Effect of receiver fluid pH on in vitro skin flux of weakly ionizable drugs

Plants used to treat skin diseases in northern Maputaland, South Africa: antimicrobial activity and in vitro permeability studies

CONTEXT

Ethnobotanical claims of medicinal plants used in northern Maputaland are limited. Objectives To establish scientific validity for a selection of the plants used in Maputaland to treat skin diseases.

MATERIALS AND METHODS

Aqueous and dichloromethane-methanol extracts were prepared from 37 plant species which were collected from four rural communities in Maputaland. Antimicrobial screening was performed on extracts against 12 dermatological relevant pathogens using the micro-titre plate dilution assay. Their combined effect was evaluated by determining the sum of the fractional inhibitory concentrations (ΣFICs). Chemical analysis was undertaken using reverse-phase high-performance liquid chromatography (RP-HPLC) and investigated in vitro across excised intact porcine skin using the ILC07 automated system.

RESULTS

The organic extract of Garcinia livingstonei T. Anderson (Clusiaceae) was found to be the most antimicrobially active displaying an average broad-spectrum MIC value of 270 μg/mL. The combination of Sclerocarya birrea (A. Rich.) Hotsch. (Anacardaceae) with Syzygium cordatum Hochst. ex C. Krauss (Myrtaceae) displayed synergistic effects. The four antimicrobially active organic extracts were found to possess mainly anthraquinones, flavonoids, tannins and saponins. The organic extracts of Kigelia africana (Lam.) Benth. (Bignoniaceae) and S. cordatum were found to have more compounds capable of permeating intact skin after 10 min of exposure.

CONCLUSION

More than 80% of the organic extracts tested displayed a correlation between the antimicrobial efficacy and the reported traditional uses of the plants. Furthermore, the traditional use of topically applied plant preparations is validated as some compounds from the active plants are capable of permeating the skin in vitro.

Nifedipine in semi-solid formulations for topical use in peripheral vascular disease: preparation, characterization, and permeation assay

Nifedipine (NFD) has been used for the treatment of cutaneous lesions caused by peripheral vascular disease and diabetic ulcers. NFD was formulated at 8% in three semi-solid formulations: Polaxamer 407 Lecithin Organogel (PLO), PLO plus Transcutol(®), and an oil-in-water (o/w) emulsion. In vitro release and permeation tests were carried out using a synthetic (cellulose acetate) or natural membrane (pig ear skin), respectively, mounted in a Franz-type diffusion cell at 37°C in a constant water bath. As a receptor solution, isotonic phosphate buffer at pH 7.4 was used. All samples were analyzed by high-performance liquid chromatography by employing a previously validated method. The drug flow values were 6.126 ± 0.288, 4.030 ± 0.081, and 6.660 ± 0.254 μg/cm(2)/h for PLO, PLO plus Transcutol(®), and o/w emulsion, respectively. The three formulations did not show significant differences in drug flow, considering p > 0.05. Furthermore, their penetration profiles in both the epidermis and dermis were statistically different. Thus, the incorporation of NFD in PLO, PLO plus Transcutol(®), and o/w emulsion changed the drug thermodynamic activity, as expected. In addition, Transcutol(®) increased the solubility of NFD in the formulation and promoted its penetration in both the epidermis and dermis.

Skin penetration flux and lag-time of steroids across hydrated and dehydrated human skin in vitro

To study the effect of hydration on skin absorption, we investigated penetration across human skin of twelve model chemicals having steroidal structure but different molecular weight and compared the steady-state penetration rate (J) and lag-time (t) across hydration intact skin (Jh and th) with that across dehydrated intact skin (Jd and td). Stratum corneum (SC) thickness of hydrated (52 microm) is 3.3 times that of dehydrated skin (16 microm). Transepidermal water loss (TEWL) of hydrated (7.6+/-2.1 g/m2/h) is twice that of dehydrated skin (3.4+/-1.6 g/m2/h, p<0.05) which are similar to in vivo values, suggesting the SC barrier function was recovered. The ratio of Jh/Jd ranged between 0.7 and 3.6 (average of 1.9). On the other hand, the ratio of th/td was almost constant (average of 0.8). Ratios of Jh/Jd and th/td were independent of MW and Ko/w. In percutaneous absorption experiments in vitro, skin was preserved in culture medium until use and SC might swell during that time. Therefore, we consider the possibility that J and t varied between hydrated and dehydrated skin. We confirmed the difference of J and t between hydrated and dehydrated skin in vitro and now need to define these results under in vivo condition.

