Neuroblastoma (NB) is the most common extracranial solid tumor in children. Since 2018,
we have led a prospective, nationwide multicenter collaborative clinical study on arsenic
trioxide(ATO) combined with chemotherapy for 4/M stage NB (NCT03503864).The results
indicate that the objective response rate (ORR) and complete remission rate (CRR) at the
end of induction (EOI) for the ATO-combined chemotherapy regimen are significantly higher
than those of the commonly used NB regimens in China, as well as the internationally
recognized rCOJEC and MSKCC-N5 regimens. Additionally, the ATO regimen demonstrates a
lower incidence of toxic side effects, providing favorable conditions for subsequent
consolidation therapy and contributing to the long-term survival of High-risk
neuroblastoma(HR-NB) patients. However, in clinical practice, we found that some children
responded poorly to ATO combined induction chemotherapy, with lesions persisting or
progressing and recurring, gradually developing into refractory cases during induction
chemotherapy. Studies show that primary/acquired resistance in NB is related to the
high-frequency clonal diversity that gradually emerges with chemotherapy.

With the advancement of precision medicine technology and in-depth research on signaling
pathways, we found the RAS-MAPK signaling pathway plays an important role in the
regulation of cell proliferation and differentiation, and this pathway is abnormally
activated in various tumors. Literature reports that 78% of refractory/recurrent NB have
activating mutations in the RAS-MAPK pathway, suggesting a close association between this
pathway and refractory/recurrent NB. In addition, in some tumor cell lines, ATO can cause
upregulation of the MAPK pathway, which may be the reason for the poor efficacy of ATO in
some HR-NB children and suggests the necessity of combining MAPK inhibitors in
ATO-containing induction chemotherapy regimens. Studies have shown that in APL cell line
NB4 and APL primary cells, MAPK (MEK) inhibitors (PD98059 and PD184352) enhance apoptosis
in ATO-treated cells by inhibiting downstream ERK1/2 and Bad phosphorylation, indicating
that combining ATO with MAPK inhibitors can further improve efficacy.

Clinical studies show that single-agent MAPK inhibitors often have poor efficacy in the
treatment of advanced or refractory tumors. Studies have found that in ALK-dependent
RAS-MAPK mutant NB cell lines, MAPK inhibitors can activate the pro-cancer PI3K-AKT
pathway through the mTORC2 complex-related subunit SIN1, and thus the MAPK inhibitor
trametinib cannot inhibit the growth of ALK-dependent NB in mouse models. Previous
studies have found that ATO can downregulate the PI3K-AKT pathway in various tumors, and
we have found through non-label quantitative proteomics technology that ATO treatment of
NB cell line SK-N-BE(2) can also cause downregulation of this pathway, and studies have
shown that ATO can inhibit the activity of the mTORC2 complex in this pathway. In
summary, we propose combining ATO with MAPK inhibitors, with the aim of using MAPK
inhibitors to counteract the adverse effects of ATO activating the MAPK pathway, while
ATO inhibits the activation of the PI3K-AKT pathway caused by MAPK inhibitors, thereby
jointly inhibiting tumor growth.

As chemotherapy progresses, tumor cells gradually accumulate DNA damage under
chemotherapy pressure, leading to a higher probability of gene mutations and clonal
evolution associated with poor prognosis, increasing the likelihood of drug resistance;
studies show that primary/acquired resistance in NB is also related to the high-frequency
clonal diversity that gradually emerges with chemotherapy; under the selective pressure
of chemotherapeutic drugs, cells with drug-resistant characteristics gradually expand and
enrich, and gradually dominate; the later the remission time during induction
chemotherapy in NB patients, the worse the prognosis; studies show that patients with
initial bone marrow minimal residual disease(MRD) positivity who remain positive after 4
courses of chemotherapy have a significantly higher recurrence rate than those who become
negative (74.1% vs. 33.3%), and the event-free survival has statistical significance; and
radical surgery performed after the 3rd or 4th course of induction chemotherapy according
to consensus and mainstream cooperative group regimens may also be one of the potential
risk factors promoting the dissemination and metastasis of NB tumor cells; therefore, we
chose to add trametinib to the medication sequence starting from the 5th course of
induction chemotherapy, for a total of 5 courses of combined medication (the 5th to 9th
courses of induction chemotherapy). The effectiveness and safety of the above combined
medication strategy need to be further verified through multicenter collaborative
clinical studies, and we expect that this combined medication regimen can improve the
treatment status and prognosis of HR-NB, a refractory tumor.

Based on the above theoretical basis, the combination of MAPK pathway inhibitors with ATO
has a synergistic effect, and we expect that the new regimen of adding the MAPK pathway
inhibitor trametinib to the original ATO combined chemotherapy regimen can improve CRR.
Therefore, this study aims to use the CRR at EOI of ATO combined chemotherapy and MAPK
signaling pathway inhibitor(for example. trametinib) in the treatment of 4/M stage NB as
the main research objective. The overall CRR at EOI for 4/M stage NB, both domestically
and internationally, will serve as an external control to assess the clinical efficacy
and safety of ATO-combined chemotherapy plus the MAPK signaling pathway inhibitor during
the induction chemotherapy phase of 4/M stage NB.