Examples of Alternative Use in Toxicology for Common Species

Experimental approaches to evaluate respiratory allergy in animal models

Asthma is defined as a chronic disease of the entire lung and asthma attacks may either be immediate, delayed or dual in onset. Allergic asthma is a complex chronic inflammatory disease of the airways and its etiology is multifactorial. It involves the recruitment and activation of many inflammatory and structural cells, all of which release mediators that result in typical pathological changes of asthma. A wealth of clinical and experimental data suggests that allergic asthma is due to an aberrant lung immune response mediated through T-helper type 2 (Th2) cells and associated cytokine-signaling pathways. The pathology of asthma is associated with reversible narrowing of airways, associated with prominent features that involve structural changes in the airway walls and extracellular matrix remodeling including abnormalities of bronchial smooth muscle, eosinophilic inflammation of the bronchial wall, hyperplasia and hypertrophy of mucous glands. The primary objective of respiratory allergy tests is to determine whether a low-molecular-weight chemical (hapten) or high-molecular-weight compound (antigen) exhibits sensitizing properties to the respiratory tract. This may range from reactions occurring in the nose (allergic rhinitis), in the bronchial airways (i.e., allergic bronchitis, asthma) or alveoli (e.g., hypersensitivity pneumonitis). Current assays utilize several phases, viz. an induction phase, which includes multiple exposures to the test compound (sensitization) via the respiratory tract (e.g., by intranasal or intratracheal instillations), by inhalation exposures or by dermal contact, and a single or multiple challenge or elicitation phase. The challenge can either be with the chemical (hapten), the homologous protein conjugate of the hapten or the antigen. The choice depends both on the irritant potency and the physical form (vapor, aerosol) of the hapten. The appropriate selection of concentrations (dosages) both for the induction and elicitation of respiratory allergy appears to be paramount for the outcome of test. Endpoints to characterize positive response range from the induction of immunoglobulins, cytokine or lymphokine patterns in serum (or the lung) to (patho-)physiological reactions typifying asthma. None of the currently applied animal models duplicate all features of human asthma. Accordingly, the specific pros and cons of the selected animal model, including protocol variables, animal species and strain selected, must be interpreted cautiously in order to arrive at a meaningful extrapolation for humans.

Inhalation studies in laboratory animals--current concepts and alternatives

Highly standardized and controlled inhalation studies are required for hazard identification to make test results reproducible and comparable and to fulfill general regulatory requirements for the registration of new drugs, pesticides, or chemicals. Despite significant efforts, the results of inhalation studies have to be analyzed judiciously due to the great number of variables. These variables may be related to technical issues or to the specific features of the animal model. Although inhalation exposure of animals mimics human exposure best, ie, error-prone route-to-route extrapolations are not necessary, not all results obtained under such very rigorous test conditions may necessarily also occur under real-life exposure conditions. Attempts are often made to duplicate as closely as possible these real-life exposure conditions of humans in appropriate bioassays. However, this in turn might affect established baseline data, rendering the interpretation of new findings difficult. In addition, specific use patterns, eg, of inhalation pharmaceuticals or pesticide-containing consumer products, may impose test agent-specific constraints that challenge traditional approaches. Moreover, specific modes of action of the substance under investigation, the evaluation of specific endpoints, or the clarification of equivocal findings in common rodent species may require exposure paradigms or the use of animal species not commonly used in inhalation toxicology. However, particularly in inhalation toxicology, the choice of animal models for inhalation toxicity testing is usually based on guideline requirements and practical considerations, such as exposure technology, expediency, and previous experience rather than validity for use in human beings. Larger animal species, apart from the welfare aspects, may require larger inhalation chambers to accommodate the animals, but for technical reasons and the difficulty of generating homogeneous exposure atmospheres in such inhalation chambers, this may jeopardize the outcome of the study. Some of the many variables and possible artifacts likely to occur in animal inhalation studies are addressed in this paper.

Assessment of respiratory hypersensitivity in guinea-pigs sensitized to diphenylmethane-4,4'-diisocyanate (MDI) and challenged with MDI, acetylcholine or MDI-albumin conjugate

Guinea-pigs were sensitized to monomeric diphenylmethane-4,4'-diisocyanate (MDI) by two intradermal injections (1-10% MDI, injection volumes of 50-100 microliters/day, on days 0, 2 and 4) or by a single brief high-concentration inhalation exposure (135 or 360 mg/m3, 15 min). Starting with day 21 following sensitization the animals were subjected to inhalation-challenge exposures (30 min) with non-irritating and irritating concentrations of the hapten (MDI). MDI-challenge concentrations ranged from 3.4 +/- 0.9 to 60 +/- 14.3 mg/m3 air. In some groups guinea-pigs were also challenged with acetylcholine (ACh) aerosol or the MDI-guinea pig serum albumin (GPSA) conjugate. Experimental findings indicated that from intradermally sensitized animals an immediate onset respiratory hypersensitivity response could only be elicited with concentrations exceeding the irritant threshold concentration for MDI, i.e. with concentrations greater than approximately 20 mg/m3 air. Guinea-pigs challenged with the MDI-GPSA conjugate (35.3 +/- 2.8 mg/m3 air) also experienced a weak immediate-type respiratory hypersensitivity response. An increased non-specific airway hyper-responsiveness following ACh-challenge was only observed from animals challenged with approximately 60 mg MDI/m3 air. The histopathological evaluation of lungs and lung-associated lymph nodes revealed an association of the increase in eosinophilic granulocytes and concentration of MDI used for challenge exposures. It appeared, in most instances, that this influx was more pronounced in animals sensitized with MDI as compared with concurrent controls challenged with the same MDI concentration. Guinea-pigs sensitized by a single 15-min inhalation exposure to either 135 or 360 mg MDI/m3 air were challenged sequentially with 12 +/- 2.1 mg MDI/m3 air, ACh and MDI-GPSA conjugate. Following the inhalation-induction, an airway hyper-responsiveness was elicited both after challenge with MDI and with the MDI-GPSA conjugate. The influx of eosinophilic granulocytes was more pronounced from animals sensitized by inhalation when compared with guinea-pigs sensitized intradermally and challenged with the same concentration of MDI. Thus, experimental findings suggest that elicitation of respiratory hypersensitivity is concentration-dependent and that challenge concentrations should slightly exceed the threshold concentration for irritation (approximately 20 mg/m3). Sensitization by inhalation increased the susceptibility to irritant stimuli and thus confounds the selection of the most appropriate concentration for challenge. However, the combined assessment of specific pathologic features such as airway eosinophilia and the evaluation of several breathing parameters during hapten- and ACh-challenge make it easier to distinguish effects caused by irritation and respiratory hypersensitivity.(ABSTRACT TRUNCATED AT 400 WORDS)