Synthesis, Cytotoxicity, and Antiplasmodial and Antitrypanosomal Activity of New Neocryptolepine Derivatives

Abstract

On the basis of the original lead neocryptolepine or 5-methyl-5H-indolo[2,3-b]quinoline, an alkaloid from Cryptolepis sanguinolenta, derivatives were prepared using a biradical cyclization methodology. Starting from easily accessible educts, this approach allowed the synthesis of hitherto unknown compounds with a varied substitution pattern. As a result of steric hindrance, preferential formation of the 3-substituted isomers over the 1-substituted isomers was observed when cyclizing N-(3-substituted-phenyl)-N'-[2-(2-trimethylsilylethynyl)phenyl]carbodiimides. All compounds were evaluated for their activity against chloroquine-sensitive as well as chloroquine-resistant Plasmodium falciparum strains, for their activity against Trypanosoma brucei and T. cruzi, and for their cytotoxicity on human MRC-5 cells. Mechanisms of action were investigated by testing heme complexation using ESI-MS, inhibition of beta-hematin formation, DNA interactions (DNA-methyl green assay and linear dichroism), and inhibition of human topoisomerase II. Neocryptolepine derivatives with a higher antiplasmodial activity and a lower cytotoxicity than the original lead have been obtained. This selective antiplasmodial activity was associated with inhibition of beta-hematin formation. 2-Bromoneocryptolepine was the most selective compound with an IC(50) value against chloroquine-resistant P. falciparum of 4.0 microM in the absence of cytotoxicity (IC(50) > 32 microM). Although cryptolepine, a known lead for antimalarials also originally isolated from Cryptolepis sanguinolenta, was more active (IC(50) = 2.0 microM), 2-bromoneocryptolepine showed a low affinity for DNA and no inhibition of human topoisomerase II, in contrast to cryptolepine. Although some neocryptolepine derivatives showed a higher antiplasmodial activity than 2-bromocryptolepine, these compounds also showed a higher affinity for DNA and/or a more pronounced cytotoxicity. Therefore, 2-bromoneocryptolepine is considered as the most promising lead from the present work for new antimalarial agents. In addition, 2-bromo-, 2-nitro-, and 2-methoxy-9-cyanoneocryptolepine exhibited antitrypanosomal activity in the micromolar range in the absence of obvious cytotoxicity.

Characterization of noncovalent complexes of antimalarial agents of the artemisinin-type and Fe(III)-heme by electrospray mass spectrometry and collisional activation tandem mass spectrometry

“…In addition, a complex of quinine with the heme dimer [(2FP Ϫ H):Qn] ϩ (m/z 1556, RA 10%) can be noted. Formation of a noncovalent complex with heme dimer or ␤-hematin has been detected previously for neocryptolepine [28]. Peaks related to heme, Artemisinin-heme mixture.…”

“…The relative binding strengths between the drugs and Fe(III)-heme was assessed using lowenergy collision-induced dissociation. This approach has previously been shown to be useful in determining the structure-activity relationship of antimalarial agents, namely, terpene isonitriles [27] and neocryptolepine derivatives [28]. We demonstrate in the present work that artemisinin-type drugs form a noncovalent complex with Fe(III)-heme in vitro, which is weaker than that with quinine.…”

Interactions of cryptolepine and neocryptolepine with unusual DNA structures

“…Neocryptolepine exhibited a lower global affinity for DNA than cryptolepine, as shown by equilibrium dialysis and mass spectrometry. This observation may be related to the reduced cytotoxicity of neocryptolepine and its derivatives as compared to cryptolepine [25]. These molecules are developed as selective antimalarial agents and should not exhibit pronounced DNA interactions.…”

“…Neocryptolepine (as well as 2-C1-, 2-Br-and 2 methoxyneocryptolepine) was obtained by organic synthesis as previously described [25]. Powders were resuspended at high concentrations (mM) in DMSO (Sigma) and then diluted in bidistilled water for further experiments.…”

Natural Products Active Against African Trypanosomes: A Step Towards New Drugs

“…The synthesis of neocryptolepine derivatives such as 2-nitroneocryptolepine 43 has led to compounds with an IC 50 on T. b. brucei of 0.7 µM whereas the IC 50 on MRC-5 cells was greater than 32 µM constituting a new lead structure with antitrypanosomal potential. 40 Tryptanthrin 44 (indolo [2,1-b]quinazoline-6,12-dione) is unusual in that its synthesis was described 50 years before it was discovered in a number of plant species. Scovill et al 41 The unsubstituted compounds were weakly active (IC 50 = 23 and 40 µM for 44 and 45 respectively).…”