Design, Synthesis and Antitubercular Activity of Certain Nicotinic Acid Hydrazides

Three series of 6-aryl-2-methylnicotinohydrazides 4a–i, N′-arylidene-6-(4-bromophenyl)-2-methylnicotino hydrazides 7a–f, and N′-(un/substituted 2-oxoindolin-3-ylidene)-6-(4-fluorophenyl)-2-methylnicotinohydrazides 8a–c were synthesized and evaluated for their potential in vitro antimycobacterial activity against M. tuberculosis. The results showed that isatin hydrazides 8a–c are remarkably more active than the parent hydrazide 4c. Hydrazides 8b and 8c exhibited the highest activity among all the tested compounds (MIC = 12.5 and 6.25 µg/mL, respectively). Compounds 8b and 8c were also devoid of apparent cytotoxicity to HT-29, PC-3, A549, HepG2 and MCF-7 cancer cell lines. Besides, 8b and 8c showed good drug-likeness scores of 0.62 and 0.41, respectively. Those two isatin hydrazides could offer an excellent framework for future development to obtain more potent antitubercular agents. The SAR study suggested that lipophilicity of the synthesized derivatives is a crucial element that accounts for their antimycobacterial activity. Finally, a theoretical kinetic study was established to predict the ADME of the active derivatives.


Introduction
The Nineteenth World Health Organization (WHO) Tuberculosis Report indicates that TB is one of the world's deadliest communicable diseases [1]. It estimates that in 2013 there were 9.0 million new cases and 1.5 million deaths from TB, including 400,000 deaths associated with co-infection with HIV. The highest rates per capita occurred in the African Region (25%), while South-East Asia, the Western Pacific and African Regions account for around 81% of all the total cases [1]. The disease is aggravated by the worldwide continuous emergence of multidrug-resistant strains of M. tuberculosis (MDR-TB), extensively drug-resistant tuberculosis (XDR-TB) and totally drug-resistant tuberculosis (TDR-TB) [2,3]. The magnitude and extent of drug-resistant strains have increased concern that TB may once again become an incurable disease [4,5]. Moreover, the increasing incidence of the disease in immunocompromised patients along with the longer durations of therapy emphasize the need for new drugs to extend the range of effective TB treatment options [6][7][8].
TB treatment is tedious, challenging and time-consuming. It requires the administration of appropriate treatment regimens for at least six months via directly observed therapy (DOT) and follow-up support [2]. Treatment regimens require a minimum six months in two separate phases. The duration of phase one is about two months and involves four drugs (isoniazid, rifampicin, pyrazinamide and ethambutol), followed by four months of phase two (using isoniazid plus rifampicin) [9]. However, the present treatment regimen has some limitations such as drug toxicity and intolerance, drug-drug interactions and poor patient adherence due to the lengthy treatment duration [10]. Therefore, there is an urgent need for the development and more efficient evaluation of new TB drugs and shorter treatment regimens. Isoniazid (INH, Figure 1), a critical frontline drug in TB treatment discovered by Dogmagk, is a prodrug that requires activation in vivo by mycobacterial catalase peroxidase (KatG) [11,12]. INH exerts its anti-tubercular activity via interference with the synthesis of mycolic acid, one of the essential chemical pathways responsible for the formation of cell walls in M. tuberculosis [13].
The enzymatic acetylation of isoniazid by N-acetyltransferase (NAT) represents a major metabolic pathway for isoniazid in humans [14], so blocking acetylation via chemical modification of the hydrazine unit with a functional group, while preserving potent antimycobacterial action, has the potential to counterbalance the known side effects of INH, improve clinical outcomes and reduce the emergence of acquired isoniazid resistance in patients. Subsequently, numerous studies have pointed out the importance of developing novel INH hydrazides as promising anti-tubercular agents ( Figure 1) [15][16][17][18][19][20][21][22].
Recently, Narang et al. [23] developed a novel series of nicotinic acid hydrazide derivatives as potential antimycobacterial agents with a general structure represented by VI (Figure 1). The results showed that the presence of lipophilic electron-withdrawing halogen groups at the para position of the phenyl ring improved the antimycobacterial activity. Concerning the related fused pyridine heterocycles, Adhikari and co-workers reported two studies on the design, synthesis and biological evaluation of two different series of new quinoline-3-carbohydrazone derivatives VII, (Figure 1), as potential antimycobacterial agents [24,25]. Aboul-Fadl et al. [26] also explored the anti-tubercular activity of Schiff bases VIII of nalidixic acid-3-carbohydrazides and isatins. On the other hand, isatin-based compounds are known to exhibit excellent anti-TB properties [27][28][29][30]. In this work, the aforementioned findings motivated us to synthesize three series of nicotinic acid hydrazide derivatives 4a-i, 7a-f and 8a-c with the aim of obtaining new antimycobacterial agents. The first series comprises different nine 6-aryl-2-methylnicotinohydrazides with free hydrazine units in a similar fashion to INH. Subsequently, two derivatives (compounds 4c and 4e) with considerable lipophilicity (LogP = 1.53 and 2.17, respectively) were chosen for further chemical modification. The non-classical ring opening bioisosterism for structures VI and VIII was adopted to develop a series of aldehyde hydrazides 7a-f and a series of isatin hydrazides 8a-c.
The in vitro cytotoxic activity of subset of compounds was evaluated against HT-29, PC-3, A549, HepG2 and MCF-7 cancer cell lines to determine the toxicity of these agents. In addition, we describe an ADME study and SAR description in order to explore the structural requirements controlling these observed antitubercular activities.

