Abstract PR03: Selective degradation of mutant PIK3CA promotes increased mutant specificity in a subset of PI3K ATP-competitive inhibitors

Activating mutations in PIK3CA are among the most significant oncogenic events across all cancers, making it an important target for drug development. Yet the application of PI3K inhibitors in the clinic has been limited by the difficulty of achieving an adequate therapeutic window, due to the critical role that PI3K signaling plays in normal physiologic processes, such as glucose homeostasis. In theory, the therapeutic window could be improved if it were possible to design mutant selective inhibitors, as has been demonstrated with other oncogenes such as EGFR. However, unlike EGFR, the most predominant PIK3CA activating mutations do not reside in the kinase active site, presenting a major challenge for rational structure-based design. Nevertheless, it was recently shown that the PI3K inhibitor taselisib is able to achieve modest levels of mutant selectivity both across cancer lines as well as in cell lines that were engineered to express mutant or wild-type PIK3CA. Taselisib was also shown to selectively induce degradation of mutant versus wild-type PIK3CA, leading to the speculation that this degradation may be responsible for the observed selectivity. In order to better understand the origins of mutant selectivity for taselisib and several other PIK3CA inhibitors, we assessed these inhibitors in a variety of biophysical and biochemical assays under conditions designed to mimic physiologic settings. In parallel, we also investigated the mechanistic basis of this selectivity in our engineered cell lines. Our results are consistent with the hypothesis that selective degradation of mutant PIK3CA is the predominant mechanism underlying mutant selectivity for this class of PIK3CA active site inhibitors.

This abstract is also being presented as Poster B03.

Citation Format: Lan Nguyen, Kyle Edgar, Kyung Song, Stephen Schmidt, Victorai Schutz, Noriko Ishisoko, Eric Torres, Akash Das, Divya Murali, Steve Sideris, Timothy Wendorff, Matt Saabye, Hans Purkey, Jawahar Sudhamsu, Steven Staben, Emily Hanan, Georgia Hatzivassiliou, Lori Friedman, Nicholas F. Endres. Selective degradation of mutant PIK3CA promotes increased mutant specificity in a subset of PI3K ATP-competitive inhibitors [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr PR03.

Selective inhibition of NF-kB inducing kinase (NIK) is therapeutically efficacious in IFNα-accelerated lupus nephritis prone mice

Systemic lupus erythematosus (SLE) is often considered a disease driven by autoreactive B cells and anti-nuclear antibodies. However, therapeutic targeting of B cells, for example through BAFF blockade, has been only partially effective in SLE. NF-kB Inducing Kinase (NIK) mediates non-canonical NF-kB signaling downstream of disease relevant TNF family members, such as BAFF, TWEAK, CD40, and OX40. We hypothesized that NIK inhibition might be more efficacious than BAFF blockade in lupus, and therefore generated a highly selective and potent small molecule inhibitor (SMI) of NIK to test this hypothesis. In cellular assays, this molecule inhibits non-canonical NF-kB signaling downstream of multiple TNFRSF family members. In order to differentiate the effects of NIK inhibition and BAFF blockade in vivo, we compared NIK inhibition with BAFF blockade in the context of NZB/W F1 lupus prone mice. As predicted, NIK inhibition recapitulated the pharmacological effects of BAFF blockade. Furthermore, NIK inhibition, but not BAFF blockade, affected T cell parameters in the spleen and pro-inflammatory gene expression in the kidney, the latter of which is partly attributable to TWEAK signaling. Finally, NIK inhibition resulted in improved survival, ameliorated renal pathology, and lower proteinuria scores. Collectively, our data suggest that NIK inhibition affects multiple disease-relevant pathways and may therefore have superior efficacy compared to BAFF inhibition in SLE.

Abstract A067: Targeting p21-activated kinases in breast cancer

Luminal breast cancer is the most commonly diagnosed cancer worldwide and mechanisms of innate and acquired resistance are not yet well understood. P21-activated kinases (PAKs) are members of the STE20 family of serine/threonine kinases that lie downstream of receptor tyrosine kinases and Rho family GTPases. PAKs regulate many cellular processes that are commonly perturbed in cancer, including migration, polarization and proliferation, and are therapeutic targets of interest. PAK1 is also overexpressed in several cancers and we recently showed that focal genomic amplification of PAK1 occurs in ~8% of luminal breast cancer (Proc Natl Acad Sci 2011; 108:7177). PAK1 amplification has also been associated with poor patient outcome and PAK1 is a putative mediator of resistance to anti-estrogen therapy (Nature 2012; 486:346). The aim of this study was to further characterize PAK family kinases in breast adenocarcinoma. Luminal breast cancer cells with PAK1 focal amplification rapidly underwent apoptosis following inhibition of this kinase. To identify potential mechanisms of acquired resistance we have derived PAK1-resistant cell lines and begun characterizing via genomic and proteomics approaches. Furthermore, in contrast to the role of PAK1 in luminal breast cancer, elevated expression of PAK4 was associated with poor outcome in basal-like breast cancer. We developed a novel small molecule inhibitor with high potency and selectivity of group II PAKs to delineate the role of PAK4 signaling in basal breast cancer. Together, our results provide evidence for dysregulation of PAK1 and PAK4 in differing subtypes of breast cancer and roles in for these kinases in cellular survival, proliferation and migration in this tumor indication.

Citation Format: Klaus Hoeflich, Christy Ong, Karen Lyle, Adrian Jubb, Wei Zhou, Adrian Harris, Marcia Belvin, Lori Friedman, Hartmut Koeppen, Joachim Rudolph, Steven Staben. Targeting p21-activated kinases in breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A067.