The putative PAINs nostotrebin 6 and derivatives from Nostoc sp. inhibit the trypanosomal cysteine protease rhodesain

The trypanosomal cysteine protease rhodesain plays a major role during parasitic infection by Trypanosoma brucei, known as human African trypanosomiasis. The enzyme is regarded as a promising target for new drugs [1]. Thus, a collection of about 670 cyanobacteria extracts was screened for inhibitory activity against rhodesain using an established assay. The screening revealed 21 media extracts with an inhibition higher than 80% at 0.2 mg/mL. The most promising extract from a Nostoc sp. strain was fractionated using flash and high-performance liquid chromatography, resulting in the isolation of 14 inhibitory compounds. Structure elucidation by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry revealed one of the compounds to be Nostotrebin 6 [2] while the others are new intriguing structurally related compounds composed of two basic building blocks that form monomeric, dimeric, or trimeric structures. Nostotrebin 6 acts as an inhibitor of acetylcholinesterase (IC50 6µM) and butyrylcholinesterase (IC50 7µM) [2] and exhibited further bioactivity; e.g. cytotoxicity and pro-apoptotic activity [3]. Due to its broad bioactivity spectrum and as, to our knowledge, no structure-activity relationship has been established so far, we assume Nostotrebin 6, a polyphenolic compound, to be a pan-assay interference (PAIN) compound.

[1] Ettari R, Tamborini L, Angelo IC, Micale N, Pinto A, Micheli C de, Conti P. The Journal of Medicinal Chemistry 2013; 56: 5637 – 5658

[2] Zelik P, Lukesova A, Cejka J, Budesinsky M, Havlicek V, Cegan A, Kopecky J. J Enzyme Inhib Med Chem 2010; 25: 414 – 420

[3] Vacek J, Hrbáč J, Kopecký J, Vostálová J. Molecules 2011; 16: 4254 – 4263