Chemistry
The synthetic pathways employed to prepare the new targeted derivatives are depicted in Schemes 1 and 2. In a one-pot three-component heterocyclocondensation process, ethyl 2-methyl-6-arylnicotinates 3a-i was obtained via the reaction of enaminones 2a-i with ethyl acetoacetate and ammonium acetate in refluxing acetic acid. Preparation of the nicotinic acid hydrazides 4a-i in 79%-90% yield was achieved via the hydrazinolysis of ester derivatives 3a-i with refluxing hydrazine hydrate (Scheme 1). The IR spectra of hydrazides 4a-i showed absorption bands due to the carbonyl group in the 1635-1664 cm −1 region, in addition to peaks in the region from 3187 to 3414 cm −1 , assigned to the NH and NH2 groups. The 1 H-NMR spectra of 4a-i showed two singlet D2O-exchangeable signals attributable to NH and NH2 protons in the δ 9.58-9.62 and 4.52-4.60 ppm region, respectively, while the methyl (-CH3) protons appeared as singlets around δ 2.59-2.61 ppm. Furthermore, the 13 C-NMR spectra of 4f-h showed signals resonating around δ 167 ppm attributable to the carbons of carbonyl groups, while the carbons of the methyl groups appeared in the δ 23.53-23.59 ppm range.
While the hydrazide 4e was reacted with different aldehydes 5a-f, the hydrazide 4c was reacted with three isatins 6a-c in ethanol in the presence of a catalytic amount of glacial acetic acid to furnish the target derivatives 7a-f and 8a-c, respectively (Scheme 2).
The IR spectra of hydrazides 7a-f revealed the presence of two peaks in the 3412-3413 and 1635-1654 cm −1 regions, assigned to the NH and carbonyl groups, respectively. The 1 H-NMR spectra of these compounds revealed D2O-exchangeable signals in the δ 11.80-12.05 ppm region which were assigned to NH protons, in addition to the signal of the methine proton (-CH=N-) in the δ 8.06-8.96 ppm region. Furthermore, the 13 C-NMR spectra of 7a-f showed two signals resonating at δ 23.41-23.47 and δ 162.00-170.39 ppm attributable to the methyl (CH3) and carbonyl (=C-C=O) carbons, respectively.

