Design, Synthesis and Docking Studies of Novel 1, 2, 3-Triazolyl Phenylthiazole Analogs as Potent Anti-HIV-1 NNRT Inhibitors

In an attempt to design and synthesize a new class of anti-HIV-1 RTIs i.e., 4-(phenyl)-N,N-bis((1-phenyl1H-1,2,3-triazol-4-yl)methyl)thiazol-2-amine derivatives, substituted 2-amino-4-phenylthiazoles were alkylated with propargyl bromide to obtain dialkyne 2-amino-4-phenylthiazoles. This dialkyne 2-amino-4-phenylthiazole was reacted with aryl azides to generate small library of 15 compounds (4a-o) by click chemistry. The obtained derivatives were studied for as an anti-HIV-1 NNRT Inhibitors. All synthesized compounds of 1,2,3-triazolylphenylthiazole series were be docked into the non-nucleoside inhibitor binding pocket (NNIBP) of HIV-1 RT and highly inhibiting derivatives studied for in vitro anti-HIV-1 assay.


Introduction
Reverse transcriptase inhibitors (RTIs) are a class of antiretroviral drugs used to treat HIV infection or AIDS, and in some cases hepatitis B. RTIs inhibit activity of reverse transcriptase, a viral DNA polymerase that is required for replication of HIV and other retroviruses. When HIV infects a cell, reverse transcriptase copies the viral single stranded RNA genome into a double stranded viral DNA. The viral DNA is then integrated into the host chromosomal DNA, which then allows host cellular processes, such as transcription and translation to reproduce the virus. RTIs block reverse transcriptase's enzymatic function and prevent completion of synthesis of the double stranded viral DNA, thus preventing HIV from multiplying.
A similar process occurs with other types of viruses. The hepatitis B virus, for example, carries its genetic material in the form of DNA, and employs a RNA dependent DNA polymerase to replicate. Some of the same compounds used as RTIs can also block HBV replication; When used in this way they are referred to as polymerase inhibitors. HIV only contains RNA and so needs to change its RNA into DNA to be able to integrate with our DNA for replication. To do this it has to first change its RNA to DNA. HIV uses a compound called reverse transcriptase to convert its RNA to DNA. Reverse transcriptase enzyme is not found in human cell without HIV, so that RT is the main target for the Anti-HIV drug synthesis.
RTIs come in three forms: Nucleoside analog reverse transcriptase inhibitors (NARTIs or NRTIs) Nucleotide analog reverse transcriptase inhibitors (NtARTIs or NtRTIs) and Nonnucleoside Reverse transcriptase inhibitors (NNRTIs). The mode of action of NRTIs and NtRTIs is essentially the same; They are analogues of the naturally occurring deoxynucleotides needed to synthesize the viral DNA and they compete with the natural deoxynucleotides for incorporation into the growing viral DNA chain. NRTI working different ways but one of the main ways is to compete with reverse transcriptase for their interaction site with HIV genetic material while NNRTIs work by sitting in a binding site in the virus structure.
However, 1,2,3-triazole ring is not found in any marketed drugs. The click chemistry improved by Sharpless et al. is an admirable approach for regioselective synthesis of 1, 2, 3-triazole ring system in presence of various functional groups. Genuine efforts have been made to integrate 1, 2, 3-triazole in existing drugs, still more research is needed to find lead molecule [18]. 1,2,3-triazole structural moiety is present in several compounds showing various biological activities including anti-HIV [19], anti-bacterial [20], anti-allergic [21], anticonvulsant [22], b-\lactamase inhibitory [23], and anti-tuberculosis activities [24], 1,2,3-triazole has been comprehensively studied due to its important applications in industrially interesting materials, such as dyes, anticorrosive agents, photo stabilizers, photographic materials, and agrochemicals [18]. Therefore, we found it interesting to design new molecules within the scope of synthetic procedure using phenylthiazole scaffold followed by suitable modification to generate diversified compounds for anti-HIV activity. In this study, we exploited click chemistry for synthesis of diversified phenylthiazole compounds mainly for the two reasons; first, it can tolerate wide range of functional groups and easy to do eco-friendly reactions at room temperature either in water or mixture of water and organic solvents, secondly; this approach will generate compounds having 1,2,3-triazole functionality rather than 1,2,4-triazole. These compounds can be studied for anti-HIV-1 RT activities.

