Objective: To test whether therapeutic diclofenac concentrations are attained in skeletal muscle tissue beneath the application site of an epicutaneously administered diclofenac foam formulation.
Methods: Diclofenac foam (5%) was administered epicutaneously at the thigh 80 mg/200 cm2 twice daily for a period of 7 days in healthy volunteers (n=12). On Day 8, 2 microdialysis probes were inserted into skeletal muscle tissue beneath the application site and an 80 mg dose was administered epicutaneously. Concentration versus time profiles in plasma and skeletal muscle were followed for 10 hours.
Results: Concentration versus time profiles were obtained for plasma and interstitial muscle fluid in all experiments. Mean Cmax in plasma was 18.75+/-4.97 ng/ml. Corresponding interstitial concentrations in skeletal muscle were significantly higher, 219.68+/-66.36 ng/ml (p=0.01). Plasma concentrations were not correlated to tissue concentrations (r=-0.08).
Conclusion: There is significant direct penetration of diclofenac into skeletal muscle following multiple epicutaneous administration. However, the concentration attained in individual subjects is not predictable and may be strongly influenced by individual skin properties.