Chemical enhancement of percutaneous absorption in relation to stratum corneum structural alterations

The outermost layer of the skin, stratum corneum (SC), provides an outstanding barrier against the external environment and is also responsible for skin impermeability toward most solutes. The barrier function is related to the unique composition of the SC lipids and their complex structural arrangement. The lipoidal matrix of the SC, therefore, is a target of penetration enhancer action. The literature on the skin barrier structure and function and on the mechanisms of action of some well established permeation promoters, with a focus on their impact on SC structural alterations, is reviewed. Data obtained from infrared, thermal, and fluorescence spectroscopic examinations of the SC and its components imply enhancer improved permeation of solutes through the SC is associated with alterations involving the hydrocarbon chains of the SC lipid components. Data obtained from electron microscopy and X-ray diffraction reveals that the disordering of the lamellar packing is also an important mechanism for increased permeation of drugs induced by penetration enhancers.

Ionization constants and distribution coefficients of phenothiazines and calcium channel antagonists determined by a pH-metric method and correlation with calculated partition coefficients

The pH-metric technique was used to determine the ionization constants and distribution coefficients of 10 phenothiazines and five ionizable calcium channel antagonists. Because the studied compounds were poorly water soluble and quite lipophilic with partition coefficients in the range of 3.5 to 5.5, organic cosolvents had to be added for the determination of the ionization constants to avoid precipitation of the free bases. The effect of the cosolvents dioxane and methanol on the extrapolation to pure water was compared. For both cosolvents a very good agreement with accessible published ionization constants was obtained, however the slope of the regression line was much smaller for dioxane, yielding more reliable estimates according to the standard deviation of the extrapolated values. Thus, dioxane might be preferable to methanol as a cosolvent for the determination of ionization constants of sparingly water soluble bases. Also the n-octanol/water partition coefficients were determined and compared with published data and values calculated with the ClogP, ACD, and HINT programs. Although the obtained values were approximate in conformity with the published data, the calculated partition coefficients differed from the experimental ones considerably for the majority of the investigated compounds. Furthermore, the ion pair partitioning and the distribution coefficients at physiological pH 7.4 were determined. The pH-dependent distribution profiles showed the strong influence of the ionization constants and of the distribution of the ion pairs on the overall distribution. This result strongly suggests that greater use should be made of measured distribution coefficients in quantitative structure-activity relationship studies. The potentiometric method is a convenient way to determine the distribution properties of drug molecules at pH values relevant for the biological system under investigation.

Mutual hairless rat skin permeation-enhancing effect of ethanol/water system and oleic acid

The mutual hairless rat skin permeation-enhancing effect of ethanol (EtOH)/water systems and oleic acid (OA) was investigated with model lipophilic (estradiol, progesterone, levonorgestrel) and hydrophilic drugs (zalcitabine, didanosine, zidovudine). The aqueous solubility and hairless rat skin permeation rate of each drug, saturated in various compositions of EtOH/water system (with and without OA), was determined at 37 degrees C. The hairless rat skin permeation rates of ethanol from EtOH/water systems (with and without OA) were also measured to investigate the skin permeation-enhancing mechanism of EtOH/water systems and OA. Both saturated solubility and steady-state permeation rates of each drug in EtOH/water systems increased exponentially as the volume fraction of ethanol increased, reached the maximum value, and then decreased with further increases in the ethanol volume fraction. Moreover, the hairless rat skin permeation rate of each drug had a good linear relationship with that of ethanol up to 70% (v/v) of ethanol in the EtOH/water system. The addition of OA in the EtOH/water system (70:30 and 60:40 for lipophilic and hydrophilic drugs, respectively) further enhanced the skin permeation rate of both ethanol and drugs. However, > 2.0% (v/v) OA was required to achieve the plateau level in the skin permeation rate of lipophilic drugs, whereas only 0.3% (v/v) OA was required for hydrophilic drugs. The skin permeation rate of ethanol also increased with the addition of OA in the EtOH/water systems (70:30 and 60:40), reached the plateau level with < 1.0% (v/v) OA, and did not significantly change with higher OA concentration. These results suggest that the addition of OA in the EtOH/water system is a useful method to enhance the hairless rat skin permeation rate of both hydrophilic and lipophilic drugs, with more enhancement for hydrophilic drugs.

