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• W1971275933 abstract "The malaria organism Plasmodium falciparum detoxifies heme released during degradation of host erythrocyte hemoglobin by sequestering it within the parasite digestive vacuole as a polymer called hemozoin. Antimalarial agents such as chloroquine appear to work by interrupting the heme polymerization process, but their efficacy has been impaired by the emergence of drug-resistant organisms. We report here the identification of a new class of antimalarial compounds, hexadentate ethylenediamine-N,N′-bis[propyl(2-hydroxy-(R)-benzylimino)]metal(III) complexes [(R)-ENBPI-M(III)] and a corresponding ((R)-benzylamino)] analog [(R)-ENBPA-M(III)], a group of lipophilic monocationic leads amenable to metallopharmaceutical development. Racemic mixtures of Al(III), Fe(III), or Ga(III) but not In(III) (R)-ENBPI metallo-complexes killed intraerythrocytic malaria parasites in a stage-specific manner, the R = 4,6-dimethoxy-substituted ENBPI Fe(III) complex being most potent (IC50∼1 μM). Inhibiting both chloroquine-sensitive and -resistant parasites, potency of these imino complexes correlated in a free metal-independent manner with their ability to inhibit heme polymerization in vitro In contrast, the reduced (amino) 3-MeO-ENBPA Ga(III) complex (MR045) was found to be selectively toxic to chloroquine-resistant parasites in a verapamil-insensitive manner. In 21 independent recombinant progeny of a genetic cross, susceptibility to this agent mapped in perfect linkage with the chloroquine resistance phenotype suggesting that a locus for 3-MeO-ENBPA Ga(III) susceptibility was located on the same 36-kilobase segment of chromosome 7 as the chloroquine resistance determinant. These compounds may be useful as novel probes of chloroquine resistance mechanisms and for antimalarial drug development. The malaria organism Plasmodium falciparum detoxifies heme released during degradation of host erythrocyte hemoglobin by sequestering it within the parasite digestive vacuole as a polymer called hemozoin. Antimalarial agents such as chloroquine appear to work by interrupting the heme polymerization process, but their efficacy has been impaired by the emergence of drug-resistant organisms. We report here the identification of a new class of antimalarial compounds, hexadentate ethylenediamine-N,N′-bis[propyl(2-hydroxy-(R)-benzylimino)]metal(III) complexes [(R)-ENBPI-M(III)] and a corresponding ((R)-benzylamino)] analog [(R)-ENBPA-M(III)], a group of lipophilic monocationic leads amenable to metallopharmaceutical development. Racemic mixtures of Al(III), Fe(III), or Ga(III) but not In(III) (R)-ENBPI metallo-complexes killed intraerythrocytic malaria parasites in a stage-specific manner, the R = 4,6-dimethoxy-substituted ENBPI Fe(III) complex being most potent (IC50∼1 μM). Inhibiting both chloroquine-sensitive and -resistant parasites, potency of these imino complexes correlated in a free metal-independent manner with their ability to inhibit heme polymerization in vitro In contrast, the reduced (amino) 3-MeO-ENBPA Ga(III) complex (MR045) was found to be selectively toxic to chloroquine-resistant parasites in a verapamil-insensitive manner. In 21 independent recombinant progeny of a genetic cross, susceptibility to this agent mapped in perfect linkage with the chloroquine resistance phenotype suggesting that a locus for 3-MeO-ENBPA Ga(III) susceptibility was located on the same 36-kilobase segment of chromosome 7 as the chloroquine resistance determinant. These compounds may be useful as novel probes of chloroquine resistance mechanisms and for antimalarial drug development. Malaria is still one of the world's most devastating infectious diseases, afflicting several hundred million people and killing close to two million children each year (1Sturchler D. Parasitol. Today. 1989; 5: 39-40Abstract Full Text PDF Scopus (114) Google Scholar). Plasmodium falciparum, the most deadly species, has become widely resistant to most available antimalarial therapies (2Wellhoner H.H. Neville Jr., D.M. Srinivasachar K. Erdmann G. J. Biol. Chem. 1991; 266: 4309-4314Abstract Full Text PDF PubMed Google Scholar, 3Olliaro P. Cattani J. Wirth D. J. Am. Med. Assoc. 1996; 275: 230-233Crossref PubMed Google Scholar). The mainstays of treatment and prophylaxis, the quinolines, are excellent parasitocidal agents for susceptible organisms but are becoming less and less efficacious. New antimalarials are desperately needed, and an agent that attacked the same vital target as chloroquine, but was not subject to the same resistance modes, would be highly desirable (4Olliaro P.L. Goldberg D.E. Parasitol. Today. 1995; 11: 294-297Abstract Full Text PDF PubMed Scopus (145) Google Scholar). Chloroquine and related drugs appear to work by accumulating to high concentrations in the parasite digestive vacuole (5Veignie E. Moreau S. Ann. Trop. Med. Parasitol. 