Synthesis and Biological Evaluation of Paclitaxel Conjugates Involving Linkers Cleavable by Lysosomal Enzymes and αVβ3‐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_vβ₃ 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_vβ₃+ U87 and in the α_vβ₃– 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_vβ₃+ U87/Relative Potency in the α_vβ₃– 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_vβ₃+ 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_vβ₃ integrin‐mediated process. [ABSTRACT FROM AUTHOR]