Radiation-agent combinations for glioblastoma: challenges in drug development and future considerations

Abstract

Glioblastoma is an aggressive disease characterized by moderate initial response rates to first-line radiation-chemotherapy intervention followed by low poor response rates to second-line intervention. This article discusses novel strategic platforms for the development of radiation-investigational agent combination clinical trials for primary and recurrent glioblastoma in a NCI-NCTN settings with simultaneous analysis of challenges in the drug development process.

INTRODUCTION

The prognosis for patients diagnosed with glioblastoma remains dismal. The overarching intent of this article is to discuss new strategic platforms for clinical development of radiation-agent clinical trials for the management of patients diagnosed with glioblastoma.

The Cancer Moonshot’s Blue Ribbon Panel report [1] emphasizes recommendations for accelerating cancer science in a single effort to bring together promising biological research and clinical development for cancer patients in the near-term. The National Cancer Institute (NCI) and its Cancer Therapy Evaluation Program (CTEP) have aligned some research efforts around a personalized medicine approach in order to predict a precise and individual response to treatment. Application of personalized medicine in glioblastoma is of primary importance to craft novel targeted therapies specifically tailored to molecular alterations that arise in a patient’s tumor at any clinical stage. Given such a directive, glioblastoma biology, biomarkers relevant to radiation-agent clinical development, and radiomics are discussed as each relates to near-term and long-term strategies for glioblastoma patient care.

BIOLOGY

Similar to most other cancers that arise from stem cells within a particular tissue or organ, glioblastoma is driven by evolving molecular alterations in glioma cells [2,3]. Clinically, glioblastoma may arise in eloquent brain, creating challenges for effective neurosurgical management because of quality of life decisions that impact surgical approach [4]. Glioblastoma infiltrates normal brain diffusely, a factor recognized in treatment failure, relapse, and eventual patient death [5]. Further, glioblastoma resides ‘behind’ a relatively impermeable blood-brain barrier that can impede targeted delivery of a biological product or a chemotherapeutic drug [6]. Absolute immune privilege does not necessarily apply to all brain areas, but effective afferent or efferent immune responses mounted alone against glioblastoma are rare [7].

BIOMARKERS

The comprehensive genomic fingerprint of glioblastoma has led to putative identification of biomarkers [3], such as the methylation status of O⁶-methylguanine-DNA methyltransferase (MGMT), a gene encoding an enzyme responsible for removing alkylating lesions in host cell DNA [8]. Genomic fingerprints are currently utilized as integral biomarkers to stratify glioblastoma patients in clinical trials and to assign care.

Contemporary large-scale genome profiling studies indicate that glioblastomas segregate into different biomarker subgroups which can be classified by mutations or by DNA methylation profiles [3]. Two predominantly pediatric glioblastomas subgroups are listed by a missense mutation in the H3F3A gene encoding histone H3.3—labelled either H3F3A K27 (particularly in pontine or thalamic brain tumors) or H3F3A G34 (in hemispheric brain tumors) [9]. A third young adult glioblastoma subgroup is categorized by a missense mutation in the IDH gene. This young adult subgroup often has an improved clinical outcome [9]. Glioblastomas that are IDH mutant often lack a molecular profile of monosomy 10, trisomy 7, and epidermal growth factor receptor (EGFR) amplification that is frequent in IDH wild-type glioblastomas. Moreover, H3F3A and IDH mutant glioblastomas often carry p53 gene alterations [9]. Platelet-derived growth factor receptor-α (PDGFRA) gene amplification, most prevalent in adolescent and young adult patients, identifies another subgroup [9]. Other subgroups include the ‘classic’ (or ‘RTK II’) and the ‘mesenchymal’ subgroups—characterized by distinct mRNA expression profiles and by RTK II tumors exhibiting EGFR amplification [9]. Whether therapies attuned to these biomarkers will elicit personalized responses remains to be determined.

CLINICAL DEVELOPMENT OF AGENTS TARGETING GLIOBLASTOMA MULTIFORME

There are a number of new strategic directions for radiation plus experimental therapeutic agents that appear particularly promising for clinical development in glioblastoma over the next several years. Examples of four settings for radiation-agent or drug-agent discovery are discussed below.

