Similarities and differences between the multidrug resistance phenotype of mammalian tumor cells and chloroquine resistance in Plasmodium falciparum

ATPase activity of purified plasma membranes and digestive vacuoles from Plasmodium falciparum

The ATPase activity of the human malaria parasite, Plasmodium falciparum was investigated using two experimental systems, (i) digestive vacuoles, and (ii) purified plasma membranes isolated from a chloroquine-sensitive and a chloroquine-resistant strain. No correlation between the level of ATPase activity and chloroquine sensitivity could be detected. In both systems, the ATPase activity of the chloroquine-resistant and -sensitive strain was decreased in the presence of the P-glycoprotein inhibitor vanadate. Susceptibility to inhibition by vanadate together with the lack of effect of ouabain implies a P-type ATPase activity in the plasma membrane. Furthermore, the inhibition of Fac8 ATPase activity by oligomycin both in the digestive vacuoles and the plasma membranes would be consistent with higher levels of Pgh1 in Fac8. Our data are consistent with the presence of a V-type H+-ATPase in the parasite food vacuole. Bafilomycin A1 and N-ethylmaleimide decreased the vacuolar ATPase activity in both chloroquine-resistant and -sensitive strains. Interestingly, a 30% decrease was observed between the ATPase activity of plasma membranes isolated from Fac8 and D10 in the presence of bafilomycin A1, suggesting the presence of a V-type ATPase in D10 plasma membrane that is underexpressed or altered in the plasma membrane of the chloroquine-resistant Fac8. The chemosensitisers tested had no effect on the ATPase activity of chloroquine-resistant P. falciparum in both systems suggesting that their activity is not mediated through an ATP-dependent mechanism. No effect was observed on the vacuolar ATPase activity in the presence of the antimalarials tested indicating that an ATP-dependent transport has not been activated.

The relationship of physico-chemical properties and structure to the differential antiplasmodial activity of the cinchona alkaloids

Background

The 8-amino and 9-hydroxy substituents of antimalarial cinchona alkaloids have the erythro orientation while their inactive 9-epimers are threo. From the X-ray structures a 90° difference in torsion angle between the N1-H1 and C9-O12 bonds in the two series is believed to be important. In order to kill the malaria parasite, alkaloids must cross the erythrocyte and parasite membranes to accumulate in the acid digestive vacuole where they prevent detoxication of haematin produced during haemoglobin breakdown.

Methods

Ionization constants, octanol/water distribution and haematin interaction are examined for eight alkaloids to explain the influence of small structural differences on activity.

Results

Erythro isomers have a high distribution ratio of 55:1 from plasma to the erythrocyte membrane, while for the more basic threo epimers this is only 4.5:1. This gives an increased transfer rate of the erythro drugs into the erythrocyte and thence into the parasite vacuole where their favourable conformation allows interaction with haematin, inhibiting its dimerization strongly (90 ± 7%) and thereby killing the parasite. The threo compounds not only enter more slowly but are then severely restricted from binding to haematin by the gauche alignment of their N1-H1 and C9-O12 bonds. Confirmatory molecular models allowed measurement of angles and bond lengths and computation of the electronic spectrum of a quinine-haematin complex.

Conclusion

Differences in the antiplasmodial activity of the erythro and threo cinchona alkaloids may therefore be attributed to the cumulative effects of lipid/aqueous distribution ratio and drug-haematin interaction. Possible insights into the mechanism of chloroquine-resistance are discussed.

Modulation of multidrug resistance by artemisinin, artesunate and dihydroartemisinin in K562/adr and GLC4/adr resistant cell lines

Overcoming MDR (multidrug resistance) phenomena is a crucial aspect of cancer chemotherapy research. Artemisinin and its derivatives have been found to inhibit the proliferation of cancer cells in the microM range. They poorly inhibited the function of P-glycoprotein and did not inhibit the function of MRP1-protein. The concentrations required to inhibit by 50% the function of P-glycoprotein are 110+/-5 microM. Artemisinin, artesunate and dihydroartemisinin efficiently decreased the mitochondrial membrane potential, leading to a decrease in intracellular ATP in all cell lines tested: by 30 to 50% at 5 microM. Artemisinin, artesunate and dihydroartemisinin increased cytotoxicity of pirarubicin and doxorubicin in P-glycoprotein-overexpressing K562/adr, and in MRP1-overexpressing GLC4/adr, with the delta(0.5) ranging from 200 to 860 nM, but not in their corresponding drug-sensitive cell lines. In this range of concentrations these compounds did not decrease the function of P-glycoprotein, suggesting a mechanism by which the drugs did not reverse MDR phenomenon at the P-glycoprotein level but at the mitochondrial level.

