Re-assessing the Role of Temozolomide in Glioblastoma Therapy

The alkylating agent Temozolomide (TMZ) is the chemotherapeutic of choice when it comes to the treatment of Glioblastoma, the most common primary brain tumour in adults, with a dismal survival prognosis of 14 months. TMZ is an orally available prodrug that is activated at physiological blood pH and – importantly – crosses the blood-brain-barrier. When taken up by the cancer cells the active compound (referred to as MTIC) methylates DNA at three distinct sites (the N-7 and O-6 positions of guanine, as well as the N-3 position of adenine). In the absence of the O ⁶ -alkylguanine DNA alkyltransferase, which is only expressed by ~ 50% of all Glioblastoma cells, the rare alteration at the O-6 positions of guanine (5 – 10% of DNA alterations induced by TMZ) cannot be efficiently repaired, leading to an arrest in the G2 phase of the cell cycle and eventually DNA double strand break-induced apoptosis.

Using established cell lines, patient-derived Glioblastoma stem cell-like cells and freshly differentiated cells, as well as physiologically relevant concentrations of TMZ, which are only rarely applied to mechanistic investigations, we compared the model outlined above and found it insufficient to describe the observed effects. In particular, the effect of TMZ cannot be attributed primarily to apoptosis, at least not at physiologically relevant concentrations, nor does it exhibit pH-dependent activation. Interestingly, while stem cell-like cells are more resistant to TMZ-induced apoptosis – as expected from the literature, the differentiated progeny exhibit a higher degree of resistance when considering total cell numbers and metabolic activity.

The effect of TMZ on Glioblastoma cells (even within a single tumour) is complex and multifaceted. As novel therapeutic strategies are evaluated in the presence of the standard therapy, it is imperative that we understand the fundamental underlying mechanisms of TMZʼs effectiveness.