Combination Adenovirus + Pembrolizumab to Trigger Immune Virus Effects

In the initial phase of the study, up to 12 evaluable subjects will be enrolled in 3 dose cohorts to determine the best dose of DNX-2401, as follows: * Cohort 1: Single dose DNX-2401 (5e8 vp) delivered intratumorally by cannula, followed by intravenous pembrolizumab every 3 weeks (Q3W) * Cohort 2: Single dose DNX-2401(5e9 vp) delivered intratumorally by cannula, followed by intravenous pembrolizumab every 3 weeks (Q3W) * Cohort 3: Single dose DNX-2401 (5e10 vp) delivered intratumorally by cannula, followed by intravenous pembrolizumab every 3 weeks (Q3W) Following the initial phase, up to 36 additional subjects diagnosed with recurrent glioblastoma or gliosarcoma will be enrolled to receive a single of DNX-2401 determined in the initial phase administered intratumorally followed by intravenous pembrolizumab every 3 weeks. All subjects will return to the clinic for study follow-up visits at regular intervals for safety monitoring, MRI scans and other assessments, for up to 2 years or until disease progression. All subjects will be followed closely for safety and survival.