TPS618 Background: GP2 is a biologic nine amino acid peptide of the HER2/ neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. Of the 146 patients that have been treated with GLSI-100 over 4 clinical trials, GLSI-100 was well-tolerated and no serious adverse events observed were considered related to the immunotherapy. Methods: This Phase III trial is a prospective, randomized, double-blinded, multi-center study. After 1 year of trastuzumab-based therapy, 6 intradermal injections of GLSI-100 or placebo will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up. Study Size – Interim Analysis: Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in invasive breast cancer free survival (IBCFS), 28 events will be required. An interim analysis for superiority and futility will be conducted when at least 14 events have occurred. This sample size provides 80% power if the annual rate of events in placebo patients is 2.4% or greater. Up to 250 non-HLA-A*02 subjects will be enrolled in an open-label arm. Eligibility Criteria: The patient population is defined by these key eligibility criteria: 1) HER2/neu positive and HLA-A*02, 2) Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy, 3) Exclude Stage IV, and 4) Completed at least 90% of planned trastuzumab-based therapy. Trial Objectives: The trial objectives are to: 1) Determine if GP2 therapy increases IBCFS, 2) Assess the safety profile of GP2, and 3) Monitor immunologic responses to treatment and assess relationship to efficacy and safety. Study Status: The study is actively recruiting and enrolling patients in the US and Europe at up to 150 sites. Contact Information: Greenwich LifeSciences, Inc., Stafford, TX; Email: Flamingo-01@greenwichlifesciences.com ; Website: greenwichlifesciences.com Funding: This trial is supported by Greenwich LifeSciences. Clinical trial information: NCT05232916 .

Abstract Background: This Phase III trial is a prospective, randomized, double-blinded, multi-center study (NCT05232916) in HLA-A*02 patients at approximately 140 sites in the US and Europe. A third non-randomized arm of approximately 250 non-HLA-A*02 patients is now fully enrolled and preliminary immune response data is presented below. GP2 is a biologic nine amino acid peptide of the HER2/neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. Methods: After standard of care neoadjuvant and adjuvant therapy, 6 intradermal injections of GLSI-100 will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up. Immune responses to GP2 were measured over time using delayed-type-hypersensitivity (DTH) skin tests and injection site reactions (ISRs). The patient population is defined by these key eligibility criteria: 1) HER2/neu positive and HLA, 2) Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy, 3) Exclude Stage IV, and 4) Completed at least 90% of planned trastuzumab-based therapy. Results: All patients (n=247) were vaccinated with GLSI-100 and continue in treatment and follow-up. A DTH reaction (redness) was used to assess in vivo immune responses in patients. The DTH orthogonal mean was measured 48-72 hours after injection. In this preliminary data analysis, there was a significant increase in percentage of subjects experiencing a DTH reaction in month 4 or month 6 compared to baseline. The frequency of DTH reactions increased by approximately 4x from 5.2% of the patients experiencing a DTH reaction at baseline, prior to any GLSI-100 administration, to 20.4% of the patients experiencing a DTH reaction in month 4 or month 6 (McNemar, p < 0.001). The study is ongoing and data collection and cleaning continue so final results may vary. Conclusions: The increase in the incidence of DTH reactions over time found in this preliminary analysis of GLSI-100 treated non-HLA-A*02 patients shows that GLSI-100 treatment should not be limited to the HLA-A*02 genotype. Subjects treated with GLSI-100 were increasingly able to mount an immune response to GP2 as evidenced in this preliminary data. Future investigations may explore the use of immune responses to assess: correlation of DTH to ISRs, immunogenicity of GLSI-100 by specific HLA type, timing of boosters to sustain immunity, clinical site performance, and the discontinuation of treatment for non-responders. Funding: This trial is supported by Greenwich LifeSciences. Citation Format: Snehal S. Patel, Jaye Thompson, F. Joseph Daugherty, Francois-Clement Bidard, William J. Gradishar, Marcus Schmidt, Miguel Martin, Joyce A. O’Shaughnessy, Hope S. Rugo, Cesar A. Santa-Maria, Laura M. Spring, Mothaffar F. Rimawi. Preliminary delayed-type-hypersensitivity immune response results from open-label arm of on-going Phase III study to evaluate the efficacy and safety of GLSI-100 (GP2 + GM-CSF) in breast cancer patients with residual disease or high-risk PCR after both neo-adjuvant and postoperative adjuvant anti-HER2 therapy, Flamingo-01 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr CT138.

