Abstract 5339: BCL2A1 is expressed in myelofibrosis specimens and JAK2-mutated UKE-1 cells, yet does not inhibit synergistic cell killing by BCL-XL inhibitor navitoclax plus JAK1/2 inhibitors, including ruxolitinib
Authors: Refici Marion , Yang Ziping , Riehm Jacob , Phillips Darren , Souers Andrew , Harb Jason 
Published: June 2022

Abstract
Myelofibrosis (MF) is driven by mutually exclusive mutations in JAK2, CALR, and MPL in hematopoietic stem cells, leading to constitutively active JAK-STAT signaling and consequently, increased proliferation and cell survival. The JAK1/2 inhibitor ruxolitinib (Rux) is standard of care in front line patients; however, JAK inhibition alone does not induce long-term remission or disease-modifying effects. The BCL-2 family pro-survival proteins BCL-XL and MCL-1 have been implicated in resistance to Rux, and combined activity between the BCL-XL/BCL-2 inhibitor navitoclax (Nav) and JAK2 inhibition has been described in preclinical models. The combination of Nav + Rux resulted in clinically meaningful responses (e.g., improvements in spleen volume, constitutional symptoms, anemia, and bone marrow fibrosis) in patients with suboptimal response to, or progression while on, Rux (Harrison et al., EHA 2021, EP1087).
Nav does not have strong affinity for MCL-1 or BCL2A1. BCL2A1 has garnered recent attention given its potential role in tumor survival and drug resistance; however, its role in MF is unknown. Given the potential of Nav + Rux in MF, we evaluated the expression of BCL-2 pro-survival proteins, including BCL2A1, in MF patient specimens and assessed the ability of BCL2A1 to serve as a resistance factor to Nav + Rux in vitro.
DNA/RNA was extracted from PBMCs from healthy donors (n=15), JAK1/2-inhibitor naïve patients with MF (n=18), and suboptimal responders or progressors (R/R MF) currently on stable dose of Rux (n=72). When compared to healthy donors, expression of BCL2L1, MCL1 and BCL2A1, but not BCL2, was higher in naïve and R/R MF patients, with greatest increase observed in BCL2L1. Increase in median expression in naïve patients vs healthy donors for BCL2L1, MCL1, and BCL2A1 was ~4-, 1.5-, and 2-fold, respectively. A similar trend was observed with R/R MF patients vs healthy donors. We next evaluated BCL-2 family expression in in vitro models of MPN. Notably, MCL1 expression was high in UKE-1, SET-2 and HEL cell lines, while high expression of BCL2A1 was only observed in UKE-1. The combination of Nav plus either Rux, fedratinib or momelitinib induced synergistic cell killing in all lines. These data suggest that although MCL1 and BCL2A1 are expressed in JAK2-mutated cells and other malignancies characterized by constitutive JAK/STAT signaling, they do not confer resistance to a Nav + JAK-inhibitor combination.
In summary, our data demonstrate dysregulated pro-survival BCL-2 family member expression, including MCL1 and BCL2A1, in MF patient samples and JAK2-mutated cell lines, and further implicate BCL-XL as a major survival factor in both naïve and Rux-treated R/R MF patients. Together these data support a Nav/JAK1/2-inhibitor combination for both JAK1/2-inhibitor naïve patients, as well as those no longer benefiting from Rux therapy.
Citation Format: Marion Refici, Ziping Yang, Jacob Riehm, Darren Phillips, Andrew Souers, Jason Harb. BCL2A1 is expressed in myelofibrosis specimens and JAK2-mutated UKE-1 cells, yet does not inhibit synergistic cell killing by BCL-XL inhibitor navitoclax plus JAK1/2 inhibitors, including ruxolitinib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5339.