Experimental
All solvents were used as commercial anhydrous grade without further purification. Aluminium sheets 20 × 20 cm, Silica gel 60 F 254 , Merck grade was used for thin layer chromatography to determine progress of reaction. Melting points were determined in open capillary tube and are uncorrected 1 H NMR spectral data were recorded on Brucker Advance spectrometer at 300 MHz and Jeol JNM ECX spectrometer at 300 MHz using TMS as an internal standard. The chemical shifts values are recorded on δ-scale in DMSO solvent. Mass spectra were taken on Polaris-Q Thermo scientific GC-MS.

General procedure for synthesis of dialkyne substituted 2-amino-4-phenylthiazoles derivatives (2a, 2b)
A mixture of aniline (0.5 mmol), allyl bromide (1.5 mmol), potassium carbonate (2 mmol), ethanol (2 mL), and water (1 mL) was added to a 50 mL round flask ask and stirred at 70 ∘ C for the desired time until complete consumption of starting material as judged by TLC. Then, the reaction mixture was condensed by evaporation of solvents under reduced pressure and was washed with saturated sodium carbonate solution and poured into a separating funnel [25].
The content was extracted with ethyl acetate (10 mL × 3), and the combined organic layers were dried over anhydrous magnesium sulfate. The solvent was removed by evaporation under reduced pressure to afford the two products (2a and 2a 1 i.e., monoalkyne substituted 2-amino-4-phenylthiazole), which were purified by column chromatography using hexane and dichloromethane (65:35) as eluent. The major product was compound 2a, which was used for synthesis of next step compounds (4a-o) which were further purified by column chromatography on silica gel using petroleum ether and ethyl acetate as the eluents.

Material and methods for docking studies
To guide the lead optimization strategy and rationalize the SARs, modeling study was performed to examine the possible binding conformations of our newly synthesized compounds and their interaction mode with RT, using Glide (Glide 5.8, Schrodinger, 2012) [28]. Structure-based docking studies were carried out to investi gate the intermolecular interaction between the ligand and the targeted enzyme. The coordinates of the non-nucleoside binding site were taken from the crystal structure of HIV-1 reverse transcriptase (RT) in complex with TMC278 (Rilpivirine) (PDB code: 2ZD1) [29]. Docking study of all the molecules from Indolyl and chromenyl xanthenone series was carried out with enzyme reverse transcriptase PDB ID: 2ZD1. The ligands were prepared by using LigPrep (LigPrep 2.5, Schrodinger, 2012) [30]. The protein was refined using the protein preparation wizard present in Maestro 9.3 (Maestro 9.3, Schrodinger, 2012) [31]. All the water molecules were deleted. H atoms were added to the protein, including the protons necessary to define the correct ionization and tautomeric states of the amino acid residues. Prime interface module incorporated in Maestro was used to add the missing residues of the side chain. Each structure minimization was carried out with the impact refinement module, using the OPLS-2005 force field to alleviate steric clashes potentially existing in the structures. Minimization was terminated when the energy converged or the root mean square deviation reached a maximum cutoff of 0.30 Å. To find out active site grid was prepared using grid generation panel of glide with the default settings. Grid is prepared for defining the binding site of native ligand on the receptor. The ligand was selected to define the position and size of the active site (Friesner et al.; Halgren et al.) [32,33]. Glide XP docking was used for docking purposes.  Table 1
The methoxy and chlorophenyl-thiazolyl nucleus moiety at of compounds 4h and 4k makes π-π interaction into the hydrophobic binding pocket, surrounded by the aromatic side chains of portion of Trp229 and Tyr 181 residue. The N-aryl substituted triazolyl ring of the compound 4k and 4h makes π-cation interaction with into the hydrophobic binding pocket, surrounded by the aromatic side chains of portion of lys103 residue. Triazolyl ring of compound 4h form the hydrogen bond interactions with the backbone N-H of Lys101 residue. The decrease in activity of compounds 4l and 4f of 4-(phenyl)-N,Nbis((1-phenyl-1H-1,2,3-triazol-4-yl)methyl)thiazol-2-amine series was due to lack of π-π interaction of the methoxy and chlorophenylthiazolyl nucleus into the hydrophobic binding pocket, surrounded by the aromatic side chains of portion of Trp229 and Tyr 181 residue and nonexistence of π-cation interaction with the hydrophobic binding pocket, surrounded by the aromatic side chains of portion of lys103 residue (Figure 3), instead it is showing π-π stacking of triazolyl ring with amino acid Trp 229 and Tyr181.
Docking score of compound 4k, 4h, 4f and 4l was found to be around -11.126, -11.126, -8.189 and -7.698 respectively, (Table 2) while of native ligand was found to be -13.413 which confirms that 4k and 4h compounds might have potent RT inhibition activity. Further, in silico binding studies suggested that inhibitors possessing π-π interaction of the phenyl-thiazolyl nucleus into the hydrophobic binding pocket,    surrounded by the aromatic side chains of portion of Trp 229 and Tyr 181 residue and π-cation interaction with the aromatic side chains of portion of lys103 residue improves the inhibitor selectivity for RT and thus helps in further drug design attempts to obtain potent 1,2,3, triazolyl-phenylthiazole derivatives.