Are ion-exchange processes central to understanding drug-resistance phenomena?

Drug resistance in malarial parasites is arguably the greatest challenge currently facing infectious disease research. In addressing this problem, researchers have been intrigued by similarities between drug-resistant malarial parasites and tumour cells. For example, it was originally thought that the role of pfMDR (Plasmodium falciparum multidrug resistance) proteins was central in conferring antimalarial multidrug resistance. However, recent work has questioned the precise role of MDR proteins in multidrug resistance. In addition, recent ground-breaking work in identifying mutations associated with antimalarial drug resistance might have led to identification of yet another parallel between drug-resistant tumour cells and malarial parasites, namely, intriguing alterations in transmembrane ion transport, discussed here by Paul Roepe and James Martiney. This further underscores an emerging paradigm in drug-resistance research.

Plasmodium berghei: identification of an mdr-like gene associated with drug resistance

Amplification, mutations, or overexpression of the pfmdr1 gene have been associated with multiple drug resistance in some strains of Plasmodium falciparum. In order to better understand this potential mechanism of drug resistance, we are currently investigating putative mdr homologues in vivo in the rodent malaria Plasmodium berghei. We have identified and partially sequenced a gene that is amplified in a MFQ-resistant (MFQr) line. Using degenerate primers, a 579-bp fragment was amplified by PCR using P. berghei genomic DNA as template. The predicted amino acid sequence shares 66% identity with the previously reported pfmdr1 gene product (Pgh1) of P. falciparum. Southern blots and slot blots of genomic DNA suggest that this gene is amplified two- to threefold in a MFQr line (N/1100), as has been previously reported in some MFQr strains of P. falciparum. The P. berghei gene was mapped to chromosome 12 in all of the lines analyzed. Furthermore, the cloned PCR product also hybridizes to chromosome 5 of the MFQr strain.

Synergy between two calcium channel blockers, verapamil and fantofarone (SR33557), in reversing chloroquine resistance in Plasmodium falciparum

This study describes the synergistic interaction of two calcium channel blockers, verapamil (VR) and SR33557 or fantofarone (SR), in reversing chloroquine resistance in Plasmodium falciparum, the causative agent of human malaria. The two calcium channel blockers exhibited an intrinsic antimalarial activity at 10 and 1 microM for verapamil and fantofarone, respectively. Isobolograms revealed that chloroquine and verapamil, and chloroquine and fantofarone, acted synergistically against chloroquine-resistant strains of P. falciparum. When used at subinhibitory concentrations, verapamil appeared 2 to 3 times more potent than fantofarone in reversing chloroquine resistance. Indeed, verapamil completely reversed the chloroquine resistance in P. falciparum, while fantofarone did so only partially. In the highly chloroquine-resistant strain FcB1, VR and SR acted synergistically to reverse CQ resistance, and the concentrations of VR used in these combinations could be reduced 10- or 100-fold (e.g. 100 nM and 10 nM) those required when this drug was used alone. In the moderately chloroquine-resistant strain K1, a combination of VR and SR for CQ resistance reversal allowed us to reduce the concentration of these chemosensitizers 1000- and 100-fold, respectively. The maximum tolerable plasma level beyond which side-effects occurred when using verapamil is 2.5 microM. Thus, the approach described, which allowed us to lower the doses of chemosensitizers, could well prevent toxic effects in humans and enlighten the advantages of polychemotherapy.

Apoptosis related to chloroquine sensitivity of the human malaria parasite Plasmodium falciparum

As chemoresistance of Plasmodium falciparum to chloroquine has arisen, new ways of combating the infection are needed. Similarities exist between the multidrug resistance of mammalian cells and chloroquine resistance of P. falciparum, based on the occurrence of internucleosomal deoxyribonucleic acid (DNA) breakdown and the ability of some anticancer drugs and chloroquine to induce apoptosis. Using chloroquine, oligonucleosomal DNA fragmentation was observed with a sensitive strain of P. falciparum, but not with a resistant one. This suggests that apoptosis may be involved in the action of chloroquine on the parasite.

