High Target Homology Does Not Guarantee Inhibition: Aminothiazoles Emerge as Inhibitors of Plasmodium falciparum.

In this study, we identified three novel compound classes with potent activity against Plasmodium falciparum, the most dangerous human malarial parasite. Resistance of this pathogen to known drugs is increasing, and compounds with different modes of action are urgently needed. One promising drug target is the enzyme 1-deoxy-d-xylulose-5-phosphate synthase (DXPS) of the methylerythritol 4-phosphate (MEP) pathway for which we have previously identified three active compound classes against Mycobacterium tuberculosis. The close structural similarities of the active sites of the DXPS enzymes of P. falciparum and M. tuberculosis prompted investigation of their antiparasitic action, all classes display good cell-based activity. Through structure-activity relationship studies, we increased their antimalarial potency and two classes also show good metabolic stability and low toxicity against human liver cells. The most active compound 1 inhibits the growth of blood-stage P. falciparum with an IC50 of 600 nM. The results from three different methods for target validation of compound 1 suggest no engagement of DXPS. All inhibitor classes are active against chloroquine-resistant strains, confirming a new mode of action that has to be further investigated.

Synthesis and biological activity of N-terminal lipidated and/or fluorescently labeled conjugates of astressin as corticotropin releasing factor antagonists.

This report describes the synthesis of eight N-terminally modified astressin analogs and their biochemical evaluation as corticotropin releasing factor (CRF) antagonists. The lipidated astressin derivatives were tested on rat CRF receptor type 1 and 2alpha and were found to be active as CRF antagonists (rCRFR1: pA(2)=7.5-8.3; rCRFR2alpha: pA(2)=7.5-9.0) with nearly equal activities as compared to unmodified astressin (rCRFR1: pA(2)=8.3+/-0.09; rCRFR2alpha: pA(2)=8.7+/-0.08).

Structure-activity studies on the corticotropin releasing factor antagonist astressin, leading to a minimal sequence necessary for antagonistic activity.

Corticotropin Releasing Factor (CRF) antagonists are considered promising for treatment of stress-related illnesses such as major depression and anxiety-related disorders. We report here the design, synthesis and biological evaluation of 91 truncated astressin analogues in order to deduce the pharmacophoric amino acid residues. Such truncated peptides may serve as valuable lead structures for the development of new small, non-peptide-based CRF antagonists. N-Terminal truncation of astressin led to active CRF antagonists that are substantially reduced in size and are selectively active at the human CRF receptor type 1 in vitro and in vivo. Subsequently, an alanine scan in combination with further truncated derivatives led to the proposal of a new pharmacophoric model of peptide-based CRF antagonists. It was found that the astressin(27-41)C sequence is the shortest active CRF antagonist. The first eight N-terminal amino acid residues were found to be an important structural determinant and were replaceable by alanine residues, thus enhancing the alpha-helical propensity. A covalent structural constraint is of utmost importance for the preorganization of the C-terminal amino acid residues. The C-terminal heptapeptide sequence, however, was found to be crucial for the antagonistic activity, since substitution or deletion of any residue led to inactive compounds.

An optimized solid phase synthesis strategy--including on-resin lactamization--of astressin, its retro-, inverso-, and retro-inverso isomers as corticotropin releasing factor antagonists.

This report describes an optimized solid phase synthesis strategy for astressin and new derivatives thereof. The synthesis is based on 9-fluorenylmethyloxycarbonyl/allyl/tert-butyl chemistry. The glutamic acid and lysine residue, which together form the cyclic constraint by coupling of their side chains, were protected by allyl functionalities during the synthesis of the linear peptide. Allyl removal by Pd(0) and the construction of the lactam bridge have been performed on-resin after completion of the chain assembly. This synthetic methodology resulted in high chemical yields (58-72%) and excellent purities of the crude peptides. The peptides were tested for their binding at the corticotropin releasing factor receptor, type 1, and their corticotropin releasing factor antagonistic activity. Furthermore, astressin and its analogs were studied by CD in order to determine the secondary structure in solution. Since the linear form of astressin and also the cyclic inverso isomer were found to be fully inactive, it can be concluded that a cyclic constraint and a right-handed alpha-helix, respectively, are of utmost importance for these peptides to act as corticotropin releasing factor antagonists.