Mechanical Properties of Healthy and ex vivo Onychomycosis Nails and the Influence of a Porphyrin-propylene Glycol Antifungal Formulation

Aims: To investigate nail penetration enhancing effectiveness of a novel drug formulation and ingredients, 40% propylene glycol (PG) and 40 µM multifunctional photosensitizer (MFPS). Proposed formulation was proven effective in photodynamic treatment (PDT) of ex vivo fungal infections of human nails (onychomycoses) and indirectly also to weaken nail’s strength.

Study Design: To directly proof the effect of the novel formulation on nail’s hardness Atomic Force Microscopy (AFM) was employed to measure small-distance forces. Based on indentations Young’s moduli of healthy and ex vivo onychomycosis nails were calculated before and after treatment with the novel formulation. Changes in young’s modulus can be related to changes in nail’s strength and thus formulation effectiveness. Additionally, AFM imaging was employed to study topographical / roughness changes resulting from the treatments to both nail types. Trichophyton mentagrophytes, representative for clinical onychomycosis, was chosen to induce nail infections.

Place and Duration of Study: AFM study was performed at Leiden Institute of Physics (Leiden University, The Netherlands). General study design, chemical and microbiological investigations took place at Erasmus MC (Rotterdam, The Netherlands).

Methodology: Tapping mode was used for imaging and contact mode for indentations. For each healthy and onychomycosis nail 5 to 6 spots of 8 x 8 µm on marked areas were studies (3-5 image scans and 5-10 indentations performed per spot). Young’s moduli were calculated using the Hertz model.

Results: MFPS, PG and MFPS/PG affected healthy nail topography; roughness remained unchanged (PG; MFPS; MFPS/PG) or decreased (MFPS; MFPS/PG). Roughness of onychomycosis nails was lower compared to healthy nails and reduced after MFPS/PG treatment. Healthy nail Young’s moduli varied (1-18 GPa) but decreased significantly after MFPS or PG treatment. Young’s moduli of onychomycosis nails were lower compared to healthy nails and seemingly increased after MFPS/PG treatment.

Conclusion: Human nails are extremely inhomogeneous (strength / topography) thereby influencing topical drug response.