These patients will have their response classified according to the definitions stated below. (Note: Patients who exhibit objective disease progression prior to the end of cycle 1 will also be considered evaluable.) 6.3.1.3 Evaluable Non-Target Disease Response: Patients who have lesions present at baseline that are evaluable but do not meet the definitions of measurable disease, have received at least one cycle of therapy, and have had their disease re-evaluated will be considered evaluable for non-target disease. The response assessment is based on the presence, absence, or unequivocal progression of the lesions. 6.3.2 Disease Parameters 6.3.2.1 Measurable disease: Based on RANO criteria [44] 6.3.3 RANO Criteria for Response Table 2: RANO Criteria for Response RANO Criteria for Response [44] CR PR SD PD T1 gadolinium enhancing disease None ≥ 50% ↓< 50% ↓ but < 25% ↑≥ 25% ↑1T2/FLAIR Stable or ↓Stable or ↓ Stable or ↓↑1 New lesion None None None Present1 Corticosteroids None Stable or ↓ Stable or ↓ n/a† Clinical status Stable or ↑Stable or ↑ Stable or ↑↓1 Requirement for response All All All Any1 70 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 RANO Criteria for Response [44] CR PR SD PD 1 Progression occurs when this criterion is present †Increase in corticosteroids alone will not be taken into account in determining progression in the absence of persistent clinical deterioration. ↓ = decreased, ↑ = increased, CR = complete response, PR = partial response, SD = stable disease, PD = progressive disease. If PD is unclear based on white matter changes, patients may be kept on trial. FLAIR, fluid-attenuated inversion recovery; NA, not applicable. 6.3.4 iRANO (Exploratory)[45] Table 3: iRANO 71 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 6.3.5 Evaluation of Best Overall Response 6.3.5.1 Duration of overall response: The duration of overall response is measured from the time measurement criteria are met for CR or PR (whichever is first recorded) until the first date that recurrent or progressive disease is objectively documented (taking as reference for progressive disease the smallest measurements recorded since the treatment started). The duration of overall CR is measured from the time measurement criteria are first met for CR until the first date that progressive disease is objectively documented. 6.3.5.2 Duration of stable disease: Stable disease is measured from the start of the treatment until the criteria for progression are met, taking as reference the smallest measurements recorded since the treatment started, including the baseline measurements. 6.3.6 Dose Limiting Toxicity Any of the following adverse events not due to disease progression or any underlying disease will be documented as a DLT during the first 4 weeks of treatment with Pembrolizumab, TMZ and HSPPC-96/Placebo (attribution should be assigned to drug most suspected) during the first post RT cycle, after completion of induction: Grade 4 neutropenia for more than 7 days Grade 3 or 4 febrile neutropenia during the first 4 weeks Grade 3 or 4 thrombocytopenia for more than 7 days Grade 4 anemia for more than 7 days Grade 3 or 4 non-hematologic adverse events. Exceptions include: Grade 3 nausea, vomiting or diarrhea will only be considered a DLT if not adequately managed with maximum supportive care within 48 hours. More than 14 days of treatment delay due to the failure to recover from attributable toxicity. Subjects experiencing irAEs due to Pembrolizumab and are successfully treated for toxicity, and able to be re-dosed will not be considered to have experienced DLTs. The frequency, grade, and nature of irAE will be recorded. 6.3.7 Progression-Free Survival Progression free survival will be defined from the time of registration to the time of confirmed progression. In cases where this might be unclear, patients may continue on trial. 6.3.8 Overall survival Defined as the time from registration to the time of death 6.3.9 Response Review Response review will be done locally and by Study Chair if response is questionable. 72 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 6.4 TOXICITY CRITERIA The following adverse event management guidelines are intended to ensure the safety of each patient while on the study. The descriptions and grading scales found in the revised NCI Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 will be utilized for AE reporting. An investigator who is a qualified physician will evaluate all adverse events according to (CTCAE), version 5.0. Any adverse event which changes CTCAE grade over the course of a given episode will have each change of grade recorded on the adverse event case report forms/worksheets. All adverse events regardless of CTCAE grade must also be evaluated for seriousness. All appropriate treatment areas should have access to a copy of the CTCAE version 5.0. A copy of the CTCAE version 5.0 can be downloaded from the CTEP web site (http://ctep.cancer.gov/protocolDevelopment/electronic_applications/ctc.htm ). 7 NIH REPORTING REQUIREMENTS/DATA AND SAFETY MONITORING PLAN 7.1 DEFINITIONS Please refer to definitions provided in Policy 801: Reporting Research Events found at: https://irbo.nih.gov/confluence/pages/viewpage.action?pageId=36241835#Policies&Guidance- 800Series-ComplianceandResearchEventReportingRequirements. 7.2 OHSRP OFFICE OF COMPLIANCE AND TRAINING / IRB REPORTING 7.2.1 Expedited Reporting Please refer to the reporting requirements in Policy 801: Reporting Research Events and Policy 802 Non-Compliance Human Subjects Research found at: https://irbo.nih.gov/confluence/pages/viewpage.action?pageId=36241835#Policies&Guidance- 800Series-ComplianceandResearchEventReportingRequirements. Note : Only IND Safety Reports that meet the definition of an unanticipated problem will need to be reported per these policies. 