SYNTHESIS AND RP-TLC LIPOPHILICITY EVALUATION OF A NOVEL FLUOCINOLON ACETONIDE SOFT DRUG DERIVATIVE

Cortienic acid was obtained by periodic acid oxida tion of fluocinolone acetonide, whereas corresponding amide was synthesi zed from the cortienic acid and ethyl ester of β-alanine by dicyclohexylcarbodiimide – hydroxybenzo triazole coupling procedure. Lipophilicity of the amide was evaluated by using reversed-phase thin-layer chromatography systems, consisting of ethanol and w ater in various ratios, and was higher in comparison to fluocinolone acetonide and cortienic acid.


INTRODUCTION
The soft drug concept was presented as part of retrometabolic drug design approach, whose aim was to obtain biologically active compounds with fewer side effects (BODOR et al., 1980;BODOR and KAMINSKI, 1980;STAŃCZAK et al., 2006;PROCOPIOU et al., 2010).The first and second generations of soft glucocorticoids (loteprednol etabonate, etiprednol dicloacetate and corresponding analogues) are cortienic acid derivatives, which are inactive and non-toxic metabolites of traditional glucocorticoids (BODOR and BUCHWALD, 2006;PAVESIO et al., 2008;CHANDEGARA and CHORAWALA, 2012).Other glucocorticoid derivatives which are easily metabolized after local administration to non-toxic and inactive metabolites could also be considered soft drugs (antedrugs) (KHAN and LEE, 2008).
Novel amides of cortienic acids (derived from hydrocortisone, prednisolone, methylprednisolone, dexamethasone and betamethasone) and esterified amino acids (methyl and ethyl esters of L-glycine, methyl ester of L-alanine, ethyl ester of β-alanine and methyl ester of L-phenylalanine) have been recently presented as potentially new soft glucocorticoids for local application to the skin.Their permeability and retention in the skin were predicted by using parallel artificial membrane permeability assay (PAMPA) and biopartitioning micellar chromatography.Quantitative structure -permeability relationship (QSPR) and quantitative structure -retention relationship (QSRR) analyses underlined lipophilicity as a physicochemical property with the highest positive influence on permeability and retention (DOBRIČIĆ et al., 2014;DOBRIČIĆ et al., 2014a).The presence of L-glycine, L-alanine and βalanine residues in the 17β side chain enables favorable molecule orientation and key binding interactions with amino acids in the glucocorticoid receptor, which was confirmed by good local anti-inflammatory activity of these derivatives.Therefore, good local anti-inflammatory activity of novel soft glucocorticoids containing these amino acids in the 17β side chain could be expected.Additionally, fewer systemic side effects (in comparison with dexamethasone) and predicted metabolic inactivation indicate that these compounds could be classified as novel soft glucocorticoids (DOBRIČIĆ et al., 2014b).
The aim of this study was to synthesize a new soft glucocorticoid, which is amide of fluocinolone acetonide -derived cortienic acid and ethyl ester of β-alanine.Lipophilicity of the amide was evaluated by using reversed-phase thin-layer chromatography (RP-TLC) and compared to fluocinolone acetonide and cortienic acid.
Melting points were determined by using Boetius PHMK 05 apparatus (Radebeul, Germany).The 1 H NMR spectra were recorded on an NMR BRUKER AVANCE III 400 spectrometer (Bruker Biospin GmbH, Rheinstetten, Germany), operating at 400 MHz.The accurate masses were determined by using Agilent 6210 Time-of-Flight mass spectrometer (Agilent Technologies, Palo Alto, CA, USA), whereas IR spectra were recorded by using ATR-FTIR spectrometer Nicolet iS10 (Thermo Scientific, Madison, WI, USA).MS/MS analyses were performed on TSQ Quantum Access MAX triple quadrupole mass spectrometer (Thermo Fisher Scientific, San Jose, USA), equipped with heated electrospray ionization source (HESI).

Synthesis of cortienic acid (CFA)
Fluocinolone acetonide (300 mg, 0.66 mmol, 1 eq) was dissolved in dioxane (2 mL).Solution of periodic acid in water (275 mg mL -1 ) was added dropwise (0.6 mL, 0.72 mmol of periodic acid, 1.1 eq).The reaction mixture was stirred at room temperature for 20 h and evaporated to dryness under reduced pressure.The crude product was washed with water (5 mL), dried, washed with chloroform (12 mL) and dried again.

