title: The antitumour drug 7-ethyl-10-hydroxycamptothecin monohydrate and its solid-state hydrolysis mechanism on heating.
abstract: 7-Ethyl-10-hydroxycamptothecin [systematic name: (4S)-4,11-diethyl-4,9-dihydroxy-1H-pyrano[3',4':6,7]indolizino[1,2-b]quinoline-3,14(4H,12H)-dione, SN-38] is an antitumour drug which exerts activity through the inhibition of topoisomerase I. The crystal structure of SN-38 as the monohydrate, C22H20N2O5·H2O, reveals that it is a monoclinic crystal, with one SN-38 molecule and one water molecule in the asymmetric unit. When the crystal is heated to 473 K, approximately 30% of SN-38 is hydrolyzed at its lactone ring, resulting in the formation of the inactive carboxylate form. The molecular arrangement around the water molecule and the lactone ring of SN-38 in the crystal structure suggests that SN-38 is hydrolyzed by the water molecule at (x, y, z) nucleophilically attacking the carbonyl C atom of the lactone ring at (x - 1, y, z - 1). Hydrogen bonding around the water molecules and the lactone ring appears to promote this hydrolysis reaction: two carbonyl O atoms, which are hydrogen bonded as hydrogen-bond acceptors to the water molecule at (x, y, z), might enhance the nucleophilicity of this water molecule, while the water molecule at (-x, y + 1/2, -z), which is hydrogen bonded as a hydrogen-bond donor to the carbonyl O atom at (x - 1, y, z - 1), might enhance the electrophilicity of the carbonyl C atom.
authors.affiliation: Graduate School of Integrated Pharmaceutical and Nutritional Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan.
authors.forename: Md Ashraf
authors.initials: MA
authors.lastname: Ali
authors.affiliation: School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan.
authors.forename: Shuji
authors.initials: S
authors.lastname: Noguchi
authors.affiliation: School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan.
authors.forename: Miteki
authors.initials: M
authors.lastname: Watanabe
authors.affiliation: School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan.
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Iwao
authors.affiliation: School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Itai
keywords: 7-ethyl-10-hydroxycamptothecin; anticancer agent; antitumour drugs; crystal structure; irinotecan; solid-state hydrolysis; synchrotron; topoisomerase I inhibitor