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Effect of Chemotherapy-Induced DNA
Repair on Oncolytic Herpes Simplex Viral Replication
Manish Aghi, Samuel
Rabkin, Robert L. Martuza
Affiliation of authors: Department
of Neurosurgery, Massachusetts General Hospital, Harvard Medical
School, Boston, MA.
Correspondence to: Manish Aghi, MD, PhD, Brain Tumor Research Center,
Massachusetts General Hospital–Simches Research Bldg., 185 Cambridge
St., CPZN-3800, Boston, MA 02114 (e-mail: maghi@partners.org
).
Background. Gliomas
treated with the alkylating agent temozolomide have
incomplete responses in part because of tumoral repair of
chemotherapy-induced DNA damage.
Data from phase I trials suggest that G207, an oncolytic
herpes simplex virus (HSV) with mutated ribonucleotide
reductase (RR) and γ34.5 genes, is safe but needs
greater viral oncolysis to be effective.
We hypothesized that temozolomide and G207 treatment
limitations could be jointly addressed using
temozolomide-induced tumor-protective DNA repair pathways
to enhance viral replication.
Methods. Human
glioblastoma cells (U87, T98, and U373) and U87 cells
transfected with the gene for the DNA repair enzyme O6-methylguanine
DNA methyltransferase (MGMT) were treated with G207 and/or
temozolomide.
Drug interactions, expression of the growth arrest DNA
damage 34 (GADD34) and RR transcripts before and after
their knockdown with short interfering RNAs, DNA strand
breaks, and apoptosis were measured using Chou–Talalay analysis,
real-time reverse transcription–polymerase chain reaction,
the comet assay, and flow cytometry, respectively.
Survival of mice (groups of ten) with intracranial U87 xenograft tumors
treated with temozolomide and/or G207 was analyzed using Kaplan–Meier
analysis.
Results. Temozolomide
exhibited strong synergy with G207 in both MGMT-negative
and the MGMT inhibitor O6-benzylguanine–treated
MGMT-expressing gliomas (Chou–Talalay combination indices
= 0.005 to 0.39) and induced GADD34 expression primarily in
nonapoptotic MGMT-negative U87 glioma cells (fold
difference = 16, 95% confidence interval [CI] = 12.6 to
20.4, compared with untreated cells).
MGMT-expressing T98 and U87/MGMT cells treated with
temozolomide plus O6-benzylguanine had higher RR
expression than untreated cells (fold difference =14.9, 95%
CI = 10.1 to 22.0 [T98]; 9.9, 95% CI = 7.0 to 13.8 [U87/MGMT]).
GADD34 and RR knockdown increased temozolomide-induced DNA
damage and inhibited the synergy of G207 and temozolomide in
U87 and O6-benzylguanine–treated U87/MGMT cells.
Mice bearing intracranial U87 tumors survived longer after
combination therapy (100% survival at 90 days) than after
single-agent therapy (median survival = 46 and 48 days with
G207 and temozolomide treatment, respectively).
Conclusions.
Temozolomide-induced DNA repair pathways vary with MGMT
expression and enhance HSV-mediated oncolysis in glioma
cells.
These findings unveil the potential of HSV to target cells
surviving temozolomide treatment.
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