Biology Reference
In-Depth Information
D159687, a UCR2-directed, partial inhibitor was 300-fold less emetic than the
D157140 or rolipram, despite equal potency in cognition assays (Table
4
). D159687
was 3,000-fold less emetic than rolipram in the beagle dog, and 500-fold less emetic
in monkey. Thus, there is a striking difference between full and partial PDE4D
selective-inhibitors directed against UCR2 with respect to tolerability. As discussed
above, PDE4 allosteric modulators have less impact on cAMP hydrolysis in a cellular
model than traditional, active site-directed compounds. Allosteric modulators that
only partially inhibit PDE4D activity may achieve improved tolerability by main-
taining temporal and spatial aspects of cAMP signaling.
7 C-Terminal-Directed Inhibitors Bind a Closed,
Symmetric PDE4 Conformer
It also may be possible to exploit for drug design a symmetric conformer of PDE4
in which the C-terminal helices cap both active sites (Fig.
3
). Naganuma and
colleagues recently reported the discovery of PDE4 inhibitors that are
100-fold
selective for PDE4B over PDE4D (Naganuma et al.
2009
). The chemical series
explored by Naganuma utilized a pyrimidine core, which provides a ring nitrogen
able to hydrogen bound to the invariant, active site glutamine, as well as a planar
aromatic scaffold that will be sandwiched by the active site P-clamp. Their pyrimi-
dine series additionally explored the SAR around a pair of aromatic arms, which our
molecular superposition studies suggest are similar to the chemotype described
above for UCR2-directed, allosteric modulators (Fig.
5
).
Our studies of Naganuma compound 33 indicate that it likely interacts with the
C-terminus of PDE4B, thereby achieving selectivity and potency (Fig.
5
). We
compared activity of compound 33 against PDE4B5, a supershort isoform, and a
truncation lacking the C-terminus. This demonstrated that the compound required
the PDE4B C-terminus and not the PDE4B UCR2 for potency and subtype selec-
tivity. We think it likely that compound 33 exploits a binding pose similar to that
which we reported for PMNPQ (PDB ID: 3G45) and a C-terminal PDE4D regulatory
helix (Burgin et al.
2010
). In that structure, the aromatic arms of PMPQ clamps a
phenylalanine in a C-terminal helix that caps the active site. The C-terminal helix
spans residues Gly593 to Thr603 (GQTEK
F
QFELTL) and contains a conserved FQF
motif. The region is homologous to Gly673 to Thr683 in PDE4B3. Thus, we think it
likely that compound 33 is acting as a bifunctional intramolecular clamp bridging
Gln615 in the PDE4B active site and an aromatic residue, likely a phenylalanine on
an
a
-helix in the PDE4B C-terminus.
Naganuma et al. report that compound 33 inhibits TNF
a
production in mice
in vitro and in vivo (Naganuma et al.
2009
). In mice, it decreases LPS-induced
TNF
a
production with a 50% inhibitory oral dose of 14 mg/kg. The compound also
decreases LPS-induced neutrophil migration to lung in ferrets at a 50% inhibitory
dose of 12.5-25 mg/kg. In ferrets, the NOEL for emesis is reported as an oral dose
>
