Arnulf Dorn

Identification of an antimalarial synthetic trioxolane drug development candidate

The discovery of artemisinin more than 30 years ago provided a completely new antimalarial structural prototype; that is, a molecule with a pharmacophoric peroxide bond in a unique 1,2,4-trioxane heterocycle. Available evidence suggests that artemisinin and related peroxidic antimalarial drugs exert their parasiticidal activity subsequent to reductive activation by haem, released as a result of haemoglobin digestion by the malaria-causing parasite. This irreversible redox reaction produces carbon-centred free radicals, leading to alkylation of haem and proteins (enzymes), one of which—the sarcoplasmic-endoplasmic reticulum ATPase PfATP6 (ref. 7)—may be critical to parasite survival. Notably, there is no evidence of drug resistance to any member of the artemisinin family of drugs. The chemotherapy of malaria has benefited greatly from the semi-synthetic artemisinins artemether and artesunate as they rapidly reduce parasite burden, have good therapeutic indices and provide for successful treatment outcomes. However, as a drug class, the artemisinins suffer from chemical (semi-synthetic availability, purity and cost), biopharmaceutical (poor bioavailability and limiting pharmacokinetics) and treatment (non-compliance with long treatment regimens and recrudescence) issues that limit their therapeutic potential. Here we describe how a synthetic peroxide antimalarial drug development candidate was identified in a collaborative drug discovery project.

An assessment of drug-haematin binding as a mechanism for inhibition of haematin polymerisation by quinoline antimalarials

Chloroquine is thought to exert its antimalarial activity by preventing the polymerisation of toxic haematin released during proteolysis of haemoglobin in the Plasmodium digestive vacuole. However, the molecular mechanisms by which this inhibition occurs and the universality of this mechanism for other quinoline antimalarials remain to be established. We demonstrate here a correlation for eight antimalarial quinolines between inhibition of haematin polymerisation in vitro and inhibition of P. falciparum growth in culture, confirming haematin polymerisation as the likely target of quinoline blood schizonticides. Furthermore, using isothermal titration microcalorimetry, a correlation was observed between the haematin binding constant of these compounds and their ability to inhibit haematin polymerisation, suggesting that these compounds mediate their activity through binding to haematin. It was also observed that the compounds bind primarily to the mu-oxo dimer form of haematin rather than the monomeric form. It is postulated that this binding inhibits haematin polymerisation by shifting the haematin dimerisation equilibrium to the mu-oxo dimer, thus reducing the availability of monomeric haematin for incorporation into haemozoin. These data reconcile the haematin polymerisation theory with the Fitch hypothesis, which states that chloroquine mediates its activity through binding to haematin.

4-aminoquinoline analogs of chloroquine with shortened side chains retain activity against chloroquine-resistant Plasmodium falciparum.

We have synthesized several 4-aminoquinolines with shortened side chains that retain activity against chloroquine-resistant isolates of Plasmodium falciparum malaria (W. Hofheinz, C. Jaquet, and S. Jolidon, European patent 94116281.0, June 1995). We report here an assessment of the activities of four selected compounds containing ethyl, propyl, and isopropyl side chains. Reasonable in vitro activity (50% inhibitory concentration, < 100 nM) against chloroquine-resistant P. falciparum strains was consistently observed, and the compounds performed well in a variety of plasmodium berghei animal models. However, some potential drawbacks of these compounds became evident upon in-depth testing. In vitro analysis of more than 70 isolates of P. falciparum and studies with a mouse in vivo model suggested a degree of cross-resistance with chloroquine. In addition, pharmacokinetic analysis demonstrated the formation of N-dealkylated metabolites of these compounds. These metabolites are similarly active against chloroquine-susceptible strains but are much less active against chloroquine-resistant strains. Thus, the clinical dosing required for these compounds would probably be greater for chloroquine-resistant strains than for chloroquine-susceptible strains. The clinical potential of these compounds is discussed within the context of chloroquines low therapeutic ratio and toxicity.

