Regina Ortmann

Acyloxyalkyl ester prodrugs of FR900098 with improved in vivo anti-malarial activity.

FR900098 represents an improved derivative of the new antimalarial drug fosmidomycin and acts through inhibition of the 1-deoxy-D-xylulose 5-phosphate (DOXP) reductoisomerase, an essential enzyme of the mevalonate independent pathway of isoprenoid biosynthesis. Prodrugs with increased activity after oral administration were obtained by chemical modification of the phosphonate moiety to yield acyloxyalkyl esters. The most successful compound demonstrated 2-fold increased activity in mice infected with the rodent malaria parasite Plasmodium vinckei.

Development of Benzophenone‐Based Farnesyltransferase Inhibitors as Novel Antimalarials

The development of farnesyltransferase inhibitors directed against Plasmodium falciparum is a strategy towards new drugs against malaria. Previously, we described benzophenone‐based farnesyltransferase inhibitors with high in vitro antimalarial activity but no in vivo activity. Through the introduction of a methylpiperazinyl moiety, farnesyltransferase inhibitors with in vivo antimalarial activity were obtained. Subsequently, a structure‐based design approach was chosen to further improve the antimalarial activity of this type of inhibitor. As no crystal structure of the farnesyltransferase of the target organism is available, homology modeling was used to reveal differences between the active sites of the rat/human and the P. falciparum farnesyltransferase. Based on flexible docking data, the piperazinyl moiety was replaced by a N,N,N′‐trimethylethylenediamine moiety. This resulted in an inhibitor with significantly improved in vitro and in vivo antimalarial activity. Furthermore, this inhibitor displayed a notable increase in selectivity towards malaria parasites relative to human cells.

The antimalarial pipeline – An update

There are quite a number of antimalarial compounds in different states of preclinical and clinical development. Among those in advanced stages, combinations of known drugs or new substances from drug classes already used in antimalarial therapy are predominant. More compounds with novel or even unknown mechanism of action are found among those compounds which are in less advanced stages of development.

Studies addressing the importance of charge in the binding of fosmidomycin-like molecules to deoxyxylulosephosphate reductoisomerase.

Fosmidomycin and its homologue FR900098 are inhibitors of 1‐deoxy‐D‐xylulose‐5‐phosphate reductoisomerase, which is part of the mevalonate‐independent isoprenoid biosynthetic pathway. Replacement of the phosphonate moiety by uncharged sulfone or sulfonamide partial structures resulted in complete loss of activity. Dropping one of the two negative charges resulted in a marked decrease in activity. Through occupation of a hydrophobic binding site, some activity could be regained, leading to compounds with micromolar activity against cultured malaria parasites.

Feeding the Antimalarial Pipeline

Malaria is still a major health problem in large parts of the world. To approach the ultimate goal of eradicating this disease, a continuous feed of the antimalarial drug pipeline is needed. Many expectations have been placed on target-based approachs after deciphering the malarial genome; however, hopes have been disappointed until now. Two research groups have recently made the results of a different approach available to the public. They have evaluated large compound libraries in a whole-cell assay against blood stages of Plasmodium falciparum, the most important malaria-causing parasite, revealing thousands of compounds with low micromolar to submicromolar activity against the parasite. These hits provide valuable starting points for further drug development that, due to the public availability of the screening data, are open for any interest research group. Malaria continues to be one of the major causes of death in the world. The World Health Organisation (WHO) has estimated the burden of malaria in 2008 to be as follows: 243 million cases, leading to 863 000 deaths. In 2004, P. falciparum was among the leading causes of death worldwide from a single infectious agent. Although considerable efforts have been invested in the development of malaria vaccines, chemotherapy will continue to be the main weapon in the fight against malaria. The armoury currently available against malaria is rather limited (for recent Reviews see Reference [2])—in fact, the last novel chemical entity (atovaquone) was introduced into therapy in 1996. Table 1 details the drugs currently in clinical use, and Figure 1 gives their structures. The combination of an artemisinin derivative with another antimalarial drug is known as artemisinin-based combination therapy (ACT) and represents the current gold standard in antimalarial treatment. Two major problems greatly reduce the therapeutic value of most of the antimalarial drugs listed in Table 1: drug toxicity/side effects and resistance. The development of resistance to any anti-infective therapy is inevitable, and antimalarial therapy is no exception. There are already signs of an emerging resistance to artemisinins, and it is no question of if but when will artemisinin resistance spread. As such, there is an urgent need for novel antimalarial agents. Table 2 gives an overview of antimalarial drug candidates in clinical development (structures are given in Figure 2). Those medicines that are in the most advanced stages of development are combinations of know substances: Euartesim (piperaquine/dihydroartemisinine); Pyramax (pyronaridine/artesunate); mefloquine/artesunate; chloroquine/azithromycin. Most of the other compounds in clinical development stem from two major classes: 4-aminoquinolines and synthetic peroxides. Only SAR97267 (formerly known as T3) and fosmidomycin act against targets as yet unexploited in antimalarial chemotherapy. To achieve the ultimate goal of eradicating malaria, a strong pipeline of innovative new medicines is mandatory. The question now arises: What is the best way to ensure a continuous feed of the pipeline with novel compounds with the potential to become a new antimalarial drug? With the sequencing of the P. falciparum genome, expectations that this will provide new targets for antimalarial drug design have been created. Indeed, a plethora of biological targets have been described, and the list of proteins that have been suggested as potential drug targets is growing. A literature survey revealed nearly 60 biological targets in P. falciparum for which inhibitors are described. However, a closer look on these inhibitors reveals that this target-based approach has not yet delivered a drug candidate. While these compounds do inhibit their molecular target, most of them fail to exert any significant activity against cultured parasites, most probably because they are unable to reach their target. Furthermore, a lot of compounds that do show whole-cell activity will never be developed into a drug due to the lack of druglike properties. In recent articles that appeared in Nature, two groups have reported the results of a different approach. Both have assayed large compound libraries against cultured malaria parasites. Compounds identified in these screens as hits have already cleared the major hurdle on which most compounds stemming from the target-based approach have failed, which is antimalarial activity in the whole-cell assay. In the first paper screening of the GlaxoSmithKline (GSK) in-house chemical library comprising nearly 2 000 000 compounds is reported. In the first screen, 13 553 compounds were identified that inhibit the growth of the chloroquine-sensitive P. falciparum 3D7 strain by more than 80 % at a concentration of 2 mm. More than 8 000 compounds also showed activity against the multidrug-resistant Dd2 strain, and fewer than 2 000 compounds displayed appreciable cytotoxicity when assayed at 10 mm against human hepatoma HepG2 cells. Of the hits identified, 82 % originate from internal company projects and thus are hitherto unknown to the public. Representatives from all antimalarial drug classes—with the exception of peroxides which are not present in the library—were recovered in the screen. The whole assembly of hits can be described by 416 molecular frameworks or 857 clusters and 1 978 singletons. Notably, hits tend to have higher molecular mass and lipo[a] Prof. Dr. M. Schlitzer, Dr. R. Ortmann Institut f r Pharmazeutische Chemie, Philipps-Universit t Marbacher Weg 6, 35032 Marburg (Germany) Fax: (+ 49) 6421-28-25953 E-mail : schlitzer@staff.uni-marbug.de

