Development of fexofenadine self-microemulsifying delivery systems: an efficient way to improve intestinal permeability

Abstract

Aim: The present study aimed to prepare and evaluate fexofenadine self-microemulsifying drug-delivery systems (SMEDDS) formulation and to determine and compare its intestinal permeability using in situ single-pass intestinal perfusion (SPIP) technique.

Methods: Fexofenadine-loaded SMEDDS were prepared and optimized. Droplet size, polydispersity index, zeta potential, drug release and intestinal permeability were evaluated.

Results: Optimized formulation consisted of 15% oil, 80% surfactant and 5% cosolvent. Droplet size and drug loading of optimized formulation was 13.77 nm and 60 mg/g and it has released 90% of its drug content. Intestinal permeability of fexofenadine was threefold enhanced in SMEDDS compared with free fexofenadine.

Conclusion: The results of our study revealed that SMEDDS could be a promising tool for oral delivery of fexofenadine with enhanced dissolution rate and intestinal permeability.

Plain language summary

Article highlights.

• A mixture of turpentine oil/castor oil (3:1) and ethyl oleate was selected as the oil phase and Tween 80 and PEG 600 are selected as the surfactant and cosolvent to prepare the fexofenadine-loaded self-microemulsifying drug-delivery systems (SMEDDS).

• 36 different SMEDDS formulations with varying ratios of oil, surfactant and cosolvent were prepared and their state of homogeneity and stability were evaluated.

• F8 formulation produced droplets with smaller sizes compared with F6, F7 and F15.

• All of the SMEDDS formulations have released more than 50% of their drug content in 30 min.

• There were no signs of creaming, phase separation, layering and droplet size change in F6, F and F8 formulations after facing to freeze–thaw and centrifugation tests.

• The ratios of corrected outlet to inlet fexofenadine concentrations obtained from SPIP studies were used to determine reaching the steady state and calculating intestinal effective permeabilities.

• An approximately threefold improvement was seen in the intestinal effective permeability of fexofenadine from SMEDDS compared with free fexofenadine suspension.

• Human intestinal permeabilities of SMEDDS formulaton and free fexofenadine suspension was predicted.

1. Background

Fexofenadine, a second-generation antihistamine compound, is a selective H₁ antagonist with no cardiovascular adverse effects (compared with terfenadine and astemizole) [1]. Since fexofenadine is not able to pass the blood–brain barrier, it does not show sedative effects unlike first generation antihistamines [2,3]. Due to the selectivity, lack of severe adverse effects and long duration of action compared with first generation antihistamines; fexofenadine is widely used for the symptomatic treatment of seasonal allergic rhinitis and chronic idiopathic urticaria [4,5].

After oral administration, fexofenadine exhibits a maximum drug concentration (C_max) of 142 ng/ml and low bioavailability of about 30%, as well as an elimination half-life of 3 to 17 h [6]. Fexofenadine is not a substrate of intestinal and hepatic cytochrome P450, therefore first pass metabolism is not considered as a cause of its low bioavailability. This low oral bioavailability has been ascribed to different properties of the drug, such as its slight solubility in water and low permeability across intestinal epithelium [7,8]. Furthermore, fexofenadine is a substrate of both P-glycoprotein (P-gp) and organic anion transporter protein (OATP) family [9]. P-gp mediated efflux in the intestinal lumen can have an essential role in the limited intestinal absorption and low bioavailability of fexofenadine [8].

Self-microemulsifying drug-delivery systems (SMEDDS) are isotropic mixtures of oily components, nonionic surfactant and co-solvents or co-surfactants [10,11] which are able to establish microemulsions with fine droplet sizes (less than 200 nm) upon exposure with the gastrointestinal fluids under mild agitations produced by peristaltic movements of the gastrointestinal (GI) tract [12,13]. The mechanism of self-emulsification is somehow similar however simpler than self-assembled polymers [14–16].

