Design, Synthesis, and Biological Evaluation of Ferrocenyl–Cyclo-(Gly‑l‑Pro) Hybrids Sensitizing Multidrug-Resistant Cancer Cells to Anticancer Agents

Ferrocenyl–cyclo-(Gly-l-Pro) hybrids as novel inhibitors of ABCB1 and ABCG2 transporters were developed. These organometallic compounds were virtually nontoxic to colon cancer cells, their multidrug-resistant (MDR) variants, and normal fibroblasts. Derivatives bearing o-, m-, or p-ferrocenylphenyl groups significantly sensitized ABCB1- and ABCG2-overexpressing cells to chemotherapeutics such as vincristine, mitoxantrone, and doxorubicin, reducing IC50 values by up to 12.7- and 10.3-fold, respectively. Notably, (S,Z)-4b, (S,Z)-4c, and (S,Z)-4d showed the strongest effects. Drug combination studies revealed synergistic interactions, particularly in vincristine-, mitoxantrone-, and etoposide-resistant cells (synergy scores: 13.6–17.05). Accumulation assays confirmed ABC transporter inhibition, with (S,Z)-4b and (S,Z)-4d increasing intracellular retention of calcein and pheophorbide A up to 3.4- and 2.9-foldcomparable to those of verapamil and Ko143. Antibody-binding assays further indicated that these hybrids act as substrates of ABCB1 and ABCG2.