Nitric oxide synthase inhibitors as potential therapeutic agents for gliomas: A systematic review

INTRODUCTION

Gliomas represent the most prevalent form of brain tumors, among which glioblastomas are the most malignant subtype. Despite advances in comprehending their biology and treatment strategies, median survival remains disappointingly low. Inflammatory processes involving nitric oxide (NO), critically contribute to glioma formation. The inducible isoform of NO synthase (iNOS) is highly overexpressed in gliomas and has been linked to resistance against temozolomide (TMZ) treatment, neoplastic transformation, and modulation of immune response. While both in vitro and in vivo studies showed the potential of iNOS inhibitors as effective treatments for gliomas, no clinical trials on gliomas have been published. This review aims to summarize the available evidence regarding iNOS as a target for glioma treatment, focusing on clinically relevant data.

METHODS

Following PRISMA guidelines, we conducted a systematic review by searching PubMed/Medline, and Embase databases in May 2023. We included studies that investigated the impact of NOS inhibitors on glioma cells using L-NMMA, CM544, PBN, 1400W or l-NAME either alone or combined with TMZ. We extracted data on the NOS inhibitor used, subtype, study setting, animal model or cell lines employed, obtained results, and safety profile. Our inclusion criteria encompassed original articles in English or Spanish, studies with an untreated control group, and a primary outcome focused on the biological effects on glioma cells.

RESULTS

Out of 871 articles screened from the aforementioned databases, 37 reports were assessed for eligibility. After excluding studies that did not utilize glioma cells or address the designated outcome, 11 original articles satisfied the inclusion and exclusion criteria. Although no NOS inhibitor has been tested in a published clinical trial, three inhibitors have been evaluated using in vivo models of intracranial gliomas. l-NAME, 1400W, and CM544 were tested in vitro. Co-administration of l-NAME, or CM544 with TMZ showed superior results in vitro compared to individual agent testing.

CONCLUSION

Glioblastomas remain a challenging therapeutic target. iNOS inhibitors exhibit substantial potential as treatment options for oncologic lesions, and they have demonstrated a safe toxicity profile in humans for other pathological conditions. Research endeavors should be focused on investigating their potential effects on brain tumors.

Harnessing oxidative stress for anti-glioma therapy

Glioma cells use intermediate levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) for growth and invasion, and suppressing these reactive molecules thus may compromise processes that are vital for glioma survival. Increased oxidative stress has been identified in glioma cells, in particular in glioma stem-like cells. Studies have shown that these cells harbor potent antioxidant defenses, although endogenous protection against nitrosative stress remains understudied. The enhancement of oxidative or nitrosative stress offers a potential target for triggering glioma cell death, but whether oxidative and nitrosative stresses can be combined for therapeutic effects requires further research. The optimal approach of harnessing oxidative stress for anti-glioma therapy should include the induction of free radical-induced oxidative damage and the suppression of antioxidant defense mechanisms selectively in glioma cells. However, selective induction of oxidative/nitrosative stress in glioma cells remains a therapeutic challenge, and research into selective drug delivery systems is ongoing. Because of multifactorial mechanisms of glioma growth, progression, and invasion, prospective oncological therapies may include not only therapeutic oxidative/nitrosative stress but also inhibition of oncogenic kinases, antioxidant molecules, and programmed cell death mediators.

The implications of nitric oxide metabolism in the treatment of glial tumors

Glial tumors are the most common intracranial malignancies. Unfortunately, despite such a high prevalence, patients' prognosis is usually poor. It is related to the high invasiveness, tendency to relapse and the resistance of tumors to traditional methods of treatment. An important link in the aspect of these issues may be nitric oxide (NO) metabolism. It is a very complex mechanism with multidirectional effects on the neoplastic process. Depending on the concentration axis, it can both exert pro-tumor action as well as contribute to the inhibition of tumorigenesis. The latest observations show that the control of its metabolism can be very helpful in the development of new methods of treating gliomas, as well as in increasing the effectiveness of the agents currently used. The influence of nitric oxide and nitric oxide synthase (NOS) activity on glioma stem cells seem to be of particular importance. The use of specific inhibitors may allow the reduction of tumor growth and its tendency to relapse. Another important feature of GSCs is their conditioning of glioma resistance to traditional forms of treatment. Recent studies have shown that modulation of NO metabolism can suppress this effect, preventing the induction of radio and chemoresistance. Moreover, nitric oxide is involved in the regulation of a number of immune mechanisms. Adequate modulation of its metabolism may contribute to the induction of an anti-tumor response in the patients' immune system.