1991; 85: 229-237Crossref PubMed Scopus (18) Google Scholar, 6Krogstad D.J. Gluzman I.Y. Herwaldt B.L. Schlesinger P.H. Wellems T.E. Biochem. Pharmacol. 1992; 43: 57-62Crossref PubMed Scopus (87) Google Scholar), the organelle in which hemoglobin is degraded and free heme is released (4Olliaro P.L. Goldberg D.E. Parasitol. Today. 1995; 11: 294-297Abstract Full Text PDF PubMed Scopus (145) Google Scholar). This heme is toxic to eukaryotic cells and is capable of killing parasites by lysing membranes (7Orjih A.U. Banyal H.S. Chevli R. Fitch C.D. Science. 1981; 214: 667-669Crossref PubMed Scopus (166) Google Scholar) and inactivating critical proteases (8Vander Jagt D. Hunsaker L. Campos N. Scaletti J. Biochim. Biophys. Acta. 1992; 1122: 256-264Crossref PubMed Scopus (47) Google Scholar, 9Gluzman I. Francis S. Oksman A. Smith C. Duffin K. Goldberg D. J. Clin. Invest. 1994; 93: 1602-1608Crossref PubMed Scopus (252) Google Scholar). To counter this, the parasite polymerizes heme into a crystalline matrix called hemozoin, in which heme units are coordinated via a unique iron-carboxylate linkage (10Slater A. Swiggard W. Orton B. Flitter W. Goldberg D. Cerami A. Henderson G. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 325-329Crossref PubMed Scopus (483) Google Scholar). A family of molecules called histidine-rich proteins has been implicated in the polymerization reaction (11Sullivan Jr., D.J. Gluzman I.Y. Goldberg D.E. Science. 1996; 271: 219-222Crossref PubMed Scopus (353) Google Scholar), and lipids may also contribute to the process (12Bendrat K. Berger B. Cerami A. Nature. 1995; 378Crossref PubMed Scopus (155) Google Scholar). It is postulated that once hemozoin formation has been initiated, extension of the polymer is able to proceed nonenzymatically (11Sullivan Jr., D.J. Gluzman I.Y. Goldberg D.E. Science. 1996; 271: 219-222Crossref PubMed Scopus (353) Google Scholar). Chloroquine can block this extension reaction, resulting in accumulation of toxic heme (11Sullivan Jr., D.J. Gluzman I.Y. Goldberg D.E. Science. 1996; 271: 219-222Crossref PubMed Scopus (353) Google Scholar, 13Chou A. Chevli R. Fitch C. Biochemistry. 1980; 19: 1543-1549Crossref PubMed Scopus (327) Google Scholar, 14Slater A.F.G. Cerami A. Nature. 1992; 355: 167-169Crossref PubMed Scopus (538) Google Scholar). 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Chem. 1983; 20: 225-336Crossref PubMed Scopus (70) Google Scholar) and their corresponding metal complexes evaluated as diagnostic agents or medicinals (23Deutsch E. Libson K. Jurisson S. Lindoy L.F. Prog. Inorg. Chem. 1983; 30: 75-139Crossref Scopus (117) Google Scholar, 24Green M.A. Welch M.J. Huffman J.C. J. Am. Chem. Soc. 1984; 106: 3689-3691Crossref Scopus (44) Google Scholar, 25Lauffer R. Chem. Rev. 1987; 87: 901-927Crossref Scopus (2006) Google Scholar, 26Kumar K. Tweedle M.F. Pure Appl. Chem. 1993; 65: 515-520Crossref Scopus (132) Google Scholar, 27Abrams M.J. Murrer B.A. Science. 1993; 261: 725-730Crossref PubMed Scopus (252) Google Scholar, 28Tsang B.W. Mathias C.J. Green M.A. J. Nucl. Med. 1993; 34: 1127-1131PubMed Google Scholar, 29Tsang B.W. Mathias C.J. Fanwick P.E. Green M.A. J. Med. Chem. 1994; 37: 4400-4406Crossref PubMed Scopus (57) Google Scholar, 30Xu J. Franklin S.J. Whisenhunt D.W. Raymond K.N. J. Am. Chem. Soc. 1995; 117: 7245-7246Crossref Scopus (172) Google Scholar). We recently reported that selected hexadentate (N4O2) ethylenediamine-bis[propyl((R)-benzylimino)]Fe(III) complexes are hydrolytically stable and possess pharmacological potential resulting from a favorable balance of hydrophobicity and delocalized monocationic charge which enhance cell membrane permeability (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar). In mammalian cells, these compounds exhibit cytotoxic activity that is modulated by expression of the multidrug resistance (MDR1) 1The abbreviations used are:MDR1multidrug resistance geneHRP IIhistidine-rich protein IIpfmdr1P. falciparum multidrug resistance genePgh1pfmdr1 gene product(R)-ENBPI(R)-thylenediamine-,N′-is[ropyl(2-hydroxy-(R)-benzylmino)](R)-ENBPA(R)-thylenediamine-,N′-is[ropyl(2-hydroxy-(R)-benzylmino)]MR0453-me-th-oxy-ENBPA Ga(III) complex. P-glycoprotein in the plasma membranes (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar). Human MDR1 P-glycoprotein is a homolog of the P. falciparum pfmdr1 gene product, Pgh1, both being integral membrane proteins of the ATP-binding cassette superfamily of membrane transporters (32Wilson C.M. Seranno A.E. Wasley A. Bogenshutz M.P. Shankar A.H. Wirth D.F. Science. 1989; 244: 1184-1186Crossref PubMed Scopus (396) Google Scholar, 33Foote S.J. Kyle D.E. Martin R.K. Oduola A.M.J. Forsyth K. Kemp D.J. Cowman A.F. Nature. 1990; 345: 255-258Crossref PubMed Scopus (509) Google Scholar, 34Ford J.M. Hait W.N. Pharmacol. Rev. 1990; 42: 155-199PubMed Google Scholar, 35Gottesman M.M. Pastan I. Annu. Rev. Biochem. 