Emerging novel agents for patients with advanced Ewing sarcoma: a report from the Children’s Oncology Group (COG) New Agents for Ewing Sarcoma Task Force

Introduction

Ewing sarcoma (ES) is a small round blue cell bone tumor most commonly occurring in adolescent and young adult (AYA) patients. ES is a classic fusion oncoprotein-driven tumor typically associated with a reciprocal translocation involving EWSR1 (chromosome 22) and the ETS family transcription factor FLI1 (chromosome 11)¹. These fusions may arise from chromoplexy events². Whereas classically no known genetic predisposition syndromes were linked to ES tumor development, recent evidence suggests that germline mutations in genes regulating DNA damage pathways are associated with an increased risk of developing ES³. The field continues to learn more about the origins, nature, and behavior of the EWS-FLI1 oncoprotein in the context of individual tumor cells and the micro-environment^4,5. This knowledge could assist in the future discovery of new therapeutic targets and in developing guidelines for risk stratification and treatment.

Currently, the strongest prognostic factor is stage at initial diagnosis. In North America, survival in localized ES improved to more than 70% 5-year event-free survival (EFS) by treating with alternating cycles of interval-compressed vincristine, doxorubicin, cyclophosphamide and ifosfamide and etoposide (VDC/IE)⁶. The Euro-Ewing 99 clinical trial improved outcomes in localized ES with high-risk features by intensification of therapy with high-dose chemotherapy with busulfan and melphalan⁷. The addition of IE cycles in earlier studies or intensification of therapy with high-dose chemotherapy has not improved outcomes for patients with metastatic disease^8,9. As patients with metastatic ES have not benefited from treatment intensification, it suggests that targeted agents will be needed for this population. As an example, an ongoing Children’s Oncology Group (COG) trial is evaluating the insulin-like growth factor 1 receptor (IGF-1R) antibody ganitumab with cytotoxic chemotherapy (ClinicalTrials.gov Identifier: NCT02306161)¹⁰. Patients with relapsed ES also have a dismal prognosis, and survival estimates are about 10%¹. Current efforts in the relapsed setting have focused on targeting EWS-FLI1 itself, targeting DNA damage vulnerabilities, or exploring immunotherapy strategies^11–20. For the purpose of this report, we consider patients with either newly diagnosed metastatic or relapsed ES to have advanced disease. There is a dire need for new therapies to improve outcomes for patients with advanced ES.

It is in this clinical climate that the COG Bone Tumor Committee established the New Agents for Ewing Sarcoma Task Force. The COG Bone Tumor Committee previously established a successful working group for drug development in osteosarcoma²¹. The purpose of this ongoing effort in ES is to bring together experts (basic scientists, experts in preclinical testing, pediatric sarcoma clinicians, and clinical investigators) with the primary goal of identifying potential agents of high priority for clinical evaluation and expeditiously incorporating these agents into clinical trials. Many new agents are being investigated in relapsed ES and many of these are listed in Table 1. In this report, we summarize our work to date, including establishing a framework to prioritize potential agents in this rare disease, characterizing challenges in understanding the disease biology, determining the bar for preclinical data, recommending an appropriate cytotoxic chemotherapy backbone on which to layer novel agents, and highlighting practical considerations for clinical trial development in the advanced ES patient population. We aim to provide clinicians and basic scientist’s insight into our approach in order to broadly facilitate discussions on moving new agents forward in this rare tumor. For additional background on ES more generally, we refer the reader to a recent review article²².

Conceptual framework for assessing novel agents in advanced Ewing sarcoma

The task force defined key criteria against which to gauge potential new agents of relevance to ES (Figure 1). These criteria were broadly divided into non-clinical and clinical criteria.

Figure 1. Paradigm for evaluation new agents for Ewing sarcoma.

Task force members proposed agents or targets (detailed in Table 1). These proposals were then each individually discussed using the step-wise approach outlined. Agents deemed worthy to move forward were then re-examined and re-vetted through this work flow as new preclinical or trial data updates became available. CTEP, Cancer Therapy Evaluation Program; EWS, Ewing sarcoma; FDA, US Food and Drug Administration.

