Research Article

Altered Skin Permeation of Finasteride Using Clove Oil, Urea, and Lyophilized Powder of Grape Seed Extract

Introduction

5α-Reductase is an enzyme produced by the skin and other body organs. This enzyme increases male traits in people by converting testosterone to dihydrotestosterone (DHT).¹ DHT produced in the skin, in addition to its physiologic effects, can lead to acne or hair loss.²

finasteride is an enzyme inhibitor that inhibits the conversion of testosterone into DHT by inhibiting 5-alpha reductase.³ It has good gastrointestinal absorption and high bioavailability (65%).^4,5 The oral or systemic administration of finasteride has many side effects.⁶ Therefore, according to the DHT function in the tissue, the use of topical products is recommended.⁷ In fact, the dermal use of finasteride can be considered a targeted drug delivery method that eliminates systemic adverse effects.⁸ finasteride has a molecular weight of 372.5 Da and low solubility in water (11.7 mg/L, log p 3). Its low molecular weight makes it a candidate for the delivery of drugs via the skin.^9-11

Two medication administration routes are transdermal and dermal.¹² The main advantages of these systems are related to the elimination of the primary liver metabolism, contact with biological conditions and gastrointestinal chemicals, undesirable events, and the ability to provide a controlled drug delivery system for medications with a short half-life and a narrow therapeutic index.¹³

The possibility of topical administration depends on the permeability of the drug and its therapeutic concentration.¹⁴ Although the nature of the skin prevents the penetration of many drugs, and drug delivery in such a way is often difficult, but it has several benefits for drug delivery.¹⁵ The main obstacle preventing dermal delivery is the stratum corneum.¹⁶

One of the approaches for increasing the absorption of drugs through the skin is the use of penetration enhancers. Many compounds have been used for this purpose, including sulfoxides, azones, pyrrolidones, alcohols and alkanols, glycols, surfactants, and terpenes.¹⁷ Previous studies have shown that natural products can be used to increase the penetration of drugs through the skin. Some studies have shown that these materials are even more permeable than other penetration enhancers. Furthermore, they are safe, non-toxic, pharmaceutically inert, non-irritant, and non-allergic.¹⁸

Clove oil is an essential oil that is used as a penetration enhancer.¹⁹ Urea and its derivatives are among the most-used and oldest permeability enhancers.²⁰ Although lyophilized powder of grape seed extract (LPGSE) has been acknowledged as a polyphenol in dermatological products, no research has been conducted on the effect of LPGSE powder on penetration enhancement.²¹

This study aims to assess the impact of various factors on finasteride’s influence on skin permeability. Also, in this study, the effect of skin penetration enhancers’ exposure times on the skin permeability of finasteride has been investigated.

Materials and Methods

Materials

Finasteride was purchased from a pharmaceutical company (Soha Helal, Tehran, Iran). Urea and clove oil were obtained from the Barij Essence Iranian Company (Kashan, Iran). LPGSE was gifted from Roshd Center (Ahvaz Jundishapur University of Medical Sciences).

Animal experiments

Male rats (200-250 g; 8-10 weeks old) were provided by the Animal Laboratory of the Ahvaz Jundishapur University of Medical Sciences. All animals were anesthetized before being sacrificed with high concentrations of thiopental sodium. After that, hairs from their abdominal skin were removed by an electric clip carefully so that the skin was not damaged. Animals were treated according to the principles of care. Approval for these studies was given by the Ethics Committee of Ahvaz Jundishapur University of Medical Sciences (Permit no.: IR.AJUMS.REC.1396.252).²²

Finasteride assay

Finasteride amounts were determined using the UV spectroscopy method at λ_max = 224 nm.²³

In vitro skin permeation experiment

Vertical glass Franz diffusion cells (surface area = 4.906 cm²) were utilized for skin permeability experiments. Whole skins were completely treated for two and four hours using 2 mL of each of penetration enhancers (Urea, LPGSE, and clove oil) on the surface of the skin at the donor section. finasteride (1% w/v) was dispersed in the distilled water and placed in the donor compartment and the receptor cell, which was filled with phosphate buffer solution (pH 7) and methanol (2:1). The receptor environment is stirred by a small magnetic bead at 300 rpm. At specified intervals (0.5, 1, 2..., 8, and 24 hours), 2 ml of the receptor media was removed and immediately replaced with a fresh phosphate buffer-methanol solution to keep sink conditions. Then, the collected samples were filtered, and finasteride amounts were determined by UV spectrometry at 224 nm.^24,25

