PDE6A and neoplasm: Similarly, Thangavelu Kavitha et al. [110] covalently attached the pH-sensitive polymer PDEA to the GO surface via in situ ATRP (atom transfer radical polymerization), successfully achieving efficient release of CPT in the acidic tumor microenvironment, this was attributed to protonation of CPT’s ring nitrogen at lower pH, which increased its hydrophilicity and solubility and weakened hydrophobic interactions with GO-PDEA, promoting release.