BCL-2 family proteins dictate survival of human multiple myeloma cells, making them attractive drug targets. Indeed, multiple myeloma cells are sensitive to antagonists that selectively target prosurvival proteins such as BCL-2/BCL-X L (ABT-737 and ABT-263/navitoclax) or BCL-2 only (ABT-199/GDC-0199/venetoclax). Resistance to these three drugs is mediated by expression of MCL-1. However, given the selectivity profile of venetoclax it is unclear whether coexpression of BCL-X L also affects antitumor responses to venetoclax in multiple myeloma. In multiple myeloma cell lines (n ¼ 21), BCL-2 is expressed but sensitivity to venetoclax correlated with high BCL-2 and low BCL-X L or MCL-1 expression. Multiple myeloma cells that coexpress BCL-2 and BCL-X L were resistant to venetoclax but sensitive to a BCL-X Lselective inhibitor (A-1155463). Multiple myeloma xenograft models that coexpressed BCL-X L or MCL-1 with BCL-2 were also resistant to venetoclax. Resistance to venetoclax was mitigated by cotreatment with bortezomib in xenografts that coexpressed BCL-2 and MCL-1 due to upregulation of NOXA, a proapoptotic factor that neutralizes MCL-1. In contrast, xenografts that expressed BCL-X L , MCL-1, and BCL-2 were more sensitive to the combination of bortezomib with a BCL-X L selective inhibitor (A-1331852) but not with venetoclax cotreatment when compared with monotherapies. IHC of multiple myeloma patient bone marrow biopsies and aspirates (n ¼ 95) revealed high levels of BCL-2 and BCL-X L in 62% and 43% of evaluable samples, respectively, while 34% were characterized as BCL-2High /BCL-X L Low . In addition to MCL-1, our data suggest that BCL-X L may also be a potential resistance factor to venetoclax monotherapy and in combination with bortezomib. Mol Cancer Ther; 15(5); 1132-44. Ó2016 AACR.

Highlights d Drug candidates optimized for ER degradation can weakly activate ER in cancer cells d ''ER degraders'' trigger interaction of ER with DNA at canonical binding sites d Impact on chromatin accessibility distinguishes ER antagonists from weak activators d Dramatic slowing of ER mobility drives ER antagonism, and precedes ER turnover

ER-targeted therapeutics provide valuable treatment options for patients with ER+ breast cancer, however, current relapse and mortality rates emphasize the need for improved therapeutic strategies. The recent discovery of prevalent ESR1 mutations in relapsed tumors underscores a sustained reliance of advanced tumors on ERα signaling, and provides a strong rationale for continued targeting of ERα. Here we describe GDC-0810, a novel, non-steroidal, orally bioavailable selective ER downregulator (SERD), which was identified by prospectively optimizing ERα degradation, antagonism and pharmacokinetic properties. GDC-0810 induces a distinct ERα conformation, relative to that induced by currently approved therapeutics, suggesting a unique mechanism of action. GDC-0810 has robust in vitro and in vivo activity against a variety of human breast cancer cell lines and patient derived xenografts, including a tamoxifen-resistant model and those that harbor ERα mutations. GDC-0810 is currently being evaluated in Phase II clinical studies in women with ER+ breast cancer.

Dysregulation of ErbB receptor tyrosine kinases is thought to promote mammary tumor progression by stimulating tumor cell growth and invasion. Overexpression and aberrant activation of ErbB2/HER2 confer aggressive and malignant characteristics to breast cancer cells, and patients displaying ErbB2-amplified breast cancer face a worsened prognosis. Recent studies have established that ErbB2 and ErbB3 are commonly co-overexpressed in breast tumor cell lines and in patient samples. ErbB2 heterodimerizes with and activates the ErbB3 receptor, and the two receptors synergize in promoting growth factor-induced cell proliferation, transformation, and invasiveness. Our previous studies have shown that the neuregulin receptor degradation protein-1 (Nrdp1) E3 ubiquitin ligase specifically suppresses cellular ErbB3 levels by marking the receptor for proteolytic degradation. Here, we show that overexpression of Nrdp1 in human breast cancer cells results in the suppression of ErbB3 levels, accompanied by the inhibition of cell growth and motility and the attenuation of signal transduction pathways. In contrast, either Nrdp1 knockdown or the overexpression of a dominantnegative form enhances ErbB3 levels and cellular proliferation. Additionally, Nrdp1 expression levels inversely correlate with ErbB3 levels in primary human breast cancer tissue and in a mouse model of ErbB2 mammary tumorigenesis. Our observations suggest that Nrdp1-mediated ErbB3 degradation suppresses cellular growth and motility, and that Nrdp1 loss in breast tumors may promote tumor progression by augmenting ErbB2/ErbB3 signaling. (Cancer Res 2006; 66(23): 11279-86)

Breast cancer remains a leading cause of cancer death in women, representing a significant unmet medical need. Here, we disclose our discovery efforts culminating in a clinical candidate, 35 (GDC-9545 or giredestrant). 35 is an efficient and potent selective estrogen receptor degrader (SERD) and a full antagonist, which translates into better antiproliferation activity than known SERDs (1, 6, 7, and 9) across multiple cell lines. Fine-tuning the physiochemical properties enabled once daily oral dosing of 35 in preclinical species and humans. 35 exhibits low drug−drug interaction liability and demonstrates excellent in vitro and in vivo safety profiles. At low doses, 35 induces tumor regressions either as a single agent or in combination with a CDK4/6 inhibitor in an ESR1 Y537S mutant PDX or a wild-type ERα tumor model. Currently, 35 is being evaluated in Phase III clinical trials.