Synthesis, characterization, and preliminary in vivo tests of new poly(ethylene glycol) conjugates of the antitumor agent 10-amino-7-ethylcamptothecin

Despite the high antitumor activity of camptothecins, few derivatives have been developed and tested for human treatment of solid tumors, due to unpredictable toxicity mainly connected to their poor water solubility. We report the conjugation of the antitumor agent 10-amino-7-hydroxy camptothecin (SN-392) to linear or branched poly(ethylene glycol)s (PEGs) of different loading capacity through a tri- or tetrapeptide spacer selectively cleaved by lysosomal enzymes (cathepsins). A synthetic strategy based on the chemoselective acylation of the aromatic amino group in the presence of the unprotected C20 tertiary alcohol allowed high overall yields. Two conjugates demonstrated good stability at physiological pH and in mouse plasma (nonspecific proteases) but slowly released the drug payload in the presence of the lysosomal enzyme cathepsin B1. Compound 3, selected for in vivo experiments, was very active against P388, P388/ADM leukaemia, and Meth A fibrosarcoma cell lines, scoring T/C% values comparable with the camptothecin derivative CPT-11. Pharmacokinetic studies indicated that 3 acts as a reservoir of 10-amino-7-ethylcamptothecin, as the mean residence time (MRT) is about 3-fold higher than that of the free drug.