Adult glioblastoma is the most common primary brain tumour. It is characterised by substantial morbidity and mortality despite multimodal therapy with surgical resection and adjuvant radiochemotherapy as standard care.1 The poor prognosis is largely due to the disease’s high frequency of recurrence, which is indicative of its intrinsic invasive properties into the peritumoral zone.2 Consequently, an unmet need exists to improve local control of glioblastoma beyond the margin of resection and to explore new treatment options targeted peritumouraly. Local therapies that can be applied during surgery are therefore well-suited to bridge the gap between initial surgical resection and subsequent radiochemotherapy. In The Lancet Oncology, Manfred Westphal and colleagues3 explore the use of so-called suicide gene therapy to address this treatment gap, describing the results of a randomised, open-label phase 3 trial (ASPECT) for the treatment of operable high-grade glioblastoma. This trial was based on previous phase 1 and phase 2 trials4—6 and relies on local injection into the resection cavity of a replication-deficient adenoviral vector encoding a herpes simplex virus thymidine kinase (HSV-tk) gene to selectively eliminate any residual glioblastoma cells. The HSV-TK catalyses the conversion of a non-toxic ganciclovir prodrug into a toxic nucleotide analogue that is incorporated into the DNA of dividing cancer cells, prompting apoptosis. This approach overcomes the typical inaccessibility of glioblastoma tumour, and brain-infiltrating, cells to most systemic therapies. Moreover, preclinical studies indicate that both the HSV-tk gene-modified cells and adjacent, non-modified dividing cells are eliminated through a so-called bystander effect that enhances the overall anti-tumour effect. This bystander effect is probably mediated by intercellular trafficking of the toxic ganciclovir metabolites through gap junctions or immune mechanisms.7, 8 Another advantage is that normal neurons do not proliferate and are therefore resistant to the ganciclovir metabolites, which improves the tumour selectivity of this treatment strategy.
The specific objective of the ASPECT trial was to determine whether ganciclovir with adenoviral HSV-tk gene therapy was better than standard care, with time to death or re-intervention as the composite primary endpoint. After the ASPECT trial had begun, temozolomide emerged as a new and effective treatment for glioblastoma and was included in both the treatment and control groups. Consequently, this invalidated the initial statistical analysis strategy. A post-hoc multivariate statistical analysis based on a Cox’s proportional hazards model was therefore needed for the composite primary endpoint, which took into consideration the use of temozolomide as a time-dependent covariate. The methylation status of the MGMTpromoter is a prognostic factor and was therefore also taken into account as a covariate in the Cox analysis. MGMT encodes a DNA-repair enzyme that removes alkyl groups from DNA. High levels of MGMT activity in glioblastoma annihilate the therapeutic effect of alkylating agents, including temozolomide, creating a temozolomide-resistant phenotype. Conversely, epigenetic silencing of the MGMT gene by promoter methylation in glioblastoma cells is associated with loss of MGMTexpression, diminished DNA-repair activity, and increased sensitivity to temozolomide. Consequently, patients with glioblastoma containing a methylated MGMT promoter benefit from temozolomide, whereas the drug has no therapeutic effect in those with an unmethylated MGMT promoter.9
In the multivariate statistical analysis of the ASPECT trial, patients in the experimental group had a favourable outcome in terms of the primary composite endpoint—time to death or re-intervention—compared with those in the standard care group (hazard ratio 1·53, 95% CI 1·13—2·07; p=0·006). One of the intriguing findings of this trial is that a post-hoc subgroup analysis showed an even more pronounced effect in a subgroup of patients with an unmethylated MGMT promoter (hazard ratio 1·72, 1·15—2·56; p=0·008). Hemiparesis, hyponatraemia, and seizures were more common in the experimental group and were mainly transient. Despite the statistically significant effect on the composite primary endpoint, the difference in overall survival between gene therapy and standard care was not statistically significant. Nevertheless, there seemed to be improved overall survival in the experimental group versus the control group in a subgroup of patients with non-methylatedMGMT, although this difference was not statistically significant. This finding needs substantiation in larger trials. The investigators recorded no between-group difference in tumour sizes at the time of re-intervention, suggesting that the time to re-intervene was not biased in favour of the treatment group.
Findings from the ASPECT trial raise several interesting hypotheses and questions. Most importantly, the post-hoc multivariate analysis suggests that patients with a glioblastoma containing an unmethylated MGMT promoter might benefit most from the proposed gene therapy strategy. This difference in regards to methylation status might ultimately create new perspectives for the treatment of patients who do not benefit from temozolomide treatment. Methylation status of the MGMTpromoter was not prespecified at the time of treatment, ruling out a possible treatment bias. Post-hoc analysis also showed that the effect of the treatment seemed to be greater in patients with a higher baseline titre of adenovirus-specific neutralising antibodies, suggesting a possible immunological bystander effect resulting from previous infection with wild-type adenovirus.8 Further studies are needed to address these different hypotheses. Findings from the ASPECT trial also indicate a need to develop new approaches that augment transduction efficiency, improve vector spread within the residual tumour tissue, and enhance bystander effects. The continuous development of these multipronged strategies represent the new for patients and their families.