7.2.2 IRB Requirements for NCI PI Reporting at Continuing Review Please refer to the reporting requirements in Policy 801: Reporting Research Events found at: https://irbo.nih.gov/confluence/pages/viewpage.action?pageId=36241835#Policies&Guidance- 800Series-ComplianceandResearchEventReportingRequirements. 7.3 NCI CLINICAL DIRECTOR R EPORTING NIH site only: Problems expeditiously reviewed by the OHSRP in the NIH eIRB system will also be reported to the NCI Clinical Director/designee; therefore, a separate submission for these reports is not necessary. 73 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 In addition to those reports, all deaths that occur within 30 days after receiving a research intervention should be reported via email to the Clinical Director unless they are due to progressive disease. To report these deaths, please send an email describing the circumstances of the death to NCICCRQA@mail.nih.gov within one business day of learning of the death. 7.4 NCI GUIDANCE FOR REPORTING EXPEDITED ADVERSE EVENTS FOR MULTI-CENTER TRIALS The site PI (at the participating BTTC site) must immediately report to the BTTC Coordinating center PI any serious adverse event, whether or not considered drug related, including those listed in the protocol or investigator brochure and must include an assessment of whether there is a reasonable possibility that the drug caused the event within 24 hours of PI awareness of the event. CCR Reportable Event Form (REF) will be used to submit Serious Adverse Events to both the CCR Office of Sponsor and Regulatory Oversight at OSROSafety@mail.nih.gov and the BTTC Coordinating Center. https://ccrod.cancer.gov/confluence/display/CCRCRO/Templates Participating BTTC sites must also report events to the Reviewing IRB as per its policy. For IND studies, the site PI will also directly submit reports to the CCR as IND sponsor per section 8.3. Please also notify the coordinating center PI and study coordinator of your submission at the time you make it. The BTTC Coordinating Center will maintain documentation of all Serious Adverse Events from each institution. The BTTC Coordinating Center will notify all investigators of any serious and unexpected adverse experiences that are possibly related to the study agent/s. The investigators are to file a copy with their protocol file and send a copy to their IRB according to their local IRB’s policies and procedures. For the time period beginning when the consent form is signed until treatment initiation, any serious adverse event, or follow up to a serious adverse event, including death due to any cause that occurs to any subject must be reported within 24 hours to the Sponsor (NCI) if it causes the subject to be excluded from the trial, or is the result of a protocol- specified intervention, including but not limited to washout or discontinuation of usual therapy, diet, placebo treatment or a procedure. For the time period beginning at treatment initiation through 90 days following cessation of treatment, or 30 days following cessation of treatment if the subject initiates new anticancer therapy, whichever is earlier, any serious adverse event, or follow up to a serious adverse event, including death due to any cause, whether or not related to the study treatments, must be reported within 24 hours to the Sponsor (NCI). Additionally, any serious adverse event, considered by an investigator who is a qualified physician to be related to study treatment that is brought to the attention of the investigator at any time following consent through the end of the specified safety follow- up period specified in the paragraph above, or at any time outside of the time period specified in the previous paragraph also must be reported immediately to the Sponsor. 74 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 All subjects with serious adverse events must be followed up for outcome. 7.4.1 Reporting Deviations and Unanticipated Problems for Multi-Center Trials Deviation or Unanticipated Problem Reports and any accompanying documentation (to include the local IRB acknowledgement of the event when applicable) are to be submitted by the participating BTTC site to the BTTC Coordinating Center using the CCR Reportable Event (REF) Form in Appendix 16.10 by email nci_bttc@mail.nih.gov or via fax at: 240-541-4432. Deviations or Unanticipated problems must be submitted to the BTTC Coordinating Center within 5 days after becoming aware of the event. When Deviations or Unanticipated Problems are reported to BTTC, but, the local IRB does not require a report, the report that is submitted to the BTTC Coordinating Center must be accompanied by a formal memo explaining the local policy and the rationale for not reporting the event to the local IRB. Neither the FDA nor the ICH GCP guidelines define the term “protocol deviation.” The definition is often left to the Lead Institution IRB. Accordingly, since NCI, Center for Cancer Research is the Coordinating Center and the Lead PI must adhere to those policies set by the NIH Intramural IRB, the definitions for unanticipated problem and protocol deviation as described by the NIH Intramural IRB will be applied for reporting purposes for all institutions participating in this study. Definitions are listed in Section 7.1. Protocol Deviations or Unanticipated problems occurring at a participating institution will be submitted to that institution’s own IRB in accordance with local policies and procedures. However, the participating institution must submit a report to the BTTC Coordinating Center even in instances where the local IRB does not require a report. NCI Center for Cancer Research BTTC Coordinating Center: Upon receipt of the deviation/unanticipated reportable event report from the participating institution, the BTTC Coordinating Center will submit the report to the Study Chair for review. Subsequently, the participating institution’s IRB deviation/unanticipated reportable event report will be submitted to the NIH Intramural IRB if it meets the criteria under Policy 801: Reporting Research Events found at: https://irbo.nih.gov/confluence/pages/viewpage.action?pageId=36241835#Policies&Guidance- 800Series-ComplianceandResearchEventReportingRequirements. 