482
C. C. Musonda et al. / Bioorg. Med. Chem. Lett. 19 (2009) 481–484
N
N
H
N
H
N
NH
N
N
N
N
O
N
F
N
F
R
N
N
Cl
5 clogP 6.4
mwt 472
4 Norastemizole
F
3 R = H
Desmethylastemizole
2 R = Me
Astemizole
clog 5.8
mwt 459
F
F
F
N
HN
N
N
N
N
N
N
N
N
N
N
N
N
N
H
N
Cl
H
8 clogP 6.6
mwt 486
6 clogP 7.4
mwt 529
7 clogP 6.6
mwt 486
Cl
Cl
Figure 2. Chemical structures of astemizole and its metabolites (2–4) and synthetic hybrids 5–8.
for the discovery of new antimalarial therapies. A similar hybrid-
ization strategy has also been employed in antimalarial peroxide
research, for example, aminoquinoline and trioxanes,7 artemisinin
and quinine,8 and aminoquinoline and tetraoxanes.9
tions to afford 11 in excellent yield. Attempted palladium-cata-
lysed Buchwald–Hartwig type amination of 11 under standard
conditions only yielded starting materials, but 12 could be ob-
tained in good yield after treatment of 11 with excess piperazine
in the absence of catalyst under microwave irradiation. Final cou-
pling of 12 with 4,7-dichloroquinoline 13 afforded target com-
pound 5.15 Amination of 13 with propanolamine 14 gave
intermediate 15 in excellent yield. Compound 6 was then obtained
by subsequent triflation of 15 to yield 16, followed by nucleophilic
substitution of the triflate with 12.
Targets 7 and 8 were synthesised in a similar fashion. The ana-
logue 7 was realised via coupling 4-aminopiperidine 17 with 13 to
give 18,16 and then reaction of 18 with 11 to give the desired prod-
uct 7. The synthesis of reverse analogue 8 began with reaction of
11 with 4-aminopiperidine 17 under microwave irradiation, to af-
ford 19 in almost quantitative yield. Coupling of 13 and 19, again
under microwave conditions, gave compound 8.
Antiplasmodial activity was determined in a CQ-resistant K1
strain of the P. falciparum parasite.17 The results of the antiplasmo-
dial activity of compounds 5–8 are shown in Table 1, along with
the value for the standard drug CQ and astemizole 2. With the
exception of 5, all compounds were 3–10 times more active than
CQ with optimum activity being seen in compound 6 (IC50
23 nM). Interestingly, the conformationally constrained amino-
piperidine linkers 7 and 8 also delivered potent K1 activity. This
is in contrast to a previously described series of CQ analogues
where acyclic linkers were found to deliver more potency that
those with a conformationally constrained piperidine or pyrroli-
dine linker.16 Importantly, the hybrid analogues displayed good
cytotoxicity profiles, with all compounds showing >100-fold selec-
tivity for antiparasitic activity over cell-based cytotoxicity.
These results suggested that hybridizing the two different units,
each of which possesses significant antiplasmodial activity, via an
appropriate linker could overcome resistance to CQ, much of which
has been ascribed to mutations in the PfCRT gene.3
Based on the in vitro P. falciparum activity, analogues 7 and 8
were progressed to a Plasmodium berghei mouse malaria model.18
Both compounds showed in vivo activity although neither was as
active as CQ (Table 2). Both 5 and 6 showed prolongation of sur-
vival (based on mean survival time) and significant reductions in
parasitemia at 4 Â 50 mg/kg (7) and 4 Â 20 mg/kg (8) ip. At this
high dose, 7 reduced parasitemia comparatively to CQ. These
in vivo results suggested that the hybridization approach outlined
The combination of two separate pharmacological agents into a
single molecule is an emerging strategy within medicinal chemis-
try and drug discovery.10 The work of Morphy and Rankovic11 and
Hopkins et al.12 has highlighted both the opportunities within this
paradigm, but also the trends towards increased molecular weight
and lipophilicity in multipharmacology ligands when compared to
known oral drugs.13 With these challenges in mind, a small set of
conjugated CQ–astemizole hybrids 5–8 was designed (Fig. 2), the
synthesis and biological evaluation of which are described in this
letter.
The plasmodia data disclosed by Chong et al.6 gave some guid-
ance on the possible strategies for conjugating astemizole and CQ
together. Both 2 and 3 had good antiplasmodial activity (in vitro
and in vivo), whereas norastemizole 4, which lacked the phenethyl
sidechain, had significantly reduced activity. This suggested to us
that alkylation of the piperidine nitrogen was important, and that
replacement of the phenethyl unit of astemizole with the quinoline
heterocycle of CQ was an approach worth investigating. This strat-
egy had the added advantage of replacing the metabolically vul-
nerable methoxyphenyl group of astemizole with a more robust
heterocyclic moiety. A basic centre was also retained in the new
analogues, as CQ is postulated to concentrate in the parasite diges-
tive vacuole by virtue of protonation under the acidic conditions
found in that compartment (pH of digestive vacuole measured at
4.7). The calculated pKas for analogues 5–8 were all above 7.0
(using ACD software) and would result in >99% protonation at
pH 4.7.
It was a significant challenge to keep these conjugated mole-
cules approaching the Ro5, and in all cases lipophilicity increased
compared to astemizole. Clearly this would need to be addressed
in further design cycles, due to the potential for high lipophilicity
to increase both off-target pharmacology and the potential for ad-
verse toxicological outcomes.14 However, it was felt the designed
set of compounds 5–8 would allow for proof-of-principle of this
hybrid approach to be established, which could then trigger further
work to improve drug-like properties.
The synthetic route employed to access the target compounds
5–8 is outlined in Scheme 1. Alkylation of 2-chlorobenzimidazole
9 was effected with 4-fluorobenzyl bromide 10 under basic condi-