LBA538 Background: This phase III trial is a prospective, randomized, double-blinded, multi-center study (NCT05232916) in HLA-A*02 patients at approximately 140 sites in the US and Europe. A third non-randomized arm of approximately 250 non-HLA-A*02 patients is now fully enrolled and preliminary immune response data is presented below. GP2 is a biologic nine amino acid peptide of the HER2/ neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. Methods: After standard of care neoadjuvant and adjuvant therapy, 6 intradermal injections of GLSI-100 will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up. Immune responses to GP2 were measured over time using delayed-type-hypersensitivity (DTH) skin tests and injection site reactions (ISRs). The patient population is defined by these key eligibility criteria: 1) HER2/neu positive and HLA, 2) Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy, 3) Exclude Stage IV, and 4) Completed at least 90% of planned adjuvant trastuzumab-based therapy. Results: All patients (n=247) were vaccinated with GLSI-100. Injection site reactions and erythema (redness) were assessed at various time points and represent an in vivo immune response in patients. The ISR orthogonal mean was measured 48-72 hours following vaccination with GLSI-100. For GP2 treated patients, there was a significant increase in the percentage of patients experiencing ISRs in the 4th, 5th or 6th vaccination compared to the ISRs from the 1st vaccination. In this preliminary analysis, the frequency of ISRs increased significantly from 20.2% of the patients experiencing an ISR after the first vaccination to 55.3% of the patients experiencing an ISR after the 4th, 5th or 6th vaccination (McNemar p < 0.001). The study is ongoing and data collection and cleaning continue, so final results may vary. Conclusions: Preliminary injection site reaction data comparing vaccination over time in GLSI-100 treated non-HLA-A*02 patients showed a significant increase in immune response. Future studies may explore the use of immune responses to assess correlation of DTH to ISRs, immunogenicity of GLSI-100 by specific HLA type, timing of boosters to sustain immunity, clinical site performance, and the discontinuation of treatment for non-responders. Funding: This trial is supported by Greenwich LifeSciences. Clinical trial information: NCT05232916 .

Abstract Background: GP2 is a biologic nine amino acid peptide of the HER2/neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. Methods: This Phase III trial is a prospective, randomized, double-blinded, multi-center study (NCT05232916). After 1 year of trastuzumab-based therapy, 6 intradermal injections of GLSI-100 or placebo will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up. Study Size - Interim Analysis: Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in invasive breast cancer free survival (IBCFS), 28 events will be required. An interim analysis for superiority and futility will be conducted when at least 14 events have occurred. This sample size provides 80% power if the annual rate of events in placebo patients is 2.4% or greater. Up to 250 non-HLA-A*02 subjects will be enrolled in an open-label arm. Eligibility Criteria: The patient population is defined by these key eligibility criteria: 1) HER2/neu positive and HLA-A*02, 2) Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy, 3) Exclude Stage IV, and 4) Completed at least 90% of planned trastuzumab-based therapy. Trial Objectives: The trial objectives are to: 1) Determine if GP2 therapy increases IBCFS, 2) Assess the safety profile of GP2, and 3) Monitor immunologic responses to treatment and assess relationship to efficacy and safety. Study Status: The study is actively recruiting and enrolling patients in the US and Europe at up to 150 sites. Contact Information: Greenwich LifeSciences, Inc. Stafford, TX Email: Flamingo-01@greenwichlifesciences.com Website: greenwichlifesciences.com Funding: This trial is supported by Greenwich LifeSciences. Citation Format: Jaye Thompson, Snehal Patel, Mira Patel, Anu Tammareddi, F. Joseph Daugherty, Mothaffar F. Rimawi. Phase III study to evaluate the efficacy and safety of GLSI-100 (GP2 + GM-CSF) in breast cancer patients with residual disease or high-risk PCR after both neo-adjuvant and postoperative adjuvant anti-HER2 therapy, Flamingo-01 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr CT227.

