Biomedical Engineering Reference
In-Depth Information
which compound
16
was prepared. This fragment contained orthogonal functionality
that could initially partake in a condensation coupling reaction with 5-bromoisatin
(
17
), followed by in situ diastereoselective pairing to afford the spiroazepineindole
19
. The importance of having access to every possible stereoisomer in a diversity
library was seen upon chiral resolution of the reaction products. Testing the 1
R
,3
S
and 1
S
,3
R
stereoisomers of
19
in vitro against the NF54 parasite strain showed that
the former was 250-fold more potent than its enantiomer (Table 17.2).
Libraries prepared using the build/couple/pair strategy facilitates subsequent
medicinal chemistry by modularizing the synthetic route. For example, Novar-
tis chemists could readily vary the size of the seven-membered ring primary hit
(spiroazepineindole
19
) to six-membered spirotetrahydro
-carboline
25
(termed
spiroindolone
) simply by employing an indoleamine with one less methylene unit
(Scheme 17.4). Thus, starting from D-tryptophanol, amine
23
could be obtained
through overall reduction to a methyl group of the hydroxymethyl group in
20
.
A diastereoselective Pictet-Spengler reaction provided the trans isomer (1
R
,3
S
)-
25
as the major component. Access to L-tryptophanol allowed for preparation of the
enantiomer, and both substrates provided the separable cis isomer, making all possi-
ble stereoisomers obtainable. In practice, due to difficulty obtaining various starting
materials, the Novartis chemists preferred a racemic route using chiral resolution to
obtain pure compounds.
DOS libraries designed to access the full complement of stereoisomers allow for
powerful SSAR and are invaluable in terms of both investigating potency (as seen
above) and pharmacokinetic properties. As shown in Table 17.2, the in vitro potencies
of the spirotetrahydro
-carboline series varied dramatically among the four possi-
ble stereoisomers. Additionally, significant differences between enantiomers were
observed with respect to metabolic stability. For example, the highly potent 1
R
,3
S
-
25
had inferior metabolic stability and high clearance relative to its enantiomer 1
S
,3
R
-
25
, which displayed excellent pharmacokinetic properties but weak potency. Novartis
chemists were able to overcome these liabilities by identifying parts of the molecule
susceptible to CYP enzyme metabolism and blocking these sites with halogens to
extend the compound's half-life. These standard medicinal chemistry techniques to
improve clearance were no doubt aided by the ability to interchange fragments or
portions of the molecule readily without having to develop new chemistry. Attempts
to remove stereochemistry, such as the C3 methyl group (compound
26
)ledtoa
decrease in activity. Compound
RS-27
, known as NITD609, has potency in the low-
nanomolar range and shows no evidence of diminished potency against a number
of drug-resistant strains, suggesting a new mechanism of action. This compound
showed potency at early and late
P. falciparum
development, in contrast to the late
(trophozoite)-stage specific activity observed with chloroquine, although NITD609
did not inhibit parasite growth as quickly as the artemisinin derivatives.
NITD609 showed no significant cytotoxicity against a panel of in vitro mamalian
cell lines and was not mutagenic. Low binding to hERG and a series of human
G-protein-coupled receptors, enzymes, and ion channels was also observed. In oral
and intravenous PK studies in mice and rats, NITD609 had a good half-life (
T
1/2
=
10 and 27.7 h, respectively), moderate volume of distribution (
V
SS
=
2.11 and 3.04