DNA methylation

Modification of gene expression, rather than the genome itself, can impact glioblastoma biology. Initial findings from phase III clinical trials suggested that the addition of an alkylating chemotherapeutic agent (such as nitrosourea) given during or after radiotherapy led to small but clinically significant gains in patient survival [10,11]. The newer chemotherapeutic agent temozolomide acts as a cytotoxic alkylating agent, which is converted at physiologic pH to a short-lived active compound, monomethyl-triazeno-imidazole-carboxamide (MTIC). Temozolomide cytotoxicity results from DNA methylation at the O⁶ or N⁷ positions of guanine, effectively blocking DNA replication [12]. A 573-patient randomized phase III clinical trial of radiotherapy alone or radiotherapy-temozolomide showed a significant 2.5-month prolongation in median survival after radiotherapy-temozolomide [13]. In a later performed subgroup analysis, patients with a methylated MGMT promoter had a median 22-month survival from radiotherapy-temozolomide, compared to a median 15-month survival after radiotherapy alone [14]. For patients with an unmethylated MGMT promoter, a smaller gain in survival was observed with the addition of temozolomide [14]. The prognostic importance of the MGMT promoter methylation status was confirmed in multiple subsequent phase III trials in newly diagnosed glioblastoma [15,16]. Emerging from such studies is the concept that the MGMT promoter methylation status might serve as an integral biomarker in an umbrella trial protocol—the rationale being, patients harboring methylated glioblastomas are most likely to benefit from temozolomide, and thus, a temozolomide-new agent combination might be warranted and studied, whereas patients having unmethylated glioblastomas appear to benefit little from temozolomide and a novel radiation-agent trial arm without temozolomide could be justified. An intriguing future direction in the methylated MGMT promoter patient populations involves methoxyamine. Methoxyamine is an oral small-molecule inhibitor that covalently binds to apurinic/apyrimidinic (AP) DNA damage sites and inhibits DNA base excision repair (BER), expanding the number of DNA strand breaks and inducing apoptosis [17]. Methoxyamine may increase the anti-tumor activity of alkylating agents like temozolomide [18], and perhaps, would be of clinical trial interest if preclinical data confirmed blood-brain barrier penetration.

Cell cycle

It is commonly held that the pathognomonic radiographic appearance of glioblastoma results from failure of blood supply to keep pace with rapid cell turnover. Glioblastoma’s propensity for rapid cell turnover has been linked to deregulation of the phosphoinositide 3-kinase (PI3K), p53, and the cell cycle retinoblastoma (Rb) protein pathways [3] (Figure 1). Attempts, to date, at targeting nodes of convergence within the PI3K pathway by small-molecule inhibitors have been unsuccessful [19–23]; however, novel approaches with improved drugs are in progress. As an example, temsirolimus acts as a rapamycin analog to pharmacologically block the PI3K pathway’s mammalian target of rapamycin (mTOR), resulting in decreased expression of downstream effectors necessary for G1 phase cell cycle progression. Preclinical data demonstrated a radiation sensitization effect of rapalogs in glioblastoma [24] and modest brain penetration by drug was observed [19]. But, a clinical trial in glioblastoma patients reported poor clinical activity using radiation plus temsirolimus [20]. Future study for the glioblastoma PI3K pathway might involve sapanisertib (TAK-228), an orally bioavailable equipotent inhibitor of the raptor-mTOR complex (TOR complex 1 or TORC1) and the rictor-mTOR complex (TOR complex 2 or TORC2), that could possibly halt tumor cell proliferation and might better activate tumor cell apoptosis [25].

Figure 1.

Pathway alterations in glioblastoma multiforme. Canonical signaling pathway and tumor suppressor pathways are summarized for glioblastoma, highlighting potential nodes of convergence amenable to blockade by experimental therapeutics as monotherapy or in combination with radiation therapy. Percentages represent the proportion of glioblastoma cases harboring amplification (+) or mutation or homozygous deletion (−) of indicated pathway node. Figure adapted from reference [3]. Abbreviations: EGFR = epidermal growth factor receptor, PDGFT = platelet-derived growth factor receptor, c-MET = hepatocyte growth factor receptor, FGFR = fibroblast growth factor receptor, PTEN = phosphatase and tensin homolog, PI3K = phosphoinositide 3-kinase, NF1 = neurofibromatosis type 1, Akt = protein kinase B, mTOR = mammalian target of rapamycin, MAPK = mitogen-activated protein kinase, MDM2 = mouse double minute 2 homolog, CDK = cyclin-dependent kinase, RNR = ribonucleotide reductase, Rb = retinoblastoma protein.

Along the same general approach, 85% of glioblastomas possess dysregulated p53 pathway effectors [3], which are appealing anticancer drug targets (Figure 1). Clinical development is in progress of the murine double minute 2 (MDM2) inhibitor AMG 232, which is an oral piperidinone binder to the MDM2 protein and disrupts its binding to the transcriptional activation domain of the tumor suppressor protein p53. By blocking a MDM2-p53 protein-protein interaction, transcriptional activity of p53 becomes restored in a manner that induces tumor cell apoptosis when DNA damage is detected [26,27]. Growing evidence suggests MDM2 amplification contributes to glioblastoma pathogenesis [3], but it is not understood whether MDM2 amplification drives an oncogenic phenotype or is a bystander of (epi-)genetic silencing [28]. Clinical development of AMG 232 in glioblastoma patients is underway.