Background: GP2 is a biologic nine amino acid peptide of the HER2/neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. Of the 146 patients that have been treated with GLSI-100 over 4 clinical trials, GLSI-100 was well-tolerated and no serious adverse events observed were considered related to the immunotherapy. Methods: This Phase III trial is a prospective, randomized, double-blinded, multi-center study. After 1 year of trastuzumab-based therapy, 6 intradermal injections of GLSI-100 or placebo will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up. Study Size – Interim Analysis: Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in invasive breast cancer free survival (IBCFS), 28 events will be required. An interim analysis for superiority and futility will be conducted when at least 14 events have occurred. This sample size provides 80% power if the annual rate of events in placebo patients is 2.4% or greater. Up to 250 non-HLA-A*02 subjects will be enrolled in an open-label arm. Eligibility Criteria: The patient population is defined by these key eligibility criteria: 1) HER2/neu positive and HLA-A*02, 2) Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy, 3) Exclude Stage IV, and 4) Completed at least 90% of planned trastuzumab-based therapy. Trial Objectives: The trial objectives are to: 1) Determine if GP2 therapy increases IBCFS, 2) Assess the safety profile of GP2, and 3) Monitor immunologic responses to treatment and assess relationship to efficacy and safety. Study Status: The study is actively recruiting and enrolling patients in the US and Europe at up to 150 sites. Contact Information: Greenwich LifeSciences, Inc. Stafford, TX Email: Flamingo-01@greenwichlifesciences.com Website: greenwichlifesciences.com Funding: This trial is supported by Greenwich LifeSciences. Citation Format: Snehal Patel, Jaye Thompson, Mira Patel, F. Joseph Daugherty, Mothaffar F. Rimawi. Phase III study to evaluate the efficacy and safety of GLSI-100 (GP2 + GM-CSF) in breast cancer patients with residual disease or high-risk PCR after both neo-adjuvant and postoperative adjuvant anti-HER2 therapy, Flamingo-01 [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P5-07-22.

Abstract Background: GP2 is a biologic nine amino acid peptide of the HER2/neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. In a prospective, randomized, single-blinded, placebo-controlled, multicenter Phase IIb study, no recurrences were observed in the HER2+ population after 5 years of follow-up, if the patient was treated with GLSI-100, survived, and was followed for more than 6 months (p = 0.0338). Immunotherapy elicited a potent response measured by skin tests and immunological assays. Of the 146 patients that have been treated with GLSI-100 over 4 clinical trials, GLSI-100 was well-tolerated and no serious adverse events observed were considered related to the immunotherapy. Methods: This Phase III trial is a prospective, randomized, double-blinded, multi-center study. After 1 year of trastuzumab-based therapy, 6 intradermal injections of GLSI-100 or placebo will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years for a total of 11 injections over 3 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up for a total of 4 years following the first year of treatment with trastuzumab-based therapy. Patients will be stratified based on residual disease status at surgery, hormone receptor status and region. Study Size – Interim Analysis: Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in invasive breast cancer free survival (IBCFS), 28 events will be required. An interim analysis for superiority and futility will be conducted when at least 14 events have occurred. This sample size provides 80% power if the annual rate of events in placebo patients is 2.4% or greater. Up to 100 non-HLA-A*02 subjects will be enrolled in an open-label arm. Eligibility Criteria: The patient population is defined by these key eligibility criteria: Objectives: Study Status: The study has been initiated at a number of sites in the US. The study is also expected to be opened in Spain, Germany, and France. Contact Information: Greenwich LifeSciences, Inc. Stafford, TX Email: Flamingo-01@greenwichlifesciences.com Website: greenwichlifesciences.com Funding: This trial is supported by Greenwich LifeSciences. Citation Format: Snehal Patel, Jaye Thompson, Mira Patel, F. Joseph Daugherty, Mothaffar Rimawi. Phase III study to evaluate the efficacy and safety of GLSI-100 (GP2 + GM-CSF) in breast cancer patients with residual disease or high-risk PCR after both neo-adjuvant and postoperative adjuvant anti-HER2 therapy, Flamingo-01 [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-19-05.

