INHIBITION OF HSP90 IN TRYPANOSOMA BRUCEI AND PHARMACOKINETIC-PHARMACODYNAMIC RELATIONSHIPS OF ANTITRYPANOSOMALS
Meyer, Kirsten Joy
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Human African trypanosomiasis (sleeping sickness), caused by the protozoan parasite Trypanosoma brucei, is fatal if untreated. Current drugs are cumbersome to deliver and severely toxic. New antitrypanosomals are greatly needed. The molecular chaperone Hsp90 is conserved and highly expressed in eukaryotes, and essential for several regulatory cell pathways. Hsp90 inhibitors were investigated for anti-trypanosomal activity. Geldanamycin, radicicol, and NVP-AUY922 had nanomolar potency against bloodstream form T. brucei in vitro; CUDC-305 and novobiocin had micromolar activity. Structure–activity studies of geldanamycin analogs found 17-AAG and 17-DMAG were most selective against T. brucei compared to mammalian cells. Both oral and parenteral 17-DMAG cured mice of a normally lethal infection of T. brucei. 17-AAG treatment sensitized trypanosomes to heat shock and caused severe cell cycle disruption. Cytokinesis and kinetoplast replication were particularly affected. RNAi of trypanosome Hsp90s, HSP83 and Tb427tmp.02.0250 (putative mitochondrial Hsp90), caused growth defects. Loss of HSP83 inhibited cytokinesis, and loss of Tb427tmp.02.0250 disrupted kinetoplast replication, similar effects to those seen with 17-AAG. These promising results support the use of inhibitors to study Hsp90 function in trypanosomes and to expand current clinical development of Hsp90 inhibitors to include T. brucei. Correct dosing can be the critical difference between efficacy and failure, not only in the total dose administered but also in the shape of the drug concentration-time curve produced. A hollow-fiber cartridge model was developed to study pharmacokinetic-pharmacodynamic relationships of antitrypanosomals. Contrasting pharmacokinetic profiles, with artificial kinetics favoring either high peak concentrations or sustained drug levels, were delivered to parasites. Simply changing the shape of exposure significantly shifted the dose-response curve of 17-AAG, affecting potency and efficacy. Hsp90 inhibitors from three chemical scaffolds were concentration-driven: significantly more efficacious applied as short-lived high peaks. For optimal efficacy, ideal pharmacokinetic properties and dosing regimens of Hsp90 inhibitors as antitrypanosomals will produce high peaks in vivo. The PK-PD system was also used to study four clinically used and two candidate antitrypanosomals. In vitro PK-PD methodology is expedient, versatile, and a valuable translational tool.