CTSB and neoplasm: Concentration gradients of specific enzymes, such as matrix metalloproteinases (MMPs), hyaluronidase (HAase), cathepsin B, and esterase, between tumor tissues and normal tissues (usually tumor tissue > normal tissue) can be used as appropriate activators for achieving localized drug release (Jhaveri et al., 2014; Ghaffari et al., 2018; Rahikkala et al., 2018).