To protect peptide pharmaceuticals against peptidases.

INTRODUCTION: The major hurdle in the application and delivery of peptide pharmaceuticals is their rapid in vivo breakdown. METHODS: We here combined two approaches to stabilize peptide pharmaceuticals, introduction of D-amino acids and cyclization, by applying an innovative enzymatic method. This method yields peptides with thioether bridges between a D-amino acid and an L-amino acid. On the basis of guidelines concerning the flanking residues of serines/threonines and cysteines, a peptide of interest is designed with serine/threonine and cysteine at appropriate positions to allow their effective participation in cyclization. In Lactococcus lactis the peptide of interest is directly or via a spacer genetically fused to a lantibiotic leader peptide which induces enzyme-catalysed synthesis of a thioether-bridged peptide. The peptide is translocated via a lantibiotic transporter, analysed by mass spectrometry and the leader peptide is removed. Because of its therapeutic relevance and terminal modifications we chose the decapeptide Luteïnizing Hormone Release Hormone (LHRH) as a test case for thioether bridge introduction. The N-terminal pyroglutamate protects against aminopeptidase activity; the amidated C-terminus, which occurs in 50% of all therapeutic peptides, precludes carboxypeptidase action and is essential for optimal receptor interaction. We had Lactococcus posttranslationally introduce a thioether bridge between residues 4 and 7 of the Leu7Cys-LHRH analog QHWSYGCRPG. The N-terminal glutamine of the thioether-bridged peptide could be converted in pyroglutamate. The introduction of the thioether bridge proved to be compatible with subsequent chemical and enzymatic amidation methods. In this way biologically produced thioether LHRH was compared with LHRH isomers obtained by base-assisted sulfur extrusion. RESULTS: Biologically produced thioether LHRH is the most stable thioether LHRH isomer with strongly enhanced proteolytic resistance compared to natural LHRH. DISCUSSION: The data convincingly demonstrate the broad perspective of stereo- and regiospecifically generating cyclized peptide pharmaceuticals with significantly enhanced therapeutic potential.