Detection of RAPD markers correlated with chloroquine resistance in Plasmodium falciparum

We described the use of the random amplified polymorphic DNA (RAPD) technique on Plasmodium falciparum DNA to detect genetic markers for chloroquine-resistant strains. Fourteen RAPD primers were tested, three of which generated banding patterns correlated with chloroquine resistance. To measure this correlation, the RAPD profiles were analyzed using the Nei and Li similarity coefficient. Detection of distinctive RAPD bands allowed us to synthesize specific PCR primers to be used on whole-blood samples. Two primer sets were synthesized and tested on sensitive and resistant strains for their ability to amplify the DNA fragment corresponding to the RAPD marker. These results suggest that RAPD and PCR techniques can be used as powerful tools for the detection of genetic markers associated with drug resistance.

Biophysical aspects of P-glycoprotein-mediated multidrug resistance

In the 45 years since Burchenal's observation of chemotherapeutic drug resistance in tumor cells, many investigators have studied the molecular basis of tumor drug resistance and the phenomenon of tumor multidrug resistance (tumor MDR). Examples of MDR in microorganisms have also become topics of intensive study (e.g., Plasmodium falciparum MDR and various types of bacterial MDR) and these emerging fields have, in some cases, borrowed language, techniques, and theories from the tumor MDR field. Serendipitously, the cloning of MDR genes overexpressed in MDR tumor cells has led to elucidation of a large family of membrane proteins [the ATP-binding cassette (ABC) proteins], an important subset of which confer drug resistance in many different cells and microorganisms. In trying to decipher how ABC proteins confer various forms of drug resistance, studies on the structure and function of both murine and human MDR1 protein (also called P-glycoprotein or P-gp) have often led the way. Although various theories of P-gp function have become popular, there is still no precise molecular-level description for how P-gp overexpression lowers intracellular accumulation of chemotherapeutic drugs. In recent years, controversy has developed over whether the protein protects cells by translocating drugs directly (as some type of drug pump) or indirectly (through modulating biophysical parameters of the cell). In this ongoing debate over P-gp function, detailed consideration of biophysical issues is critical but has often been neglected in considering cell biological and pharmacological issues. In particular, P-gp overexpression also changes plasma membrane electrical potential (delta psi zero) and intracellular pH (pHi), and these changes will greatly affect the cellular flux of a large number of compounds to which P-gp overexpression confers resistance. In this chapter, we highlight these biophysical issues and describe how delta psi zero and pHi may in fact be responsible for many MDR-related phenomena that have often been hypothesized to be due to direct drug translocation (e.g., drug pumping) by P-gp.

The antiparasite effects of cyclosporin A: possible drug targets and clinical applications

1. The immunosuppressive drug cyclosporin A, and some of its nonimmunosuppressive derivatives, are potent inhibitors of a range of parasites of humans. 2. Cyclosporin A and the structurally unrelated immunosuppressant FK506 are known to act on T-lymphocytes as complexes with their binding proteins, cyclophilins and FKBPs, respectively. 3. Cyclophilins and FKBPs have been structurally identified in a number of parasites and, in some instances, are believed to play roles in the antiparasitic actions of these drugs. 4. Nonimmunosuppressive cyclosporins and FK506 derivatives may have clinical potential in certain parasitic diseases, especially malaria and schistosomiasis, and identification of the targets of these drugs in parasites may lead to development of novel chemotherapeutic agents.

Analysis of pfmdr1 and drug susceptibility in fresh isolates of Plasmodium falciparum from subsaharan Africa

Resistance of Plasmodium falciparum to many therapeutic agents is an increasing problem in most endemic areas. The role of the mdr-like gene products of P. falciparum in resistance to quinoline-containing compounds is not clear. The purpose of this study was to further examine the role of pfmdr1 in drug resistance in fresh clinical isolates originating from Africa. Drug susceptibility testing (chloroquine, mefloquine, halofantrine and quinine) and a molecular analysis of pfmdr1 was completed for 51 fresh clinical isolates. A statistical association between the chloroquine sensitivity phenotype and an intragenic allele of pfmdr1 was noted at a position, amino acid 86, which was previously associated with chloroquine resistance. There was little variation in the other intragenic alleles previously associated with chloroquine resistance. No correlation between pfmdr1 intragenic allelic variation and susceptibility to mefloquine, halofantrine or quinine was found. There was no association between gene copy number of pfmdr1 and any drug resistant phenotype in an analysis of selected isolates. This, along with other data, suggests that mefloquine resistance may have arisen by two different mechanisms in African and Southeast Asian isolates. Much more variability in the polyasparaginated region of the pfmdr1 gene was noted in this study than previously reported. In addition, fingerprint analysis using multiplex PCR revealed considerable genetic variability among these isolates.