Background: GP2 is a biologic nine amino acid peptide of the HER2/neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. In a prospective, randomized, single-blinded, placebo-controlled, multicenter Phase IIb study, no recurrences were observed in the HER2+ population after 5 years of follow-up, if the patient was treated with GLSI-100, survived and was followed for more than 6 months (p = 0.0338). Immunotherapy elicited a potent response measured by skin tests and immunological assays. Of the 146 patients that have been treated with GLSI-100 over 4 clinical trials, GLSI-100 was well-tolerated and no serious adverse events were observed considered related to the immunotherapy. Method: This Phase III trial is a prospective, randomized, double-blinded, multi-center study. After 1 year of trastuzumab-based therapy, 6 intradermal injections of GLSI-100 or placebo will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years for a total of 11 injections over 3 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up for a total of 4 years following the first year of treatment with trastuzumab-based therapy. Patients will be stratified based on residual disease status at surgery, hormone receptor status and region. Study Size – Interim Analysis: Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in invasive breast cancer free survival (IBCFS), 28 events will be required. An interim analysis for superiority and futility will be conducted when at least 14 events have occurred. This sample size provides 80% power if the annual rate of events in placebo patients is 2.4% or greater. Up to 100 non-HLA-A*02 subjects will be enrolled in an open-label arm. Eligibility Criteria: The patient population is defined by these key eligibility criteria: 1. HER2/neu positive and HLA-A*02 2. Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy 3. Exclude Stage IV 4. Completed at least 90% of planned trastuzumab-based therapy Trial Objectives: 1. To determine if GP2 therapy increases IBCFS 2. To assess the safety profile of GP2 3. To monitor immunologic responses to treatment and assess relationship to efficacy and safety Contact information: Snehal Patel Greenwich LifeSciences, Inc. Stafford, TX Email: snehal.patel@greenwichlifesciences.com Website: greenwichlifesciences.com Funding: This trial is supported by Greenwich LifeSciences. Citation Format: Snehal Patel, Jaye Thompson, Mira Patel, F. Joseph Daugherty, C. Kent Osborne, Mothaffar Rimawi. Phase III study to evaluate the efficacy and safety of GLSI-100 (GP2 + GM-CSF) in breast cancer patients with residual disease or high-risk PCR after both neo-adjuvant and postoperative adjuvant anti-HER2 therapy, Flamingo-01 [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT2-10-06.

TPS1110 Background: GP2 is a biologic nine amino acid peptide of the HER2/ neu protein delivered in combination with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) that stimulates an immune response targeting HER2/neu expressing cancers, the combination known as GLSI-100. In a prospective, randomized, single-blinded, placebo-controlled, multicenter Phase IIb study, no recurrences were observed in the HER2+ population after 5 years of follow-up, if the patient was treated with GLSI-100, survived and was followed from more than 6 months ( p = 0.0338). Immunotherapy elicited a potent response measured by skin tests and immunological assays. Of the 146 patients that have been treated with GLSI-100 over 4 clinical trials, GLSI-100 was well-tolerated and no serious adverse events were observed considered related to the immunotherapy. Methods: This Phase 3 trial is a prospective, randomized, double-blinded, multi-center study. After 1 year of trastuzumab-based therapy, 6 intradermal injections of GLSI-100 or placebo will be administered over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years for a total of 11 injections over 3 years. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up for a total of 4 years following the first year of treatment with trastuzumab-based therapy. Patients will be stratified based on residual disease status at surgery, hormone receptor status and region. Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in invasive breast cancer free survival (IBCFS), 28 events will be required. An interim analysis for superiority and futility will be conducted when at least 14 events have occurred. This sample size provides 80% power if the annual rate of events in placebo patients is 2.4% or greater. Up to 100 non-HLA-A*02 subjects will be enrolled in an open-label arm. Eligibility Criteria: The patient population is defined by these key eligibility criteria: HER2/neu positive and HLA-A*02; Residual disease or High risk pCR (Stage III at presentation) post neo-adjuvant therapy; Exclude Stage IV; Completed at least 90% of planned trastuzumab-based therapy. Trial Objectives: To determine if GP2 therapy increases IBCFS; To assess the safety profile of GP2; To monitor immunologic responses to treatment and assess relationship to efficacy and safety. Accrual: Site selection and study start-up is in progress at multiple sites. Target enrollment is 598 subjects. Clinical trial information: 05232916.

