The prolyl oligopeptidase inhibitor KYP-2047 is not readily bioavailable to bloodstream form trypanosomes and human myelocytic cells

Introduction: Only three drugs are currently available for treatment of the neurological stage of human African sleeping sickness caused by the protozoan parasite Trypanosoma brucei. As these drugs have serious side effects and are difficult to administer, new and safe anti-trypanosomal medications are urgently required. Research in recent years has shown that prolyl oligopeptidase (POP) inhibitors are promising trypanocidal agents. As the novel POP inhibitor KYP-2047 can cross the blood-brain barrier, we aimed to test whether this compound would prove to be a promising anti-trypanosomal drug candidate. Materials and Methods: The efficacy of KYP-2047 to inhibit trypanosome and human POP was evaluated with cell lysates using the fluorogenic peptide substrate Suc-Gly-Pro-Leu-Gly-Pro-AMC. The trypanocidal and cytotoxic activity of KYP-2047 was studied in vitro using bloodstream forms of T. brucei and human myelocytic leukemia HL-60 cells. The bioavailability of KYP-2047 to T. brucei and HL-60 cells was determined by incubation of cells with the inhibitor for 24 h followed by measuring the residual POP activity. Results: KYP-2047 inactivated POP in cell lysates of T. brucei and HL-60 cells with IC50 values in the mid nanomolar range indicating that the compound is a very potent inhibitor of the trypanosome and human enzyme. However, KYP-2047 did not affect the growth of T. brucei and HL-60 cells. Upon incubation of the cells with 100 μM KYP-2047, POP activity was inhibited between 20-80% which is too low to have any effect on cell growth. Conclusion: The absence of trypanocidal and cytotoxic activity of KYP-2047 is due to low bioavailability of the inhibitor to bloodstream forms of T. brucei and human HL-60 cells.