The Discovery of Piragliatin, a Glucokinase Activator - Accounts in Drug Discovery Case Studies in Medicinal Chemistry - page 64

However, several of these analogs were also less potent than the parent
unsubstituted thiazole in vitro and in vivo, had poor pharmacokinetic proper-
ties, and were not considered further. Hence, we reinvestigated alternative
heterocyclic ring systems as replacements to the thiazole moiety.
We observed earlier during SAR studies that introduction of a small elec-
tron-withdrawing group such as a halogen or cyano group next to the methyl
sulfone moiety on R 3 generally led to a modest improvement in potency
(Table 3.1). More interestingly, when the thiazole of 9a was replaced by a six-
membered heterocycle, such as a pyridine, pyrazine, pyrimidine, or quinoline,
there was a significant loss of activity, whereas incorporation of a 3-chloro-4-
(methylsulfonyl)phenyl ring was found to enhance potency at least 10-fold over
the corresponding 4-(methylsulfonyl)phenyl derivative of these six-membered
heterocycles. From these observations, and extensive comparison of in vitro
profiles, in vivo potency, rodent pharmacokinetics, and five-day high-dose rat
exploratory toxicity profiles of several compounds, we selected 27 (RO0505082)
as a development candidate. This compound was found to be as ecacious as
compound 9a in reducing fasting and post-prandial glucose levels and was
profiled in a rat toxicity study. In a rodent 14-day safety range finding study at
doses up to 600mg kg 1 , there was no evidence for hepatic lipidosis, supporting
our hypothesis that the lipidosis observed with 9a was related to metabolic
thiourea formation rather than GK activation.
However, 27 was found to be a potent inhibitor of the hERG potassium
channel (IC 20 E

3 mM) and showed activity in a rabbit Purkinje fiber assay
(EC 50 @1Hz ¼ 8.2 mM), suggesting a high potential for cardiovascular risk
based on the low margin between the plasma ecacious exposure levels and
hERG IC 20 values. In addition, 27 caused a time-dependent inhibition of
CYP3A4 (52% inh @ 24min, 10 mM incubation), indicating a potential for
causing drug-drug interactions and undesired non-specific covalent modifica-
tion of proteins, which could lead to unpredictable idiosyncratic toxicity. These
findings were considered to pose an unacceptable risk for further development
and consequently 27 was abandoned.
Detailed characterization of the metabolites of 27 from liver microsomal
studies, in vivo rat PK, and safety studies showed a significant formation of
alcohol and ketone metabolites resulting from oxidation of the cyclopentyl ring
at C2 and C3 (Figure 3.14). The extent of metabolism in liver microsomal
studies was found to be 76% in human and rat, while in dog it was 56%.
Initially, the authentic diastereomeric mixtures of metabolites were synthesized
for potency and in vitro safety evaluations. 20 The diastereomeric mixtures
of the C2-hydroxyl 28 and C3-hydroxyl 29 metabolites were less potent in GK
activation compared to the parent 27 (Figure 3.15). However, the diaster-
eomeric mixtures of C2- (30) and C3- (31) ketone metabolites had comparable
potency to 27. Both the hydroxyl and keto metabolites had reduced hERG
inhibition and CYP 3A4 time-dependent inhibition compared to the parent 27
(Figure 3.15). We chose to further evaluate the C3-keto metabolite 31 over the
C2-keto metabolite 30 in view of the potential of the C2-ketone to promote
epimerization of the adjacent chiral center.