Targeting Homologous Recombination by Pharmacological Inhibitors Enhances the Killing Response of Glioblastoma Cells Treated with Alkylating Drugs

Mol Cancer Ther. 2016 Nov;15(11):2665-2678. doi: 10.1158/1535-7163.MCT-16-0176. Epub 2016 Jul 29.


Malignant gliomas exhibit a high level of intrinsic and acquired drug resistance and have a dismal prognosis. First- and second-line therapeutics for glioblastomas are alkylating agents, including the chloroethylating nitrosoureas (CNU) lomustine, nimustine, fotemustine, and carmustine. These agents target the tumor DNA, forming O6-chloroethylguanine adducts and secondary DNA interstrand cross-links (ICL). These cross-links are supposed to be converted into DNA double-strand breaks, which trigger cell death pathways. Here, we show that lomustine (CCNU) with moderately toxic doses induces ICLs in glioblastoma cells, inhibits DNA replication fork movement, and provokes the formation of DSBs and chromosomal aberrations. Since homologous recombination (HR) is involved in the repair of DSBs formed in response to CNUs, we elucidated whether pharmacologic inhibitors of HR might have impact on these endpoints and enhance the killing effect. We show that the Rad51 inhibitors RI-1 and B02 greatly ameliorate DSBs, chromosomal changes, and the level of apoptosis and necrosis. We also show that an inhibitor of MRE11, mirin, which blocks the formation of the MRN complex and thus the recognition of DSBs, has a sensitizing effect on these endpoints as well. In a glioma xenograft model, the Rad51 inhibitor RI-1 clearly enhanced the effect of CCNU on tumor growth. The data suggest that pharmacologic inhibition of HR, for example by RI-1, is a reasonable strategy for enhancing the anticancer effect of CNUs. Mol Cancer Ther; 15(11); 2665-78. ©2016 AACR.

MeSH terms

  • Animals
  • Antineoplastic Agents, Alkylating / pharmacology*
  • Apoptosis / drug effects
  • Apoptosis / genetics
  • Cell Line, Tumor
  • Cell Survival / drug effects
  • Chromosome Aberrations / drug effects
  • DNA Damage
  • DNA Modification Methylases / metabolism
  • DNA Repair
  • DNA Repair Enzymes / metabolism
  • DNA Replication / drug effects
  • DNA-Binding Proteins / metabolism
  • Disease Models, Animal
  • Glioblastoma / drug therapy
  • Glioblastoma / genetics
  • Glioblastoma / metabolism
  • Homologous Recombination / drug effects*
  • Humans
  • Lomustine / pharmacology
  • MRE11 Homologue Protein
  • Mice
  • Rad51 Recombinase / metabolism
  • Tumor Suppressor Proteins / metabolism
  • Xenograft Model Antitumor Assays


  • Antineoplastic Agents, Alkylating
  • DNA-Binding Proteins
  • MRE11 protein, human
  • Tumor Suppressor Proteins
  • Lomustine
  • DNA Modification Methylases
  • MGMT protein, human
  • Rad51 Recombinase
  • MRE11 Homologue Protein
  • DNA Repair Enzymes