Figure 12: Interaction of the polymer prodrug with the cell. Targeting antibody in the conjugate interacts with its receptor in the cell membrane. Polymer conjugate enters the cell by receptor-mediated pinocytosis. (A) Drug is released in endosomes as a result of pH drop from 7.4 (external pH) to 5~6 (pH in endosomes) in case of the conjugates with pH-sensitive spacer. (B) Drug is released in secondary lysosome due to low pH (acid-sensitive conjugates) or due to enzymolysis (conjugates with spacers tailor-made as a substrate for any lysosomal enzyme).