Once localized in the tumour, a release mechanism must then be employed for polymeric prodrugs based on PEG or other polymers such as N-(2-hydroxypropyl) methacrylamide HMPA e.g., exploiting lysosomal enzymes such as Cathepsin B with a peptide linker of sequence GFLG [44,47,48], or lower pH levels of tumour cells with a hydrazone linker [49]. Here, CTSB is linked to neoplasm.