Angewandte
Chemie
scaffold diversity of the computationally designed chemo-
types might help us find molecules with improved 5-HT2B
selectivity. Therefore, we employed the de novo design
software DOGS[19] coupled to the GP prediction models to
generate NCEs, taking 239 GPCR-targeting and FDA-
approved small molecules as design templates (GPCR
framework diversity of de novo-designed molecules by
expanding the search space with up to 58 tractable reaction
types for stepwise fragment growing.[19] Furthermore, we
employed a large template set (SARfari), as target promis-
cuity is common among GPCR ligands.[2c] From the DOGS
runs we obtained 111854 unique, synthetically accessible and
GPCR-ligand-like designs. A total of 25363 Murcko scaffolds
with profound dissimilarity to the design templates and
annotated high-affinity 5-HT2B ligands in ChEMBL (average
structural Tanimoto similarity = 0.13 Æ 0.07 and 0.17 Æ 0.04,
respectively) highlight the scaffold-hopping capabilities and
intrinsic explorative nature of the DOGS algorithm.
We subjected all designed NCEs to the PAINS[20] and
ADMET[21] filters to eliminate potentially undesirable small
molecules and scored the remaining 78468 designs with the
GP target prediction models. In line with previous findings in
which ca. 30% of the Novartis corporate compound library
binds to 5-HT2B at 10 mm,[13] the computationally designed
molecules presented an average pAffinity value of 5.0 Æ 0.6.
For the training data we computed an average pAffinity value
of 5.1 Æ 0.5, suggesting no bias in the design and scoring
algorithms towards 5-HT2B ligands over other GPCRs. We
selected compounds 5–8 for subsequent synthesis according
to either pAffinity and/or GPCR-panel selectivity criteria.
Although no 5-HT2B selectivity was forecast for 5 (Fig-
ure 3A), we chose it because of its
Consequently, we prioritized compounds 6–8 based on GPCR
panel selectivity, including the 5-HT2A and D2 receptors
(Figure 3A and SI). Compounds 6 and 7 were designed from
the low-affinity 5-HT2B ligands rizatriptan and metaraminol,
respectively. Molecule 6 had already been disclosed as a 5-
HT2B receptor agonist,[25] but its selectivity over the dopa-
mine D2 receptor remained unknown. Although limited
utility as a drug lead may be foreseen for 6, due to 5-HT2B
agonism, the herein experimentally confirmed potent func-
tional effect (EC50 = 18 Æ 3 nm; Figure 2B) and GPCR panel
selectivity (Figure 3C, SI) may still warrant high value as
a chemical probe. A two-step synthetic route afforded 7.
Despite its excellent overall selectivity (Figure 3C), the
moderate 5-HT2B binding affinity (Ki = 1700 Æ 25 nm; Fig-
ure 3A) was translated into an undesired partial agonistic
effect (EC50 = 2365 Æ 243 nm; Figure 3B). Irrespective of the
functional activity of the profiled ligands, the computer-
assisted design method (DOGS) fulfilled our primary goal of
designing GPCR-tailored ligands with accurately predicted
selectivity profiles. Compound retrieval by library enumera-
tion and similarity searching would probably have failed to
identify 6–7 as screening candidates due to their low structural
similarity to the design templates as well as the ChEMBL
training data (Table 2). Our final focus was on the piperazine
derivative 8, which we acquired in flow. Binding assessment
against the 5-HT2 receptor panel revealed selectivity toward
5-HT2B (Ki = 251 Æ 0.02 nm, LE = 0.42; 5-HT2A: Ki = 3383 Æ
1100 nm; 5-HT2C: Ki = 18100 Æ 1570 nm), a potent antagonis-
tic effect in vitro (IC50 = 611 Æ 240 nm, Figure 3B) and func-
tional selectivity against a panel of GPCR off-targets and the
hERG cardiac potassium channel (Figure 3C). These results
are important taking into account that 8 presented no
chemical similarity to aripipra-
zole[24] (Tanimoto similarity =
0.51) and its predicted 5-HT2B
binding affinity (pAffinity = 6.8).
Analogously to its template
(Table 2) and as predicted by the
GP models, compound 5 showed
binding affinity for 5-HT2A-B (Fig-
ure 3A). Our results suggest the
importance of structural features in
aripiprazole for potent 5-HT2B
binding and overall 5-HT2 receptor
subtype selectivity. In an orthogo-
nal assay, 5 showed a strong 5-HT2B
antagonistic effect (IC50 = 225 Æ
26 nm; Figure 3B), fully corrobo-
rating receptor binding. The
remaining pooled designs were
analyzed for potential 5-HT2B
selectivity over other GPCRs and
Figure 3. A) Predicted affinity (pAffinity) and experimental pKi values obtained for 5–8 and template
structures through radioligand assays against 5-HT2A–C. Ligand efficiency LE=(À1.4 logKi)/number
heavy atoms. B) Cell-based functional effect (IP1 quantification by HTRF detection, see SI) of 5–8
against 5-HT2B. Compounds 5 and 8 are antagonists of 5-HT2B, whereas 6 and 7 are full and partial
agonists, respectively. C) Cell-based functional effects (%antagonism and agonism) of 6–8 against
selected off-targets (10 mm, n=2), suggesting selectivity for the 5-HT2B receptor. Note: Full details on
hERG. Although no design
revealed clear predicted selectivity
over the 5-HT2A/C and D2 receptors
simultaneously, the engagement of
5-HT2C is apparently uncorrelated
with known adverse drug reactions. controls are provided in the SI; For hERG, %inhibition is given.
Angew. Chem. Int. Ed. 2015, 54, 1551 –1555
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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