Transdermal Glyceryl Trinitrate and Clonidine Good bioavailability for both

In 2 recent studies, the bioavailability of glyceryl trinitrate [nitroglycerin] and clonidine from transdermal drug delivery systems was investigated. In a 3-way crossover study 'Transderm-Nitro' was compared with 'Nitro-Dur' and 'Nitro-disc' . No statistically significant difference among the preparations could be determined since the plasma drug concentrations varied greatly within and between subjects for each transdermal product. 'Transderm-Nitro' was the only preparation containing a drug reservoir and a discrete rate-controlling element in its design, and with this product the AUC was highest and the coefficient of variation indicated least variability in plasma concentrations among subjects. Transdermal preparations give continuous prolonged delivery of glyceryl trinitrate for 24 hours and this is an enormous advantage for a drug with a half-life measured in minutes.

In a 2-way crossover study 'Catapres-TTS', which has a similar design to that of 'Transderm-Nitro', was compared with oral clonidine. With the transdermal preparation, plasma drug levels reached a steady-state value within 3 days and remained constant throughout the study period. However, plasma levels with oral administration of clonidine showed large fluctuations. The constant rate of drug input achieved with 'Catapres­TIS' minimises side effects such as dry mouth and drowsiness while maintaining an antihypertensive effect.

These studies indicate that the bioavailability of a drug from rate-controlled transdermal dosage forms is consistent in a patient throughout the lifetime of the preparation. The rate of drug input may be increased by increasing the area of preparation applied to the skin. Shaw. JE. American Hearl Journal 108. 217 (Jul 1984)

0156-2703/84/ 0922-0015/0$01.00/0 © ADIS Press INPHARMA ® 22 Sep 1984 15