A new method for long-term inhalation toxicity studies for bronchospasmolytic aerosol formulations in dogs

In routine inhalation toxicity studies laboratory animals are exposed under dynamic exposure conditions to different steady-state concentrations of a test compound. The relevant exposure concentration is the analytically determined time-weighted average concentration in the vicinity of the breathing zone of the animals. If the intended use of a bronchospasmolytic aerosol is taken into account, a non-steady-state exposure regimen might be more appropriate. Using this approach local effects of high aerosol concentrations on the respiratory tract were investigated in dogs by daily head/nose-only exposure for 1 h for 4 weeks. Four groups of four dogs were exposed to 20, 40 and 80 bursts of a bronchodilator formulation using a modified metered dose inhaler. The analytically determined mean concentration of the active ingredient (prostaglandin) in the breathing zone was 43.1, 92.2 and 193.9 micrograms l-1 air, respectively. Maximum concentrations were determined by simulation. The aerosol was in the respirable range (mass median aerodynamic diameter approximately 1.2 microns, geometric standard deviation approximately 1.4). A kinetic model was developed to simulate the mean and maximum non-steady-state concentration for an exposure regimen of 20, 40 and 80 bursts per hour under dynamic exposure conditions. The model was based on first-order inhalation chamber kinetics. The kinetic model was validated experimentally by comparing measured mean and simulated mean concentrations. In all exposure groups the simulated maximum concentrations were ca. 80 mg of test compound (formulation) per litre of air. Plasma levels were elevated in a dose-dependent manner as evidence of the validity of the test model employed.