Carboxylesterase 3 is an enzyme that in humans is encoded by the CES3 gene.

This gene encodes a member of the carboxylesterase large family. The family members are responsible for the hydrolysis or transesterification of various xenobiotics, such as cocaine and heroin, and endogenous substrates with ester, thioester, or amide bonds. They may participate in fatty acyl and cholesterol ester metabolism, and may play a role in the blood-brain barrier system. This gene is expressed in several tissues, particularly in colon, trachea and in brain, and the protein participates in colon and neural drug metabolism. Multiple alternatively spliced transcript variants encoding distinct isoforms have been reported, but the biological validity and/or full-length nature of some variants have not been determined.[provided by RefSeq, Jun 2010].
Function: Involved in the detoxification of xenobiotics and in the activation of ester and amide prodrugs. Shows low catalytic efficiency for hydrolysis of CPT-11 (7-ethyl-10-[4-(1-piperidino)-1-piperidino]-carbonyloxycamptothecin), a prodrug for camptothecin used in cancer therapeutics.