Applications of Microorganisms in the Discovery of Pharmaceuticals

APPLICATIONS OF MICROORGANISMS IN THE DISCOVERY OF PHARMACEUTICALS

Microbial display platforms expressing recombinant polypeptides (peptides, antibodies, enzymes) on their surface are emerging as invaluable tools for the investigation of protein–protein interactions and can serve as biological combinatorial libraries for the discovery of new therapeutics. To date three microbial display platforms have been described: phage, bacterial, and yeast display. All three technologies share the common principle of a direct link between genotype and phenotype affording the identification of the displayed polypeptide by gene sequencing. Of the three technologies described, it is probably phage display that has witnessed the most widespread application.

A)   Phage Display

The filamentous bacteriophage readily accepts relatively large insertions of additional genetic material into its genome, which allows for the display of polypeptides as fusions with bacteriophage coat proteins. Phage libraries containing a repertoire of many billions of viral particles that each display a unique polypeptide sequence can be subjected to an affinity selection process against a target of interest. Those phage clones that display a polypeptide that strongly interacts with the target can be recovered and amplified for further rounds of selection before the direct link between genotype and phenotype is exploited to identify the polypeptide displayed.

Although to date no phage-derived product has reached market, a number of clinical trials are currently under way using polypeptides derived from phage display: DX-88 (Ecallantide, Dyax Corp) is a highly specific and potent inhibitor of kallikrein (K_i ∼20–40 pM). Kallikrein is a key molecule in the regulation of infl ammatory and blood clotting processes and plays a role in a number of autoimmune and infl ammatory conditions. DX-88 has successfully completed phase III trials in the treatment of hereditary angiodema and phase I/II trials for the prevention of blood loss in on-pump coronary artery bypass graft surgery. Compstatin (Potentia Pharmaceutics), a cyclic 13-mer peptide originally identified from phage display, is a selective and potent inhibitor (K_d ∼0.13 μM) of C3 protein in the complement cascade and will shortly enter clinical trials for the treatment of age-related macular degeneration.