Design, Synthesis, and Biological Evaluation of Ferrocenyl–Cyclo-(Gly-<scp>l</scp>-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 <i>o</i>-, <i>m</i>-, or <i>p</i>-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, (<i>S</i>,<i>Z</i>)-<b>4b</b>, (<i>S</i>,<i>Z</i>)-<b>4c</b>, and (<i>S</i>,<i>Z</i>)-<b>4d</b> 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 (<i>S</i>,<i>Z</i>)-<b>4b</b> and (<i>S</i>,<i>Z</i>)-<b>4d</b> 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.