The inhibition of acetylcholinesterase by irinotecan and related camptothecins: key structural properties and experimental variables

Irinotecan (CPT‐11), a water‐soluble and semi‐synthetic topoisomerase I poison of the camptothecin family, has activity against both adult and paediatric malignancies. Recently, we demonstrated that CPT‐11 (lactone) is also a potent inhibitor of human acetylcholinesterase (AChE) at clinically relevant concentrations. Attachment of heterocyclic and branched amino groups onto the camptothecin back‐bone continues to be a strategy for the synthesis of water‐soluble analogues, but this may lead to undesirable inhibition of AChE. In this study, we screened a range of camptothecin analogues, degradation products and metabolites for their ability to inhibit AChE. Those compounds possessing N‐substitutions at C‐10 were all found to inhibit AChE in a similar kinetic manner to CPT‐11, but with a broad range of potencies. It is recognized that the charge‐state is important for ligands that bind to the peripheral anionic site and we postulated that the protonated distal piperidine of CPT‐11 would be important. To address this question, an N‐methyl piperidinium iodide analogue was synthesized and tested. This derivative inhibited electric eel AChE with an inhibition constant (K_i) of 1 nM. Kinetic and deacylation experiments demonstrated that it acted relatively less as an inhibitor of deacylation than CPT‐11. Overall, our experiments reveal that nitrogenous substitutions at the permissive C‐10 of the camptothecin backbone may lead to AchE inhibition, particularly if they involve a quaternary nitrogen.