From bolinaquinone, a marine hydroxyquinone sesquiterpene, to naphthoquinones with clathrin inhibitory effects

Bolinaquinone (BLQ, 1), a hydroxyquinone sesquiterpene, has been reported to inhibit clathrin mediated endocytosis. As there is no reported clathrin IC₅₀ or commercial source of BLQ, we examined the related marine hydroxysesquiterpenes (5-10) and noted good levels of CME inhibition in a subset of these analogues. Off-target inhibition of dynamin GTPase, another key endocytosis protein, was also noted. Given the potentially complex synthesis of BLQ analogues, density functional theory (DFT) computational analysis of BLQ (1) was applied to determine potential structural modifications that would retain the electronic and conformation requirements of BLQ, but offer a more rapid access to analogue development. This analysis suggested that a minimal pharmacophore comprising substituted naphthoquinones such as 11 met our requirements. Focused library synthesis and biological screening identified naphthoquinone based BLQ analogues with excellent inhibition of the clathrin-amphiphysin protein-protein interaction (NTD-PPI, measured by ELISA) and in cell CME inhibition. Replacement of BLQ's decalin moiety gave two naphthalene-1,4-dione analogues, 2-((3-hydroxyphenyl)amino) 12 and 2-((3-hydroxy-4-methylphenyl)amino) 13, with modest inhibition of the clathrin N-terminal-amphiphysin protein-protein interaction (NTD-PPI), but with off-target inhibition of dynamin. Further modifications yielded 2-chloro-3-((3-hydroxyphenyl)amino) 23, with an NTD-PPI IC₅₀ = 2.77 ± 0.9 μM, with reduced dynamin inhibition. Further modification afforded 2-chloro-3-((4-hydroxyphenyl)amino) 26 and 5-((3-chloro-1,4-dioxo-1,4-dihydronaphthalen-2-yl)amino) 27 that were dynamin inactive and 19 μM potent clathrin inhibitors (NTD-PPI). Compound 26 retuned excellent inhibition of clathrin mediated endocytosis in U2OS cells (IC₅₀ = 2.2 ± 0.9 μM). Molecular docking and dynamic analysis with BLQ (1) and 23 across the three published clathrin crystal structures was consistent with these analogues preferentially binding within clathrin's site 1. Clathrin plays a role in cell division and quinones are known to be cytotoxic; this was confirmed herein against a broad panel of cancer cell lines. While we do not directly link the cytotoxic effects of these analogues with their clathrin inhibition, this does, as with all small molecule probes, support caution in their use. Ideally small molecule probes spanning multiple chemotypes (<i>e.g.</i> the Pitstop 1 and Pitstop 2 families of clathrin inhibitors) should be used to minimise off target effects and provide compelling information of target specific biological effects.