1993; 62: 385-427Crossref PubMed Scopus (3567) Google Scholar). While P-glycoprotein is an outwardly directed drug transporter (35Gottesman M.M. Pastan I. Annu. Rev. Biochem. 1993; 62: 385-427Crossref PubMed Scopus (3567) Google Scholar, 36Piwnica-Worms D. Rao V. Kronauge J. Croop J. Biochemistry. 1995; 34: 12210-12220Crossref PubMed Scopus (108) Google Scholar), Pgh1 has been localized to the digestive vacuole membrane and implicated in chloroquine sequestration (37Cowman A.F. Karcz S. Galatis D. Culvenor J.G. J. Cell Biol. 1991; 113: 1033-1042Crossref PubMed Scopus (253) Google Scholar), possibly as an inwardly directed transporter (38van Es H. Karcz S. Chu F. Cowman A. Vidal S. Gros P. Schurr E. Mol. Cell. Biol. 1994; 14: 2419-2428Crossref PubMed Scopus (84) Google Scholar). Pgh1 has been proposed to contribute to chloroquine resistance in various clinical isolates of P. falciparum (33Foote S.J. Kyle D.E. Martin R.K. Oduola A.M.J. Forsyth K. Kemp D.J. Cowman A.F. Nature. 1990; 345: 255-258Crossref PubMed Scopus (509) Google Scholar), although pfmdr1 has been unlinked from chloroquine resistance in a genetic cross (39Wellems T.E. Panton L.J. Gluzman I.Y. do Rosario V.E. Gwadz R.W. Walker-Jonah A. Krogstad D.J. Nature. 1990; 345: 253-255Crossref PubMed Scopus (442) Google Scholar). multidrug resistance gene histidine-rich protein II P. falciparum multidrug resistance gene pfmdr1 gene product (R)-thylenediamine-,N′-is[ropyl(2-hydroxy-(R)-benzylmino)] (R)-thylenediamine-,N′-is[ropyl(2-hydroxy-(R)-benzylmino)] 3-me-th-oxy-ENBPA Ga(III) complex. Because members of the N4O2 class of metal(III) complexes are known to interact with at least one ATP-binding cassette transporter, we originally conceived that these compounds could be designed to target ATP-binding cassette homologs in P. falciparum As potential antimalarial agents, the hexadentate metal(III) complexes with the general structure shown in Fig. 1 offer tremendous flexibility, since their metal binding affinities can be varied by inserting appropriate donor atoms to match the requirements of various coordinated metals, including Fe(III), thereby enhancing stability of the intact complex (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar, 40Wong E. Liu S. Lugger T. Hahn F.E. Orvig C. Inorg. Chem. 1995; 34: 93-101Crossref Scopus (55) Google Scholar). We report that these compounds comprise a new class of readily synthesized antimalarials that blocks hemozoin formation. Additionally, while several agents were effective against chloroquine-sensitive and -resistant clones, one agent showed paradoxically selective toxicity solely against chloroquine-resistant clones in a non-pfmdr1-dependent manner. All reagents, solvents, and metal salts (nitrates and acetylacetonates of Al(III), Fe(III), Ga(III), and In(III)) were obtained from Aldrich or Alpha Chemical Co. as analytical grade materials. Heptadentate precursors of the desired Schiff base hexadentate ligands (1, Fig. 1) were obtained by condensation of the appropriate linear tetramine and three equivalents of substituted salicylaldehyde in ethanol or in dry methylene chloride as described previously (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar). A bis-salicylaldamine analog of 1 was obtained by condensation of the appropriate linear tetramine and two equivalents of 3-methoxy-substituted salicylaldehyde followed by in situ reduction of the Schiff base as described (40Wong E. Liu S. Lugger T. Hahn F.E. Orvig C. Inorg. Chem. 1995; 34: 93-101Crossref Scopus (55) Google Scholar). 2V. Sharma, A. Beatty, D. E. Goldberg, and D. Piwnica-Worms, manuscript submitted. Heptadentate Schiff base precursors and Schiff base reduced analogs were characterized by 1H and 13C NMR, infrared (IR), and mass spectrometry (fast atom bombardment-high resolution mass spectrometry) at the Washington University Resource for Biomedical and Bioorganic Mass Spectrometry Facility (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar). Metal complexes were obtained through the reactions of Schiff base precursors and reduced analogs with the appropriate metal salts in ethanol as described previously (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar). Resulting metal(III) complexes were characterized by 1H NMR (except for Fe(III) complexes), IR, fast atom bombardment-low resolution spectrometry and elemental analysis establishing structural identity and confirming purity. Plasmodium falciparum lines (HB3, FCR-3, Indochina-1/CDC (Indo-1), Dd2 and progeny of the HB3xDd2 cross (39Wellems T.E. Panton L.J. Gluzman I.Y. do Rosario V.E. Gwadz R.W. Walker-Jonah A. Krogstad D.J. Nature. 1990; 345: 253-255Crossref PubMed Scopus (442) Google Scholar)) were grown in intra-erythrocytic culture by the method of Trager and Jensen (41Trager W. Jensen J.B. Science. 1976; 193: 673-675Crossref PubMed Scopus (6219) Google Scholar). Cultures were maintained at 5% parasitemia, 2% hematocrit using human serum and erythrocytes, in a 3% oxygen, 3% carbon dioxide atmosphere. Synchronization of developmental stage was achieved by sorbitol treatment (42Lambros C. Vanderberg J. J. Parasitol. 