A Comparison and Analysis of Several Ways to Promote Haematin (Haem) Polymerisation and an Assessment of Its Initiation In Vitro

We compared several methods for producing haematin polymerisation at physiological temperatures (i.e., 37 degrees) and found that a trophozoite lysate-mediated reaction was inappropriate for measuring compound inhibition of haematin polymerisation. Using this method, we obtained significantly higher IC50 values (concentration inhibiting haematin polymerisation by 50%) for certain compounds than when other methods were used, including a food vacuole lysate-mediated reaction. This difference was probably due to the binding of these compounds to cytosolic parasite proteins, as proteinase K treatment of the trophozoite lysate reversed this effect. The initiation of haematin polymerisation was also investigated using several assays. It was found that haematin polymerisation occurred spontaneously, in the absence of preformed haemozoin, over a period of several days, but that the process was more rapid when an acetonitrile extract of malarial trophozoites was added. This extract contained no detectable protein, and its activity could be replicated using an extract from uninfected erythrocytes and by using lipids. We therefore postulate that no protein or parasite-specific material is absolutely required for the initiation of haematin polymerisation. The formation of beta-haematin de novo using the acetonitrile extract is more pH-dependent than the generation of newly synthesised beta-haematin from preformed haemozoin and cannot proceed much above pH = 6. We postulate that the initiation of haematin polymerisation is more sensitive to the equilibrium of haematin between its monomeric and mu-oxo dimer form and requires a higher concentration of monomer than for the elongation phase of polymerisation.

Distamycin-induced inhibition of homeodomain-DNA complexes.

The mobility shift assay was used to study the competition of the minor groove binder distamycin A with either an Antennapedia homeodomain (Antp HD) peptide or derivatives of a fushi tarazu homeodomain (ftz HD) peptide for their AT‐rich DNA binding site. The results show that distamycin and the homeodomain peptides compete under the conditions: (i) preincubation of DNA with distamycin and subsequent addition of HD peptide; (ii) simultaneous incubation of DNA with distamycin and HD peptide; and (iii) preincubation of DNA with HD peptide and subsequent addition of distamycin. There is also competition when using a peptide which lacks the N‐terminal arm of ftz HD that is involved in contacts in the minor groove. It is proposed that the proteins binding affinity is diminished by distamycin‐induced conformational changes of the DNA. The feasibility of the propagation of conformational changes upon binding in the minor groove is also shown for the inhibition of restriction endonucleases differing in the AT content of their recognition site and of their flanking DNA sequences. Thus, it is demonstrated that minor groove binders can compete with the binding of proteins in the major groove, providing an experimental indication for the influence of biological activities exerted by DNA ligands binding in the minor groove.

The Structure−Activity Relationship of the Antimalarial Ozonide Arterolane (OZ277)

The structure and stereochemistry of the cyclohexane substituents of analogues of arterolane (OZ277) had little effect on potency against Plasmodium falciparum in vitro. Weak base functional groups were not required for high antimalarial potency, but they were essential for high antimalarial efficacy in P. berghei-infected mice. Five new ozonides with antimalarial efficacy and ADME profiles superior or equal to that of arterolane were identified.

Hematin Polymerization Assay as a High-Throughput Screen for Identification of New Antimalarial Pharmacophores

ABSTRACT Hematin polymerization is a parasite-specific process that enables the detoxification of heme following its release in the lysosomal digestive vacuole during hemoglobin degradation, and represents both an essential and a unique pharmacological drug target. We have developed a high-throughput in vitro microassay of hematin polymerization based on the detection of 14C-labeled hematin incorporated into polymeric hemozoin (malaria pigment). The assay uses 96-well filtration microplates and requires 12 h and a Wallac 1450 MicroBeta liquid scintillation counter. The robustness of the assay allowed the rapid screening and evaluation of more than 100,000 compounds. Random screening was complemented by the development of a pharmacophore hypothesis using the “Catalyst” program and a large amount of data available on the inhibitory activity of a large library of 4-aminoquinolines. Using these methods, we identified “hit” compounds belonging to several chemical structural classes that had potential antimalarial activity. Follow-up evaluation of the antimalarial activity of these compounds in culture and in thePlasmodium berghei murine model further identified compounds with actual antimalarial activity. Of particular interest was a triarylcarbinol (Ro 06-9075) and a related benzophenone (Ro 22-8014) that showed oral activity in the murine model. These compounds are chemically accessible and could form the basis of a new antimalarial medicinal chemistry program.