e12519 Background: Delayed type hypersensitivity (DTH) skin tests to GP2 were conducted in the randomized, active-controlled, single-blinded, multicenter Phase IIb trial investigating GLSI-100 (GP2+GM-CSF) administered in the adjuvant setting to node-positive and high-risk node-negative breast cancer patients with tumors expressing HER2. Previously it has been reported that subjects with baseline response to GP2 by DTH were more likely to experience a recurrence sooner than those without a positive baseline response. It was of interest to determine if this response was independent of other known prognostic factors. Methods: Patients were randomized and received GLSI-100 (500 mcg GP2: 125 mcg GM-CSF) or control (GM-CSF only) via 6 intradermal injections every 3-4 weeks as part of the Primary Immunization Series (PIS) for 6 months and 4 booster injections every 6 months thereafter. GP2 DTH skin tests were placed at baseline and after the 6th dose. After 48-72 hours, the largest perpendicular diameters of induration were measured and the orthogonal mean was calculated. Induration of over 5mm was considered a positive response. The relationship between prognostic factors such as demographic and disease characteristics and the baseline DTH response were investigated by logistic regression. Results: The study enrolled 180 patients who were HER2 3+ positive and low HER2 expressors (1-2+). After 5 years of follow-up, the Kaplan-Meier estimated 5-year DFS rate in the 46 HER2 3+ patients treated with GLSI-100, if completing the PIS, was 100% versus 89.4% (95% CI:76.2, 95.5%) in the 50 patients treated with GM-CSF (p = 0.0338). Just under 23% of subjects experienced an immune response to GP2 by DTH at baseline. The incidence was similar in patients who were HER2/neu positive and those considered low expressors and was equally distributed across GLSI-100 and placebo-treated subjects. Known prognostic factors such as hormone receptor status, stage at presentation, node status, etc. were evenly distributed across subjects that experienced a baseline DTH response to GP2 and those who did not. Each prognostic factor was investigated in a logistic model to determine if it predicted baseline DTH immune status. The prognostic factors did not predict baseline DTH immune response in the logistic models. There appears to be no correlation between baseline immune response to GP2 and known prognostic factors. Conclusions: Baseline GP2 immune response as measured by the delayed-type hypersensitivity test may be an independent prognostic factor for recurrence. Knowledge of this GP2 immune response may identify a patient with increased risk of rapid recurrence. Clinical trial information: 00524277.

The authors did not submit an updated abstract. The original abstract should be considered final. Citation Format: Snehal S. Patel, David B. McWilliams, Mira S. Patel, Jaye Thompson, F. Joseph Daugherty. GP2 immune response a predictor of recurrence in a phase IIb study evaluating HER2/neu peptide GP2 (GLSI-100) vs. GM-CSF alone after adjuvant trastuzumab in HER2 positive women with breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT161.

Background: GP2 is a biologic nine amino acid peptide of the HER2/neu protein delivered in combination with an FDA-approved immunoadjuvant Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF, Sargramostim, Leukine) that stimulates an immune response targeting HER2/neu expressing cancers. In a prospective, randomized, single-blinded, placebo-controlled, multicenter Phase IIb clinical trial, no recurrences were observed in the HER2/neu positive adjuvant setting after median 5 years of follow-up, if the HLA-A*02 patient received the 6 primary intradermal GLSI-100 injections over the first 6 months (p = 0.0338) in a pre-specified subgroup analysis. Furthermore, the immunotherapy elicited a potent immune response measured by local skin tests and immunological assays. Of the 138 patients that have been treated with GLSI-100 to date over 4 clinical trials, GLSI-100 was well-tolerated and no serious adverse events were observed related to the immunotherapy. This Phase III trial aims to reproduce the Phase IIb trial and will explore the use of GLSI-100 as adjuvant therapy to increase invasive disease-free survival in HER2/neu positive and HLA-A*02 patients, post-surgery and following the first year of treatment with any trastuzumab-based therapy. Method: This Phase 3 trial is a prospective, randomized, double-blinded, multi-center study. After 1 year of trastuzumab-based therapy or an approved biosimilar, 6 intradermal injections of GLSI-100 or placebo (Bacteriostatic Saline/WFI) will be administered for the primary immunization series over the first 6 months and 5 subsequent boosters will be administered over the next 2.5 years for a total of 11 injections over 3 years of treatment. The participant duration of the trial will be 3 years treatment plus 1 additional year follow-up for a total of 4 years following the first year of treatment with trastuzumab-based therapy. An interim analysis is planned, and patients will be stratified based on prior and current treatments. Study Size - Interim Analysis: Approximately 498 patients will be enrolled. To detect a hazard ratio of 0.3 in IDFS, 28 events will be required. An interim analysis for superiority and futility will be conducted when at least half of those events, 14, have occurred. This sample size provides 80% power if the annual rate of events in placebo-treated patients is 2.4% or greater. Eligibility Criteria: The patient population is defined by these key eligibility criteria: Trial Objectives: Contact information: Website: greenwichlifesciences.com. Funding: This trial is supported by Greenwich LifeSciences. Citation Format: Snehal S Patel, David B McWilliams, Christine T Fischette, Jaye Thompson, F. Joseph Daugherty, C. Kent Osborne, Mothaffar F Rimawi. A randomized, multicenter, placebo-controlled, phase III study to evaluate the efficacy and safety of HER2/neu peptide GLSI-100 (GP2 + GM-CSF) in patients with residual disease or high-risk PCR after both neo-adjuvant and postoperative adjuvant anti-HER2 therapy [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT1-18-07.