In vitro anti-HIV
According to the docking study of synthesized compounds, some of it showed the high inhibition activity and some with low activity. From the above conclusion, we studied in vitro anti-HIV assay for particular compounds to verify their activity. The HIV-RT inhibition assay was performed by using an RT assay kit (Roche), and the procedure for assaying RT inhibition was performed as described in the kit protocol (Roche Kit) [34]. The compounds presented in this study namely 24-(phenyl)-N,N-bis((1-phenyl-1H-1,2,3-triazol-4-yl)methyl)thiazol-2- amine derivatives (4k, 4h, 4o, 4f, 4l) were evaluated for anti-HIV-1 activity by using enzymatic (RT) and cell based assays. The HIV-1 RT inhibition activity range for these compounds showed from 64-92% inhibition at 100 μg/ml concentrations. The compounds 4k and 4h showed highest inhibitory activity both in docking as well cell based study (91.45%, 90.0%) respectively, were 4o, 4f and 4l shows low activity (86.67, 66.88% and 64.71%) respectively, whereas the control NNRTI marketed drug nevirapine showed 99.15% inhibition at 100 μg/ml concentration. The enzyme assay results demonstrated that the compound 4k and 4h were more potent than remaining derivatives comparing against reverse transcriptase enzyme. Subsequently, the inhibitory activity of HIV-1 viral replication was also assessed by cellbased assay. The results are summarized in Table 3 along with standard nevirapine as reference drug. In the cell based assay, the compounds 4k and 4h were the most potent inhibitors of HIV-1 replication against HIV-1 IIIB (EC 50 =0.65 and 0.93 μg/ml respectively; the selectivity index (SI)=62.00 and 41.82 respectively; CC 50 with HIV-1 IIIB=40.3 and 38.9 μg/ml respectively) and HIV-1 ADA5 (EC 50 =1.02 and 0.25 μg/ ml;the selectivity index (SI)=40.98 and 180.8 respectively; CC 50 =41.08; c d a b Figure 3: Two (3a, 3b) and three-dimensional (3c and 3d) view for decrease in activity of compound 4l and 4f respectively. a Data represent the mean of two and three independent assays for EC 50 and CC 50 , respectively; b EC 50 is the 50% effective concentration required to reduce HIV-1 induced cytopathic effect of HIV-1 IIIB and HIV-1 ADA5; c The CC 50 is the 50% cytotoxic concentration for HIV-1 IIIB and HIV-1 ADA5; d Selectivity index ratio CC 50 /EC 50

Conclusion
A series of new 1,2,3-triazolyl-phenyl thiazole hybrids bearing different aryl triazolyl/phenylthiozole moieties were synthesized and evaluated as potent inhibitors of human immunodeficiency virus type-1 (HIV-1). Based upon the preliminary molecular docking studies of these new 1,2,3-triazolyl-phenyl thiazole hybrids, some structural requirements for high potency against HIV-1 were rationalized. In the series of 1,2,3-triazolyl-phenyl thiazole, compound 4k and 4h identified as potent inhibitor against the strains HIV-1 IIIB and HIV-1 ADA5. The decrease in activity of compound 4f and 4l 1,2,3, triazolylphenylthiazole was due to lack of π-cation interaction with lys103. This study suggested that inhibitors' possessing π-cation interaction with lys103 and π-π interaction with the aromatic side chains of Trp 229 and Tyr 181 improves the inhibitor selectivity for RT.

Supporting Information
It includes docking score table and full characterization of synthesized compounds.