title: Radiolytic and cellular reduction of a novel hypoxia-activated cobalt(III) prodrug of a chloromethylbenzindoline DNA minor groove alkylator.
abstract: Metabolic reduction can be used to activate prodrugs in hypoxic regions of tumours, but reduction by ionising radiation is also theoretically attractive. Previously, we showed that a cobalt(III) complex containing 8-hydroxyquinoline (8-HQ) and cyclen ligands releases 8-HQ efficiently on irradiation in hypoxic solutions [Ahn G-O, Ware DC, Denny WA, Wilson WR. Optimization of the auxiliary ligand shell of cobalt(III)(8-hydroxyquinoline) complexes as model hypoxia-selective radiation-activated prodrugs. Radiat Res 2004;162:315-25]. Here we investigate an analogous Co(III) complex containing the potent DNA minor groove alkylator azachloromethylbenzindoline (azaCBI, 1) to determine whether it releases 1 on radiolytic and/or enzymatic reduction under hypoxia. Monitoring by HPLC, the azaCBI ligand in the Co(III)(cyclen)(azaCBI) complex (2) slowly hydrolysed in aqueous solution, in contrast to the free ligand 1 which readily converted to its reactive cyclopropyl form. Irradiation of 2 (30-50 microM) in hypoxic solutions released 1 with yields of 0.57 micromol/J in formate buffer and 0.13 micromol/J in human plasma. Using bioassay methods, cytotoxic activation by irradiation of 2 at 1 microM in hypoxic plasma was readily detectable at clinically relevant doses (> or = 1 Gy), with a estimated yield of 1 of 0.075 micromol/J. Release of 1 from 2 was also observed in hypoxic HT29 cultures without radiation, with subsequent conversion of 1 to its O-glucuronide. Surprisingly, overexpression of human cytochrome P450 reductase in A549 cells did not increase the rate of metabolic reduction of 2, suggesting that other reductases and/or non-enzymatic reductants are responsible. Thus the cobalt(III) complex 2 is a promising prodrug capable of being activated to release a very potent cytotoxin when reduced by either ionising radiation or cells under hypoxic conditions.
authors.affiliation: Auckland Cancer Society Research Centre, The University of Auckland, Private Bag 92019, Auckland, New Zealand.
authors.forename: G-One
authors.initials: GO
authors.lastname: Ahn
authors.forename: K Jane
authors.initials: KJ
authors.lastname: Botting
authors.forename: Adam V
authors.initials: AV
authors.lastname: Patterson
authors.forename: David C
authors.initials: DC
authors.lastname: Ware
authors.forename: Moana
authors.initials: M
authors.lastname: Tercel
authors.forename: William R
authors.initials: WR
authors.lastname: Wilson
mesh_terms: D000477:Alkylating Agents / Q000493:pharmacokinetics / Q000494:pharmacology*; D001391:Azo Compounds / Q000493:pharmacokinetics / Q000494:pharmacology*; D001711:Biotransformation; D015687:Cell Hypoxia; D002851:Chromatography, High Pressure Liquid; D003035:Cobalt / Q000737:chemistry*; D019073:HT29 Cells; D006801:Humans; D007211:Indoles / Q000493:pharmacokinetics / Q000494:pharmacology*; D013058:Mass Spectrometry; D011355:Prodrugs / Q000737:chemistry*; D011839:Radiation, Ionizing
chemical_list: D000477:Alkylating Agents; D001391:Azo Compounds; D007211:Indoles; D011355:Prodrugs; C511528:azachloromethylbenzindoline; D003035:Cobalt