Pembrolizumab and Vorinostat Combined With Temozolomide for Newly Diagnosed Glioblastoma

200 mg intravenously (IV) every 3 weeks. The dose of pembrolizumab will remain the same throughout study treatment. During the maintenance phase, participants will receive pembrolizumab (for 12 months).

Dose Escalation Level -1 vorinostat 100 mg/day by mouth on days 1-5 every week during radiotherapy starting on first day of radiotherapy and 300 mg/day 1 week on 1 week off after radiotherapy. Level 1:vorinostat 200 mg/day by mouth on day 1-5 every week during radiotherapy, starting on first day of radiotherapy; and 300 mg/day 1 week on 1 week off after radiotherapy. Dose Escalation Level 2: vorinostat 300 mg/day by mouth on days 1-5 every week during radiotherapy, starting on first day of radiotherapy; and 400 mg/day 1 week on 1 week off after radiotherapy.

All participants will receive standard Temozolomide: Temozolomide (chemotherapy) 75 mg/m\^2/day by mouth administered during the course of radiotherapy. Temozolomide will be administered continuously from Day 1 of radiotherapy to the last day of radiation. Maintenance Phase: temozolomide will start 4 weeks (+/- 3 days) after last dose of radiotherapy and will continue for 6 cycles post radiotherapy (150-200 mg/m\^2/day, days 1-5 every 4 weeks) as per standard of care. During the maintenance phase, participants will receive Temozolomide (for the first 6 months).

All participants will receive standard radiotherapy: a total dose of 60 Gy administered in daily doses of 2 Gy, typically on a 5 days on / 2 days off schedule over 6 - 7 weeks.