1979; 65: 418-420Crossref PubMed Scopus (2855) Google Scholar). Parasite growth inhibition and half-maximal inhibitory concentration values (IC50) of antagonists were determined by measuring [3H]hypoxanthine incorporation (43Desjardins R.E. Canfield R.J. Haynes J.D. Chulay J.D. Antimicrob. Agents Chemother. 1979; 16: 710-718Crossref PubMed Scopus (2263) Google Scholar). Parasites were incubated with antagonist for 24 h starting at the late ring stage, and then [3H]hypoxanthine was added for 4 h at the mid-trophozoite stage before harvesting parasites and assaying for incorporated radioactivity. To assay for verapamil reversal, parasites were incubated with antagonist ± verapamil for 72 h, and then [3H]hypoxanthine was added. All drugs were added as 1:1000 dilutions of a 10 mM dimethyl sulfoxide stock. Vehicle alone had no effect on [3H]hypoxanthine incorporation. The procedure used was modified from those of Slater and Cerami (14Slater A.F.G. Cerami A. Nature. 1992; 355: 167-169Crossref PubMed Scopus (538) Google Scholar) and Dorn et al. (44Dorn A. Stoffel R. Matile H. Bubendorf A. Ridley R. Nature. 1995; 374: 269-271Crossref PubMed Scopus (362) Google Scholar). Hemin (50 μM) was incubated with parasite-derived hemozoin (2 μg/ml) in 100 mM sodium acetate, pH 5.0 (final volume 0.5 ml). After 16 h at 37°C, product was harvested by centrifugation at 4°C, 15,000 × g for 30 min. The pellet was resuspended by brief sonication in 1 ml of 2.5% sodium dodecyl sulfate (SDS) in 0.1 M sodium bicarbonate, pH 9.1, with incubation at 37°C for 30 min. Centrifugation was repeated and the resultant pellet washed in 1 ml of 2.5% SDS with sonication. After centrifugation, the pellet was solubilized with 2.5% SDS, 50 mN NaOH (1 h at room temperature with intermittent mixing). Product was measured spectrophotometrically (A) at 400 nm. Control incubations had no added hemin or hemin alone, and the sum of their adsorption values was subtracted. Individual inhibitors or chloroquine was added directly to the incubations and was compared with incubations containing drug vehicle (dimethyl sulfoxide) alone. The histidine-rich protein II (HRP II)-mediated polymerization assay was performed as described previously (11Sullivan Jr., D.J. Gluzman I.Y. Goldberg D.E. Science. 1996; 271: 219-222Crossref PubMed Scopus (353) Google Scholar) using 500 pmol of protein incubated at 37°C for 16 h in a 1-ml reaction with 50 μM hematin. The digestive vacuole proteases plasmepsins I and II and falcipain were purified and assayed using [14C]globin substrate as described previously (9Gluzman I. Francis S. Oksman A. Smith C. Duffin K. Goldberg D. J. Clin. Invest. 1994; 93: 1602-1608Crossref PubMed Scopus (252) Google Scholar). Inhibitors were added as 1:1000 dilutions of a 10 mM dimethyl sulfoxide stock. All reagents and starting materials were inexpensive and commercially available, favorable practical considerations for the development of these agents. The synthesis and characterization of these compounds are described elsewhere (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar, 40Wong E. Liu S. Lugger T. Hahn F.E. Orvig C. Inorg. Chem. 1995; 34: 93-101Crossref Scopus (55) Google Scholar). An appealing property of the ethylenediamine-N,N′-bis[propyl(2-hydroxy-(R)-benzylimino)] ligand 1 ((R)-ENBPI; Fig. 1) and the corresponding Schiff base reduced amino analog ((R)-ENBPA; Fig. 1) is their ability to efficiently form stable “holo-complexes” with a wide range of metal ions (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar, 40Wong E. Liu S. Lugger T. Hahn F.E. Orvig C. Inorg. Chem. 1995; 34: 93-101Crossref Scopus (55) Google Scholar). Specifically, we synthesized complexes of the general structure 2 coordinating Al(III), Fe(III), Ga(III), and In(III) from the reactions of heptadentate Schiff base precursors and Schiff base amino derivatives with corresponding metal(III) acetylacetonates or hydrated metal(III) salts in refluxing ethanol or methanol, respectively. Formation of these metal(III) complexes from the ligands 1 (Fig. 1) results in formation of racemic mixtures of stereoisomers (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar, 45Sarma B.D. Bailar J.C. J. Am. Chem. Soc. 1955; 77: 5476-5480Crossref Scopus (189) Google Scholar). Because of the importance of Fe(III) in malarial metabolism and relative biocompatibility of this metal with the host, Fe(III) compounds were the target leads tested against P. falciparum trophozoites in intra-erythrocytic culture. P. falciparum killing curves for the 4,6-dimethoxy-ENBPI Fe(III) derivative are shown in Fig. 2A IC50 values of 1 to 1.5 μM were obtained against chloroquine-sensitive HB3 as well as against resistant FCR-3, Indo-1, and Dd2 lines. Control experiments with chloroquine confirmed that the HB3 line was sensitive, whereas the others