High levels of exopeptidase activity are present in rat and canine bronchoalveolar lavage fluid

Abstract

Although reduced loss from porteolytic activity is a potential advantage of the repiratory route for peptide and protein drug delivery, except for elastase and enkephalinase, few data are availabe regarding relevamt enzyme levels. Protease activity in brochoalvelar lavage (BAL) fuid from rat and dog has been examined to determined whether degradation is potential limiting factor of peptide s deliverd via the lung in these animal models. Normal levels of endopeptidases cleaving substrates preferred by elastase, trypsin, and chymotrypsin are low in BAL fluid of both species, consistent with preliminary published data indicating that the stability of proteins may be much greater in the respiratory than the gastrointestinal tract. Aminopeptidase, dipeptidyl peptidase IV, and prolyl carboxypeptidase activity are found in BAL fluid: aminopeptidase levels are suffciently high to be a potential limiting factor for peptide absorption after pulmonary administration in either species. In situ assays confirm that aminopeptidase activity detected in rat lavage fluids is only a fraction of that present at the surface of cells lining the respiratory tract. Whil low levels of active endoproteases probably do not pose a major enzymatic barrier, peptides susceptible to exopeptidase may require protection against degradation e.g., by structural modification or protease inhibition, for optimal delivery via the respiratory tact.