Radiolytic and cellular reduction of a novel hypoxia-activated cobalt(III) prodrug of a chloromethylbenzindoline DNA minor groove alkylator

Biochem Pharmacol. 2006 Jun 14;71(12):1683-94. doi: 10.1016/j.bcp.2006.03.007. Epub 2006 Mar 14.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkylating Agents / pharmacokinetics
  • Alkylating Agents / pharmacology*
  • Azo Compounds / pharmacokinetics
  • Azo Compounds / pharmacology*
  • Biotransformation
  • Cell Hypoxia*
  • Chromatography, High Pressure Liquid
  • Cobalt / chemistry*
  • HT29 Cells
  • Humans
  • Indoles / pharmacokinetics
  • Indoles / pharmacology*
  • Mass Spectrometry
  • Prodrugs / chemistry*
  • Radiation, Ionizing


  • Alkylating Agents
  • Azo Compounds
  • Indoles
  • Prodrugs
  • azachloromethylbenzindoline
  • Cobalt