BCL-2 Inhibitors May Provide Avenue of Attack Against High-Risk Neuroblastoma

Anthony Faber, PhD

Member, Developmental Therapeutics Research Program

Virginia Commonwealth University (VCU) Massey Cancer Center

Assistant Professor, Philips Institute for Oral Health Research, VCU School of Dentistry

Madhu Gowda, MD

Pediatric Hematologist-Oncologist

Children’s Hospital of Richmond at VCU and Massey Cancer Center

Assistant Professor, Department of Pediatrics, VCU School of Medicine Richmond, VA

Neuroblastoma is a rare cancer that develops in very early forms of nerve cells in an embryo or fetus. Among children, it is the most common extracranial solid tumor and typically occurs before the age of 5 years. Every year in the United States, about 700 patients are diagnosed with neuroblastoma. Of these children, one-third have low- or intermediate-risk neuroblastoma with an excellent prognosis (90%-95% survival rate) with minimal treatments. Unfortunately, the remaining two-thirds have high-risk disease and a survival rate of only about 50% despite multimodal therapy.¹

Targeted Treatments Emerge

The current treatment for children with high-risk neuroblastoma includes high-dose chemotherapy, surgery, stem cell transplantation, radiation therapy, isotretinoin, and immunotherapy. It is an intense regimen that has significant adverse effects (AEs) and toxicity to the kidneys, heart, and lungs. The physical and emotional impact on children and their families is tremendous. Even with these therapies, nearly 50% of the children cannot be cured and those who go into remission are at high risk for long-term treatment-related AEs.However, the news is not all grim. There have been some recent treatment advancements. For instance, metaiodobenzylguanidine (MIBG) is a radioactive compound used for the diagnosis of neuroblastoma. By substituting the radioactive iodine for a different isotope, MIBG can be used to deliver targeted radiation therapy, to which about 30% of patients with relapsed neuroblastoma respond.²

A second approach to specifically attack neuroblastoma is through the chimeric antibody dinutuximab, which targets disialoganglioside GD2, found richly on neuroectodermal tissue. Longer event-free survival and progression-free survival have been demonstrated in children given the combination of dinutuximab, cytokines, and isotretinoin compared with isotretinoin alone.³ Additionally, Yael P. Mossé, MD, from Children’s Hospital of Philadelphia (CHOP), has recently published research showing that the ALK inhibitor crizotinib may restore sensitivity to common chemotherapy agents in preclinical neuroblastoma models with ALK mutations.⁴

Venetoclax Looks Promising

Mossé and colleagues also recently published results from the New Approaches to Neuroblastoma Therapy (NANT) trial, which found that alisertib, an Aurora-A kinase inhibitor, demonstrated activity against neuroblastoma and was tolerable in patients with relapsed or refractory neuroblastoma. The trial had an overall response rate of nearly 32%, with a response rate of 50% at the maximum tolerated dose.⁵Some of the most lethal forms of neuroblastoma are often associated with amplification of the transcription factor MYCN, and researchers at Massey Cancer Center and the Philips Institute for Oral Health Research have been investigating new therapies for MYCN-amplified neuroblastoma. In fact, our laboratory is working on the development of targeted therapies for currently undruggable cancers—those that are driven by genomic abnormalities—such as amplification of MYCN, for which there are no available drugs to directly target the genetic abnormality.

Essentially, it comes down to a chemistry problem, since many of the genomic events driving the formation and growth of these cancers have been identified, but the proteins they encode are not yet druggable. High on this list are transcription factor-driven cancers, like MYCN-amplified neuroblastomas. In collaboration with Mossé as well as scientists at VCU Massey Cancer Center and a group led by Cyril Benes, PhD, at the Center for Molecular Therapeutics at Massachusetts General Hospital (MGH) Cancer Center, we are studying novel drug combinations that work specifically in MYCN-amplified neuroblastomas.

