Novel elastase inhibitors from marine cyanobacteria: Structural diversity, target-bound crystal structures and cellular effects

We report the isolation and structure determination of six new dolastatin 13 analogs given the trivial names symplostatins 5–10, the molecular basis of their anti-proteolytic activity, and characterization of their cytoprotective effects. IC₅₀ determination for symplostatin 5 against a panel of 26 serine proteases revealed potent and selective anti-proteolytic activity against elastase with low-nanomolar IC₅₀, and to a lesser extent against chymotrypsin, while IC₅₀s for the other serine proteases tested occurred only at high-micromolar concentrations. Crystallization of the elastase-inhibitor complex at 1.5 Å resolution offered insights on key pharmacophores for this class of serine protease inhibitor, with the 2-amino-butenoic acid (Abu) moiety being central, while other functionalities can be tuned to achieve further improvements in potency and selectivity. The structural basis of elastase inhibition was also corroborated by structure-activity relationship studies derived from in vitro enzyme inhibition assays. This family of compounds can also alleviate the effects of exogenous elastase on mammalian cells, and elastase-induced cell injury was attenuated with inhibitor treatment.