A number of fexofenadine-loaded nano-formulations have been developed and reported in literature including fexofenadine solid lipid nanoparticles [17], fexofenadine-loaded chitosan coated solid lipid nanoparticles [18], Cubosomes [19], nanostructured lipid carriers (NLCs) [20], fexofenadine albumin nanoparticles loaded in gel [21] and fexofenadine-encapsulated poly(lactic-co-glycolic acid) nanoparticles [22]. However, some limitations and drawbacks are associated with these studies, e.g., difficulties in large scale manufacture of cubosomes due to complicated phase behavior [23], intended non oral route of administration (albumin nanoparticles loaded in gel [21] are intended for topical delivery), limited encapsulation efficiency and low potential of industrial manufacture compared with SMEDDS [17,18,20]. Additionally, except in the case of cubosomes, the effect of nano-formulations on intestinal permeability of fexofenadine have not been studied yet.

SMEDDS formulations are reported to enhance the oral bioavailability of different drug compounds either via solubility or permeation enhancement [24–26]. In several studies the impact of SMEDDS on intestinal permeability of various drugs have been evaluated either by in vitro, ex vivo or in situ techniques. Oridonin [27], canagliflozin [28], resveratrol [29], sulpiride [30], olmesartan medoxomil [31] are some examples of lipophilic drugs showed improved intestinal permeation after being loaded in a SMEDDS formulation.

Furthermore, self-emulsifying excipients exhibit efflux pump inhibiting properties, which makes them appropriate drug delivery systems for oral delivery of substrates of efflux transporters [32,33]. This bioavailability enhancing properties as well as the simple manufacturing and scale up of SMEDDS formulations made them promising carriers for wide range of drugs having limited solubility and/or permeability including fexofenadine.

In situ single-pass intestinal perfusion (SPIP) in rat model is one of the various methods for determination of intestinal permeability [34]. Due to the capability of this technique to maintain the normal physiologic conditions and almost intact blood supply [35], this method is considered as an established method for determination of intestinal permeability [36]. Since a high correlation had been obtained between the intestinal rat permeability determined by SPIP method and human intestinal permeability and fraction oral dose absorbed in human [37], SPIP has been used in this study as a reliable method to determine the intestinal effective permeability of fexofenadine.

The aim of the present study was to design, prepare and evaluate the physicochemical properties of fexofenadine SMEDDS formulations and to determine the intestinal permeability of fexofenadine loaded in SMEDDS formulation and compare it with plain fexofenadine using SPIP technique.

2. Materials & methods

2.1. Materials

Fexofenadine was obtained from DaroPakhsh Co., Tehran, Iran (laboratory grade, purity 98%). Canola oil, olive oil, turpentine oil, cottonseed oil, rose oil, soy oil, sesame oil, sunflower oil, corn oil and propylene glycol was purchased from Sepidaj, Tehran, Iran. Ethyl oleate, Tween 80, Tween 20, MCT, Isopropyl myristate, Brij 35, Span 80, Poly ethylene glycol (PEG) 200, PEG 400, PEG 600, PEG 1500, PEG 3350, PEG 4000 and glycerol triacetate was supplied from Merck, Darmstadt, Germany.

2.2. Solubility measurements

Solubility of fexofenadine in different oils, surfactants and cosolvents were determined using the shake flask method [38]. An excess amount of fexofenadine was added to 1 ml of each excipient in screw-caped vials. The vials were incubated for 72 h in a shaker incubator at 25°C (100 rpm) and then centrifuged at 10000 rpm for 10 min. Supernatants were filtered through a 0.45 μm syringe filter and the amount of fexofenadine in each sample was determined spectrophotometrically after dilution with ethanol.

2.3. Formulation design

In order to estimate the acceptable range of each formulation component, 36 formulations with different ratios of oils, surfactant and cosurfactant were designed using a factorial design approach as summarized in Supplementary File S1 and 1 g of each formulation was prepared and left undisturbed for 24 h. Then the formulations were centrifuged at 10000 rpm for 10 min and evaluated for creaming and phase separation. Afterward the ternary phase diagram was prepared according to the results of stability and droplet size tests.

