e14022 Background: Novel cytotoxic drugs are commonly first studied in patients with solid tumors who have recurrent disease. If high response rates and durable remissions are observed, further studies are conducted in previously untreated cancers where the novel agent can be combined with radiation or administered as neoadjuvant therapy (NAT) to reduce tumor size before surgery or radiation and to limit systemic dissemination. While NAT can provide critical information on response rates and survival in previously untreated patients, it can add toxicities and delay the administration of standard therapies. Neoadjuvant chemotherapy is now standard-of-care in many systemic cancers but is rarely considered in patients with glioblastoma (GBM). This literature review was conducted to describe factors leading to NAT playing an increasingly important role in systemic cancers and a diminishing role in patients with GBM. Methods: Prospective trials published between 1980 and 2023 where patients with newly diagnosed systemic cancers and GBM received NAT prior to radiation were identified in PubMed. Results: During these 43 years, over 5,000 NAT trials in patients with systemic cancers were published. As a result of these studies, NAT is now considered standard therapy for 28 different cancers. In contrast, during the same years only 51 prospective studies were reported evaluating the efficacy of NAT in newly diagnosed GBM. These clinical trials accrued a total of 2,047 patients and evaluated the efficacy of nitrosoureas, temozolomide, platinum-based agents, and targeted therapies. These studies peaked before 2005, when combining temozolomide and radiation was shown to improve survival, and declined thereafter. NAT temozolomide was studied in 12 trials (641 patients) resulting in a median response rate of 35% (range 8.7-51%) and a median overall survival of 12.5 (range 6-22) months. NAT trials in patients with GBM were not associated with unexpected toxicities, major reductions in survival, or poor patient accrual. Conclusions: As in other solid tumors, NAT in patients with GBM is relatively safe and feasible. Enthusiasm for this approach has been limited by the paucity of active agents in recurrent glioblastoma that could be subsequently evaluated in newly diagnosed patients. However, the neoadjuvant setting is ideally suited to rapidly screening novel cytotoxic agents for efficacy as it allows: 1) radiographic response rates in previously untreated patients to be the primary endpoint, 2) small sample sizes, and 3) efficacy results in 2-3 months when patients are transitioned to standard care. Given the well documented safety and feasibility history of using NAT in patients with solid tumors and GBM, this approach has the ability to rapidly identify potentially active, novel cytotoxic agents, especially in patients with MGMT unmethylated, IDH wildtype GBM who are unlikely to benefit from temozolomide.