were chloroquine-resistant (Table I, bottom). While the 3-methoxy-ENBPI Fe(III) analog was as potent as the 4,6-dimethoxy analog against the chloroquine-sensitive line, the agent was less effective against the resistant FCR-3 and Indo-1 lines (Table I). Parasite death as measured by the [3H]hypoxanthine incorporation assay correlated well with direct blood smear counts. The drugs were also observed to be most effective at the mid-trophozoite stage. For example, using the 4,6-dimethoxy derivative, more mature parasites were resistant and less mature parasites grew normally until they developed into trophozoites, at which point they were killed by the treatment (Fig. 2B). Parasites cultured in the presence of drug showed greatly diminished hemozoin formation. In contrast, control parasites matured normally, developing a large digestive vacuole filled with hemozoin pigment, and then underwent normal schizogony.Table IAntimalarial activity of (R)-ENBPI metal(III) complexes against chloroquine-sensitive (CQS) and -resistant (CQR) P. falciparum clones in vitroMetal(R)-ENBPI complexesConcentrationClonesSubstitutionsHB3 (CQS)FCR-3 (CQR)Indo-1 (CQR)R3R4R6μM% growth inhibitionFe(III)OMeHH590.431.151.6Fe(III)HOMeOMe599.996.196.1In(III)HOMeOMe5NAANAANAAChloroquine0.0274.5NAANAA Open table in a new tab We therefore assessed the ability of drug to block heme polymerization in an in vitro assay. Heme was incubated with pre-formed hemozoin under acidic conditions in the presence of varying concentrations of the 4,6-dimethoxy-ENBPI Fe(III) complex. The compound was a potent polymerization inhibitor with an IC50 of ∼4 μM (Table II). Control experiments confirmed an identical IC50 for chloroquine. Recent data suggest that the parasite's HRP II can initiate heme polymerization in the Plasmodium digestive vacuole (11Sullivan Jr., D.J. Gluzman I.Y. Goldberg D.E. Science. 1996; 271: 219-222Crossref PubMed Scopus (353) Google Scholar). The 4,6-dimethoxy-ENBPI Fe(III) complex showed equivalent potency in blocking polymerization using the HRP II assay (IC50∼ 4 μM).Table IIInhibition of Plasmodium culture (HB3) and hemozoin formation by (R)-ENBPI metal(III) complexes correlates with MDR1mediated drug resistanceENBPI ligandMetalParasite culture IC50Hemozoin IC50MDR1-mediated fold resistanceμMμM(R) = 3-MeOFe(III)2477Ga(III)>20502.6Al(III)>20302.6In(III)>20>501.4(R) = 4,6-Di-MeOFe(III)14>200Ga(III)32>200Al(III)33>200In(III)>20401.0 Open table in a new tab To determine whether coordination of Fe(III) per se was critical for antimalarial activity or whether overall conformations of the intact metallopharmaceutical conferred the desired effect, parasite culture and heme polymerization inhibition curves were also generated for 3-methoxy- and 4,6-dimethoxy-ENBPI complexes using a variety of coordinated metals including Ga(III), Al(III), and In(III) (Table II). An excellent correlation was found between inhibition of hemozoin formation and ability to kill the Plasmodium culture for all complexed metal species. In particular, note that the 3-methoxy-ENBPI Ga(III) complex was a relatively poor inhibitor of both HB3 culture growth and heme polymerization, whereas the 4,6-dimethoxy-ENBPI Ga(III) analog was a potent inhibitor in both assays. Furthermore, the 3-methoxy-ENBPI Ga(III) complex was equally ineffective against the chloroquine-resistant FCR-3 line, whereas the 4,6-dimethoxy-ENBPI Ga(III) analog was equally potent (data not shown). Interestingly, the potency of inhibition of both heme polymerization and HB3 Plasmodium culture correlated well with the fold resistance to the agents induced by expression of the human MDR1 P-glycoprotein in a cancer cell cytotoxicity assay (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar) (Table II). To further determine if demetallation reactions were potentially involved in the mechanism of inhibition of heme polymerization, a variety of metal salts were directly tested in the hemozoin assay (Table III). Although ferric Fe(III) and Al(III) cations were potent inhibitors of the process (IC50 values of 0.7 μM and 1.5 μM, respectively), Ga(III) had little effect (IC50 = 100 μM), despite the substantial antagonist activity of (R)-ENBPI Ga(III) complexes. Conversely, while the In(III) cation showed modest inhibition (IC50 = 25 μM), the (R)-ENBPI In(III) complexes were completely ineffective. With the exception of modest activity of Fe(II), a variety of other physiological dicationic metals were ineffective (Table III). Control experiments demonstrated that the counter anions (iodide, nitrate, and perchlorate) were also without effect (data not shown). Therefore, the lack of correlation between potency of metal(III) salts and their corresponding (R)-ENBPI metal(III) complexes for inhibition of heme polymerization again pointed toward the intact holo-complex as the biochemically active component.Table IIIEffect of metal salts (10 μM) on heme polymerizationMetal% inhibition of heme