CST4 and breast cancer: In analogy to the natural pan-cathepsin inhibitor cystatin, Mikhaylov et al. loaded the small molecule broad spectrum inhibitor JPM-565 into PEGylated liposomes together with ferrimagnetic iron oxide nanoparticles yielding “ferri-liposomes”, which allowed tumor-directed MRI-based imaging, and resulted in local cathepsin inhibition and finally reduced mammary tumor growth [88].