A Randomised, Open-label, Multicentre Phase III Clinical Study to Evaluate the Efficacy and Safety of JS105 Combined With Dalpiciclib and Fulvestrant Compared With Dalpiciclib and Fulvestrant in Patients With PIK3CA-mutated, HR-positive, HER2-negative Recurrent or Metastatic Breast Cancer.

Patients will receive oral JS105 on Days 1-28 of each 4-week cycle.

Patients will receive oral Dalpiciclib for 3 weeks, followed by 1 week off in each 4-week cycle；

Patients will receive intramuscular Fulvestrant on day 1 and day 15 of the first cycle and then on day 1 of each subsequent 4-week cycle.