with stably overexpressed human MRP4. Other anticancer agents such as paclitaxel, cyclophosphamide, and carboplatin were also included.

METHODS

HepG2 cells were transfected with an empty vehicle plasmid (V/HepG2) or human MRP4 (MRP4/HepG2). The resistance profiles of test drugs in exponentially growing V/HepG2 and MRP4/HepG2 cells were examined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazonium bromide (MTT) assay with 4 or 48 h exposure time of the test drug in the absence or presence of various MRP4 inhibitors. The accumulation of CPT-11, SN-38, and paclitaxel by V/HepG2 and MRP4/HepG2 cells was determined by validated high-performance liquid chromatography methods.

RESULTS

Based on the resistance folds from the MTT assay with 48 h exposure time of the test drug, MRP4 conferred resistance to CPTs tested in the order 10-OH-CPT (14.21) > SN-38 carboxylate (9.70) > rubitecan (9.06) > SN-38 lactone (8.91) > CPT lactone (7.33) > CPT-11 lactone (5.64) > CPT carboxylate (4.30) > CPT-11 carboxylate (2.68). Overall, overexpression of MRP4 increased the IC50 values 1.78- to 14.21-fold for various CPTs in lactone or carboxylate form. The resistance of MRP4 to various CPTs tested was significantly reversed in the presence of dl-buthionine-(S,R)-sulfoximine (BSO, a gamma-glutamylcysteine synthetase inhibitor), MK571, celecoxib, or diclofenac (all MRP4 inhibitors). In addition, the accumulation of CPT-11 and SN-38 over 120 min in MRP4/HepG2 cells was significantly reduced compared to V/HepG2 cells, whereas the addition of celecoxib, MK571, or BSO significantly increased their accumulation in MRP4/HepG2 cells. There was no significant difference in the intracellular accumulation of paclitaxel in V/HepG2 and MRP4/HepG2 cells, indicating that P-glycoprotein was not involved in the observed resistance to CPTs in this study. MRP4 also conferred resistance to cyclophosphamide and this was partially reversed by BSO. However, MRP4 did not increase resistance to paclitaxel, carboplatin, etoposide (VP-16), 5-fluorouracil, and cyclosporine.

CONCLUSIONS

Human MRP4 rendered significant resistance to cyclophosphamide, CPT, CPT-11, SN-38, rubitecan, and 10-OH-CPT. CPT-11 and SN-38 are substrates for MRP4. Further studies are needed to explore the role of MRP4 in resistance, toxicity, and pharmacokinetics of CPTs and cyclophosphamide.

Cellular export of drugs and signaling molecules by the ATP-binding cassette transporters MRP4 (ABCC4) and MRP5 (ABCC5)

Like other members of the multidrug resistance protein (MRP)/ABCC subfamily of ATP-binding cassette transporters, MRP4 (ABCC4) and MRP5 (ABCC5) are organic anion transporters. They have, however, the outstanding ability to transport nucleotides and nucleotide analogs. In vitro experiments using drug-selected or -transfected cells indicated that these transport proteins, when overexpressed, can lower the intracellular concentration of nucleoside/nucleotide analogs, such as the antiviral compounds PMEA (9-(2-phosphonylmethoxyethyl)adenine) or ganciclovir, and of anticancer nucleobase analogs, such as 6-mercaptopurine, after their conversion into the respective nucleotides. This may lead to an impaired ability of these compounds to inhibit virus replication or cell proliferation. It remains to be tested whether antiviral or anticancer chemotherapy based on nucleobase, nucleoside, or nucleotide precursors can be modulated by inhibition of MRP4 and MRP5. MRP4 also seems to be able to mediate the transport of conjugated steroids, prostaglandins, and glutathione. Furthermore, cyclic nucleotides (cyclic adenosine monophosphate and cyclic guanine monophosphate) are exported from cells by MRP4 and MRP5. This may modulate the intracellular concentration of these important mediators, besides the action of phosphodiesterases, as well as provide extracellular nucleotides for a possible paracrine action. In this line, tissue distribution and subcellular localization of MRP4 and MRP5 specifically in smooth muscle cells (MRP5), platelet-dense granules (MRP4), and nervous cells (MRP4 and MRP5), besides the capillary endothelium, point not only to a possible function of these transporters as exporters in cellular defense, but also to a physiological function in signaling processes.

