Measurement and correlation of solubility of diosmin in four pure solvents and β-cyclodextrin solution at 298.15 K to 333.15 K

Md Khalid Anwer ,Faiyaz Shakeel *

1 Department of Pharmaceutics,College of Pharmacy,Salman Bin Abdulaziz University,Al-Kharj,Saudi Arabia

2 Center of Excellence in Biotechnology Research,King Saud University,Riyadh 11451,Saudi Arabia

3 Department of Pharmaceutics,College of Pharmacy,King Saud University,Riyadh 11451,Saudi Arabia

Keywords:Apelblat model Cyclodextrin Diosmin Solubility Thermodynamics

A B S T R A C T The aim of present study was to measure and correlate the solubility of poorly water-soluble flavonoid diosmin in water,ethanol,isopropyl alcohol(IPA),polyethylene glycol-400(PEG-400)and β-cyclodextrin(β-CD)aqueous solution(0.02 mol·L?1).The solubility of diosmin was measured using the shake flask method from(298.15 to 333.15)K at atmospheric pressure.The experimental solubilities of diosmin were regressed by the modified Apelblat model with a relative deviation in the range of 0.048%to 5.940%.The correlation coefficients were observed in the range of 0.9957 to 0.9995.The solubility of diosmin was found to be increased with temperature in all sample matrices investigated.The mole fraction solubility of diosmin was found to be higher in β-CD aqueous solution and PEG-400 as compared to water,ethanol and IPA.Based on solubility data of present study,diosmin was considered as practically insoluble in water,insoluble in ethanol&IPA and soluble in PEG-400 and β-CD aqueous solution.

1.Introduction

The IUPAC name of diosmin is 3,5,7-trihydroxy-4-methoxy flavone-7 rutinoside and its molecular structure is presented in Fig.1[1].It occurs as a pale yellow needle and its molecular formula and molar mass are C28H32O15and 608.54 g·mol?1,respectively[2,3].It has been reported as poorly water-soluble(practically insoluble) flavonoid which shows great biological activities such as anti-inflammatory,antioxidant,antimutagenic and anticancer activities[1,3–5].Duetoitspoor solubility in water,it suffers from poor dissolution in water and also in most of the organic solvents which in turn results in poor in vivo absorption/bioavailability[1,6].Poor solubility and poor bioavailability are the major hindrances in formulation development of bioactive natural compounds including flavonoids such as diosmin[1,7].Moreover,most of the bioactive compounds are also extracted using toxic solvents such as methanol,chloroform,dichloromethane and ether from their respective plant source[8,9].These toxic solvents could not be utilized in the formulation development of natural bioactive compounds due to their regulatory and toxicity issues.Several non-toxic and green solvents such as ethanol,propylene glycol(PG)and polyethylene glycol-400(PEG-400)have been investigated in enhancing the solubility of various poorly water-soluble drugs/pharmaceuticals[10–13].Therefore,in present study,various green solvents such as ethanol,isopropyl alcohol(IPA),PEG-400 and β-cyclodextrin(β-CD)aqueous solution were selected to get solubilization of diosmin.These green solvents could help in the formulation development of diosmin.Various approaches such as micronization,phytosomes and nanosuspensions have been investigated for solubility/dissolution and bioavailability enhancement of diosmin[1,6,7].However,the temperature dependent solubility data of diosmin in water,ethanol,IPA,PEG-400 and β-CD aqueous solution are not available in literature so far.Various mathematical models have been developed for the correlation of experimental solubility data of drugs with calculated one,but the modified Apelblat model is the commonly applied model among them[10–18].Therefore,the aim of this article was to measure and correlate the temperature dependent solubility data of diosmin in water,ethanol,IPA,PEG-400 and β-CD aqueous solution(0.02 mol·L?1)from 298.15 to 333.15 K at atmospheric pressure.

Fig.1.Molecular structure of diosmin.

2.Materials and Methods

2.1.Materials

Diosmin(mass fraction purity of 0.94),β-CD(mass fraction purity of 0.98),PEG-400(mass fraction purity of 0.9998)and ethanol(mass fraction purity of 0.997)were purchased from Sigma Aldrich(St.Louis,MO).IPA(mass fraction purity of 0.99)was purchased from BDH Laboratory(Liverpool,UK).Distilled water was collected from distillation unit in the laboratory.

