1. Objectives
1.1. Primary objective
To evaluate the objective response rate (ORR) based on RECIST 1.1 in patients with hormone receptor-positive hypermutated metastatic breast cancer identified by whole exome sequencing
1.2. Secondary objectives
Clinical benefit rate (CBR) based on RECIST 1.1
Reaction period (DoR)
Disease control rate (DCR) based on RECIST 1.1
Progression-free survival (PFS) based on RECIST 1.1
Toxicity evaluation based on NCI-CTCAE v5.0

2. Background
2.1. Hormone receptor positive breast cancer
About 70 percent of breast cancer patients express estrogen receptors or progesterone receptors. Currently, the standard hormone therapy in hormone receptor positive metastatic breast cancer include sequential use of selective estrogen receptor modulators (tamoxifen), nonsteroidal aromatase inhibitors (AIs; anastrozole, letrozole), steroidal aromatase inhibitors (exemestane), and selective estrogen receptor downregulators (fulvestrant). However, primary, intrinsic or secondary resistance to hormone therapy is a clinically important issue.
2.2. Chemotherapy for hormone-positive breast cancer
Once resistance to endocrine therapy develops or there is a visceral crisis regardless of sensitivity to endocrine therapy, cytotoxic chemotherapy is the preferred treatment. Taxane or Antracycline is the most popular drug among cytotoxic chemotherapy for the first treatment in advanced breast cancer, even for HR-positive breast cancer. 
2.3. Development status of PD-1/PD-L1 blockades 
Clinical trials of PD-1/PD-L1 inhibitors in breast cancer are mainly performed in triple negative cancer. As a result of the KEYNOTE-12 study, Pembrolizumab alone showed a response rate of 18.5% in patients with metastatic triple-negative breast cancer who previously failed various treatments, and the response was sustained for a considerable period of time in some patients (San Antonio Breast Cancer Symposium 2014). However, there are few clinical studies on PD-1/PD-L1 blockers alone in hormone receptor-positive breast cancer.

3. Hypothesis
The response rate (ORR) based on RECIST 1.1 of Pembrolizumab and Paclitaxel combination therapy in patients with hormone receptor-positive hypermutated metastatic breast cancer confirmed by whole exome sequencing has clinical significance

4. Clinical Research Plan
4.1. Prescreening to enrichment immunogenicity
-In order to enrich the immunogenicity ('immunogenecity enrichment') among hormone receptor-positive breast cancer patients known to have generally low immunogenicity, patients with nonsynonymous mutation ≥ 70 per tumor through biomarker prescreening (defined as 'hypermutated') will be enrolled in this clinical trial.
-The reference point of 70 was determined as the top 20% nonsynonymous mutation burden by comparing and analyzing the entire in-house exome sequence data with TCGA and our in-house exome pipeline.
4.2. An adjusted approach to estimating the top 20% mutation burden threshold ('adapted approach')
-Since 16 cases of in-house total exome data are not sufficient, the boundary values will be recalculated when 30 cases (second calculation) and 60 cases (third analysis) of total exome sequencing are performed in the biomarker prescreening step. In the next case, the new threshold will be used to determine whether to administer an immune checkpoint inhibitor.
4.3. Finding the threshold of the mutation burden that best predicts the objective response rate or clinical benefit rate.
Patients selected for the treatment phase will receive pembrolizumab treatment, and the objective response rate (ORR, overall response rate) and clinical benefit rate (CBR, clinical benefit rate) (CBR, clinical benefit rate) through the ROC curve for patients receiving clinical trial drugs after all the treatment phases have been completed. We will find a cutoff value that best correlates with the benefit rate.

