Synthesis and Structural Characterization of Ricin Inhibitors Targeting Ribosome Binding Using Fragment-Based Methods and Structure-Based Design

Ricin toxin A subunit (RTA) is the
catalytic subunit of ricin,
which depurinates an adenine from the sarcin/ricin loop in eukaryotic
ribosomes. There are no approved inhibitors against ricin. We used
a new strategy to disrupt RTA–ribosome interactions by fragment
screening using surface plasmon resonance. Here, using a structure-guided
approach, we improved the affinity and inhibitory activity of small-molecular-weight
lead compounds and obtained improved compounds with over an order
of magnitude higher efficiency. Four advanced compounds were characterized
by X-ray crystallography. They bind at the RTA–ribosome binding
site as the original compound but in a distinctive manner. These inhibitors
bind remotely from the catalytic site and cause local conformational
changes with no alteration of the catalytic site geometry. Yet they
inhibit depurination by ricin holotoxin and inhibit the cytotoxicity
of ricin in mammalian cells. They are the first agents that protect
against ricin holotoxin by acting directly on RTA.