2.2.Measurement of diosmin solubility

The mole fraction solubility of diosmin in four different pure solvents namely water,ethanol,IPA&PEG-400 and β-CD aqueous solution(0.02 mol·L?1)was measured by the shake fl ask method(from 298.15 to 333.15 K)at an atmospheric pressure of 0.1 MPa[11,12].In order to perform solubility experiments,the excess amount of diosmin was added in 5 ml of each pure solvent and β-CD aqueous solution(0.02 mol·L?1)in 10 ml capacity flasks in triplicate.The samples in each fl ask were equilibrated by continuous shaking in an isothermal mechanical shaker bath(Julabo,PA)at 100 r·min?1for 72 h to reach equilibrium[19].The temperature of isothermal mechanical shaker bath was maintained with thermostatic bath equipped with shaker.After 72 h(enough time to reach equilibrium),all the sample fl asks were taken out from the shaker and allowed to settle diosmin(solute)particles for 2 h at the bottom of each flask[11,15,17].All the sample fl asks were centrifuged at 5000 r·min?1for 20 min, filtered through 0.45μm filter paper and supernatant from each sample was diluted suitably with respective solvent and subjected for analysis of diosmin content using high performance liquid chromatography(HPLC)method at the detection wavelength of 346 nm[20].The standard uncertainty for the temperatures u(T)was observed as±0.2 K.However,the relative standard uncertainty in solubility ur(xe)was observed as 1.4%.The experimental mole fraction solubility(xe)of diosmin in each sample was calculated using Eq.(1)[12,17]:

w here,m1and m2represent the masses of diosmin(g)and respective solvent(water,ethanol,IPA&PEG-400)orβ-CD(g),respectively.M1and M2represent the molecular masses of diosmin(g·mol?1)and respective solvent(water,ethanol,IPA&PEG-400)or β-CD,respectively.

2.3.HPLC analysis of diosmin

The diosmin content in all investigated samples was determined by reported HPLC method with slight modifications as per our laboratory conditions[20].Waters HPLC system equipped with Waters 600 controller pump,auto-sampler(Waters 717 plus) fitted with a 20 μl loop and Waters 486 tunable absorbance detector were used.The chromatographic column used was are versed-phase C18(150 mm×4.6 mm,particlesize 5 μm).The isocratic mobile phase consisted binary mixture of methanol and water(45:55,by volume)was delivered via a stainless steel analytical column at flow rate of 0.6 ml·min?1.The wavelength of variables UV–VIS detector was set at 346 nm.The analysis was carried out at ambient temperature and the volume of injection was 5 μl.

3.Results and Discussion

3.1.Solubility data of diosmin

Preliminary experiments were performed to optimize the concentration of β-CD aqueous solution.For this purpose,phase solubility studies were performed at various concentrations of β-CD.From the phase solubility curves,the maximum solubility of diosmin was observed in 0.02 mol·L?1aqueous solution of β-CD(unpublished data).Hence,this aqueous solution of β-CD(0.02 mol·L?1)was selected in present study.The mole fraction solubility data of diosmin in four pure solvents(water,ethanol,IPA&PEG-400)and β-CD aqueous solution(0.02 mol·L?1)from 298.15 to 333.15 K at atmospheric pressure of 0.1 MPa are listed in Table 1 The experimental solubility of diosmin was found to be increased with temperature in all pure solvents and β-CD aqueous solution from 298.15 to 333.15 K.The solubility(g·g?1)of diosmin was found to be highest in PEG-400(1.29 × 10?3g·g?1at 298.15 K)follow ed by β-CD aqueous solution(0.99 × 10?3g·g?1at 298.15 K),IPA(0.04 × 10?3g·g?1at 298.15 K),ethanol(0.03 × 10?3g·g?1at 298.15 K)and water(0.02 × 10?3g·g?1at 298.15 K)from 298.15 to 333.15 K.However,the mole fraction solubility of diosmin was found to be highest in β-CD aqueous solution(1.85×10?3at 298.15 K)follow ed by PEG-400(8.47× 10?4at 298.15 K),IPA(3.95× 10?6at 298.15 K),ethanol(2.57×10?6at 298.15 K)and water(1.25× 10?6at 298.15 K)from 298.15 to 333.15 K as shown in Table 1.The mole fraction solubilities of diosmin in β-CD aqueous solution and PEG-400 were significantly higher than its solubility in ethanol,IPA and water at all temperatures investigated.How ever,the differences in mole fraction solubility of diosmin in β-CD aqueous solution and PEG-400 were not significant.The higher solubilities of diosmin in β-CD aqueous solution and PEG-400 were probably due to their lower polarities as compared to higher polarity of water[11,12].The highest mole fraction solubility of diosmin in β-CD aqueous solution was attributed due to the formation of soluble inclusion complex between diosmin and β-CD aqueous solution as well as its highest molecular mass(1134.98 g·mol?1)as compared to PEG-400(molecular mass 400 g·mol?1),ethanol(molecular mass 46.069 g·mol?1),IPA(molecular mass 60.1 g·mol?1)and water(molecular mass 18.015 g·mol?1).Temperature-dependent mole fraction solubility of data of diosmin in water,ethanol,IPA,PEG-400 and β-CD aqueous solution is not available in literature so far.How ever,it has been reported as poorly soluble/practically insoluble in water and insoluble in most of the organic solvents[1,7].Based on solubility results of present study,diosmin has been considered as practically insoluble in water,insoluble in ethanol&IPAand soluble in PEG-400&β-CD aqueous solution.

