TRYPTASE INHIBITORS BLOCK ALLERGEN-INDUCED AIRWAY AND INFLAMMATORY RESPONSES IN ALLERGIC SHEEP

Tryptase, a mast cell serine protease, has been implicated in the pathophysiology of allergic asthma, but formal evidence to support this hypothesis has been limited by the lack of specific inhibitors for use in vivo. Therefore, in this study we examined the effects of two inhibitors of tryptase, APC 366 [N-(1-hydroxy-2-naphthoyl)-L-arginyl-L-prolinamide hydrochloride] and BABIM [bis(5-amidino-2-abenzimidazolyl)methane] on antigen-induced early and late responses, airway responsiveness as measured by carbachol provocation, microvascular permeability as measured by bronchoalveolar lavage (BAL) albumin concentrations, and tissue eosinophilia from biopsies in allergic sheep. APC 366 and BABIM were administered by aerosol in all experiments. In vehicle control trials, antigen challenge resulted in peak early and late increases in specific lung resistance (SR(L)) of (mean +/- SE, n = 6) 259 +/- 30% and 183 +/- 27% over baseline, respectively. Treatment with APC 366 (9 mg/3 ml H2O given 0.5 h before, 4 h after, and 24 h after antigen challenge) slightly reduced the peak early response (194 +/- 41%), but significantly inhibited the late response (38 +/- 6%, p < 0.05 versus control trials). Twenty-four hours after challenge, APC 366 also completely blocked the antigen-induced airway hyperresponsiveness to inhaled carbachol observed in the control trial. In the APC 366 trial, PC400 (the cumulative number of carbachol breath units [BU] that increased SR(L) by 400%) was 22.3 +/- 3.3 before and 22.6 +/- 3.6 BU after challenge as compared with the control trial where PC400 was 20.4 +/- 3.2 before and fell to 7.0 +/- 1.5 BU after challenge (p < 0.05 versus baseline and drug treated). Similar results were obtained with BABIM (9 mg/3 ml) using the same treatment regimen. APC 366 also showed antiinflammatory activity by significantly inhibiting post antigen-induced BAL albumin and tissue eosinophilia when compared with control trials. Prophylactic administration of APC 366 (18 mg twice a day for 3 d plus 18 mg 0.5 h before antigen challenge) provided a significantly greater reduction in the early response (SR(L) 310 +/- 62 control trials and 100 +/- 10 drug treated, p < 0.05) as compared with the acute treatment. This multiple treatment regimen also blocked the late airway response and the postchallenge airway hyperresponsiveness as well. These results suggest that mast cell tryptase plays an important role in airway responses, including airway inflammation, to inhaled antigen. Inhibition of this enzyme may represent a new target for therapeutic intervention in the treatment of asthma.