Yes / It is 40 years since the publication of the patent that announced the imidazotetrazines temozolomide and mitozolomide to the world and 30 since the discovery that they function as prodrugs of alkyldiazonium reactive intermediates. Temozolomide combined with radiation is established as the first-line treatment for glioma but despite the attentions of the inventors and others, further examples of this intriguing ring system have yet to enter the clinic.

Yes / Silane-substituted imidazotetrazines 1,2 were investigated for their activity as anticancer prodrugs related to temozolomide (TMZ). The TMS-derivative 1 showed an activity profile against TMZ susceptible and resistant cell lines very similar to TMZ; in contrast, the SEM-derivative 2 showed activity irrespective of MGMT expression or MMR deficiency (Table). Probing the prodrug activation mechanism by NMR kinetic studies determined that the TMS compound 1 follows a reaction pathway and time-course very similar to temozolomide. 1H-NMR spectra of the reaction mixture showed considerable incorporation of deuterium into the final alkylation products of the reaction (methanol and methyl phosphate) as had previously been shown for temozolomide (Wheelhouse, R.T., et al. Chem. Commun. 1993, 15, 1177–1178). The SEM-derivative 2 reacted more rapidly than TMZ or TMS-derivative 1. Somewhat surprisingly, the silane remained intact throughout the experiment and the observed reaction was the hydrolysis of the imidazo-tetrazine to ultimately release formaldehyde hydrate and 2-TMS-ethanol. In conclusion, TMS-derivative 1 is a diazomethane precursor with prodrug activation mechanism, kinetics and anti-cancer activity in vitro similar to TMZ. In contrast, the SEM derivative 2 was more rapidly hydrolysed, a precursor of 2-TMS-ethanol and had activity in vitro different from TMZ. 2-TMS-ethanol was previously reported as a non-toxic compound in mice (Voronkov, M.G., et al. Dokl. Akad. Nauk SSSR 1976, 229, 1011–1013) and is known as a substrate for alcohol dehydrogenase (Zong, M.-H., et al. Appl. Microbiol. Biotechnol. 1991, 36, 40–43) and as a modest inhibitor of acetylcholinesterase (Aberman, A., et al. Biochim. Biophys. Acta 1984, 791, 278–280; Cohen, S.G., et al. J. Med. Chem. 1985, 28, 1309–1313).