Engineered Nanobodies Targeting Endogenous Digoxin-like Substances Function as Novel Immune Checkpoints Inhibitors and Enhance Immunotherapy against Cancer Metastasis

Digoxin is a well-recognized cardiovascular medication; however, epidemiological investigations have associated its clinical application with an elevated risk of malignancy. To elucidate it, we discovered, for the first time, that therapeutic digoxin can facilitate tumor metastasis by acting as an inducer of immune checkpoints, including PD-1, PD-L1/2, CTLA-4, LAG-3, ICOS, TIM-3, and TIGIT, among others. The PD-1/PD-L1 axis plays a pivotal role. Digoxin upregulates PD-L1 production in cancer cells through multiple mechanisms, including digoxin-PKM2 interaction, liquid-liquid phase separation, and Na,K-ATPase degradation. Notably, endogenous digoxin (ED) was significantly upregulated in breast cancer, correlating with increased metastasis. When specific nanobodies (Nbs) were developed to functionally inhibit ED in vivo, pulmonary tumor metastasis was reduced. However, the efficacy of ED-targeting Nbs was limited by endogenous ouabain (EO), another hormone structurally similar to digoxin. To address this, Nbs co-targeting ED and EO demonstrated an increased anti-metastatic effect and a notable reduction in immune checkpoint expression. Furthermore, Nbs simultaneously targeting ED, EO, and PD-1 exhibited significant synergy in suppressing tumor progression with a favorable safety profile. In conclusion, therapeutic digoxin and ED are potent inducers of immune checkpoints. Engineered nanobodies co-targeting cardiotonic hormones (ED, EO) and immune checkpoints present a promising novel approach in the treatment of breast cancer metastasis.