HS-10342 is a selective CDK4/6 kinase inhibitor. This study is conducted to evaluate the safety and efficacy of HS-10342 at repeated doses.

HS-10342-201 is a single- arm, open- label, multicenter, phase 2 study in patients with hormone receptor positive (HR positive), human epidermal growth factor receptor 2 negative (HER2 negative) advanced and/or metastatic breast cancer who have had disease progression after endocrine therapy. The efficacy is evaluated as monotherapy, and the primary endpoint is ORR.

Inclusion Criteria:

1. Have a diagnosis of Hormone Receptor Positive(HR+), Human Epidermal Growth Factor Receptor 2 Negative (HER2-) breast cancer.
2. Recurrent, locally advanced, unresectable or metastatic breast cancer with disease progression following anti-estrogen therapy.
3. Prior treatment with chemotherapy regimens, No more than 2 prior chemotherapy regimens in the metastatic setting.
4. At least one extracranial measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) criteria version 1.1.
5. recovered from the acute effects of therapy with toxicity resolving to baseline or grade 1 except for residual alopecia and peripheral neuropathy.
6. Adequate function of major organs.

Exclusion Criteria:

1. Has received or is undergoing the following treatments:

   1. Currently receiving or have received any CDK4/6 inhibitors;
   2. Receiving/received antitumor therapy within 14 days or 5 half-lives, before the initial dose whichever is the longer;
   3. Radiotherapy with a limited field of radiation for palliation within 14 days of the initial dose of study drug, or received more than 30% of the bone marrow irradiation, or large-scale radiotherapy within 28 days of the initial dose.;
   4. Major surgery within 4 weeks of the initial dose of study drug;
   5. Brain metastases unless asymptomatic, stable, and not requiring steroids for at least 2 weeks prior to start of study treatment. Meningeal or brainstem metastases. Spinal cord compression;

2. Abnormal liver and kidney functions that are known to affect drug metabolism and excretion:

3. History of other primary malignancies.

4. Participating in other clinical studies.