7.5 NIH REQUIRED DATA AND SAFETY MONITORING PLAN The clinical research team will have a teleconference every other week when patients are being actively treated on the trial to discuss each patient. All data will be collected in a timely manner and reviewed by the Principal investigator or a lead associate investigator. Events meeting requirements for expedited reporting as described in section 7.2.1 will be submitted within the appropriate timelines. The Principal Investigator will review adverse event and response data on each patient to ensure safety and data accuracy. The Principal Investigator will personally conduct or supervise the investigation and provide appropriate delegation of responsibilities to other members of the research staff. 75 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 7.5.1 Data Safety Monitoring Board (DSMB) As this is a prospective, double-blinded, randomized controlled trial, the NCI/CCR DSMB will be used to monitor the trial. This monitoring will be done for futility, as well as for safety. Since the meetings of the NCI DSMB are annual, futility monitoring may be performed by the DSMB at a special meeting requested by the study team. These ad-hoc DSMB meetings may convene by phone in the event of rapid accrual. The interim data analyses needed for the DSMB futility monitoring will be performed by the BTTC biostatistician. Subjects will be randomized without stratification on a 1:1 basis, using variable block sizes. The first block will be of size n=6 in order to facilitate the evaluation of toxicity and safety in the first 3 patients on each arm. The futility monitoring will be performed as follows: at the first DSMB meeting after which 22 patients have been enrolled into each of the experimental arms, given an experimental arm, (i) if 16 or more patients survive at 8 months, additional 23 patients will be enrolled into that arm; (ii) if 7 or more patients fail to survive at 8 months, the experimental arm will be terminated early. If neither of these two conditions are satisfied, in other words, some patients are followed less than 8 months and their survival at 8 month is unknown, the posterior probability Pr(OS8 < 0.75 | data) will be evaluated using the method of Cai, Liu and Yuan (2014) [48]. If Pr(OS8 < 0.75 | data) > 0.6, the experimental arm will be terminated early; otherwise additional 23 patients will be enrolled into that arm. Note that this posterior stopping rule is consistent with rules (i) and (ii). Interim outcome results will not be revealed to the investigators of the trial; results will be presented to the investigators prior to final accrual to the trial only if the DSMB recommends early termination of the trial. The DSMB will review the rules for early termination for toxicity and provide feedback to the BTTC Biostatistician, the Study Chair, and lead PI who will confirm the condition for early termination. Once confirmed, the Biostatistician will then communicate to CRO (Central Registration Office) to break the blind and provide the listing to the BTTC Biostatistician and Study Chair. The evaluation for safety will be performed independently for each of the two experimental arms as follows: For each of the experimental arms, stop the enrollment if [# of patients with DLT]/[# of patients evaluated] ≥ 2/3, 4/6, 5/10 or 8/20; otherwise continue the enrollment. In the case that one arm is discontinued due to futility or safety, enrollment would shift to the remaining arm as the primary objective of this trial is not to compare two arms. 7.5.2 Patient-Reported Outcomes Received MDASI-BT forms will be checked versus the timing schedule and considered as valid if they fall within 14 days of the scheduled assessment window. Compliance rates will be calculated as the number of received valid forms over the number of expected forms. Differences between groups in compliance will be tested by use of Fisher’s exact test at every time point. 76 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 We will use descriptive statistics to describe how patients rate symptom severity and interference with function at each time point. Error bar graphs for each of the symptoms will be constructed at each time point. The proportion of patients rating their symptoms to be 5 or greater (on a 0-10 scale) will also be reported. We will construct individual patient profiles for each of the selected symptoms to describe the individual patients’ patterns of change over time. We will calculate the mean symptom severity and mean symptom interference at the time of clinical evaluation. Estimates of differences in the mean symptom severity and mean symptom interference between responders and non-responders will be estimated in the intent to treat population. All patients with at least one valid questionnaire will be included in the analyses. Questionnaires completed at study registration will be considered baseline. All questionnaire data received after randomization will be used in the primary analyses. Differences of at least 2 points will be classified as the minimum clinically meaningful change in the symptom severity and symptom interference measures. For example, an increase of 2 points or more would mean a moderate improvement, whereas a decrease of 2 points or more would be interpreted as moderate worsening. For individual symptoms, a rise in a symptom score means deterioration, whereas a reduced score means improvement of the specific symptom. 