MITOTIC VULNERABILITIES
AND DRUG RESISTANCE

The antimitotic paclitaxel is used extensively to treat several cancers, such as breast, prostate and ovarian, including high-grade serous ovarian cancer (HGSOC), earning it the moniker of a blockbuster drug. However, not every tumour responds and we still have no way of predicting which patients will likely benefit. Therefore, although almost all patients with HGSOC receive paclitaxel, a substantial proportion are not benefiting, whilst being exposed to associated toxicity.

In the meantime, our understanding of paclitaxel’s mode-of-action and resistance has advanced, presenting opportunities for biomarker-driven strategies to improve paclitaxel-based therapy. Thus, more discerning use of paclitaxel could present new opportunities for women with HR-proficient HGSOC. Our living biobank of ex vivo cultures from patients with HGSOC provides a unique opportunity to guide development of predictive biomarkers for paclitaxel sensitivity, and to evaluate appropriate drugs as strategies to mitigate paclitaxel resistance.

The BCL2L1 gene, encoding pro-survival Bcl-xL, is frequently amplified in HGSOC, and Bcl-xL-high tumours are less sensitive to paclitaxel. We found that MYC primes paclitaxel-induced apoptosis by suppressing Bcl-xL and upregulating the pro-death factors Bim, Bid and Noxa. Subsequently, we showed that the selective Bcl-xL inhibitor WEHI-539 sensitizes cells to the mitotic blockers paclitaxel and nocodazole, and second generation anti-mitotics targeting Eg5, Cenp-E, and Plk1, but not to mitotic drivers, e.g. Aurora A, Aurora B and Mps1 inhibitors.

This differential is due to compensation by pro-survival Mcl-1; during a mitotic delay induced by mitotic blockers, Mcl-1 is degraded such that survival becomes critically dependent on Bcl-xL. By contrast, when cells are driven through mitosis, Mcl-1 remains intact, sustaining viability despite inhibition of Bcl-xL. Our work therefore provides a mechanistic rationale explaining why pharmacological inhibition of Bcl-xL synergizes with paclitaxel, contributing to a growing rationale for exploring Bcl-xL as a biomarker and target to enhance paclitaxel chemotherapy.

Drug efflux pumps have also emerged as contributing to paclitaxel resistance in HGSOC, in particular the ABC transporters. It was recently reported that translocations of ABCB1 were present in 18.5% of recurrent HGSOC, leading to increased expression. Thus, transporters have potential as biomarkers for drug resistance and indeed, ABCB1 expression is associated with poor outcome in ovarian cancer.

A wide range of drugs target ABC transporters with varying specificity and potency, and many are effective in pre-clinical models. Unfortunately, early clinical studies evaluating drug pump inhibitors to enhance chemotherapy were unsuccessful. However, these studies did not select patients based on expression of the targeted transporter. Now that more advanced technologies for measuring gene expression are available, there is a compelling case to revisit drug pump inhibitors to explore biomarker-based strategies to improve paclitaxel-based therapy.

We are interrogating the RNAseq and SWATH-MS datasets from our patient-derived HGSOC ex vivo cultures to determine the relative expression of pro-survival and ABC family members and correlating this with paclitaxel EC50 generated by drug-sensitivity profiling of the cultures. Samples with up-regulated transporters or Bcl-xL can then be re-challenged with corresponding inhibitors to determine if this restores paclitaxel sensitivity. We are also creating a panel of model cell lines expressing ABC transporter cDNAs under tetracycline control to confirm on-target action of drug pump inhibitors to be used, for example elacridar is used to target ABCB1 (Figure).

Key references:

• Topham C, Tighe A, Ly P, et al. MYC Is a Major Determinant of Mitotic Cell Fate. Cancer Cell. 2015; 28:129–140.

• Bennett A, Sloss O, Topham C, et al. Inhibition of Bcl-xL sensitizes cells to mitotic blockers, but not mitotic drivers. Open Biol. 2016; 6:160134.