Ritonavir enhances the efficacy of amprenavir: A promising combination therapy by targeting Leishmania DNA topoisomerase I for treatment of visceral leishmaniasis.

Visceral leishmaniasis (VL) is an opportunistic infection in HIV patients with higher relapse and mortality rate. The number of HIV-VL patients is comparatively higher in areas where both infections are endemic. However, the conventional chemotherapeutic agents have limited success due to drug toxicity, efficacy variance and overall cost of treatment. Therefore, it is crucial to discover and develop newer potent antileishmanial agents for successful eradication of the disease. Our previous report, for the first time showed the leishminicidal effect of amprenavir (APV) mediated by inhibition of L.donovani Topoisomerase I (LdTopILS). So, we intended to demonstrate the effect of APV in combination with ritonavir (RTV). The present study revealed that the complete catalytic inhibition of LdTopILS by APV (10 μM) in combination with RTV (5 μM), compare to APV (20 μM) as previously reported (Roy et al., 2021). Moreover, APV (5 μM) in combination with RTV (4 μM) exhibited promastigote inhibition with IC₅₀ values of 2.4 ± 0.6 μM at 12 h and 1.6 ± 0.7 μM at 24 h, respectively. The study was extended in animal model where the in vivo antileishmanial efficacy of APV-RTV in BALB/c mice demonstrated that treatment of APV in combination with RTV led to significant splenic and hepatic protection as compared to single dose of APV. Moreover, the antileishmanial activity of APV in combination with RTV was exerted via inhibition of LdTopILS at much lower concentration of APV and this inhibition of the enzyme induced programmed cell death in Leishmania parasites by generating oxidative stress within the cells. From the in vitro, ex vivo and in vivo studies, it was indicated that lower dose of APV in combination with RTV elevated the effective killing of the parasites as compared to the single higher dose of APV. Thus, the current study highlights repurposing of available protease inhibitors in combination, which might be exploited further for the therapeutic development against VL as well as HIV-VL co-infection.