Toward Hypoxia-Selective DNA-Alkylating Agents Built by Grafting Nitrogen Mustards onto the Bioreductively Activated, Hypoxia-Selective DNA-Oxidizing Agent 3‑Amino-1,2,4-benzotriazine 1,4-Dioxide (Tirapazamine)

Tirapazamine
(3-amino-1,2,4-benzotriazine 1,4-dioxide) is a heterocyclic
di-N-oxide that undergoes enzymatic deoxygenation
selectively in the oxygen-poor (hypoxic) cells found in solid tumors
to generate a mono-N-oxide metabolite. This work
explored the idea that the electronic changes resulting from the metabolic
deoxygenation of tirapazamine analogues might be exploited to activate
a DNA-alkylating species selectively in hypoxic tissue. Toward this
end, tirapazamine analogues bearing nitrogen mustard units were prepared.
In the case of the tirapazamine analogue 18a bearing
a nitrogen mustard unit at the 6-position, it was found that removal
of the 4-oxide from the parent di-N-oxide to generate
the mono-N-oxide analogue 17a did indeed
cause a substantial increase in reactivity of the mustard unit, as
measured by hydrolysis rates and DNA-alkylation yields. Hammett sigma
values were measured to quantitatively assess the magnitude of the
electronic changes induced by metabolic deoxygenation of the 3-amino-1,2,4-benzotriazine
1,4-dioxide heterocycle. The results provide evidence that the 1,2,4-benzotiazine
1,4-dioxide unit can serve as an oxygen-sensing prodrug platform for
the selective unmasking of bioactive agents in hypoxic cells.