Clinical trials of adenoviral-mediated suicide gene therapy of malignant gliomas

A preclinical assessment of the safety and biodistribution of an adenoviral vector containing the herpes simplex virus thymidine kinase gene (Cerepro) after intracerebral administration

BACKGROUND

Cerepro (sitimagene ceradenovec) is an adenoviral vector containing herpes simplex virus thymidine kinase gene (HSV-tk), which is being developed for the treatment of high-grade glioma with oral ganciclovir (GCV). The nonclinical safety and biodistribution of Cerepro were assessed following intravenous (i.v.) or intracerebral (i.c.) injection.

METHODS

Crl : WI(GLX/BRL/Han) rats (n = 198) were injected i.c. or i.v. with Cerepro or vehicle control, with GCV by intraperitoneal (i.p.) injection to selected groups. Safety was assessed by observation of animal behaviour and post mortem histology. Antibody response was assessed, and biodistribution measured using the quantitative polymerase chain reaction (PCR) and reverse transcriptase-PCR in blood and tissues.

RESULTS

Following i.v. or i.c. injection, there was no antibody response and no effect on behaviour, body weight, food consumption or haematological and clinical chemistry parameters. Minor needle track changes were observed in control and Cerepro-i.c. injection groups. Transient myeloid hyperplasia was observed in five of the 24 animals in the i.v. injection group and spleen weight increased in both the i.c. and i.v. groups. Cerepro was detected in the brain and at low levels in blood and spleen following i.c. injection, decreasing with time. Following i.v. injection, Cerepro was detected in viscera and blood, decreasing with time. Transcription of Cerepro was detected in the brain following i.c. injection, with lower levels in spleen; following i.v. injection, transcription was seen in viscera. Germline integration was not seen.

CONCLUSIONS

Intracerebral injection of Cerepro is safe and produces a high level of transgene expression in the brain, with limited biodistribution.

AdvHSV-tk gene therapy with intravenous ganciclovir improves survival in human malignant glioma: a randomised, controlled study

Malignant glioma is a devastating brain tumor with no effective treatment. This randomised, controlled study involved 36 patients with operable primary or recurrent malignant glioma. Seventeen patients were randomized to receive AdvHSV-tk gene therapy (3 x 10(10) pfu) by local injection into the wound bed after tumor resection, followed by intravenous ganciclovir (GCV), 5 mg/kg twice daily for 14 days. The control group of 19 patients received standard care consisting of radical excision followed by radiotherapy in those patients with primary tumors. The primary end-point was survival as defined by death or surgery for recurrence. Secondary end-points were all-cause mortality and tumour progression as determined by MRI. Overall safety and quality of life were also assessed. Findings were also compared with historical controls (n = 36) from the same unit over 2 years preceding the study. AdvHSV-tk treatment produced a clinically and statistically significant increase in mean survival from 39.0 +/- 19.7 (SD) to 70.6 +/- 52.9 weeks (P = 0.0095, log-rank regression vs. randomized controls). The median survival time increased from 37.7 to 62.4 weeks. Six patients had increased anti-adenovirus antibody titers, without adverse effects. The treatment was well tolerated. It is concluded that AdvHSV-tk gene therapy with GCV is a potential new treatment for operable primary or recurrent high-grade glioma.

Emerging molecular therapies for brain tumors

This article briefly reviews the basic research findings on brain tumors made in the last decade and how they are now impacting clinical trials. Major improvements in our understanding of the genetic, molecular, and biological mechanisms that lead to brain tumor development have been made. These include mechanistic insights on how tumor cells overcome cell cycle control, evade programmed cell death, induce blood vessel formation, and escape immune regulation. These advances have been tremendously accelerated by the availability of novel whole-genome analysis technologies. These findings are now being translated into innovative clinical trials that provide new hope for patients with these devastating diseases.

Gene therapy for neurological disease

Significant advances have been made in the last 20 years in understanding the basic biology of the normal nervous system and in elucidating molecular and cellular mechanisms underlying neurological disease. This progress has generated, for the first time, a realistic possibility of treating what have historically been common and tragically untreatable diseases of the nervous system. In particular, therapeutic delivery of genes to the degenerating, injured or developmentally-deficient nervous system offers the potential to prevent cell death, induce new growth and restore function. Clinical trials of gene therapy are beginning to move forward in several neurological disorders. We have thereby begun the transition to molecular-based medicine which has the potential to alter the landscape and prognosis of neurological disease.