polymerizationFe2+54 ± 2Fe3+100 ± 0Al3+94 ± 12In3+22 ± 1Ga3+0 ± 0Co2+−1 ± 12Cu2+−3 ± 2Ni2+−9 ± 2Zn2+5 ± 4Ca2+−6 ± 5Mn2+2 ± 1Mg2+0 ± 1 Open table in a new tab Because of the association between (R)-ENBPI metal(III) complexes and P-glycoprotein transport, and since the agents appeared to target the digestive vacuole in Plasmodium culture, assays were performed to examine possible inhibition of vacuolar protease action. The 4,6-dimethoxy-ENBPI Fe(III) complex at 10 μM had no effect on the globin cleavage activities of aspartic proteases plasmepsins I and II nor of the cysteine protease falcipain (data not shown), each purified from cultured parasites. To explore additional aspects of structure-activity relationships, the reduced amine 3-methoxy-ENBPA Ga(III) complex (MR045) was synthesized. As with its imine analog (Table II), there was relatively poor activity against the chloroquine-sensitive HB3 clone in culture (Fig. 3; IC50> 20 μM). However, and quite remarkably, the compound was active against the chloroquine-resistant Dd2 clone (Fig. 3) as well as other resistant lines (IC50 = 0.5-1 μM). Similarly, the agent was a potent inhibitor of hemozoin formation in vitro (IC50∼1 μM; data not shown). Control experiments confirmed that verapamil, a known reversal agent (46Martin S.K. Oduola A.M. Milhous W.K. Science. 1987; 235: 899-901Crossref PubMed Scopus (505) Google Scholar), indeed reversed chloroquine resistance in Dd2, while showing no effect on HB3 (Dd2: chloroquine (40 nM), 10% growth inhibition; verapamil (0.6 μM), 25%; chloroquine (40 nM) plus verapamil (0.6 μM), 87%. HB3: chloroquine (40 nM), 99% growth inhibition; verapamil (0.6 μM), 6%; chloroquine (40 nM) plus verapamil (0.6 μM), 99%. S.E. ± 2%; n = 3 each). However, verapamil did not alter MR045 susceptibility in chloroquine-resistant Dd2 nor reverse MR045 resistance of HB3 (Dd2: MR045 (1 μM), 99% growth inhibition; MR045 (1 μM) plus verapamil (0.6 μM), 99%. HB3: MR045 (1 μM), 11% growth inhibition; verapamil (0.6 μM), 12%; MR045 (1 μM) plus verapamil (0.6 μM), 34%. S.E. ± 2%; n = 3 each). To reconcile these hemozoin and verapamil data requires that the (R)-ENBPA Ga(III) complex, but not chloroquine, was acted upon by a verapamil-insensitive export or sequestration mechanism in HB3 parasites, whereas chloroquine, but not the Ga(III) complex, was acted upon by a verapamil-sensitive mechanism in Dd2 parasites. To further characterize the basis of this selectivity, the 3-methoxy-ENBPA Ga(III) complex was tested against 21 independent recombinant progeny of a genetic cross between sensitive HB3 and chloroquine-resistant Dd2 lines (39Wellems T.E. Panton L.J. Gluzman I.Y. do Rosario V.E. Gwadz R.W. Walker-Jonah A. Krogstad D.J. Nature. 1990; 345: 253-255Crossref PubMed Scopus (442) Google Scholar). The pfmdr1 gene, which encodes Pgh1, has been shown not to map with the chloroquine resistance phenotype in this cross. In particular, a BamHI polymorphism at the 3′ end of pfmdr1 that uniquely identifies the inherited Dd2 parental gene is found in four clones (TC-05, QC-23, SC-01, and 3B-D5) of eight chloroquine-resistant progeny and in three clones (3B-B1, B1-SD, and TC-08) of eight chloroquine-sensitive progeny indicating that a gene separate from pfmdr1 is the major determinant of chloroquine resistance (39Wellems T.E. Panton L.J. Gluzman I.Y. do Rosario V.E. Gwadz R.W. Walker-Jonah A. Krogstad D.J. Nature. 1990; 345: 253-255Crossref PubMed Scopus (442) Google Scholar). In addition to these 16 published clones, included herein are five more progeny from this same cross (C188, CH3-61, 116, C408, and D43) in which the inheritance linkage of the chloroquine resistance phenotype has been further refined. Chloroquine-resistant clone C408 and chloroquine-sensitive clone C188 contain genetic crossover markers that localize the chloroquine resistance determinant to a 36-kilobase segment of chromosome 7. 3X.-Z. Su, L. A. Kirkman and T. E. Wellems, unpublished observations. Control culture growth experiments confirmed chloroquine sensitivity for three clones (C188, CH3-61, 116; chloroquine (40 nM): >97 ± 1% growth inhibition) and chloroquine resistance for the remaining two clones (C408 and D43; chloroquine (40 nM): 3 ± 2% and 14 ± 5% growth inhibition, respectively). As shown in Fig. 4, all of the chloroquine-sensitive progeny were resistant to the ENBPA Ga(III) complex, whereas all of the chloroquineresistant progeny were susceptible to the new agent at 5 μM. This evidence indicated that the chloroquine resistance locus and the MR045 susceptibility locus of chromosome 7 were exclusively co-inherited in these haploid progeny. Disruption of hemozoin formation within the digestive vacuole requires that agents such as chloroquine be transported across several membrane bilayers, including the erythrocyte plasma membrane, the parasitophorous vacuolar membrane, the parasite plasma membrane, and finally, the digestive vacuole membrane. Detailed