Synthesis and Biological Evaluation of Paclitaxel Conjugates Involving Linkers Cleavable by Lysosomal Enzymes and α V β 3 ‐Integrin Ligands for Tumor Targeting

Two cyclo [DKP‐RGD]‐PTX (PTX = paclitaxel) and two cyclo [RGDfK]‐PTX conjugates containing the Gly‐Phe‐Leu‐Gly (GFLG) linker, which is cleavable by lysosomal enzymes, were synthesized and compared to two cyclo [DKP‐RGD]‐Val‐Ala‐PTX conjugates. The conjugates were evaluated for their ability to inhibit biotinylated vitronectin binding to the isolated α v β 3 receptor, retaining good binding affinity, in the same nanomolar range of the free ligands. Cell viability assays were performed for the six conjugates in the α v β 3 + U87 and in the α v β 3 – HT29 cell lines. Loss of potency was observed for all the conjugates, attenuated by the presence of a tetraethylene glycol (PEG‐4) spacer. A good Targeting Index (TI = Relative Potency in the α v β 3 + U87/Relative Potency in the α v β 3 – HT29) was displayed by the conjugates, in particular by cyclo [DKP‐RGD]‐PEG‐4‐Val‐Ala‐PTX 9 (TI = 533). This conjugate was tested in the α v β 3 + U87 cell line in the presence of 50‐fold excess free cyclo [DKP‐RGD] ligand 2 . In this competition experiment, a fivefold decrease of the conjugate cytotoxicity was calculated, suggesting that the conjugate is possibly internalized by an α v β 3 integrin‐mediated process.