Pharmacokinetics of amitriptyline in rabbit skin and plasma following iontophoretic administrations

Several researches show that the skin can play very important role as a port of entry of therapeutic agents into the body for the systemic as well as topical delivery. In the skin structure, the upper layer of the skin called stratum corneum poses a barrier to the entry of many therapeutic active moieties. The iontophoresis application technique can enhance transdermal drug delivery while drug application by passive diffusion is not enough.;Amitriptyline (AMT) is a tricyclic antidepressant (TCA) and has been widely used in the treatment of major depressive disorders. It has potent voltage-gated Na+ channels blocking effect and due to such efficacy, it can be used as local analgesic effects in human skin. AMT is positive by charged drug at the physiological pH which fulfills the requirement for its iontophoretic application. The purpose of this study is to investigate the feasibility and mechanisms of iontophoretic delivery of AMT to rabbit dermis and plasma.;A simple and selective high performance liquid chromatographic method for the determination of AMT in dialysate and rabbit plasma samples has been used. The Lower limit of quantitation (LLOQ) for dialysate samples was 5 ng/ml and for plasma samples, it was 0.1 microg/ml. The calibration curves were linear (R was always greater than 0.998) for both samples between the wide working ranges of 5--10,000 ng/ml dialysate concentrations and 10--0.1 microg/ml plasma concentrations.;A new plasma separation method for AMT was invented during this course work. Plasma extraction efficiency was 80 +/- 3% (n = 9).;In-vitro microdialysis studies of AMT recovery and extraction were conducted at 35°C to validate the method before in vivo studies. In vitro extraction and retrodialysis were identical, 64 +/- 6% (n = 9) and 67 +/- 7% (n = 10) respectively. Linear microdialysis probes had a 10 mm window made of polyacrylonitrile hollow fibres with a 50 KDa molecular weight cut off.;In vivo studies were performed on three female, pathogen-free New Zealand albino rabbits and were conducted by inserting two microdialysis probes into the upper dorsal shaved skin of tranquilized rabbits. After one hour, one probe was used for in vivo retrodialysis study. Retrodialysis was performed at a control site for each experiment at the same experimental conditions as at the sampling site to estimate the recovery factor. In vivo retrodialysis recovery was 89 +/- 2% (n = 7). Another probe was used for iontophoresis study. Iontophoresis was performed at 100, 200, or 300 microA/cm2 anodic constant-current densities for 60 minutes in three rabbits according to a randomized crossover design. The iontophoresis cartridge consisted of a stainless steel electrode (3.14 cm2) covered with a pad that was filled with 0.3 ml of a 4.3% AMT Glycerine/Water (50:50) solution via a syringe. Dialysis samples were collected every 8 min (flow rate: 2 microL/min) for 5 hours and analyzed for AMT via a validated HPLC assay. Blood samples were collected serially for 4 hours but plasma concentrations were always below LLOQ (0.1microg/ml).;The dialysate data were transformed to skin interstitial fluid concentration by dividing them by the in vivo microdialysis recovery factor. Skin concentrations from iontophoresis experiments were processed by non-compartmental Pharmacokinetic modeling using WinNolin. AUG and Cmax were increased exponentially with current density while Tmax remained unchanged.;Data shows that Rabbit skin well tolerated iontophoresis of AMT. AMT can be administered transdermally with minimum local and systemic side effect by anodic iontophoresis. There was no skin concentration of AMT observed after passive delivery. This application method can produce significant skin concentration compared to passive penetration and is likely to produce local analgesic effects.