Treatment

1. Pembrolizumab
Pembrolizumab 200 mg is administered intravenously over 30 minutes every 3 weeks. As much as possible, the infusion time of 30 minutes is adjusted, but the infusion time is allowed to vary by -5 min/+10 min because the infusion pump may be variable for each laboratory. The Pharmacy Manual contains specific instructions for the preparation and administration of pembrolizumab injection. Specific instructions for the reconstitution of pembrolizumab and the preparation and administration of infusion fluids are provided in the pharmacy manual.
2. Paclitaxel
Paclitaxel is administered at a dose level of 80 mg/m2 by intravenous infusion for more than 1 hour according to local practice and product labeling information. Paclitaxel will be administered on days 1, 8 and 15 of each 28-day cycle, separate from the pembrolizumab cycle. If paclitaxel cannot be administered due to an adverse reaction on the 8th day, it is “skip” and the administration is continued on the 15th day. All participants should be given oral or intravenous steroids and antihistamines as pre-treatment drugs according to approved product labeling information and/or standard practice. The administration of additional pretreatment drugs follows standard practice guidelines.

Arm Label

Target Number of Participant

Arm Type

Arm Description

Pembrolizumab 200mg q3wks, Paclitaxel 80mg/m2 on Day 1,8, 15 q4wks

Gender

Age

Description

1. Histologically or cytologically confirmed premenopausal or postmenopausal hormone receptor-positive stage 4 breast cancer patients.
*Definition of hormone receptor positivity: Immunochemical staining using tumor samples, when the degree of nuclear staining of tumor cells in ER or PR is between 1 and 100% (ASCO/CAP guideline 2020)
2. Patients who have given written informed consent for this clinical study.
3. Patients who did not receive systemic cytotoxic chemotherapy in metastatic state. Hormone therapy and anti-HER2 targeted therapy are not limited.
4. Patients over 19 years of age as of the date of written informed consent.
5. The criteria for hypermutated are satisfied based on whole exome sequencing.
* Criteria for overmutation: The number of nonsynonymous mutations is initially set as 70 or more, and is applied based on the top 20% or higher through interim analysis for every 30 cases. That is, the standard may be changed every about 30 cases.
6. Patients with metastatic tumors that can be biopsied for complete exome sequencing. Biopsy of breast or axillary lymph node tumors is also allowed for patients with local recurrence after surgery or in stage 4 from diagnosis.
7. Patients with one or more measurable lesions based on RECIST v1.1. Tissues that have undergone biopsy are considered measurable lesions if not resected.
8. Patients whose disease progression has been confirmed since the most recent treatment (not applicable in the prescreening stage, ie, tissue biopsy is possible while maintaining the existing treatment).
9. Patients with an expected life span of 12 weeks or more.
10. ECOG is 0 to 1.
11. The laboratory test conducted within 10 days prior to the start of treatment meets the criteria described in the protocol.
12. In the case of female patients of childbearing potential, a urine test or serum pregnancy test confirmed negative pregnancy reaction within 72 hours prior to the first administration of the test drug. If the urine test is positive or negative and it is difficult to confirm, a serum pregnancy test is required.
13. For female patients of childbearing potential, willingness to use the appropriate method of contraception described in the protocol by day 180 after the last dose of the investigational drug.
*If abstinence is the patient's personal routine and the preferred method of contraception, that method of contraception is also acceptable.