Table 1Experimental(x e)and calculated mole fraction solubilities(x)of crystalline diosmin in four pure solvents and β-cyclodextrin aqueous solution at temperatures T=298.15 to 333.15 K and pressure p=0.1 MPa

3.2.Correlation and thermodynamic modeling of diosmin solubility

The experimental solubilities of diosmin were correlated with the modified Apelblat model because of its simplicity and wider range of applications[12,14,17].According to this model,the mole fraction solubility of diosmin can be expressed using Eq.(2)[11,14,16]:

w here,x is the solubility of diosmin calculated by Apelblat equation(calculated/Apelblat solubility)and T is the absolute temperature(K).The parameters A,B&C are adjustable model parameters which were determined by multivariate regression analysis of experimental data with the help of Eq.(2).How ever,the calculated/modified Apelblat solubilities(x)of diosmin were calculated with the help of model parameters A,B&C using Eq.(2).The percentage of relative deviation(AD)between xeand x was calculated using Eq.(3).The resulting data of xe,x and AD(%)in water,ethanol,IPA,PEG-400 and β-CD aqueous solution from 298.15 to 333.15 K at atmospheric pressure are listed in Table 1.

The AD values were observed lower in PEG-400(0.274%to 0.451%)andβ-CD aqueous solution(0.048%to 0.545%)as compared to relatively higher values in water,ethanol and IPA from 298.15 to 333.15 K.However,the highest AD values were observed in water(0.441%to 5.940%)from 298.15 to 333.15 K.Overall,the AD value was found to be less than 6%in all sample matrices investigated.The values of model parameters A,B&C along with correlation coefficients(R2)in water,ethanol,IPA,PEG-400 and β-CD aqueous are listed in Table 2.The R2value for diosmin in ethanol and IPA was found to be 0.9975.How ever,the R2values for diosmin in water,PEG-400 and β-CD aqueous solution were observed in the range of 0.9957–0.9995(Table 2).Overall,these resultsindicated good fitting of experimental solubility data in water,ethanol,IPA,PEG-400 and β-CD aqueous solution.

Table 2The modified Apelblat parameters(A,B and C)and correlation coefficients(R2)for diosmin in different solvents

3.3.Thermodynamic parameters for diosmin dissolution

The dissolution behavior of diosmin(solid)into water,ethanol,IPA,PEG-400 and β-CD aqueous solution(liquids)can be expressed as[14,17,18]:

The molar enthalpy(ΔH0)and entropy(ΔS0)for diosmin dissolution in pure solvents and β-CD aqueous solution were calculated using Eqs.(4)and(5),respectively:

where,R is the universal gas constant and other parameters are already defined in the previous text.The values of ΔH0and ΔS0for diosmin dissolution were found to be higher in water,ethanol and IPA as compared to PEG-400 and β-CD aqueous solution from 298.15 to 333.15 K.The ΔH0values for diosmin dissolution in water,ethanol,IPA,PEG-400 and β-CD aqueous solution were observed in the range of 42.03 to 57.10 kJ·mol?1,42.19 to 52.74 kJ·mol?1,39.24 to 51.26 k J·mol?1,3.03 to 5.01 kJ·mol?1and 8.93 to 11.30 k J·mol?1from 298.15 to 333.15 K,respectively.How ever,the ΔS0values for diosmin dissolution in water,ethanol,IPA,PEG-400 and β-CD aqueous solution were observed in the range of 126.18 to 191.51 J·mol?1·K?1,126.65 to 176.89 J·mol?1·K?1,131.64 to 153.86 J·mol?1·K?1,10.18 to 15.06 J·mol?1·K?1and 29.98 to 33.95 J·mol?1·K?1from 298.15 to 333.15 K,respectively.The ΔH0and ΔS0values for diosmin dissolution in PEG-400 and β-CD aqueous solution were significantly reduced as compared to water,ethanol and IPA.These results clearly indicated that relatively low energy is required for solubilization of diosmin in PEG-400 and β-CD aqueous solution as compared to water,ethanol and IPA.Overall,the positive values of ΔH0and ΔS0in all sample matrices indicated that diosmin dissolution was endothermic and an entropydriven process in sample matrices investigated.

4.Conclusions

In the present study,the measurement and correlation of diosmin solubility in water,ethanol,IPA,PEG-400 and β-CD aqueous solution was performed from 298.15 to 333.15 K at atmospheric pressure using the shake flask method.The mole fraction solubility of diosmin was found to be increased with temperature in all sample matrices investigated.The mole fraction solubility of diosmin was significantly higher in β-CD aqueous solution and PEG-400 as compared to water,ethanol and IPA.The experimental solubilities of diosmin were correlated well with the modified Apelblat model from 298.15 to 333.15 K with the correlation coefficients in the range of 0.9957 to 0.9995.Based on these results,diosmin has been considered as practically insoluble in water,insoluble in ethanol&IPA and soluble in PEG-400 and β-CD aqueous solution.

Acknowledgments

This project was supported by the Deanship of Scientific Research,Salman Bin Abdulaziz University,Al-Kharj,Saudi Arabia(47H/1433).