As a third alternative, clinical investigation has begun into inhibitors of cyclin-dependent kinases to target the deregulation of the Rb pathway in glioblastoma (Figure 1). An agent like palbociclib, an oral inhibitor of cyclin-dependent kinase 4 (CDK4) and 6 (CDK6) [29], may hold promise in a radiation-agent management strategy for glioblastoma due to the deletion of CDKN2A and amplification of CDK4 observed in glioblastomas [30].

Angiogenesis

Developing new or pruning old cerebral blood vessels generate tell-tale signs of glioblastoma tumorigenesis [31,32]. Bevacizumab, a recombinant humanized monoclonal antibody directed against the vascular endothelial growth factor (VEGF), a pro-angiogenic cytokine, binds to VEGF and prevents VEGF coupling with its cell surface receptor, thereby slowing the growth and the maintenance of tumor blood vessels. Bevacizumab has short-lived survival benefits in second-line therapy for patients with recurrent glioblastoma [33] and no overall survival benefit for patients in first-line disease management [15,16]. Attempts at targeting nodes of convergence within the VEGF receptor pathway by small-molecule inhibitors have been ineffective [34–37]. Modifying a stem cell-like transition following VEGF receptor blockade, such as through disruption of the Hedgehog (Hh) signaling pathway, may be more enticing in the near future. Hh is vital to the pattern and embryonic development of many organs, and both Hh ligand-dependent and ligand-independent activation promote tumorigenesis and VEGF-mediated angiogenesis [38]. Study of one such inhibitor, vismodegib, an orally bioavailable small-molecule inhibitor of the Hh signaling pathway, has found that pharmacologically blocked Hh-ligand cell surface receptor signaling disrupts neuronal stem cell-like properties [39]. Whether Vismodegib has clinical activity in prospective glioblastoma clinical trials remains to be tested. Retrospective data indicates that Hedgehog signaling is an active and important pathway in glioblastoma [40].

Immunity

Tumor vascular and blood-brain integrity impact efferent and afferent immune responses against glioblastoma [7]. Expression and signaling activity of immune checkpoint effectors that inhibit CD8+ T cells—in particular, the cell surface programmed cell death protein 1 (PD-1)—have emerged as actionable targets in glioblastoma. Usually, activated PD-1 negatively regulates CD8+ T cell activation and serves a key role in glioblastoma’s evasion of the immune system. Pembrolizumab is a humanized monoclonal antibody directed against PD-1 with immune checkpoint blocking and anticancer activity. Given a prominent role of PD-1 signaling in glioblastoma [41], there may be a future role for immune checkpoint inhibitors like pembrolizumab for glioblastoma, both as monotherapy and in combination with other anticancer agents (clinicaltrials.gov, NCT02337491). Much is to be learned in the immunotherapy oncology research space. Clinical trial results are eagerly anticipated, and particularly of prime interest, is when ionizing radiation is used in combination because it can cause robust immune-modulation and modification of the blood-brain barrier.

NEW COMPLIMENTARY TOOLS AND CLINICAL METHODS IN MANAGEMENT OF GBM

Radiomics

Beyond therapeutic developments stemming from novel combinatorial strategies of different therapeutic agents with radiation therapy, radiomics research can be equally important to lead incremental gains. Radiomics enables high-output extraction of medical image quantitative features, providing a comprehensive non-invasive assessment of tumor phenotype [42]. Radiomics has spurred interest in near-term research initiatives that include photon and particle radiotherapy delivery platforms driven by treatment planning software utilizing (a) biologic rather than physical dose [43], (b) grid therapy [44], and (c) avoidance algorithms for eloquent brain [45]. How best to integrate radiomics into clinical trials remains under study.

Clinical trial design for glioblastoma

As the proportion of unselected patients truly benefiting from use of any specific targeted agent is expected to be relatively low, a trial design with a properly selected limited sample size could quickly generate clinical activity signal or screen out an inactive agent. An appealing trial design could be the enrichment or patient-matched trial design. In the enrichment trial design, only patients with a biomarker-positive status are enrolled and are treated with the targeted biologic agent regimen. Simon [46] and Maitouram [47] showed that the enrichment trial design has a higher relative efficiency in terms of required sample size and number of patients to be screened, when compared to a traditional randomized clinical trial design without incorporating biomarkers. However, study successes depend on proper biomarker enrichment strategies to correctly identify the patient-matched population. When multiple potentially therapeutic agents honing-in on similar pathways are under clinical development, it is critical to optimally utilize limited patient and clinical trial infrastructure resources. Typically, a phase II selection design can facilitate the prioritization among competing targeted biologic agents. One phase II selection design example is the “pick-the-winner” design [48]. In this particular design, patients are randomized to two or more treatment groups. Within each treatment group, the outcome is compared to a historical control group rather than each other. This affords common single-arm trial designs (e.g., Simon’s two-stage design) to be used for each group rather than a more complex format. If more than one competing targeted biologic agent shows efficacy, a winner will be declared for further investigation based on the best outcome estimate. This design always selects a winner, even with nominal difference, and is efficient to narrow choice of targeted biologic agent for phase III studies.

