To investigate the ability of microdialysis technique to detect in vivo skin metabolism of dexamethasone sodium phosphate

Drugs are often applied to the skin to target local diseases or to be absorbed systemically as an alternative to oral or intravenous administration. The epidermis and dermis components of the skin contains a number of enzymes for either Phase I metabolism (e.g., CYP450, esterases, hydrolases, alcohol and aldehyde dehydrogenase) and Phase II (e.g., glutathione transferases, glucoronyl transferases or N-acetyl transferases). The metabolism of xenobiotic compounds in the skin is primarily intended to detoxify reactive chemicals, but in some cases results in sensitization reactions or enhanced local activity or systemic toxicity. Therefore the knowledge of cutaneous metabolism in the skin is a critical determinant for the safe and efficient use of local skin treatment with topically applied drugs.;Dexamethasone sodium phosphate (DSP) is a water-soluble prodrug of Dexamethasone (DXM) as it is readily converted to the active form (DXM) in-vivo by esterases. DSP iontophoresis is routinely used in clinical practice by physiotherapists to decrease pain and inflammation in musculoskeletal disorders. Recently, in our laboratory we studied the kinetic of DSP in rabbit and human skin following iontophoresis using cutaneous microdialysis as sampling technique. During these studies we have detected DXM together with DSP in most dialysate samples. The objective of the research presented in this thesis is to find out whether the detected DXM is due to skin metabolism or to chemical degradation as DSP converts to DXM also in solution. Three female pathogen-free New Zealand albino rabbits were inserted with three microdialysis probes according to a cross over design. The probes were perfused with DSP solutions for 4 hours. The same procedure was repeated inserting the microdialysis probes in a hollow box that contained only air. Microdilaysis samples for both rabbit and hollow box were collected at predetermined frequency for four hours and analyzed by a validated, reversed phase chromatography with UV detector. Mobile phase composition was 61% methanol: 39% 10 mM Tetrabutyl ammonium hydrogen sulphate (pH 3) for both analyte. Calibration curves were linear in the range 10000-100 ng/mL and 10000-50 ng/mL for DSP and DXM respectively. Dialysate data for rabbit and hollow box were analyzed for DSP and DXM content by HPLC. The percentage DXM detected in the dialysates from the rabbits' skin was then compared with that measured from the boxes using the statistical packages SAS and SPSS. The results of this study show that the conversion of DSP to DXM was always slightly higher in rabbits than that in the hollow boxes but was not statistically significant (P> 0.05) possibly due to a large variability in the data. In conclusion, most of the DXM detected in skin dialysates is due to natural degradation of DSP in solution rather than to skin metabolism.