In Vitro Antimycobacterial Activity
Biological assays were performed at the Regional Center for Mycology and Biotechnology (RCMB), Al-Azhar University (Cairo, Egypt). Target compounds 4a-i, 7a-f and 8a-c were evaluated in vitro for their anti-tubercular activity against M. tuberculosis (RCMB 010126) using the microplate Alamar blue assay (MABA) [31]. Isoniazide and pyrazinamide were used as reference drugs. The results as percent inhibition and minimum inhibitory concentration (MIC) are presented in Table 1.
With regard to the issue of activity, valuable information about cell wall structure, host-pathogen interactions and drug targets provide opportunities for rational drug design strategies focused on drug lipophilicity. Increases in lipophilic character, however, result in changes in pathways of diffusion across the cell wall, enhancing the contribution of diffusion through the lipid domain, so increasing the lipophilicity of an antimycobacterial agent enhances its efficacy [32][33][34][35]. The calculated lipophilicity (miLogP) values are listed in Table 1.
Drug-likeness model scores were computed for all the compounds using the MolSoft software and are presented in Table 1. Drug-likeness models help to optimize the pharmacokinetic and pharmaceutical properties, for example, solubility, chemical stability, bioavailability and distribution profile of compounds [36]. Counterparts having zero or negative values should not be considered as drug-like candidates. Compounds 7f and 8a-c possessed the maximum drug-likeness score ranging from 0.41 to 0.88. Isoniazide -0.75 a : Calculated by [37]; b : Calculated by [38]; NA: No Activity (>100 µg/mL). Table 1, compound 8c emerged as the most potent analog with good antimycobacterial activity (MIC = 6.25 µg/mL). Compound 8b also possessed reasonable activity with a MIC value of 12.5 µg/mL. Besides, derivatives 4a, 4b, 4f and 8a displayed moderate activity (MIC = 25 µg/mL). On the other hand, compounds 4d, 4e, 4g-i, 7d and 7f exhibited modest antimycobacterial activity with MIC values ranging from 50 to 100 µg/mL.

Structure Activity Relationships (SAR)
Observing the results, we could deduce valuable data about the structure activity correlations of the tested compounds. Firstly, we explored the impact of substitution of the 4-position of the phenyl group in the first series compounds 4a-i. Incorporation of an unsubstituted phenyl group led to compound 4a with good activity against M. tuberculosis (MIC = 25 µg/mL). Introduction of a fluorine atom in the 4-position (compound 4c) led to complete loss of activity, suggesting that the presence of a strongly electron-withdrawing group is not favorable to the activity. Meanwhile, compounds 4d and 4e bearing more lipophilic chlorine and bromine substituents at the same position, elicited better activity (MIC = 50 and 100 µg/mL, respectively) than that of analog 4c. Thence, the order of activities of the halogenated members in the first series, decreased in the order of Br > Cl > F, indicating that the increased lipophilicity is a crucial element for the antitubercular activity. Conversely, substitution at the 4-position with electron-donating groups as methyl and methoxy groups (compounds 4b and 4f) retained the activity at MIC = 25 µg/mL. Interestingly, di-and trimethoxy substitutions (compounds 4g and 4h) led to decrease in the lipophilicity with subsequent decrease in the antimycobacterial activity (MIC = 50 µg/mL). Furthermore, the bioisosteric replacement of the phenyl group with a 2-thienyl group (compound 4i) led to a decrease in the activity.
Considering the aldehyde hydrazides of the second series 7a-f, antimycobacterial activity was only observed in counterparts 7d and 7f (MIC = 100 µg/mL). The remaining hydrazides exhibited no activity. Noteworthily, compounds 7d and 7f are the most lipophilic analogs in this series with LogP values of 6.23 and 6.13, respectively.
Finally, the effect of substitution of the 5-position of the incorporated isatin moiety in hydrazides 8a-c was investigated. Compound 8a with an unsubstituted isatin moiety showed good anti-TB activity (MIC = 25 µg/mL). The introduction of a chlorine substituent at the 5-position (compound 8b) resulted in increased activity with MIC = 12.50 µg/mL. Moreover, incorporation of a bromine atom (compound 8c) caused a remarkable increase of activity against M. tuberculosis (MIC = 6.25 µg/mL). To summarize, the order of activities of the isatin hydrazides of the third series decreased in the order Br > Cl > H, confirming the importance of lipophilicity for the antimycobacterial activity.
In general, the active members in the second series, compounds 7d and 7f, are less active than their parent compound 4e, indicating that condensation of free hydrazides with aldehydes results in a sharp decrease in activity, whilst, the compounds of the third series are remarkably more potent than their parent compound 4c, hinting that condensation with isatins greatly enhances the antitubercular activity.

