Int J Pept Res Ther
effective against multi-resistant Gram-positive bacteria
bind and stabilize the inactive conformation of calcium
channel pump proteins thereby acting as calcium channel
blockers useful for the treatment of cardiovascular diseases
(Janis et al. 1987). More than 20 years have passed after
the introduction of the prescription drug nifedipine (1, aka:
Adalat, Procardia, Afeditab, Nifediac, etc. Fig. 1) and
many 1,4-DHP analogs have been synthesized since then.
Several second-generation commercial products have also
appeared on the market (e.g., 2, 3, Fig. 1, Bossert and Vater
1989).
(
Hancock 2001; Niu et al. 2012).
Antibacterial agents can be classified in three major
groups according to their mechanism of action: (a) cell wall
synthesis and disruption, (b) protein synthesis, and
(
c) DNA synthesis. One of the most promising approaches
in the development of antibacterial agents of broad action
is the application of a model based on cationic oligopep-
tides capable of disrupting the bacterial cell wall. Cationic
peptides can be found in all forms of life as natural defense
against pathogens (Brown and Hancock 2006; Jenssen
et al. 2006). Inspection of naturally-occurring antibacterial
peptides shows a general pattern of short amino acid se-
quences including two cationic amino acids, like lysine or
arginine, bracketing a sequence of two to three hy-
drophobic amino acids (Faccone et al. 2014; Toke 2005;
Uggerhoj et al. 2015). Many naturally-occurring antibac-
terial peptides acquire a well-defined three-dimensional
structure only upon binding to a membrane such as a
bacterial cell wall, when the side chains of the cationic
amino acids interact with negatively charged phosphodi-
ester groups in the membrane (Zaiou 2007). For cationic
peptides having an a-helix secondary structure, the mini-
mal separation distance between two cationic residues lo-
In the past, 1,4-DHPs were subjected to Mannich reac-
0
tion yielding 2-alkyl-1-(1 -dihydropyridinyl methyl)
benzimidazoles having significant antibacterial activity
(Mane et al. 1995). Since then, focus has also been given to
the ability of 1,4-DHP derivatives to revert multi-drug re-
sistance (Carosati et al. 2012).
The nature of the hydrophobic linker is not the main
factor determining the antibacterial activity of cationic
peptides. Therefore, it is tempting and intriguing to ex-
amine the possibility of using 1,4-DHP, an accessible
scaffold with advantageous pharmacological properties, in
the preparation of cationic peptidomimetics with a poten-
tially broad bactericide spectrum. We concentrated our
efforts on the implementation of the 1,4-DHP scaffold as a
bridge between two cationic Lys residues at different dis-
tances, mimicking one or two turns of the a-helix. Since in
some cases antibacterials can cause hemolytic anemia, we
decided to test the minimal microbial inhibitory concen-
tration (MIC) of the synthesized peptidomimetic com-
pounds as well as the ability to induce lysis of human red
blood cells (RBC), as a measure of toxicity.
˚
cated on the same polar face is 5.4 A corresponding to one
turn of the helix. The second step in this molecular ladder
˚
represents a vertical distance of 10.8 A corresponding to
two turns of the helix.
One of the main disadvantages of peptide drugs is their
potential metabolic instability while being quickly cleared
from the body. A peptidomimetic approach designed to
improve the metabolic stability and retain the desired an-
tibacterial activity seems very attractive (Marr et al. 2006).
We reasoned that a 1,4-dihydropyridine (1,4-DHP) scaffold
could be used to mimic the hydrophobic peptide gap be-
tween two cationic residues. The 1,4-DHP structure is a
well-known pharmacophore considered to be a ‘‘privileged
structure’’ that, when appropriately substituted, can cause
different biological responses. 1,4-DHPs were first reported
by Fleckenstein (1983) for the treatment of coronary dis-
eases (Neal et al. 2000). Specifically, 4-Aryl-1,4-DHPs of
the nifedipine type (1, 2, and 3 in Fig. 1) were found to
The synthesized cationic peptidomimetcs include a
central hydrophobic 1,4-DHP moiety flanked at both sides
by units of the same dipeptide in a Cterm-to-N-[1,4-DHP]-
N-to-Cterm fashion. The selected dipeptides are based on
Lys-Leu-OMe and Pro-Lys-OMe, which are commercially
available. In the Lys-Leu-OH derivative, the two cationic
Lys residues can be directly connected to the 1,4-DHP
scaffold. The resulting molecule can adopt a conformation
that aligns the two cation bearing side chains separated by a
˚
distance of 5.5–6.0 A, as estimated from a 3D minimized
model using the MM2 program as implemented by
Chem3D (ChemOffice Ultra 10.0, CambrigeSoft). A dis-
˚
tance of about 11 A can be achieved in a similar way, in the
NO2
Pro-Lys-OH derivative (Fig. 2).
NO2
The main features of our design are the use of two ca-
tionic Lys residues, separated by a vertical distance which
resembles one or two a-helix full turns, committed to the
disruption of the bacterial membrane, and the hydrophobic
O
O
O
O
R
R
N
O
O
O
O
N
H
N
H
1,4-DHP core with its outstanding pharmacokinetic fea-
1
; nifedipine; R=Me,2-NO 2
; nitrendipine; R=Et,3-NO 2
3; nicadipine
tures bridging between the two cations.
2
The synthesis of the 1,4-DHP moiety is based on a
general strategy which relies on the Hantzsch reaction
Fig. 1 4-Aryl-1,4-dihydropyridines
1
23