Abstract 3919: Sensitization of pancreatic cancer stem cells to gemcitabine by Chk1 inhibition

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL We have previously shown that inhibition of Chk1 (checkpoint kinase 1), a key regulator of DNA damage response/repair, sensitizes pancreatic cancer cells and tumors to gemcitabine. However the role of Chk1 inhibition in the sensitivity of pancreatic CSC's (cancer stem cells) to gemcitabine is not known. Pancreatic CSC's have been reported to be resistant to chemo- and radio-therapy via enhanced DNA damage response/repair. Therefore, in the present study we have tested whether inhibition of Chk1 by AZD7762 (AstraZeneca), currently in Phase I clinical trials, would selectively sensitize pancreatic cancer stem cells to gemcitabine. We utilized two patient-derived xenograft models (designated patient J and F) and the previously identified pancreatic CSC markers CD24, CD44, and ESA. Following tumor initiation, NOD/SCID mice were treated with gemcitabine (90mg/kg, Mon & Thu) and/or AZD7762 (20mg/kg, Mon, Tues, Thu, & Fri) for three cycles. At the end of treatment tumors were harvested and the percentage of CD24, CD44 and ESA positive cells was determined. We found that the percentage of CD24, CD44, ESA positive cells was significantly reduced in response to gemcitabine+AZD7762 (1.3 ± 0.17%, p<0.05) compared to vehicle (7.5 ± 1.1%), gemcitabine (5.5 ± 1.2%), or AZD7762 (6.1 ± 0.79%) treatment. Similarly, in patient F xenografts gemcitabine+AZD7762 significantly reduced the percentage of CD24, CD44, ESA positive cells (1.0 ± 0.1%, p<0.05) compared to vehicle (4.5 ± 0.3%), gemcitabine (2.0 ± 0.2%), or AZD7762 (1.8 ± 0.2%) treatment. In order to begin to determine whether gemcitabine+AZD7762 inhibited the tumor initiating capacity of pancreatic CSC's, secondary mice were implanted with limiting dilutions of primary-treated, patient J, bulk tumor cells (1000-250,000) which contained known quantities of CD24, CD44, ESA positive cells (10-25,000). At 10 weeks post-implantation 98% of the untreated tumors had initiated, while 57% and 89% of tumors originating from gemcitabine- or AZD7762-treated, respectively, primary tumors had initiated. Gemcitabine+AZD7762 treatment of primary tumors had the greatest effect, reducing secondary tumor initiation to 15%. Further analysis of these data revealed that for the same number of stem cells implanted (3150) from gemcitabine- versus gemcitabine+AZD7762- treated primary tumors, secondary tumor initiation was 83% versus 43%, respectively. Taken together, these data demonstrate that AZD7762 not only reduces the percentage of pancreatic CSC's in response to gemcitabine, but also inhibits their ability to initiate secondary tumors. Furthermore, these results suggest that Chk1 inhibition may selectively sensitize pancreatic cancer stem cells to gemcitabine, thus making Chk1 a potentially important therapeutic target for inhibiting pancreatic tumor growth and preventing tumor recurrence. Supported by Astra Zeneca and NIH R01CA78554. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3919. doi:10.1158/1538-7445.AM2011-3919