TRANSPORT AND METABOLIC PATHWAY OF THYMOCARTIN (TP4) IN EXCISED BOVINE NASAL-MUCOSA

Thymocartin (TP4, Arg-Lys-Asp-Val) is the 32-35 fragment of the natura lly occuring thymic factor (thymopoietin). Here studies on the nasal t ransport and metabolism of TP4 were performed. Freshly excised bovine nasal mucosa was taken as a model membrane. For permeation studies typ ical donor-receiver experiments (side-by-side) and finite-dose experim ents with small volumes of highly concentrated solutions were carried out. The metabolic pathway of TP4 in nasal mucosa was found to occur a ccording to a typical aminopeptidase cleavage pattern, stepwise formin g Lys-Asp-Val and Asp-Val. TP4 metabolism experiments under reflection kinetics showed a saturation profile above 0.5 mu mol mL(-1). A non-l inear kinetic model consisting of three steps in sequence was sufficie nt to describe the kinetics: for the first step saturable Michaelis-Me at kinetics, and for the second and the third step first-order kinetic s were assured. The model was capable of simultaneously fitting the da ta for the full range of initial concentrations from 0.1 up to 1.0 mu mol mL(-1). Saturation kinetics was also found to be the prominent fea ture of the permeation experiments performed. In the lower concentrati on range (< 0.4 mu mol mL(-1)), transport of TP4 across nasal mucosa w as controlled by metabolism, in the higher concentration range (>0.85 mu mol mL(-1)) diffusion control became more important. We conclude th at enhancement of absorption can be achieved when nasal aminopeptidase s are Saturated, e.g. at high TP4 concentrations.