Replacements There will be no replacements in this study. SELLAS Life Sciences Group Galinpepimut -S Protocol : SLS17 -201/MK3475 -770 Protocol v1.1 (Amendment 1.0) 12 April 2019 Page 133 5. Study Treatments 5.1. Method of Assigning Patient s to Treatment Once the investigator confirms that the patient has met all eligibility criteria, the patient will be enrolled in the study by the local investigators and their teams . Trial treatment should begin on the day of enrollment or as close as possible to the date on which the patient is enrolled. The investigator is responsible to maintain appropriate records and ensure appropriate supply, storage, handling, distribution and usage of trial treatments is in accordance with the protocol and any applicable laws and regulations. 5.2. Treatments Administered Refer to Table 8 (Schedule of Assessments) for additional details. 5.2.1. Galinpepimut -S Each of the 4 peptides will be supplied at a concentration of 0.2 mg/vial. Aft er reconstitution with 0.6 mL sterile water for injection (SWFI ), each peptide will be at a concentration of 0.4 mg/mL (400 μg/mL of each peptide) combined in a single sterile solution in a buffer consisting of 10 mM histidine, pH 6.0 + 0.05% ethylene diamine tetra acetic acid ( EDTA ). As galinpepimut-S will comprise 50% (0.5 mL) of the study treatment injection administered, the actual dose of each peptide is 200 μg (with the total 4 active peptides equaling 800 μg). On galinpepimut-S administration days, blood samples will be collected before study treatment administration. Before injection, study staff will assess the immunization site. Patients will receive an injection of GM -CSF (refer to Section 5.2.3). A 1-mL dose of galinpepimut-S will be administered subcutaneously (SC) on the arm, leg, or torso of the patient, and should be positioned at least 5 cm away from the previous site of the study treatment injection. A maximum of 16 total injections will be administered as follows: SELLAS Life Sciences Group Galinpepimut -S Protocol : SLS17 -201/MK3475 -770 Protocol v1.1 (Amendment 1.0) 12 April 2019 Page 134 • First 2 galinpepimut-S injections: every 3 weeks (×2). Thereafter, galinpepimut-S will also be administered every 3 weeks to coincide with pembrolizumab treatment as detailed below: • Injections 3 to 6: every 3 weeks (between Weeks #6 and #15). The first series of 6 injections of galinpepimut-S (I + II) define the initial immunization induction phase. • Injections 7 to 12: every 3 weeks (between Weeks #21 and #36). The second series of injections of galinpepimut-S define the early immune booster p hase. • Injections 13 to 18: every 12 weeks (between Weeks #48 and #84). The series of the last 4 injections of galinpepimut-S define the late immune booster phase. • As galinpepimut-S will be co-administered with Montanide, specific information about this emulsifying agent is provided in the Montanide Investigator Brochure. Every effort should be made to follow the administration schedule for galinpepimut-S and pembrolizumab. If a patient misses a scheduled administration of either the combination or pembrolizumab alone, the drug(s), should be administered as soon as possible. If the patient is more than one-half the time interval to the next scheduled administration, the patient should skip that preplanned administration and have the next administration on schedule. Galinpepimut-S will be administered 30-60 minutes after the completion of IV infusion of pembrolizumab on Day 1 of each cycle during which the 2 drugs are being co -administered. Refer to Section 5.2.2 for Montanide ISA 51 VG administration. 5.2.2. Montanide ISA 51 VG Galinpepimut-S treatment preparation will require reconstitution of the lyophilized powder with SWFI , followed by a mixture of the peptide solution of galinpepimut -S with the SELLAS Life Sciences Group Galinpepimut -S Protocol : SLS17 -201/MK3475 -770 Protocol v1.1 (Amendment 1.0) 12 April 2019 Page 135 immunologic adjuvant Montanide ISA 51 VG as an emulsion to a total volume of 1 mL . The study treatment solution will comprise 50% (0.5 mL) of galinpepimut-S and 50% (0.5 mL) Montanide. Galinpepimut-S includes concomitant administration of Montanide (adjuvant) on Day 1 of all sessions of administration of this direct immunizer. Galinpepimut-S will be subsequently mixed in a water-in-oil emulsion with the oily adjuvant Montanide ISA 51 at a 1:1 ratio. The resulting final emulsion will be administered to the patient SC as a 1 mL injection. 5.2.3. Sargramostim (GM -CSF) Patients will receive 70 μg sargramostim (GM-CSF) SC on Day -2 and on Day 1 before each galinpepimut-S injection. Patients will remain in the clinic for approximately 30 minutes following receipt of GM-CSF. This injection will occur at the same anatomical site of the next planned study treatment injection.