Biology Reference
In-Depth Information
highly resistant to breakdown by PDEs (Beltman et al.
1995
; Butt et al.
1995
; Poppe
et al.
2008
; Sekhar et al.
1996
). Low doses of such compounds would be predicted
to work on two target proteins to synergistically foster increased cN signaling
through that pathway. Examples include YC-1 and BAY 41-2272, both of which
are NO-independent activators of soluble guanylyl cyclase and act to elevate cGMP
in platelets, inhibit platelet aggregation, and lower blood pressure (Friebe et al.
1998
; Galle et al.
1999
; Mullershausen et al.
2004
; Stasch and Hobbs
2009
); each
also acts as a PDE inhibitor. Thus, the biological actions result from the simulta-
neous increase in cGMP production and inhibition of cGMP breakdown. The
combined effects of such compounds at two points in a pathway would foster
a synergistic increase in cN signaling and potentially produce meaningful changes
in signaling even at low ligand concentrations.
4.3
Interaction with Other Proteins
Interaction of PDE inhibitors with proteins other than PDEs is a serious problem that
is quite commonly ignored by investigators. However, this oversight compromises
many studies since many investigators accept the hyperbole asserting that
compounds are truly “specific and selective” for the targeted PDE.
The use of zaprinast, which has long been touted as a selective inhibitor for
PDE5/PDE6, is a prime example of such a problem. The interpretation of the results
of the vast majority of studies where zaprinast has been used attributes its effects
entirely to inhibition of PDE5. Its action to also inhibit PDE1 with reasonable
potency is largely ignored. However, zaprinast has recently been shown to be
a potent agonist for GPR35 (EC
50
as low as 16 nM), a widely expressed receptor
for kynurenic acid (Ohshiro et al.
2008
; Taniguchi et al.
2006
). In contrast to the
widely and erroneously accepted action of zaprinast to exclusively inhibit PDE5
(IC
50
~ 130 nM), promote elevation of cNs and lower calcium, its action on GPR35
inhibits adenylyl cyclase activity and induces intracellular calcium mobilization.
Notably, these effects are not mimicked by two potent PDE5/PDE6 inhibitors that
structurally differ from zaprinast. In referring to the earlier studies, the authors of
the report note “it may be necessary to repeat those experiments (
done only with
zaprinast
) with different structural classes of selective PDE inhibitors.”
Theophylline, a natural compound and one of the first PDE inhibitors to be
discovered, is still in wide use for investigational purposes and various clinical
treatments; its biological effects are mediated by actions as an antagonist of A
1
-
adenosine receptors or as a nonspecific inhibitor of PDEs. Relatively high plasma
concentrations of theophylline (10-20 mg/L, which translates to ~50-100
m
M)
are required to elicit significant bronchodilation (Barnes
2006
). Based on IC
50
values of theophylline for several PDEs, it is entirely possible that this level
could significantly diminish PDE activity, thereby promoting cN elevation (Butcher
and Sutherland
1962
; Dent and Rabe
1996
), and it is well accepted that the relation-
ship between the extracellular concentration of a PDE inhibitor and its intracellular
