EGF receptor inhibitors in the treatment of glioblastoma multiform: Old clinical allies and newly emerging therapeutic concepts

Glioblastoma multiform (GBM) is the most common malignant brain tumour in adults. Despite decades of experimentation to improve the outcome of patients with GBM this highly aggressive tumour remains fatal. Primary GBM are often characterized by the over-expression of epidermal growth factor (EGF) receptor/HER1 and/or its mutational variants, with ligand-independent, constitutively active EGF receptor vIII variant most frequently observed in GBM. EGF receptor signalling can promote tumorigenesis by increasing cell proliferation, tissue invasion, neoangiogenesis, tumour cell chemoresistance, and by inhibiting apoptosis of cancer cells. EGF receptor was the first receptor to serve as target for cancer therapy of many solid tumours. After 2 decades of intensive targeting of EGF receptor for molecular therapy, several anti-EGF receptor inhibitors are now available in the clinic. Therapeutic strategies to target EGF receptor and EGF receptor mutant forms in GBM include humanized monoclonal antibodies, tyrosine kinase inhibitors, and RNAi compounds. However, despite the fact that most EGF receptor-directed glioma therapies to date have focused on single therapeutic agents, a multi-directional approach involving targeted inhibition of multiple signalling pathways has emerged as a more robust therapeutical approach. Furthermore, the emergence of the hypothesis of «brain cancer stem cells» in the bulb of GBM identifies this population of cells with self-renewal capacity as novel obligatory targets for efficient cure of GBM. Here we summarize current findings on the clinical role of these EGF receptor inhibitory therapeutic agents in the treatment of GBM.