Synthesis and growth-inhibitory activities of imidazo[5,1-d]-1,2,3,5-tetrazine-8-carboxamides related to the anti-tumour drug temozolomide, with appended silicon, benzyl and heteromethyl groups at the 3-position

A series of 3-(benzyl-substituted)-imidazo.5,1-d]-1,2,3,5-tetrazines (13) and related derivatives with 3-heteromethyl groups has been synthesised and screened for growth-inhibitory activity in vitro against two pairs of glioma cell lines with temozolomide-sensitive and -resistant phenotypes dependent on the absence/ presence of the DNA repair protein O-6-methylguanine-DNA methyltransferase (MGMT). In general the compounds had low inhibitory activity with GI(50) values > 50 mu M against both sets of cell lines. Two silicon-containing derivatives, the TMS-methylimidazotetrazine (9) and the SEM-analogue (10), showed interesting differences: compound (9) had a profile very similar to that of temozolomide with the MGMT+ cell lines being 5 to 10-fold more resistant than MGMT-isogenic partners; the SEM-substituted compound (10) showed potency across all cell lines irrespective of their MGMT status.