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.