Characterization of chloroquine-hematin μ-oxo dimer binding by isothermal titration calorimetry

Numerous studies indicate that a key feature of chloroquines (CQ) antimalarial activity is its interaction with hematin. We now characterize this CQ-hematin interaction in detail using isothermal titration calorimetry (ITC). Between pH 5.6 and 9.0, association constants (K(a) values) for enthalpy-driven CQ-hematin mu-oxo dimer binding fell in the narrow range of 2.3-4.4 x 10(5) M(-1). It is therefore probable that CQ-hematin mu-oxo dimer binding affinity does not diminish at the pH range (4.8-5.4) of the parasite food vacuole. The binding affinity was unaffected by high salt concentrations, suggesting that ionic interactions do not contribute significantly to this complexation. With increasing ionic strength, the entropic penalty of CQ-hematin mu-oxo dimer binding decreased accompanied by increased hematin mu-oxo dimer aggregation. A stoichiometry (n) of 1:4 in the pH range 6.5-9.0 indicates that CQ binds to two hematin mu-oxo dimers. At pH 5.6, a stoichiometry of 1:8 suggests that CQ binds to an aggregate of four hematin mu-oxo dimers. This work adds further evidence supporting the hypothesis that CQ impedes hematin monomer incorporation into hemozoin by producing a forward shift in the hematin monomer-hematin mu-oxo dimer equilibrium, contributing to a destructive accumulation of soluble forms of hematin in the parasite and leading to its death by hematin poisoning.

De Novo design, synthesis, and in vitro evaluation of a new class of nonpeptidic inhibitors of the malarial enzyme plasmepsin II.

Malaria, a life-threatening disease caused by parasites of the genus Plasmodium, affects 500 million people annually, of which more than one million die. The emergence of multi-drugresistant strains of Plasmodium falciparum, the parasite that causes the deadliest form of malaria, exacerbates the situation and necessitates new medicines with novel modes of action. Plasmepsin II (PII ; EC3.4.23.39), a parasitic aspartic protease involved in the hemoglobin degradation process that takes place in an acidic vacuole, has been identified as a potential target for antimalarial therapy. Several groups reported PII inhibitors that mimic the natural substrate and display up to single-digit nanomolar activity. Inhibition of PII is expected to block the life cycle of the parasite. Here we report the synthesis and in vitro evaluation of a new class of nonpeptidic PII inhibitors developed with the help of structure-based de novo design that show up to single-digit micromolar inhibitory activities. A major conformational change around the active site of the human aspartic protease renin (EC3.4.23.15) upon complexation of 3,4-disubstituted piperidines has been observed, which unveils unexpected flexibility of the enzyme. The flap that lies over the catalytic dyad, and a tryptophan side chain of the core domain, move and thereby unlock a new hydrophobic pocket (flap pocket). The high sequence homology between renin and PII prompted us to hypothesize that an induced-fit adaptation such as that of the active site of renin might also be operative [3] H. Zhu, M. Bilgin, R. Bangham, D. Hall, A. Casamayor, P. Bertone, N. Lan, R. Jansen, S. Bidlingmaier, T. Houfek, T. Mitchell, P. Miller, R. A. Dean, M. Gerstein, M. Snyder, Science 2001, 293, 2101. [4] A.-C. Gavin, M. Bˆsche, R. Krause, P. Grandi, M. Marzioch, A. Bauer, J. Schultz, J. M. Rick, A.-M. Michon, C.-M. Cruciat, M. Remor, C. Hˆfert, M. Schelder, M. Brajenovic, H. Ruffner, A. Merino, K. Klein, M. Hudak, D. Dickson, T. Rudi, V. Gnau, A. Bauch, S. Bastuck, B. Huhse, C. Leutwein, M.-A. Heurtier, R. R. Copley, A. Edelmann, E. Querfurth, V. Rybin, G. Drewes, M. Raida, T. Bouwmeester, P. Bork, B. Seraphin, B. Kuster, G. Neubauer, G. Superti-Furga, Nature 2002, 415, 141. [5] M. Mann, R. C. Hendrickson, A. Pandey, Annu. Rev. Biochem. 2001, 70. [6] A. Dongre, J. K. Eng, J. R. Yates, Trends Biotechnol. 1997, 15, 418. [7] M. R. Wilkins, K. L. Williams, R. D. Appel, D. F. Hiochstrasser, Proteome Research: New Frontiers in Functional Genomics, Springer, Berlin, 1997. [8] Y. Ho, A. Gruhler, A. Heilbut, G. D. Bader, L. Moore, S.-L. Adams, A. Millar, P. Taylor, K. Bennett, K. Boutilier, L. Yang, C. Wolting, I. Donaldson, S. Schandorff, J. Shewnarane, M. Vo, J. Taggart, M. Goudreault, B. Muskat, C. Alfarano, D. Dewar, Z. Lin, K. Michalickova, A. R. Willems, H. Sassi, P. A. Nielsen, K. J. Rasmussen, J. R. Andersen, L. E. Johansen, L. H. Hansen, H. Jespersen, A. Podtelejnikov, E. Nielsen, J. Crawford, V. Poulsen, B. D. S ̆rensen, J. Matthiesen, R. C. Hendrickson, F. Gleeson, T. Pawson, M. F. Moran, D. Durocher, M. Mann, C. W. V. Hogue, D. Figeys, M. Tyers, Nature 2002, 415, 180. [9] L. Wang, A. Brock, B. Herberich, P. G. Schultz, Science 2001, 292, 498. [10] L. Wang, A. Brock, P. G. Schultz, J. Am. Chem. Soc. 2002, 124, 1836. [11] J. W. Chin, S. W. Santoro, A. B. Martin, D. S. King, L. Wang, P. G. Schultz, J. Am. Chem. Soc. 2002, 124, 9026. [12] J. C. Kauer, S. Erickson-Viitanen, H. R. Wolfe, W. F. DeGrado, J. Biol. Chem. 1986, 261, 10695. [13] J. W. Chin, A. B. Martin, D. S. King, L. Wang, P. G. Schultz, Proc. Natl. Acad. Sci. USA 2002, 99, 11020. [14] G. Dorman, G. D. Prestwich, Biochemistry 1994, 33, 5661. [15] D. A. Fancy, K. Melcher, S. A. Johnston, T. Kodadek, Chem. Biol. 1996, 3, 551. [16] Y. Maru, D. E. Afar, O. N. Witte, M. Shibuya, J. Biol. Chem. 1996, 271, 15353. [17] M. A. McTigue, D. R. Williams, J. A. Tainer, J. Mol. Biol. 1995, 246, 21. [18] W. A. Houry, D. Frishman, C. Eckerskorn, F. Lottspeich, F. U. Hartl, Nature 1999, 402, 147. [19] The near-UV irradiation per unit area (fluence) at 365 nm used in these studies for a five-minute exposure was approximately 10 ± 30 kJm . Classic studies on the effects of bacterial exposure at this wavelength have shown that lethal effects of such radiation on bacterial cells require fluences one to two orders of magnitude greater than this value. This observation raises the possibility that the phenotypes of live cells may be altered by photocrosslinking between protein surfaces in vivo. [20] B. Alberts, Cell 1998, 92, 291.

