Introduction:

In about 15% to 20% of breast cancer cases, human epidermal growth factor receptor 2 (HER2) over-expression or gene-amplification is associated with poor prognosis. Thanks to the development of target therapies, HER2 positive patients can be managed using HER2-targeting drugs. There are several kinds ofHER2 inhibitors, such as trastuzumab, lapatinib, and pyrotinib. Pyrotinib which exert different functions, of note, the latest generation of the drug, is an irreversible small-molecule tyrosine kinase inhibitor targeting epidermal growth factor receptor (EGFR) (HER1) and/or HER2 and/or HER4. Both lapatinib and pyrotinib potentially target EGFR and/or HER2, but in some instances, induces different responses of patients with EGFR and/or HER2 mutations. This is attributed to the different mutations in EGFR and HER2 genes, which may form distinct types of HER2 dimers, with different binding capacities to drugs.

Patient concerns:

Five years ago, a patient underwent a radical mastectomy in an external hospital. Results of the resection histopathology revealed an invasive ductal carcinoma, pT3N0M0, stage IIB, HER2 positive. The lady patient received 6 cycles of adjuvant chemotherapy and was subjected to adjuvant trastuzumab therapy for 1 year. After a regular 1-year follow-up and in March 2018, she complained of chest pain and visited our hospital. We diagnosed her with metastatic breast cancer, positive for HER2.

Diagnosis:

positron emission tomography/computed tomography showed multiple metastases in the lung and sternum, while the breast lesions did not progress, the curative effect of which we evaluated as a progressive disease. Then, lapatinib integrated with chemotherapy was administered to the patient. After 5 cycles of the treatment, the patient experienced lower back pain. Through CT examination, it was revealed that she had multiple metastases in the lung and sternum, in addition to new metastases in the lumbar spine and right lobe of the liver. Moreover, magnetic resonance imaging revealed multiple metastases in the brain, and the disease further progressed. The results of circulating tumor DNA assays showed that other than HER2 amplification, novel EGFR-ZNF880 fusion and EGFR E114K mutations developed.

Interventions:

The patient was administered with a combination of pyrotinib with chemotherapy.

Outcomes:

After 2 months of pyrotinib treatment, the metastases of the lung, sternum, lumbar spine, and right lobe of the liver disappeared. Also, the size of the brain metastases reduced while bone metastases were relieved. The curative effect was evaluated as a partial response. Following the results of circulating tumor DNA assays, HER2 amplification, EGFR-ZNF880 fusion, and EGFR E114K mutations disappeared. However, since a small lesion was present in the brain, the patient was subjected to radiotherapy in the head. Notably, after 9 months treatment with pyrotinib, enhanced CT indicated that tumors in the breast, liver, both lungs, brain, and bone were under control. The patient continually received oral pyrotinib, however, a new brain lesion appeared 6 months later. Overall, we managed to regulate the efficacy of pyrotinib for up to 15 months.

Conclusion:

This case report demonstrates that EGFR-ZNF880 fusion and EGFR E114K mutations may contribute or lead to the formation of a special HER2 dimer, which is rapidly resistant to lapatinib but sensitive to pyrotinib. Of note, this is the first report that such a new fusion has been found.