Eribulin

Eribulin is a microtubule inhibitor that inhibits polymerization of tubulin subunits and differs from other microtubule inhibitors by preventing lengthening and shortening of microtubules during division. Aggregation of unstable tubulin polymers ultimately results in cellular apoptosis^32–35. EWS-FLI1 is known to drive the expression of proteins that regulate microtubule stability¹⁵. Preclinical testing of ES cell lines demonstrated apoptosis, induced through the Bcl-2 pathway³⁶. Eribulin was also evaluated by the Pediatric Preclinical Testing Program (PPTP); four out of five ES xenografts demonstrated a complete response to treatment³⁷. In sarcoma treatment, eribulin is FDA-approved for adult patients with liposarcoma who previously received an anthracycline³⁸. The COG performed a phase 1 trial of eribulin in children with advanced solid tumors; one of the evaluable patients with ES experienced a partial response for four cycles³⁹. An ongoing phase 2 trial (ClinicalTrials.gov Identifier: NCT03441360) is assessing eribulin in patients with relapsed ES⁴⁰. An ongoing phase 1/2 trial (ClinicalTrials.gov Identifier: NCT03245450) is evaluating eribulin in combination with irinotecan, although these agents are not expected to have significant overlapping toxicities⁴¹. Furthermore, vincristine and topoisomerase 1 inhibitors act synergistically in xenograft models and in pediatric clinical trials; therefore, eribulin with irinotecan may also have clinical synergy^42,43.

Eribulin shows strong preclinical data in ES, is an FDA-approved agent for another sarcoma indication, and has led to at least one monotherapy response in relapsed ES. We will recommend further trial development with eribulin as a component of combination therapy pending the results of current trials.

Dinutuximab

GD2 is a disialoganglioside that is expressed on the surface of tumors of neural crest origin, such as neuroblastoma⁴⁴. The precise cell of origin for ES is unknown; however, it is thought to be a neural crest or mesenchymal-derived tumor^45–47. ES tumors have been evaluated for GD2 expression, and results range from no detectable surface expression to diffuse/intense staining in some tumors^48–51. It is not known whether GD2 expression changes upon relapse or whether levels differ in patients with newly diagnosed metastatic ES. Several available antibodies in the clinic are known to bind to this target, including dinutuximab, which is approved by the FDA for use in neuroblastoma. There is currently no preclinical evidence for utility of dinutuximab in ES.

The clinical evidence supporting dinutuximab borrows heavily from another GD2-positive pediatric cancer, neuroblastoma. The combination of irinotecan, temozolomide, and dinutuximab has been shown to demonstrate significant clinical benefit in children with advanced neuroblastoma⁵². Pediatric dosing, schedules, and toxicities are well documented with this therapy combination. Patients with relapsed neuroblastoma are generally younger than patients with relapsed ES, and there is the potential for different toxicity in older patients. As some ES tumors express GD2, determining whether the addition of dinutuximab to chemotherapy extends to ES is a logical clinical trial question. Dinutuximab is FDA-approved and also available in the CTEP portfolio. As demonstrated in neuroblastoma trials, dinutuximab is both feasible and more effective in combination with irinotecan and temozolomide (IT).

The task force determined that the available clinical data in another GD2-positive tumor treated with this combination are sufficiently strong to nominate this approach for clinical evaluation in ES. Irinotecan and temozolomide are agents routinely used in relapsed ES therapy, and a next-step trial could determine the efficacy of dinutuximab added to IT. The combination would be strategically combining immune-based treatment with traditional cytotoxic therapy. Given the current inconsistency in GD2 immunohistochemistry staining, it would not be possible to select patients upfront on the basis of tumor GD2 expression.

Anti-angiogenic tyrosine kinase inhibitors

A large body of work highlights the role of angiogenesis in ES^53–56. Several tyrosine kinase inhibitors (TKIs) targeting angiogenesis have been developed, although most are multi-targeted TKIs that target other receptor tyrosine kinases (RTKs). RTKs are important regulators of cell growth, proliferation, and survival. Aberrant RTK signaling resulting from amplification, mutation, or overexpression has been implicated in many cancers, including ES⁵⁷.

