Inetetamab in combination with rapamycin and chemotherapy for trastuzumab‐treated metastatic human epidermal growth factor receptor 2‐positive breast cancer with abnormal activation of PI3K/Akt/mTOR pathway

Abstract Background Human epidermal growth factor receptor 2 (HER2) overexpression is an independent prognostic factor of poor prognosis and a predictor of efficacy of anti‐HER2 therapy. A limited number of patients can receive standard second‐line therapy (DS‐8201 or T‐DM1) for metastatic HER2‐positive in some parts of the world, including China, due to many factors, such as cost–benefit ratios. Case A 51‐year‐old premenopausal woman was diagnosed with HER2‐positive breast cancer. The pathological stage was ypT3N2M0 and stage IIIA. Trastuzumab targeted therapy combined with goserelin depot was started along with letrozole endocrine therapy. After eight courses of treatment, magnetic resonance imaging (MRI) examination revealed new multiple metastases in the liver, and progression disease (PD) was evaluated. Due to abnormal activation of the phosphatidylinositol 3‐kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway in the patient, treatment was changed to the mammalian target of rapamycin (mTOR) inhibitor combined with the anti‐HER‐2 agents inetetamab and paclitaxel, while partial response (PR) was evaluated after 6 cycles of treatment. As the patient was hormone receptor (HR) positive, treatment was changed to the inetetamab + rapamycin + exemestane regimen. The lesion continued to shrink and PR was evaluated for 8 cycles. The original regimen was continued, PR was evaluated after 12 courses of treatment. The abdominal MRI performed showed an increase in the volume of intrahepatic multiple metastatic tumor lesion. Efficacy was used to assess for PD and the progression‐free survival (PFS) was 317 days. Conclusion A phosphatidylinositol‐4, 5‐bisphosphate 3‐kinase catalytic subunit alpha (PIK3CA) mutation in trastuzumab‐treated metastatic HER2‐positive breast cancer female had a long PFS by treating with the mammalian target of rapamycin inhibitor in combination with the anti‐HER‐2 agent inetetamab.


| INTRODUCTION
Breast cancer is one of the most frequently diagnosed life-threatening malignancies in women both in China and abroad. 1 Human epidermal growth factor receptor 2 (HER2) is a transmembrane protein with receptor tyrosine kinase activity and a member of the epidermal growth factor receptor family, overexpressed in 15%-20% of patients with invasive breast cancers, contributing to poor prognosis for patients. 2 Currently, anti-HER2 therapy has been extensively used in clinical practice and trastuzumab had become the standard postoperative targeted therapy for HER2-positive breast cancer, however, the emergence of drug resistance has made it challenging for patients to benefit from trastuzumab in the long term. 3 Mutations in the phosphatidylinositol-4, 5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) gene are currently one of the known mechanisms of resistance to anti-HER-2 therapy and are associated with a poorer prognosis. 4 Inetetamab is a trastuzumab analogue that has the exact F(ab')2 of trastuzumab fusing into a different IgG1 Fc allotype, with two amino acid variants that differ from trastuzumab, which could be used for treatment after trastuzumab resistance. 5 This study reported a case of PIK3CA mutation in HER-2 positive breast cancer patients whose phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway was activated abnormally after the primary resistance to trastuzumab, treated with the mammalian target of rapamycin (mTOR) inhibitor in combination with the anti-HER-2 agent inetetamab and the second-line nab-paclitaxel, the progression-free survival (PFS) was 317 days and tolerable adverse effects. This study is designed to evaluate the efficacy and safety of inetetamab combined with rapamycin and chemotherapy regimens and to explore new treatment options for trastuzumabresistant disease with the goal of improving prognosis.

| Chief complain
On May 5, 2020, a 51-year-old non-menopausal woman came to Anqing Municipal Hospital for examination due to a mass in the left breast that has developed for more than 2 months.

| Present patient history
The patient had a history of type 2 diabetes mellitus for 10 years and received insulin hypoglycemic treatment with stable blood glucose control, while a history of hypertension for more than 8 years and amlodipine hypoglycemic treatment with stable blood pressure control. The patient denied other specific medical histories.

| Examination
The mammography showed hypoechoic masses in the left breast

| Treatment
The patient refused to use targeted therapy due to financial reasons and was administered with six courses of chemotherapy of the TTC regimen (docetaxel 75 mg/kg d1 + pirubicin 50 mg/kg d1 + cyclophosphamide 500 mg/kg d1 q3w). Chemotherapy was initiated on May 13, 2020, and completed on August 30, 2020. The patient experienced adverse effects but was well tolerated.

