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Catapres‑TTS ® (clonidine transdermal system) is a transdermal system providing continuous systemic delivery of clonidine for 7 days at an approximately constant rate. Clonidine is a centrally acting alpha‑agonist hypotensive agent. It is an imidazoline derivative with the chemical name 2, 6‑dichloro‑N‑2‑imidazolidinylidenebenzenamine and has the following chemical structure: System Structure and Components Catapres‑TTS ® is a multi-layered film, 0.2 mm thick, containing clonidine as the active agent. The system areas are 3.5 cm 2 (Catapres‑TTS ® ‑1), 7.0 cm 2 (Catapres‑TTS ® ‑2) and 10.5 cm 2 (Catapres‑TTS ® ‑3) and the amount of drug released is directly proportional to the area (see Release Rate Concept ). The composition per unit area is the same for all three doses. Proceeding from the visible surface towards the surface attached to the skin, there are four consecutive layers: 1) a backing layer of pigmented polyester and aluminum film; 2) a drug reservoir of clonidine, mineral oil, polyisobutylene, and colloidal silicon dioxide; 3) a microporous polypropylene membrane that controls the rate of delivery of clonidine from the system to the skin surface; 4) an adhesive formulation of clonidine, mineral oil, polyisobutylene, and colloidal silicon dioxide. Prior to use, a protective slit release liner of polyester that covers the adhesive layer is removed. Cross Section of the System: cross-section-table Release Rate Concept Catapres‑TTS ® (clonidine transdermal system) is programmed to release clonidine at an approximately constant rate for 7 days. The energy for drug release is derived from the concentration gradient existing between a saturated solution of drug in the system and the much lower concentration prevailing in the skin. Clonidine flows in the direction of the lower concentration at a constant rate, limited by the rate‑controlling membrane, so long as a saturated solution is maintained in the drug reservoir. Following system application to intact skin, clonidine in the adhesive layer saturates the skin site below the system. Clonidine from the drug reservoir then begins to flow through the rate‑controlling membrane and the adhesive layer of the system into the systemic circulation via the capillaries beneath the skin. Therapeutic plasma clonidine levels are achieved 2 to 3 days after initial application of Catapres‑TTS ® . The 3.5, 7.0, and 10.5 cm 2 systems deliver 0.1, 0.2, and 0.3 mg of clonidine per day, respectively. To ensure constant release of drug for 7 days, the total drug content of the system is higher than the total amount of drug delivered. Application of a new system to a fresh skin site at weekly intervals continuously maintains therapeutic plasma concentrations of clonidine. If the Catapres‑TTS ® is removed and not replaced with a new system, therapeutic plasma clonidine levels will persist for about 8 hours and then decline slowly over several days. Over this time period, blood pressure returns gradually to pretreatment levels. catapres-tts-structure