2.4. Preparation of fexofenadine-loaded self-microemulsifying formulations

A constant amount (60 mg) of fexofenadine was accurately weighed and dissolved in 1 ml of ethanol. Oil, surfactant and cosurfactant were combined in desired ratios according to the experimental design, mixed on a magnetic stirrer and heated to 40°C in order to prepare 940 g of homogenous blank SMEDDS formulation. Fexofenadine was added to the formulation and continuously stirred and heated until complete solubilization of the drug.

In order to determine the maximum loading capacity of formulations, increasing amounts of fexofenadine (30, 60 and 120 mg) were added to 1 g aliquots of optimized formulations. Each aliquot was evaluated for signs of precipitation.

2.5. Particle size & zeta potential measurement

Droplet size of the emulsion droplets were determined using a Malvern Zetasizer Nano-ZS instrument. Briefly, 1 g of each formulation was added to 250 ml 0.1 N HCl in a volumetric flask and shaken gently. Then the droplet size and polydispersity index (PDI) were analyzed by the dynamic light scattering technique.

2.6. Stability studies

Optimized formulations were centrifuged at 5000 rpm for 30 min and then evaluated for any signs of creaming, phase separation and layering.

The formulations which pass the centrifugation test successfully, were subjected to three freeze–thaw cycles. Briefly 2 g of each formulation was incubated at -4°C for 48 h followed by incubating at 40°C for 48 h. Afterward, the samples were centrifuged at 4000 rpm for 5 min and were evaluated for phase separation, creaming, layering and size changes.

2.7. In vitro dissolution

Dissolution studies were performed using dissolution apparatus I (basket) for capsules and dissolution apparatus II (paddle) for commercial fexofenadine tablets. Optimized formulations (500 mg) containing 30 mg fexofenadine were filled in hard gelatin capsules. Commercial fexofenadine tablets (Telfast^® 30 mg) as well as plain fexofenadine powder (30 mg) filled hard gelatin capsules were also used for in vitro dissolution studies. The dissolution medium was 900 ml of 0.1 N hydrochloric acid. Capsules were immersed in the dissolution medium using sinkers. Dissolution studies were performed at 37°C and the rotation speed was 50 rpm. At determined time points (10, 30, 60 and 120 min), 5 ml aliquots were collected and replaced with the same volume of fresh medium to maintain the sink conditions. The aliquots were filtered through 0.45 μm Millipore syringe filters and analyzed with HPLC method to measure the fexofenadine content. In order to characterize and compare the dissolution curves, area under the dissolution curve (AUC), dissolution efficiency (DE) and mean dissolution time (MDT) were assessed using DDSolver^® add-in program. These parameters were calculated using a model-independent nonparametric method based on the linear trapezoidal rule.

To study the release kinetics of fexofenadine from SMEDDS formulations, the release data were fitted to the following equations [39] using DDSolver^® add-in program:

• Zero-order equation: F = K₀.t

  Where F represents the fraction of drug released up to time (t) and k₀ is the apparent dissolution rate constant.

• First-order equation: ln (1-F) = -K₁.t

  Where k₁ is the first-order release rate constant.

• Higuchi's equation: F = K_H. t^1/2

  Where K_H stands for the Higuchi release rate constant.

• Korsmeyer–Peppas semi-empirical model: lnF = ln K_KP + n ln t

  Where K_KP is a constant incorporating the structural and geometric characteristics of the drug dosage form and “n” is the release exponent, indicating drug release mechanism.

2.8. HPLC analysis of fexofenadine

A liquid chromatographic system (Knauer V7603) equipped with a pump (Smartline pump 1000), a rheodyne 7725 manual injector and a UV-vis detector. A cyano column (4.6 × 250 mm, particle size 5 μm) was used for the chromatographic analysis. The flow rate was set at 1 ml/min and column temperature was 35°C. The mobile phase consisted of methanol and 0.1% triethylamine in water (pH adjusted to 3 with orthophosphoric acid) (80:20% v/v). The detection wavelength was 221 nm and the injection volume was 20 μl.

