Biomedical Engineering Reference
In-Depth Information
derivatization, which in the case of
31
was achieved using an isocyanate. Subsequent
removal of the Boc group allowed for derivatization at the amine side chain. Finally,
the alcohol at C1 could be deprotected using DDQ, and nitrogen-based functional
groups could be accessed with DPPA under Mitsunobu conditions.
Broad Institute scientists initially explored the aniline site (Table 17.3), and while
all analogs saw a loss of potency, preparation of the 4-amino-3,5-dimethylisoxazoyl
urea analog
40
provided valuable aqueous solubility. Attempted optimization of the
amine side chain resulted in a loss of biological activity for all compounds, even with
the regioisomeric 2-fluorophenylsulfonamide
42
, suggesting that this portion of the
molecule contains an essential pharmacophore.
Substituents of the amide side chain were then investigated (Table 17.4). Interest-
ingly, the
para
-methoxybenzyl ether
47
, which is accessed en route to alcohol
31
, had
subnanomolar potency (EC
50
=
0.18 nM). A range of groups were appended to the
alcohol and showed similar improvements, but lacked solubility in water. The solu-
tion to the compound's poor solubility came from the introduction of nitrogen-based
functional groups. As shown in Table 17.3, introduction of an azide produced a highly
potent analog (
51
) but one still lacking aqueous solubility. Staudinger reduction, fol-
lowed by bis-methylation of the resulting amine, affords compound
54
with both
subnanomolar potency (EC
50
=
0.54 nM) and solubility in water (water solubility
=
124
M), achieving the initial objective of discovering a highly potent antimalarial
with good aqueous solubility. This compound is more potent than chloroquine or
artesunate and is similar in potency to atovoquone. Compound
54
is a promising
antimalarial with a unique chemotype and subnanomolar activity in multiple malaria
strains. This compound is soluble in water and nontoxic to erythrocytes and HepG2
cells. Studies to discover the mechanism of action for this new class of antimalarial
agents are currently in progress.
17.4 CASE STUDY 3: TARGETING PROTEIN-PROTEIN AND
PROTEIN-DNA INTERACTIONS
Considerable attention has been cast of late on protein-protein interactions (PPIs)
and their ability to be perturbed by small molecules. PPIs play a critical role in many
biological processes, and the ability to stabilize [49] or disrupt these interactions is an
approach to developing new therapeutics [50]. Recent literature has estimated about
130,000 PPIs in the human interactome, of which only about 8% have been identified
[51]. In the past, these targets were largely avoided, due to the thought that the protein
surfaces were too large and featureless to be influenced by a small molecule. However,
evidence suggests that defined areas termed
hot spots
, on the protein surface are ideal
for small-molecule interactions and can have a large influence on the function of
the protein [52]. Another preconceived notion of these targets being “undruggable”
could also have arisen on past failures of HTS campaigns. However, recent successes
in disrupting PPIs either with structurally more complex compounds or those beyond
the rule of 5 (BRo5) has indicated that the existing screening collections may have
been a contributing factor [53-55].