1. Women of childbearing potential who have a positive urine pregnancy test within 72 hours prior to the first administration of the test treatment. If a urine test cannot confirm negative, a serum pregnancy test should be performed. In this case, if the serum pregnancy test result is positive, the participant must be excluded from participation in the test.
2. Has a history of receiving treatment with drugs targeting anti-PD-1, anti-PD-L1, anti-PD-L2 agents, or other stimulating or co-inhibiting T-cell receptors (eg CTLA-4, OX 40, CD137).
3. In the case of receiving prior systemic chemotherapy including study drugs within 4 weeks prior to randomization [In the case of kinase inhibitors or other short half-life drugs, less than 5 times the half-life before randomization has passed]
4. Previous radiation therapy within 2 weeks of starting treatment. Participants must recover from all radiation-related toxicity, do not need corticosteroids, and must not have had radiation pneumonia. A 1-week withdrawal period is permitted for palliative radiation (less than 2 weeks of radiation treatment) for non-CNS diseases.
5. Received live vaccine within 30 days prior to the first administration of the investigational drug.
-Note: Because seasonal influenza vaccines are generally inactivated flu vaccines, clinical participation is permitted. However, intranasal influenza vaccines (e.g. Flu-Mist®) are herbal poisoned vaccines and are not allowed to participate in clinical trials.
6. If there is a currently participating clinical study, participated in a clinical trial for an investigational drug within 4 weeks prior to the first administration of the investigational drug, and received trial treatment or used an investigational medical device.
-Note: Patients currently participating in observational studies can enroll in this study if 4 weeks have passed since the last drug administration.
7. Has been diagnosed with immunodeficiency or has been chronically receiving systemic steroid therapy (in excess of a dose equivalent to 10 mg per day of prednisone) or other immunosuppressive therapy in any form within 7 days prior to first administration of the test treatment.
8. Other invasive malignancies are known that are ongoing or require active treatment within the past 2 years.
Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, epidermal bladder cancer, or in situ cervical carcinoma are not eligible for curative treatment.
9. Active central nervous system (CNS) metastases and/or carcinoma meningitis are excluded from clinical trials. Patients treated for past brain metastasis are stable (if there is no progression of the disease on imaging for at least 4 weeks prior to the first administration of the clinical trial drug, and neurological symptoms have recovered to the baseline level), new metastasis to the brain, and If there is no enlargement of metastases and no steroids have been used for at least 7 days prior to clinical treatment, they can participate in clinical trials. Carcinoma meningitis is excluded from this study regardless of its clinical stability.
10. Previous history of hypersensitivity to Pembrolizumab or its excipients (grade 3 or higher)
11. In case of hypersensitivity to Paclitaxel or its excipients (grade 2 or higher peripheral neurosis) or contraindications to Paclitaxel
12. Active autoimmune diseases requiring systemic treatment within the past 2 years (eg, use of disease modulators, corticosteroids or immunosuppressive drugs) are excluded from clinical studies. Replacement therapy (eg thyroxine, insulin or physiological corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic therapy.
13. Patients with high blood pressure (including unstable angina pectoris, myocardial infarction, stroke and transient ischemic attack) confirmed or uncontrolled hypertension that occurred within 24 weeks prior to participation in the clinical trial, and according to the judgment of the investigator. Nevertheless, those with SBP> 140 mmHg or DBP> 90 mmHg)
14. Excluding active non-infectious pneumonia and those with medical history
15. Excluding active infection patients requiring systemic treatment
16. Patients with a history of immunodeficiency virus (HIV 1/2 or HTLV-1 antibody)
17. In case of active hepatitis B (HBs antigen+) or hepatitis C (anti-HCV antibody+) test within 180 days of test drug administration
However, registration is possible if the following items are proven within 60 days of administration of the test drug.
-Registration is possible when HBs antigen+ is inactive without treatment (HBV DNA <20 IU/mL) or virus concentration is not detected while antiviral treatment is performed (HBV DNA <20 IU/mL).
 -In the case of anti-HCV antibody+, HCV RNA PCR test is performed, and if negative, registration is possible. However, registration is not possible if the anti-HCV antibody+ has not been tested for HCV RNA PCR.
18. Active tuberculosis
19. Participation in the clinical trial itself is not suitable for the patient in the opinion of those with abnormalities in conditions, treatments, and laboratory tests that may confuse the results of the clinical trial or interfere with the patient's participation in the entire clinical trial period. Those who are judged not suitable for study
20. Mental illness or substance abuse disorder.
21. Current pregnant or breastfeeding female patients or patients of childbearing potential who are not willing to continue to use effective contraception from the time of pre-screening or screening visit until 180 days after the final administration of the test drug.
22. Received an allogeneic tissue/solid organ transplant.

Objective response rate based on RECIST 1.1

Full dosing period

Clinical benefit ratio based on RECIST 1.1

Full dosing period

Duration of response based on RECIST 1.1

Full dosing period

Disease control rate based on RECIST 1.1

Full dosing period

Progrssion-free survival based on RECIST 1.1

Full dosing period

Toxicity based on NCI-CACAE v5.0

Full dosing period