Renal tubular transporters and antiviral drugs: an update

Systemic disposition of antiviral drugs partly depends on renal handling of these compounds. There are some known, functionally characterized anionic and cationic transporters with varying substrate specificities for those drugs: human organic anion transporter (OAT) family (hOAT1-3) and human organic cation transporter (OCT) family (hOCT1-3), which mediate the intracellular flux, and adenosine 5'-triphosphate (ATP) binding cassette transporter family (P-glycoprotein, MRP2-5), which mediate the cellular efflux of antiviral drugs. The peptide transporter (PEPT1-2) mediate bi-directional facilitated diffusion of valacyclovir. All these transporters are expressed in the kidney. Organic anion and cation transporters primarily localize to the basolateral membrane of renal epithelial cells while ATP-binding cassette transporters primarily localize to the apical membrane. These transporters work in concert to mediate renal intracellular concentration of occurring antiviral drugs. Along with drug-metabolizing enzymes, these transporters are important determinants of drug effectiveness and toxicity. This review examines the role that these transporters play in renal disposition of antiviral drugs.

Determination of adefovir in human plasma by liquid chromatography/tandem mass spectrometry: application to a pharmacokinetic study

A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated to determine the concentrations of adefovir [9-(2-phosphonylmethoxyethyl)adenine, PMEA] in human plasma. After one-step protein precipitation of plasma samples by methanol, adefovir was analyzed by LC/MS/MS using positive electrospray ionization. Chromatography was performed on a C18 column. The extraction recoveries of adefovir were found to be 85.1-89.3%. Adefovir was stable under routine laboratory conditions. A minimal matrix effect resulting in a slight ionization enhancement of adefovir (<10.9%) was observed, which did not markedly affect the behavior of the calibrations curves and accuracy and precision data. The method had a chromatographic run time of 7.8 min and a linear calibration curve over the concentration range 1.5-90 ng/mL for adefovir. The lower limit of quantification of the method was 1.5 ng/mL. The intra- and inter-day precision was less than 8.4%. These results indicated that this LC/MS/MS method has high selectivity and efficiency, and acceptable accuracy, precision and sensitivity. The validated LC/MS/MS method has been successfully used in a pharmacokinetic study in healthy volunteers treated with oral adefovir dipivoxil at 10 and 20 mg.

Multidrug resistance protein (MRP) 4- and MRP 5-mediated efflux of 9-(2-phosphonylmethoxyethyl)adenine by microglia

The pathogenesis of human immunodeficiency virus (HIV)-associated dementia has been linked to microglial responses after infection. We have recently confirmed expression of several ATP-dependent efflux transporters in microglia, namely, multidrug resistance protein 1 (MRP1) and P-glycoprotein (P-gp). In the present study, we investigated whether cultured rat microglia express two additional MRP family members, rMRP4 and rMRP5. Using reverse transcriptase-polymerase chain reaction, rMRP4 and rMRP5 mRNA was detected in primary cultures of microglia and in a rat microglia cell line, MLS-9. Western blot analysis further confirmed protein expression of the two MRP isoforms in MLS-9 cells. Bis(pivaloxymethyl)-9-(2-phosphonylmethoxyethyl)adenine [bis(POM)PMEA], a lipophilic ester prodrug of the well characterized MRP4 and 5 substrate 9-(2-phosphonylmethoxyethyl)adenine (PMEA), was chosen to examine transport characteristics in MLS-9. Using thin layer chromatography, we verified that more than 90% of radioactivity recovered in MLS-9 loaded with 1 microM [(3)H]bis(POM)PMEA for 1 h under ATP-depleting conditions was converted to PMEA. Efflux of PMEA by MLS-9 cell monolayers was ATP-dependent, glutathione-independent, and significantly inhibited by several MRP inhibitors (i.e., sulfinpyrazone, genistein, indomethacin, and probenecid) as well as the antiretroviral drug azidothymidine-monophosphate. Similar results were not observed in MRP1- or P-gp-overexpressing cell lines, suggesting that PMEA is not a substrate for either P-gp or MRP1. These studies provide further evidence that microglia express multiple subfamilies of ATP-binding cassette transporters (i.e., P-gp, MRP1, MRP4, and MRP5) that could restrict permeation of several different classes of antiretroviral drugs in a brain cellular target of HIV-1 infection.