Differential scanning calorimeter (DSC)

To investigate the changes in the skin structure after the pretreatment via penetration enhancers, the DSC technique was employed. Full skin samples were first treated with 2 mL of each penetration enhancers for a period of two and four hours, respectively. Approximately 6-10 mg of skin samples were placed into the sealed aluminum pans. At the same time, a blank pan was used as a reference. The skin samples were exposed to heat ranging from 20 to 200°C (scan speed: 5°C/min). All experiments were completed at least three times. In order to ensure the accuracy and repeatability of the data, the DSC analyzer was calibrated using the indium standard.^26,27

FT-IR spectroscopy

Skin samples were treated using clove oil, urea, or LPGSE for two and four hours. The samples were then vacuum-dried (650 mm Hg, 25°C) for one hour and finally stored in desiccators to remove traces of the solvent. The skin samples were analyzed in a 4000 to 500 cm ^-1 scan range using an FT-IR apparatus (Uker, Vertex70, Germany).²⁸

Statistics and data analysis

The steady-state flux (mg/cm²/h) was calculated from the linear section of the slope of the penetration curve.

The permeability coefficient factor (K_p, cm/h) of finasteride by the skin was calculated as follows ().²⁹

Where J_ss is the rate of the steady-state (flux) and C_v is the initial concentration of finasteride. The enhancement ratio (E_R) is the permeability parameter after the skin had been treated with penetration enhancers divided by the same parameters for non-treated skin (control) ().²⁵

Diffusivity was calculated using .³⁰

Where, h is the thickness of the skin, and the lag time (Tlag) parameter was calculated by extrapolating the line of steady-state onto the time axis.

Statistical analysis was performed using a one-way ANOVA, and P < 0.05 was considered significant.³¹

Results and Discussion

Effect of penetration enhancers on the skin permeation of finasteride

The permeability parameters after skin exposure with penetration enhancers are shown in Table 1. All enhancers affected the flux to a greater extent than diffusivity. The results indicate that all enhancers increased permeability through the skin significantly when compared with control. Two and four hours of pretreatment using clove oil increased the flux of finasteride by 10.09 and 14.35 times, respectively. These results indicate that the time of exposure to clove oil can improve the penetration of finasteride into the skin.

Two and four hours of pretreatment with urea increased the flux of finasteride by 2.51 and 2.18 times, respectively. These results indicate that time of exposure to urea can decrease the penetration of finasteride into the skin. Thus, it seems that the effects of the penetration enhancer of urea are reversible after two hours.

Two and four hours of pretreatment with LPGSEs (1% solution), which has been used for the first time to enhance penetration in the present study, increased the flux of finasteride by 2.647 and 5.935 times, respectively.

T_Lag refers to the limited time that a drug appears in the systemic circulation following extravascular administration. This parameter reflects the processes associated with the adsorption phase from the drug delivery system and the migration of the adsorption surface. Failure to determine T_Lag can lead to incorrect estimations of pharmacokinetic parameters.³² The greatest reduction in T_Lag is related to two-hour LPGSEs, and so this uptake can greatly reduce the duration of transdermal drug delivery. The highest drug diffusivity (ER_D = 31.5) was recorded when urea was used for two hours.

Differential scanning calorimetry

The thermal behavior of various penetration enhancers that were used to pretreat the skin was evaluated based on mean phase transition temperature (Tm) and enthalpy (ΔH). The displacement of the phase transition temperature to lower temperatures indicates a lipid bilayer disorder and an irreversible denaturation of the protein structure in the stratum corneum. Meanwhile, the decrease in the ΔH indicates lipid fluidization in the lipid bilayer and lipid-protein complex in the stratum corneum.³³ In previous studies, Tm₁ was attributed to the temperature of the lipid transfer from lamellar to irregular state. Tm₂ represents the melting point of the keratein-lipid complex or the disturbance in the polypeptide groups of lipids, and Tm₃ is the discontinuous denaturation temperature of the protein in the stratum corneum.³⁴

The amount of mean phase transition temperature and the enthalpy value after the skin was pretreated with penetration enhancers are shown in Table 2. In previous studies, Vaddi et al reported three endothermic phase transition temperatures for skin (i.e., 62, 79, and 95°C),²⁸ while Shakeel et al observed four changes in the endothermic phase transition temperatures (i.e., 34, 82, 105, and 114°C).³⁵ Kaushik et al established three endothermic phase transition temperature ranges for stratum corneum (i.e., 59-63°C (Tm₁), 75-82°C (Tm₂), and 99.5-120°C (Tm₃). Tm3 occurred throughout the irreversible denaturation of proteins in SC.³⁶

