The results indicated that PDOX was not hydrolyzed (no observable changes) over 6 - 7 h in human plasma without CTSB, but hydrolyzed in 16 min in CTSB rich conditions, and the appearance of DOX correlated precisely with the disappearance of PDOX at 495 nm, suggesting that PDOX could be stable in blood circulation and effectively released at the CTSB-rich tumor site. Here, CTSB is linked to neoplasm.