Table 1. Inhibition of human recombinant PDE5A1 by TOP-N53, TOP-52, and reference PDE5 inhibitors. IC50 (half-maximum inhibition) values were calculated from concentration-dependent inhibition curves recorded at 0.5 µM cGMP and shown as means from at least 3 independent experiments.
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TOP-52, the active metabolite which is formed together with nitric oxide from TOP-N53 after intracellular bioactivation is a very potent PDE5 inhibitor (Table 1). The increased activity over TOP-N53 can be explained by the additional binding interaction with the polar residuesSer 661 and Gln 663 (Fig. 6). The intracellular release of locally active nitric oxide together with the very potent PDE5 inhibitor TOP-52 makes Topadur’s newly invented enzyme regulation system unique and led to unprecedented efficacy to elevate intracellular cGMP levels.
TOP-N53 was a potent inhibitor of PDE5 reflected by an IC50 of 1.18 nM. TOP-52, its main metabolite after the release of NO was revealed to be an even more potent inhibitor of PDE5 with an IC50 of 0.1 nM. In fact, compared with a range of marketed PDE5 inhibitors, TOP-52 was the most potent molecule (Table 1).
*In cellular assays, TOP-N53 was by far more effective than its corresponding PDE5 inhibitor TOP-52 or reference PDE5 inhibitors to increase cGMP as exemplified in washed human platelets (Fig. 7), likely reflecting the synergism inherent in TOP-N53 (Fig. 2).
