RTK-Dependent Inducible Degradation of Mutant PI3Kα Drives GDC-0077 (Inavolisib) Efficacy.

PIK3CA is one of the most frequently mutated oncogenes; the p110a protein it encodes plays a central role in tumor cell proliferation. Small-molecule inhibitors targeting the PI3K p110a catalytic subunit have entered clinical trials, with early-phase GDC-0077 studies showing antitumor activity and a manageable safety profile in patients with PIK3CA-mutant breast cancer. However, preclinical studies have shown that PI3K pathway inhibition releases negative feedback and activates receptor tyrosine kinase signaling, reengaging the pathway and attenuating drug activity. Here we discover that GDC-0077 and taselisib more potently inhibit mutant PI3K pathway signaling and cell viability through unique HER2-dependent mutant p110a degradation. Both are more effective than other PI3K inhibitors at maintaining prolonged pathway suppression. This study establishes a new strategy for identifying inhibitors that specifically target mutant tumors by selective degradation of the mutant oncoprotein and provide a strong rationale for pursuing PI3Kα degraders in patients with HER2-positive breast cancer. SIGNIFICANCE: The PI3K inhibitors GDC-0077 and taselisib have a unique mechanism of action; both inhibitors lead to degradation of mutant p110a protein. The inhibitors that have the ability to trigger specific degradation of mutant p110a without significant change in wild-type p110a protein may result in improved therapeutic index in PIK3CA-mutant tumors.See related commentary by Vanhaesebroeck et al., p. 20.This article is highlighted in the In This Issue feature, p. 1.

GDC-9545 (Giredestrant): A Potent and Orally Bioavailable Selective Estrogen Receptor Antagonist and Degrader with an Exceptional Preclinical Profile for ER+ Breast Cancer.

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 ESR1Y537S mutant PDX or a wild-type ERα tumor model. Currently, 35 is being evaluated in Phase III clinical trials.

Therapeutic Ligands Antagonize Estrogen Receptor Function by Impairing Its Mobility.

Estrogen receptor-positive (ER+) breast cancers frequently remain dependent on ER signaling even after acquiring resistance to endocrine agents, prompting the development of optimized ER antagonists. Fulvestrant is unique among approved ER therapeutics due to its capacity for full ER antagonism, thought to be achieved through ER degradation. The clinical potential of fulvestrant is limited by poor physicochemical features, spurring attempts to generate ER degraders with improved drug-like properties. We show that optimization of ER degradation does not guarantee full ER antagonism in breast cancer cells; ER "degraders" exhibit a spectrum of transcriptional activities and anti-proliferative potential. Mechanistically, we find that fulvestrant-like antagonists suppress ER transcriptional activity not by ER elimination, but by markedly slowing the intra-nuclear mobility of ER. Increased ER turnover occurs as a consequence of ER immobilization. These findings provide proof-of-concept that small molecule perturbation of transcription factor mobility may enable therapeutic targeting of this challenging target class.

Maternal help-seeking for child developmental concerns: Associations with socio-demographic factors.

AIM: To examine socio-demographic factors associated with maternal help-seeking for child developmental concerns in a longitudinal birth cohort study. An understanding of these factors is critical to improving uptake of services to maximise early identification and intervention for developmental concerns. METHODS: A birth cohort was recruited from the post-natal wards of two teaching hospitals and through community nurses in South Western Sydney, Australia, between November 2011 and April 2013. Of the 4047 mothers approached, 2025 consented to participate (response rate = 50%). Socio-demographic and service use information was collected after the child's birth and when the child was 18 months of age. Sources of help were divided into three categories (formal health services, other formal services and informal supports) and compound variables were created by summing the number of different sources identified by mothers. RESULTS: Significantly more sources of help were intended to be used and/or actually accessed by mothers born in Australia, whose primary language was English, with higher levels of education and annual household income, and among mothers of first-born children. CONCLUSIONS: Developmental concerns are known to increase with increased psychosocial adversity. Our findings of reduced intent to access and use of services by socio-economically disadvantaged families and those from culturally and linguistically diverse backgrounds suggests that an inverse care effect is in operation whereby those children with the greatest health needs may have the least access to services. Possible explanations for this, and recommendations for improving service accessibility for these populations through targeted and culturally appropriate services, are discussed.

The PI3K inhibitor taselisib overcomes letrozole resistance in a breast cancer model expressing aromatase.

