Biology Reference
In-Depth Information
The potency of sildenafil for inhibition of the isolated catalytic domain is essen-
tially the same as that for the PDE5 holoenzyme, but the potency of vardenafil
inhibition of the isolated catalytic domain is 10- to 40-times less than that for the
holoenzyme. In fact, it has the same potency for inhibition of the isolated catalytic
domain as that of sildenafil. In the X-ray crystal structures of the PDE5 catalytic
domain in complex with the respective inhibitors, the contacts between each of the
inhibitors and the catalytic site are very similar (Sung et al. 2003 ; Wang et al. 2006 ).
The higher potency of vardenafil is retained in constructs of the catalytic domain
conjoined with GAF-B indicating that in some instances structural features outside
the catalytic domain contribute importantly to inhibitor potency and selectivity
(Blount et al. 2006 ). Thus, comparing potencies of sildenafil and vardenafil, two
closely related compounds, or predicting potencies using only the isolated catalytic
domain would have missed important elements that contribute to the higher potency
of vardenafil for the holoenzyme.
Given that many compound screens have been performed using truncated PDEs
that lack their regulatory domains and because different PDE isoforms are char-
acterized by subtle differences in conformation, this approach may have hindered
identification of more selective compounds for certain PDE families and variants
within PDE families.
2.3 Posttranslational Modification and Occupancy
of Allosteric Sites
Posttranslational modification of a PDE may have profound effects on the potency
of selected groups of compounds. This has been clearly demonstrated for both the
PDE4 and the PDE5 families. PKA phosphorylation of PDE4 affects the potency of
several compounds in a complex manner (Alvarez et al. 1995 ; Burgin et al. 2010 ;
Hoffmann et al. 1998 ; Houslay and Adams 2003 , 2010 ; Laliberte et al. 2002 ).
PKA phosphorylation of long PDE4 isoforms, in general, increases the affinity for
the prototypical inhibitor rolipram and alters the kinetics of this inhibition (Fig. 4 ).
In the same vein, phosphorylation of PDE4D3 increases its sensitivity to inhibition
by RS-25344 (~100-fold) and RS-33793 (~330-fold) and phosphorylation of
PDE4A4 increases the potencies of ( R )- and ( S )-rolipram but does not affect the
potencies of CDP-840 or SB-207499. Screening for compound potency with a
phosphorylated PDE4 isoform is a strategy that has been adopted to identify
compounds with high potency, but this has had little success in improving the
therapeutic window of this class of compounds. Phosphorylation also modulates
PDE5 enzyme functions, with detectable changes in the conformation of the
enzyme (Bessay et al. 2008 ; Corbin et al. 2000 ). These changes in conformation,
in turn, are associated with an increase in the affinity of PDE5 for substrate as well
as for inhibitors, such as sildenafil. The concept has been proposed that when PDE5
inhibitor is present in cells, cGMP level and phosphorylation of PDE5 by PKG is
increased, which in turn increases PDE5 inhibitor binding at the catalytic site, that
Search WWH ::




Custom Search