mechanisms of chloroquine transport and their contributions to drug resistance have yet to be fully understood. Chloroquine is modestly lipophilic and possesses titratable protons that confer a net positive charge in acid environments. Thus, it has been proposed that diffusive transmembrane transport and drug trapping may account for concentrative accumulation of chloroquine within the digestive vacuole (reviewed in Ref. 47Slater A. Pharmacol. & Ther. 1993; 57: 203-235Crossref PubMed Scopus (320) Google Scholar). The net cationic charge provided by protonation of the agent or binding to high affinity sites within the vacuole (ferriprotoporphyrins or hemozoin) may prevent back diffusion of the compound (13Chou A. Chevli R. Fitch C. Biochemistry. 1980; 19: 1543-1549Crossref PubMed Scopus (327) Google Scholar, 48Sullivan D.J.J. Gluzman I.Y. Russell D.G. Goldberg D.E. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 11865-11870Crossref PubMed Scopus (446) Google Scholar). Other contributions to chloroquine uptake and antimalarial action may involve its properties as an amphiphilic cation which enables adsorption onto phospholipid bilayers (49Wernsdorfer W. McGregor I.(eds) Malaria: Principles and Practice of Malariology. Churchill Livingstone, New York1988Google Scholar) and, as with many hydrophobic compounds possessing a delocalized monocationic charge (50Lichtshtein D. Kaback H.R. Blume A.J. Proc. Natl. Acad. Sci. U. S. A. 1979; 76: 650-654Crossref PubMed Scopus (218) Google Scholar, 51Chernoff D.M. Strichartz G.R. Piwnica-Worms D. Biochim. Biophys. Acta. 1993; 1147: 262-266Crossref PubMed Scopus (30) Google Scholar), may allow permeation across membranes and concentrative accumulation within cell interiors in response to the negative transmembrane potentials generated by living cells (52Ritchie R.J. Prog. Biophys. Mol. Biol. 1984; 43: 1-32Crossref PubMed Scopus (77) Google Scholar). Additionally, membrane transporters may contribute either directly or indirectly as has been proposed, for example, with pfmdr1 (38van Es H. Karcz S. Chu F. Cowman A. Vidal S. Gros P. Schurr E. Mol. Cell. Biol. 1994; 14: 2419-2428Crossref PubMed Scopus (84) Google Scholar, 53Foote S. Cowman A. Acta Trop. 1994; 56: 157-171Crossref PubMed Scopus (120) Google Scholar). Thus, interfering with or bypassing any one of the steps that regulate drug permeation or the site of action could confer chloroquine resistance. We report herein the identification of a new class of antimalarial compounds designed to permeate bilayers that demonstrate potent activity in Plasmodium culture and, significantly, against both chloroquine-sensitive and chloroquine-resistant clones. Antimalarial potency of these compounds in culture correlated with their ability to inhibit heme polymerization, the most potent leads being the 4,6-dimethoxy-ENBPI Fe(III) and the 3-methoxy-ENBPA Ga(III) complexes. For the imino complexes, agents that lack significant parasitocidal activity in Plasmodium culture do not inhibit hemozoin formation in vitro It is unlikely that this can be attributed to differential demetallation reactions of the agents, since this study demonstrated poor correlation between metal(III) salts and intact ENBPI metal(III) complexes for inhibition of hemozoin formation under acidic conditions, and furthermore, the agents have been documented by 1H NMR and UV-visible spectroscopy to be hydrolytically stable at neutral pH (37°C) for 72 h (54Sharma V. Rao V. Reichert D. Crankshaw C. Welch M. Piwnica-Worms D. J. Nucl. Med. 1996; 37: 71-72PubMed Google Scholar). Molecular configuration may be relevant. Crystal structures of (R)-ENBPI Fe(III) and Ga(III) analogs show a trans configuration for the phenolic oxygens around the central coordination sphere (as in Fig. 1) (29Tsang B.W. Mathias C.J. Fanwick P.E. Green M.A. J. Med. Chem. 1994; 37: 4400-4406Crossref PubMed Scopus (57) Google Scholar, 55Ito T. Sugimoto M. Ito H. Toriumi K. Nakayama H. Mori W. Sekizaki M. Chem. Lett. 1983; : 121-124Crossref Google Scholar). However, preliminary molecular modeling data suggest that the identity of the coordination metal in the central core has a profound influence on conformation of the molecule (54Sharma V. Rao V. Reichert D. Crankshaw C. Welch M. Piwnica-Worms D. J. Nucl. Med. 1996; 37: 71-72PubMed Google Scholar). Given the larger ionic radii of six-coordinated In(III) versus Fe(III) and Ga(III) (0.81 Å versus 0.65 and 0.62 Å, respectively) (56Shannon R.D. Acta Crystallogr. A. 1976; 32: 751-767Crossref Scopus (54783) Google Scholar), molecular modeling data suggest that steric constraints favor the cis configuration for the (R)-ENBPI Indium(III) compounds (54Sharma V. Rao V. Reichert D. Crankshaw C. Welch M. Piwnica-Worms D. J. Nucl. Med. 1996; 37: 71-72PubMed Google Scholar). Thus, several compounds preferring the trans configuration are recognized by human MDR1 P-glycoprotein (31Sharma V. Crankshaw C. Piwnica-Worms D. J. Med. Chem. 