Design, Synthesis, and Biological Evaluation of Benzylamino-Methanone Based Cholesteryl Ester Transfer Protein Inhibitors

Cholesteryl ester transfer protein (CETP) is a glycoprotein involved in transporting lipoprotein particles and neutral lipids between high-density lipoprotein (HDL) and low density lipoproteins (LDL) and therefore its a proper target for treating dyslipidemia and related disorders. Guided by our previosuly-reported pharmacophore and QSAR models for CETP inhibition, we synthesized and bioassayed a series of benzylamino-methanones. The most potent illustrated 30% CETP inhibition at 10 μM.


Introduction
Atherosclerosis is the main cause for arterial dysfunction that limits blood flow to vessels in the peripheral vasculature and is finally marked as coronary artery disease (CAD) [1]. A number of epidemiological studies have established an inverse relationship between serum high-density lipoprotein (HDL) cholesterol levels and the incidence of ischemic heart disease [2]. HDL removes excess cholesterol from peripheral tissues to the liver for biliary elimination [3]. CETP, a 476-residue glycoprotein, is engaged in interchanging lipoprotein particles and neutral lipids, including cholesteryl esters, phospholipids and triglycerides between HDL and low density lipoproteins (LDL). CETP, as OPEN ACCESS revealed by X-ray crystallography (PDB code: 2OBD, resolution 2.2 Å), has a huge highly lipophilic binding site capable of binding up to four lipid molecules [4]. In human plasma, CETP plays a probably proatherogenic task by moving cholesteryl esters from HDL to very-low density lipoprotein (VLDL) and LDL particles, thereby lowering atheroprotective HDL cholesterol and raising proatherogenic VLDL and LDL cholesterols. Obviously, the risk of CAD is proportional to the plasma levels of CETP [5]. Actually, It is rather frequent within the CAD population to have elevated CETP plasma protein levels that are 2-to 3-fold higher than concentrations typically found in the plasma of normal subjects (1-3 µg/mL) [6]. Indication exists that the outcomes of CETP activity may depend on the metabolic setting, particularly on triglyceride levels. Therefore, pharmacological CETP inhibition may reduce the risk of CAD in humans, but only in those with high triglyceride levels [5].
The inaccessibility of a reasonable high resolution crystallographic structure for CETP combined with its large binding pocket locked up most modeling-related discovery projects to ligand-based approaches particularly quantitative structure-activity relationship analysis (QSAR) [7][8][9][10][11]. Earlier, we have developed ligand-based three-dimensional (3D) pharmacophores integrated within a selfconsistent QSAR model for CETP inhibitors. The pharmacophore models were used as 3D search queries to mine 3D libraries for new CETP inhibitors, while the QSAR model predicted their biological activities and therefore prioritize them for in vitro evaluation [12]. Additionally, we recently described the synthesis of several benzylidene-amino methanones, (including 1, Figure 1), as representatives of a new series of simple CETP inhibitors [12]. Herein we describe our efforts to optimize the activity of this series of compounds through reduction of the imine double bond to the corresponding amine analogues (compounds 19-30, Scheme 2). The new compounds are expected to have better anti-CETP bioactivities compared to the previously synthesized rigid benzylidene-amino methanones due to the enhanced flexibility of the amino analogues, which should allow better fit values against the pharmacophores i.e. to fit both QSARemerged pharmacophores instead of mapping just one of them. Furthermore, amino-derivatives are more stable than imines in aqueous conditions [13]. On the other hand, reduction of imines into amines can alter their physicochemical properties such as lipophilicity and basicity of the nitrogen, whereby the amines are more water soluble and more basic than their imine analogues. This property modification can influence the anti-CETP activity of the synthesized compounds. The synthesis commenced by preparing different substituted imine intermediates 8-11 (Scheme 1). Imines are typically formed by reversible acid-catalyzed condensation of amines and aldehydes with extrusion of water through either azeotropic distillation or by employing chemical drying agents [14]. Subsequently, the imine intermediates were reduced to their corresponding amines 12-15 (Scheme 1) which were then used to prepare the final benzylamino-methanones, in modest to reasonable yields, via Friedel-Crafts acylation of the substituted benzene derivatives 16-18 in the presence of polyphosphoric acid (PPA), as reported in Scheme 2 [15]. The structures proposed for compounds 19-30 were confirmed via elemental analyses, IR and 1 H-and 13 C-NMR spectroscopy (see the Experimental section).

Results and Discussion
In the current work, the imine intermediates were prepared from reaction of trifluoro-mtolualdehyde, trifluoro-p-tolualdehyde, 3-methoxybenzaldehyde and 4-tert-butylbenzaldehyde with the methyl ester of 4-aminobenzoic acid (3) as illustrated in Scheme 1 [16]. The best yield was obtained when 3, dissolved in DMF, reacted with trifluoro-p-tolualdehyde to yield 15 (92.2%). Afterward, the imine intermediates were reduced to their corresponding amines, which were then used to prepare the final benzylamino-methanones. The best yield was obtained upon reacting 4-aminobenzoic acid derivative 15 with toluene to yield 22 (96%). Scheme 2 shows the new benzylamino-methanone derivatives 19-30, while

