Pilot Study: Busulfan-Based Haploidentical Stem Cell Transplantation Plus GD2 Antibody Therapy for High-Risk Neuroblastoma

Overview

High-risk neuroblastoma remains one of the most difficult childhood cancers to treat. It is an aggressive tumor of the developing nervous system, usually arising in infants and young children, and very high-risk cases often do poorly even with intensive multimodal therapy. This pilot study explored a combined strategy that brought together myeloablative busulfan-based haploidentical stem cell transplantation and GD2-directed antibody immunotherapy with dinutuximab beta. The goal was to improve long-term disease control in children whose disease was resistant to standard induction therapy and whose stem cells were not suitable for standard autologous transplantation.

Why this approach was considered

In very high-risk neuroblastoma, tumor cells frequently carry biologic features associated with treatment resistance, including alterations in RAS or p53 signaling and mechanisms that maintain telomeres, allowing the cancer to keep dividing. Even with current intensive therapy, event-free survival in these children is often below 20%. For many patients, the usual treatment path includes chemotherapy, surgery, radiation or metaiodobenzylguanidine-based therapy, and consolidation with high-dose chemotherapy followed by autologous stem cell rescue. However, some children cannot collect enough hematopoietic stem cells for autologous transplantation, or their disease remains too difficult to control. That creates a major therapeutic gap.

Haploidentical stem cell transplantation, or haplo-SCT, uses stem cells from a partially matched family donor, often a parent. In addition to restoring blood cell production after intensive chemotherapy, this approach can provide a graft-versus-tumor effect, meaning donor immune cells may help attack residual cancer cells. It may also enhance antibody-dependent cellular cytotoxicity, a mechanism by which antibody therapy helps immune cells recognize and destroy tumor cells. These immune advantages made haplo-SCT an attractive partner for GD2-targeted immunotherapy.

What was studied

The report describes a 5-patient pilot cohort with very high-risk neuroblastoma. All children received systemic induction or salvage therapy and local treatment according to national guidelines. Before transplantation, they underwent conditioning with busulfan, melphalan, fludarabine, and antithymocyte globulin. This was followed by T-cell- and B-cell-depleted haploidentical stem cell transplantation. After engraftment, the patients received six cycles of dinutuximab beta, an anti-GD2 monoclonal antibody used to target neuroblastoma cells.

Two of the five patients also received [131I]MIBG therapy before the transplant regimen. [131I]MIBG is a targeted radiopharmaceutical that delivers radiation to neuroblastoma cells and is sometimes used in difficult relapsed or refractory cases. The treatment plan therefore combined cytotoxic therapy, cellular replacement, donor-derived immune effects, and antibody-based immunotherapy in a single sequential strategy.

Key findings

All five patients successfully engrafted, showing that the transplant platform was feasible despite the intensity of the regimen and the high-risk nature of the disease. At the time of reporting, three of the five patients were alive and remained in their first complete remission for 7.3 years, 6.3 years, and 1.5 years after haplo-SCT. Two patients experienced treatment-related events: one had a relapse and one died from a non-relapse cause.

These results are notable because durable remission in this subgroup is usually difficult to achieve. The combination of myeloablative busulfan-based therapy with haploidentical transplantation and dinutuximab beta appeared not only technically possible but also clinically promising. While the cohort was very small, the long remission durations in several patients suggest that the approach may offer meaningful benefit for selected children with otherwise poor prognosis.

How the treatment may work

The strategy is built on complementary mechanisms. Busulfan and melphalan provide intensive cytoreduction, aiming to eliminate as much tumor burden as possible. Fludarabine and antithymocyte globulin help shape the transplant environment and control immune complications. The haploidentical graft then reconstitutes the blood and immune system using donor cells that may contribute anti-tumor activity.

Dinutuximab beta targets GD2, a surface antigen highly expressed on neuroblastoma cells. Once bound to GD2, the antibody can recruit immune effector cells and trigger killing of tumor cells through antibody-dependent cellular cytotoxicity. In a haplo-SCT setting, donor-derived immune cells may potentially amplify this effect. In theory, this combination could create a stronger post-transplant anti-neuroblastoma response than either strategy alone.

Clinical significance

This pilot experience is important for several reasons. First, it offers an option for children who are not candidates for conventional autologous stem cell transplantation because they cannot mobilize enough stem cells. Second, it shows that a fully donor-based transplant approach can be integrated with modern anti-GD2 immunotherapy. Third, it supports the concept that immunologic graft effects may improve outcomes in a disease where relapse remains the major cause of failure.

For families and clinicians facing very high-risk neuroblastoma, any strategy that can improve the chance of sustained remission deserves careful attention. However, because this was only a five-patient study, the findings should be viewed as early evidence rather than definitive proof.

Limitations

The main limitation is sample size. With only five patients, it is not possible to determine how much of the benefit came from the transplant platform, the antibody therapy, prior therapies such as [131I]MIBG, or patient-specific biology. There was also no randomized comparison group, which makes it impossible to directly compare this strategy with standard autologous consolidation or other salvage approaches.

Another limitation is the complexity of the regimen. Myeloablative conditioning, haploidentical transplant, and antibody therapy all carry risks, including infection, organ toxicity, graft complications, and treatment-related mortality. Careful patient selection and close supportive care are essential. The study also reflects experience from highly specialized centers, so the results may not be immediately generalizable to all hospitals.

What this means for future care

The early results justify further study in a larger controlled trial. If confirmed, this strategy could become an important option for children with very high-risk neuroblastoma, especially those who cannot undergo standard autologous transplant or whose disease is refractory to induction therapy. Future research will need to clarify which patients are most likely to benefit, how to optimize conditioning intensity, how best to sequence dinutuximab beta with transplant, and how to reduce toxicity while preserving anti-tumor effect.

It will also be important to compare this approach with other emerging treatments, including novel immunotherapies, CAR T-cell strategies, and improved targeted radiopharmaceutical regimens. Neuroblastoma treatment continues to move toward more personalized therapy based on tumor biology and immune vulnerability, and this study adds to that evolving landscape.

Conclusion

In this pilot study, primary busulfan-based haploidentical stem cell transplantation followed by dinutuximab beta was feasible and showed encouraging long-term disease control in a small group of children with very high-risk neuroblastoma. All patients engrafted, and three remained in durable first complete remission for years after transplant. Although confirmation in a larger trial is needed, the findings suggest that combining transplant immunology with GD2-targeted antibody therapy may improve outcomes in a population with historically poor survival.