∙ Patients with recurrent or progressive pediatric high-grade glioma (pHGG)

∙ Patients must be age 3-25 years

∙ Diagnosis:

∙ Patients must have had a prior histologically-diagnosed pHGG(including but not limited to: Astrocytoma WHO Grade 3 or 4 and Glioblastoma WHO Grade 4 by histopathology or molecular studies, per 2021 WHO Classification of Tumors of the CNS57, WHO CNS5).

• Patients with M+ disease without gliomatosis cerebri (see definition under exclusion criteria) ARE eligible.

• Recurrent pHGG involving the midline structures other than those intrinsically located within the pons ARE eligible.

• Patients with mismatch repair deficient (MMRD) tumors refractory to immune checkpoint inhibitors ARE eligible.

∙ Patients must have recurred or progressed after receiving surgery/biopsy and radiation therapy as frontline standard-of-care treatments in primary disease.

∙ Patients must have MRI evidence of probable recurrent pHGG. Patients must be clinically eligible for standard-of-care surgical resection/biopsy and sterile collection of tumor material in a manner suitable for RNA extraction, amplification, and loading of lipid particles.

∙ Performance Level Karnofsky ≥ 60% for patients ≥ 16 years of age and Lansky ≥ 60% for patients ≤ 16 years of age

• Participants who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing the performance score.

• Participants with post-surgical neurological deficits should have deficits that are stable for a minimum of 2 week prior to enrollment.

∙ Prior Therapy

∙ Patients must have recovered from all acute toxic effects of all prior anti-cancer therapy (all adverse events must have improved to grade 1 or better):

• Cytotoxic chemotherapy or other anti-cancer agents known to be myelosuppressive: ≥ 21 days after the last dose of myelosuppressive chemotherapy. If questions, the agent and duration can be discussed with the study chair.

• Anti-cancer agents not known to be myelosuppressive (e.g., not associated with reduced platelet or ANC counts): ≥ 14 days after the last dose of agent. If questions, the agent and duration can be discussed with the study chair.

• Antibodies: ≥ 21 days must have elapsed from infusion of last dose of antibody, and toxicity related to prior antibody therapy must be recovered to Grade ≤ 1.

• Corticosteroids: All systemically administered corticosteroids must be stable or decreasing for ≥ 1 week prior to enrollment, with a maximum dexamethasone dose of 2.8 mg/m2/day. Corticosteroid physiologic replacement therapy for management of pituitary/adrenal axis insufficiency and/or topical administration (e.g. inhaled or dermatologic) is allowed.

• Hematopoietic growth factors: ≥14 days after the last dose of a long-acting growth factor (e.g., pegfilgrastim) or ≥7 days for short-acting growth factor. For agents that have known adverse events occurring beyond 7 days after administration, this period must be extended beyond the time during which adverse events are known to occur.

• Interleukins, Interferons, and Cytokines (other than hematopoietic growth factors): ≥ 21 days after the completion of interleukins, interferon, or cytokines.

• Stem cell infusions (with or without TBI):

‣ Autologous stem cell infusion including boost infusion: ≥ 30 days.

• Cellular Therapy: ≥ 42 days after the completion of any type of cellular therapy (e.g., modified T cells, NK cells, dendritic cells, etc.)

• XRT/External Beam Irradiation, including Protons: ≥ 90 days after local XRT unless recurrence is a new enhancement on MRI outside the radiation treatment field; ≥ 150 days after TBI, craniospinal XRT or if radiation to ≥ 50% of the pelvis.

• Radiopharmaceutical therapy (e.g., radiolabeled antibody): ≥ 42 days after systematically administered radiopharmaceutical therapy.

• Other therapeutic clinical trials: ≥ 14 days after last dose of investigational agent, unless otherwise defined above.

• Prior use of RNA-LP therapy: Patients must not have received prior exposure to pp65-directed therapy or any RNA-LP therapy.

∙ Organ Function Requirements

∙ Adequate bone marrow function as defined as:

• Absolute neutrophil count (ANC) ≥ 1,000/µl

• Platelets ≥ 100,000/µl (transfusion-independent, defined as not receiving platelet transfusions for at least 7 days prior to enrollment)

• Hemoglobin ≥ 8 g/dL (transfusion-independent, defined as not receiving packed red blood cell transfusions for at least 7 days prior to enrollment)

∙ Adequate renal function as defined as:

• A creatinine based on age/gender

• Creatinine clearance or radioisotope GFR ≥ 70 mL/min/1.73 m2

∙ Adequate liver function as defined as:

• Total bilirubin ≤ 3x institutional upper limits of normal for age

• ALT ≤ 5x institutional upper limits of normal for age

• AST ≤ 5x institutional upper limits of normal for age Adequate pulmonary function defined as baseline pulse oximetry of at least 92% on room air.

∙ All patients must be willing to take an antiepileptic medication such as levetiracetam for the duration of RNA-LP vaccinations.

∙ Contraception

• Women of childbearing potential must agree to use of at least 2 forms of acceptable contraceptive methods or abstinence to avoid pregnancy throughout the study and for at least 24 weeks after the last dose of study drug.

• Men with female partners of childbearing potential must agree to use of at least 2 forms of acceptable contraceptive methods or abstinence throughout the study and should avoid conceiving children for at least 24 weeks following the last dose of study drug.

