A comparison of test methods for determining in vitro drug release from transdermal delivery dosage forms

Three test methods for determining in vitro drug release rate from transdermal delivery dosage forms were tested for equivalency of results, ease of implementation and precision. The ‘paddle-over-disk’ (POD) method is under consideration by the USP as a standarized method for release-rate testing of all transdermal delivery dosage forms. The ‘reciprocating disk’ (RD) and ‘diffusion cell’ (DC) methods are both commonly employed throughout the pharmaceutical industry. The three methods were demonstrated to be equivalent in terms of release rate profile (curve shape) and total drug released over the lifetime of the dosage form tested (Transderm-Scop). The precision for the RD method as measured by the mean relative standard deviation over all time points was 4.6%; the precision of the POD method was 5.4% and that for the DC method was 6.7%. Steady-state flux values derived from the POD and RD methods were equivalent (∼4 μg cm −2 h −1) but were ∼25% greater than the steady-state flux value derived from the DC method (∼3 μg cm −2 h −1). All three methods gave results which were within the specifications of the manufacturer (CIBA-GEIGY). The POD method was the easiest to use on a routine basis, required the least amount of specialized equipment and most resembled the current test methodology for dissolution testing of other dosage forms such as tablets or capsules.