Contribution of various instrumental methods to transformation/metamorphosis assessment of hydrophilic gels during skin application

INTRODUCTION The metamorphosis of topical products is a relatively new concept that has grown in importance during the past few years [1]. It is defined by the phenomenon in which the primary packaged formulation changes its composition and/or microstructure during application. This change could be caused by different internal/external stimuli, most commonly, but not limited to the evaporation of highly volatile ingredients. As a consequence, there is a high possibility of a discrepancy between the desired release profile of the active substance and the obtained one. In addition to the pharmacokinetic profile changes, metamorphosis can also cause changes in the rheological, texture and cosmetic properties of the preparation [2,3]. According to the most recent EMA Draft Guideline on quality and equivalence of topical products from 2018, it is necessary to examine metamorphosis when assessing the bioequivalence of (trans)dermal products [1]. At this moment, a part of the scientific community suggests only two methods for metamorphosis assessment: nanothermal analysis and photothermal microspectroscopy. Both methods use probes to detect crystals of the drug substance in the deeper layers of the stratum corneum [4]. However, these methods require very expensive, sophisticated equipment, and they have not yet been formalized as methodologies used for this purpose. The aim of this work was to evaluate the possibility of using rheological, tribological and mass loss testing as single or combined methods for the assessment of metamorphosis of topical hydrogels. MATERIALS AND METHODS Materials Carbopol® 934 was obtained from Lubrizol (USA), while propylene glycol, triethanolamine, isopropanol, sodium hydroxide, methyl and propyl paraben were purchased from Sigma Aldrich (Germany). A model substance, diclofenac-sodium, was kindly donated by Hemofarm (Serbia). Sample preparation The concentration of isopropanol, as a model easily volatile ingredient, was varied in the range 0-15% (w/w). In the sample F1 (Table 1), the concentration of the gelling agent was also varied, as another parameter of interest for valid transformation analysis. The gels were prepared in accordance with the usual compendial guidelines (DAC/NRF). Rheological characterization Measurements were preformed using a Rheolab MC 120 rotational rheometer (Paar Physica, Germany), with a cone/plate system with a diameter of 50 mm, at an angle of 1° and a sample thickness of 0.05 mm, at a temperature of 20±0.1°C. Tribological study Test was performed in vivo in 4 female healthy volunteers, at the forearm skin, using Frictiometer® FR700 (Courage+Khazaka, Germany) equipped with a plain, smooth Teflon (PTFE) disk. Study was performed both in finite and infinite dosing conditions. Measurements were performed at 90 rpm for 100 s, with one measurement taken per second. Mass loss analysis The test was performed at a temperature of 32±0.1°C in a closed system of the device Orbital Shaker Incubator ES 20 (Biosan, Latvia). Each sample was applied in the quantity of 1 g, in a thin layer on the glass substrate and placed in a closed chamber system. The mass of the samples was measured on the ABJ 120-4M analytical scale (Kern & Sohn GmbH, Germany) during two hours in 15-minute intervals. RESULTS AND DISCUSSION The greatest potential for instrumental transformation analysis has been demonstrated by the simple mass loss study. In Figure 1, it can be clearly seen that samples F3-F6, which contain isopropanol, have reached the "plateau" sooner than samples F1-F2. The slope of the curve (transformation rate) between 45 and 60 min of drying is also significantly different for the samples F1- F2 and F3-F6. This method additionally allows discerning regions that represent the secondary and third (residual) formulations. Part of the curve between 15 and 45 min, when the mass loss is the most significant, correlates with forming the secondary formulation. The region between 45 and 120 min, depending on the very sample, represents the process of forming the residual formulation, after all the volatile ingredients have evaporated. On the other hand, rheological test has shown somewhat different results. Flow curves (Figure 2) represent changes in the microstructure that formulations go through during the test. Expectedly, gels that contain isopropanol go through more drastic changes. In the descending part of the curve unsymmetrical regions could be spotted, relative to the ascending part. This phenomenon could not be noted in the flow curves of the formulations F1 and F2. Furthermore, maximal and minimal viscosities and hysteresis area were the parameters that failed to show great potential for in depth assessment of a vehicle’s metamorphosis.A tribological study carried out under finite dosing conditions (3 mg/cm2 ; Figure 3a) provided more informative results, especially for the samples’ F3-F6 process of metamorphosis. The levels of changes in the friction value correlate well with the concentration of isopropyl alcohol. Therefore, it can be seen that the changes of this parameter are more intensive for formulations F5-F6 compared to formulations F3-F4, that contained isopropyl alcohol in lower concentrations. The same study, performed under infinite dosing conditions (10 mg/cm2 ; Figure 3b), showed no apparent potential for these purposes.CONSLUSION The results have showed that all three methods in a certain sense contribute to the examination of the metamorphosis of carbomer gels. Certain time points during mass loss and friction tests correlate well in terms of the exact onset of each transformation phase