Letrozole is a commonly used treatment option for metastatic hormone receptor-positive (HR+) breast cancer, but many patients ultimately relapse. Due to the importance of phosphoinositide-3 kinase (PI3K) in breast cancer, PI3K inhibitors such as taselisib are attractive for combination with endocrine therapies such as letrozole. Taselisib was evaluated as a single agent and in combination with letrozole in a breast cancer cell line engineered to express aromatase. The combination of taselisib and letrozole decreased cellular viability and increased apoptosis relative to either single agent. Signaling cross-talk between the PI3K and ER pathways was associated with efficacy for the combination. In a secreted factor screen, multiple soluble factors, including members of the epidermal and fibroblast growth factor families, rendered breast cancer cells non-responsive to letrozole. It was discovered that many of these factors signal through the PI3K pathway and cells remained sensitive to taselisib in the presence of the soluble factors. We also found that letrozole resistant lines have elevated PI3K pathway signaling due to an increased level of p110α, but are still sensitive to taselisib. These data provide rationale for clinical evaluation of PI3K inhibitors to overcome resistance to endocrine therapies in ER+ breast cancer.

GDC-0941, a novel class I selective PI3K inhibitor, enhances the efficacy of docetaxel in human breast cancer models by increasing cell death in vitro and in vivo.

PURPOSE: Docetaxel is a front-line standard-of-care chemotherapeutic drug for the treatment of breast cancer. Phosphoinositide 3-kinases (PI3K) are lipid kinases that regulate breast tumor cell growth, migration, and survival. The current study was intended to determine whether GDC-0941, an orally bioavailable class I selective PI3K inhibitor, enhances the antitumor activity of docetaxel in human breast cancer models in vitro and in vivo. EXPERIMENTAL DESIGN: A panel of 25 breast tumor cell lines representing HER2+, luminal, and basal subtypes were treated with GDC-0941, docetaxel, or the combination of both drugs and assayed for cellular viability, modulation of PI3K pathway markers, and apoptosis induction. Drug combination effects on cellular viability were also assessed in nontransformed MCF10A human mammary epithelial cells. Human xenografts of breast cancer cell lines and patient-derived tumors were used to assess efficacy of GDC-0941 and docetaxel in vivo. RESULTS: Combination of GDC-0941 and docetaxel decreased the cellular viability of breast tumor cell lines in vitro but to variable degrees of drug synergy. Compared with nontransformed MCF10A cells, the addition of both drugs resulted in stronger synergistic effects in a subset of tumor cell lines that were not predicted by breast cancer subtype. In xenograft models, GDC-0941 enhanced the antitumor activity of docetaxel with maximum combination efficacy observed within 1 hour of administering both drugs. GDC-0941 increased the rate of apoptosis in cells arrested in mitosis upon cotreatment with docetaxel. CONCLUSION: GDC-0941 augments the efficacy of docetaxel by increasing drug-induced apoptosis in breast cancer models.

Active PI3K pathway causes an invasive phenotype which can be reversed or promoted by blocking the pathway at divergent nodes.

The PTEN/PI3K pathway is commonly mutated in cancer and therefore represents an attractive target for therapeutic intervention. To investigate the primary phenotypes mediated by increased pathway signaling in a clean, patient-relevant context, an activating PIK3CA mutation (H1047R) was knocked-in to an endogenous allele of the MCF10A non-tumorigenic human breast epithelial cell line. Introduction of an endogenously mutated PIK3CA allele resulted in a marked epithelial-mesenchymal transition (EMT) and invasive phenotype, compared to isogenic wild-type cells. The invasive phenotype was linked to enhanced PIP(3) production via a S6K-IRS positive feedback mechanism. Moreover, potent and selective inhibitors of PI3K were highly effective in reversing this phenotype, which is optimally revealed in 3-dimensional cell culture. In contrast, inhibition of Akt or mTOR exacerbated the invasive phenotype. Our results suggest that invasion is a core phenotype mediated by increased PTEN/PI3K pathway activity and that therapeutic agents targeting different nodes of the PI3K pathway may have dramatic differences in their ability to reverse or promote cancer metastasis.

GDC-0980 is a novel class I PI3K/mTOR kinase inhibitor with robust activity in cancer models driven by the PI3K pathway.

Alterations of the phosphoinositide-3 kinase (PI3K)/Akt signaling pathway occur broadly in cancer via multiple mechanisms including mutation of the PIK3CA gene, loss or mutation of phosphatase and tensin homolog (PTEN), and deregulation of mammalian target of rapamycin (mTOR) complexes. The dysregulation of this pathway has been implicated in tumor initiation, cell growth and survival, invasion and angiogenesis, thus, PI3K and mTOR are promising therapeutic targets for cancer. We discovered GDC-0980, a selective, potent, orally bioavailable inhibitor of Class I PI3 kinase and mTOR kinase (TORC1/2) with excellent pharmacokinetic and pharmaceutical properties. GDC-0980 potently inhibits signal transduction downstream of both PI3K and mTOR, as measured by pharmacodynamic (PD) biomarkers, thereby acting upon two key pathway nodes to produce the strongest attainable inhibition of signaling in the pathway. Correspondingly, GDC-0980 was potent across a broad panel of cancer cell lines, with the greatest potency in breast, prostate, and lung cancers and less activity in melanoma and pancreatic cancers, consistent with KRAS and BRAF acting as resistance markers. Treatment of cancer cell lines with GDC-0980 resulted in G1 cell-cycle arrest, and in contrast to mTOR inhibitors, GDC-0980 induced apoptosis in certain cancer cell lines, including those with direct pathway activation via PI3K and PTEN. Low doses of GDC-0980 potently inhibited tumor growth in xenograft models including those with activated PI3K, loss of LKB1 or PTEN, and elicited an exposure-related decrease in PD biomarkers. These preclinical data show that GDC-0980 is a potent and effective dual PI3K/mTOR inhibitor with promise for the clinic.