1996; 39: 3483-3490Crossref PubMed Scopus (60) Google Scholar), are able to block heme polymerization, and can produce parasite death as shown herein, whereas those favoring the cis configuration cannot, potentially tying together these various biological processes into a common metabolic pathway. While data suggest that conformations of the intact holo-complexes confer pharmacological activity, additional experiments will be required to fully evaluate the potential for acid hydrolysis and demetallation reactions in the digestive vacuole on the overall mechanism of action. The IC50 values for culture inhibition are in some cases slightly lower than those for in vitro heme polymerization. Several possibilities could explain this observation. First, the conditions for heme polymerization in the digestive vacuole could be sufficiently different from those in the assay to change the IC50 value. Second, the compounds could be accumulated against a concentration gradient in the digestive vacuole. Third, it is possible that blocking only a small amount of heme polymerization leads to build-up of enough toxic free heme to kill the intact parasites. The (R)-ENBPI metal(III) complexes had equivalent potency in the pre-formed hemozoin-initiated polymerization assay as in the HRP II-mediated assay. These were similar to results obtained for chloroquine. It has been shown that heme polymerization can be initiated by HRP II-mediated bonding of hemes (11Sullivan Jr., D.J. Gluzman I.Y. Goldberg D.E. Science. 1996; 271: 219-222Crossref PubMed Scopus (353) Google Scholar). Once nucleation has occurred, polymerization can proceed nonenzymatically. Since both the (R)-ENBPI metal(III) complexes and chloroquine work on the hemozoin-seeded (protein-free) as well as the protein-mediated reactions, this suggests that the blockade is either at the nonenzymatic polymer extension phase of the reaction or a direct effect on the heme substrate. Detailed kinetics will be required to distinguish these possibilities. An unanticipated result of this study, as shown in Fig. 4, was the observation that one (R)-ENBPA Ga(III) complex (MR045), which possessed poor antimalarial activity against sensitive clones, showed selective and potent activity against all chloroquine-resistant progeny of a genetic cross (39Wellems T.E. Panton L.J. Gluzman I.Y. do Rosario V.E. Gwadz R.W. Walker-Jonah A. Krogstad D.J. Nature. 1990; 345: 253-255Crossref PubMed Scopus (442) Google Scholar). This suggested that a gene for 3-methoxy-ENBPA Ga(III) susceptibility is located on the same 36-kilobase segment of chromosome 7 as the chloroquine resistance gene and likely is the same gene. Of note, the Schiff base (R)-ENBPI Ga(III) analog of this compound showed relatively poor cytotoxicity against both resistant and sensitive lines. Thus, the data indicate that the mechanism that confers chloroquine resistance in these organisms is selectively targeted by the 3-methoxy-ENBPA Ga(III) complex. One possibility is that a transport system is altered in chloroquine resistance (57Krogstad D.J. Gluzman I.Y. Kyle D.E. Oduola A.M.J. Martin S.K. Milhous W.K. Schlesinger P.H. Science. 1987; 238: 1283-1285Crossref PubMed Scopus (467) Google Scholar), by mutation of either the transporter or an associated transport regulator. Because chloroquine resistance maps away from the pfmdr1 gene in this cross (39Wellems T.E. Panton L.J. Gluzman I.Y. do Rosario V.E. Gwadz R.W. Walker-Jonah A. Krogstad D.J. Nature. 1990; 345: 253-255Crossref PubMed Scopus (442) Google Scholar), the data further imply that the chloroquine resistance activity cannot be attributed to the Pgh1 transporter. Additionally, unlike chloroquine, selective blockade of the chloroquine-resistant Dd2 line by MR045 was verapamil-insensitive. Thus, the unusual properties of 3-methoxy-ENBPA Ga(III) can be exploited to probe the mechanisms of chloroquine resistance. Clearly, additional experiments must be done to better understand the targeting of these agents, their mechanism of selectivity, and their bioavailability. There is great potential to vary the tetramine backbone and to substitute in a variety of positions on the aromatic ring in this class of compounds. The agents are relatively easy and inexpensive to synthesize, which is a crucial feature for success of a useful reagent or antimalarial drug. It is encouraging that a group of agents with scaffolds capable of incorporating biologically compatible metals like Fe(III) can act against the same putative molecular target as chloroquine but are not susceptible to the same drug resistance mechanisms. We thank Carolyn Crankshaw for assistance with data analysis." @default.
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• W1971275933 title "Probing the Chloroquine Resistance Locus of Plasmodium falciparum with a Novel Class of Multidentate Metal(III) Coordination Complexes" @default.
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