Abstract A17: Inhibiting polo-like kinases for the treatment of pancreatic cancer: Targeting kinase outliers for personalized therapy.

Purpose: To apply a high-throughput, sequencing-based method towards the systematic discovery of pancreatic cancer drivers in the setting of oncogenic KRAS mutations. Background: Many cancers harbor “dependence” on one or a few genetic or signaling abnormalities, a concept known as “oncogene addiction.” Identifying and targeting these specific genetic drivers can therefore have dramatic therapeutic effects. A well-known example is the use of imatinib, a kinase inhibitor of BCR-ABL, which has significantly improved survival in patients with this driver of chronic myeloid leukemia. In the case of pancreatic cancer, the most obvious target is the KRAS mutation, which is present in greater than 90% of patients. However, attempts to directly target the KRAS oncogene and its related pathways have so far been unsuccessful. As such, pancreatic cancer continues to be the fourth leading cause of cancer-related death in the United States, and there remains a strong motivation for identifying new targets for intervention. With the availability of high-throughput techniques, it is now possible to efficiently sequence a pancreatic cancer patient’s entire transcriptome and thereby identify genes that have higher relative levels of expression. These “outliers” may represent additional drivers of that patient’s malignant phenotype beyond the KRAS oncogene. Such outlier expression would be a strong predictor for oncogene addiction and therefore an ideal target for therapy. Methods: We carried out transcriptome sequencing of 21 pancreatic cancer cell lines, and analyzed relative gene expression levels to identify kinases with outlier expression. Multiple samples displayed outlier expression of the polo-like kinase (PLK) family genes, which have previously been identified in the literature as KRAS synthetic lethals. Knockdown of PLK1 and PLK2 were carried out using On-Target Plus siRNAs in pancreatic cancer cell lines and validated by qRT-PCR. Cell counts were performed at 2, 4, and 6 days post-transfection. Cell lines were also treated with the PLK inhibitor, BI 6727. Phenotypic response was measured by cell count at 12, 24, 48, and 72 hours. Using lentiviral constructs, we then created stable pancreatic cancer cell lines expressing doxycycline-inducible TRIPZ shKRAS. shKRAS induction by doxycycline (1 ug/mL) was combined with BI 6727 treatment (0-30 nM). Phenotypic response was measured by WST-1 assay at 0, 1, 3, and 5 days. Results: PLK2 siRNA knockdown inhibited cell proliferation only in those pancreatic cancer cell lines that had PLK2 outlier expression. Conversely, PLK1 siRNA knockdown markedly inhibited cell proliferation across all cell lines tested, regardless of outlier expression. However, this effect was less prominent in KRAS wild-type cell lines. Treatment with BI 6727 decreased cell proliferation and increased apoptosis across all cell lines tested, though cell lines without PLK outlier expression had higher IC 50 . All samples with KRAS dependence had PLK1 or PLK2 outlier expression. In these cell lines, combining KRAS knockdown and BI 6727 treatment had an additive inhibitory effect on cell growth. Conclusions: KRAS-dependent pancreatic cancer cell lines are also PLK-driven, with outlier expression of PLK1 or PLK2. In these samples, KRAS knockdown and PLK inhibition synergistically inhibit cell proliferation in vitro. This supports the role of the polo-like kinases serving as KRAS-synthetic lethals. Our data demonstrate the potential of combining KRAS silencing and sample-specific outlier kinase inhibition to affect cancer growth in a personalized manner. Citation Format: Iris Wei, Vishal Kothari, Sunita Shankar, Shanker Kalyana-Sundaram, Catherine Grasso, Dan Robinson, Xuhong Cao, Diane Simeone, Arul Chinnaiyan, Chandan Kumar-Sinha. Inhibiting polo-like kinases for the treatment of pancreatic cancer: Targeting kinase outliers for personalized therapy. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Progress and Challenges; Jun 18-21, 2012; Lake Tahoe, NV. Philadelphia (PA): AACR; Cancer Res 2012;72(12 Suppl):Abstract nr A17.