Temozolomide

Glioblastoma (glioblastoma multiforme) is the most common and aggressive adult primary brain tumour, accounting for 45.6% of all primary malignant brain tumours. Primarily defined histopathologically by necrosis and microvascular proliferation (WHO grade IV classification), glioblastomas are commonly treated through radiotherapy and concomitant alkylation-based chemotherapy with temozolomide. Temozolomide (TMZ) is a small (194 Da) lipophilic alkylating agent of the imidazotetrazine class that is stable at acidic pH, allowing for both oral and intravenous dosing, and can cross the blood-brain barrier to affect CNS tumours. After absorption, TMZ undergoes spontaneous nonenzymatic breakdown at physiological pH to form 5-(3-methyltriazen-1-yl) imidazole-4-carboxamide (MTIC), which then reacts with water to produce 5-aminoimidazole-4-carboxamide (AIC) and a highly reactive methyl diazonium cation. Brain tumours such as glioblastoma typically possess a more alkaline pH than healthy tissue, favouring TMZ activation within tumour tissue.

The methyl diazonium cation is highly reactive and methylates DNA at the N7 position of guanine (N7-MeG, 70%), the N3 position of adenine (N3-MeA, 9%), and the O6 position of guanine (O6-MeG, 6%). Although more prevalent, N7-MeG and N3-MeA are rapidly repaired by the base excision repair pathway and are not primary mediators of temozolomide toxicity, although N3-MeA lesions are lethal if not repaired. By comparison, repair of O6-MeG requires action by the suicide enzyme methylguanine-DNA methyltransferase (MGMT), which removes the methyl group to restore guanine. If not repaired by MGMT, O6-MeG mispairs with thymine, activating the DNA mismatch repair (MMR) pathway that removes the thymine (not the O6-MeG), resulting in futile cycles of repair and eventual DNA strand breaks leading to apoptosis. As MMR activity is crucial for temozolomide cytotoxicity, cells that have reduced or absent MGMT function and an intact MMR pathway are the most sensitive to temozolomide treatment. Glioblastomas that upregulate MGMT downregulate MMR or alter both are resistant to TMZ, leading to treatment failure.

More recently, increased interest has also been shown in the immunomodulatory effects of TMZ, related to its myelosuppressive effects. Counterintuitively, lymphodepletion may enhance the antitumour effects of cellular immunotherapy and improve the dynamics of memory cells by altering tumour-specific versus tumour-tolerant populations. The depletion of tumour-localized immunosuppressive Treg cells may contribute to an improved response to immunotherapy. Hence, TMZ treatment may also form the backbone of immunotherapy strategies against glioblastoma in the future.