Abstract B245: Synergistic antiproliferative effects of the P21-activated kinase (PAK) inhibitor, PF-03758309, and standard-of-care agents in colorectal cancer models.

Abstract

Abstract Background: The p21-activated kinase (PAK) family of serine/threonine kinases is overexpressed in multiple cancer types including breast, ovarian, colorectal, thyroid, and pancreatic. The two groups of PAK proteins, Group I (PAK 1–3) and Group II (PAK 4–6), are critical mediators of cell survival, motility, mitosis, transcription, and translation. The goal of this study was to assess the efficacy of a PAK inhibitor, PF-03758309, with irinotecan in colorectal cancer (CRC). Methods: A panel of 27 colorectal cancer (CRC) cell lines were exposed to increasing doses of PF-03758309 (0.015–1 μM) for 72 hours and analyzed for inhibition of proliferation using the sulforhodamine B (SRB) assay. Cell lines were designated sensitive (S), intermediate (I), or resistant (R) based on IC50's < 0.125 μM, ≥ 0.125 and ≤ 1 μM, or > 1 μM, respectively. Two sensitive and two resistant cell lines were exposed to three different concentrations of either the PAKi, irinotecan or all possible combinations. Proliferation was assessed by the SRB assay and synergy was calculated using the Calcusyn software. Apoptosis was also assessed using the luminescent Caspase 3/7 assay. Athymic nude mice were injected with primary patient derived explants. Once the tumors reached 200–300mm3 the mice were dosed with PF-03758309, irinotecan, or cetuximab or the combination. Tumors were measured twice weekly and growth curves were analyzed. Results: Synergy was observed in both PF-03758309 sensitive and resistant cell lines, though the greatest synergy was seen in one of the more sensitive lines. Apoptosis was also increased in the combination when compared to the single agents. Of the explants treated, one of them demonstrated a synergistic/super-additive effect when dosed with the combination of irinotecan. One of the explants also demonstrated an added benefit with the combination of PF-03758309 and cetuximab. Further investigation into the differential sensitivities seen with PF-03758309 revealed that the more resistant CRC cell lines had higher expression of MDR1. When this was modulated either by shRNA knockdown or by addition of verapamil, we saw a shift to a more sensitive phenotype. Conclusions: The combination of the PAKi, PF-03758309 with irinotecan resulted in synergistic anti-proliferative effectsand apoptosis in CRC cell lines regardless of sensitivity. Similar effects were observed in patient-derived CRC explants with PF-03758309 and standard of care agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B245.