Roles of peptidyl-prolyl cis-trans isomerase and calcineurin in the mechanisms of antimalarial action of cyclosporin A, FK506, and rapamycin

The immunosuppressive peptide cyclosporin A inhibits the growth of malaria parasites in vitro and in vivo, but little is known about its mechanism of antimalarial action. The immunosuppressive action of cyclosporin A is believed to result from binding of the drug to cyclophilins (intracellular peptidyl-prolyl cis-trans isomerases), and inhibition of the protein phosphatase calcineurin by the cyclosporin A-cyclophilin complex. Two immunosuppressive macrolides, FK506 and rapamycin, bind to a distinct isomerase, FKBP12, and the FK506-FKBP complex also inhibits calcineurin. Calcineurin itself is apparently involved in signal transduction between the T-cell membrane and nucleus, and its inhibition blocks T-cell activation. Rapamycin inhibits a later step in T-cell proliferation. Peptidyl-propyl cis-trans isomerase activity was detected in extracts of Plasmodium falciparum. It was completely inhibited by concentrations of cyclosporin A above 0.1 microM, but not by FK506 or rapamycin, and probably represented one or more cyclophilins. Comparison of the antimalarial and anti-isomerase activities of a series of cyclosporin analogues failed to reveal a correlation between the two properties. Cyclosporin A and its more active 8'-oxymethyl-dihydro-derivative, in combination with the cyclophilin-containing P. falciparum extract, inhibited the protein phosphatase activity of bovine calcineurin. Therefore inhibition of a putative P. falciparum calcineurin by a complex of CsA and cyclophilin might be responsible for the antimalarial action of the drug. The most active cyclosporin, however, was a 3'-keto-derivative of cyclosporin D (SDZ PSC-833) which inhibited P. falciparum growth with a 50% inhibitory concentration (IC50) of 0.032 microM (compared with 0.30 microM for cyclosporin A), but was a poor inhibitor of the parasite isomerase. 3'-Keto-cyclosporin D has negligible immunosuppressive activity, but it strongly inhibits the P-glycoprotein of multi-drug resistant mammalian tumour cells. FK506 and rapamycin were also active antimalarials (IC50 of 1.9 and 2.6 microM, respectively) but in the absence of detectable FKBP in P. falciparum extracts, their mechanisms of antimalarial action remain unclear.

Expression of the plasmodial pfmdr1 gene in mammalian cells is associated with increased susceptibility to chloroquine

Chloroquine (CQ)-resistant (CQR) Plasmodium falciparum malaria parasites show a strong decrease in CQ accumulation in comparison with chloroquine-sensitive parasites. Controversy exists over the role of the plasmodial pfmdr1 gene in the CQR phenotype. pfmdr1 is a member of the superfamily of ATP-binding cassette transporters. Other members of this family are the mammalian multidrug resistance genes and the CFTR gene. We have expressed the pfmdr1-encoded protein, Pgh1, in CHO cells and Xenopus oocytes. CHO cells expressing the Pgh1 protein demonstrated an increased, verapamil-insensitive susceptibility to CQ. Conversely, no increase in drug susceptibility to primaquine, quinine, adriamycin, or colchicine was observed in Pgh1-expressing cells. CQ uptake experiments revealed an increased, ATP-dependent accumulation of CQ in Pgh1-expressing cells over the level in nonexpressing control cells. The increased CQ accumulation in Pgh1-expressing cells coincided with an enhanced in vivo inhibition of lysosomal alpha-galactosidase by CQ. CHO cells expressing Pgh1 carrying two of the CQR-associated Pgh1 amino acid changes (S1034C and N1042D) did not display an increased CQ sensitivity. Immunofluorescence experiments revealed an intracellular localization of both mutant and wild-type forms of Pgh1. We conclude from our results that wild-type Pgh1 protein can mediate an increased intracellular accumulation of CQ and that this function is impaired in CQR-associated mutant forms of the protein. We speculate that the Pgh1 protein plays an important role in CQ import in CQ-sensitive malaria parasites.

