Tadalafil to Overcome Immunosuppression During Chemoradiotherapy for IDH-wildtype Grade III-IV Astrocytoma

Inclusion Criteria: * Histologically proven diagnosis of newly diagnosed supratentorial high-grade astrocytoma (WHO grade III-IV), excluding astrocytoma of brainstem and cerebellum. However, supratentorial astrocytoma with extension to the brainstem and cerebellum is allowed at discretion of the PI. Gliosarcoma or other subvariants are allowed, including the newly defined "diffuse astrocytoma, IDH-wildtype, with molecular features of glioblastoma, WHO grade IV" (Brat et al., 2018). * Must have recovered from the effects of surgery, postoperative infection, and other complications sufficiently that they can proceed with RT and TMZ. -≥ 18 years of age. * Eligible for and planning to receive standard fractionated RT of 60 Gy with concurrent TMZ. * Karnofsky performance status ≥ 60. * Available archival formalin-fixed paraffin-embedded (FFPE) tumor blocks. * Adequate organ and bone marrow function as defined below: * Absolute neutrophil count (ANC) ≥ 1,500 cells/mm3; * Platelets ≥ 100,000 cells/mm3; * Hemoglobin \> 9.0 g/dL (Note: the use of transfusion or other intervention to achieve Hgb \>9.0 g/dL is acceptable); * Total bilirubin ≤ 1.5 upper limit of normal (ULN) * AST (SGOT) and ALT (SGPT) ≤ 3 x ULN * Creatinine ≤ 1.5 ULN or creatinine clearance ≥ 60 mL/min * If there is history of human immunodeficiency virus (HIV) infection, patients must be on effective antiretroviral therapy, and HIV viral load must be undetectable within 6 months of study enrollment. * If there is history of chronic hepatitis B virus (HBV) infection, patients must have either been treated or are on suppressive therapy (as indicated), and HBV viral load must be undetectable. * If there is history of hepatitis C virus (HCV) infection, patients must have been treated, and HCV viral load must be undetectable. * Females of childbearing potential (defined as a female who is non-menopausal or surgically sterilized) must be willing to use an acceptable method of birth control (i.e., hormonal contraceptive, intra-uterine device, diaphragm with spermicide, condom with spermicide, or abstinence) for the duration of the study. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. * Able to understand and willing to sign an IRB-approved written informed consent document (legally authorized representative permitted). Exclusion Criteria: * Prior cranial RT or RT to the head and neck where potential field overlap may exist * Gliomatosis, leptomeningeal, or metastatic involvement. * High-grade glioma with known IDH mutation. IDH status could be determined by either immunohistochemistry (IDH1-R132H mutation) or sequencing (including other uncommon variants of IDH1 and IDH2 mutations) as evaluated routinely for clinical diagnosis using a CLIA-approved assay. * Known severe hypersensitivity to tadalafil or other PDE5 inhibitors, including history of hypotension, priapism (painful erection \> 4 hours duration), blindness, or hearing loss during prior treatment with tadalafil or other PDE5 inhibitors. * Concurrent nitrate, alpha-blocker, guanylate cyclase stimulators (eg, riociguat), or cytochrome P-450 3A4 (CYP3A4) inhibitor use. CYP3A4 inhibitors include ketoconazole, itraconazole, and ritonavir. * Severe, active co-morbidity, defined as follows: * Unstable angina, angina requiring treatment with nitrates, positive cardiac stress test without evidence of subsequent effective cardiac intervention within 90 days of planned tadalafil administration * Myocardial infarction, coronary artery bypass graft surgery, or percutaneous coronary angioplasty or stent within the 90 days of planned tadalafil administration * New York Heart Association grade II or greater congestive heart failure within 6 months * Serious and inadequately controlled arrhythmia * Hypotension (\<90/50 mm Hg) or uncontrolled hypertension (\>170/100 mm Hg) * Left ventricular outflow obstructions, such as aortic stenosis * Stroke within the last 6 months * Acute bacterial or fungal infection requiring intravenous antibiotics. * Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects; note, however, that laboratory tests for coagulation parameters are not required for entry into this protocol. * Active peptic ulcer disease. * End-stage renal disease (ie, on dialysis or dialysis has been recommended). * Unilateral blindness, hereditary retinal disorder, including retinitis pigmentosa. * Patients treated on any other therapeutic clinical protocols within 30 days prior to registration. * Inability to undergo contrast-enhanced MRI (e.g., due to safety reasons, such as presence of a pacemaker, or severe claustrophobia). * Pregnant or breastfeeding. Women of childbearing potential must have a negative pregnancy test within 14 days of study entry. * Patients with psychiatric illness/social situations, including alcohol or drug abuse that in the investigator's opinion will prevent administration or completion of protocol therapy.