The decoupling of the proliferative and anti apoptotic effects of estrogen shows that continuing estrogen deprivation in adding a PI3K inhibitor and progressing GW9508 885101-89-3 individuals might be a technique worth testing. The optimum endocrine mix with PI3K inhibition in cells resistant to estrogen deprivation can be a important consideration since the overwhelming majority of patients with high level breast cancer have been treated with an aromatase inhibitor in the adjuvant setting. Treatment options include an anti estrogen or therapy with low dose estradiol. We modeled these secondline strategies in different LTED cell lines, one where ER expression was maintained and one where it was lost, as a way to replicate the clinical observation that upon disease progression ER is down-regulated in a proportion of cases. Both LTED lines were found to be fairly resistant to PI3K inhibitors compared with the parental lines, consistent with reports that acquiring the ability to develop in the lack of estrogen is associated with increased Metastasis PI3K and MAPK signaling. The use of fulvestrant effectively sensitized MCF7 LTED cells to both BKM120 and BGT226, but, consistent with a vital role for ligand separate ER activity in PI3K inhibitor resistance. The use of estradiol to return the LTED phenotype, followed by re association of estrogen deprivation, is a possible alternative approach, but, the recovery of sensitivity to PI3K inhibition with this method appeared less profound than with fulvestrant treatment. Taken together our data provide a reason for combining estrogen deprivation with PI3K inhibitors for treating PIK3CA mutant estrogen dependent, ERpositive cancers and for the mix of fulvestrant with PI3K inhibitors in patients with ER beneficial, VX-661 aromatase chemical resistant illness. However, further studies is going to be essential to effortlessly translate these pre-clinical data to the clinical setting. These reports could include additional pre-clinical modeling in PIK3CA wild-type estrogen starvation resistant tumor lines to ascertain whether PIK3CA mutation is necessary in endocrine resistant cancers to confer PI3K chemical sensitivity. Additionally, integrating biomarker analysis in early stage PI3K chemical tests may assist in pinpointing patients most likely to reap the benefits of these therapeutic agents. To address the prevalence of the target population for a fulvestrant/PI3K inhibitor trial for second line treatment of ER positive PIK3CA mutant relapsed disease, we analyzed 51 advanced disease biopsies from both ERpositive and ER negative cases for PIK3CA mutation and correlated findings with the medical trajectory of the patients. While patients with ER positive PIK3CA mutant tumors tended to relapse later than patients with ER bad or ER positive PIK3CA wild type tumors, the PIK3CA mutation prevalence in ER positive relapsed disease was high.