The aim of this study was to investigate the synergistic effect of quinacrine and a novel apigenin dimer (compound 9d) on reversing pentamidine resistance of Leishmania parasites.Methods
Pentamidine-resistant cell lines, LePentR50 and LdAG83PentR50, were generated by gradually increasing pentamidine pressure on wild-type promastigotes. We tested the effects of different combinations of quinacrine and an apigenin dimer on modulating the pentamidine resistance levels of LePentR50 and LdAG83PentR50 using an MTS proliferation assay. We then measured the accumulation level of pentamidine using HPLC. The fractional inhibitory concentration index (FICI) method was used to evaluate the interaction between quinacrine and the apigenin dimer on reversing pentamidine resistance in Leishmania.Results
LePentR50 and LdAG83PentR50 promastigotes were ∼8.6- and 4.6-fold more resistant to pentamidine than their wild-type parents. Amastigotes derived from LePentR50 and LdAG83PentR50 were also pentamidine-resistant. We found that quinacrine can increase the susceptibility of Leishmania to pentamidine. Quinacrine, when used at 6 µM, can increase the IC50 of pentamidine by 3.8-, 3.4-, 3.5- and 6.3-fold in wild-type Leishmania enriettii Le, LePentR50, wild-type Leishmania donovani LdAG83 and LdAG83PentR50, respectively. Quinine, quinidine and verapamil did not show any sensitizing effect. The sensitizing effect of quinacrine was: (i) dose-dependent; (ii) not associated with an increase in pentamidine accumulation; and (iii) only observed in pentamidine-resistant but not sodium stibogluconate-resistant or vinblastine-resistant parasites. Other than quinacrine, we also found that an apigenin dimer (compound 9d), previously shown to be able to inhibit ABCB1-mediated cancer drug resistance in mammalian cells, can also increase the pentamidine susceptibility of Leishmania. 9d, when used at 6 µM, can increase the IC50 of pentamidine by 2.5-, 4.2-, 1.6- and 1.9-fold in Le, LePentR50, LdAG83 and LdAG83PentR50, respectively. Unlike quinacrine, sensitization by 9d was accompanied by an increase in pentamidine accumulation, presumably due to the inhibition of an ABC transporter. Using the FICI method, we found that quinacrine and 9d can act synergistically. When they are used in a 1:1 ratio, they sensitize LePentR50 to pentamidine by 19-fold, with an FICI of 0.48 (P < 0.005), indicating that they might act synergistically.Conclusions
Our findings support the notion that the pentamidine susceptibility of Leishmania is mediated by multiple targets. Quinacrine and apigenin dimer 9d, each inhibiting its own target, can have a synergistic effect when used together to sensitize Leishmania to pentamidine.