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Deoxyhypusine synthase (DHPS) is the primary enzyme responsible for the hypusine modification and, thereby, activation of the eukaryotic translation initiation factor 5A (eIF5A), which is key in regulating the protein translation processes associated with tumor proliferation. Although DHPS inhibitors could be a promising therapeutic option for treating cancer, only a few studies reported druglike compounds with this inhibition property. Thus, in this work, we designed and synthesized a new chemical series possessing fused ring scaffolds designed from high-throughput screening hit compounds, discovering a 5,6-dihydrothieno[2,3-c]pyridine derivative (26d) with potent inhibitory activity; furthermore, the X-ray crystallographic analysis of the DHPS complex with 26d demonstrated a distinct allosteric binding mode compared to a previously reported inhibitor. These findings could be significantly useful in the functional analysis of conformational changes in DHPS as well as the structure-based design of allosteric inhibitors.

Deoxyhypusine synthase (DHPS) utilizes spermidine and NAD as cofactors to incorporate a hypusine modification into the eukaryotic translation initiation factor 5A (eIF5A). Hypusine is essential for eIF5A activation, which, in turn, plays a key role in regulating protein translation of selected mRNA that are associated with the synthesis of oncoproteins, thereby enhancing tumor cell proliferation. Therefore, inhibition of DHPS is a promising therapeutic option for the treatment of cancer. To discover novel lead compounds that target DHPS, we conducted synthetic studies with a hit obtained via high-throughput screening. Optimization of the ring structures of the amide compound (2) led to bromobenzothiophene (11g) with potent inhibitory activity against DHPS. X-ray crystallographic analysis of 11g complexed with DHPS revealed a dramatic conformational change in DHPS, which suggests the presence of a novel allosteric site. These findings provide the basis for the development of novel therapy distinct from spermidine mimetic inhibitors.

Eukaryotic initiation factor 4A-3 (eIF4A3) is an Asp-Glu-Ala-Asp (DEAD) box-family adenosine triphosphate (ATP)-dependent RNA helicase. Subtypes eIF4A1 and eIF4A2 are required for translation initiation, but eIF4A3 participates in the exon junction complex (EJC) and functions in RNA metabolism including nonsense-mediated RNA decay (NMD). No small molecules for NMD inhibition via selective inhibition of eIF4A3 have been discovered. Here, we identified allosteric eIF4A3 inhibitors from a high-throughput screening campaign. Chemical optimization of the lead compounds based on ATPase activity yielded compound 2, which exhibited noncompetitive inhibition with ATP or RNA and high selectivity for eIF4A3 over other helicases. The optimized compounds suppressed the helicase activity of eIF4A3 in an ATPase-dependent manner. Hydrogen/deuterium exchange mass spectrometry demonstrated that the deuterium-incorporation pattern of compound 2 overlapped with that of an allosteric pan-eIF4A inhibitor, hippuristanol, suggesting that compound 2 binds to an allosteric region on eIF4A3. We examined NMD activity using a luciferase-based cellular reporter system and a quantitative real-time polymerase chain-reaction-based cellular system to monitor levels of endogenous NMD substrates. NMD suppression by the compounds correlated positively with their ATPase-inhibitory activity. In conclusion, we developed a novel eIF4A3 inhibitor that targets the EJC. The optimized chemical probes represent useful tools for understanding the functions of eIF4A3 in RNA homeostasis.