Pazopanib primarily targets VEGFR-1 and -2, PDGFR-α and -β, and c-Kit⁵⁸. Pazopanib has gained FDA approval for treatment of refractory soft tissue sarcoma (STS) in adult patients. A phase 3 clinical trial for patients with advanced STS demonstrated improved progression-free survival (PFS) (4.6 months) in the pazopanib arm compared with placebo (1.6 months)⁵⁹. The PPTP evaluation of pazopanib revealed a statistically prolonged EFS in ES xenografts but no objective responses⁶⁰. Several case reports have demonstrated partial responses in patients with ES; however, resistance seems to develop with prolonged use^61–63. Cabozantinib targets VEGFR2, c-MET, and AXL and is FDA-approved for the treatment of medullary thyroid cancer and renal cell carcinoma in adults. A phase 2 study of cabozantinib in patients with recurrent ES showed tumor control with 9 (27.7%) partial responses and 10 (30.3%) with stable disease⁶⁴. Pediatric phase 2 dosing is established for both pazopanib and cabozantinib^65,66. Regorafenib is an oral multikinase inhibitor that targets VEGFR-1-3, FGFR1, PDGFR-α and -β, CSFR-1, and c-Kit. It has FDA approval for use in metastatic colorectal carcinoma, gastrointestinal stromal tumor, and hepatocellular carcinoma. SARC024 evaluated regorafenib in 30 patients with ES. The median PFS was 3.6 months, and the median duration of response was 5.5 months⁶⁷.

At least three anti-angiogenic TKIs have shown clinical monotherapy activity in relapsed ES. This clinical activity together with preclinical rationale and drug availability should motivate evaluation of this class of agents in combination with cytotoxic chemotherapy or other targeted agents or as a potential maintenance therapy in future trials.

CDK4/6 inhibitors

CDKN2A is a gene that codes for two proteins: p16 and p14arf. Both proteins are tumor suppressors; p16 inhibits cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) by phosphorylating the RB protein, thus preventing cell cycle progression⁶⁸. About 13 to 30% of ES tumors have deletions in CDKN2A^69–71. CDKN2A deletion does not appear to be associated with clinical outcome⁷². Owing to the alteration of p16 in a subset of ES, there is clinical interest in CDK4/6 inhibitors that target and inhibit this pathway. The cyclin D1 gene has been shown to be associated with a superenhancer in ES⁷³. ES cells consequently have an activated cyclin D1/CDK4 pathway and require CDK4 and cyclin D1 for growth. In vivo data demonstrated decreased tumor growth and prolonged survival with CDK4/6 inhibition⁷³. These data provide a rationale for use even in the absence of CDKN2A deletions⁷⁴.

Currently, three CDK4/6 inhibitors have been FDA-approved for advanced breast cancer: abemaciclib, palbociclib, and ribociclib. These agents each have ongoing or completed single-agent pediatric phase 1 trials (ClinicalTrials.gov Identifiers: NCT02644460, NCT01747876, and NCT02255461)^75–77. As in adults, the predominant toxicity seen in children is hematologic⁷⁸. Since single-agent therapy is rarely curative in sarcomas, there is little interest in evaluating single-agent CDK4/6 inhibitors in patients with ES. Combination with conventional cytotoxic chemotherapy agents, many of which are dependent on S-phase cycling, may be challenging because of antagonistic mechanisms⁷⁹. Furthermore, the significant myelosuppression seen with CDK4/6 inhibitors complicates combinatorial therapy. Without significant adult data evaluating CDK4/6 inhibitors with chemotherapy, we await the results of a planned trial evaluating palbociclib with IT before recommending a chemotherapy combination approach in advanced ES (ClinicalTrials.gov Identifier: NCT03709680)⁸⁰. There are other trials in the US combining CDK4/6 inhibitors with agents, including MEK and mTOR inhibitors (ClinicalTrials.gov Identifiers: NCT03387020, NCT03114527, and NCT02703571)^81–83. Other preclinical data suggest that dual inhibition of CDK4/6 and IGF-1R may be a combination to consider in ES, as CDK4/6 drug resistance is mediated by activation of IGF-1R signaling⁸⁴. In sum, CDK4/6 inhibitors have potential efficacy in ES but identifying the appropriate combination therapy has been challenging. We recommend additional preclinical and clinical data before moving this class of drugs forward in first-relapse or metastatic ES.

