Biological potency is expressed in International Units (IU) as tested against the World Health Organization (WHO) First International Reference Standard. The specific activity of GM-CSF is approximately 2.8 x 106 IU/mL. See GM-CSF (Leukine® (sargramostim)) package insert and labeling (Appendix E) for additional details. The E75 vaccine acetate drug substance is a 9-amino acid peptide produced by solid-phase peptide synthesis. Nelipepimut-S acetate drug product is manufactured by Oso Biopharmaceuticals Manufacturing, Albuquerque, NM as a 1.5 mg/mL solution in 1 mL water for injection (WFI) in a 2-mL glass vial. Each mg of Nelipepimut-S acetate is equivalent to 0.94 mg of Nelipepimut S peptide. Hence, the vaccine concentration is equivalent to 1.41 mg/mL Nelipepimut-S peptide. The control is WFI. Prior to administration, 1,065 mcg (0.8 mL) of the E75 acetate or control is mixed by the pharmacist or research nurse with 1 mL of reconstituted lyophilized Leukine® GM-CSF (reconstituted as 250 mcg/mL in solution). Immediately after mixing, the study drug is withdrawn into four 1 mL syringes and within 6 hours of 25 Protocol: 2014-0443 Version 2.1 Version Date: 11 April 2016 mixing, a total of 1.6 mL is administered as 4 intradermal injections to deliver a total of 1,000 mcg of E75 peptide vaccine (1,065 mcg of E75 acetate). The control vial contains 1 mL WFI. The detailed guidance for mixing E75 (or control) solution from lyophilized Leukine® 250μg and test drug (E75 solution or control) is stated in Appendix D. For DTH skin test, E75 acetate solution (1.5mg/1mL vial) is supplied in DTH skin test kit. Detailed mixing instruction for DTH skin test prep is stated in Appendix D. If dose reduction of GM-CSF is required based on >100 mm x 100 mm induration, and/or ≥Grade 3 toxicity at the injection site after the previous dose, the liquid Leukine® vial is diluted with an additional 1.0 mL WFI, using aseptic technique, and mixed gently by rolling the Leukine vial between your hands. Allow any foaming to dissipate before continuing. Do not invert the vial. Subsequently, 1 mL is withdrawn and discarded and the remaining 1 mL is used for mixing following the same methodology above to make a 1.6 mL Leukine + E75 mixture. Further dilutions (approximately 1:1) may be required if >10 cm erythema and induration and/or ≥ Grade 3 toxicity are observed on subsequent doses, the details of which will be provided in study instructions. 4.3.2.2 Storage For detailed storage instruction of the vaccine and its components (E75 acetate in WFI and GM-CSF) please follow the instructions in the Investigator’s Brochure. Stability data support storage of E75 acetate in WFI and control to E75 acetate in WFI at -20°C (± 5°C) or 5°C (± 3°C). Store study drug (E75 acetate in WFI, control to E75 acetate in WFI and GM-CSF) as per appropriate storage temperature designated on clinical carton label. 4.3.3 Inoculation series – administration Patients will receive 4 intradermal inoculations on the anterior or medial side of the same thigh. The general area of inoculation will be at a location midway between the inguinal ligament and the knee preferably, but may be given in the arm. The 1.6 mL by volume vaccine (or GM-CSF alone) will be administered intradermally in four equal inoculums at four different sites in a square configuration 5 cm from each other. Inoculations will be given every three weeks and will be administered in the same lymph node draining area (same arm or leg). Timing of the inoculation series with trastuzumab treatment will occur as outlined in Section 3.1 (see also Appendix B). The dose will be 1000 mcg of E75 peptide and 250 mcg of GM-CSF. The research nurse coordinator will administer the inoculations sterilely in the vaccine or clinical laboratory facility located at each study site. For female patients with childbearing potential, a urine 26 Protocol: 2014-0443 Version 2.1 Version Date: 11 April 2016 pregnancy test will be performed before each inoculation. If this test is positive at any time, the patient will be discontinued from the study. 4.3.4 Booster inoculations After completion of the six-inoculation PVS, patients will then receive a total of four booster inoculations to be administered, respectively, at 6, 12, 18, and 24, months +/- 2 weeks from the date of completion of the PVS (Appendix B). One booster inoculation will be administered at each time point noted above ± two weeks and administered in the same extremity as the primary series.