Ester groups in polymer prodrugs deriving from the "drug-initiated" method can be efficiently cleaved by enzymes providing they are accessible and solvated enough. This is what has been shown by a series of studies aiming to establish a structure-drug release-cytotoxicity relationships. One of them reported the synthesis of polymer prodrugs based on cladribine (CdA), an anticancer agent used in the treatment of some leukemias. CdA was conjugated to AMA-SG1 through either a methyl-substituted ester bond (CdA-ester-AMA-SG1) or a diglycolate bond (CdA-digly-AMA-SG1), which is known to be more hydrophilic and labile. Both drug-alkoxyamines were used to polymerize isoprene by NMP, producing CdA-ester-PI and CdA-digly-PI polymer prodrugs, respectively (Figure ). The nanoparticles obtained by nanoprecipitation exhibited a diameter in the 110-160 nm range, with no release of CdA in human serum from CdA-ester-PI (< 1% after 24 h) conversely to 20 % from CdA-digly-PI. This result was assigned to the difference in steric hindrance, but also to a greater solvation and lability of the diglycolate linker. Interestingly, no obvious difference of release in PBS was observed (< 1 and < 3 %, respectively, after 24 h), confirming the enzymatic-driven cleavage of the ester groups. In line with the release experiments, CdA-digly-PI nanoparticles showed significant cytotoxicity on L1210 cells, whereas CdA-ester-PI nanoparticles did not induce any cell death. As expected, when developing similar polymer prodrugs with a strongly hydrophobic drug such as Ptx, by polymerizing isoprene by NMP from a Ptx-digly-AMA-SG1 initiator (Figure ), the resulting Ptx-digly-PI nanoparticles led to much lower release of Ptx in human serum compared to CdA-digly-PI counterparts This result supported the detrimental effect of a hydrophobic environment around the ester group, preventing efficient enzymatic cleavage. Interestingly, this could be improved by growing a hydrophilic polymer such as POEGMA from Ptx (Figure ). The Ptx-digly-POEGMA polymer prodrugs were able to self-assemble into nanoparticles of 133-151 nm in diameter, which led to 32 % release of Ptx in human serum after 24 h. This showed that the detrimental effect of using hydrophobic drugs can be compensated by the use of hydrophilic polymers when constructing the polymer prodrugs by the "drug-initiated" method. Importantly, the released Ptx in PBS reached 14 % after 24 h, which is higher than any PI-based prodrugs. In relation to previous results, the supramolecular organization of polymer prodrug nanoparticles derived from the "drug-initiated" method can also greatly affect the drug release kinetics. This has been shown with heterotelechelic polymer prodrugs in which the chain end of Gem-amide-PI prodrugs has been functionalized with another anticancer drug such as Dox or Lapatinib (Lap), for combination therapy purposes. This was achieved by a general postfunctionalization approach via the nitroxide exchange reaction using 2,2,6,6tetramethylpiperidin-1-yl)oxy (TEMPO) nitroxide previously functionalized by the drug of interest (Figure ), which can also be adapted to targeting ligands, 201 as well as fluorescent dyes for in vitro and in vivo imaging. While Gem was linked to PI through an amide linker, Dox and Lap were conjugated through either an amide or an amide-diglycolate linker.