Abstract 1801: Sequential inhibition of BCL-XL and MCL1 elicits massive apoptosis in anaplastic thyroid cancer cells
Authors: Shanja-Grabarz Xhesika , Cidado Justin , Cristofano Antonio Di 
Published: September 2020

Abstract
Despite advancements in cancer therapy, anaplastic thyroid cancer (ATC) remains a lethal disease. In fact, the effect of both current chemoradiation regimens and of investigative targeted therapies is at best cytostatic, as shown by the systematic lack of durable responses and the rarity even of achieving stable disease. While efficient induction of apoptosis would be the ideal and definitive therapeutic approach for this aggressive tumor, ATC cells are remarkably resistant to therapy-induced apoptotic stimuli, thanks to the high rate of TP53 loss, hallmark EMT, and frequent overexpression of anti-apoptotic BCL2 family members.
BH3 mimetics are small molecules that bind to anti-apoptotic BCL2 proteins in the docking site where the BH3 domains of the death-promoting members of the family bind, thus releasing the latter from sequestration to induce apoptosis. This concept has been successfully applied to hematological malignancies, with the recent approval of venetoclax, a BCL2 inhibitor, for the treatment of chronic lymphocytic leukemia and acute myeloid leukemia.
Solid tumors have instead proven very refractory to these approaches, and the clinical activity of BH3 mimetics is extremely limited.
To determine the effect of targeting key anti-apoptotic proteins that protect ATC cells from apoptosis, we have studied the response of a large panel of ATC cell lines to novel MCL1 (AZD5991) and BCL-2/BCL-XL (AZD4320) inhibitors.
We found that every cell line tested was modestly sensitive to each inhibitor in monotherapy, and that the AZD5991 and AZD4320 showed identical or quite similar EC50s in the low micromolar range. These data strongly suggest that ATC cells are in fact co-addicted to MCL1 and BCL-XL, and that both proteins are essential to buffer the pro-apoptotic signals induced with the transformed state. This notion, however, cannot be readily translated into a therapeutic opportunity because concomitant treatment at doses that exhibit monotherapy activity results in rapid lethal toxicity.
To begin exploring how to mitigate this toxicity, we hypothesized that inhibition of one critical anti-apoptotic protein (i.e. BCL-XL), in co-addicted cells, might shift the cells' dependence on the other protein (i.e. MCL1). Thus, we have tested whether intermittent and sequential dosing in vitro could improve efficacy compared to monotherapy, providing an opportunity for a therapeutic margin.
Strikingly, pre-treatment with a dose of the BCL-2/BCL-XL inhibitor that has no effect in monotherapy resulted in an approximately 50-fold decrease in EC50 for the MCL1 inhibitor, and in massive caspase 3-mediated apoptosis occurring within 6-8 hours from MCL1 inhibition.
These results provide a compelling proof-of-principle suggesting that ATC cells can be effectively killed using a rationally designed drug administration strategy that targets critical components of the cells' antiapoptotic machinery.
Citation Format: Xhesika Shanja-Grabarz, Justin Cidado, Antonio Di Cristofano. Sequential inhibition of BCL-XL and MCL1 elicits massive apoptosis in anaplastic thyroid cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1801.