2.9. Single pass intestinal permeability studies

Male Wistar rats weighing between 200–300 g were used for the SPIP studies. The rats had no access to food 12 h prior to SPIP experiment (with free access to water). Rats were anesthetized by an IP injection of ketamine - xylazine (100 mg/kg; 10 mg/kg), then a midline incision was made in the abdomen and an approximately 10 cm jejunal segment was selected, isolated and cannulated with silicone tubing on both ends. First, the cannulated segment was rinsed with saline 37°C and then with drug-free perfusion buffer to clean the segment till the perfusate became clear. Then the perfusion buffer containing either plain fexofenadine powder or fexofenadine loaded SMEDDS formulation were perfused through the cannulated segment using a syringe pump at a flow rate of 0.2 ml/min. The perfusates were collected from the outlet tubing every 10 min during a 90-min time period. To maintain the natural moisture in the exposed intestinal area, it was covered with a saline-moisturized sterile gauze pad and a 10 × 10 cm piece of Parafilm^® film. The whole experiment was performed carefully to lessen the damage during the surgical operation and keep the intestinal blood circulation intact. A schematic representation of the procedure is presented in Supplementary File S2. At the end of the experiment, the length of the cannulated segment was carefully measured and recorded for further calculations. The whole procedure was approved by the Ethics Committee (approval number: IR.KUMS.REC.1400.099), Kermanshah University of Medical Sciences, Kermanshah, Iran.

2.10. Data analysis

Effective intestinal permeability of fexofenadine was calculated using the steady state concentrations of fexofenadine in the samples collected during time intervals. P_eff values were calculated as stated by the parallel tube method with the following equation:

$$P_{eff}=-\text{Q} ln\left[\frac{{\text{C}}_{\text{out} \left(\text{corr}\right)}}{{\text{C}}_{\text{in}}}\right]/2\pi \text{rl}$$

In this equation Q is the perfusion flow rate (0.2 ml/min). C_in stands for inlet fexofenadine concentration (μg/ml). r is the radius of the rat intestine (cm) and l is the length of the intestinal segment (cm). C_out(corr) is outlet concentration of fexofenadine which is corrected for water flux using gravimetric method (μg/ml) with the following equation.

$${\text{C}}_{\text{out}\left(\text{corr}\right)}={\text{C}}_{\text{outmeasured}}\times \frac{{\text{Q}}_{\text{out}}}{{\text{Q}}_{\text{in}}}$$

In which Q_in was the perfusion flow rate entering the intestine and Q_out was the measured perfusate exit flow net weight of each sample/10 min, assumed density of 1.0 g/mL for the specified 10 min time intervals.

Moreover, human intestinal effective permeabilities (P_{eff (human)}) of free fexofenadine and SMEDDS were predicted based on the following equation [37,40].

$${\text{P}}_{\text{eff} \left(\text{human}\right)}=11.4{\text{P}}_{\text{eff} \left(\text{rat}\right)}-0.0003$$

Student's t-test was used to investigate the significance of difference between intestinal permeabilites of plain fexofenadine suspension and fexofenadine SMEDDS formulation. A p-value < 0.05 was considered to be statistically significant.

3. Results

In order to choose the most appropriate composition of formulation ingredients and to ensure the maximum drug payload, the solubility of fexofenadine was determined in various oils, surfactants and cosurfactants or cosolvents. Solubility of fexofenadine in water was found to be 0.169 mg/ml. The results of solubility studies were summarized in Figure 1. As it is shown in the figure, comparing the oils, the maximum drug solubility was demonstrated in ethyl oleate followed by castor oil and turpentine oil. Among the tested surfactants and cosolvents, the solubility of fexofenadine is higher in Tween 80 and PEGs (PEG 200, 400 and 600) respectively. According to the results of solubility measurements in addition to some preliminary tests about the compatibility and emulsification ability of these excipients, a mixture of turpentine oil/castor oil (3:1) and ethyl oleate was selected as the oil phase of SMEDDS formulations. Tween 80 and PEG 600 are chosen as the surfactant and cosolvent of choice to prepare the fexofenadine-loaded SMEDDS formulations.

