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Page 280 Cancer Drug Resist 2018;1:266-302 I http://dx.doi.org/10.20517/cdr.2018.18
1 Bioscience, Oncology, IMED Biotech Unit, AstraZeneca, Cambridge, UK
2 Bioscience, Oncology, IMED Biotech Unit, AstraZeneca, Alderley Park, UK
3 Discovery Sciences, IMED Biotech Unit, AstraZeneca, Cambridge, UK
4 Wellcome Trust Sanger Institute, Cambridge, UK

The PI3K-AKT-mTOR pathway is frequently hyper-activated in breast cancer and several inhibitors tar-
geting the PI3K pathway, including AKT (AZD5363) and PI3Kβ (AZD8186), are in clinical development.
A better understanding of the mechanisms of resistance to PI3K pathway inhibitors is critical for the
development of combination strategies. To this end, we performed genome-wide CRISPR-Cas9 knock-
out screens to identify genes that mediate drug resistance to AZD5363 and AZD8186 in three PTEN-null
breast cancer cell lines. The screens have confirmed a number of known resistance genes and also revealed
a number of novel mediators. Moreover, our screens identified a number of “sensitiser” genes. Eighteen of
these hits have inhibitors in clinical development and we have performed a combination screen to identify
the compounds that give the best synergy with AZD5363/AZD8186 across the breast cancer cell lines. The
most striking combination was with the Mcl-1 inhibitor, AZD5991. Mechanistic work is ongoing and our
data suggest that AZD5363/AZD8186 “prime” cells for apoptosis and combined inhibition with AZD5991
drives a rapid apoptotic response. Furthermore, these combinations induce regressions in a breast cancer
xenograft model. Overall, our CRISPR screening data provide new insights into the pathways that drive
resistance and have identified novel synergistic combinations.

28. Acquired resistance mechanisms to β-tubulin inhibitors in triple negative breast cancer

3
3
2
Helen E. Grimsley , Jindrich Cinatl Jr , Catherine Harper-Wynne , Karina Cox , Martin
1
1
1
Michaelis , Mark N. Wass , Michelle D. Garrett 1
1 School of Biosciences, University of Kent, Canterbury, UK
2 Institut für Medizinische Virologie, Klinikum der Goethe-Universität, Frankfurt am Main, Germany
3 Maidstone and Tunbridge Wells NHS Trust
Microtubules are dynamic components of the cytoskeleton, and are common targets for anticancer thera-
pies. Disruption of mitotic processes through inhibition of β-tubulin, an essential building block of the
mitotic spindle, leads to mitotic catastrophe. β-tubulin inhibitors; Eribulin and Paclitaxel are common
drugs for the treatment of triple negative breast cancer. Triple negative breast cancer exhibits an aggressive
metastatic phenotype and patients are commonly known to relapse due to acquired drug resistance. Here
we start to elucidate mechanisms of resistance to β-tubulin inhibitors which may provide opportunities to
strategically alter their therapeutic use in patients. In vitro assays demonstrated similar phenotypes in the
Eribulin-adapted and Paclitaxel-adapted cell lines, distinct from the parental. Cross-resistance was found
upon treatment with Eribulin and Paclitaxel in the drug-adapted cell lines. Remarkably, whole exome se-
quencing analysis showed only a small number of variants gained in the drug-adapted cell lines, with even
less shared in respective drug-adapted cell lines. Our results suggest that although similar responses in the
drug-adapted cell lines are seen when treated with Eribulin and Paclitaxel, the acquired resistance mecha-
nisms are most likely to be different.

29. The role of SOX4 in drug-resistant chronic myeloid leukemia

Jorge Luis Jimenez Macias, April J. Baral, Peter Laslo

Section of Experimental Haematology, Leeds Institute of Molecular Medicine, St James’s University Hospital,
Leeds, UK