Serpin B9-IN-1 (BTCA) is an inhibitor that specifically targets Serpin B9 (SB9); SB9 is a natural inhibitor of granzyme B (GrB) , but may promote the metastasis of lung cancer cells into the bone marrow. SB9-overexpressing cancer cells promote proliferation and metastasis in an immune cell-dependent manner by binding to GrB. Inhibition of SB9 by Serpin B9-IN-1 significantly inhibits immunotherapy of lung cancer bone metastases in the caudal artery (CA) mouse model (LCBM)^[1].