Figure 1.

The solubility of fexofenadine in different oils and surfactants (37°C; mean ± SD, n = 3).

36 different SMEDDS formulations with varying ratios of oil, surfactant and cosolvent were prepared and their state of homogeneity and stability were evaluated. From the evaluated formulations four of them (F6, F7, F8 and F15) were selected for undergoing further tests, as they have shown more homogeny appearance comparing the other formulations and there were no signs of phase separation in them after mixing the ingredients.

The ability of these selected formulations in dispersing fast and complete after addition to water or 0.1 N HCl and producing a homogenous microemulsion were observed and ternary phase diagram was plotted as shown in Figure 2.

Figure 2.

Ternary phase diagram (dark area shows microemulsions zone).

Afterward, the four selected formulations were loaded with three different amounts of fexofenadine to estimate the optimum drug loading. Any signs of precipitation were not observed in formulation aliquots containing 30 and 60 mg of drug and therefore 60 mg/g was chosen as the optimum drug loading amount.

Droplet sizes and polydispersity indexes of 4 selected fexofenadine loaded SMEDDS formulations were measured and the results have shown in Table 1. Formulations F6, F7 and F8 formed microemulsions with very fine droplets upon dilution (lower than 20 nm), particle size of droplets formed by dilution of F15 was 197.22 nm. Accordingly, the polydispersity indexes of F6, F7 and F8 were lower than 0.25 which represented a narrow droplet size distribution and microemulsions with almost homogenous droplets (a sample particle size distribution plot is represented as Supplementary File S3). F15 exhibits a PDI value of 0.461 meaning the formation of a more polydisperse microemulsion.

Table 1.

Droplet sizes and polydispersity indexes of 4 selected fexofenadine loaded self-microemulsifying drug-delivery systems formulations.

Formulation	Size (nm)	Polydispersity index (PDI)
F6	19.17	0.207
F7	15.97	0.155
F8	13.77	0. 091
F15	48.86	0.505

In order to evaluate the thermodynamic stability of formulations, optimized drug-loaded SMEDDS formulations were centrifuged at 5000 rpm for 30 min. There were no signs of creaming, phase separation and layering in formulations.

The optimized formulations were subjected to three consecutive freeze–thaw cycles. None of the formulations have shown creaming, layering and phase separation. As it is shown in Figure 3, droplet sizes of the formulations have not revealed significant changes after the freeze thaw experiment except formulation no. 15.

Figure 3.

Particle size changes after three consecutive freeze–thaw cycles (mean ± SD, n = 3).

The compatibility of optimized SMEDDS formulations with hard gelatin capsule shells was evaluated for a period of 3 months. There were no signs of leakage or any other physical instabilities in the four optimized formulations. Based on the results of particle size and stability studies, it was concluded that F15 could be omitted from the rest of the studies.