Resistance of the Burkholderia cepacia complex to fosmidomycin and fosmidomycin derivatives

The Burkholderia cepacia complex (BCC) is a group of 17 closely related opportunistic pathogens that are able to infect the respiratory tract of cystic fibrosis patients. BCC bacteria are intrinsically resistant to many antibiotics and are therefore difficult to eradicate. Fosmidomycin could be a new therapeutic agent to treat BCC infections as it inhibits 1-deoxy-d-xylulose-5-phosphate reductoisomerase (Dxr), a key enzyme in the non-mevalonate pathway essential in BCC bacteria for isoprenoid synthesis. In this study, the antimicrobial activity of fosmidomycin and eight fosmidomycin derivatives towards 40 BCC strains was investigated. All BCC strains were resistant to fosmidomycin, although addition of glucose-6-phosphate reduced the minimum inhibitory concentration values of FR900098, the fosmidomycin acetyl derivative, from 512 mg/L to 64 mg/L for Burkholderia multivorans and B. cepacia. This enhanced activity was linked to increased expression of the genes involved in glycerol-3-phosphate transport, which appears to be the only route for fosmidomycin import in BCC bacteria. Furthermore, upregulation of a fosmidomycin resistance gene (fsr) encoding an efflux pump was observed during fosmidomycin and FR900098 treatment. These results strongly suggest that the observed resistance in BCC bacteria is due to insufficient uptake accompanied by fosmidomycin and FR900098 efflux.

Structure–Activity relationships of novel anti-Malarial agents: Part 5. N-(4-acylamino-3-benzoylphenyl)-[5-(4-nitrophenyl)-2-furyl]acrylic acid amides

We have developed the [5-(4-nitrophenyl)-2-furyl]acrylic acid substituted benzophenone 4g as a novel lead for anti-malarial agents. Here, we demonstrated that the acyl residue at the 2-amino group of the benzophenone core structure has to be a phenylacetic acid substructure substituted in its para-position with methyl or other substituents of similar size. The trifluoromethyl substituted derivative displayed an IC(50) of 47 nM against the multi-drug resistant Plasmodium falciparum strain Dd2.

Structure-Based Optimization of Aldose Reductase Inhibitors Originating from Virtual Screening

Virtual screening discovered two prospective hits as potential leads for aldose reductase inhibition. Based on their crystal structures with the enzyme, a systematic optimization has been performed to reveal a first structure–activity relationship. A central thiophen moiety and a terminal nitro group exhibit the best binding properties.

Antimalarial and antitrypanosomal activity of a series of amide and sulfonamide derivatives of a 2,5-diaminobenzophenone.

Here, we describe a series of readily obtainable benzophenone derivatives with antimalarial and antitrypanosomal activity. The most active compounds display submicromolar activity against Plasmodium falciparum. Micromolar activity is obtained against Trypanosoma brucei. Main problem of the compounds is low selectivity. However, there are indications that separation of antimalarial and cytotoxic activity might by possible. In addition, some compounds inhibit human ABC transporter with nanomolar activity.

2-Acylamino-5-chlorobenzophenones with enhanced selectivity towards malaria parasites.

Previously we described a series of 5-acylaminobenzophenones with considerable antimalarial activity. Unfortunately, most compounds also displayed high cytotoxicity resulting in low selectivity towards malaria parasites. Through the replacement of the 5-acylamino moiety by simple chlorine and further modifications of the 2-acylamino residue we could obtain inhibitors with improved selectivity towards malaria parasites combined with an acceptable reduction of antimalarial activity.