Expression of the plasmodial pfmdr1 gene in mammalian cells is associated with increased susceptibility to chloroquine

Chloroquine (CQ)-resistant (CQR) Plasmodium falciparum malaria parasites show a strong decrease in CQ accumulation in comparison with chloroquine-sensitive parasites. Controversy exists over the role of the plasmodial pfmdr1 gene in the CQR phenotype. pfmdr1 is a member of the superfamily of ATP-binding cassette transporters. Other members of this family are the mammalian multidrug resistance genes and the CFTR gene. We have expressed the pfmdr1-encoded protein, Pgh1, in CHO cells and Xenopus oocytes. CHO cells expressing the Pgh1 protein demonstrated an increased, verapamil-insensitive susceptibility to CQ. Conversely, no increase in drug susceptibility to primaquine, quinine, adriamycin, or colchicine was observed in Pgh1-expressing cells. CQ uptake experiments revealed an increased, ATP-dependent accumulation of CQ in Pgh1-expressing cells over the level in nonexpressing control cells. The increased CQ accumulation in Pgh1-expressing cells coincided with an enhanced in vivo inhibition of lysosomal alpha-galactosidase by CQ. CHO cells expressing Pgh1 carrying two of the CQR-associated Pgh1 amino acid changes (S1034C and N1042D) did not display an increased CQ sensitivity. Immunofluorescence experiments revealed an intracellular localization of both mutant and wild-type forms of Pgh1. We conclude from our results that wild-type Pgh1 protein can mediate an increased intracellular accumulation of CQ and that this function is impaired in CQR-associated mutant forms of the protein. We speculate that the Pgh1 protein plays an important role in CQ import in CQ-sensitive malaria parasites.

Relationship of global chloroquine transport and reversal of resistance in Plasmodium falciparum

Control of falciparum malaria has become almost impossible in many areas due to the development of resistance to chloroquine and other antimalarial drugs. Verapamil and a number of unrelated compounds which chemosensitise multi-drug resistant cancer cells also enhance chloroquine susceptibility in Plasmodium falciparum. Chloroquine is accumulated to lower levels in resistant plasmodia, hence the reversal of chloroquine resistance has been attributed to the ability of chemosensitising agents to increase the amount of chloroquine accumulated by the resistant parasite. We have conducted a detailed examination of the effect of verapamil on chloroquine sensitivity and its relationship to chloroquine accumulation. The ability of verapamil to increase steady-state chloroquine accumulation was found to be totally insufficient to explain the increase in chloroquine sensitivity caused by the drug. In contrast, when chloroquine accumulation was increased by raising the pH gradient, the corresponding shifts in sensitivity to chloroquine could be accurately predicted. These results were confirmed with other classes of chemosensitisers and we conclude that an alternative mechanistic explanation is required to completely explain the reversal of chloroquine resistance in P. falciparum.

Rapid chloroquine efflux phenotype in both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. A correlation of chloroquine sensitivity with energy-dependent drug accumulation

Recent reports suggest that lower levels of chloroquine accumulation in chloroquine-resistant isolates of Plasmodium falciparum are achieved by energy-dependent chloroquine efflux from resistant parasites. In support of this argument, a rapid chloroquine efflux phenotype has been observed in some chloroquine-resistant isolates of P. falciparum. In this study, no relationship was found between chloroquine sensitivity and the rate of [3H]chloroquine efflux from four isolates of P. falciparum with a greater than 10-fold range in sensitivity to chloroquine. All the isolates tested displayed the rapid efflux phenotype, irrespective of sensitivity. However, chloroquine sensitivity of these isolates was correlated with energy-dependent rate of drug accumulation into these parasites. Verapamil and a variety of other compounds reverse chloroquine resistance. The reversal mechanism is assumed to result from competition between verapamil and chloroquine for efflux protein translocation sites, thus causing an increase in steady-state accumulation of chloroquine and hence a return to sensitivity. Verapamil accumulation at a steady-state is increased by chloroquine, possibly indicating competition for efflux of the two substrates. Increases in steady-state verapamil concentrations caused by chloroquine were identical in sensitive and resistant strains, suggesting that similar capacity efflux pumps may exist in these isolates. These data suggest that differences in steady-state chloroquine accumulation seen in these isolates can be attributed to changes in the chloroquine concentrating mechanism rather than the efflux pump. It seems likely that chloroquine resistance generally in P. falciparum, results at least in part from a change in the drug concentrating mechanism and that changes in efflux rates per se are insufficient to explain chloroquine resistance.

Hypothesis: impaired immunity as a factor which contributes to the spread of drug-resistance

Evidence has accrued to indicate that host defence mechanisms enhance the efficacy of many of the drugs used to treat infectious diseases. Because of this, and also because of the likelihood of increased pathogen loads in immunoincompetent hosts, some infections are less likely to be completely cured by normal regimens of chemotherapy in individuals with drastically impaired immune responsiveness. In such circumstances natural selection could result in the accelerated emergence of drug-resistance pathogens.