Key Points Axl inhibition by BGB324 is active in FLT3-mutated and FLT3 wild-type AML, and presence of Axl and Gas6 are required for therapeutic efficacy. AML cells educate BMDSCs to secrete Gas6, which mediates leukemia cell proliferation and therapy resistance.

Intrinsic and adaptive resistance hampers the success of antiangiogenic therapies (AAT), especially in breast cancer where this treatment modality has proven largely ineffective. Therefore, novel strategies to improve the efficacy of AAT are warranted. Solid tumors such as breast cancer are characterized by a high infiltration of myeloid-derived suppressor cells (MDSC), which are key drivers of resistance to AAT. Therefore, we hypothesized that all- retinoic acid (ATRA), which induces differentiation of MDSC into mature cells, could improve the therapeutic effect of AAT. ATRA increased the efficacy of anti-VEGFR2 antibodies alone and in combination with chemotherapy in preclinical breast cancer models. ATRA reverted the anti-VEGFR2-induced accumulation of intratumoral MDSC, alleviated hypoxia, and counteracted the disorganization of tumor microvessels. Mechanistic studies indicate that ATRA treatment blocked the AAT-induced expansion of MDSC secreting high levels of vessel-destabilizing S100A8. Thus, concomitant treatment with ATRA holds the potential to improve AAT in breast cancer and possibly other tumor types. Increasing the therapeutic efficiency of antiangiogenic drugs by reducing resistance-conferring myeloid-derived suppressor cells might improve breast cancer treatment. http://cancerres.aacrjournals.org/content/canres/78/12/3220/F1.large.jpg .