Characterization of the transport of nucleoside analog drugs by the human multidrug resistance proteins MRP4 and MRP5

The human multidrug resistance proteins MRP4 and MRP5 are organic anion transporters that have the unusual ability to transport cyclic nucleotides and some nucleoside monophosphate analogs. Base and nucleoside analogs used in the chemotherapy of cancer and viral infections are potential substrates. To assess the possible contribution of MRP4 and MRP5 to resistance against these drugs, we have investigated the transport mediated by MRP4 and MRP5. In cytotoxicity assays, MRP4 conferred resistance to the antiviral agent 9-(2-phosphonomethoxyethyl)adenine (PMEA) and high-performance liquid chromatography analysis showed that, like MRP5, MRP4 transported PMEA in an unmodified form. MRP4 also mediated substantial resistance against other acyclic nucleoside phosphonates, whereas MRP5 did not. Apart from low-level MRP4-mediated cladribine resistance, the cytotoxicity of clinically used anticancer nucleosides was not influenced by overexpression of MRP4 or MRP5. In contrast, MRP5 mediated efflux of the pyrimidine-based antiviral 2',3'-dideoxynucleoside 2',3'-didehydro-2',3'-dideoxythymidine 5'-monophosphate (d4TMP) and its phosphoramidate derivative alaninyl-d4TMP from cells loaded with the 2',3'-didehydro-2',3'-dideoxythymidine prodrugs cyclosaligenyl-d4TMP and aryloxyphosphoramidate d4TMP (So324), respectively. Moreover, only inside-out membrane vesicles derived from MRP5-overexpressing cells accumulated alaninyl-d4TMP. Cellular efflux and vesicular uptake studies were carried out to further compare transport mediated by MRP4 and MRP5 and showed that dipyridamole, dilazep, nitrobenzyl mercaptopurine riboside, sildenafil, trequinsin and MK571 inhibited MRP4 more than MRP5, whereas cyclic nucleotides and monophosphorylated nucleoside analogs were equally poor inhibitors of both pumps. These results strongly suggest that the affinity of MRP4 and MRP5 for nucleotide-based substrates is low.

Therapeutic and biological importance of getting nucleotides out of cells: a case for the ABC transporters, MRP4 and 5

The energy dependent transport of drugs contributes to cellular resistance and is undoubtedly a prime suspect in chemotherapeutic failure of a variety of disease processes. Early studies focused on a single gene, the multidrug resistance gene, MDR1, as a main contributor to chemotherapeutic failure. However, the multifaceted nature of cellular resistance lead to the discovery of the MRP gene. This pivotal finding and the concurrent rapid development of gene databases lead to the expansion of the MRP gene family. The purpose of this review is to discuss two of the recently described MRP family members that were orphans until their role in drug resistance was discovered. This review will provide an overview of the current state of our understanding of MRP4 and 5.

Role of MRP4 and MRP5 in biology and chemotherapy

Nucleotide efflux (especially cyclic nucleotides) from a variety of mammalian tissues, bacteria, and lower eukaryotes has been studied for several decades. However, the molecular identity of these nucleotide efflux transporters remained elusive, despite extensive knowledge of their kinetic properties and inhibitor profiles. Identification of the subfamily of adenosine triphosphate (ATP) binding cassette transporters, multidrug resistance protein (MRP) subfamily, permitted rapid advances because some recently identified MRP family members transport modified nucleotide analogs (ie, chemotherapeutic agents). We first identified, MRP4, based on its ability to efflux antiretroviral compounds, such as azidothymidine monophosphate (AZT-MP) and 9-(2-phosphonyl methoxyethyl) adenine (PMEA), in drug-resistant and also in transfected cell lines. MRP5, a close structural homologue of MRP4 also transported PMEA. MRP4 and MRP5 confer resistance to cytotoxic thiopurine nucleotides, and we demonstrate MRP4 expression varies among acute lymphoblastic leukemias, suggesting this as a factor in response to chemotherapy with these agents. The ability of MRP4 and MRP5 to transport 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) suggests they may play a biological role in cellular signaling by these nucleotides. Finally, we propose that MRP4 may also play a role in hepatic bile acid homeostasis because loss of the main bile acid efflux transporter, sister of P-glycoprotein (SPGP) aka bile-salt export pump (BSEP), leads to a strong compensatory upregulation in MRP4 expression. Cumulatively, these studies reveal that the ATP-binding cassette (ABC) transporters MRP4 and MRP5 have a unique role in biology and in chemotherapeutic response.

A human lymphocyte based ex vivo assay to study the effect of drugs on P-glycoprotein (P-gp) function

PURPOSE

The effect of drugs on P-glycoprotein (P-gp) is normally studied in transfected or overexpressing cell lines derived from tumor cells or animal tissue. We wanted to develop an assay using normal healthy human tissue to study and characterize the drug-transporter interaction.

