All subjects received trilaciclib lead-in (240mg/m2) single dose monotherapy, followed by trilaciclib plus anthracycline/cyclophosphamide (Trilaciclib (240mg/m2) + doxorubicin (60 mg/m2) + cyclophosphamide (600 mg/m2) + pembrolizumab (per Investigator discretion; 400mg)), then trilaciclib plus taxane chemotherapy (Trilaciclib (240mg/m2) + paclitaxel (80 mg/m2) + carboplatin (per Investigator discretion; AUC 1.5) + pembrolizumab (per Investigator discretion; 400mg).

Trilaciclib: Trilaciclib is administered IV as monotherapy during the lead-in phase and administered prior to chemotherapy on each day chemotherapy is administered during the treatment phase.

Cylophosphamide: Cyclophosphamide administered IV every 2 weeks for the first 4 cycles (1-4), each cycle 2 weeks in length.

Doxorubicin: Doxorubicin administered as an IV bolus every 2 weeks for the first 4 cycles (1-4), each cycle 2 weeks in length.

Paclitaxel: Paclitaxel administered weekly for the last 12 cycles (cycles 5-16), each cycle 1 week in length.

Carboplatin (Investigator discretion): Carboplatin, if given, is administered IV weekly at the start of paclitaxel administration, for the last 12 cycles (cycles 5-16).

Pembrolizumab (Investigator discretion): Pembrolizumab, if given, is administered IV every 6 weeks throughout the treatment phase (cycles 1, 4, 9, 15).

For Stage I - Stage III, cancer is present. The higher the number, the bigger the tumor size and the more it has spread into nearby tissues.

• Stage I - cancer is present but is limited to the place where it started; also referred to as early-stage cancer.
• Stage II - cancer is present and has spread to nearby tissue
• Stage III - cancer is present and has spread to nearby tissue with a larger tumor than Stage II

Per the American Cancer society, cancer cells are given a grade when removed from the breast and checked in a lab. Grading is determined by how much the cancer cells look like normal cells. Grading is used to predict outcome (prognosis) and to help the doctor determine the best course of treatment.

• Grade I - cancer is slower-growing and less likely to spread
• Grade 2 - cancer is grower faster than Grade 1 but slower than Grade 3
• Grade 3 - faster-growing cancer that's more likely to spread

Histopathological typing is completed by looking at the morphology, or the size, shape, location, and structure, of cancer cells under a microscope. A biopsy, or a procedure to remove tissue or cells for testing, is undergone to obtain cells to view. Cancer tissue with lobular carcinomas generally have cells in a single-file line and occur in fatty tissue, where ductal carcinomas have cells which re-form glandular structures and are generally more likely to create a mass.

ECOG Performance Scale measures how cancer impacts a patient's daily activities. A higher score indicates a lower ability to function independently and the patient's cancer is having a significant impact on their day-to-day activities. For this trial, scores of 0 to 1 were required to participate, indicating a patient was fully active or able to carry out work of a light or sedentary nature. The scale goes from 0 (fully active) to 5 (deceased).

All subjects received trilaciclib lead-in (240mg/m2) single dose monotherapy, followed by trilaciclib plus anthracycline/cyclophosphamide (Trilaciclib (240mg/m2) + doxorubicin (60 mg/m2) + cyclophosphamide (600 mg/m2) + pembrolizumab (per Investigator discretion; 400mg)), then trilaciclib plus taxane chemotherapy (Trilaciclib (240mg/m2) + paclitaxel (80 mg/m2) + carboplatin (per Investigator discretion; AUC 1.5) + pembrolizumab (per Investigator discretion; 400mg).

This trial investigated the mechanism of action of trilaciclib, and not safety profile comparison between trilaciclib treated patients receiving or not receiving pembrolizumab, nor comparison of the safety profile between trilaciclib treated subjects receiving or not receiving carboplatin. This was a single-arm study; only 1 treatment arm/group was prospectively defined. Therefore safety was reported in accordance to the study design (i.e., a single arm/group). Additionally, 21 of 24 subjects enrolled were treated with pembrolizumab and 21 of 24 patients enrolled were treated with carboplatin. Thus, the safety reporting as a single group is representative of the vast majority of participants (21/24 (87.5%)).