LAUSR

Simple Summary Glioblastoma (GBM) is the most common and aggressive adult-type diffusely infiltrating glioma. These tumors invariably develop resistance to standard treatment with radiation and temozolomide, leading to recurrence and almost always fatal outcomes. In vivo models of such recurrences are limited, and new therapies for recurrent GBM are usually tested on therapy-naïve preclinical models, which do not accurately predict outcomes in clinical trials. Experimental therapies which are effective against therapy-naïve tumor models in mice often fail to achieve survival benefit in patients with recurrent, therapy-resistant GBMs. In this study, we developed multiple treatment-resistant GBM models by exposing patient-derived xenografts (PDX) of GBM to radiation and temozolomide. These therapy-resistant PDX reflect key genetic and phenotypic features of recurrent GBM in patients. These PDX models are stable and expandable, and can serve as a valuable tool for testing new therapies in a setting that more accurately models GBM that recurs after front-line therapy. Abstract Adult-type diffusely infiltrating gliomas, of which glioblastoma is the most common and aggressive, almost always recur after treatment and are fatal. Improved understanding of therapy-driven tumor evolution and acquired therapy resistance in gliomas is essential for improving patient outcomes, yet the majority of the models currently used in preclinical research are of therapy-naïve tumors. Here, we describe the development of therapy-resistant IDH-wildtype glioblastoma patient-derived xenografts (PDX) through orthotopic engraftment of therapy naïve PDX in athymic nude mice, and repeated in vivo exposure to the therapeutic modalities most often used in treating glioblastoma patients: radiotherapy and temozolomide chemotherapy. Post-temozolomide PDX became enriched for C>T transition mutations, acquired inactivating mutations in DNA mismatch repair genes (especially MSH6), and developed hypermutation. Such post-temozolomide PDX were resistant to additional temozolomide (median survival decrease from 80 days in parental PDX to 42 days in a temozolomide-resistant derivative). However, temozolomide-resistant PDX were sensitive to lomustine (also known as CCNU), a nitrosourea which induces tumor cell apoptosis by a different mechanism than temozolomide. These PDX models mimic changes observed in recurrent GBM in patients, including critical features of therapy-driven tumor evolution. These models can therefore serve as valuable tools for improving our understanding and treatment of recurrent glioma.