Our research, featured on the cover of Cancer Cell, demonstrated that neuroblastoma cells are highly sensitive to the drug venetoclax (Venclexta).⁶ Venetoclax is an FDA-approved targeted therapy that inhibits the protein B-cell lymphoma 2 (BCL- 2), a key regulator of apoptosis (cell death). We found that venetoclax kills neuroblastoma cells preferentially in the presence of amplified MYCN due to higher cellular levels of the proapoptotic protein NOXA, which is a sensitizer to venetoclax. In fact, prior research by Hisashi Harada, PhD, of the Philips Institute for Oral Health Research at the VCU School of Dentistry, and Geoffrey Krystal, MD, PhD, of VCU Massey Cancer Center, shed light on the important link between NOXA and BCL-2 inhibitor sensitivity several years ago.⁷

Overall, this current finding underlies an intriguing strategy for the development of targeted therapies for cancers driven by amplified transcription factors: when there is that much of the transcription factor being made by the cell, it inevitably will fall onto the genome in many different places. In most instances, this will lead to high expression of a gene that provides a Darwinian survival advantage for the cell. Once this gene is identified, there may be an available targeted therapy that blocks it. In this way, one can easily see how the oncogenic transcription factor can be attacked indirectly by targeting an important downstream effector gene. In other cases, such as the one we describe here, the transcription factor binds to and upregulates a gene with an unexpected opposing survival effect which, once identified (in this case, NOXA), may be a sensitizer to an available targeted therapy (ie, venetoclax).

Once we determined why venetoclax was effective against MYCN-amplified neuroblastoma, we began collaborating with Erin Coffee, PhD, and Carlotta Costa, PhD, at MGH Cancer Center, to find other drugs that might increase its effectiveness. Eventually, we discovered that alisertib complements venetoclax and that the combination is very effective at killing neuroblastoma tumors in advanced mouse models.

We attribute the synergism between the 2 drugs to a further imbalance of the BCL-2 family member proteins. We only observed the toxic effects of the drug combination on the MYCN-amplified neuroblastomas; the combination was not very effective against neuroblastomas without amplified MYCN. Of note, some of the benefit of alisertib is most likely due to its direct effect on the MYCN protein, since destabilization of Aurora-A kinase leads to a decrease in MYCN protein expression.⁸

Combinations Explored

Although alisertib is currently being investigated in clinical trials by Mossé and her colleagues at CHOP, it is too early to tell if there will be a future trial of alisertib and venetoclax in neuroblastoma. We and other groups are actively exploring several other preclinically effective combination therapies utilizing venetoclax, underscoring our belief that venetoclax has a place for treating neuroblastoma. These include combining other Aurora kinase inhibitors with venetoclax.Outside of that strategy, we are also utilizing preclinical models of advanced neuroblastoma developed from tumor samples from relapsed patients provided through the Children’s Oncology Group Cell Culture and Xenograft Repository by C. Patrick Reynolds, MD, PhD, director of The Cancer Center at Texas Tech University Health Sciences Center School of Medicine. Early experiments in these models, in collaboration with Mossé’s group, have demonstrated that venetoclax synergizes with a combination of topotecan and cyclophosphamide, standard chemotherapeutic drugs.

Additionally, there seems to be a rationale for pairing venetoclax with MDM2 inhibitors. Although mutation/deletion of the TP53 gene has been implicated in most cancers, it is usually preserved in neuroblastoma. When p53 is intact as in neuroblastoma, it can be upregulated as a signal to the cell to stop proliferating and die. MDM2 is one of the key molecules that prevents p53 from being upregulated, but MDM2 inhibitors can only lead to upregulation of p53 when p53 is preserved in the cell. So far, we have had very promising results testing combinations of venetoclax and MDM2 inhibitors against p53-preserved neuroblastoma, and the synergy again appears to be centered on the BCL-2 family of proteins.

Overall, we feel there may be a place for venetoclax and other BCL-2 inhibitors under development in the future care of patients with advanced neuroblastoma. We plan to continue pursuing this research thanks to funding from VCU Massey Cancer Center, the Rally Foundation for Childhood Cancer Research and Truth 365, Alex’s Lemonade Stand, Wipe Out Kids’ Cancer, and the George and Lavinia Blick Research Fund.

As researchers, we know it must be unbearable for pediatric oncologists and everyone involved in the treatment of these patients to run out of options. We desperately need new, more effective therapies and are hopeful that our efforts will help ease the burden caused by this disease.

References

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