Enhanced human nail drug delivery: nail inner drug content assayed by new unique method

The purpose of this study was to develop an assay method of the human inner nail plate and to compare nail drug penetration by a penetrating enhancing formulation (the test carrier formulation). The test carrier and saline formulations were tested using radiolabeled urea, ketoconazole, and salicylic acid. After twice dosing daily for 7 days on human nail plates, the under inner section of the nail plate was assayed for absorbed drug content using a unique drilling/removal system. Results show that the weight-normalized radioactivity contents of three chemicals in the inner intermediate nail plate center in the carrier formulation were two fold higher than those from saline (p < 0.05). Total radioactivity recovery of dosed [(14)C]-salicylic acid was 89 +/- 2% in the carrier formulation and 88 +/- 5% in saline. In saline formulation, salicylic acid showed greater binding to the outer nail, making it less bioavailable for the inner nail area. This didn't occur with carrier formulation. In conclusion, topical treatment of nail diseases such as onychomycosis is not yet sufficiently effective, likely because of minimal drug penetration into the inner nail plate where the disease perpetuates. The assay system has the unique characteristic of being able to assay the inner part of the nail where the disease resides.