In vitro dissolution profiles of optimized SMEDDS formulations F6, F7 and F8 was determined and compared with pure fexofenadine (30 mg fexofenadine filled in a gelatin capsule) and the commercial Telfast^® tablets. All of the SMEDDS formulations have released more than 50% of their drug content in 30 min. SMEDDS formulation F8 has released approximately 83.70% of its fexofenadine content at the end of 120 min, while formulation F6 and F7 have released 60.8 and 79.51% of their drug content respectively. Pure fexofenadine and commercial tablet have revealed 21.1% and 47.4% of drug release at the end of 120 min experiment period. The in vitro dissolution profiles of 3 SMEDDS and commercial tablet formulation as well as pure drug were shown in Figure 4. Area under dissolution curves (AUC), dissolution efficiencies (DE) and mean dissolution time were calculated for each dissolution profile and compared. In all three SMEDDS formulations, AUC and DE significantly increased in comparison to pure fexofenadine and Telfast tablets (P < 0.05). Meanwhile, Mean dissolution time of all SMEDDS were shorter than pure fexofenadine and Telfast tablets demonstrating the better dissolution performance of SMEDDS formulations. The rank order of DE values for different SMEDDS were as follow as: pure fexofenadine powder < Telfast tablet < F6 < F8 < F7, furthermore, the rank order of MDT values were: F6 < F7 < F8 < Telfast tablet < pure fexofenadine powder. Table 2 summarizes the assessed dissolution parameters.

Figure 4.

The in vitro dissolution profiles of 3 self-microemulsifying drug-delivery systems and commercial tablet formulation and pure drug (mean ± SD, n = 3).

Table 2.

Dissolution parameters (AUC, DE% and MDT) of self-microemulsifying drug-delivery systems formulations as well as commercial tablet and pure drug as well as goodness of fit represented by (R²) values for different release kinetics models (mean ± SD, n = 3).

Formulation code	AUC	DE (%)	MDT (min)	Zero order R2	First order R2	Higuchi R2	Korsmeyer-Peppas R2
Fexo powder	1628.25 ± 11.13	13.57 ± 0.09	42.84 ± 11.07	–	–	–	–
F6	5925.11 ± 147.58	49.38 ± 1.23	22.59 ± 3.57	0.22	0.63	0.84	0.97
F7	7311.43 ± 464.14	60.93 ± 3.87	28.05 ± 10.81	0.13	0.65	0.73	0.95
F8	6761.51 ± 424.09	56.35 ± 3.53	39.22 ± 14.32	0.65	0.89	0.95	0.98
Commercial tablet	3753.45	31.28	40.82	–	–	–	–

The dissolution profiles of F6, F7 and F8 formulations were fitted to four mathematical models as mentioned above and the model having the highest R² were considered as the best fitted model. As it is represented in Table 2, the release of fexofenadine from F6, F7 and F8 were best fitted the Korsmeyer–Peppas model. Diffusional exponents (n) were 0.26, 0.23 and 0.44 for F6, F7 and F8 respectively.

Based on the physicochemical characteristics and results of dissolution and stability studies, fexofenadine-loaded SMEDDS formulation F8 was chosen as the final optimized formulation to be subjected to the SPIP studies. Fexofenadine inlet and outlet concentrations in all samples was measured by HPLC method (chromatograms of fexofenadine standard solution in PBS and fexofenadine -loaded SMEDDS perfusate (outlet sample) were represented as Supplementary File S4). The outlet concentrations were corrected for water flux according to the above-mentioned equations. The ratio of corrected outlet to inlet fexofenadine concentration was plotted and used to determine reaching the steady state. Effective intestinal permeabilities of SMEDDS formulation were determined and compared with that of plain fexofenadine. Intestinal permeabilities, drug release and some physicochemical properties of optimal SMEDDS formulation (F8) versus plain aqueous fexofenadine suspension was summarized in Table 3.

Table 3.

Intestinal permeability and other properties of optimal self-microemulsifying drug-delivery systems formulation (F8) vs. fexofenadine aqueous suspension.

Formulation	Drug (mg)	Oil+Smix (mg)	Size (nm)	PDI	Cumulative % released	Peff (cm/s)
F8	30	470	13.77	0.091	83.70 ± 19.67	3.14 × 10-4 ± 1.13 × 10-4
Aqueous suspension	30	–	–	–	21.10 ± 2.85	1.12 × 10-4 ± 4.75 × 10-5

4. Discussion

Fexofenadine is known to exhibit low aqueous solubility; however different water solubility amounts for it were reported in the literature [41–43]. Due to these characteristics, fexofenadine presents itself as a promising candidate for incorporation in the SMEDDS formulations.