PARP inhibitors

Much enthusiasm has surrounded poly-ADP-ribose polymerase 1 (PARP1) inhibitors in ES. PARP plays a significant role in DNA repair, particularly with single-strand DNA damage. Inhibition of PARP proteins can cause persistent single-strand breaks, ultimately resulting in cellular apoptosis. EWS-FLI1 interacts with PARP1, influencing its transcriptional activity, and ES tumors have high levels of PARP mRNA and protein activity⁸⁵. Several PARP inhibitors, including olaparib, rucaparib, talazoparib, and niraparib, have received FDA approval for the treatment of ovarian or breast cancer in adult patients. Interest in PARP inhibitors in ES resulted from preclinical data demonstrating sensitivity in ES cell lines⁸⁶. A phase 2 trial with olaparib monotherapy quickly opened for adults with relapsed ES. Of the 12 patients enrolled, none had objective responses and four patients had stable disease. The median time to progression was 5.7 weeks²⁵. The PPTP and others evaluated PARP inhibitors in combination with DNA-damaging chemotherapy and demonstrated activity in ES xenografts^87–89.

Owing to these promising preclinical data, a series of successor trials have evaluated PARP inhibitors in combination with irinotecan, temozolomide, or IT (ClinicalTrials.gov Identifiers: NCT02116777, NCT01858168, NCT02044120, and NCT02392793)^90–93. While there have been hints of clinical activity, preliminary presentations of the toxicity data have shown that the myelosuppression of PARP inhibitors with cytotoxic chemotherapy is limiting dose intensity in these combination trials⁹⁴. This drug class demonstrates that strong preclinical activity and rationale may not always predict clinical feasibility, particularly in a heavily pretreated, relapsed patient population. At this time, we recommend awaiting results from ongoing clinical trials prior to moving this class of drugs further in advanced ES clinical trials.

The cytotoxic backbone: trial recommendations

We also discussed the chemotherapy backbone onto which selected agents could be added for patients with first recurrent ES. Currently, there is no established standard backbone for patients with recurrent or refractory ES. Topotecan with cyclophosphamide has shown activity in patients with relapsed ES with response rates between 23 and 41%^95–100. Owing to this promising activity, the COG conducted a pilot trial adding vincristine, topotecan, and cyclophosphamide (VTc) to interval-compressed ES therapy. This combination was tolerable, but hematologic toxicity was between 44 and 63%¹⁰¹. The Euro Ewing Consortium is evaluating topotecan and cyclophosphamide in the rEECur trial (EudraCT number: 2014-000259-99) for recurrent ES. A randomized COG phase 3 trial, AEWS1031, evaluated the efficacy of adding VTc to the interval-compressed five-drug backbone, and results are pending. Owing to the significant hematologic toxicity of this combination, incorporating additional agents to this backbone may prove challenging but could be considered for the appropriate novel agents.

The combination of vincristine and IT (VIT) has also shown activity in patients with relapsed ES. VIT has demonstrated response rates between 29 and 63% in relapsed or refractory patients, although the actual VIT response rate has not yet been investigated in a prospective randomized trial^102–105. This chemotherapy has schedule-dependent synergy^106,107. The combination of drugs also has limited overlapping toxicity. Diarrhea and abdominal pain are the most common dose-limiting toxicities of irinotecan^108,109. The major dose-limiting toxicity of temozolomide is myelosuppression^110–112. Currently, the Euro Ewing Consortium is evaluating IT in relapsed ES. IT has a strong history of efficacious combination with monoclonal antibodies or targeted therapies in other diseases^52,113,114. Currently, there are ongoing trials for patients with ES combining IT with PARP inhibitors or Vigil autologous vaccine (ClinicalTrials.gov Identifiers: NCT02044120, NCT01858168, and NCT03495921)^90,92,93,115. Owing to the reported efficacy of IT in ES and successful combination with other novel therapeutics, we believe IT (with or without vincristine) may be a preferred and feasible cytotoxic backbone for future combination clinical trials in first recurrent ES.

