Real-time, high-resolution imaging of tumor cells in genetically engineered and orthotopic models of thyroid cancer

Genetically engineered and orthotopic xenograft mouse models have been instrumental for increasing our understanding of thyroid cancer progression and for the development of novel therapeutic approaches in a setting that is more physiologically relevant than the classical subcutaneous flank implants. However, the anatomical location of the thyroid gland precludes a non-invasive analysis at the cellular level of the interactions between tumor cells and the surrounding microenvironment and does not allow a real-time evaluation of the response of tumor cells to drug treatments. As a consequence, such studies have generally only relied on endpoint approaches, limiting the amount and depth of the information that could be gathered. Here we describe the development of an innovative approach to imaging specific aspects of thyroid cancer biology, based on the implantation of a permanent, minimally invasive optical window that allows high-resolution, multi-day, intravital imaging of the behavior and cellular dynamics of thyroid tumors in the mouse. We show that this technology allows visualization of fluorescently tagged tumor cells both in immunocompetent, genetically engineered mouse models of anaplastic thyroid cancer (ATC) and in immunocompromised mice carrying orthotopic implanted human or mouse ATC cells. Furthermore, the use of recipient mice in which endothelial cells and macrophages are fluorescently labeled allows the detection of the spatial and functional relationship between tumor cells and their microenvironment. Finally, we show that ATC cells expressing a fluorescent biosensor for caspase 3 activity can be effectively utilized to evaluate, in real-time, the efficacy and kinetics of action of novel small molecule therapeutics. This novel approach to intravital imaging of thyroid cancer represents a platform that will allow, for the first time, the longitudinal, in situ analysis of tumor cell responses to therapy and of their interaction with the microenvironment.

Abstract 1801: Sequential inhibition of BCL-XL and MCL1 elicits massive apoptosis in anaplastic thyroid cancer cells

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.

Abstract LB-089: Direct BAX activation effectively induces apoptosis in anaplastic thyroid cancer cells

Despite recent advancements in chemotherapy and radiation, anaplastic thyroid cancer (ATC) remains an almost invariably lethal disease with an average life expectancy of 2-6 months. Current interventions, including surgery are palliative, underlining the urgent need for novel, effective approaches.

Targeting the apoptosis pathway using BH3 mimetics is effective in hematologic malignancies. However, in solid tumors, the response to these compounds is usually limited and short-lived. In addition, most ATCs have lost p53, resulting in a significant dampening of the pro-apoptotic signals deriving from chemotherapy or radiation.

We hypothesized that a small molecule that directly binds to and activates BAX, bypassing the need for upstream activating signals, might effectively induce apoptosis in ATC cells.

We used a novel activator, EGAV1, that is derived from the previously described BTSA1 molecule, and a panel of 10 ATC cell lines, representing the entire mutation spectrum observed in patients.

We generated dose-response profiles of these cell lines both in traditional 2D cell cultures and in 3D spheroid cultures, and found that EGAV1 induces prompt apoptosis in all cell lines. However, co-immunoprecipitation assays showed that EGAV1-activated BAX can still be sequestered by BCL-XL, which may reduce its efficacy.

Thus, we tested the ability of the BH3 mimetic Navitoclax (ABT-263) to increase EGAV1 efficacy by inhibiting BCL-XL and BCL2.

In fact, we found a dramatic decrease in the EGAV1 IC50 in the presence of a low dose of Navitoclax that has no effect as single agent. Also, we found that the efficacy of Navitoclax was equally amplified by a low dose of EGAV1.

Time-course analysis of caspase 3/7 activity confirmed that these compounds induce prompt apoptosis, peaking between four and eight hours after treatment.

Taken together these data suggest that there is an important therapeutic opportunity in targeting the apoptosis pathway in ATC with a BAX activator accompanied by a BH3 mimetic.

Citation Format: Xhesika Shanja-Grabarz, Francesca Di Cristofano, Evripidis Gavathiotis, Antonio Di Cristofano. Direct BAX activation effectively induces apoptosis in anaplastic thyroid cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-089.