DSC thermograms of skin samples pretreated by water and penetration enhancers are shown in . In this study, two intraperitoneal transitions of DSC thermograms from hydrated rat skin (about 67°C (Tm₁) and 112°C (Tm₂)) were observed. The transmission of Tm₁ and Tm₂ seems to be due to the melting of lipids and the irreversible discontinuity of intracellular keratin or melting of lipid (keratin protein) protein. Changes in Tm to lower temperatures can be seen as a two-layer lipid insufficiency and denaturation of proteins in the stratum corneum skin layer, while a decrease in ΔH is related to lipid fluidization in the lipid layer.^28,35,37-39

Close

Figure 1. DSC Thermogram of whole skin abdominal rat after pretreatment with (A): Water, (B): Clove oil, (C): Urea and (D): LPGSE.

The results of the DSC diagram pretreatment skin by clove oil (2 and 4 hours) indicate decreased Tm₂ and ΔH₂ levels. These decreases reflect the melting of the keratin-lipid complex or disorder in the head polar lipids. They also indicate lipid fluidization in lipid bilayers and lipid-protein complexes in the stratum corneum.

The results of the DSC thermogram of rat skin pretreated with urea (4 hours) suggests that lipid structure is well-regulated in the lipid bilayer. However, a significant decrease in the amount of ΔH₁indicates the fluidization of the lipid-protein complex in the stratum corneum. The results of the DSC of pretreatment rat skin with urea (4 hours) are largely consistent with the FT-IR results for urea (4 hours). Urea also increases permeability by facilitating the hydration of the stratum corneum and forming hydrophilic channels in this layer. The urea enhancement mechanism involves both hydrophilic activity and lipid disorders in the stratum corneum.⁴⁰

DSC results from skin pretreated with LPGSE showed an increase in Tm₁ and Tm₂ when compared to the control conditions. The results also showed significant increases in ΔH₁ and ΔH₂, which do not indicate a disorder in the lipid layer of the skin and the lipid fluidization in the lipid-protein complex in the stratum corneum. These results indicate that LPGSE does not affect the lipid and protein parts of the skin.

FT-IR spectroscopy

and Tables 3-5 show the spectral analysis of the samples concerning any change in the position of the peaks, as well as their severity from the intensity of 3500 cm ^-1-500 cm ^-1. Each type of O-H and N-H is found in protein, water, and fat between 3000 and 3600 cm, while the symmetrical and asymmetric stretching bands of the restorative methyl groups of fats are presented at 2948.02 and 2821.65 cm ^-1. C = O stretching of lipid ester was shown in SC at 1723.62 cm. Also, the stretching of lipid esters in the corneal layer, as well as amide I (stretching C = O) and amide II (stretching C-N) from the secondary spiral structure, was observed in epidermal keratin at 1652.78 and 1572.27 cm ^-1.^37,41

Close

Figure 2. FT-IR spectra of whole skin abdominal rat after pretreatment with (A): Water (B): Clove oil (2 h), (C): Clove oil (4 h), (D): Urea (2 h), (E): Urea (4 h), (F): LPGSE. (2 h), (G): LPGSE (4 h).

To investigate the mechanism of penetration enhancers’ effects on skin structure by the FT-IR method, shifts in the position of absorbing bands were generally considered to be higher and lower wavelengths and the change in the position of the peaks. If the absorption band is transmitted to higher wavelengths (blue shift), this indicates that the bilayers are fluid in the stratum corneum layer membrane, which impairs the barrier properties and potentially increases the passage of the drug from the skin. On the other hand, a shift of the absorption band towards the lower wavelength (red shift) indicates the reordering of the lipid bilayer groups of the stratum corneum, which ultimately prevents the introduction of drugs into the skin.⁴²

If the penetration enhancer affects the lipid bilayer of the stratum corneum, the phase transition temperature of the lipids increases or decreases in peak wavelengths at 2920 cm ^-1, 2850 cm ^-1, and 1738 cm ^-1. As the height (or intensity) or position of the absorbing bands shows the amount of lipids or proteins in the horny layer, increasing the peak intensity indicates a strengthened lipid structure and causes changes in drug permeability. The decrease in peak intensity, on the other hand, represents a weakening of the lipid structure in the horny layer, which increases the permeability of the drug.⁴³

