PEP-CMV Vaccine Targeting CMV Antigen to Treat Newly Diagnosed Pediatric HGG and DIPG and Recurrent Medulloblastoma

This phase II clinical trial will have 3 strata in order to assess the efficacy of a highly immunogenic CMV-directed peptide vaccines in children with (1) recurrent medulloblastoma (rMB), (2) newly-diagnosed high-grade gliomas (HGG) and (3) newly-diagnosed diffuse intrinsic pontine glioma (DIPG). Each stratum will run independently with a different endpoint and statistical design. Within each stratum, the populations that may be used for analysis are defined as: * Safety Analysis: Patients who receive at least 1 dose of the treatment will be used for safety analyses. * Efficacy Analysis: Patients who receive at least 1 dose of the treatment will be used for efficacy analyses. Stratum I: Patients with recurrent medulloblastoma with measurable disease (see eligibility) can be enrolled at any point following recurrence regardless of any prior therapy. For the purpose of this study, recurrence will be defined as a new lesion confirmed by biopsy or resection, positive cerebrospinal fluid (CSF) cytology, or recurrent/progressive tumor on MRI. Strata II \[CLOSED on 19Aug2026\] and III: Patients with newly-diagnosed high-grade glioma \[CLOSED on 19Aug2026\] or DIPG may be enrolled any time within 42 days after completing radiation. Cycle 1 (Induction cycle) is 77±2 days. Patients will receive one course of temozolomide 200 mg/m2/day x 5 days on Days 1-5 of cycle 1 and receive PEP-CMV vaccine intradermally at dose level 1 (250 μg/m2 ) on Days 21, 35 ±2 days, and 49 ±2 days. After the induction cycle which is 77 ± 2 days, each subsequent cycle is 28 days. Starting in cycle 2, PEP-CMV vaccine is administered intradermally every 28 days on day 1 of each course. Patient can continue to receive PEP-CMV every 28 days for a total of 24 cycles. Patients will receive a tetanus (Td) booster (Td 5 flocculation units, Lf) at the time of enrollment. Immunotherapy begins with a Td pre-conditioning vaccine (Td 1 Lf, in 0.4 mL of saline) delivered i.d. at the RIGHT groin site of the vaccine injection 6-24 hours prior to the first vaccine on day 21. The PEP-CMV vaccine will be administered as follows: 250 µg/m2 (up to a maximum of 500 µg) of Component A mixed with Montanide ISA-51 (1:1 volume ratio) intradermally administered half in the RIGHT groin and half in the LEFT groin.