Deferoxamine: Stimulation of hematin polymerization and antagonism of its inhibition by chloroquine

The iron chelator deferoxamine enhances the clearance of Plasmodium falciparum parasitemia and may be useful in drug combinations for the treatment of cerebral malaria. However, the deferoxamine-chloroquine drug combination is antagonistic, or at best additive, against P. falciparum in vitro. As chloroquine is thought to exert its antimalarial activity by interacting with hematin released from the proteolytic degradation of hemoglobin in the parasite food vacuole, we hypothesized that deferoxamine might interfere with the ability of chloroquine to inhibit hematin polymerization, since it was reported that deferoxamine interacts with hematin. Therefore, we assessed deferoxamine-hematin binding in more detail and investigated the effect of deferoxamine on hematin polymerization in the presence and absence of chloroquine. Isothermal titration calorimetry (ITC) experiments demonstrated an enthalpy-driven deferoxamine:hematin mu-oxo dimer binding with an association constant of 2.8 x 10(4) M(-1) at pH 6.5, a binding affinity 14-fold lower than that measured for chloroquine. At least two of the three hydroxamic acid functional groups of deferoxamine must be unionized for effective binding. We also discovered that deferoxamine antagonized chloroquine-mediated inhibition of hematin polymerization. Unexpectedly, deferoxamine increased the concentration of soluble forms of hematin and enhanced the rate of hematin polymerization. Deferoxamine also could initiate hematin polymerization. In contrast, chloroquine decreased the concentration of soluble forms of hematin and inhibited hematin polymerization. This work supports the postulate that initiation of hematin polymerization requires a higher concentration of soluble hematin monomer than does the elongation phase of polymerization and provides one possible explanation for the observed antagonism between deferoxamine and chloroquine against parasites in culture.