Permeability of ketorolac acid and its ester analogs (prodrug) through human cadaver skin

The in vitro skin permeabilities of ketorolac acid (KA), a potent nonsteroidal analgesic, and its two ester analogs as prodrug through human cadaver skin were investigated. The two esters of KA, namely, the ethyl ester (KEE) and [(N,N-dimethylamino)carbonyl]methyl ester (KDAE), were selected. The melting temperature of the two esters was significantly lower than that of ketorolac free acid. The partition coefficients (KO/W) were 600, 3541, and 124 for KA, KEE, and KDAE, respectively. The enzymatic hydrolysis of KEE and KDAE by human pooled serum at 37 degrees C was investigated. The esters were hydrolyzed to KA by the serum esterases; the metabolic rate constants were 0.0418 and 0.0148 min-1 for KDAE and KEE, respectively. The serum half-life of KDAE was about 3 times shorter than KEE. When split-thickness cadaver skin was incubated with ester solution at 32 degrees C, the enzymatic hydrolysis of these esters was observed. The metabolic rate in the skin, however, was significantly lower than in the human pooled serum. The skin permeations of KA, KEE, and KDAE through heat-separated epidermis from propylene glycol (PG), PG/glyceryl monocaprylate (GMC) (9:1), and PG/Azone (19:1) vehicle mixtures were evaluated using modified Franz flow-through diffusion cells. The skin fluxes of KA, KEE, and KDAE from PG/GMC (9:1) were 50 +/- 10, 15 +/- 4, and 57 +/- 6 micrograms/cm2/h, respectively. KA was detected in the receiver compartment, albeit to a lesser extent. In conclusion, KDAE appeared to be a better ester prodrug than KEE because it exhibited relatively higher skin flux and faster enzymatic hydrolysis by human serum to liberate the parent drug.

Transdermal delivery of buprenorphine through cadaver skin

The skin permeation of buprenorphine base and HCl salt through cadaver skin was investigated. The octanol-water partition coefficient and solubilities of both buprenorphine free base and HCl salt were determined at 32 degrees C. As expected, buprenorphine free base was more lipophilic than its HCl salt and was practically insoluble in aqueous buffer at pH 8.7. The drug solubility decreased exponentially as the pH of the solution increased, whereas the permeability coefficient increased as the donor solution pH decreased. The skin flux of buprenorphine.HCl was significantly higher than that of the free base from propylene glycol/lauric acid vehicle mixtures. Buprenorphine base permeation through tape-stripped epidermis suggested that in addition to stratum corneum, viable epidermis presented a significant diffusion barrier because of the very low aqueous solubility of the free base observed. The mean steady-state skin fluxes of buprenorphine.HCl were 20.3 and 29.7 micrograms/cm2/h from propylene glycol:lauryl alcohol: ethanol (80:15:5) and propylene glycol: propylene glycol monolaurate: water (80:15:5) vehicle mixtures, respectively. The skin flux of buprenorphine.HCl from various monolithic matrix patches was also evaluated. When capric acid, lauric acid, and lauryl alcohol were separately incorporated into an adhesive matrix, the skin flux of buprenorphine.HCl was enhanced by a factor of 2 to 3.5. Finally, based on the total body clearance and minimum effective concentration of buprenorphine, a transdermal delivery rate of 2.5 micrograms/cm2/h from a 20-cm2 patch was estimated. The in vitro skin permeation data clearly suggest that transdermal delivery of buprenorphine is feasible to achieve a desired systemic analgesic effect.