Peptides are a promising biopharmaceutical modality. Peptides are relatively straightforward to synthesize, they have low off-target toxicity and, thanks to their peptidic nature, they can bind protein targets tightly and with high specificity. Many natural peptides or protein fragments bind to important drug targets, and these peptides can be converted into potent, bioactive drugs and chemical tools. But what if there is an interesting protein target that does not have a natural peptide binding partner? This is where peptide display steps in.
Peptide display methods search very large libraries of random, artificial peptide sequences to find new binding molecules. The size of these libraries is very large indeed: billions (109) to trillions (1012) of unique peptides. To make and test these incredible numbers, methods of synthesis are borrowed from nature: DNA encoding for randomized amino acid sequences are prepared and provided to ribosomes, either in, or outside of, the cell. Elegant biotechnology is then used to ‘tag’ each unique peptide with its encoding DNA, allowing the peptides to be screeded for binding as a mixed pool – much faster than one-at-a-time testing. DNA sequencing can then be used later to identify the top binders from these mixtures, and these top peptides can then be synthesized at a larger scale, by solid phase peptide synthesis, for further testing and optimization.
At the Center for Biopharmaceuticals, we have two in-house methods of peptide display: phage display (Lohse group) and mRNA display (Rogers group). In phage display, each peptide is synthesized in E.coli as part of the coat of a phage particle. The encoding DNA ends up enclosed withning the phage particle, acting as the DNA tag needed for peptide screening. In mRNA display can be combined with ‘flexizyme’ technology to include unnatural or non-canonical amino acids, allowing for, amongst other things, the screening of cyclic peptides for potent drug-like molecules.