Protocol Title: Determination of Tretinoin Bioequivalence in Man by Stratum Corneum Tape Stripping

Products: Retin-A Gel 0.025%

Avita Gel 0.025%

Study Center: Dermtech International

San Diego, CA 92128

Investigators: Paul A. Lehman M.Sc.

Thomas J. Franz M.D.

Study Design: Protocol Outline: This was a single center, double-blind, side-by-side comparison of two tretinoin 0.025% gels. The products were randomly applied (20 FL dose) to eight 4 cm2 sites each on the ventral aspect of both forearms of 36 subjects. An elevated, non-occlusive guard was then placed over the dosed sites. The sites were placed side by side as two columns of four each (medial and lateral) on both forearms. The two products were always paired such that if one was assigned to the medial site of a given proximal6distal location, the other was assigned to the lateral site. A single 4 cm2 site was placed on the upper arm as a control (untreated) site. The movement of tretinoin into and out of the stratum corneum was determined at 0.5, 1, 2, and 4 hours (uptake phase); and 8, 12, 24, and 48 hours (elimination phase). For convenience, one arm of each subject was randomly selected for the uptake phase and the opposite arm for the elimination phase.



Uptake Phase: At each time point of the uptake phase, one set of paired sites were first blotted three times with KimWipes® and wiped once with a dry cotton-tipped swab, then stripped 22-times with Transpore™ Tape. Each tape strip was cut to be slightly larger than the 4 cm2 site so that the entire dosed area was stripped.

Elimination Phase: The elimination phase was initiated by removing unabsorbed surface drug from all sites on the elimination arm at four hours using the same procedure outlined in the uptake phase. In addition, all sites were stripped twice with Transpore™ Tape. Subsequently, at 8, 12, 24, and 48 hours one of the paired sites were again stripped 20-times.

Assay Procedures: Tretinoin was assayed by reverse phase HPLC. Both the all-trans and 13-cis isomers were quantified. Neither the KimWipes® or cotton-tipped swabs used to clean the sites were analyzed, nor were the first two tapes strips taken from all sites. The remaining 20 strips were combined in two groups of ten, and extracted for 48 hours in 10 mL of acetonitrile. An 8 mL aliquot was taken to dryness by vacuum centrifugation, reconstituted in 20 FL acetonitrile and analyzed.


Results: Pilot Variability Study: Prior to the pivotal study, a pilot study in six subjects was conducted to determine site-to-site variability. All sixteen sites on each subject were dosed with 20 FL Retin-A Gel 0.025% and tretinoin content determined at all sites at 6 hours. All other procedures (removal of unabsorbed drug, tretinoin analysis) were the same as for the pivotal study.


Total Tretinoin Recovery*

(m grams/4 cm2)


Strips 3-12

Strips 13-12

Medial Forearm Sites

0.230 ± 0.023

0.028 ± 0.005

Lateral Forearm Sites

0.221 ± 0.040

0.028 ± 0.004

Right Forearm Sites

0.222 ± 0.013

0.026 ± 0.001

Left Forearm Sites

0.226 ± 0.033

0.030 ± 0.003

All Sites Combined

0.224 ± 0.023

0.028 ± 0.003

* Mean ± SD






Results: Pivotal Study


Figure 1: Summary Results: By Strip Set

Mean Tretinoin (m g) recovered from 36 subjects


Figure 2: Summary Results: All Strips

Mean Retinoic acid (m g) recovered from 36 subjects


Conclusion: Tretinoin uptake into the stratum corneum is greater from Avita Gel 0.025% than Retin-A Gel 0.025%. The two products are not bioequivalent.