Abstract Background: Delayed type hypersensitivity (DTH) skin tests in the randomized, active-controlled, single-blinded, multicenter Phase IIb trial investigating GLSI-100 (GP2+GM-CSF) administered in the adjuvant setting to node-positive and high-risk node-negative breast cancer patients with tumors expressing any degree of HER2 (immuno-histochemistry [IHC] 1-3+) (NCT00524277) have been analyzed. The trial enrolled HLA-A*02 patients randomized to receive GLSI-100 versus GM-CSF alone. The trial's primary objective was to determine if treatment with GLSI-100, a HER2-derived peptide, reduces recurrence rates. Analyses for this trial showing GLSI-100 to be efficacious, safe and immunogenic have been previously reported by Patel et al. and Mittendorf et al.. Methods: Consented patients were randomized and scheduled to receive GLSI-100 (500 mcg GP2: 125 mcg GM-CSF) or control (GM-CSF only) via 6 intradermal injections every 3-4 weeks as part of the Primary Immunization Series (PIS) for the first 6 months and 4 booster intradermal injections every 6 months thereafter. Boosters were introduced during the trial, thus some patients did not receive all 4 boosters. DTH skin tests were assessed at baseline and after the 6th dose with the orthogonal mean of each skin reaction measured 48-72 hours after injection using the sensitive ballpoint-pen method.. Results: The study enrolled 180 patients across 16 clinical sites with both HER2 3+ positive and low HER2 expressors (1-2+). After 5 years of follow-up, the Kaplan-Meier estimated 5-year DFS rate in the 46 HER2 3+ patients treated with GLSI-100, if the patient completed the PIS, was 100% versus 89.4% (95% CI:76.2, 95.5%) in the 50 placebo patients treated with GM-CSF (p = 0.0338). GLSI-100 was shown to be well tolerated with no SAEs deemed related to study medication and elicited a potent immune response measured by local skin tests and immunological assays. Injection site reactions were common, occurring in almost 100% of patients treated with either GLSI-100 or GM-CSF alone. Previous publications have reported the increase in DTH response reported among patients after treatment with GLSI-100. However, it was of interest to understand the positive DTH responses to GP2 noted at baseline. 22.8% of patients reacted to GP2 at baseline with induration of 5mm or greater. In the subgroup of patients who later experienced a breast cancer recurrence, 36.4% (8/22) had such a baseline response. Analysis of the time to recurrence among those recurring found that the median time to recurrence was 0.6 years for those with a baseline response while those that did not have a positive baseline DTH response to GP2 took 1.2 years to recur.. Conclusions: This study demonstrated that GLSI-100 safely elicited a potent immune response as evidenced by increased DTH skin responses with treatment paired with improved disease-free survival. It is theorized that a positive baseline DTH skin test to GP2 may be evidence of an existing immune response to GP2 associated with residual disease, impending recurrence, or prior treatments. Further studies assessing if GP2 immune response is an important prognosticator of cancer disease state or recurrence are planned. Citation Format: Snehal S Patel, David B McWilliams, Mira S Patel, Christine T Fischette, Jaye Thompson, F. Joseph Daugherty. Analysis of GP2 immune response and relationship to recurrence in a prospective, randomized, placebo-controlled, single-blinded, multicenter, phase IIb study evaluating the reduction of recurrences using HER2/neu peptide GP2 (GLSI-100) vs. GM-CSF alone after adjuvant trastuzumab in HER2 positive women with operable breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-29.