Synthesis of amide (FAEA)
CFA (61 mg, 0.14 mmol, 1 eq) was dissolved in DMF (2 mL) and the solution was cooled at 0 o C. Subsequently, DCC (58 mg, 0.28 mmol, 2 eq) and HOBt (29 mg, 0.21 mmol, 1.5 eq) were added.The mixture was stirred at the same temperature for 1 h and after that it was maintained at temperature not exceeding 8 o C for 20 h.β-alanine ethyl ester hydrochloride (22.2 mg, 0.14 mmol, 1 eq) was dissolved in DMF (1 mL), TEA was added (39 µL, 0.28 mmol, 2 eq) and this mixture was cooled at 0 o C. Finally, the mixture of CFA, DCC and HOBt was filtered and added dropwise.The final reaction mixture was stirred at 0 o C for 1 h and it was maintained at temperature not exceeding 8 o C for 20 h.The reaction mixture was filtered, evaporated to dryness under reduced pressure and purified by using column chromatography (the mobile phase was chloroform/methanol 99:1 (v/v)).Final purification was performed using preparative thin layer chromatography (the mobile phase was chloroform/methanol 95:5 (v/v)).The purified compound was recrystallized in the mixture of water and methanol.

RP-TLC evaluation of lipophilicity
The TLC plates were spotted with 1.0 µL of freshly prepared solutions of tested compounds in ethanol (1 mg mL -1 ) and developed by the ascending technique in glass TLC chambers with glass lids.After development, spots were detected under UV light (λ=254 nm).The R F values were calculated according to the equation ( 1): (1) l -migration distance of tested compound l 0 -migration distance of solvent front The R M values were calculated according to the equation ( 2). (2)

RP-TLC evaluation of lipophilicity
Chromatographic behavior of FAEA, FA (parent glucocorticoid) and CFA (intermediate in the FAEA synthesis and potential metabolite of FAEA) was tested in four RP-TLC systems, consisting of ethanol and water in different ratios: 40:50, 50:50, 60:40 and 70:30 (v/v).For each compound, R M values were calculated and plotted against the percentage of ethanol (Fig. 3).RP-TLC parameters, used for the evaluation of lipophlicity of FA, CFA and FAEA, are presented in Table 1.

Synthesis of CFA and FAEA
Periodic acid oxidation is the most commonly used procedure for the synthesis of cortienic acids.In this study, CFA was synthesized by periodic acid oxidation of fluocinolone acetonide and the highest yield was obtained when dioxane was used as a solvent.Alternatively, oxidation could be performed by introducing air in the solution of fluocinolone acetonide in the presence of potassium-carbonate or by using potassium superoxide (ALVAREZ, 1980;ASHTON et al., 1994).
FAEA was synthesized by using two-step DCC-HOBt coupling procedure, which resulted in higher yields than the single-step EDC-HOBt coupling procedure when similar compounds were synthesized (DOBRIČIĆ et al., 2014b).In the first step, CFA was esterified with HOBt (in the presence of DCC) and in the second step FAEA was synthesized in the reaction between CFA-HOBt ester and ethyl ester of β-alanine (FORMSTECHER et al., 1980).

RP-TLC evaluation of lipophilicity
The shake flask method, based on partitioning of substances between octanol and water, is considered standard method for the estimation of lipophilicity.However, it is time consuming and requires large amounts of solvents and tested compounds.(3) FAEA has the highest values of and and the highest absolute value of a, indicating higher lipophilicity of this derivative in comparison with FA and CFA.Therefore, it could also be expected that FAEA has favorable properties for local application to the skin.

Figure 3 .
Figure 3. R M values versus the ethanol content; A) FA, B) CFA and C) FAEA Alternatively, chromatographic methods (HPLC and TLC) can be used for fast and simple evaluation of lipophilicity of various groups of drugs (MAES et al., 1998; CSERMELY et al., 2008; ODOVIĆ et al., 2009).RP-TLC parameters used for the estimation of lipophilicity were C 0 (R M value when the ethanol content is 0% (curve intercept)), a (slope of the curve) and (MORAK et al., 2007; STAREKA et al., 2013; BIEGANOWSKA et al., 1995).Correlation coefficients ranged from 0.9889 to 0.9995, which means there are linear relationships between R M values and ethanol contents.Therefore, can be calculated by extrapolation of the curves to the y axes.Parameter is calculated according to the equation (3): Cortienic acid was synthesized by periodic acid oxidation of fluocinolon acetonide (yield: 44%) and used for the synthesis of amide with ethyl ester of β-alanine (DCC-HOBt coupling procedure; yield: 70%).Chromatographic behavior of fluocinolone acetonide, cortienic acid and the amide was tested in RP-TLC systems consisting of ethanol and water in various ratios (40:50, 50:50, 60:40 and 70:30 (v/v)).Calculated RP-TLC parameters ( , a and ) indicate that FAEA should have the highest lipophilicity and favorable properties for local skin application.

Table 1 .
RP-TLC parameters calculated for FA, CFA and FAEA