Abstract A24: Novel small molecule PAK4 allosteric modulators show potency against oncogenic K-Ras driven pancreatic cancer
Authors: AZMI ASFAR S. , Senapedis William T. , Landesman Yosef , Baloglu Erkan , Bao Bin , Wu Jack , Ori Kalid , Shacham Sharon , Kauffman Michael , Mohammad Ramzi M. 
Published: May 2015

Introduction: In spite of the well-recognized role of oncogenic K-Ras in the establishment and sustenance of the highly fatal disease, pancreatic cancer (PC), development of clinical agents that can tame this master regulator have been unsuccessful. This is primarily because, unlike other druggable targets, K-Ras lacks an ideal binding pocket that can be used to design small molecule drugs. Therefore, there is an urgent need to identify novel druggable sites in K-Ras or to develop agents that can target key effector proteins downstream of K-Ras signaling. To this end, we are pursuing p21-activated kinase 4 (PAK4) as a novel target for PC. PAK4 acts as a key effector of Rho family GTPases downstream of K-Ras and is found over-expressed in most of the available PC cell lines but not in normal human pancreatic ductal epithelial cells (HPDE). Gene copy number amplification studies in PC patient cohorts has shown amplification of PAK4. Most importantly, RNA interference of PAK4 suppresses PC cell proliferation making PAK4 an attractive therapeutic target within the K-Ras signaling network. Nevertheless, the previously developed PAK4 Type I ATP competitive inhibitor (PF-3758309; tested in non-pancreatic models) was evaluated in a Phase 1 study and showed undesirable pharmacokinetic properties as well as no objective responses and was subsequently discontinued. In order to fill this scientific void, we evaluated a new class of PAK4 allosteric modulators in pancreatic cancer models.

Experimental Procedure: Using multiple molecular biology techniques we tested the Pak4 modulators' activities (in the presence and absence of -ve and +ve controls) in a panel of PC cells lines, PAK4 over-expressing Gemcitabine resistant (GEM-R) PC models and highly resistant flow sorted PC stem cells (CSC). CSC's are triple positive for CD133+CD44+EpCam+ and undergo epithelial-to-mesenchymal transition (EMT). The toxicity and efficacy of these PAK4 modulators were evaluated in sub-cutaneous mouse models of PC.

Results: The novel, orally available PAK4 allosteric modulators (KPT-7189, KPT-8752) show anti-proliferative activity against different PC cell lines (AsPC-1, Colo-357, MiaPaCa-2, L3.6pl and HPAC IC50s &lt;250nM) sparing HPDE (IC50s 5 fold higher). Cell growth inhibition was concurrent with apoptosis induction and suppression of colony forming abilities in 5 different PC cell lines (not in HPDE). KPT-7189 reduced PAK4 protein levels along with the inhibition of proliferative and anti-apoptotic signals downstream of PAK4. Molecularly, PAK4 RNA interference enhanced KPT-7189 activity and co-immunoprecipitation experiments showed disruption of PAK4 complexes (p65, Bcl-2 and vimentin). KPT-7189 inhibited spheroid forming ability of CSCs and reversed the epithelial-to-mesenchymal (EMT) phenotype. Confocal, western blot and RT-PCR analysis of KPT-7189 treated CD33+CD44+EpCam+ spheroids showed suppression of EMT and CSC markers EpCAM, vimentin and snail with re-expression of epithelial phenotype promoter E-cadherin. A similar compound, KPT-8752, with desirable PK properties was well tolerated in mice with the absence of any clinical signs of toxicity up to 200 mg/kg oral daily dose. Pre-clinical animal efficacy trial in sub-cutaneous, orthotopic (developed from human primary cells) and LSL-K-Ras G12D/+;LSL-Trp53R172H/+;Pdx-1-Cre transgenic mouse models are ongoing.

Conclusion: This is the first proof of concept study demonstrating the anti-proliferative effects of novel allosteric modulators of PAK4, a downstream effector of K-Ras, in pancreatic cancer that warrants further clinical investigations.

Citation Format: ASFAR S. AZMI, William T. Senapedis, Yosef Landesman, Erkan Baloglu, Bin Bao, Jack Wu, Kalid Ori, Sharon Shacham, Michael Kauffman, Ramzi M. Mohammad. Novel small molecule PAK4 allosteric modulators show potency against oncogenic K-Ras driven pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr A24. doi: 10.1158/1557-3125.RASONC14-A24