| DISCUSSION
Breast cancer is a type of malignant tumor with the highest morbidity and mortality in women. 6  consistent with that of Pierobon et al., 11 which indicated that PIK3CA mutation was found in 62.5% of patients with liver metastasis, while the rate of PIK3CA mutation was only 5.5% in patients with nonhepatic lesions, demonstrating that breast cancer patients carrying PIK3CA mutation was significantly greater among liver metastases compared with the other metastatic sites. In conjunction with the medical history, the patient was considered to be resistant to trastuzumab and endocrine treatment. Although HER2-targeting antibody trastuzumab confers a substantial benefit for patients, acquired resistance remains has become a key challenge in clinical practice. Chinese guidelines also recommend pyrrolitinib + capecitabine as treatment for primary trastuzumab resistance, but both pyrrolitinib and capecitabine are associated with high incidence of grade 3 diarrhea and poor patient compliance. Thus, trastuzumab resistance is an important clinical problem and management for patients with primary trastuzumab resistance is still an unmet clinical need.
PIK3CA mutation is one of the common mechanisms of HER-2 resistance, with a mutation rate of 12%-39% in HER-2-positive breast cancer. 12 Loibl et al. conducted a meta-analysis that involving 967 patients with HER2-positive breast cancer and found that patients with PIK3CA mutations treated with neoadjuvant chemotherapy combined with anti-HER2 therapy (even with a dual-targeted regimen) had significantly lower rates of pathologic complete remission than those with wild-type PIK3CA (16.2% vs. 29.6%, p < .001). 13 In the correlation analysis of the CLEOPATRA study, 32% of patients with metastatic HER-2-positive breast cancer had a (D) PIK3CA mutation and the PFS in patients with PIK3CA mutation of targeted therapy was significantly lower than those with wild-type PIK3CA. 14,15 Ioannis et al. 16 had also found HER2 cell line HCC1954 carry a strong gain-of-function PIK3CA mutation exhibited sustained MTOR signaling. PIK3CA mutations are associated with activation of the PI3K/AKT/mTOR pathway, leading to abnormal activation of this pathway and in turn stimulates tumor proliferation, metastasis, and invasion. Accumulating evidence suggests that PIK3CA mutations are associated with a poorer prognosis and are more likely to lead to drug resistance. 17 Preclinical studies have implicated that the combination of PI3K/ AKT/mTOR pathway inhibitors can enhance the antiproliferative activity of anti-HER-2 therapy and overcome resistance to anti-HER-2 therapy. [18][19][20][21] The BOLERO-3 study showed that everolimus (mTOR inhibitors) in combination with trastuzumab and vincristine significantly prolonged the PFS of HER2-positive, trastuzumab-resistant patients who previously received paclitaxelbased chemotherapy (7.00 months vs. 5.78 months, p = .0067). 22 Meanwhile, PIK3CA gene mutations were associated with endocrine resistance. The BELERO-2 study showed that for patients with breast cancer who experienced relapse or PD after NSAID treatment, the PFS was significantly prolonged in the everolimus combined with exemestane group compared with that in the placebo combined with exemestane group (7.8 months vs. 3.2 months, p < .01). 23 However, the clinical application of everolimus is limited due to its many adverse reactions, poor tolerance, and a higher rate of discontinuation and dose reduction. 24 Rapamycin is another mTOR inhibitor that targets the PI3K/AKT/ mTOR pathway and blocks downstream signaling. As an anti-tumor treatment, rapamycin is usually highlighted for antitumor proliferation and anti-tumorigenic properties and fewer adverse effects, better patient tolerance, and the potential to improve prognosis notably. 25 Inetetamab (Septrin ® ), the first recombinant human monoclonal antibody developed and marketed in China, has the same two antigen binding fragments with 214 amino acids each, but with amino acid modifications to the fragment crystallizable region and manufacturing process optimization, exhibiting the same HER2 antigen binding affinity and greater antibody-dependent cell-mediated cytotoxicity as trastuzumab. 26 Results from the Lan et al. 27 had highlighted that inetetamab combined with pyrotinib and capecitabine showed complete tumor growth inhibition, and better antitumor activity than trastuzumab combined with pertuzumab plus capecitabine. In this study, the patient developed liver metastasis and was accompanied by PIK3CA gene mutation after trastuzumab treatment, treated with mTOR inhibitor in combination with the anti-HER-2 agent inetetamab.
The choice of combination chemotherapy drugs has remained a tough problem in combination cancer therapy. In a study of patients with metastatic breast cancer, results suggested that the efficacy of albumin paclitaxel was not affected in patients previously treated with paclitaxel-based drugs 28 with higher efficacy. 29 Therefore, the chemotherapeutic agent of choice was albumin paclitaxel monotherapy. Ultimately, the optimal regimen for patient was rapamycin combined with inetetamab combined with albumin paclitaxel monotherapy. The patient was well tolerated and had a progression-free survival of 317 days.
The prospective, randomized controlled clinical trial of the patients enrolled in this case report aimed to explore the efficacy and  (Table 1), and tumor indexes were considered insensitive and were not used as a monitoring index.

| CONCLUSION
In this study, patient who progressed after trastuzumab treatment and had abnormal activation of the PI3K/Akt/mTOR pathway were selected to receive inetetamab combined with rapamycin to reverse drug resistance and restore the tumor sensitivity to anti-HER2 monoclonal antibodies, achieving 317 days of PFS and tolerable adverse effects, and improving the prognosis. This was only a case study; a study on the efficacy and safety of inetetamab combined with rapamycin and chemotherapy in patients with metastatic HER2-positive breast cancer with abnormal activation of the PI3K/AKT/MTOR pathway who have progressed after trastuzumab treatment is still ongoing. We look forward to the final results of this study, which will provide a new treatment option for patients with HER-2-positive PI3K mutations.

AUTHOR CONTRIBUTIONS
All authors had full access to the data in the study and take responsibility for the integrity of the data and the accuracy of the data analy-