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 3H-FA, indicate that internalization is receptor-mediated and that Eu3+2 shows high selectivity and specificity for FR.