The FT-IR spectrum of the pretreated urea skin and the decreases in the peak heights of the asymmetric C-H, symmetric C-H, C = O, and amid I, II bands have been created. It seems that the effect of urea enhancement manifests mostly in the lipid and protein content of the stratum corneum and interferes with the stratum corneum tissue keratein. The transfer of absorption bands in symmetric and asymmetric stretching CH regions in the FT-IR spectrum of skin (which was treated by urea to lower wavelengths (red shift)) shows the reordering of bilayer lipid and protein groups.⁴⁴

Delgado-Charro et al have shown that the use of urea as a penetration enhancer in the solvent system has different effects; however, this is generally the case with lipid disorders in the skin.⁴⁵ Williams and Barry argued that enhancing the permeability of urea, due to increased moisturizing effects, increases the levels of water in the stratum corneum.¹⁷ Shaikh et al have shown that urea increases permeability by facilitating the hydration of the stratum corneum and by forming the hydrophilic channel channels in this layer. As such, they stated that the urea absorption mechanism involves hydrophilic activity as well as a lipid disorder in the layer stratum corneum.⁴⁶

In this study, all of the penetration enhancers with different effect times (except for LPGSE (4 h)) transferred the absorption band of non-symmetric C-H to higher wavelengths. This transition is due to the fluidity of the bilayers in the stratum corneum and the impairment of barrier properties, which ultimately increases the passage of medicine into the skin.

The FT-IR spectrum of clove oil-treated skin indicates that this compound affects the lipid layer of the stratum corneum. The pretreatment time of this compound had a significant effect on the permeability of finasteride on rat skin.

The FT-IR spectrum of the pretreated skin with LPGSE shows that this combination affects the lipid section and (to some extent) the protein section of the skin.

Conclusion

In this study, it was found that all penetration enhancers significantly increased the penetration of finasteride into the skin of the rats. The enhancer effects of clove oil and LPGSE were increased by lengthening the pretreatment time. However, lengthening the pretreatment time did not significantly influence the effect of urea. This result could indicate a reversible increase in urea from increasing the pretreatment time. The results indicate that pretreatment time can increase the permeability of drugs into the skin. Therefore, in order to increase efficacy, the optimal pretreatment time should be determined.

Two-layer fat disorder and lipid fluidization in two layers of lipids and lipid proteins are the main causes of ER_flux and ER_D. The main barrier to the penetration of finasteride is the skin’s layers. Therefore, a combination of urea, clove oil, and LPGSE can help to achieve suitable skin formulation.

Acknowledgments

This paper is derived from the pharm D thesis of one of the authors (Saeed Mohamad Soleymani). Ahvaz Jundishapour University of Medical Sciences is acknowledged for providing financial support.

Author Contributions

Conceptualization: Anayatollah Salimi.

Data curation: Saeed Mohammad Soleymani.

Formal Analysis: Anayatollah Salimi.

Funding acquisition: Hamid Mohammad Soleymani.

Investigation: Saeed Mohammad Soleymani.

Methodology: Anayatollah Salimi.

Project administration: Anayatollah Salimi.

Resources: Saeed Mohammad Soleymani.

Software: Hamid Mohammad Soleymani.

Supervision: Anayatollah Salimi.

Validation: Saeed Mohammad Soleymani.

Visualization: Hamid Mohammad Soleymani.

Writing – original draft: Saeed Mohammad Soleymani.

Writing – review & editing: Hamid Mohammad Soleymani.

Ethical Issues

Ethical approval was granted by Ethics Committee of Ahvaz Jundishapour University of Medical Sciences (Ethics No. IR.AJUMS.REC.1396.252).

Conflicts of Interest

The authors declare no conflict of interest.

