Synthesis and Biological Evaluation of Paclitaxel Conjugates Involving Linkers Cleavable by Lysosomal Enzymes and αVβ3- Integrin Ligands for Tumor Targeting

摘要

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_vβ_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_vβ_(3+) U87 and in the α_vβ_(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_vβ_(3+) U87/Relative Potency in the α_vβ_(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_vβ_(3+) U87 cell line in the presence of 50-fold excess free cyclo[DKPRGD] 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_vβ_3 integrinmediated process.