Nuclear phospho-Akt increase predicts synergy of PI3K inhibition and doxorubicin in breast and ovarian cancer.

The phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is frequently disrupted in cancer and implicated in multiple aspects of tumor growth and survival. In addition, increased activity of this pathway in cancer is associated with resistance to chemotherapeutic agents. Therefore, it has been hypothesized that PI3K inhibitors could help to overcome resistance to chemotherapies. We used preclinical cancer models to determine the effects of combining the DNA-damaging drug doxorubicin with GDC-0941, a class I PI3K inhibitor that is currently being tested in early-stage clinical trials. We found that PI3K inhibition significantly increased apoptosis and enhanced the antitumor effects of doxorubicin in a defined set of breast and ovarian cancer models. Doxorubicin treatment caused an increase in the amount of nuclear phospho-Akt(Ser473) in cancer cells that rely on the PI3K pathway for survival. This increased phospho-Akt(Ser473) response to doxorubicin correlates with the strength of GDC-0941's effect to augment doxorubicin action. These studies predict that clinical use of combination therapies with GDC-0941 in addition to DNA-damaging agents will be effective in tumors that rely on the PI3K pathway for survival.

Predictive biomarkers of sensitivity to the phosphatidylinositol 3' kinase inhibitor GDC-0941 in breast cancer preclinical models.

PURPOSE: The class I phosphatidylinositol 3' kinase (PI3K) plays a major role in proliferation and survival in a wide variety of human cancers. A key factor in successful development of drugs targeting this pathway is likely to be the identification of responsive patient populations with predictive diagnostic biomarkers. This study sought to identify candidate biomarkers of response to the selective PI3K inhibitor GDC-0941. EXPERIMENTAL DESIGN: We used a large panel of breast cancer cell lines and in vivo xenograft models to identify candidate predictive biomarkers for a selective inhibitor of class I PI3K that is currently in clinical development. The approach involved pharmacogenomic profiling as well as analysis of gene expression data sets from cells profiled at baseline or after GDC-0941 treatment. RESULTS: We found that models harboring mutations in PIK3CA, amplification of human epidermal growth factor receptor 2, or dual alterations in two pathway components were exquisitely sensitive to the antitumor effects of GDC-0941. We found that several models that do not harbor these alterations also showed sensitivity, suggesting a need for additional diagnostic markers. Gene expression studies identified a collection of genes whose expression was associated with in vitro sensitivity to GDC-0941, and expression of a subset of these genes was found to be intimately linked to signaling through the pathway. CONCLUSION: Pathway focused biomarkers and the gene expression signature described in this study may have utility in the identification of patients likely to benefit from therapy with a selective PI3K inhibitor.

Antisense oligonucleotides to N-methyl-D-aspartate receptor subunits attenuate formalin-induced nociception in the rat.

Noxious peripheral stimuli increase the sensitivity of central nociceptive neurons to subsequent noxious stimuli. This occurs in part through activation of spinal N-methyl-D-aspartate (NMDA) receptors. These receptors are heteromeric complexes of NMDA-R1 and NMDA-R2 A--D subunits. NMDA-R1 is necessary for the formation of functional NMDA receptors whereas the R2 subunits (A-D) modify the properties of the receptor. However, the role of the various receptor subtypes in nociception has not been established. In this study, we used intrathecally administered phosphodiester antisense oligonucleotides (ODEs) to examine the role of the NMDA-R1, NMDA-R2C and NMDA-R2D subunits in the mediation of formalin-induced nociception in the rat. The antisense ODEs against the NMDA-R1 and NMDAR-2C subunits reduced nociceptive behaviors whereas the corresponding sense ODEs had no effect. In contrast, nociception was unaffected by the antisense ODE to NMDAR-2D. Using an RNase protection assay, we also found that each antisense ODE selectively decreased the level of the corresponding mRNA in the lumbar spinal cord but that the sense ODEs had no such effect. Accordingly, these data provide evidence that the R1 and R2C subunits, but not R2D, of the NMDA receptor participate in the development of formalin-induced nociception.