According to the ternary phase plot, higher ratios of surfactant are required for producing more homogenous formulations with lower emulsification time.

Based on the results of size analysis, F8 formulation produced droplets with smaller sizes compared with 3 other ones, but the size difference between F6, F7 and F8 was not significant.

Considering both droplet size and PDI, F8 was able to form microemulsions with smaller and more homogenous droplets. This could be attributed to the higher ratio of Tween 80 in its formulation. The more surfactant used in the formulation leads to the more reduction in interfacial tension and makes the formulation able to produce a more stable microemulsion with smaller droplets and more narrow size distribution upon dispersion in aqueous media.

The observed enhancement in the dissolution of fexofenadine from SMEDDS formulations could be attributed to the solubilizing nature of the SMEDDS components as well as the large surface area produced subsequent to the dispersion of nano-sized droplets in dissolution media.

Korsmeyer–Peppas model was found to be the best fitting model for all three formulations. In this model the value of exponent (n) can be used to characterize different release mechanisms. A Fickian or quasi-Fickian diffusion mechanism of drug release can be considered when the value of n is less than 0.45 [44,45].

It has been reported in literature that the rate controlling step in the oral administration of fexofenadine is its permeability from the intestinal barrier [46,47]. Furthermore the great part of administered fexofenadine is excreted by biliary and urinary excretion and metabolism by hepatic and intestinal cytochrome P450 take nearly no part in its elimination. Therefore contribution of first pass metabolism in low oral bioavailability of fexofenadine is not the case [9,48].

In addition to its low passive permeability; which is likely due to hydrophilicity; fexofenadine is also a well-established substrate of P-glycoprotein [49]. Administration of fexofenadine along with some P-gp inhibitors such as verapamil [50,51], ketoconazole [9], itraconazole [52], have reported to significantly enhance the bioavailability of fexofenadine. Furthermore it has been shown that using of permeability enhancers like Cremophor RH 40 could improve the intestinal permeability of fexofenadine even after increasing the dose of drug [7]. Cremophor RH 40 is both a solubility and permeability enhancer and a P-gp inhibitor. Enhancement of solubility and permeability using the prevalent excipients of the drug delivery system is a favorable approach.

During the formulation of SMEDDS, nonionic surfactants are preferred over ionic surfactants as they are less toxic [53]. Tween 80 which is used in this study is a nonionic surfactant inducing less irritation in vivo. Performing oral mucosal or skin irritation study is not usually common in SMEDDS development studies, however there are already independent and comprehensive studies on cytotoxicity of surfactants used in SMEDDS [54,55], considering the results of these studies and comparing the toxic concentration with concentration of surfactant in the GI after being diluted with GI liquids and bearing in mind that SMEDDS usually administered as soft gel capsules and the formulation components have not been in contact with oral mucosa upon administration, it seems that oral or GI mucosal toxicity would not be of great concern, however complete and comprehensive toxicity investigations can be performed as independent studies in the future.

The significant increase in the effective intestinal permeability could be a result of the presence of Tween 80 as a nonionic surfactant in the formulation. Tween 80 is considered as an absorption enhancer as well as a P-gp inhibitor [56,57], since fexofenadine is introduced as a P-gp substrate in the literature [58], the permeability enhancement can be at last partly due to the efflux pump inhibitor function of the formulation components. This result ties well with previous studies wherein Tween 80 had improved the apical to basolateral permeability and oral bioavailability of digoxin, an efflux pump substrate, via inhibition of P-gp [59]. It has been shown that nonionic detergents like tween 80 are able to influence the membrane packing density as well as to inhibit the ATP-binding cassette transporters [60] and therefore could enhance the drug absorption via influx and efflux mediated mechanisms.

