This Phase 2 single arm trial in patients with rGBM will characterize the efficacy, safety, tolerability and initial efficacy of lerapolturev intratumoral infusion followed by intravenous pembrolizumab 14 to 28 days later, and every 3 weeks, thereafter.

Inclusion Criteria:

1. ≥ 18 years of age.

2. Recurrent supratentorial glioblastoma (GBM) confirmed via prior histology by the site's neuropathologist or designate.

   • Histologically confirmed recurrent glioblastoma (rGBM) within 6 weeks of Lerapolturev infusion will not require a biopsy to confirm active tumor prior to catheter placement
   • Progression of primary glioblastoma or transformation from a lower grade to a higher grade is acceptable for recurrence and as for primary glioblastoma, must be confirmed via prior histology by site pathologist

3. Enhancing lesion ≥1 cm shortest diameter to ≤ 5.5 cm longest diameter in all planes.

4. Before catheter placement based on screening MRI and at the time of catheter placement via CT prior to infusion, neurosurgical investigator must confirm placement of infusion catheter within or through the progressive enhancing tumor is feasible and at a safe distance relative to eloquent brain function, with the tip of the catheter being placed:

   1. Within the enhancing portion or in the vicinity of enhancement of target lesion (ie, infiltrative disease).
   2. ≥ 0.5 cm from ventricles.
   3. ≥ 1 cm deep into the brain.
   4. ≥ 0.5 cm from corpus callosum.

5. First or second relapse supported by MRI or CT scan; relapse is defined as progression following initial/prior therapy(ies).

6. Failed previous first line therapy: maximum surgical resection and radiotherapy (RT) (plus concomitant chemotherapy followed by maintenance chemotherapy if unknown or methylated O6-methylguanine-DNA methyltransferase (MGMT) promoter). Patients who begin but do not complete chemotherapy/RT may still be considered for eligibility at the discretion of Sponsor.

7. Karnofsky Performance Status ≥ 70 at screening and baseline.

8. Undergone prior vaccination against PV and received a boost immunization with trivalent inactivated poliovirus vaccine (IPOL®) at least 1 week, but less than 6 weeks, prior to administration of Lerapolturev. Note: Patients who are unsure of their prior vaccination status/who have not been vaccinated must provide proof of vaccination and/or evidence of anti-PV immunity prior to enrollment, as applicable.

9. Ability to safely discontinue anti-coagulant therapy(ies) prior to biopsy/catheter placement, as required per site/surgical guidelines.

10. Hemoglobin ≥ 9 g/dL prior to biopsy/catheter placement.

11. Platelet count ≥ 100,000/μL (unsupported); ≥ 125,000/ μL (can be supported via platelet transfusion) at biopsy/catheter placement.

12. Absolute Neutrophil Count (ANC) ≥ 1000/μL prior to biopsy/catheter placement.

13. Creatinine ≤ 1.2 x Upper Limit of Normal (ULN) prior to biopsy/catheter placement.

14. Total bilirubin, Alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) ≤ 2.5 x ULN prior to biopsy/catheter placement.

15. Prothrombin time (PT) and activated Partial Thromboplastin Time (aPTT) ≤ 1.2 x ULN prior to biopsy/catheter placement.

16. If undetectable antitetanus toxin (ATT) Immunoglobulin G (IgG) at screen, Tdap booster vaccine ≥ 1 week prior to biopsy/catheter placement.

17. Patients must be willing and able to understand and provide written informed consent.

Exclusion Criteria:

1. Received prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti- CTLA-4 antibody (including ipilimumab or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways) ≤ 12 weeks prior to lerapolturev infusion (Note: does not apply for patients treated with pembrolizumab under this protocol who are eligible for lerapolturev retreatment). Note: patients who had previously permanently discontinued any anti-PD-1 or PD-L1 therapy due to severe or life-threatening immune-related adverse events are excluded.

