Focused library development of our lead 2-cyano-3-(1-(3-(dimethylamino)propyl)-2-methyl-1H-indol-3-yl)-N-octylacrylamide (2) confirmed the tertiary dimethylamino-propyl moiety as critical for inhibition of dynamin GTPase. The cyanoamide moiety could be replaced with a thiazole-4(5H)-one isostere (19, IC(₅₀(dyn I)) = 7.7 μM), reduced under flow chemistry conditions (20, IC(₅₀(dyn I)) = 5.2 μM) or replaced by a simple amine. The latter provided a basis for a high yield library of compounds via a reductive amination by flow hydrogenation. Two compounds, 24 (IC(₅₀ (dyn I)) = 0.56 μM) and 25 (IC(₅₀(dyn I)) = 0.76 μM), stood out. Indole 24 is nontoxic and showed increased potency against dynamin I and II in vitro and in cells (IC(₅₀(CME)) = 1.9 μM). It also showed 4.4-fold selectivity for dynamin I. The indole 24 compound has improved isoform selectivity and is the most active in-cell inhibitor of clathrin-mediated endocytosis reported to date.