Glioblastoma multiforme (GBM) is the most prevalent primary brain tumor and also one of the most difficult human malignancies to manage. In fact, only 7 months was added to the mean survival rate of patients with GBM during the last seven decades Citation[1,2]. The location of tumor, which impacts on poor penetration of drugs through the blood–brain barrier and also within the tumor; well-known resistance to chemotherapy and radiation therapy also in part because of the location; the infiltrative nature reflected by tumor cells migrating away from the primary site and embedding into normal brain parenchyma; genetic, morphological and molecular heterogeneity; and extremely well developed although improperly working neovasculature all contribute to a dismal outcome of GBM patients. Recent years have brought more interest in studying GBM among cancer researchers generating new invaluable knowledge. This new knowledge at genetic, morphological, cellular and molecular levels provides a basis for rational drug development approaches. The prospect of finding better treatments for GBM is now certainly more realistic.
Just about everything that relates to GBM pathobiology and its clinical course invites thinking about specific targeting of more than one tumor compartment/target and more than one mechanism controlling the pathobiology of GBM, hence its maintenance and progression. This truly prompts researchers to consider combinatorial therapy or a cocktail of drugs. The current standard of care itself is a combination of different modalities of treatment, such as surgery, chemotherapy and radiation, but they do not address fully, as they cannot, the formidable complexity of GBM. In this editorial, it is argued that combinatorial treatment can and should be applied utilizing various classes of drugs that are directed against different tumoral compartments and different aspects of cellular/molecular composition and pathobiological importance. A combinatorial (i.e., a cocktail) drug approach would be relevant within a specific group of drugs as well as among them. This would imply that perhaps not only a cocktail of drugs (nowadays rationally designed) against specific tumor compartments/functions will be needed, but also such a cocktail will need to include cocktails from each specific type of agents.
A novel approach to GBM treatment is the use of molecularly targeted recombinant cytotoxins. Recombinant cytotoxins are targeted to GBM tumor cells by the means of a ligand or antibody that binds an internalized plasma membrane receptor. They deliver modified bacterial toxins as active, catalytic portions of the cytotoxins Citation[3]. Several cytotoxins have been in clinical trials during the last decade or so even though none of them were designed for the treatment of brain tumors Citation[3]. The most striking preclinical and clinical results were obtained with a cytotoxin composed of a wild-type IL-13 and PE38QQR derivative of Pseudomonas exotoxin A Citation[4]. An uncommon efficacy seen in patients with recurrent GBM in Phase I and II trials with IL13-PE38QQR Citation[5] prompted development of Phase III trial that ended in late 2006. The drug demonstrated significantly better progression-free survival in patients receiving the cytotoxin versus current standard of care Citation[6]. This efficacy was obtained even though intratumoral drug delivery through convection-enhanced delivery was not monitored, and patients were not selected for the overexpression of one of the drug’s targets, IL-13 receptor-α2 (IL-13Rα2). Here is the first reason why we should think about a cocktail of cytotoxins rather than an individual one. IL-13Rα2 is overexpressed in approximately 75% of patients with GBM Citation[7–10] although its gene is expressed at lower levels Citation[11], not an uncommon scenario. Thus, at least a quarter, and perhaps even more than that, of patients with GBM are expected to be poor or nonresponders to therapies targeting IL-13Rα2. This calls for prescreening patients before applying therapy, but then a sizable subset of patients will not have a chance to receive a drug based on a promising therapeutic strategy. One can envision that addition of another cytotoxin would make a difference. The prerequisite would be that there is another target in GBM that is not completely overlapping with the expression of IL-13Rα2 and its distribution. EphA2 tyrosine kinase receptor is the second receptor that is highly overexpressed in patients with GBM and its expression is linked to a worse outcome Citation[12,13]. A vast majority of GBM patients overexpress either IL-13Rα2 or EphA2, but importantly approximately 95% overexpress two receptors Citation[10]. Hence, a very small fraction of patients would be potential nonresponders if a cocktail of two cytotoxins, the ones targeting IL-13Rα 2 and EphA2, was administered. A prototype cytotoxin against EphA2 receptor has already been generated and tested Citation[14]. In addition, a combination of two cytotoxins or double-targeting in GBM treatment makes the cytotoxins somehow more effective antitumor agents Citation[15,16].
