Pharmacological properties of indolone- N -oxides controlled by a bioreductive transformation in red blood cells?

Indolone-N-oxides, long known for their biological activities, possess remarkable anti-infectious properties. With the aim of improving the pharmacological and antimalarial properties of indolone-N-oxide derivatives (INODs), 6-(4-chlorophenyl)-7H-[1,3]dioxolo[4,5-f]indol-7-one-5-oxide, compound 1, was selected to study its penetration and biotransformation in red blood cells (RBC) in vitro. Compound 1 accumulated inside RBCs and was rapidly bio-transformed giving a major fluorescent metabolite, the dihydroanalogue, 1-HH, identified after extraction, through LC-MS and NMR analyses. This bioreductive transformation was (i) observed with other INOD derivatives (ii: 1–7); (ii) observed in normal, β-thalassemic and Plasmodium falciparum infected RBCs; (iii) temperature and thiol-dependent; (iv) not observed with heat-denatured RBCs, suggesting an enzyme-dependent biotransformation. The dihydro form, 1-HH, has antiplasmodial activity but lower than the parent compound. Since the RBCs represent 99% of the total cellular space of blood in humans, this leads to extensive metabolism of indolone-N-oxide type compounds. Given the redox events occurring in Plasmodium infected RBCs, this bioreductive transformation may be pivotal for parasite redox balance and antiplasmodial activity. However, it may be a drawback when other pharmacological properties of INODs are investigated. These results show the importance of RBCs as an in vitro model to study the biotransformation of drugs, especially antimalarial drugs in the early discovery stages.