L. Gao et al.
Bioorganic & Medicinal Chemistry Letters 40 (2021) 127909
Using this strategy, previous studies from our research group have
developed series of novel compounds based on benzoisox-
demonstrated best in vitro activities, and exhibited potent antipsychotic
effect in vivo behavioral studies. Especially, compound 4w exhibited
pro-cognitive properties and negligible side effects (weight gain and
hyperprolactinemia). As a result, the compound 4w was developed as an
atypical antipsychotic candidate for the treatment of schizophrenia
based on the D2, 5-HT1A, 5-HT2A and H3 multi-target profile.
a
azoleylpiperidine. Several of these compounds significantly alleviate
schizophrenia-like symptoms (inhibit the apomorphine-induced climb-
ing behavior and MK-801-induced hyperactivity), are associated with
lower weight gain and lower prolactin levels, displayed procognition
properties in a novel object recognition task in rats, and have obvious
therapeutic effects.13–16 And their binding affinities for the various re-
ceptors that associated with schizophrenia were summarized in the
Table 1. Compound S1, S2 and S3 all showed high affinity for D2, 5-HT1A
and 5-HT2A receptors. In addition, compounds S1 and S3 exhibited good
affinity for D3 receptors, S2 and S3 showed high strong 5-HT6 receptor
potency. Moreover, all of them showed lower affinities for the off-target
receptors (5-HT2c, α1 and H1). Based on this research, we’re trying to
endow the new compounds with additional H3 receptor potency, as a
large amount of scientific data have demonstrated that actions at the
histamine H3 and 5-HT6 receptor may constitute a promising approach
to treat the cognitive symptoms in schizophrenia17,18. The histamine H3
The syntheses of intermediates (1a, 1b, 2a-2af, 3) and the com-
pounds 4a-4w are illustrated in Scheme 1. The 1a, 1b were synthesized
through benzoisoxazoleylpiperidine reacted with appropriate hal-
oalkanes in the presence of anhydrous potassium carbonate. The com-
pounds 2a-2ab were prepared from p-hydroxy benzaldehyde and its
derivatives via the Mannich reaction in the presence of pyrrolidine,
piperidine, morpholine, or their derivatives and sodium triacetoxybor-
ohydride. 2ac-2af prepared by reaction of pyrrolidine, piperidine,
morpholine, or their derivatives with 4-(2-bromoethyl) phenol in the
presence of diisopropylethylamine. There are two methods to obtain the
desired 4a-4w. For Method A, it’s suitable for synthesize the important
intermediate which without substituent on the benzene ring. For
example, intermediate 3 was obtained by reaction of 1a with p-
hydroxybenzaldehyde in the presence of potassium carbonate, further-
more, get the desired product 4a and 4j. The method B was utilized to
prepare the compounds with substituents on benzene ring. By using this
method, it’s easy to synthesize purity and separate the intermediates
2ac-2af. The intermediates 1a and 1b prepared from method A were
reacted with the derivatives of 2a-2ab and 2ac-2af yielded the target
compounds 4a-4w. Combination of two methods provides an effective
way to synthesize the different target compounds, saved lot of tediously
separate process.
receptor is a member of the G protein-coupled receptor (GPCR) family19
.
It is an important regulator of several neurotransmitters, including DA,
5-HT, acetylcholine, and norepinephrine20. Pharmacological studies
have shown that H3 receptor antagonists and inverse agonists have po-
tential applications in the therapy of various CNS disorders, including
cognitive deficits, depression, Alzheimer’s disease, Parkinson’s disease,
epilepsy, and schizophrenia21. For example, the H3 antagonist Bavisant
currently on the market has been used as a therapeutic intervention for
the promotion of wakefulness, and animal studies have already
demonstrated that the H3 receptor antagonists ABT-239 and A-431404
have the potential to ameliorate cognitive deficits relevant to schizo-
phrenia whereas risperidone and olanzapine do not 22,23. Many anti-
schizophrenia drugs available have no or low H3R affinity, such as
chlorpromazine and risperidone24,25, even the novel atypical antipsy-
chotics aripiprazole5. Therefore, adding on H3 receptor antagonism to
mixed D/5-HT receptors antagonist profile will contribute to the
cognitive function for patients with schizophrenia, and with batter
therapeutic effects.
The SARs of the synthesized compounds were preliminarily evalu-
ated in vitro, and the compounds with desired activities were selected
and further characterized for safety in terms of pharmacology. Finally,
the most promising candidate was subjected to behavioral study and
pharmacokinetic evaluation.
The in vitro evaluations results (Ki values) were shown in Table 2. All
of the compounds exhibited affinities for D2, 5-HT1A and 5-HT2A to some
extent. In addition to these three receptors, these compunds also showed
affinity for the H3 receptor; however, this affinity varied between the
pharmacophores. From an integral view, most of the compounds dis-
played higher affinities for D2, 5-HT1A, and 5-HT2A receptors, which may
be attributed to the benzoisoxazoleylpiperidine groups in their
structure.
To verify this multi-receptor affinity strategy, a series of novel
compounds was synthesized (Table 2) by using the molecular hybridi-
zation method26,27. This strategy for design multi-target ligands as
shown in Fig. 1, the benzoisoxazoleylpiperidine groups were reserved as
optimal structure to guarantee their effects on D2, 5-HT1A and 5-HT2A
receptors as they are crucial DA and 5-HT pharmacophores of com-
mercial atypical antipsychotics such as risperidone, paliperidone and
brexpiprazole. In addition, the privileged fragments of reported H3 re-
ceptor antagonists13 (S4 and Pitolisant) were introduced. Furthermore,
the molecular structures of the novel compounds were further optimized
according to pharmacological actions and pharmacodynamics to maxi-
mize the therapeutic benefits. Finally, extensive structure–activity
relationship (SAR) studies, including in vitro receptor profiles and in vivo
behavioral studies, were conducted. Among these compounds, 4w
The effects of cyclic N-heterocyclic links to phenoxy structures were
assessed and the present results showed the cyclic N-heterocyclic play an
important role in SARs. Most of compounds with a morpholinyl substi-
tution exhibited higher affinity for the desired four receptors than with
pyrrolidine and pyridine substitution (e.g. 4q vs 4d and 4i. Table 2), but
the compound with N-methyl piperazinyl and 4-methylpiperidyl sub-
stitution displayed higher affinities for H3 receptor (4j vs 4r-4u). When
pyrrolidinyl in compound 4e was replaced by N-methyl piperazine, it
formed compound 4t, which showed improved affinities for all the four
Table 1
Binding Affinities for the Receptors Associated with Schizophrenia of Compounds S1-S3.
Compound
Receptor affinity Ki (nM)
D2
D3
4.3
5-HT1A
3.3
5-HT2A
0.3
5-HT2c
1700.7
5-HT6
228.4
α1
H1
S1
2.6
2569.2
1125.3
S2
S3
5.0
2.9
35.7
1.7
5.9
8.6
0.3
0.7
1584.1
616.0
0.5
5.6
337.2
43.1
1998.0
630.1
2