In Vitro Cytotoxicity
In vitro cytotoxicity of the most active antitubercular compounds 8a-c was examined against HT-29 colon cancer and PC-3 prostate cancer cell lines using a sulforhodamine B (SRB) colorimetric assay as described by Skehan et al. [39]. Besides, these compounds were evaluated in a previous study for their cytotoxicity against HepG-2 liver cancer, A-549 human lung cancer and MCF-7 breast cancer cell lines [40]. Doxorubicin was included in the experiment as a reference cytotoxic compound. The results were expressed as growth inhibitory concentration (IC50) values which represent the compound concentration required to produce a 50% inhibition of cell growth after 72 h of incubation compared to untreated control (Table 2). Interestingly, none of the tested compounds displayed any significant cytotoxicity, thereby providing a high therapeutic index.

ADME Study
The ADME of the biologically active counterparts 4a, 4b, 4d-i, 7d, 7f and 8a-c was predicted via a theoretical kinetic study performed by means of the Discovery Studio software (Table 3). Both AlogP98 and PSA_2D descriptors were calculated to evaluate the lipophilicity and polar surface area. Also, solubility, absorption and CYP2D inhibition levels were predicted. Active members of the first series were expected to have good solubility, while compounds 7d and 7f showed very low solubility levels and compounds 8a-c showed low solubility levels. All the examined compounds showed good absorption levels, except compound 7d that displayed a moderate absorption level. Finally, with exception to compounds 4b-f, all members were predicted to be CYP2D non-inhibitors.

General Information
Melting points were measured with a Stuart melting point apparatus and are uncorrected. Infrared (IR) spectra were recorded as KBr disks using a Perkin Elmer FT-IR Spectrum BX apparatus (Akron, OH, USA). NMR Spectra were recorded on a Bruker AV-500 MHz NMR spectrometer (Billerica, MA, USA). 1 H-NMR spectra were run at 500 MHz and 13 C spectra was run at 125 MHz in deuterated dimethylsulfoxide (DMSO-d6). Chemical shifts are expressed in δ values (ppm) using the solvent peak as internal standard. All coupling constants (J) values are given in Hertz. The abbreviations used are as follows: s, singlet; d, doublet; m, multiplet. Microanalyses were carried out using Perkin Elmer PE 2400 CHN Elemental Analyzer and the results were within ±0.4%. Reaction courses and product mixtures were routinely monitored by thin layer chromatography (TLC) on silica gel precoated F254 plates Merck (Merck KGaA, Darmstadt, Germany). Unless otherwise noted, all solvents and reagents were commercially available and used without further purification.

General Procedure for Synthesis of N′-arylidene-6-(4-bromophenyl)-2-methylnicotinyl
Hydrazides 7a-f To a stirred solution of the hydrazide 4e (5 mmol) in hot ethanol (20 mL), aldehydes 5a-f (5 mmol) and catalytic amount of glacial acetic acid were added. The reaction mixture was heated under reflux for 4 h. The precipitate formed was collected by filtration while hot, washed with hot ethanol, dried and crystallized from ethanol/DMF to afford compounds 7a-f in 65%-80% yield.

General Procedure for Preparation of Target Compounds 8a-c
Indoline-2,3-dione derivative 6a-c (1 mmol) was added to a suspension of 6-(4-fluorophenyl)-2methylnicotinohydrazide (4c, 1 mmol) in ethanol (10 mL) and a catalytic amount of glacial acetic acid. The reaction mixture was refluxed for 4 h. The precipitate formed was collected by filtration while hot, washed with hot ethanol, dried and crystallized from ethanol/DMF to furnish compounds 8a-c. The physical properties and spectral data of 8a-c were identical with those reported [40]. The yields of compounds 4a-c ranged from 75%-80%.