An ideal scenario would be to have a cocktail of anti-GBM cytotoxins targeted against receptors that are overexpressed in 100% of patients at any given time. The third receptor, besides IL-13Rα2 and EphA2, highly overexpressed in GBM but not normal brain is currently being searched for as a suitable candidate for the development of another specific and potent cytotoxin. It is likely that we will identify such a target, since a protein, a transcription factor, the overexpression of which complements that of IL-13Rα2 and EphA2, has been found. Fos-related antigen 1, together with IL-13Rα2 and EphA2 constitutes the first trimolecular signature of GBM identified as such Citation[10]. A combination of drugs targeting this trio of factors will not miss a single patient with GBM, but obviously a cytotoxin cannot be generated against Fra-1.
It is now possible to specifically target a neovasculature (vessels formed during tumor initiation and progression) of GBM with drugs. Clinical trials demonstrate that such a treatment tightens up neomicrovasculature and tumor vessels become even less permeable Citation[17]. One could consider intratumoral delivery of recombinant cytotoxins under such conditions in expectation of longer residence time within the tumor and its vicinity with less leakage into the periphery. This requires having cytotoxins of neutral activity towards normal brain, such as those targeted against IL-13Rα2 and EphA2. Clinical examination of a cocktail of cytotoxins/antiangiogens is warranted.
We hypothesized that GBM is powered, like a sports car, by multiple engine cylinders and it might be necessary to knock down most of them in order to effectively stop or at least weaken it Citation[18]. Biological forces moving this engine are the intracellular signaling pathways. Recent extensive research in this area of investigation fully supports the notion that an Achilles heel of GBM has not yet been identified, but rather multiple complex mechanisms operating in GBM that maintain the tumor and cause it to progress Citation[19]. Thus, inhibiting one activated system may slow the work temporarily but will not stop the engine. Preferably, two or three pathways should be attacked concomitantly or consecutively. This is another example of combinatorial therapy in which a cocktail of intracellular pathway inhibitors might need to be employed in the treatment of GBM.
And finally, I would like to make a case for having drugs for the treatment of GBM available off-the-shelf. Our current understanding of the disease and current ability to generate therapeutic compounds makes combinatorial therapy an approach of choice. The next decade or so of research will indicate which combinations/cocktails of drugs belonging to one group of compounds, such as small molecule inhibitors or recombinant cytotoxins, will work most effectively in patients. During this process, combining a cocktail of recombinant cytotoxins with a cocktail of small molecule inhibitors may make therapeutic sense in that the cytotoxins will eliminate as many tumor cells as they can, but even a residual disease will ascertain the recurrence, and hence needs to be taken care of. Small molecule inhibitors could then be introduced into the treatment regimen. Or, a multivalent vaccine following the cytotoxins/inhibitors will be necessary to be combined with the preceding treatment Citation[20]. This is again in order to eliminate surgically inaccessible, chemotherapy-resistant/inaccessible and radiation-resistant/inaccessible tumors. The cost of developing off-the-shelf cocktails might actually be less than making individualized target identification for choosing an individual patient-profiled cocktail, and certainly more applicable in parts of the world where individualized medicine will not have economic support for a long time to come.
Financial & competing interests disclosure
Dr Waldemar Debinski is a consulting scientific advisor and a shareholder in Targepeutics, Inc. Dr Debinski is an inventor on the patent applications submitted or issued, which are also on the subject of this paper (however, Penn State University and Wake Forest University own the patents). Dr Debinski’s work has never been supported by any industrial entity. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
References
- Stupp R, Mason WP, van den Bent MJ et al. Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N. Engl. J. Med.352, 987–996 (2005).
- Gallia GL, Brem S, Brem H. Local treatment of malignant brain tumors using implantable chemotherapeutic polymers. J. Natl Compr. Canc. Netw.3, 721–728 (2005).
- Debinski W. Local treatment of brain tumors with targeted chimera cytotoxic proteins. Cancer Invest.20, 801–809 (2002).
