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 naturally occurring thymic factor (thymopoietin). Here studies on the nasal transport and metabolism of TP4 were performed. Freshly excised bovine nasal mucosa was taken as a model membrane. For permeation studies typical donor-receiver experiments (side-by-side) and finite-dose experiments with small volumes of highly concentrated solutions were carried out. The metabolic pathway of TP4 in nasal mucosa was found to occur according to a typical aminopeptidase cleavage pattern, stepwise forming Lys-Asp-Val and Asp-Val. TP4 metabolism experiments under reflection kinetics showed a saturation profile above 0.5 mumol mL-1. A non-linear kinetic model consisting of three steps in sequence was sufficient to describe the kinetics: for the first step saturable Michaelis-Meat kinetics, and for the second and the third step first-order kinetics were assured. The model was capable of simultaneously fitting the data for the full range of initial concentrations from 0.1 up to 1.0 mumol mL-1. Saturation kinetics was also found to be the prominent feature of the permeation experiments performed. In the lower concentration range (< 0.4 mumol mL-1), transport of TP4 across nasal mucosa was controlled by metabolism, in the higher concentration range (> 0.85 mumol mL-1) diffusion control became more important. We conclude that enhancement of absorption can be achieved when nasal aminopeptidases are saturated, e.g. at high TP4 concentrations.