Peptide Bioconjugates of Electron-Poor Metallocenes: Synthesis, Characterization, and Anti-Proliferative Activity

We report the synthesis of metallocene compounds Cp2M with two different electron-withdrawing substituents on both cyclopentadienyl rings (hexafluoroacetone (HFA) and chlorobenzoyl (1-5); HFA and COOH (6 and 7), M=Fe or Ru). The COOH-containing derivatives were used to synthesize peptide bioconjugates with enkephalin (8 and 9) and neurotensin (10 and 11) as well as fluorescein-labeled neurotensin (12). All the molecules were fully characterized, including X-ray structures for 6 and 7. The physicochemical properties (lipophilicity and electrochemistry) and cytotoxicity on MCF-7, HT-29, and PT-45 cancer cells were evaluated for selected compounds. Electrochemical investigation by cyclic voltammetry revealed that all bis-substituted metallocenes are up to 300 mV harder to oxidize compared to the monosubstituted 2-ferrocenylhexafluoropropan-2-ol (FcHFA: Delta E-0(f)=214 mV; disubstituted derivatives: up to Delta E-0(f) = 512 mV; both vs. FcH(0/+)). For the bis-substituted compounds, logP determinations by RP-HPLC showed increased lipophilicity in comparison to the monosubstituted FcHFA and RcHFA. Cellular uptake was investigated by fluorescence microcopy, and this revealed endosomal entrapment for 12.