Folic Acid-Conjugated Europium Complexes as Luminescent Probes for Selective Targeting of Cancer Cells

We report the synthesis of three optical probes (Eu3+?1, Eu3+?2, and Eu3+?3) having a luminescent Eu complex (signaling unit) bonded in different positions to folic acid (FA), the folate receptor (FR) targeting unit. The structures of the two regioisomers Eu3+?1 and Eu3+?2 were assigned by mass spectrometric experiments. The optical properties and stability of these probes were assessed in phosphate-buffered saline, cell culture medium, rat serum, and cellular lysate, and results indicated that they are chemically and photophysically stable. Cytotoxicity was studied with ovarian cancer cells having high (SKOV-3), intermediate (OVCAR-3), low (IGROV-1), or null (A2780) expression of FRs. The internalized probe, evaluated in SKOV-3, IGROV-1, and A2780 cells, was in the order Eu3+?2 > Eu3+?1 > Eu3+?3. No internalization was observed for A2780 cells. Such results, together with those obtained in competition experiments of FA versus Eu3+?2 and FA or Eu3+?2 versus 3 H-FA, indicate that internalization is receptor-mediated and that Eu3+?2 shows high selectivity and specificity for FR.