One caveat of a pick-the-winner design is that the statistical significance comparison is not the purpose of the study, and thus, it is not properly powered for this type of endpoint. When the treatment mechanism of targeted biologic agents is cytostatic, response endpoints may not be suitable. Progression-free survival (PFS) in these cases may capture the treatment effects better than response endpoints. However, historical PFS data are not always readily available and are commonly subject to patient selection bias and sensitive to miscellaneous prognostic factors. In this latter case, a randomized design with a concurrent control arm is needed to protect against such biases. A randomized phase II screening design [49], which applies less stringent type I error rate and power allows preliminary comparison between new targeted biologic agent versus control regimen, provides insight into whether a targeted biologic agent warrants further definitive evaluation in phase III studies. For the designs mentioned above, efficacy evaluations are fixed at (a) the number of treatment groups, (b) the number of patients within treatment group, and (c) pre-specified time points.

An adaptive design clinical trial for human drugs and biological products has been defined as a prospectively planned opportunity for modification of one or more aspects of trial design based upon interim in-trial subject data [50]. For this definition, the term ‘prospectively’ implies that the modification was preplanned and specified in detail before unblinded analysis.

The traditional clinical development plan for oncology agents typically involves ‘isolated’ trial phases—from dose finding (pilot or phase I) to efficacy screening (single-arm or randomized phase II) to confirmatory (phase III) trials. Lead time between phases, duplicated efforts for trial protocol development, and unawareness of data collected on earlier phase trials are the major inefficiencies of such a clinical development program. Nowadays, seamless trial designs which integrate both early and late phase trial hypotheses under an umbrella trial protocol are a new trend in oncology. Under umbrella trial protocols, data collected in the ‘learning’ early phases (i.e., phase I or II studies in traditional paradigm) informs final analyses. During the transition from the early phase to the late phase of an umbrella trial protocol, the seamless design allows adaptability within a single protocol for (a) testing different hypotheses, (b) selecting different endpoints, (c) integrating biomarker validations as results mature, and (d) adjusting agent dose and intensity as safety data matures. A biomarker-enriched glioblastoma clinical trial (clinicaltrials.gov, NCT02152982) trial is employing a seamless phase II/III design to expeditiously and to efficiently answer the question if blocking DNA damage repair by using the poly(ADP-ribose) polymerase (PARP) inhibitor veliparib can improve survival outcome in MGMT methylated newly diagnosed glioblastoma patients.

New collaborations for glioblastoma science and clinical development

As new settings for radiation-agent discovery evolve, there is an increasing impetus for collaborations among research stakeholders in academia, industry, and government. For example, NCI CTEP and the NCI Molecular Radiation Therapeutics (MRT) of the Radiation Research Program facilitates a working group of peers with expertise in primary brain tumors (and brain metastasis) whose tasks include scientific comment early-on in radiation-agent clinical development plans, harmonization of pre-clinical data and radiobiological biomarkers in trials strongly supported by providing inputs for the evaluation of radiation-agent preclinical performance. Such input impacts the enthusiasm for the overall radiation-agent clinical development process at the intramural agent project team, extramural agent project team, and NCI CTEP consensus reviews. In addition, RRP-MRT play an important role in bridging companies that are funded with SBIR projects from NCI with the members of working groups within MRT with a focus to promote cancer drug development with radiation. In this mission, MRT also interacts with FDA to discuss and educate particularly guidelines for non-clinical and clinical considerations. A recent publication from FDA highlights the existing regulatory framework in which cancer drugs may be developed for use in combination with radiation therapy and describe the mechanisms in place for sponsors to obtain feedback from the Agency during the process of drug development [51]. Moving forward, NCI CTEP MRT working group participation is encouraged among the research community to help develop optimal smart clinical trial designs that can potentially lead to the development of new standard-of-care glioblastoma management.

CONCLUSION

In summary, these are exciting times for personalized medicine initiatives in glioblastoma cancer care. Novel early-phase and late-phase clinical trial designs investigating experimental therapeutics paired with radiotherapy hold promise for effective therapy against glioblastoma over the next five years.