Antimycobacterial Activity
The M. tuberculosis (RCMB 010126) strain was provided from culture collection of the Regional Center for Mycology and Biotechnology (RCMB), Al-Azhar University (Cairo, Egypt). Isoniazide and pyrazinamide were used as reference drugs. Antimycobacterial activity of the synthesized compounds was evaluated using the microplate Alamar blue assay (MABA) which was performed in black, clear-bottomed, 96 well microplates (in order to minimize background effects). Outer perimeter wells were filled with sterile water to prevent dehydration in experimental wells. Initial compounds dilutions were prepared in dimethyl sulfoxide and subsequent twofold dilutions were performed in the microplates. 0.1 ml of 10 5 CFU/mL Mycobacterium tuberculosis inoculum was added to wells, additional control wells consisted of bacteria only (B). Plates were incubated at 37 °C. Starting at day 4 of incubation, 20 µL of alamarBlue solution (Alamar Biosciences/Accumed, Westlake, OH, USA) and 12.5 µL of 20% Tween 80 were added added to the entire plate. Plates were then incubated at 37 °C, and results were recorded at 24 h post-reagent addition at 590 nm. Percent inhibition was defined as: 1 − (mean of test well/mean of B wells) × 100. Visual MICs were defined as the lowest concentration of drug that prevented a color change.

In Vitro Cytotoxic Activity
HT-29 colon cancer and PC-3 prostate cancer cell lines were obtained from the National Cancer Institute (Cairo, Egypt). HT-29 and PC-3, cells were grown in RPMI-1640. Media were supplemented with 10% heat-inactivated FBS, 50 units/mL of penicillin and 50 g/mL of streptomycin and maintained at 37 °C in a humidified atmosphere containing 5% CO2. The cells were maintained as a "monolayer culture" by serial subculturing. Cytotoxicity was determined using the SRB method as previously described by Skehan et al. [39]. Exponentially growing cells were collected using 0.25% trypsin-EDTA and seeded in 96-well plates at 1000-2000 cells/well in supplemented DMEM medium. After 24 h, cells were incubated for 72 h with various concentrations of the tested compounds as well as doxorubicin as the reference compound. Following 72 h of treatment, the cells were fixed with 10% trichloroacetic acid for 1 h at 4 °C. Wells were stained for 10 min at room temperature with 0.4% SRB dissolved in 1% acetic acid. The plates were air dried for 24 h, and the dye was solubilized with Tris-HCl for 5 min on a shaker at 1600 rpm. The optical density (OD) of each well was measured spectrophotometrically at 564 nm with an ELISA microplate reader (ChroMate-4300, Palm City, FL, USA). The IC50 values were calculated according to the equation for Boltzmann sigmoidal concentration-response curve using the nonlinear regression models (Graph Pad, Prism Version 5). The results reported are means of at least three separate experiments. Significant differences were analyzed by one-way ANOVA wherein the differences were considered to be significant at p < 0.05.

Conclusions
In summary, we have synthesized eighteen derivatives based on the nicotinic acid hydrazide scaffold and evaluated their antimycobacterial activity. From the obtained results, it was obvious that condensation of free hydrazides with aldehydes sharply decreased the activity, while isatin hydrazides enhanced the activity. Compounds 8b and 8c emerged as the most potent counterparts among all the tested compounds (MIC = 12.5 and 6.25 µg/mL, respectively). They also displayed good drug-likeness scores of 0.62 and 0.41, respectively. The cytotoxicity of hydrazides 8a-c was evaluated against HT-29, PC-3, A549, HepG2 and MCF-7 cancer cell lines. None of the tested hydrazides exhibited cytotoxicity up to 200 µM. The importance of lipophilicity for the antimycobacterial activity was explored via a SAR study. Finally, a theoretical kinetic study was established to predict the ADME of the active derivatives.