Antiplasmodial thiostrepton derivatives: proteasome inhibitors with a dual mode of action.

Causing more than one million deaths annually, tropical malaria is a fundamental threat to human health worldwide. Successful chemotherapeutic treatment is increasingly complicated by frequent drug resistance of the malaria parasites Plasmodium Tsp. To counter this, different antimalaria drugs like quinine, mefloquine, and artemisinine are administered increasingly in combination. In addition, potent lowcost antibiotics such as doxycycline, clindamycin, and azithromycin are often applied. 3] The effectiveness of these inhibitors of bacterial protein biosynthesis against malaria is generally explained by the high similarity shared by 70S bacterial ribosomes and mitochondrial or apicoplast ribosomes from the eukaryotic parasite. Protein translation is then locally inhibited in these organelles. 4] A hallmark of this activity is the late onset of their antiplasmodial action, which typically occurs only four days after infection of the red blood cells. 5] This so-called “delayed death effect” is ascribed to the distribution of defective apicoplasts into daughter merozoites during replication of the erythrocytic parasite. The readily accessible thiostrepton (1) was identified early on as a very potent antibiotic with strong activity against many Gram-positive bacteria. It belongs to the large family of thiopeptide antibiotics, which are highly modified macrocyclic peptide natural products produced by ribosomal peptide biosynthesis. Thiostrepton blocks translation in bacteria by binding tightly to the GTPase-associated center of the 70S ribosome. Studies on the action of 1 in eukaryotic cells reported activity associated with immunomodulation, cancer cell proliferation, and growth inhibition of Plasmodium falciparum. 14] Thiostrepton (1) was proven to suppress protein translation in the apicoplast. 14b,c,15] However, immediate killing of the parasites led to the antimalarial effect, in contrast to the action of other ribosomal inhibitors. A delayed death effect was never observed for 1, but the reasons for these deviating properties of thiostrepton remained unclear. Here, we report on semisynthetic thiostrepton derivatives with enhanced potency against P. falciparum, desribe initial structure–activity patterns, and show that the activity of these compounds is tightly linked to the inhibition of the 20S proteasome. To investigate the antimalarial profile of thiostrepton in detail and to elucidate its potential for possible applications, we needed synthetic access to a series of derivatives. Extending previous studies on thiostrepton semisynthesis, we found that the configurationally labile thiazoline unit of 1–3 (ring C) can be selectively oxidized to the corresponding thiazole (Scheme 1). This modification conferred improved chemical stability to the compounds. We then tested a range of lipophilic and hydrophilic derivatives on the human malaria pathogen P. falciparum. This initial screening suggested that hydrophobic extensions at the dehydroamino acid terminus improved the antiplasmodial properties. A focused collection of candidate compounds was then synthesized based on combinations of tail truncation, oxidation, and addition of lipophilic thiols to the terminal dehydroamino acid (Scheme 1, Table 1). All compounds were obtained in good yield, purified by preparative HPLC, and characterized by NMR spectroscopy, HPLC, and HRMS (see the Supporting Information). Compounds 1–14 were then tested for growth inhibition of P. falciparum. Synchronized ring stages were studied at a parasitemia of 1 %, and parasite viability was monitored by measuring the activity of Plasmodium-specific lactate dehydrogenase (see the Supporting Information). In line with previous reports, 14a] we found that 1 suppressed parasite growth with an IC50 of 10 mm in our assay. No delayed death effect was observed (data not shown). Optimum substituents R* seemed to have medium chain lengths (5 and 14). Longer (6), sterically more-demanding (10, 13), and polar (11) appendages were less effective. The B-ring-opened com[*] S. Schoof, B. Ellinger, S. Baumann, M. Potowski, Prof. Dr. Y. Najajreh, Dr. H.-D. Arndt Technische Universit t Dortmund, Fakult t Chemie Otto-Hahn-Strasse 6, 44221 Dortmund (Germany) and Max-Planck-Institut f r Molekulare Physiologie Otto-Hahn-Strasse 11, 44227 Dortmund (Germany) Fax: (+ 49)231-133-2498 E-mail: hans-dieter.arndt@mpi-dortmund.mpg.de