Although the original addition of IE to VDC did show improved outcomes for localized disease, patients with upfront metastatic ES saw no clinical benefit from this addition^8,116,117. Given these data, the group also discussed eliminating or reducing IE cycles for metastatic patients to decrease toxicity, allowing the addition of other targeted agents to VDC in future trials. Furthermore, early data from a single-institution trial including patients with upfront metastatic ES (ClinicalTrials.gov Identifier: NCT01864109) demonstrate the feasibility of incorporating IT into standard upfront therapy for metastatic ES¹¹⁸. Altering the number of IE cycles and adding IT to the upfront chemotherapy backbone are both considerations for further trial design considering drug synergism and overlapping toxicities of the targeted agents being studied.

Additional key clinical trial design considerations

In addition to establishing recommendations for advanced ES cytotoxic backbones, we examined other important aspects of trial design including patient randomization, maintenance therapy, and specific differences in trial design approach for patients with upfront metastatic versus relapsed ES. Given the rarity of this tumor, we continue to support collaborative, multi-national trials in order to study this cancer in a timely and inclusive manner.

With a goal of testing novel agents in patients with advanced ES, we propose several potential trial designs depending upon the agent and specific population of interest. The newly diagnosed metastatic population presents two potential opportunities for evaluating new agents. For agents expected to combine well with conventional cytotoxic chemotherapy, we recommend a randomized design comparing chemotherapy alone versus chemotherapy plus a novel agent. Depending upon the available evidence supporting the agent, the statistical parameters for such a trial may incorporate a phase 2 design with early stopping rules or a phase 3 trial to show definitive efficacy. For agents difficult to combine with chemotherapy, those that may be administered on a chronic schedule, or those with expected efficacy in minimal residual disease settings, a maintenance design could be considered. In this design, patients in a radiographic complete remission would continue treatment with the novel agent upon completion of frontline therapy. Both approaches are used in the ongoing COG trial of ganitumab. Given the paucity of clinical data for maintenance strategies in ES, this approach is best studied in the context of a randomized study, comparing maintenance with no maintenance or comparing two different promising maintenance strategies. Likewise, the type of maintenance regimen should be carefully considered in light of concerns about toxicity and adherence in an AYA population¹¹⁹.

For the relapse population, there are two main design considerations. Given that ES often remains chemotherapy-responsive at first relapse, trial designs should incorporate cytotoxic chemotherapy. Therefore, testing of new agents can be performed as a single-arm trial combining a new agent with a standard backbone, and inference about activity of the combination is based on strong historical data for the backbone regimen. Alternatively, new agents may be tested in a randomized manner, either as a definitive comparison against the backbone alone or in a selection design comparing two novel agents added to the same backbone. Promising agents that are unlikely to pair well with chemotherapy (for example, antagonism; overlapping toxicity) may be best evaluated as monotherapy in a second relapse population.

Conclusions and future directions

We have generated a robust group and infrastructure for vetting novel agents for evaluation in patients with ES. Future efforts by our group and other groups may use this paradigm as new agents or concepts become available. We discussed several strategies that were determined to be too early in development for immediate translation into a clinical trial for the newly diagnosed metastatic or first recurrent populations. Examples of these strategies include lysine-specific demethylase 1A (LSD1) inhibition, inhibition of interaction between EWSR1-FLI1 protein with RNA helicase, epigenetic modification of chromatin modeling resulting in EWS-FLI1 suppression, inhibition of transcriptionally active CDKs, and immunotherapy approaches in ES^17,120–122. The first two of these approaches are particularly noteworthy as they target fundamental functions of the fusion oncoprotein that drives this disease. The task force will continue to discuss the progress of these and other emerging strategies.