Received for publication December 3, 2004.
Revised March 15, 2005.
Accepted for publication March 16, 2005.

The crystal structure of the complex of the anticancer prodrug CPT-11 with Torpedo acetylcholinesterase provides a molecular explanation for its cholinergic action

Abstract

The anticancer prodrug CPT-11 is a highly effective camptothecin analog that has been approved for the treatment of colon cancer. It is hydrolyzed by carboxylesterases to yield SN-38, a potent topoisomerase I poison. However, upon high dose i.v. administration of CPT-11, a cholinergic syndrome is observed that can be ameliorated by atropine. Previous studies have indicated that CPT-11 can inhibit acetylcholinesterase (AChE) and here we provide a detailed analysis of the inhibition of AChE by CPT-11 and by structural analogs. These studies demonstrate that the terminal dipiperidino moiety in CPT-11 plays a major role in enzyme inhibition, and this has been confirmed by X-ray crystallographic studies of a complex of the drug with Torpedo californica AChE. Our results indicate that CPT-11 binds within the active site gorge of the protein in a similar fashion to that observed with the Alzheimer drug, donepezil. The 3-D structure of the CPT-11/AChE complex also permits modeling of CPT-11 complexed with mammalian butyrylcholinesterase and carboxylesterase, both of which are known to hydrolyze the drug to the active metabolite. Overall, the results presented here clarify the mechanism of AChE inhibition by CPT-11, and detail the interaction of the drug with the protein. These studies may allow the design of both novel camptothecin analogs that would not inhibit AChE, and of new AChE inhibitors based upon the camptothecin scaffold.

Key words: Structure determinations, X-ray crystallography, Carboxylesterase, Structure/function/mechanism