The PARP inhibitors, veliparib and olaparib, are effective chemopreventive agents for delaying mammary tumor development in BRCA1-deficient mice

Abstract

Poly-ADP ribose polymerase (PARP) inhibitors are effective for the treatment of BRCA-deficient tumors. Women with these mutations have an increased risk of developing breast cancer and would benefit from effective chemoprevention. This study examines whether the PARP inhibitors, veliparib and olaparib, delay mammary gland tumor development in a BRCA1-deficient (BRCA1(Co/Co);MMTV-Cre;p53(+/-)) mouse model. In dose de-escalation studies, mice were fed with control, veliparib (100 mg/kg diet), or olaparib (200, 100, 50, or 25 mg/kg diet) continuously for up to 43 weeks. For intermittent dosing studies, mice cycled through olaparib (200 mg/kg diet) for 2 weeks followed by a 4-week rest period on control diet. To examine biomarkers, mice were fed with olaparib using the intermittent dosing regimen and mammary glands were evaluated by immunohistochemistry. In mice treated with veliparib or olaparib (200 mg/kg diet), the average age of the first detectable tumor was delayed by 2.4 and 6.5 weeks, respectively, compared with controls. Olaparib also increased the average lifespan of mice by 7 weeks. In dose de-escalation studies, lower concentrations of olaparib delayed tumor development but were less effective than the highest dose. When fed intermittently, olaparib delayed the onset of the first palpable tumor by 5.7 weeks and significantly reduced proliferation and induced apoptosis in hyperplastic mammary glands. In summary, veliparib and olaparib are effective for delaying tumor development and extending the lifespan of BRCA1-deficient mice, and intermittent dosing with olaparib was as effective as continuous dosing. These results suggest that the use of PARP inhibitors is a promising chemopreventive option.

Fig. 1

Continuous feeding of veliparib and olaparib diets significantly delayed tumor development in BRCA1^Co/Co;MMTV-Cre;p53^+/− mice. Mice were fed with either control diet, 100 mg/kg veliparib in diet A, or 200 mg/kg of olaparib in diet B, starting when mice were 10 weeks old, and tumor development was assessed by weekly palpation. No tumors were found before the mice were 20 weeks of age. n=15, 16 in the control and veliparib groups respectively in A; n=17 for both olaparib and its respective control group in B. * p < 0.05 versus control group in A and ** p < 0.001 versus control group in B.

Fig. 2

Dose de-escalation studies with BRCA1-deficient mice fed continuously with 100 mg/kg, 50 mg/kg and 25 mg/kg olaparib diets. Mice were fed with control, 100 mg/kg A, 50 mg/kg B, or 25 mg/kg C, of olaparib diets starting when mice were 10 weeks old and palpated for tumors weekly. n=14–19 mice in the control groups; n=14–20 mice for the olaparib groups. * p < 0.05 versus control group.

Fig. 3

Intermittent treatment with olaparib delayed tumor development in BRCA1^Co/Co;MMTV-Cre;p53^+/− mice. A, Intermittent dosing schedule for olaparib. All mice were fed with either control or olaparib diet starting at 10 weeks of age. The control group was fed with control diet throughout the study. Mice in the intermittent dosing regimen underwent cycles of 2 weeks on olaparib diet followed by 4 weeks on control diet for up to 43 weeks in the chemoprevention study and for 18–30 weeks in the biomarker studies. B, Mice fed using the dosing schedule described in panel A were palpated weekly to assess tumor development. n=19, 18 for control and olaparib treatment group respectively. * p < 0.001 versus control group.

Fig. 4

Intermittent treatment with olaparib for 24–30 weeks reduced cell proliferation in mammary glands of BRCA1-deficient mice. A, Mice on either control or olaparib (200 mg/kg diet) fed intermittently for 24 or 30 weeks were injected with bromodeoxyuridine (BrdU) and analyzed by immunohistochemistry to measure cell proliferation (magnification, x 200; insets x 500). B, BrdU-positive cells (stained brown with diaminobenzidine) in hyperplastic and ductal areas were quantitated and expressed as a percentage of BrdU-positive cells over total number of cells (stained purple with hematoxylin) counted. n=6 per group at 24 weeks and n=5 mice per group at 30 weeks; more than 3500 and 1000 cells from hyperplastic areas and ducts were counted in both groups at each time point. * p < 0.05 versus control in hyperplastic areas; # p < 0.05 versus control in ducts.

Fig. 5

Intermittent treatment with olaparib for 30 weeks induced apoptosis in hyperplastic areas. A, Mammary tissues from BRCA1^Co/Co;MMTV-Cre;p53^+/− mice fed with either control or olaparib intermittently in diet (200 mg/kg) for 24 or 30 weeks were harvested and processed for TUNEL staining to measure apoptosis. TUNEL-positive cells in hyperplastic and ductal areas were visualized with diaminobenzidine (brown), followed by counterstaining with methyl green (green) (magnification, x 200; insets x 500). B, TUNEL-positive cells were quantitated and expressed as a percentage of TUNEL-positive cells at 24 or 30 weeks over total number of cells counted. n=5–6 mice per group at each time point; more than 1800 cells in hyperplastic areas and 950 cells in ducts were counted in both groups.* p < 0.05 versus control.