title: Overcoming multiple drug resistance mechanisms in medulloblastoma.
abstract: BACKGROUND
  Medulloblastoma (MB) is the most common malignant paediatric brain tumour. Recurrence and progression of disease occurs in 15-20% of standard risk and 30-40% of high risk patients. We analysed whether circumvention of chemoresistance pathways (drug export, DNA repair and apoptotic inhibition) can restore chemotherapeutic efficacy in a panel of MB cell lines.
  
  
  RESULTS
  We demonstrate, by immunohistochemistry in patient tissue microarrays, that ABCB1 is expressed in 43% of tumours and is significantly associated with high-risk. We show that ABCB1, O6-methylguanine-DNA-methyltransferase (MGMT) and BCL2 family members are differentially expressed (by quantitative reverse transcription polymerase chain reaction, Western blotting and flow cytometry) in MB cell lines. Based on these findings, each pathway was then inhibited or circumvented and cell survival assessed using clonogenic assays. Inhibition of ABCB1 using vardenafil or verapamil resulted in a significant increase in sensitivity to etoposide in ABCB1-expressing MB cell lines. Sensitivity to temozolomide (TMZ) was MGMT-dependent, but two novel imidazotetrazine derivatives (N-3 sulfoxide and N-3 propargyl TMZ analogues) demonstrated ≥7 fold and ≥3 fold more potent cytotoxicity respectively compared to TMZ in MGMT-expressing MB cell lines. Activity of the BAD mimetic ABT-737 was BCL2A1 and ABCB1 dependent, whereas the pan-BCL2 inhibitor obatoclax was effective as a single cytotoxic agent irrespective of MCL1, BCL2, BCL2A1, or ABCB1 expression.
  
  
  CONCLUSIONS
  ABCB1 is associated with high-risk MB; hence, inhibition of ABCB1 by vardenafil may represent a valid approach in these patients. Imidazotetrazine analogues of TMZ and the BH3 mimetic obatoclax are promising clinical candidates in drug resistant MB tumours expressing MGMT and BCL2 anti-apoptotic members respectively.
authors.forename: Ramadhan T
authors.initials: RT
authors.lastname: Othman
authors.forename: Ioanna
authors.initials: I
authors.lastname: Kimishi
authors.forename: Tracey D
authors.initials: TD
authors.lastname: Bradshaw
authors.forename: Lisa C D
authors.initials: LC
authors.lastname: Storer
authors.forename: Andrey
authors.initials: A
authors.lastname: Korshunov
authors.forename: Stefan M
authors.initials: SM
authors.lastname: Pfister
authors.forename: Richard G
authors.initials: RG
authors.lastname: Grundy
authors.forename: Ian D
authors.initials: ID
authors.lastname: Kerr
authors.affiliation: Children's Brain Tumour Research Centre, Division of Child Health, Obstetrics and Gynaecology, School of Medicine, University of Nottingham, QMC, Nottingham NG7 2UH, UK. beth.coyle@nottingham.ac.uk.
authors.forename: Beth
authors.initials: B
authors.lastname: Coyle
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000378:metabolism; D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects; D002478:Cells, Cultured; D002528:Cerebellar Neoplasms / Q000473:pathology*; D002648:Child; D002675:Child, Preschool; D015254:DNA Modification Methylases / Q000378:metabolism*; D045643:DNA Repair Enzymes / Q000378:metabolism*; D003606:Dacarbazine / Q000031:analogs & derivatives / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007211:Indoles; D008297:Male; D008527:Medulloblastoma / Q000473:pathology*; D015778:Minor Histocompatibility Antigens; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D011758:Pyrroles / Q000494:pharmacology; D000077204:Temozolomide; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins / Q000378:metabolism*
chemical_list: C513055:ABCB1 protein, human; D018435:ATP Binding Cassette Transporter, Subfamily B; D018906:Antineoplastic Agents, Alkylating; C096816:BCL2-related protein A1; D004791:Enzyme Inhibitors; D007211:Indoles; C579095:MCL1 protein, human; D015778:Minor Histocompatibility Antigens; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019253:Proto-Oncogene Proteins c-bcl-2; D011758:Pyrroles; D025521:Tumor Suppressor Proteins; D003606:Dacarbazine; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D045643:DNA Repair Enzymes; C520962:obatoclax; D000077204:Temozolomide