References

1. Human type 3 5α-reductase is expressed in peripheral tissues at higher levels than types 1 and 2 and its activity is potently inhibited by finasteride and dutasteride. Horm Mol Biol Clin Investig 2010; 2(3):293-9. doi: 10.1515/hmbci.2010.035 [Crossref]
2. Clinical biochemistry of dihydrotestosterone. Ann Clin Biochem 2013; 50(Pt 2):95-107. doi: 10.1258/acb.2012.012159 [Crossref]
3. An overview on 5alpha-reductase inhibitors. Steroids 2010; 75(2):109-53. doi: 10.1016/j.steroids.2009.10.005 [Crossref]
4. 5-alpha reductase inhibitors and erectile dysfunction: the connection. J Sex Med 2008; 5(12):2917-24. doi: 10.1111/j.1743-6109.2008.01001.x [Crossref]
5. Foye WO. Foye’s Principles of Medicinal Chemistry. 60th ed. Lippincott Williams & Wilkins; 2008.
6. Finasteride and sexual side effects. Indian Dermatol Online J 2012; 3(1):62-5. doi: 10.4103/2229-5178.93496 [Crossref]
7. A systematic review of topical finasteride in the treatment of androgenetic alopecia in men and women. J Drugs Dermatol 2018; 17(4):457-63.
8. Targeted drug delivery to the skin and deeper tissues: role of physiology, solute structure and disease. Clin Exp PharmacolPhysiol 1997; 24(11):874-9. doi: 10.1111/j.1440-1681.1997.tb02708.x [Crossref]
9. Transdermal drug delivery: basic principles for the veterinarian. Vet J 2006; 172(2):218-33. doi: 10.1016/j.tvjl.2005.09.006 [Crossref]
10. Stability-indicating micellar elektrokinetic chromatography method for determination of finasteride in pharmaceutical preparations. J LiqChromatogrRelat Technol 2013; 36(6):781-91. doi: 10.1080/10826076.2012.673214 [Crossref]
11. In vitro percutaneous permeation and skin accumulation of finasteride using vesicular ethosomal carriers. AAPS PharmSciTech 2008; 9(3):860-5. doi: 10.1208/s12249-008-9124-y [Crossref]
12. Dermal and transdermal drug delivery systems: current and future prospects. Drug Deliv 2006; 13(3):175-87. doi: 10.1080/10717540500455975 [Crossref]
13. Review on: recent trend on transdermal drug delivery system. J Drug DelivTher 2012; 2(1):66-75. doi: 10.22270/jddt.v2i1.74 [Crossref]
14. Skin permeation: the years of enlightenment. Int J Pharm 2005; 305(1-2):2-12. doi: 10.1016/j.ijpharm.2005.07.014 [Crossref]
15. Vesicular carriers for dermal drug delivery. Expert Opin Drug Deliv 2009; 6(8):813-25. doi: 10.1517/17425240903071029 [Crossref]
16. Transdermal drug delivery: penetration enhancement techniques. Curr Drug Deliv 2005; 2(1):23-33. doi: 10.2174/1567201052772915 [Crossref]
17. Penetration enhancers. Adv Drug Deliv Rev 2012; 64 Suppl:128-37. doi: 10.1016/j.addr.2012.09.032 [Crossref]
18. Natural products as potential drug permeation enhancer in transdermal drug delivery system. Arch Dermatol Res 2014; 306(5):419-26. doi: 10.1007/s00403-014-1445-y [Crossref]
19. Essential oils and their constituents as skin penetration enhancer for transdermal drug delivery: a review. J Pharm Pharmacol 2015; 67(4):473-85. doi: 10.1111/jphp.12334 [Crossref]
20. Williams AC. Urea and its derivatives as penetration enhancers. In: Smith EW, Maibach HI, eds. Percutaneous Penetration Enhancers. Boca Raton, FL: CRC Press; 1995. p. 289-96.
21. Delivery of natural polyphenols by polymeric nanoparticles improves the resistance of endothelial progenitor cells to oxidative stress. Eur J Pharm Sci 2013; 50(3-4):393-9. doi: 10.1016/j.ejps.2013.08.008 [Crossref]
22. Preparation and characterization of cyanocobalamin (vit B12) microemulsion properties and structure for topical and transdermal application. Iran J Basic Med Sci 2013; 16(7):865-72. doi: 10.22038/ijbms.2013.1126 [Crossref]
23. UV spectrophotometric method for simultaneous determination of finasteride and tamsulosin in combined dosage form. Int J Pharm Biol Sci 2011; 1(3):303-10.
24. Effects of the various solvents on the in vitro permeability of indomethacin through whole abdominal rat skin. Annu Res Rev Biol 2015; 5(4):335-46. doi: 10.9734/arrb/2015/13410 [Crossref]
