Biology Reference
In-Depth Information
endogenously-generated DNA single-strand breaks go
unrepaired when PARP is inhibited, leading to stalled
replication forks and replication-associated DNA
double-strand breaks. In normal cells these lesions are
repaired and replication is re-started by the process of
HR but in BRCA1/2-defective cells the lesions persist
and the cell dies. Carriers of BRCA1/2 mutations have
one functional allele, and can therefore conduct HR in
normal tissues, but in the tumor cells the second allele
is lost either through LOH or a second mutation,
rendering them defective in HR. These studies were
very exciting because they indicated for the first time
that inhibition of PARP could be selectively toxic to at
least a sub-set of tumors.
The studies described above were conducted in
hamster and mouse cells where the BRCA status had
been altered by mutagenesis or genetic manipulation
and it does not necessarily follow that human cancers,
which have evolved to survive despite a defect in
BRCA1/2, will be equally sensitive. Indeed, results
from studies examining the sensitivity of human cancer
cell lines to PARP inhibitors have been conflicting. For
example, De Soto
et al.
found that human
BRCA1
-posi-
tive,
BRCA1
heterozygous, and
BRCA1
-negative breast
cancer cell lines were not selectively sensitive to
three different PARP inhibitors (3-aminobenzamide,
NU1025, and AG14361).
138
In addition, Gallmeier
et al.
demonstrated that the
BRCA2
-defective human pancre-
atic cancer CAPAN-1 cells were not sensitive to the
PARP inhibitor NU1025 (139). In contrast to these data,
a recent study using a panel of nine human cancer cell
lines with differing BRCA status; including the pancre-
atic carcinoma Capan-1 cells and the BRCA1 defective
HCC1937 breast cancer cell line used in the De Soto
study did show that the PARP inhibitor AG014699 was
selectively toxic to cell lines and xenograft models
with defective BRCA 1 and 2.
140
Interestingly this study
also demonstrated that the BRCA1 methylation silenced
breast cancer UACC3199 cell line was also sensitive to
PARP inhibition suggesting a wider role for these agents
in non-BRCA mutated cancers, and there is emerging
clinical evidence to confirm this with the observation
by Gelmon
et al.
that responses to the PARP inhibitor,
olaparib, were observed in patients with high grade
serous ovarian cancer without a BRCA-defective
background.
141
This observation was first tested clinically with ola-
parib. The phase I study of this agent had an expanded
cohort of patients with known germ line mutations in
BRCA1 or BRCA2 genes, and presumed loss of the
second allele as a tumor-forming event.
142
This study
used an oral formulation of the compound and explored
dosing from 10 mg daily for two out of three weeks,
increasing to 600 mg twice daily on a continuous dosing
schedule to achieve optimal PK and PD parameters.
Dose-limiting toxicities were myelosuppression and
central nervous system side effects. The recommended
phase II dose was 400 mg twice daily as continuous
dosing. Nine patients developed confirmed partial
responses on this phase I study, all of these had
confirmed BRCA mutations, and this represented
a 39% response rate (9/23) in this population. Toxicities
were similar in the BRCA mutated and normal popula-
tion, so there was no suggestion that normal tissue
toxicity night be worse in germ line mutation carriers.
The investigators also demonstrated an increase in
g
-H2AX foci in plucked eyebrow hair follicles 6 hours
after olaparib treatment. These foci indicate the accumu-
lation of DNA double-strand breaks, indicating a proof
of mechanism of the process of
synthetic lethality
where
preservation of SSB by PARP inhibition leads to the
formation of DNA DSB. It must be noted that this mech-
anistic proof was demonstrated in normal tissue not in
the tumor. It is not clear from the publication whether
this effect was seen in BRCA carriers with a heterozygote
defect in all cells, or in all patients groups. The data do
raise a concern over the potential dangers of continuous
dosing over a long period if there is accumulation of
DNA damage within normal tissue.
Two phase II studies of olaparib in BRCA1 or BRCA2
mutant carriers in breast and ovarian cancer respectively
have confirmed these data. Both these studies explored
response and toxicity in two sequential cohorts of
patients treated with 400 mg twice daily and 100 mg
twice daily. The activity as a single agent was confirmed
in the 400 mg cohorts but there was less activity in the
lower dose cohort, suggesting that the degree of PARP
inhibition is important for response. In the study
reported by Dr Tutt and colleagues, 27 patients with
metastatic breast cancer were treated at each of the
doses. The response rate in the 400 mg cohort was
41%, falling to 22% with the lower dose.
143
Toxicities
were less on the lower dose cohort but mild overall
with fatigue, nausea and vomiting the commonest toxic-
ities with this well-tolerated agent. This dose response
was confirmed in the ovarian study where 33 patients
were treated at 400 mg bd and 24 at 100 mg bd with
a 33% confirmed partial response rate at the higher
dose and 13% at the lower dose.
144
Responses were
seen in both BRCA 1 and BRCA 2 carriers, and in
patients with both platinum sensitive and resistant
Summary of Single Agent Clinical Trials
The exciting promise of synthetic lethality to HR-
defective tumors, as well as the chemo- and radio-poten-
tiation demonstrated in the preclinial arena led to
a significant number of clinical trials of PARPi both as
single agent in selected populations and in combination
studies.
