(M1030-06-35) Physicochemical Characterisation of Piroctone Olamine for Topical Delivery to the Skin

Purpose: Dandruff and the more severe related condition, seborrheic dermatitis affects a high proportion of the population at some point in their life. Focus has been traditionally placed on the clinical improvement of the condition. However, limited research has been conducted to investigate the interaction of anti-dandruff actives with the skin. Piroctone olamine, also known as Octopirox ®(OPX) is the monoethanolamine salt of piroctone. It is an antifungal agent widely used for the management of dandruff. The aim of the present work was to characterise the physicochemical properties of piroctone olamine and to conduct pre-formulation studies for development of novel topical formulations of this active.

Methods: A high-performance liquid chromatography (HPLC) method was developed and validated for analysis of the molecule. Validation of the HPLC method was conducted according to ICH guidelines. Melting point was determined using differential scanning calorimetry (DSC). The samples were then heated from 20 to 140 °C at 10 °C min^-1 under nitrogen gas (50 ml min^-1) in a DSC Q2000 from (TA Instruments, USA). Data analysis was performed with TA Universal Analysis software. The distribution coefficient (log D_(O/PBS)) was determined in water and PBS using the shake flask method. The organic phase used was n-octanol and all experiments were performed at room temperature (21 ± 2 °C). The amount of active in each phase in the different ratios was determined using HPLC to calculate the log_(O/PBS). The solubility was determined experimentally. An amount of active was added to 0.5 ml of solvent in flat bottom tubes. The tubes were kept at 32° ± 1 °C for 48 h in a water bath. More active was added if needed until saturation and the caps were covered with Parafilm™. The tubes were centrifuged and the supernatant was then withdrawn and suitably diluted. Finally, the samples were analysed by HPLC. All measurements were conducted in triplicate. The stability of the compound was determined in a range of solvents. Solutions of 1 mg ml^-1 were prepared in the relevant solvents in glass vials, closed with plastic caps and covered in Parafilm™. Solutions were kept and stirred throughout the experiment at 32 ± 1 °C. Aliquots were taken at 24, 48 and 96 h and diluted accordingly to fit the calibration curve. The HPLC analysis was performed following the method described earlier.

Results: Based on experiments with DSC the melting point was observed at an onset temperature of 132.4°C.
A HPLC method was developed and validated for the analysis of the molecule. Samples within the lower end of the calibration curve showed lower degrees of precision and accuracy, which may reflect their proximity to the limit of detection of 0.5 μg ml^-1. Linearity was confirmed from 0.5 to 50 μg ml^-1. As OPX has two ionisable microspecies at different pH values, the logD is appropriate for assessment of octanol/PBS partition behaviour of the molecule. The LogD was determined to be 1.84. This means the piroctone ion presents a higher distribution in octanol. The compound had the highest solubility in methanol (278.4 mg ml^-1) and propylene glycol (PG), with a value of 248.8 mg ml^-1, so the former solvent was selected for mass balance evaluation during skin permeation studies. The lowest solubility for OPX was in dimethyl isosorbide (9.9 mg ml^-1), Labrafac™ (3.6 mg ml^-1) and isostearyl isostearate (0.5 mg ml^-1). Stability of the active at 32 ± 1 °C in several relevant solvents for in vitro studies was studied over 4 days. OPX showed stability in ethanol and PG, while a notable decrease in the amounts of OPX was observed in PBS and water.

Conclusion: The physicochemical properties of OPX were characterised to find suitable excipients able to target the epidermis for topical delivery. An HPLC method for the active ingredient was developed and validated. Melting point, log D, solubility and stability studies were evaluated since these physicochemical properties were not available in the literature. OPX dissociates into two ionised species in solution, piroctone and ethanolamine. The melting point was determined and log D value at physiological pH was confirmed to be >1. Additionally, the low water solubility of the molecule must be addressed for future in vitro penetration studies where sink conditions will be required. Stability was shown in solution for up to 4 days, which will allow the performance of permeation studies and analytical quantification within a known time frame of stability. Future work will build on the results reported here for the development and evaluation of novel topical formulations of OPX.

References: Paz-Alvarez M, Pudney PDA, Hadgraft J, Lane ME. Topical delivery of climbazole to mammalian skin. Int J Pharm. 2018 Oct 5;549(1-2):317-324

Acknowledgments: This work was supported by Unilever [grant number 553834 ]; and the Biotechnology and Biological Sciences Research Council (BBSRC) [grant number 561672]

Solubility of OPX in relevant solvents at 32 ± 1 °C. n=3, mean ± SD



Stability of OPX in selected solvents at 32 ± 1 °C, n=3, mean ± SD



HPLC method validation results for quantitative analysis of OPX. Mean ± SD, precision expressed in RSD%