An effective treatment could be found for one of the most aggressive pediatric cancers, alveolar rhabdomyosarcoma (aRMS). These kinds of tumors, which develop mainly in skeletal muscle, are characterized by the presence of the PAX3-FOXO1 fusion protein. Scientists at St Jude Children's Research Hospital published in the July 13, 2022, issue of Science Translational Medicine how they eliminated the tumor in a mouse model by inhibiting an enzyme to which PAX3-FOXO1 binds.

Rhabdomyosarcoma (RMS) has two histological variants, embryonic (eRMS) and alveolar (aRMS), which has higher mortality.

The degree of differentiation of the rhabdomyoblasts that form the tumor determines its classification. Embryonic tumors have few differentiated cells, while alveolar tumors have alveolar-like structures or small cavities.

The chimeric protein resulting from the union of PAX3 and FOXO1 comes from a fusion gene formed by a reciprocal translocation of chromosomes 2 and 13. This PAX3-FOXO1 protein only exists in aRMS and could be related to the worse prognosis associated with this variety of the disease, more aggressive than the fusion-negative embryonic form.

The investigators discovered that, for aRMS cells to spread, and the tumor to grow, an epigenetic alteration is needed. The histone lysine demethylase 4B (KDM4B) interacted by physically binding to PAX3-FOXO1. To inhibit it and see what happened, they used the experimental drug QC-6352 (Celgene).

This inhibitor is specific to the KDM4B enzyme. Its use in preclinical tests with mice (subcutaneous xenograft models) reduced tumor growth without producing side effects. In addition, the researchers combined these tests with the application of two chemotherapy drugs (vincristine [VCR] and irinotecan [IRN]), enhancing their results.

"The KDM4B inhibitor reduced expression of PAX3-FOXO1, the key molecular driver of aRMS. In the absence of adjuvant chemotherapy, the inhibitor also reduced expression of proteins such as MYOD1, MYCN that are essential to aRMS cell survival," the director of this study, Jun Yang, assistant faculty member in the Department of Surgery at St. Jude Children’s Research Hospital, told BioWorld. Regarding the reduction of the tumor by combining KDM4B inhibition with chemotherapy, Yang confirmed that "there was nearly complete tumor elimination when KDM4 inhibition was combined with chemotherapy."

PAX3-FOXO1 is essential for the development of the tumor in aRMS, as it regulates the expression of the oncogenes IGF2, FGFR4, MYCN, MET or ALK and, in addition, it acts as a transcription factor in a network that directs the myogenic program, a network necessary for the survival of aRMS cells. Although there are different preclinical models and clinical trials oriented toward PAX3-FOXO1 downstream, to date, no strategies have been identified to interfere in this network of transcription factors in which PAX3-FOXO1 participates.

"The main modalities for aRMS treatment are chemotherapy and radiation therapy, which broadly target rapidly dividing cells," Yang commented to remark on how the therapeutic target they have identified differs from other treatments for aRMS. "Our approach is to target more specifically the epigenetic modifiers of the tumor cell genome. The strategy shuts down expression of key cancer drivers in the tumor cells, resulting in enhanced antitumor activity and fewer treatment side effects."

Although PAX3-FOXO1 is specific to aRMS, Yan and his colleagues have described a mechanism that can be used as a therapy in other aggressive tumors. "KDM4 histone lysine demethylases have been reported to be important in several cancers. Therefore, QC-6352 could have broad antitumor activity across tumors of different histologies."

In addition to using VCR and IRN chemotherapies in these preclinical tests, other drugs could be tested to check their suitability combined with inhibition of the enzyme. "We are trying to find other potential drug combinations to achieve the maximal anticancer effect. We observed that QC-6352 induced a type I interferon response, suggesting that KDM4 inhibition could potentially enhance immunotherapy," Yang remarked.

There are still a few steps before starting a clinical trial with this novel therapeutic target. "The preclinical model we used involved immune-deficient mice. We will test if this combination works even better in transgenic mouse models that have intact immune systems," Yang said. "We also need to find biomarker(s) to stratify patients who may have maximal benefit from this inhibitor, and pharmacodynamic marker(s) to verify the action of inhibitor in tumor cells, in future clinical trials," he added.