S-31. Proteases of RNA viruses as drug targets

The majority of recent viral outbreaks has been caused by RNA viruses. Yet, for most viral infections, no causative therapy is available. Hence, there is an urgent need for discovering and developing new antivirals. For the past few years, my laboratory has been working on the structure-based design and the chemical synthesis of inhibitors of proteases from coronaviruses and picorna/enteroviruses.

Coronaviruses have been in the spotlight since the SARS outbreak of the year 2003. These viruses feature the largest genome of all plus-stranded RNA viruses (27–31 kb of ssRNA). Open reading frame 1 covers 2/3 of the entire genome and codes for two polyproteins, pp1a and pp1ab, the latter of which arises through a ribosomal frameshift during translation. These polyproteins are processed by two or three viral proteases, yielding the non-structural proteins (Nsp1-16) required for virus replication. In most coronaviruses, these cleavage reactions are performed by three cysteine proteases, two of which are of the papain type (PL1^pro and PL2^pro) and one of the chymotrypsin type (M^pro). In SARS-CoV, there is only one PL^pro. The main protease of Transmissible Gastroenteritis Virus (TGEV) was the first protein of any coronavirus to have its three-dimensional structure determined. This was followed by the structures of the homologous enzyme from human coronavirus 229E and from the SARS coronavirus. Soon it became clear that the main protease is an attractive target for the design of anticoronaviral inhibitors. The latest inhibitors will be discussed, and a number of their complexes with the M^pro will be presented.

Some of our inhibitors also exhibit an interesting activity against coxsackievirus B3 (CVB3) and caliciviruses. Hence, we have determined the crystal structure of the 3C protease of CVB3 and designed inhibitors against this target. Latest developments will be discussed.