(P 237) SECONDARY REARRANGEMENT CREATING A PLEC-EML4-ALK DOUBLE FUSION AS A MECHANISM OF RESISTANCE TO CRIZOTINIB IN AN EML4-ALK-POSITIVE INFLAMMATORY MYOFIBROBLASTIC TUMOR

Objective: ALK rearrangement/fusion is detected in greater than 50% of inflammatory myofibroblastic tumor (IMT) cases. Inhibition of ALK fusion protein with small molecule tyrosine kinase inhibitors (TKIs) is effective in positive cases, and consequently crizotinib, a first generation inhibitor of ALK, is now regulatory agency-approved in this setting. However, acquired resistance to targeted therapy remains a clinical challenge. Resistance mechanisms to crizotinib, predominantly investigated in the setting of non-small cell lung cancer, include activation of bypass signaling, ALK kinase domain mutation, and amplification of the ALK fusion gene. Amplification and the resulting increased protein expression of the fusion is believed to reduce the ability of crizotinib to saturate its target, thus permitting enough signaling to promote tumor growth. Here we present a case of EML4-ALK-positive IMT in a 61-year-old female who demonstrated an initial response to crizotinib. Disease progression occurred following 5 cycles of crizotinib, prompting additional molecular testing to determine the mechanism of resistance.

Methods: Both the initial diagnostic sample and biopsy at time of resistance to crizotinib were assessed by the ArcherDx FusionPlex Solid Tumor next-generation sequencing-based gene fusion assay. Briefly, total nucleic acid (RNA and DNA) was extracted from enriched tumor material and submitted to library preparation using the FusionPlex kit. Libraries were sequenced via the Illumina platform and raw sequence data was analyzed by the ArcherDx analysis algorithm (v6.2.3).

Results: The initial diagnostic sample demonstrated evidence of a classic EML4-ALK fusion in which exon 20 of ALK was fused with exon 2 of EML4 (variant v5a). RNA reads extending 5’ through EML4 exon 2 clearly read into the expected bases in EML4 exon 1. The same assay was performed on the sample obtained at the time of resistance. In this sample, exon 20 of ALK remained fused with exon 2 of EML4, but exon 2 of EML4 now demonstrated fusion to PLEC (several distinct isoforms were detected involving various exons of PLEC, all confirmed to be in frame). The fusion to an additional 5’ gene and thus new promoter was theorized to result in increased expression (immunohistochemical studies are ongoing) that would mimic the well-described amplification of the EML4-ALK fusion as a mechanism of resistance to crizotinib. Since newer generation ALK inhibitors are more potent than crizotinib and known to overcome this resistance mechanism, the patient was initiated on alectinib therapy and demonstrated an on-going partial response per RECIST criteria.

Conclusion: Secondary rearrangement involving EML4-ALK which places the fusion gene under control of a new promoter may be a novel mechanism of resistance to crizotinib in IMT (and potentially other ALK-fusion-driven malignancies). Alectinib, a more potent ALK inhibitor, overcame resistance in this case. More broadly, this study exemplifies the importance of molecular testing in the setting of resistance to targeted therapy to inform clinical decision making.