Glioblastoma multiforme (GBM) is the most common and aggressive brain tumour in adults. Unfortunately, to date, despite improvements in surgical removal techniques, followed by radiotherapy and chemotherapy, the survival of GBM patients is inexorably short. GBM is characterised by rapid cell proliferation, abnormal development of blood vessels that provide sustenance to the tumour, and a particularly immunosuppressive tumour microenvironment that prevents the development of an adequate immune response. However, the possibility of reversing this latter characteristic by using immunostimulant molecules is limited by the toxicity associated with their systemic delivery. For these reasons, the development of effective immunotherapies is extremely difficult.

In a study published in the prestigious scientific journal Science Translational Medicine, a group of researchers from the San Raffaele-Telethon Institute for Gene Therapy (SR-Tiget) demonstrated the efficacy - in an experimental model of glioblastoma multiforme - of a gene therapy platform that enables the targeted, selective and adjustable delivery of immunostimulatory molecules - in particular interferon alpha and interleukin 12 - into the tumour. The researchers succeeded in preventing systemic toxicity and modulating the tumour microenvironment from immunosuppressive to activating the anti-tumour response, thereby inhibiting neoplastic growth. The research - co-ordinated by Luigi Naldini, director of SR-Tiget and professor at the Vita-Salute San Raffaele University, and Nadia Coltella, researcher at SR-Tiget - was supported by the AIRC Foundation for Cancer Research, which has been contributing to the research of Luigi Naldini's group for years for the development of new anti-tumour gene therapies.

The gene therapy platform developed at San Raffaele

Researchers at San Raffaele decided to exploit the characteristics of specific immunostimulant molecules (in particular, interferon alpha and interleukin 12), which are able to act at different levels simultaneously. Interferon alpha, for example, can inhibit the growth of tumour blood vessels as well as activate and restore the functionality of the immune system. 'In order to prevent the onset of toxic effects in healthy tissues, however, it remains essential that these molecules, which belong to the cytokine family, are released in a selective and targeted manner at the tumour level, and that exposure to them occurs in the same way,' specifies Nadia Coltella.

"For several years, we have been working on the development of a cancer-targeted gene therapy strategy. We engineer with lentiviral vectors haematopoietic stem cells, which give rise to all blood cells, including monocytes, which spread throughout the tissues, becoming macrophages and contributing to the turnover of these cells and to the immune response. The vectors are designed in a way that they express the therapeutic cytokines exclusively in the monocytes derived from the engineered stem cells that reach the tumour site,' explains Luigi Naldini. In fact, during the growth phase, the tumour spontaneously attracts monocytes and macrophages, which in this case are used as a sort of 'Trojan horse', releasing interferon alpha and interleukin 12 at the site.

In addition to the tumour-specificity provided by the vector design, the results of this new study has also added the capacity to temporally regulate cytokine release to the gene therapy platform. This was achieved by modifying the cytokine and making it unstable and ineffective until a drug is administered that stabilises it and makes it functional. The release of the therapeutic cytokines by tumour macrophages therefore only occurs after the drug is administered. 'This allows us to add an extra layer of control to activate or inactivate cytokine release depending on therapeutic needs and glioblastoma growth, thus making the platform inducible and adjustable over time," adds Naldini.

The results of the study

"Having replicated in the laboratory a highly aggressive form of glioblastoma similar to the human disease, the results obtained are encouraging. Indeed, we have shown that gene therapy with interferon alpha, in both the original and inducible versions, is capable of reprogramming the immune cells that infiltrate glioblastoma in a pro-inflammatory and anti-tumour direction. We also observe the disappearance of a population of pro-tumour macrophages, usually associated with a worse prognosis in GBM patients,' says Nadia Coltella.

Filippo Birocchi, first author of the article, adds: "The reduction in tumour mass and the increase in long-term survival in experimental models of the disease are significant. In some cases we could observe the total disappearance of GBM and the development of an anti-tumour immune memory".

The SR-Tiget researchers are meanwhile continuing the development of the platform and are investigating how to further enhance its efficacy by combining it with other immunotherapy strategies, such as CAR-T cells directed against specific target antigens expressed by tumour cells.