METHODS

Lymphocytes were isolated from healthy human blood. The effect of inhibitors of P-gp (cyclosporine, tacrolimus, verapamil, quinidine, vinblastine) and of other transporters (indomethacin, probenecid, sulfinpyrazone) on intracellular accumulation of rhodamine 123 was evaluated by flow cytometry.

RESULTS

The efflux of rhodamine 123 was inhibited by P-gp inhibitors in a saturable, concentration-dependent manner. The potency of inhibition of P-gp was cyclosporine > tacrolimus > quinidine > verapamil > vinblastine. Vinblastine inhibited P-gp at lower concentrations, whereas at high concentrations, there was an activation of rhodamine 123 efflux from lymphocytes. The multidrug resistance associated protein (MRP) inhibitors, sulfinpyrazone and probenecid, did not have any significant effect on intracellular accumulation of rhodamine 123, but indomethacin caused a concentration-dependent increase in retention of rhodamine 123, indicating the involvement of other uncharacterized transporters.

CONCLUSIONS

Lymphocytes can serve as a model tissue for studying modulation of P-gp activity by drugs. Both inhibitors and inducers of P-gp activity can be evaluated.

Resistance of hepatitis B virus to antiviral drugs: current aspects and directions for future investigation

Despite the existence of vaccines, chronic hepatitis B virus (HBV) infection remains a major health problem worldwide. Interferon therapy successfully controls infection in only a small percentage of chronically infected individuals. The recent approval of the nucleoside analogue lamivudine for the treatment of chronic HBV infection has ushered in a new era of antiviral therapy. While lamivudine is highly effective at controlling viral infection short-term, prolonged therapy has been associated with an increasing incidence of viral resistance. Thus, it appears that lamivudine alone will not be sufficient to control chronic viral infection in the majority of individuals. In addition to lamivudine, several new nucleoside and nucleotide analogues that show promising antihepadnaviral activity are in various stages of development. Lamivudine resistance has been found to confer cross-resistance to some of these compounds and it is likely that resistance to newer antivirals may also develop during prolonged use. Drug resistance therefore poses a major threat to nucleoside analogue-based therapies for chronic HBV infection. Fortunately, combination chemotherapy (antiviral therapy with two or more agents) can minimize the chance that resistance will develop and can be expected to achieve sustained reductions in viral load, provided that suitable combinations of agents are chosen. Here we review the basis of drug resistance in HBV, with emphasis on aspects that are likely to affect drug choice in future.

Multidrug-resistance protein 5 is a multispecific organic anion transporter able to transport nucleotide analogs

Two prominent members of the ATP-binding cassette superfamily of transmembrane proteins, multidrug resistance 1 (MDR1) P-glycoprotein and multidrug resistance protein 1 (MRP1), can mediate the cellular extrusion of xenobiotics and (anticancer) drugs from normal and tumor cells. The MRP subfamily consists of at least six members, and here we report the functional characterization of human MRP5. We found resistance against the thiopurine anticancer drugs, 6-mercaptopurine (6-MP) and thioguanine, and the anti-HIV drug 9-(2-phosphonylmethoxyethyl)adenine (PMEA) in MRP5-transfected cells. This resistance is due to an increased extrusion of PMEA and 6-thioinosine monophosphate from the cells that overproduce MRP5. In polarized Madin-Darby canine kidney II (MDCKII) cells transfected with an MRP5 cDNA construct, MRP5 is routed to the basolateral membrane and these cells transport S-(2,4-dinitrophenyl)glutathione and glutathione preferentially toward the basal compartment. Inhibitors of organic anion transport inhibit transport mediated by MRP5. We speculate that MRP5 might play a role in some cases of unexplained resistance to thiopurines in acute lymphoblastic leukemia and/or to antiretroviral nucleoside analogs in HIV-infected patients.

Cellular Factors for Resistance against Antiretroviral Agents

Substantial advancements have been made in our understanding of the complex replication cycle of, and immunopathology associated with HIV infection as well as the drugs used to treat the disease. The nucleoside reverse transcriptase inhibitors remain the cornerstones of current antiviral treatment modalities. Unfortunately, their long-term use often leads to adverse reactions and the emergence of virus mutants with decreased susceptibility to therapeutic agents. In addition to viral resistance, prolonged antiviral treatment may affect metabolic changes in the host cells that can diminish the efficacy of the treatment. Thus, both viral and cellular resistance mechanisms must be considered in the context of failing antiviral chemotherapy. This review article concerns the intracellular pharmacology of antiviral nucleoside analogues in human lymphoid cells and the possible impact of a newly identified nucleotide transporter on drug resistance.

