For grade 3 or 4 allergic reactions considered related to HSPPC-96/Placebo, discontinue HSPPC- 96/Placebo. 3.10.6 Vaccine Injection Site Reactions For grade 1 or 2 injection site reactions, continue on study; make sure to rotate injection sites as instructed in Administration Section 14.3.8. Manage symptoms according to institutional procedures. For grade 3 or 4 injection site reactions, discontinue HSPPC-96/Placebo. For other grade 1 or 2 non-hematologic toxicity, continue on protocol once toxicity is resolved to < grade 1. For other grade 3 or 4 non-hematologic toxicity considered at least possibly related to HSPPC-96, discontinue HSPPC-96/Placebo. 54 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 3.11 STUDY CALENDAR Screening7 Treatment Follow Up Time PeriodPre- Surgery ScreeningRT + TMZ + Pembrolizumab16TMZ+Pembrolizumab +/- HSPPC-96/Vaccine (6 cycles)End of Treatment or DiscontinuationLong-term Follow-up8 Scheduling Window (Days)+/-14 of signing consentSurgery - 7 (before treatment)HSPPC-96/Vaccine Induction or Break14 (4 weeks)- 7 (before treatment)+/- 330 days after last dose Every 8-16 weeks Informed Consent X Medical history X Prior & concomitant medications reviewX Assess and document first post- protocol treatment X Assess and document Adverse Events12X12X12X12X X Physical, KPS & neurological exam13 X X13X13X13X X Vital Signs and Weight1 X X X X X X Pregnancy test2X X2X2X2 PT/INR or aPTT X X CBC with diff X X4X4X4X X Comprehensive Chemistry panel9 X X5 X5 X5X Urinalysis X X T3, T4, and TSH X5X5X5X X Tumor Imaging3X X3X3X3X X Archival or Collection Tumor X MDASI-BT11X X X X Correlative Studies Blood CollectionX6X6X6X6 55 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 Screening7 Treatment Follow Up Time PeriodPre- Surgery ScreeningRT + TMZ + Pembrolizumab16TMZ+Pembrolizumab +/- HSPPC-96/Vaccine (6 cycles)End of Treatment or DiscontinuationLong-term Follow-up8 Scheduling Window (Days)+/-14 of signing consentSurgery - 7 (before treatment)HSPPC-96/Vaccine Induction or Break14 (4 weeks)- 7 (before treatment)+/- 330 days after last dose Every 8-16 weeks Drug Administration X15X10X10 Assess and document Survival StatusX 1 Includes vital signs (pulse, respirations, blood pressure) and height (baseline only) and weight. 2 Serum or urine test for females of child-bearing potential prior to the start of RT+TMZ+Pembro and at the start of every cycle. 3 By MRI (unless contraindicated): For subjects not randomized: An MRI will be done pre surgery, post surgery (within 72 hours), 4 weeks post RT, and then every 9 weeks +/- 1 week. For subjects who are randomized: An MRI will be done pre surgery, post surgery (within 72 hours), within +/- 10 days of last dose of vaccine or placebo induction, and then every 9 weeks +/- 1 week. This schedule of MRIs is consistent with standard of care. 4 A CBC with differential will be done weekly during RT and then weekly after RT for the first year and then every 2 weeks for the year after if the patient remains on Pembrolizumab. 5 Chemistry should be done at beginning of RT, Day 21 of RT +/- 3 days, at completion of RT, and on Days 1(+/- 1 day) and Day 29 (- 7 days) of each cycle. 6 Randomized - 8x8mL CPT blue and black top tubes (LabConnect Kit B) will be used to collect correlative blood samples at the following time points for patients 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. Ancillary - 8x8mL CPT blue and black top tubes (LabConnect Kit B) will be used to collect correlative blood samples at the following time points for patients not randomized (ancillary cohort): 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. 7 Screening labs will be completed within (+/-) 14 days of signing consent. 8 Patients will be followed for overall survival. Patients withdrawn from treatment will follow the same schedule. 9 Comprehensive Serum Chemistry panel=Na, K, CL, Bicarbonate, BUN/Cr, AST, ALT, ALP T. Bili, Calcium, Albumin. 56 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 10 HSPPC-96 Vaccine/Placebo given weekly post RT x 4 weeks, then 21 days (+/- 24 hours) after the day 5 and day 33 doses of TMZ of each cycle. TMZ doses Cycle 1, days 1-5 at 150 mg/m2. If tolerated, the dose will be escalated to 200 mg/m2 for all remaining doses (This would begin with Cycle 1 days 29-33.). Pembrolizumab dosed on day 1, 22, and 43 (+/- 24 hours for each timepoint) of RT then day 1, 22, 43 (+/- 24 hours for each timepoint) of each 9 week cycle. 11 MDASI-BT assessments will take place along with imaging studies. MDASI-BT will need to be given prior to informing the patients about the results of the imaging study, within +/- 1 week of the MRI assessment date. 12 Assess patients for adverse events during Physical Exam and treatment days. 13 Physical, KPS and neurological exam every 3 weeks (- 3 days to + 7 days) during RT and then every 4 weeks (+/- 7 days) after RT. If needed, telehealth assessments will be allowed for the physical, KSP & neurological exams. For additional details on allowed telehealth assessments, please see Section 3.9. 14 For those not randomized (ancillary arm), no drug administration during the 4 weeks prior to Pembro and TMZ, therefore no laboratory tests or scheduled assessments are required. 15 TMZ may be given night before RT (within 24 hours before day 1). 16 Baseline assessments will be done prior to the start of RT+TMZ+Pembro (see 3.7). 57 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 3.12 SURGICAL GUIDELINES Surgical guidelines will be according to Standard of Care at each institution. 3.13 COST AND COMPENSATION 3.13.1 Costs Subjects costs will be based on local guidelines as described in the site specific consent.