Tight-Binding Small-Molecule Carboxylesterase 2 Inhibitors Reduce Intracellular Irinotecan Activation

As
the primary enzyme responsible for the activatable conversion
of Irinotecan (CPT-11) to SN-38, carboxylesterase 2 (CES2) is a significant
predictive biomarker toward CPT-11-based treatments for pancreatic
ductal adenocarcinoma (PDAC). High SN-38 levels from high CES2 activity
lead to harmful effects, including life-threatening diarrhea. While
alternate strategies have been explored, CES2 inhibition presents
an effective strategy to directly alter the pharmacokinetics of CPT-11
conversion, ultimately controlling the amount of SN-38 produced. To
address this, we conducted a high-throughput screening to discover
18 small-molecule CES2 inhibitors. The inhibitors are validated by
dose-response and counter-screening and 16 of these inhibitors demonstrate
selectivity for CES2. These 16 inhibitors inhibit CES2 in cells, indicating
cell permeability, and they show inhibition of CPT-11 conversion with
the purified enzyme. The top five inhibitors prohibited cell death
mediated by CPT-11 when preincubated in PDAC cells. Three of these
inhibitors displayed a tight-binding mechanism of action with a strong
binding affinity.