7.6 EXPLORATORY OBJECTIVES & ENDPOINTS PDL1 expression and T-Cell infiltrate in tumor will be correlated using Spearman correlation coefficients. Peripheral T-Cell activation and tryptophan metabolites will be summarized by reporting descriptive statistics such as median and inter-quartile range for all time points. 8 SPONSOR PROTOCOL/SAFETY REPORTING 8.1 DEFINITIONS 8.1.1 Adverse Event Any untoward medical occurrence in a patient or clinical investigation subject administered a pharmaceutical product and which does not necessarily have a causal relationship with this treatment. An adverse event (AE) can therefore be unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease, temporally associated with the use of a medicinal (investigational) product, whether or not related to the medicinal (investigational) product (ICH E6 (R2)). 8.1.2 Serious Adverse Event (SAE) An adverse event or suspected adverse reaction is considered serious if in the view of the investigator or the sponsor, it results in any of the following: Death, A life-threatening adverse event (see section 8.1.3) Inpatient hospitalization or prolongation of existing hospitalization oA hospitalization/admission that is pre-planned (i.e., elective or scheduled surgery arranged prior to the start of the study), a planned hospitalization for pre-existing condition, or a procedure required by the protocol, without a serious deterioration in health, is not considered a serious adverse event. 77 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 oA hospitalization/admission that is solely driven by non-medical reasons (e.g., hospitalization for patient or subject convenience) is not considered a serious adverse event. oEmergency room visits or stays in observation units that do not result in admission to the hospital would not be considered a serious adverse event. The reason for seeking medical care should be evaluated for meeting one of the other serious criteria. Persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions A congenital anomaly/birth defect. Important medical events that may not result in death, be life-threatening, or require hospitalization may be considered a serious adverse drug experience when, based upon appropriate medical judgment, they may jeopardize the patient or subject and may require medical or surgical intervention to prevent one of the outcomes listed in this definition. Note: In addition to the above criteria, adverse events meeting either of the below criteria, although not serious per ICH definition, are reportable to the Merck in the same timeframe as SAEs to meet certain local requirements. Therefore, these events are considered serious by Merck for collection purposes. Is a new cancer (that is not a condition of the study); Is associated with an overdose. Refer to Section 8.1.1 “Adverse Events” for additional details regarding each of the above criteria. 8.1.2.1 Protocol-Specific Exceptions to Serious Adverse Event Reporting Efficacy endpoints as outlined in this section will not be reported to Merck as described in Immediate Reporting of Adverse Events to the Sponsor (NCI), unless there is evidence suggesting a causal relationship between the drug and the event. Any such event will be submitted to the Merck within 24 hours. Specifically, the suspected/actual events covered in this exception include any event that is disease progression of the cancer under study. Hospitalization related to convenience (e.g. transportation issues, etc.) will not be considered a SAE. 8.1.3 Life-threatening An adverse event or suspected adverse reaction is considered "life-threatening" if, in the view of either the investigator or sponsor, its occurrence places the patient or subject at immediate risk of death. It does not include an adverse event or suspected adverse reaction that, had it occurred in a more severe form, might have caused death. (21CFR312.32) 8.1.4 Severity The severity of each Adverse Event will be assessed utilizing the CTCAE version 5. 78 Abbreviated Title: GBM Pembro HSPPC Version Date: 03/26/2024 8.1.5 Relationship to Study Product All AEs will have their relationship to study product assessed using the terms: related or not related. Related – There is a reasonable possibility that the study product caused the adverse event. Reasonable possibility means that there is evidence to suggest a causal relationship between the study product and the adverse event. Not Related – There is not a reasonable possibility that the administration of the study product caused the event. 8.1.6 Events Of Clinical Interest (ECI) For This Trial Include: an overdose of Merck product, as defined in Section 8.1.7 Definition of an Overdose for This Protocol and Reporting of Overdose to the Sponsor, that is not associated with clinical symptoms or abnormal laboratory results. an elevated AST or ALT lab value that is greater than or equal to 3X the upper limit of normal and an elevated total bilirubin lab value that is greater than or equal to 2X the upper limit of normal and, at the same time, an alkaline phosphatase lab value that is less than 2X the upper limit of normal, as determined by way of protocol-specified laboratory testing or unscheduled laboratory testing. *Note: These criteria are based upon available regulatory guidance documents.