LBA550 Background: Injection site reactions (ISR) of booster injections in the randomized, active-controlled, single-blinded, multicenter Phase IIb trial of GLSI-100 (GP2+GM-CSF) administered in the adjuvant setting to node-positive and high-risk node-negative breast cancer patients with tumors expressing HER2 have been analyzed. It was demonstrated that GLSI-100 safely elicited a potent immune response versus placebo, along with data showing ISRs correlate to immune response data such as Delayed Type Hypersensitivity (DTH). Methods: Patients were randomized and received GLSI-100 (500 mcg GP2: 125 mcg GM-CSF) or control (GM-CSF) via 6 intradermal injections every 3-4 weeks as part of the Primary Immunization Series (PIS) for 6 months and 4 booster injections every 6 months thereafter. Injection site reactions were assessed 48-72 hours after the injection. Booster injections were introduced mid-trial so not all patients were dosed with boosters. The largest perpendicular diameters of induration were measured, and the orthogonal mean was calculated to quantify ISRs, providing potential assessments of peak and nadir immunity during the PIS and booster phases of treatment. Results: The study enrolled 180 patients who were HER2 3+ positive and low HER2 expressors (1-2+). After 5 years of follow-up, the Kaplan-Meier estimated 5-year DFS rate in the 46 HER2 3+ patients treated with GLSI-100, who were in the efficacy population, was 100% versus 89.4% (95% CI:76.2, 95.5%) in the 50 patients treated with GM-CSF (p = 0.0338). A patient is in the efficacy population if they were treated, followed, and remained disease free over the first 6 months. Boosters were administered to 53 HER2 3+ patients (54%). The median peak ISR during the PIS in GLSI-100 treated patients was 92.1 mm versus 60.5 mm in the GM-CSF only treated control patients. Peak ISR in GLSI-100 treated patients occurred throughout the 10-dose dosing period, including during boosters. During the 6 dose PIS, 84% of the peak ISRs occurred between doses 3 through 6. Each booster is administered 6 months after the last injection, thus the booster ISR is likely measuring the nadir immune response. When comparing the peak ISR during PIS to the boosters, the nadir ISRs of the boosters were approximately 20 mm smaller. Conclusions: Administering GLSI-100 boosters at 6 month intervals to patients produced a consistent nadir ISR approximately 20 mm lower than the maximum PIS ISR of 92.1 mm which is still larger than the maximum ISR in GM-CSF only patients of 60.5 mm. A patient’s immune response a month after booster dosing would theoretically be the peak ISR, which will be measured in future trials by measuring T-cell response and DTH one month after booster injections, further helping to evaluate booster strategies to sustain peak immunity over longer periods of time. Clinical trial information: 00524277.

Abstract The authors did not submit an updated abstract. The original abstract should be considered final. Citation Format: Snehal S. Patel, David B. McWilliams, Mira S. Patel, Jaye Thompson, F. Joseph Daugherty. Injection site reactions correlate to delayed type hypersensitivity tests and suggest that GP2 reverses immune suppression of trastuzumab-treated HER2 positive patients in a phase IIb study evaluating HER2/neu peptide GP2 (GLSI-100) vs. GM-CSF after adjuvant trastuzumab in HER2 positive women with breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT166.

542 Background: The final analysis of the GP2 prospective, randomized, placebo-controlled, single-blinded, multicenter Phase IIb trial investigating GP2+GM-CSF administered in the adjuvant setting to node-positive and high-risk node-negative breast cancer patients with tumors expressing any degree of HER2 (immuno-histochemistry [IHC] 1-3+) ( NCT00524277) is now complete with 5 year follow-up. The trial enrolled HLA-A02 patients randomized to receive GP2+GM-CSF versus GM-CSF alone. It was previously reported that completion of the GP2+GM-CSF Primary Immunization Series (PIS) reduced recurrence rates to 0% over a 5 year follow-up period in HER2 3+ patients, who received a standard course of trastuzumab after surgery. Methods: Each enrolled and consented patient was randomly scheduled to receive a total of 6 GP2+GM-CSF (500 mcg GP2: 125 mcg GM-CSF) or GM-CSF only intradermal injections every 3-4 weeks as part of the PIS for the first 6 months and 4 GP2+GM-CSF or GM-CSF only booster intradermal injections every 6 months thereafter. Boosters were introduced during the trial, thus some patients did not receive all 4 boosters. Injection sight reactions were measured. Results: Safety data was analyzed to assess local and systemic toxicity of each treatment arm. Most subjects completed the planned PIS, 81 (91.0%) GP2+GM-CSF and 86 (94.5%) GM-CSF only. In addition, 77 GP2+GM-CSF and 80 GM-CSF only subjects received all 4 booster injections. The most common local toxicities were erythema, induration and pruritis and they occurred with similar frequency in the two treatment arms. Local reactions were reported by almost all subjects over the course of vaccinations. Occurring in a smaller percentage of subjects, the most common systemic toxicities were fatigue, headache, and myalgia/arthralgia, again with similar incidence by treatment group. The majority of all events reported were of Grade 1 mild severity (GP2+GM-CSF 92.5%, GM-CSF only 90.6%). Only 5 events in 4 subjects were considered Grade 3: induration and maculopapular rash/pruritis, in two GP2+GM-CSF subjects and chest pain and hypersensitivity reaction in two GM-CSF only subjects. The incidence of local reactions minimally increased with subsequent vaccinations however, the types of events remain unchanged. No serious adverse events were reported over the full 5 year treatment and follow-up periods. Conclusions: The study confirms the finding from the Phase I trial evaluating GP2+GM-CSF that the vaccine is safe and well-tolerated. The majority of patients experienced only mild local and systemic toxicities. Importantly, toxicities in the GP2+GM-CSF group were comparable to those seen in the GM-CSF only group, suggesting the toxicities are attributable to GM-CSF. Clinical trial information: NCT00524277.

