Abstract
The percutaneous absorption rate of the topical formulation of a drug (PAR) hasbeen calculated and expressed in percentage using the amount of the drug applied (Da) and the amount of the drug collected (Dc) as in the following equation:
apparent PAR= (Da-Dc) /Da
where Dc is measured by swabbing the drug remaining on the skin after a certain periodof time has passed following application . One problem is that after swabbing, still a slighttrace of the drug remains on the skin, even when it is carefully swabbed, this trace mightcause an erroneous estimation of PAR, especially for drugs with a low PAR . To improvethe accuracy in the estimation of PAR, we introduced the idea of “collection rate at time0” into the above equation as follows:
collection rate at time 0=Dc at time 0/Da
“Collection rate at time 0” indicates the technical efficacy of swabbing the drug appliedto the skin. With the above equations, we calculated the corrected PAR for 5 g of 1 %omoconazole nitrate cream applied in a single dose to the skin of the subject's back over24 hours in 9 healthy male volunteers. Three of the 9 volunteers served as the subjects forthe calculation of “collection rate at time 0” . The mean corrected PAR (±SD) for 1%omoconazole nitrate cream was found to be 13.6 (±6.3) %.The difference between thecorrected and apparent PAR ranged from 2.5 to 3.0 %. If this expanded equation isemployed, it would make a large difference in the estimation of PAR of the topicalformulation of a drug with a low PAR or a drug poorly absorbed by a particularindividual. With the “collection rate at time 0” incorporated into the calculation of PAR, even a simple calculation method using swabbing will provide more precise and usefulinformation for pharmacokinetic studies of the topical formulation of a drug .
