Targeting Integrin alpha(V)beta(3) with Theranostic RGD-Camptothecin Conjugates Bearing a Disulfide Linker: Biological Evaluation Reveals a Complex Scenario

Theranostic RGD-camptothecin conjugates, possessing a disulfide linker and a fluorescent naphthalimide moiety, were synthesized and biologically evaluated. The conjugates showed nanomolar affinity for the purified alphaVbeta3-integrin receptor. For antiproliferative assays, the U87 human glioblastoma were chosen as alphaVbeta3-expressing cells, whereas a non alphaVbeta3-expressing clone (U87 b3-KO) was generated as negative control. Although the U87 beta3-KO cells treated with the conjugates showed a statistically significant reduced fluorescence intensity (in the range 7-12%) compared to the parental U87, internalization of the conjugates was clearly observed in both cell lines. Stability studies showed premature cleavage of the disulfide linker in the cell media, with consequent release of free camptothecin. Consistent with the results of the internalization and stability studies, the conjugates did not show significant selectivity against the U87 cells compared to the U87 beta3-KO clone.