Human intestinal permeabilities of SMEDDS formulation and plain fexofenadine suspension was also predicted using the rat intestinal permeabilities obtained from SPIP experiment and showed in Figure 5. As it is revealed in Figure 5 an approximately threefold increase of fexofenadine intestinal permeability was observed after incorporation of drug in the self-emulsifying delivery system. The results of student's t-test showed that the enhancement of intestinal permeability using SMEDDS formulation was significant (p < 0.05). Intestinal permeability of fexofenadine loaded SMEDDS formulation have not been previously reported in literature, however Intestinal effective permeability of plain fexofenadine (P_eff) was determined and reported in some previous studies, e.g., 0.537 ± 0.035 × 10^-4 cm/s (ileum) [61], 7.04 ± 0.56 × 10^-6 cm/s (jejunum) [8] and 0.670 ± 0.090 × 10^-4 cm/s (ileum) [62]. Compared with 1.12 ± 0.475 × 10^-4 cm/s in our study, the difference between fexofenadine P_eff values of different studies could be attributed to the difference in perfused intestinal segments as many studies demonstrated regional differences in drug intestinal permeability. Moreover, the variation in permeability values, however, appears to be related to the laboratories conducting the experiments. This demonstrates that comparing permeability results between laboratories cannot yield valid conclusions [63]. Human intestinal permeability of fexofenadine have also been reported in a few existing studies. Effective permeability of fexofenadine determined by in vivo single-pass perfusion of the proximal jejunum in humans was 0.07 × 10^-4 [64] and 0.11 × 10^-4 cm/s [65] in two previous studies. Furthermore, the predicted human effective permeability coefficient have been estimated as 2.61 ± 0.18 × 10^-5 cm/s [8] while the predicted human P_eff value in the present study have been calculated as 9.74 ± 5.4 × 10^-4 cm/s. The sources of approximately variable rat and human permeability results in different studies on one drug molecule, could be considered as the entering drug concentration, the pH of the drug solution, the rat strain, the experimental method and experience of performing personnel and the intestinal region. Therefore, it is preferred to interpret the permeability data from individual studies, separately [63].

Figure 5.

Effective intestinal permeabilities of self-microemulsifying drug-delivery systems formulation and fexofenadine aqueous suspension in rat (observed) and in human (predicted; mean ± SD, n = 4).

5. Conclusion

SMEDDS consisted of 15% oil phase (10% of a 1:3 mixture of castor oil and turpentine and 5% of ethyl oleate), 80% Tween 80 and 5% PEG 600 was selected as the optimized formulation considering the fine droplet size as well as appropriate stability. Our findings showed that the in vitro drug dissolution from SMDDS formulations was both faster and more efficient in comparison to fexofenadine powder and commercial tablet. Consequently, it has been revealed that loading in the optimized SMEDDS presents an approximately threefold improve the intestinal effective permeability of fexofenadine compared with plain fexofenadine suspension. Therefore, our study suggested that the SMEDDS could be a promising carrier for improving the oral bioavailability of fexofenadine.

Funding Statement

The authors would like to thank the Research Council of Kermanshah University of Medical Sciences (Grant Number: 4000233) for financial support of this work.

Supplemental material

Supplemental data for this article can be accessed at https://doi.org/10.1080/20415990.2024.2363635

Author contributions

A Barfar performed the experiments and carried out the analysis. Z Islambulchilar drafted the manuscript. Z Islambulchilar and S Mirzaeei designed the study. S Mirzaeei conceived the original idea and supervised the whole project.

Financial disclosure

The authors would like to thank the Research Council of Kermanshah University of Medical Sciences (Grant Number: 4000233) for financial support of this work. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Competing interests disclosure

The authors have no competing interests or relevant affiliations with any organization or entity with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, stock ownership or options and expert testimony.

Writing disclosure

No writing assistance was utilized in the production of this manuscript.

Ethical conduct of research

The protocol was approved by the Local Ethical Committee of Kermanshah University of Medical Sciences; approval number: IR.KUMS.REC.1400.099.