On the Critical Issues in Temozolomide Research in Glioblastoma: Clinically Relevant Concentrations and MGMT-independent Resistance

Abstract

The current standard first-line treatment for adult patients with newly diagnosed glioblastoma includes concurrent radiotherapy and daily oral temozolomide (TMZ), followed by adjuvant TMZ. As a prodrug, TMZ undergoes spontaneous hydrolysis generating a methylating agent. O6-methylguanine is considered the most preponderant toxic damage mechanism at therapeutically relevant TMZ doses, whereas MGMT, which encodes the O6-methylguanine-DNA methyltransferase DNA repair enzyme, is the most relevant resistance mechanism. Speculations on clinically relevant TMZ concentrations, cytotoxic and cytostatic effects of TMZ, and resistance mechanisms exist in the literature. Here, we raise the following principal issues: What are the clinically relevant TMZ concentrations in glioma patients, and which TMZ-induced molecular lesion(s) and corresponding resistance mechanism(s) are important for TMZ therapeutic effects at clinically relevant concentrations? According to clinical data from patients with glioblastoma, the mean peak TMZ concentrations in the peritumoral tissue might be much lower (around 5 µM) than usually used in in vitro research, and may represent only 20% of systemic drug levels. According to in vitro reports, single-dose TMZ at concentrations around 5 µM have minimal, if any, effect on apoptosis and/or senescence of glioblastoma cell lines. However, the clinically relevant concentrations of TMZ are sufficient to radiosensitize both MGMT-positive and -negative cell lines in vitro. It is speculated that a single DNA repair protein, MGMT, is highly efficient in protecting cells against TMZ toxicity. However, an endogenous level of MGMT protein expression is not universally correlated with TMZ responsiveness, and MGMT-independent mechanisms of TMZ resistance exist.

Considering the Experimental Use of Temozolomide in Glioblastoma Research

“…Hypermethylation of the promoter region of MGMT is found in 30-60% of all GB patients [18]. MGMT, however, is a highly debated predictive biomarker for therapy response, as the MGMT status does not universally correlate with TMZ responsiveness [19].…”

“…As the precise nature of this drug's effect on the cells might be still not fully understood, it is important to create the correct experimental context, as TMZ is also an important component of complex combination therapies, such as the RIST (rapamycin, irinotecan, sunitinib, TMZ) protocol or the CUSP9 (coordinated undermining of survival paths with nine repurposed drugs) approach [20,21]. Importantly, in this context TMZ has been shown, even at low concentrations, to enhance the response of GB cells to radiation, or, in combination with a pharmacological PI3K/mTOR inhibitor, to break the increased apoptosis-resistance of certain GB cell populations [19,22].…”

Comment in Response to “Temozolomide in Glioblastoma Therapy: Role of Apoptosis, Senescence and Autophagy etc. by B. Kaina”

“…It is with great pleasure that we acknowledge the fact that our review on Temozolomide (TMZ) has initiated a discussion [1][2][3]. This had been our intent, since a discussion on TMZ is long overdue.…”

“…To optimize these combination therapies, we need to understand exactly not only what the individual components do, but also when they do it, the mode of action and pharmacodynamics, which will be essential for complex therapeutic approaches. The timeliness of this debate is best summarized by Stepanenko and Chekhonin in their conclusion [3]: " . .…”

“…In addition to Stepanenko and Chekhonin's discussion on clinically relevant concentrations of TMZ and the rather disappointing role of MGMT as a predictor for TMZ sensitivity in Glioblastoma [3], we would like to address two further points of difference between Kaina's analysis of the literature and our own.…”