Abstract Background: The final analysis of the GP2 prospective, randomized, placebo-controlled, single-blinded, multicenter Phase IIb trial investigating GP2+GM-CSF administered in the adjuvant setting to node-positive and high-risk node-negative breast cancer patients with tumors expressing any degree of HER2 (immuno-histochemistry [IHC] 1-3+) (NCT00524277) is now complete with 5 year follow-up. The trial enrolled HLA-A02 patients randomized to receive GP2+GM-CSF versus GM-CSF alone. The trial's primary objective was to determine if treatment with GP2, a HER2-derived peptide, reduces recurrence rates. Three and 4 year interim analyses for this trial and 3 Phase I studies showing GP2 to be safe and immunogenic have been previously reported by Mittendorf et al. Methods: Each enrolled and consented GP2-treated patient was scheduled to receive a total of 6 GP2+GM-CSF (500 mcg GP2: 125 mcg GM-CSF) intradermal injections every 3-4 weeks as part of the Primary Immunization Series (PIS) for the first 6 months and 4 GP2+GM-CSF booster intradermal injections every 6 months thereafter. Boosters were introduced during the trial, thus some patients did not receive all 4 boosters. Each enrolled and consented placebo patient was scheduled to receive a total of 6 GM-CSF only intradermal injections every 3-4 weeks for the first 6 months and 4 GM-CSF only booster intradermal injections every 6 months thereafter. Results: This 168 patient (ITT: n=180) basket trial across 16 clinical sites explored 96 HER2 3+ patients, who received a standard course of trastuzumab after surgery and subsequently completed the full PIS or placebo, starting the PIS at median 17.1 months after surgery, and 72 HER2 1-2+ patients, who did not receive trastuzumab after surgery and subsequently completed the full PIS or placebo, starting the PIS at median 10.8 months after surgery. Since GP2 is synergistic with trastuzumab, and the HER2 1-2+ patients did not receive trastuzumab, it was prespecified to compare recurrence rates ITT versus per protocol in these 2 distinct, independently reported populations, excluding those patients who did not complete the PIS. GP2 was shown to be well tolerated with no SAEs and elicited a potent immune response measured by local skin tests and immunological assays, which suggest peak immunity is reached at 6 months upon completion of the PIS. After 5 years of follow-up, the Kaplan-Meier estimated 5-year DFS rate in the 46 HER2 3+ patients treated with GP2+GM-CSF, if the patient completed the PIS, was 100% versus 89.4% (95% CI:76.2, 95.5%) in the 50 placebo patients treated with GM-CSF (p = 0.0338). The treated versus placebo HER2 3+ patients were well-matched, where 53% were stage T1, 41% were stages T2-T4, 55% were node positive, 58% were HR positive and received endocrine therapy, 77% received adjuvant radiation, 77% received adjuvant chemotherapy, and 89% received trastuzumab. After 5 years of follow-up, the Kaplan-Meier estimated 5-year DFS rate in the 35 HER2 1-2+ patients treated with GP2+GM-CSF, if the patient completed the PIS, was 77.1% (95% CI:59.5, 87.9%) versus 77.6% (95% CI:60.1, 88.2%) in the 37 placebo patients treated with GM-CSF (p = 0.9142). Conclusions: This study demonstrated that completion of the GP2+GM-CSF PIS safely elicited a potent immune response and reduced recurrence rates to 0% in HER2 3+ patients, who received a standard course of trastuzumab after surgery. A pivotal Phase III trial is being initiated to treat HER2 3+ patients in the neoadjuvant setting. GP2 also may be effective when used in parallel to trastuzumab based therapeutics or in combination with trastuzumab based therapeutics in HER2 1-2+ or other HER2 expressing cancers. Citation Format: Snehal S Patel, David B McWilliams, Marisa S Patel, Christine T Fischette, Jaye Thompson, F Joseph Daugherty. Five year median follow-up data from a prospective, randomized, placebo-controlled, single-blinded, multicenter, phase IIb study evaluating the reduction of recurrences using HER2/neu peptide GP2 + GM-CSF vs. GM-CSF alone after adjuvant trastuzumab in HER2 positive women with operable breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-23.