Previous published data from our group showed the encouraging in vitro activities of six phenolic temozolomide (TMZ) ester analogues (ES8–ES12 and ES14) with up to a five-fold increase in potency compared to TMZ against glioblastoma multiform cell lines and TMZ-resistant O6-methylguanine-DNA methyl transferase (MGMT)-positive primary cells. This evidence concerns the gene MGMT and glioblastoma.