25. Influence of chemical permeation enhancers on the in vitro skin permeation of minoxidil through excised rat skin: a mechanistic study. Iran J Pharm Sci 2018; 14(1):97-110. doi: 10.22034/ijps.2018.32053 [Crossref]
26. A microemulsion system for controlled corneal delivery of Timolol. Int Res J Pharm Appl Sci 2013; 3(4):32-9.
27. The effect of diethylene glycol monoethyl ether on skin penetration ability of diclofenac acid nanosuspensions. Colloids Surf B Biointerfaces 2018; 162:8-15. doi: 10.1016/j.colsurfb.2017.11.012 [Crossref]
28. Terpenes in propylene glycol as skin-penetration enhancers: permeation and partition of haloperidol, Fourier transform infrared spectroscopy, and differential scanning calorimetry. J Pharm Sci 2002; 91(7):1639-51. doi: 10.1002/jps.10160 [Crossref]
29. Passive skin penetration enhancement and its quantification in vitro. Eur J Pharm Biopharm 2001; 52(2):103-12. doi: 10.1016/s0939-6411(01)00166-7 [Crossref]
30. Characterisation of colloidal drug delivery systems from the naked eye to Cryo-FESEM. Micron 2007; 38(8):796-803. doi: 10.1016/j.micron.2007.06.008 [Crossref]
31. Formulation, characterization and in vitro/ex vivo evaluation of trolamine salicylate-loaded transfersomes as transdermal drug delivery carriers. Int J Pharm Sci Res 2018; 9(9):3725-31. doi: 10.13040/ijpsr.0975-8232.9(9).3725-31 [Crossref]
32. The impact of lag time on the estimation of pharmacokinetic parameters. I. One-compartment open model. Pharm Res 1993; 10(7):1031-6. doi: 10.1023/a:1018970924508 [Crossref]
33. Polymerization of benzalkonium chloride-type monomer and application to percutaneous drug absorption enhancer. J Control Release 1990; 13(1):63-71. doi: 10.1016/0168-3659(90)90075-5 [Crossref]
34. Percutaneous permeation modifiers: enhancement versus retardation. Expert Opin Drug Deliv 2008; 5(5):517-29. doi: 10.1517/17425247.5.5.517 [Crossref]
35. Skin permeation mechanism and bioavailability enhancement of celecoxib from transdermally applied nanoemulsion. J Nanobiotechnology 2008; 6:8. doi: 10.1186/1477-3155-6-8 [Crossref]
36. Percutaneous penetration modifiers and formulation effects. Int J Pharm 2010; 386(1-2):42-51. doi: 10.1016/j.ijpharm.2009.10.052 [Crossref]
37. Chemical enhancement of percutaneous absorption in relation to stratum corneum structural alterations. J Control Release 1999; 59(2):149-61. doi: 10.1016/s0168-3659(98)00187-4 [Crossref]
38. The effect of various vehicles on the naproxen permeability through rat skin: a mechanistic study by DSC and FT-IR techniques. Adv Pharm Bull 2016; 6(1):9-16. doi: 10.15171/apb.2016.003 [Crossref]
39. Differential scanning calorimetry of human and animal stratum corneum membranes. Int J Pharm 1998; 168(1):17-22. doi: 10.1016/s0378-5173(98)00078-7 [Crossref]
40. Chemical penetration enhancers for transdermal drug delivery systems. Trop J Pharm Res 2009; 8(2):173-9. doi: 10.4314/tjpr.v8i2.44527 [Crossref]
41. Lipid thermotropic transitions in human stratum corneum. J Invest Dermatol 1986; 86(3):255-9. doi: 10.1111/1523-1747.ep12285373 [Crossref]
42. Microemulsion formulation for enhanced absorption of poorly soluble drugs. I. Prescription design. J Control Release 2002; 81(1-2):65-74. doi: 10.1016/s0168-3659(02)00049-4 [Crossref]
43. Structural studies of Aerosol OT reverse micellar aggregates by FT-IR spectroscopy. J Phys Chem 1989; 93(21):7409-16. doi: 10.1021/j100358a032 [Crossref]
44. Molecular interactions of penetration enhancers within ceramides organization: a FTIR approach. Eur J Pharm Sci 2009; 36(2-3):192-9. doi: 10.1016/j.ejps.2008.10.010 [Crossref]
45. Delivery of a hydrophilic solute through the skin from novel microemulsion systems. Eur J Pharm Biopharm 1997; 43(1):37-42. doi: 10.1016/s0939-6411(96)00016-1 [Crossref]
46. Nanoparticle encapsulation improves oral bioavailability of curcumin by at least 9-fold when compared to curcumin administered with piperine as absorption enhancer. Eur J Pharm Sci 2009; 37(3-4):223-30. doi: 10.1016/j.ejps.2009.02.019 [Crossref]