(E) (F)
As can be noticed from Table 1, the benzylamino-methanone compounds have enhanced flexibility reflected by their fit values against Hypo12/4 in comparison to their benzylidene-amino methanones which do not fit Hypo12/4.
In this work, we intended to make the compounds more flexible in order to enhance their pharmacophore mapping in an attempt to increase their anti-CETP bioactivities, but it seems that the entropic cost contribution overcomes the enhanced flexibility contribution to the bioactivity. Therefore, pharmacophore fitness is just one of many factors that can influence the activity.
The results of anti-CETP activity tests, presented in Table 1, demonstrate that compound 26 exhibited appreciable activity against CETP. Although our newly synthesized benzylamino-methanones are of lower potency than some published CETP inhibitors, these derivatives are characterized by their novel scaffold that could be a promising lead for further optimization.
The new compounds 19-30 were synthesized to explore how the CETP inhibitory activity is affected by the expansion of the structure, i.e. meta or para substitution, and the electronic properties of the substituent, i.e. electron donating or with-drawing group. As a general trend, the inhibitory activity of compound 26, that is para-substituted (R) with a trifluoromethyl electron withdrawing group and fully extended, decreases with the trifluoromethyl group on the meta position as in compound 25. Furthermore, meta and para substitution (R) with electron donating groups such as t-Bu in compounds 20 and 24, and methoxy in compound 23 seems to conserve the anti-CETP activity. Moreover, mono-(X) or di-substitution (X and Y) of the aromatic ring, as in compounds 24 and 28, respectively, appears not to have a considerable effect on the inhibitory activity.
Compounds 19-30 were tested against CETP at 10 μM concentrations and exhibited anti-CETP activity ranging from 9.8 to 29.9 %. Compound 26 displayed the best activity as reported in Table 1. Furthermore, in vitro IC 50 values were determined for the most active compounds and approximately 3 to 9 fold differences were observed between QSAR-based IC 50 estimates and the experimental IC 50 values.

Conclusions
In conclusion, we have successfully achieved synthetic exploration of a new series of aromatic amines as CETP inhibitors. They showed comparable activities to their benzylidene-amino methanones analogues [12] which reveals that flexibility of these amines was not enough to influence their antiCETP activity. We are currently in the process of preparing new compounds of better bioactivity profiles.

General methods
Melting points were measured using Gallenkampf melting point apparatus and are uncorrected. 1 H-NMR and 13 C-NMR spectra were collected on a Varian Oxford NMR 300 spectrometer. The samples were dissolved in CDCl 3 . Mass spectrometry was performed using LC Mass Bruker Apex-IV mass spectrometer utilizing an electrospray interface. Infrared spectra were recorded using Shimadzu IRAffinity-1 spectrophotometer. The samples were dissolved in CHCl 3 and analysed as thin solid films using NaCl plates. Analytical thin layer chromatography (TLC) was carried out using pre-coated aluminum plates and visualized by UV light (at 254 and/ or 360 nm). Elemental analysis was performed using EuroVector elemental analyzer. Chemicals and solvents were purchased from the corresponding companies (Sigma-Aldrich, Riedel-de Haen, Fluka, BDH Laboratory Supplies and Promega Corporation) and were used in the experimentation without further purification.

General procedure for the synthesis of benzylamino-methanones 19-30
4-Aminobenzoic acid derivative 12-15 (2 mmol) was dissolved in cyclohexane (10 mL), and polyphosphoric acid (6.5 g) was added. Then a benzene derivative 16-18 (20 mmol) was added. The mixture was stirred carefully at 90-110 °C overnight and then poured on crushed ice. The solution was carefully made alkaline with 25% ammonia and then extracted with CHCl 3 (3 × 20 mL). The combined extracts were dried on anhydrous Na 2 SO 4 and filtered.

CETP inhibition assay
CETP inhibitory bioactivities were assayed by fluorescent-CE transfer employing commercially available kit (BioVision, Mountain View, CA, USA). The assay kit is based on donor molecule containing fluorescent self-quenched neutral lipid that is transferred to an acceptor molecule in the presence of CETP (from rabbit serum). CETP-mediated transfer of the fluorescent neutral lipid to the acceptor molecule results in increase in fluorescence. Inhibition of CETP will prevent lipid transfer and therefore decrease fluorescence intensity.
The assay procedure can be described briefly as follows: an aliquot of rabbit serum (1.5 µL) was added to testing sample (160 µL). Then the master mix, provided in the assay kit (donor molecule, acceptor molecule and assay buffer, 20 µL) was added, mixed well, and the volume was completed to 203 µL with the provided assay buffer. After incubation at 37 °C for 1 hour, fluorescence intensity (Excitation λ: 465 nm; Emission λ: 535 nm) was read in a FLX800TBI Microplate Fluorimeter (BioTek Instruments, Winooski, VT, USA).
The tested compounds were initially dissolved in DMSO to yield 10 mM stock solutions and subsequently diluted to the required concentrations using distilled deionized water. The final concentration of DMSO was adjusted to 0.1%. The percentage of residual activity of CETP was determined for each compound by comparing the activity of CETP in the presence and absence of the tested compound. Positive controls were tested to assess the degree of CETP inhibition by 0.1% DMSO. CETP was not affected by DMSO. Negative controls lacking rabbit serum were used as background. All measurements were conducted in duplicates.