∙ All patients and/or their parents or legal guardians must have the ability to understand and the willingness to sign a written informed consent/assent document.

∙ Patients with osteosarcoma (OSA)

∙ Age: Patients must be age 3-39 years. Diagnosis

• For Arms 1 and 2: Patients must be clinically eligible for standard-of-care surgical resection of suspected OSA recurrence with pulmonary-only metastases.

• For Arm 3: Patients must undergo standard-of-care biopsy of suspected or known recurrent, unresectable OSA.

∙ Disease Status

• For Arm 1: Patients must be eligible for single-sided thoracotomy for planned surgical resection of all OSA pulmonary metastases.

• For Arm 2: Patients must be eligible for staged two-sided thoracotomies for planned surgical resection of all OSA pulmonary metastases.

• For Arm 3: Patients must have unresectable OSA. Patients must have sufficient disease on diagnostic contrast-enhanced MRI wherein surgical biopsy is feasible.

∙ Patients must have sterile collection of tumor material in a manner suitable for RNA extraction, amplification, and loading of lipid particles.

∙ Performance Level: Karnofsky ≥ 60% for patients \> 16 years of age and Lansky ≥ 60% for patients \< 16 years of age.

• NOTE: Participants who are unable to walk because of amputation, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing the performance score.

∙ Prior Therapy: Patients must have recovered from all acute toxic effects of all prior anti-cancer therapy (all adverse events must have improved to grade 1 or better):

⁃ Cytotoxic chemotherapy or other anti-cancer agents known to be myelosuppressive: ≥ 14 days after the last dose of myelosuppressive chemotherapy. If questions, the agent and duration can be discussed with the study chair.

⁃ Anti-cancer agents not known to be myelosuppressive (e.g., not associated with reduced platelet or ANC counts): ≥ 7 days after the last dose of agent. If questions, the agent and duration can be discussed with the study chair.

⁃ Antibodies: ≥ 21 days must have elapsed from infusion of last dose of antibody, and toxicity related to prior antibody therapy must be recovered to Grade ≤ 1.

⁃ Corticosteroids: All systemically administered corticosteroids must be stable or decreasing for ≥ 1 week prior to enrollment, with a maximum dexamethasone dose of 2.8 mg/m2/day. Corticosteroid physiologic replacement therapy for management of pituitary/adrenal axis insufficiency and/or topical administration (e.g. inhaled or dermatologic) is allowed.

⁃ Hematopoietic growth factors: ≥ 14 days after the last dose of a long-acting growth factor (e.g., pegfilgrastim) or ≥ 7 days for short-acting growth factor. For agents that have known adverse events occurring beyond 7 days after administration, this period must be extended beyond the time during which adverse events are known to occur.

⁃ Interleukins, Interferons, and Cytokines (other than hematopoietic growth factors): ≥ 21 days after the completion of interleukins, interferon, or cytokines.

⁃ Stem cell infusions (with or without TBI): Autologous stem cell infusion including boost infusion: ≥ 30 days.

⁃ Cellular Therapy: ≥ 42 days after the completion of any type of cellular therapy (e.g., modified T cells, NK cells, dendritic cells, etc.).

⁃ XRT/External Beam Irradiation, including Protons: ≥ 90 days after local XRT unless recurrence is a new enhancement on MRI outside the radiation treatment field; ≥ 150 days after TBI or if radiation to ≥ 50% of the pelvis; ≥ 42 days if other substantial BM radiation.

‣ Radiopharmaceutical therapy (e.g., radiolabeled antibody): ≥ 42 days after systematically administered radiopharmaceutical therapy.

‣ Other therapeutic clinical trials: ≥ 14 days after last dose of investigational agent, unless otherwise defined above

‣ Prior use of RNA-LP therapy: Patients must not have received prior exposure to pp65-directed therapy or any RNA-LP therapy.

∙ Organ Function Requirements

∙ Adequate bone marrow function as defined as:

• Absolute neutrophil count (ANC) ≥ 1,000/µl

• Platelets ≥ 100,000/µl (transfusion-independent, defined as not receiving platelet transfusions for at least 7 days prior to enrollment)

• Hemoglobin ≥ 8 g/dL (transfusion-independent, defined as not receiving packed red blood cell transfusions for at least 7 days prior to enrollment)

∙ Adequate renal function as defined as:

• A creatinine based on age/gender

• Creatinine clearance or radioisotope GFR ≥ 70 mL/min/1.73 m2

∙ Adequate liver function as defined as:

• Total bilirubin ≤ 3x institutional upper limits of normal for age

• ALT ≤ 5x institutional upper limits of normal for age

• AST ≤ 5x institutional upper limits of normal for age Adequate pulmonary function defined as baseline pulse oximetry of at least 92% on room air.

∙ Contraception

• Women of childbearing potential must agree to use of at least 2 forms of acceptable contraceptive methods or abstinence to avoid pregnancy throughout the study and for at least 24 weeks after the last dose of study drug.

• Men with female partners of childbearing potential must agree to use of at least 2 forms of acceptable contraceptive methods or abstinence throughout the study and should avoid conceiving children for at least 24 weeks following the last dose of study drug.

∙ All patients and/or their parents or legal guardians must have the ability to understand and the willingness to sign a written informed consent/assent document.