Targeting DNA Repair Pathway In The Treatment Of Breast Cancer:Preclinical And Clinical Study

Enhanced cytotoxicity of PARP inhibition in breast cancer cells by ATM inhibitorRecent years’study have shown promising activity of Poly(ADP-ribose) polymerase (PARP) inhibitors in patients with BRCA1/2 mutation-associated ovarian and breast cancers. The mechanism underlying this strategy is based on the concept of synthetic lethality, which centers on targeting two separate molecular pathways that are nonlethal when disrupted independently, but are lethal when inhibited simultaneously. BRCAl and BRCA2 both function in the homologous recombination (HR) pathway for the repair of DNA double strand breaks (DSBs), while PARP-1 is a key mediator in base excision repair (BER) pathway that repairs single strand breaks (SSBs). PARP-1 deficiency leads to the persistence of SSBs, which converted to DSBs that are eventually repaired by the error free HR pathway at the replication fork. Consequently, cells that are deficient in BRCA1 or BRCA2 have defective HR and are highly sensitive to PARP-1 inhibition due to the persistence of unrepaired DSBs. Loss of 53BP1 can partially restore the HR defect in BRCA1-deleted cells and reverts their hypersensitivity to DNA-damaging agents including PARP inhibitors. ATM is a key regulator of the cellular response to genotoxic stress, in particular DNA DSBs. Following DNA damage, ATM phosphorylates multiple downstream effector proteins including the checkpoint kinase Chk2, DNA repair factors including BRCA1 and critical transcriptional regulators such as p53. ATM is frequently altered or deleted in many solid tumors and hematologic malignancies. Since studies have shown that ATM is also required for a functional HR pathway, we sought to examine whether ATM inhibition in breast cancer cell lines confers sensitivity to PARP inhibitors. We also sought to investigate the role of 53BP1 in the synergy of ATM and PARP pathway.Breast cancer cell lines were assayed for cytotoxicity upon treatment with PARP inhibitor Olaparib and ATM inhibitor Ku-55933. Ku-55933 sensitized both triple-negative and non-triple-negative breast cancer cell lines to Olaparib treatment, as assessed by MTS, colony formation and apoptosis assays. Interestingly, the synergistic effect seems to be more prominent in triple-negative cell lines. Additionally, Co-treatment of Olaparib and Ku-55933 inducesy-H2AX foci formation in MCF-7 and CAL-51 cell lines, indicating the formation of DSBs. To investigate the role of 53BP1 in the synergy of ATM and PARP, we used shRNA lentiviral vectors to knockdown 53BP1 expression in MCF-7 and CAL-51 cell lines. Upon 53BP1 knockdown, the synergistic cytotoxicity of Olaparib and Ku-55933 was reduced, as assessed by MTS and colony formation assays.Taken together, Ku-55933 enhanced Olaparib in inhibition of cell proliferation and colony formation in triple-negative and non-triple-negative breast cancer cell lines. Down regulation of 53BP1 can partially reverse the synergistic cytotoxicity of ATM inhibition and PARP inhibition.Comparative effectiveness of platinum based chemotherapy versus non-platinum based chemotherapy for triple negative breast cancer with metastases confined to the lungsObjective:TNBC accounts for 15% to 20% of newly diagnosed breast cancer cases. It is characterized by an early onset of recurrence between the first and third year after diagnosis followed by a sharp decrease in subsequent years. Compared with other subtypes, metastatic TNBC tends to be more aggressive and more likely to involve visceral organs, particularly lungs and brain. Chemotherapy remains to be the treatment of choice for patients with TNBC. Several preclinical studies have demonstrated the peculiar sensitivity of TNBC cell lines to DNA-damaging agents, indicating the potential value of platinum drugs in the treatment of TNBC. However, clinical data at present is contradictory and inconclusive. While large case series of lung metastasis from breast cancer have been reported, no published data are yet available on the outcome of platinum-based chemotherapy (PBCT) in lung metastasis from TNBC. We reviewed clinical data on patients with lung metastasis from TNBC over 8 years, and assessed the efficacy of first-line PBCT in these patients.Methods:Sixty-five eligible patients were divided into platinum treated group and non-platinum treated group according to the first-line therapy. The main combination used in the platinum-based group were cisplatin or carboplatin plus taxanes, gemcitabine, and vinorelbine. Cisplatin was administrated at a dose of 75mg/m2 on day 1 or 25mg/m2 /day from day 1 to day 3, every 3 weeks. Carboplatin was delivered at AUC=5 on day 1, every 3 or 4 weeks. Tumor response to chemotherapy was assessed by computed tomography scan every two cycles during treatment and every 3 months after discontinuation of treatment. Objective responses (OR) to chemotherapy were evaluated using RECIST guidelines; complete response (CR), partial response (PR) and stable disease (SD) had to be maintained at 4 weeks after the first documented response. Additional radiological tests were carried out if necessary. The efficacy of chemotherapy was analyzed according to overall survival (OS), progression-free survival (PFS) and OR.Results:Patient characteristics were similar between the two groups. In the platinum treated group (n=32),2 patients (6.3%) achieved CR,16 (50.0%) PR, and 11 (34.4%) SD, 3 patients (9.4%) had PD. In the non-platinum treated group,2 patients (6.1%) achieved CR,6 (18.2%) PR,16 (48.5%) SD, and 9 (27.3%) had PD. Meidan PFS was significantly longer in the platinum treated group than the non-platinum treated group (10 months vs 6.0 months, P=0.012); OS was also improved (32 months vs 22 months, P=0.006). Cox multivariate analysis including for local-regional lymphnodes involvement, symptoms related to lung metastasis, first-line platinum based chemotherapy, disease free interval, size and number of lung lesions revealed that first-line platinum treatment was independently associated with OS.Conclusions:First-line platinum based chemotherapy improved PFS and OS in patients with lung metastases from TNBC.