Integrin-targeted delivery into cancer cells of a Pt(iv) pro-drug through conjugation to RGD-containing peptides

Abstract

Conjugates of a Pt(IV) derivative of picoplatin with monomeric (Pt–c(RGDfK), 5) and tetrameric (Pt–RAFT-{c(RGDfK)}₄, 6) RGD-containing peptides were synthesized with the aim of exploiting their selectivity and high affinity for α_Vβ₃ and α_Vβ₅ integrins for targeted delivery of this anticancer metallodrug to tumor cells overexpressing these receptors. Solid- and solution-phase approaches in combination with click chemistry were used for the preparation of the conjugates, which were characterized by high resolution ESI MS and NMR. α_Vβ₃ and α_Vβ₅ integrin expression was evaluated in a broad panel of human cancer and non-malignant cells. SK-MEL-28 melanoma cells were selected based on the high expression levels of both integrins, while CAPAN-1 pancreatic cancer cells and 1BR3G fibroblasts were selected as the negative control. Internalization experiments revealed a good correlation between integrin expression and the cellular uptake of the corresponding fluorescein-labeled peptides and that the internalization capacity of the tetrameric RGD-containing peptide was considerably higher than that of the monomeric one. Cytotoxic experiments indicated that the antitumor activity of picoplatin in melanoma cells was increased by 2.6-fold when its Pt(IV) derivative was conjugated to c(RGDfK) (IC₅₀ = 12.8 ± 2.1 μM) and by 20-fold when conjugated to RAFT-{c(RGDfK)}₄ (IC₅₀ = 1.7 ± 0.6 μM). In contrast, the cytotoxicity of the conjugates was inhibited in control cells lacking α_Vβ₃ and α_Vβ₅ integrin expression. Finally, cellular uptake studies by ICP-MS confirmed a good correlation between the levels of expression of integrins, intracellular platinum accumulation and antitumor activity. Indeed, accumulation and cytotoxicity were much higher in SK-MEL-28 cells than in CAPAN-1, being particularly higher in the case of the tetrameric conjugate. The overall results highlight that the great ability of RAFT-{c(RGDfK)}₄ to bind to and to be internalized by integrins overexpressed in SK-MEL-28 cells results in higher accumulation of the Pt(IV) complex, leading to a high antitumor activity. These studies provide new insights into the potential of targeting α_Vβ₃ and α_Vβ₅ integrins with Pt(IV) anticancer pro-drugs conjugated to tumor-targeting devices based on RGD-containing peptides, particularly on how multivalency can improve both the selectivity and potency of such metallodrugs by increasing cellular accumulation in tumor tissues.