Gene therapy for human malignant brain tumors

PURPOSE

Brain tumors were the first human malignancy to be targeted by therapeutic transfer of nucleic acids into somatic cells, a process also known as gene therapy. Malignant brain tumor cells in the adult brain have some unique biologic features, such as high mitotic activity on an essentially postmitotic background and virtually no tumor spread outside of the central nervous system. Brain tumors seem therefore to offer major advantages in the design of tumor-selective gene therapy strategies, and the role of gene therapy in malignant glioma has been investigated since the late 1980s, initially in numerous laboratory studies and later on in clinical trials.

DESIGN

Retrovirus has been one of the earliest recombinant virus vectors used in brain tumors. Experiments in cell culture and in animal models have demonstrated the feasibility of retrovirus-mediated transduction and subsequent killing of glioma cells by toxic transgenes. Phase I and II clinical studies in patients with recurrent malignant glioma have shown a favorable safety profile and some efficacy of retrovirus-mediated gene therapy. However, the only prospective, randomized, phase III clinical study of retrovirus gene therapy in primary malignant glioma failed to demonstrate significant extension of progression-free or overall survival. Adenovirus- and herpes simplex virus type 1-based vectors have been actively investigated along with retrovirus, but their clinical use is still limited, mostly because of safety concerns. To increase efficacy, novel generations of therapeutic adenovirus and herpes simplex virus type 1 rely more on genetically engineered and tumor-selective lytic properties and less on the actual transfer of therapeutic genes.

CONCLUSIONS

The failure of most clinical gene therapy protocols to produce a significant and unequivocal benefitto brain tumor patients seems to be mainly due to the low tumor cell transduction rates observed in vivo, but it may also depend on the respective physical delivery strategy of the vector. Standard radiologic criteria for assessing the efficacy of clinical treatments may also not be fully applicable to the specific metabolic changes and blood-brain barrier permeability phenomena caused in brain tumors by virus-mediated gene therapy. Clinical trials in malignant glioma have nevertheless produced a substantial amount of data and have contributed to the continuous improvement of vector systems, delivery methods, and clinical protocols.

T-cell immune responses in the brain and their relevance for cerebral malignancies

In order that cellular immune responses afford protection without risk to sensitive normal tissue, they must be adapted to individual tissues of the body. Nowhere is this more critical than for the brain, where various passive and active mechanisms maintain a state of immune privilege that can limit high magnitude immune responses. Nevertheless, it is now clear that immune responses are induced to antigens in the brain, including those expressed by cerebral malignancies. We discuss hypotheses of how this can occur, although details such as which antigen presenting cells are involved remain to be clarified. Antitumor responses induced spontaneously are insufficient to eradicate malignant astrocytomas; many studies suggest that this can be explained by a combination of low level immune response induction and tumor mediated immunosuppression. A clinical objective currently pursued is to use immunotherapy to ameliorate antitumour immunity. This will necessitate a high level immune response to ensure sufficient effector cells reach the tumor bed, focused cytotoxicity to eradicate malignant cells with little collateral damage to critical normal cells, and minimal inflammation. To achieve these aims, priority should be given to identifying more target antigens in astrocytoma and defining those cells present in the brain parenchyma that are essential to maintain antitumour effector function without exacerbating inflammation. If we are armed with better understanding of immune interactions with brain tumor cells, we can realistically envisage that immunotherapy will one day offer hope to patients with currently untreatable neoplastic diseases of the CNS.

Growth-factor gene therapy for neurodegenerative disorders

Preclinical neuroscience has advanced rapidly over the past two decades. New approaches for treating neurological disease, including gene-based therapies, nervous-system growth factors, stem cells, novel vaccines, and modulation of the immune system, offer the potential to prevent cell loss and degeneration in the brain, rather than attempting to compensate for loss after it has occurred. I will review one of these prospective therapies: growth-factor gene therapy for Alzheimer's disease, an approach that is currently the subject of a phase I clinical trial. Other disease targets for gene therapy will also be discussed, including Parkinson's disease, Huntington's disease, inborn errors of metabolism, and cancer. The progress of gene-therapy clinical trials is aiding the transition to molecular and gene-targeted therapeutic approaches which have the potential to improve dramatically the prognosis of neurological disease.