Phase 3 Randomized Study of GLSI-100 (GP2 + Sargramostim) in HER2/neu Positive Breast Cancer with Residual Disease or High-Risk PCR Post-Trastuzumab Therapy

The primary objective of this study is to evaluate the **efficacy** of GLSI-100 compared to placebo in subjects with **HLA-A*02 positive** and **HER2/neu positive breast cancer** who are at high risk of disease recurrence. This includes patients presenting with stage I, II, or III disease with residual disease at surgery or stage III with pathologic complete response (pCR) at surgery, who have completed both neoadjuvant and postoperative adjuvant trastuzumab-based standard of care therapy. The clinical relevance of this objective lies in potentially improving outcomes for patients with a high risk of recurrence, thereby addressing a significant unmet need in breast cancer management.

Secondary objectives include:

• Assessing secondary measures of efficacy of GLSI-100 compared to placebo in the specified patient population.
• Evaluating changes in quality of life using the EORTC Core Quality of Life questionnaire (QLQ-C30) and FACT-GP5.
• Assessing the safety and tolerability of GLSI-100 in the study population.

The clinical trial involves a total of **350 participants** diagnosed with **breast cancer**. The study population includes both male and female subjects, aged over 18 years, who have completed neoadjuvant and postoperative adjuvant trastuzumab-based standard of care therapy. Participants are required to have a histologically confirmed diagnosis of HER2/neu positive primary breast cancer and must be HLA-A*02-positive unless enrolled in the third non-HLA-A*02 arm. The trial targets individuals with a high risk of disease recurrence, specifically those with stage I, II, or III breast cancer at presentation with residual disease at surgery, or stage III at presentation with a pathologic complete response at surgery. Participants must have no clinical evidence of residual or persistent breast cancer and should have an ECOG performance status of 0-2, indicating they are fully active or capable of self-care. Adequate organ function is also a requirement. The trial does not include a vulnerable population, and participants must be willing to use highly effective contraception if of childbearing potential. The selection process ensures that participants can begin study therapy within one year of completing adjuvant trastuzumab-based therapy, although concurrent endocrine therapy is allowed, while concurrent neratinib is prohibited.

The clinical trial is designed as a **randomized**, double-blind, placebo-controlled, Phase 3 study to evaluate the efficacy and safety of the investigational product GLSI-100, which consists of GP2 and GM-CSF, in subjects with **HER2-positive breast cancer**. The trial aims to assess the efficacy of GLSI-100 compared to placebo in HLA-A*02 positive and HER2/neu positive breast cancer subjects who have a high risk of disease recurrence. The study will include participants who have completed both neoadjuvant and postoperative adjuvant trastuzumab-based therapy. The trial is expected to commence recruitment on February 29, 2024, and is estimated to conclude by February 28, 2030.

Participants will be involved in the study for a maximum treatment period of 36 months. The study will include several key visits: an initial screening visit to confirm eligibility based on inclusion criteria such as age, pregnancy status, and histologically confirmed diagnosis of HER2/neu positive primary breast cancer. Following the screening, participants will be randomized to receive either the investigational product or a placebo, administered **intradermally**. Regular follow-up visits will be scheduled to monitor the participants' health, assess the primary endpoint of invasive breast cancer-free survival (IBCFS), and evaluate secondary endpoints such as invasive disease-free survival (IDFS), overall survival, quality of life, and the frequency of adverse events.

The end-of-study visit will occur at the conclusion of the treatment period or upon early termination. Conditions that may lead to early termination from the study include the occurrence of serious adverse events, withdrawal of consent, or any clinical evidence of residual or persistent breast cancer. Participants will be monitored for safety and efficacy throughout the study, with immune response measured by Delayed-Type Hypersensitivity (DTH) tests and immunologic assays. The trial will ensure that all procedures adhere to ethical standards and regulatory requirements, maintaining the integrity and scientific validity of the study.

The clinical trial involves the administration of the experimental medication **GLSI-100**, which is a combination of **GP2** and **GM-CSF**. The active substance **GP2** is chemically identified as **L-isoleucyl-L-isoleucyl-L-seryl-L-alanyl-L-valyl-L-valyl-glycyl-L-isoleucyl-L-leucine**. This compound is provided in the form of an **injection** and is administered **intradermally**. The maximum daily dose is set at 500 mEq/µg, with a total maximum dose of 18,000 mEq/µg over a treatment period of 36 months. The formulation is not specifically designed for pediatric use and is classified as a chemical product. The administration schedule and participant compliance are monitored throughout the trial to ensure adherence to the dosing regimen.

Another experimental treatment used in the study is **Leukine**, which contains the active substance **sargramostim**. This medication is provided as a **powder for solution for injection** and is also administered **intradermally**. The maximum daily dose for Leukine is 125 mEq/µg, with a total maximum dose of 4,500 mEq/µg over the same 36-month treatment period. Like GLSI-100, Leukine is not formulated for pediatric use and is categorized as a chemical product. The administration of Leukine follows a strict dosing schedule, and participant compliance is closely monitored to ensure the integrity of the trial data.

The study also includes a **placebo** control, which is a commercially available 0.9% sterile saline solution with marketing authorization. This placebo is utilized to provide a baseline for evaluating the efficacy and safety of the experimental treatments. The placebo is administered in a manner consistent with the experimental treatments to maintain the study's double-blind design. Compliance with placebo administration is monitored to ensure consistency across all study participants.

The efficacy of the investigational product GLSI-100 will be assessed in a randomized, multicenter, placebo-controlled, Phase 3 clinical trial involving subjects with **HER2/neu** positive breast cancer. The primary endpoint for evaluating efficacy is the Invasive Breast Cancer-Free Survival (IBCFS), defined as the time from randomization until the occurrence of ipsilateral invasive breast cancer recurrence, ipsilateral local-regional invasive breast cancer recurrence, distant recurrence, contralateral invasive breast cancer, or any cause mortality. Secondary endpoints include the Invasive Disease-Free Survival (IDFS), overall survival (OS), quality of life assessments using QLQ-C30 and FACT-GP5, and the frequency of adverse events (AEs) and serious adverse events (SAEs) characterized by type, severity, seriousness, duration, and relationship to study treatment.

Exploratory endpoints will involve measuring immune response through Delayed-Type Hypersensitivity (DTH) tests and immunologic assays. The efficacy assessments will be conducted at specified timepoints throughout the study duration, with data collection and analysis adhering to rigorous clinical trial standards. The trial aims to provide comprehensive insights into the efficacy and safety of GLSI-100 in comparison to a placebo in the specified patient population.