The Leishmania isoform of the 90kDa Heat Shock Protein (LsHsp90), a chaperone known to assist the folding of more than 200 client proteins, was reported to be generally involved in parasite differentiation from promastigote to amastigote possessing a pivotal role during heat-induced cellular stress. Moreover, it was demonstrated that an impair of the native functions of LsHsp90 through the action of active-site inhibitors can exert a detrimental effect on the natural parasite life-cycle ultimately leading to its death.
Celastrol is natural triterpene exhibiting a plethora of in vitro and in vivo activities. Among them, this pentacyclic compound is reported to possess a promising antiproliferative activity thanks to its ability of interacting with the chaperone cycle of the human isoform of Hsp90 (hHsp90). Moreover, celastrol derivatives (e.g. the methyl ester pristimerin, Figure 1) have also exhibited an interesting antiprotozoal activity.
With the aim of building a target-oriented approach to treat Leishmania infections based on the inhibition of LsHsp90, we prepared two basic carboxamides celastrol derivatives (SS-1 and SS-2) to enhance its leishmanicidal activity and selectivity of action by deducting its unspecific cytotoxicity (measured as IC50 on HMEC-1 cell lines). Accordingly, celastrol and the two basic derivatives SS-1 and SS-2 were in vitro tested for their leishmanicidal activity against promastigotes of Leishmania tropica and L. infantum and, in parallel, their mechanism of action was investigated as well via ad hoc in vitro experiments using a recombinant Hsp90 from L. braziliensis (LbHsp90).
In virtue of their pH sensitive basic heads, both SS-1 and SS-2 were found to be more potent (IC50 in the nanomolar range) and selective leishmanicidal agents than celastrol itself. Furthermore, we were able to demonstrate that SS-1 and SS-2 successfully (in vitro) inhibited the native kinase activity of LbHsp90 highlighting the key role of the inhibition of this chaperone in their mechanism of action