- Debinski W, Obiri NI, Powers SK, Pastan I, Puri RK. Human glioma cells over-express receptor for IL13 and are extremely sensitive to a novel chimeric protein composed of IL13 and Pseudomonas exotoxin. Clin. Cancer Res.1, 1253–1258 (1995).
- Kunwar S, Prados MD, Chang SM et al. Direct intracerebral delivery of cintredekin besudotox (IL13-PE38QQR) in recurrent malignant glioma: a report by the Cintredekin Besudotox Intraparenchymal Study Group. J. Clin. Oncol.25, 837–844 (2007).
- Kunwar S, Westphal M, Medhorn M et al. Results from PRECISE: a randomized Phase 3 study in patients with first recurrent GBM comparing Cintredekin Besudotox administered via convection-enhanced delivery with Gliadel wafers. Neuro-oncology9, 531 (2007).
- Saikali S, Avril T, Collet B et al. Expression of nine tumour antigens in a series of human glioblastoma multiforme: interest of EGFRvIII, IL-13Rα2, gp100 and TRP-2 for immunotherapy. J. Neurooncol.81, 139–148 (2007).
- Zhang JG, Eguchi J, Kruse C et al. Antigenic profiling of glioma cells to generate allogeneic vaccines or dendritic cell-based therapeutics. Clin. Cancer Res.13, 566–575 (2007).
- Liu TF, Tatter SB, Willingham MC, Yang MY, Hu J, Frankel A. Growth factor expression varies among high grade gliomas and normal brain: epidermal growth factor receptor has excellent properties for interstitial fusion protein therapy. Mol. Cancer Ther.2(8), 783–787 (2003).
- Wykosky J, Gibo DM, Stanton C, Debinski W. IL-13 Receptor α-2, EphA2, and Fra-1 as molecular denominators of high-grade astrocytomas and specific targets for combinatorial therapy. Clin. Cancer Res.14, 199–208 (2008).
- Nutt CL, Mani DR, Betensky RA et al. Gene expression-based classification of malignant gliomas correlated better with survival than histological classification. Cancer Res.63, 1602–1607 (2003).
- Wykosky J, Gibo DM, Stanton C, Debinski W. EphA2 as a novel molecular marker and target in glioblastoma multiforme. Mol. Cancer Res.3, 541–551 (2005).
- Liu F, Park PJ, Lai W et al. A genome-wide screen reveals functional gene clusters in the cancer genome and identifies EphA2 as a mitogen in glioblastoma. Cancer Res.66, 10815–10823 (2006).
- Wykosky J, Gibo DM, Debinski W. A novel, potent, and specific ephrinA1-based cytotoxin against EphA2 receptor-expressing tumor cells. Mol. Cancer Ther.6, 3208–3218 (2007 )
- Liu TF, Willingham MC, Tatter SB et al. Combination fusion protein therapy of refractory brain tumors: demonstration of efficacy in cell culture. J. Neurooncol.65(1), 77–85 (2003).
- Stish BJ, Chen H, Shu Y, Panoskaltsis-Mortari A, Vallera DA. Anti-glioblastoma effect of a recombinant bi-specific cytotoxin co-targeting human IL-13 and EGF receptors in a mouse xenograft model. J. Neurooncol.87(1), 51–61 (2008).
- Batchelor TT, Sorensen AG, di Tomaso E et al. AZD2171, a pan-VEGF receptor tyrosine kinase inhibitor, normalizes tumor vasculature and alleviates edema in glioblastoma patients. Cancer Cell11(1), 83–95 (2007).
- Debinski W, Slagle-Webb B, Achen MG et al. VEGF-D is an X-linked/AP-1 regulated putative onco-angiogen in human glioblastoma multiforme. Mol. Med.7, 598–608 (2001).
- Mischel PS, Cloughesy T. Using molecular information to guide brain tumor therapy. Nat. Clin. Pract. Neurol.2(5), 232–233 (2006).
- Debinski W. Personalized, multivalent, and more affordable: the globalization of vaccines. Clin. Cancer Res.11, 5663–5664 (2005).