Interestingly, this increase in clonogenicity was observed not only in cells from mice with homozygotic loss of BRCA1 but also in cells from mice with hetero zygotic loss of BRCA1, indicating that loss of a single allele, which is a situation analogous then to a human BRCA1 mutation carrier, leads to an increase in colony formation. Next, we examined whether treatment with erlotinib was similarly effective in murine MECs as it was in hMECs, and we found that colony formation was suppressed effectively at 1 uM erlotinib in the medium. On the basis of these findings, we tested the efficacy of erlotinib for the primary prevention of breast cancer in BRCA1 mutant mice.

EGFR inhibitor erlotinib prevents the development of ER negative, but not of ER positive, breast cancers in BRCA1 mutant mice Inhibitors,Modulators,Libraries Starting at age 3 months, MMTV Cre BRCA1 flox flox p53 mice were treated with either the EGFR inhibitor erlotinib at 100 mg kg day orally or vehicle control as dosed previously. End points were tumor free survival and tolerability of the prophylactic erlotinib treatments. The mice tolerated the Inhibitors,Modulators,Libraries treatments well, with the only adverse effect being partial alopecia in about 30% of the mice. Mice were examined daily, and tumors were diagnosed by palpa tion. Upon necropsy, tumors were counted, fixed and examined for ER expression. Survival analysis showed a median disease free survival of 365 days in the erlotinib treated cohort versus 256 days in the control cohort, that is, erlotinib treatments delayed tumor devel opment by an average of 3 months.

Only 19 tumors were observed in the erlotinib treated cohort versus 31 tumors in the control cohort, a significant reduction. Inhibitors,Modulators,Libraries Upon necropsy, tumors were fixed and pro cessed for immunohistochemistry. As expected on the basis of previous studies, Inhibitors,Modulators,Libraries the mice in the control cohort developed both ER positive and ER negative breast cancers, with a predominance of ER negative tumors. Interestingly, while the number of ER positive tumors was not significantly different in both cohorts, the num ber of ER negative breast cancers was sharply reduced in the erlotinib treated cohort, indicating that erlotinib was effective in preventing the emergence of ER negative, but not ER positive, breast cancers in this mouse model. Importantly, EGFR staining showed that the erlotinib treated cohort had a much lower number of EGFR posi tive tumors than the control group, again confirming that erlotinib treatments selected for EGFR negative tumors.

ALDH1 staining was observed in nests and at the edges of Inhibitors,Modulators,Libraries the tumors in clusters and was highly variable among tumors. There was a trend toward lower ALDH1 expression in the erlotinib treated cohort, selleck kinase inhibitor however, given the high variability of ALDH1 expression, statisti cal significance was not reached.