N6-cyclopropyl-PMEDAP: a novel derivative of 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP) with distinct metabolic, antiproliferative, and differentiation-inducing properties

N6-Cyclopropyl-PMEDAP (cPr-PMEDAP) is a novel derivative of the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP). Its cytostatic activity was found to be 8- to 20-fold more pronounced than that of PMEDAP and equivalent to that of the guanine derivative 9-(2-phosphonylmethoxyethyl)guanine (PMEG) against a variety of tumor cell lines. Unlike PMEDAP, but like PMEG, cPr-PMEDAP was equally cytostatic to wild-type and 9-(2-phosphonylmethoxyethyl)adenine/PMEDAP-resistant variants of the human erythroleukemia K562 and the murine leukemia L1210 cell lines. Also, cPr-PMEDAP and PMEG proved to be equipotent inducers of K562 and rat choriocarcinoma RCHO cell differentiation, whereas the differentiation-inducing activity of PMEDAP was 5- to 25-fold less pronounced. Furthermore, compared to PMEDAP, cPr-PMEDAP and PMEG were 10- to 25-fold more potent in inhibiting the progression of K562 cells through the S phase of the cell cycle, resulting in a marked accumulation of the four 2'-deoxyribonucleoside 5'-triphosphate pools. The biological effects of cPr-PMEDAP, but not PMEDAP, were reversed by the adenylate deaminase inhibitor 2'-deoxycoformycin (dCF). Formation of the deaminated derivative of cPr-PMEDAP (i.e. PMEG) was demonstrated in crude extracts from K562 and L1210 cells and in metabolism studies with radiolabeled cPr-PMEDAP and PMEG. This is the very first example of an acyclic nucleoside phosphonate analogue that is susceptible to deamination. However, cPr-PMEDAP was not recognized as a substrate by purified adenosine deaminase or by adenylate deaminase. These findings might point to an as yet unidentified cellular enzyme, sensitive to dCF but different from the common adenosine and AMP deaminases. Our data demonstrate the superior antiproliferative and differentiation-inducing effects of cPr-PMEDAP on tumor cells, as compared to the parent compound PMEDAP, based on the unique metabolic properties of this novel compound.

Impact of 9-(2-phosphonylmethoxyethyl)adenine on (deoxy)ribonucleotide metabolism and nucleic acid synthesis in tumor cells

Following exposure to 9-(2-phosphonylmethoxyethyl)adenine (an inhibitor of the cellular DNA polymerases alpha, delta and epsilon), human erythroleukemia K562, human T-lymphoid CEM and murine leukemia L1210 cells markedly accumulated in the S phase of the cell cycle. In contrast to DNA replication, RNA synthesis (transcription) and protein synthesis (mRNA translation) were not affected by 9-(2-phosphonylmethoxyethyl)-adenine. The ribonucleoside triphosphate pools were slightly elevated, while the intracellular levels of all four deoxyribonucleoside triphosphates were 1.5-4-fold increased in 9-(2-phosphonylmethoxyethyl)adenine-treated K562, CEM and L1210 cells. The effect of 9-(2-phosphonylmethoxyethyl)adenine on de novo (thymidylate synthase-mediated) and salvage (thymidine kinase-mediated) dTTP synthesis was investigated using radio-labelled nucleoside precursors. The amount of thymidylate synthase-derived dTTP in the acid soluble pool was 2-4-fold higher in PMEA-treated than in untreated K562 cells, which is in accord with the 3-4-fold expansion of the global dTTP level in the presence of 9-(2-phosphonylmethoxyethyl)adenine. Strikingly, 2-derived dTTP accumulated to a much higher extent (i.e. 16-40-fold) in the soluble dTTP pool upon 9-(2-phosphonylmethoxyethyl)adenine treatment. In keeping with this finding, a markedly increased thymidine kinase activity could be demonstrated in extracts of 9-(2-phosphonylmethoxyethyl)adenine-treated K562 cell cultures. Also, in the presence of 200 microM 9-(2-phosphonylmethoxyethyl)adenine, 14-fold less thymidylate synthase-derived but only 3-fold less thymidine kinase-derived dTTP was incorporated into the DNA of the K562 cells. These data show that thymidine incorporation may be inappropriate as a cell proliferation marker in the presence of DNA synthesis inhibitors such as 9-(2-phosphonylmethoxyethyl)adenine. Our findings indicate that 9-(2-phosphonylmethoxyethyl)adenine causes a peculiar pattern of (deoxy)ribonucleotide metabolism deregulation in drug-treated tumor cells, as a result of the metabolic block imposed by the drug on the S phase of the cell cycle.