MTOR and glioblastoma: Research on polytherapies for GBM has proposed various combinations, such as pairing mTOR inhibitors with mitogen-activated protein kinase (MEK), cyclin-dependent kinase 4 and 6 (CDK4/6), mouse double minute 2 homolog (MDM2), signal transducer and activator of transcription 3 (STAT3), or growth hormone-releasing hormone (GHRH) inhibitors [35,36,37,38,39].