ATP-Binding Cassette Transporters/genetics; Brain Neoplasms/pathology; DNA Methylation/drug effects; Drug Resistance, Multiple/drug effects; Drug Resistance, Neoplasm/drug effects; Glioblastoma/pathology; Melatonin/pharmacology; Neoplasm Proteins/genetics; Neoplastic Stem Cells/drug effects; ATP Binding Cassette Transporter, Subfamily G, Member 2; ATP-Binding Cassette Transporters/metabolism; ATP-Binding Cassette Transporters/physiology; Antineoplastic Agents/administration & dosage; Antineoplastic Agents/pharmacology; Brain/drug effects; Brain/metabolism; Brain/pathology; Brain Neoplasms/genetics; Cell Line, Tumor; DNA Methylation/physiology; Drug Evaluation, Preclinical; Drug Resistance, Multiple/genetics; Drug Resistance, Neoplasm/genetics; Drug Synergism; Gene Expression Regulation, Neoplastic/drug effects; Glioblastoma/genetics; Humans; Melatonin/administration & dosage; Neoplasm Proteins/metabolism; Neoplasm Proteins/physiology; Neoplastic Stem Cells/metabolism; Neoplastic Stem Cells/pathology; Neoplastic Stem Cells/physiology; Promoter Regions, Genetic/drug effects