Modification with organometallic compounds improves crossing of the blood-brain barrier of [Leu5]-enkephalin derivatives in an in vitro model system.

Enkephalin peptides are thought to be suitable vectors for the passage of the blood-brain barrier (BBB). Modifications that do not alter the amino acid sequence are often used to improve the permeation through living membrane systems. As a new type of modification we introduce organometallic compounds, in particular ferrocene carboxylic acid. Derivatives of [Leu5]enkephalin were synthesised and labelled with organometallic compounds by using solid-phase synthesis techniques. All new metal-peptide bioconjugates were comprehensively characterised by HPLC, NMR spectroscopy and mass spectrometry and found to be at least 95% pure. For the first time, permeation coefficients in a BBB model for organometal-peptide derivatives were determined in this work. The uptake and localisation of fluorescein-labelled enkephalins was monitored by fluorescence microscopy on three cancer cell lines. Octanol/H2O partition coefficients of the compounds were measured by HPLC. The introduction of the organometallic moiety enhances the uptake into cells and the permeation coefficient of [Leu5]-enkephalin. This could be due to an increase in lipophilicity caused by the organometallic label. The metal-peptide conjugates were found to be nontoxic up to mM concentrations. The low cytotoxicity encourages further experiments that could take advantage of the selectivity of enkephalin derivatives for opioid receptors.