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Tirapazamine is metabolized to its DNA-damaging radical by intranuclear enzymes
J W Evans1, K Yudoh, Y M Delahoussaye
1Department of Radiation Oncology, Stanford University School of Medicine, California 94305, USA.
Abstract:
Tirapazamine (TPZ), a new anticancer drug that is currently in Phase II and III clinical trials, has a unique mechanism of action. Its cytotoxicity is selective for hypoxic cells in solid tumors and results from DNA damage produced by a free radical, which is generated by enzymatic reduction of the parent molecule. However, there is no agreement as to which enzyme(s) are involved. Here, we have measured both DNA damage and TPZ metabolism in A549 human lung cancer cells and in isolated nuclei derived from the cells. We show that, although the nuclei metabolize TPZ at a rate that is only 20% of that of whole cells, they have levels of DNA damage that are similar to those of the cells. We also show that TPZ radicals that are formed outside nuclei do not contribute to intranuclear DNA damage. Thus, essentially all of the DNA damage from TPZ results from radicals generated within the nucleus, and the 80% of the drug metabolism that occurs in the cytoplasm is probably irrelevant for the activity of this drug in killing hypoxic cells.
Insights
Tirapazamine (TPZ) drug activity in cancer cells stems from DNA damage caused by radicals generated within the nucleus. Cytoplasmic metabolism of TPZ is likely irrelevant to its therapeutic effect on hypoxic cells.
Area of Science:
- Pharmacology
- Cancer Biology
- Molecular Toxicology
Background:
- Tirapazamine (TPZ) is an anticancer drug with selective cytotoxicity for hypoxic tumor cells.
- Its mechanism involves DNA damage induced by free radicals generated from TPZ.
- The specific enzymes responsible for TPZ reduction remain unidentified.
Purpose of the Study:
- To investigate the cellular location of TPZ metabolism and DNA damage induction.
- To determine the contribution of nuclear versus cytoplasmic TPZ metabolism to cytotoxicity.
Main Methods:
- TPZ metabolism and DNA damage were measured in A549 human lung cancer cells and isolated nuclei.
- Enzymatic reduction and radical formation of TPZ were assessed in both whole cells and nuclear fractions.
Main Results:
- Intranuclear TPZ metabolism accounted for significant DNA damage, despite lower metabolic rates compared to whole cells.
- Extranuclear TPZ radicals did not contribute to DNA damage within the nucleus.
- Approximately 80% of TPZ metabolism occurring in the cytoplasm was found to be irrelevant to DNA damage.
Conclusions:
- TPZ-induced DNA damage and cytotoxicity are primarily mediated by radicals generated within the cell nucleus.
- Cytoplasmic metabolism of TPZ is likely not essential for its anticancer activity against hypoxic cells.