Biology Reference
In-Depth Information
of 100 mg/kg. Thus, the therapeutic margin in ferret is four- to tenfold. This is
the first exploration of PDE4B pharmacology in preclinical models. Although
compound 33 did cause emesis in ferret at a high dose, the compound is only
~120
selective for PDE4B over PDE4D. Thus, it is not clear whether PDE4B
inhibition was producing emesis or whether the PDE4D inhibition was sufficient to
reach a threshold for emesis. Perhaps, compounds with even greater selectivity for
PDE4B will be found to have proportionately lower potential for emesis.
8 Conclusions
Our new structures of the PDE4 catalytic unit include, for the first time, the key
portion of UCR2, which characterizes PDE4 super-short isoforms. This provides a
functional regulatory domain, where we demonstrate (Burgin et al
2010
) that
this portion of UCR2 is able to cap the PDE4 active site. In this, we develop a
dual-gating model of PDE4 regulation where enzyme activity is controlled by such
“capping” in a way that can be regulated by previously described changes in the
phosphorylation status of the enzyme and fine-tuned by interaction with specific
accessory proteins.
Three different PDE4 conformers have been identified that can be exploited for
the design of subtype-selective or kinetically novel PDE4 allosteric modulators.
The first is an open conformer in which both active sites are accessible to substrate
and are catalytically independent. The second is a symmetrical, closed conformer in
which a C-terminal regulatory helix closes intramolecularly across the active of
each PDE4 monomer. In this conformation, the enzyme is completely inhibited
with both active sites closed to cAMP. The third is an asymmetric PDE4 dimer in
which one UCR2 is closed intermolecularly across the active site of the opposite
monomer.
PDE4 subtype selectivity can be achieved by designing compounds that interact
selectively with the gating helices in UCR2 or at the C-terminus. Subtype-selectivity
can be achieved in the case of the UCR2-binding site by exploiting the Phe/Tyr
polymorphism in PDE4D that is unique to primates. How subtype-selectivity is
achieved by compounds that engage the C-terminal gating helix (Naganuma et al.
2009
) awaits detailed structural information.
In the open PDE4 conformer, the traditional approach to the design of competitive,
active site-directed inhibitors has yielded potent compounds that nonselectively
inhibit all PDE4 subtypes. PDE4 competitive inhibitors have shown therapeutic
benefit in human clinical trials, primarily in respiratory diseases such as asthma
and chronic obstructive pulmonary disorder (Giembycz
2002
; Lipworth
2005
).
However, the clinical dose for such actions of these compounds appears to be
limited by emesis.
By weakening the interaction with the active site P-clamp, UCR2-directed,
allosteric modulators can be designed that only partially inhibit PDE4 hydrolysis
of cAMP. As only one active site can be capped in the dimer, allosteric modulators
