Subjects may receive other medications that the investigator deems to be medically necessary. The Exclusion Criteria describes other medications which are prohibited in this trial. There are no prohibited therapies during the Post-Treatment Follow-up Phase. 60 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 4.3 DIET/ACTIVITY/OTHER CONSIDERATIONS 4.3.1 Diet Subjects should maintain a normal diet unless modifications are required to manage an AE such as diarrhea, nausea or vomiting. 4.3.2 Contraception Pembrolizumab may have adverse effects on a fetus in utero. Furthermore, it is not known if pembrolizumab has transient adverse effects on the composition of sperm. For this trial, male subjects will be considered to be of non-reproductive potential if they have azoospermia (whether due to having had a vasectomy or due to an underlying medical condition). Female subjects will be considered of non-reproductive potential if they are either: (1) postmenopausal (defined as at least 12 months with no menses without an alternative medical cause; in women < 45 years of age a high follicle stimulating hormone (FSH) level in the postmenopausal range may be used to confirm a post-menopausal state in women not using hormonal contraception or hormonal replacement therapy. In the absence of 12 months of amenorrhea, a single FSH measurement is insufficient.); OR (2) have had a hysterectomy and/or bilateral oophorectomy, bilateral salpingectomy or bilateral tubal ligation/occlusion, at least 6 weeks prior to screening; OR (3) has a congenital or acquired condition that prevents childbearing. Female and male subjects of reproductive potential must agree to avoid becoming pregnant or impregnating a partner, respectively, while receiving study drug and for 120 days after the last dose of study drug by complying with one of the following: (1) practice abstinence† from heterosexual activity; OR (2) use (or have their partner use) acceptable contraception during heterosexual activity. Acceptable methods of contraception are‡: Single method (one of the following is acceptable): ointrauterine device (IUD) ovasectomy of a female subject’s male partner ocontraceptive rod implanted into the skin Combination method (requires use of two of the following): odiaphragm with spermicide (cannot be used in conjunction with cervical cap/spermicide) ocervical cap with spermicide (nulliparous women only) ocontraceptive sponge (nulliparous women only) 61 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 omale condom or female condom (cannot be used together) ohormonal contraceptive: oral contraceptive pill (estrogen/progestin pill or progestin-only pill), contraceptive skin patch, vaginal contraceptive ring, or subcutaneous contraceptive injection †Abstinence (relative to heterosexual activity) can be used as the sole method of contraception if it is consistently employed as the subject’s preferred and usual lifestyle and if considered acceptable by local regulatory agencies and ERCs/IRBs. Periodic abstinence (e.g., calendar, ovulation, sympto-thermal, post-ovulation methods, etc.) and withdrawal are not acceptable methods of contraception. ‡If a contraceptive method listed above is restricted by local regulations/guidelines, then it does not qualify as an acceptable method of contraception for subjects participating at sites in this country/region. Subjects should be informed that taking the study medication may involve unknown risks to the fetus (unborn baby) if pregnancy were to occur during the study. In order to participate in the study subjects of childbearing potential must adhere to the contraception requirement (described above) from the day of study medication initiation (or 14 days prior to the initiation of study medication for oral contraception) throughout the study period up to 120 days after the last dose of trial therapy. If there is any question that a subject of childbearing potential will not reliably comply with the requirements for contraception, that subject should not be entered into the study. 5 BIOSPECIMEN COLLECTION 5.1 CORRELATIVE STUDIES FOR RESEARCH/PHARMACOKINETIC S TUDIES The purpose of the correlative studies is to determine if there is a correlation with clinical outcome of PD1 or PDL1 expression and T-Cell infiltrate in pathology from first, and if applicable, second surgical specimens with outcome. oTo assess tumor tissue for neo-antigens (initial tumor pathology). oTo collect peripheral monocytes for PD-L1 levels to confirm that low level expression leads to better outcome and if higher levels that are treated with Pembrolizumab might improve outcome. To perform other immune monitoring assessments including, but are not limited to: whole exome sequencing (WES), immune phenotyping of immune cell subsets, functional characterization of immune cell subsets, measurement of immune mediators and the assessment of circulating tumor DNA (ctDNA). oTo correlate MGMT and IDH status with outcome (local pathology report). At each timepoint noted in the study calendar, correlative blood will be collected in 8 mononuclear cell preparation tubes (LabConnect Kit B, 8mL CPT™ blue and black top tubes) and processed for the isolation of both PBMCs and plasma. As detailed in the LabConnect lab manual, Correlative blood will be collected at NCI and participating BTTC sites and sent to: NCI Frederick Lab Leidos Biomedical Research 62 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 Attn: Theresa Burks 1050 Boyles Street Bldg. 469/Room 121 Frederick, MD 21702 Phone 301-846-5125, or 301-846-6865 Email: burkst@mail.nih.gov, cc: jon.inglefield@nih.gov 5.1.1 Collection of correlative blood time points: NOTE: The schedule of correlative blood sample collection should be held/delayed when treatment dosing is held. See section 5 Biospecimen Collection, and at the end of Table 1, and/or the calendar. For participants randomized to study vaccine or placebo: 1) Pre-RT treatment (RT+TMZ+Pembro) 2) Post-RT treatment (RT+TMZ+Pembro)/Pre-vaccine (or placebo) induction 3) Post Vaccine (or placebo) induction/Pre-cycle 1 maintenance 4) Every Day 1 on Cycle 2, C4, C6, C8 (if continuing on Pembrolizumab); then, at discretion of Study Chair and BTTC Coordinating Center, every other cycle 5) At time of progression or at end of treatment For participants not randomized (ancillary cohort 3): 1) Pre-RT treatment (RT+TMZ+Pembro) 2) Pre-cycle 1 maintenance 3) Every Day 1 on Cycle 2, C4, C6, C8 (if continuing on Pembrolizumab); then, at discretion of Study Chair and BTTC Coordinating Center, every other cycle 4) At time of progression or at end of treatment LabConnect (Fee for Service Provider) will supply kits and track shipments for correlative blood collection. (See LabConnect lab manual.) 5.1.2 PD-L1 expression on tumor cells A collection of sectioned slides from tumor samples for the purpose of immunohistochemical (IHC) analysis will be sent for PD-L1 analysis at the end of the study.