Conjugates of 2,4-Dihydroxybenzoate and Salicylhydroxamate and Lipocations Display Potent Antiparasite Effects by Efficiently Targeting the Trypanosoma brucei and Trypanosoma congolense Mitochondrion

Abstract

We investigated a chemical strategy to boost the trypanocidal activity of 2,4dihydroxybenzoic acid (2,4-DHBA)-and salicylhydroxamic acid (SHAM)-based trypanocides with triphenylphosphonium and quinolinium lipophilic cations (LC). Three series of LC conjugates were synthesized that were active in the submicromolar (5a-d and 10d-f) to low nanomolar (6a-f) range against wild-type and multi-drug resistant strains of African trypanosomes (Trypanosoma brucei brucei and T. congolense). This represented an improvement in trypanocidal potency of at least 200fold, and up to >10,000-fold, compared with the non-LC coupled parent compounds 2,4-DHBA and SHAM. Selectivity over human cells was >500 and reached >23,000 for 6e. Mechanistic studies showed that 6e did not inhibit the cell cycle but affected parasite respiration in a dose-dependent manner. Inhibition of the trypanosome alternative oxidase (TAO) and the mitochondrial membrane potential was also studied for selected compounds. We conclude that effective mitochondrial targeting greatly potentiated the activity of these compound series.

Alternative oxidase inhibitors: Mitochondrion‐targeting as a strategy for new drugs against pathogenic parasites and fungi

“…The model supported the enzymological inhibitory kinetics of the derivatives, which characterized a number of them as competitive inhibitors of TAO . Due to the availability of the crystal structure of TAO in the ligand‐free and inhibitor‐bound forms, there are renewed efforts toward utilizing the structure data for rational design of new inhibitors with improved potency and drug‐like properties above AF and SHAM …”

“…However, until recently only the full‐length protein had been produced and studied by the various authors, despite difficulties associated with its stability, solubility, and yield, and although it is clearly desirable to conduct inhibitor studies with the physiologically relevant form of the protein. When a ΔMTS‐rTAO, lacking the first 75 nucleotides, was produced, it was found to have relatively higher activity, solubility, and stability compared with the full‐length enzyme …”

“…The low solubility and toxicity of SHAM have stimulated the design of SHAM derivatives that are more potent, less toxic, and display improved solubility (see Section ) . Likewise, efforts to find alternative TAO inhibitors has led to the discovery of AF, and its safety and efficacy have been tested in mice infected with African trypanosomes .…”

“…As discussed above, known inhibitors of TAO (SHAM and AF) had earlier been discovered, making SADD targeting of TAO much easier. Guided by the TAO structure, efforts are currently ongoing to modify AF and SHAM structures to obtain more potent inhibitors with better drug‐like properties …”

“…In particular, the triphenylphosphonium (TPP) cation, which holds a charge that is delocalized over a large and hydrophobic surface area, can cross lipid bilayers by noncarrier mediated transport electrophoretically driven by the plasma and mitochondrial transmembrane potentials (Figure ) . Other cations such as quinolinium salts are also effective mitochondrion‐targeting groups …”