2. Excluded are:

   1. Neoplastic lesions in the brainstem, cerebellum, or spinal cord.
   2. Radiological evidence of active/growing multifocal disease: no size increase > 0.5 cm in any direction of any other enhancing non-target lesions present at baseline confirmed via most recent, prior, consecutive MRIs at least 3 months apart.
   3. Tumors with ≥ 1cm of contrast-enhancing tumor component crossing the midline (crossing the corpus callosum).
   4. Extensive subependymal disease: multiple lesions or lesions covering > 50% of subependymal space. Tumor touching subependymal space allowed.
   5. Extensive leptomeningeal disease: multiple lesions or lesions covering > 50% of leptomeninges. Tumor touching leptomeninges allowed.

3. Has received systemic immunosuppressive treatments other than systemic corticosteroids (eg, methotrexate, chloroquine, azathioprine) within six months of Lerapolturev infusion.

4. Requires treatment with high dose systemic corticosteroids, defined as dexamethasone > 4 mg/day or equivalent, within 2 weeks of Lerapolturev infusion.

5. Prior interstitial brachytherapy, implanted chemotherapy, stereotactic radiosurgery or therapeutics delivered by local injection or CED, including Lerapolturev (except for qualifying patients being retreated with Lerapolturev within this trial).

6. Pregnant and/or breast feeding female; patient/female partner of childbearing potential who is unwilling to utilize protocol-defined acceptable form of contraception for duration of study.

7. Impending/life-threatening cerebral herniation syndrome, per neurosurgeon/designate.

8. Severe, active co-morbidity, defined as follows:

   1. Infection requiring IV treatment/unexplained febrile illness (Tmax > 99.5°F/37.5°C)
   2. Known immunosuppressive disease/human immunodeficiency virus infection
   3. Known active hepatitis B or C infection via positive viral DNA or RNA, respectively
   4. Unstable or severe intercurrent medical conditions such as severe heart disease (New York Heart Association Class 3 or 4)
   5. Known lung disease with forced expiratory volume in 1st second of expiration < 50%
   6. Uncontrolled diabetes mellitus (eg, hemoglobin A1C level > 7.0% with treatment)
   7. History of other malignancy requiring active treatment within 2 years of biopsy/catheter placement with the exception of those with a negligible risk of metastasis or death (eg, resected cutaneous basal cell carcinoma, or other cancers with 5-year overall survival (OS) of >90%)

9. Known albumin allergy.

10. Uncontrolled unexplained bleeding and/or hemoptysis within 4 weeks of planned lerapolturev infusion.

11. Inability to undergo brain MRI with and without contrast. History of severe/anaphylactic reaction to gadolinium contrast agent is excluded. Mild allergy (eg, rash) acceptable with prophylactic acetaminophen and diphenhydramine.

12. History of neurological complications due to PV infection.

13. Not recovered from toxic side effects (alopecia acceptable) and/or no current or prior tumor treatments within the following timeframe relative to biopsy/catheter placement:

    1. Chemotherapy or bevacizumab ≤ 4 weeks (except for nitrosourea (6 weeks) or metronomic dosed chemotherapy/targeted therapies such as daily temozolomide, etoposide or cyclophosphamide (1 week)).
    2. Tumor treating fields ≤ 7 days.
    3. RT of brain ≤ 12 weeks, except for progressive disease outside of the radiation field or 2 progressive scans at least 4 weeks apart or histopathologic confirmation.

14. History of agammaglobulinemia.

15. Known hypersensitivity to pembrolizumab, or any components of pembrolizumab.

16. Active autoimmune disease requiring systemic immunomodulatory treatment within the past 12 months; physiologic replacement therapy (eg, thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment.

17. History of other malignancy requiring active treatment within 2 years of biopsy/catheter placement with the exception of those with a negligible risk of metastasis or death (eg, resected cutaneous basal cell carcinoma, or other cancers with 5-year OS of >90%)