Rapalogues as hCES2A Inhibitors: In Vitro and In Silico Investigations.

BACKGROUND AND OBJECTIVE: Rapamycin and its semi-synthetic analogues (rapalogues) are frequently used in combination with other prescribed medications in clinical settings. Although the inhibitory effects of rapalogues on cytochrome P450 enzymes (CYPs) have been well examined, the inhibition potentials of rapalogues on human esterases have not been investigated. Herein, the inhibition potentials and inhibitory mechanisms of six marketed rapalogues on human esterases are investigated. METHODS: The inhibitory effects of six marketed rapalogues (rapamycin, zotarolimus, temsirolimus, everolimus, pimecrolimus and tacrolimus) on three major esterases, including human carboxylesterases 1 (hCES1A), human carboxylesterases 2 (hCES2A) and butyrylcholinesterase (BuChE), were assayed using isozyme-specific substrates. Inhibition kinetic analyses and docking simulations were performed to investigate the inhibitory mechanisms of the rapalogues with strong hCES2A inhibition potency. RESULTS: Zotarolimus and pimecrolimus displayed strong inhibition of human hCES2A but these agents did not inhibit hCES1A or BuChE. Further investigation demonstrated that zotarolimus could strongly inhibit intracellular hCES2A in living HepG2 cells, with an estimated IC50 value of 4.09 µM. Inhibition kinetic analyses revealed that zotarolimus inhibited hCES2A-catalyzed fluorescein diacetate hydrolysis in a mixed manner, with the Ki value of 1.61 µM. Docking simulations showed that zotarolimus could tightly bind on hCES2A at two district ligand-binding sites, consistent with its mixed inhibition mode. CONCLUSION: Our findings demonstrate that several marketed rapalogues are potent and specific hCES2A inhibitors, and these agents can serve as leading compounds for the development of more efficacious hCES2A inhibitors to modulate the pharmacokinetic profiles and toxicity of hCES2A-substrate drugs (such as the anticancer agent irinotecan).

[Prey selection by tiger frog larvae (Hoplobatrachus chinensis) of two sympatric anuran species' tadpoles].

We examined the prey selection and behavioral responses of tiger frog Hoplobatrachus chinensis larvae exposed to unpalatable and palatable sympatric prey tadpoles, Bufo melanostictus and Pelophylax nigromaculatus. We found that after a short exposure to the toxic toad tadpoles B. melanostictus, predators may learn to decrease going after unpalatable prey, subsequently it seems they may express short-term behavioral memory in order to avoid the toxic prey. In general, H. chinensis showed no preference for either any of the two prey species, which may be the result of P. nigromaculatus using behavioral performance and chemical defense as antipredatation strategies. These results facilitate further investigation of other aspects of the behavioral ecology of these three anuran species and hint at some potentially interesting possibilities of memory in choice of prey which may suggest further study.