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R.H. Cote, Q. Feng, B.A. Valeriani; Relative Potency of Various Classes of Phosphodiesterase (PDE) Inhibitors for Rod and Cone Photoreceptor PDE . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1524.
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Purpose: PDEs are clinical targets for a variety of diseases, because of the central importance of cyclic nucleotides in signal transduction pathways. PDE inhibitors have therefore become increasingly important in drug therapies targeting cyclic nucleotide metabolism. There is inadequate information about the ability of isozyme-selective PDE inhibitors to inhibit rod and cone photoreceptor PDE (PDE6), and visual disturbances have been reported to accompany administration of PDE inhibitors to humans. Furthermore, in lieu of the ability to express recombinant PDE6 and generate site-directed mutants, structure/activity relationships of PDE inhibitors can provide insight into the catalytic site of the enzyme. We have undertaken a systematic study of the ability of isozyme-selective PDE inhibitors to bind to the active site of rod and cone PDE6. Methods: Mammalian rod and cone PDE6 were isolated from bovine retinas. Cone PDE6C was separated from rod PDE6R by Q-Sepharose chromatography, followed by hydroxyapatite and gel filtration chromatography. Activity measurements were performed using standard procedures [Cote, Meth. Enzymol. 315, 646 (2000)]. Results: As expected from the structural and biochemical similarities of PDE5 and PDE6, PDE5-selective inhibitors were most potent in inhibiting PDE6. While sildenafil was several-fold more effective in inhibiting PDE5 compared to PDE6C and PDE6R, zaprinast inhibited both isoforms of PDE6 ~7-fold better than PDE5. Another PDE5 inhibitor, E4021, showed <2-fold preference for PDE5 versus PDE6. None of the PDE5 inhibitors discriminated PDE6C from PDE6R to a significant extent. Interestingly, PDE1 - PDE4-selective inhibitors were more likely to discriminate rod and cone PDE6 than PDE5 inhibitors. The classical PDE1-specific inhibitor, 8-methoxymethyl-IBMX, inhibited PDE6C (Ki = 0.34 µM) 8-fold more potently than PDE6R, whose Ki (2.8 µM) was similar to published values for PDE1. Likewise, the PDE3 inhibitor, cilostamide, was 4-fold more potent in inhibiting PDE6C compared to PDE6R; in this case, inhibitor potency for PDE3 was more than 1000-fold greater than for PDE6. PDE2- and PDE4-selective inhibitors inhibited PDE6C ~2-fold more potently than PDE6R. Conclusions: PDE6 activity in retinal rods and cones may be adversely affected by therapeutic PDE1 and PDE5 inhibitors, thereby impairing visual function. The ability of isozyme-selective PDE inhibitors to discriminate rod and cone PDE6 will provide new insights into structural differences of the active sites of these closely related enzymes.
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