Inhalation testing parameters

For the 28-/90-day studies, separate guidelines are available for studies using oral administration, dermal application, or inhalation. The principle of these study protocols is identical although the updated protocols for oral administration includes additional parameters compared to those for dermal and inhalation administration, see Table 4.12. The 28-day inhalation test guideline is currently undergoing revision, see Table 2.4. 

The AEGL-1 concentration was based on a 1-hour (h) no-effect concentration of 8,000 parts per million (ppm) in healthy human subjects (Emmen et al. 2000). This concentration was without effects on pulmonary function, respiratory parameters, the eyes (irritation), or the cardiovascular system. Because this concentration is considerably below that causing any adverse effect in animal studies, an intraspecies uncertainty factor (UF) of 1 was applied. The intraspecies UF of 1 is supported by the absence of adverse effects in therapy tests with patients with severe chronic obstructive pulmonary disease and adult and pediatric asthmatics who were tested with metered-dose inhalers containing HFC-134a as the propellant. Because blood concentrations in this study approached equilibrium following 55 minutes (min) of exposure and effects are determined by blood concentrations, the value of 8,000 ppm was made equivalent across all time periods. The AEGL-1 of 8,000 ppm is supported by the absence of adverse effects in experimental animals that inhaled considerably higher concentrations. No adverse effects were observed in rats exposed at 81,000 ppm for 4 h (Silber and Kennedy 1979) or in rats exposed... 

Data adequacy The key study was well designed and conducted and documented a lack of effects on heart and lung parameters as well as clinical chemistry. Pharmacokinetic data were also collected. The compound was without adverse effects when tested as a component of metered-dose inhalers on patients with COPD. Animal studies covered acute, subchronic, and chronic exposure durations and addressed systemic toxicity as well as neurotoxicity, reproductive and developmental effects, cardiac sensitization, genotoxicity, and carcinogenicity. The values are supported by a study with rats in which no effects were observed during a 4-h exposure to 81,000 ppm. Adjustment of the 81,000 ppm concentration by an interspecies and intraspecies uncertainty factors of 3 each, for a total of 10, results in essentially the same value (8,100 ppm) as that from the human study. ... 

In critical evaluation of the effect of a gas, vapor, or aerosol inhaled in to the respiratory tract of an animal, the dosimetric method has been recommended (Oberst, 1961). However, due to the complexity of measuring the various parameters simultaneously, only a few studies on gaseous drugs or chemicals have employed the dosimetric method (Weston and Karel, 1946 Adams et al., 1952 Leong and MacFarland, 1965 Landy et al., 1983 Stott and McKenna, 1984 Dallas et al., 1986, 1989). For studies on liquid or powdery aerosols, modified techniques such as intratracheal instillation (Brain et al., 1976) or endotracheal nebulization (Leong et al., 1988) were used to deliver an exact dose of the test material into the lower respiratory tract (LRT) while bypassing the URT and ignoring the ventilatory parameters. 

While irritancy resulting from the above reflex reaction is one cause of altered respiratory parameters during exposure, there are many others. These include other types of reflex response, such as bronchoconstriction, the narcotic effects of many solvents, the development of toxic signs as exposure progresses, or simply a voluntary reduction in respiratory rate by the test animal due to the unpleasant nature of the inhaled atmosphere. The extent to which these affect breathing patterns and hence inhaled dose can only be assessed by actual measurement. 

Principles of fluid and particle dynamics in the respiratory tract (physical and anatomical parameters) are also discussed, as they are the starting point for the development of drug products for inhalation. In fact, they set the conditions used for in vitro and in vivo testing of inhalation systems and define the specifications for new inhalation systems. 

As mentioned above, a NOAEL can usually not be derived from the classic test guideline methods for skin and eye irritation. Based on information from acute and/or repeated dose toxicity smdies using inhalation, it may be possible to derive a NOAEL and/or LOAEL for respiratory tract irritation. In such smdies, the slope of the dose-response curve is a particularly useful parameter as it indicates the extent to which reduction of exposure will reduce the irritative response the steeper the slope, the greater the reduction in response for a particular finite reduction in exposure. 

Although a variety of interpretations have been issued, reversibility to bronchodilators is considered to be present when the FEV i increases by 200 ml and 12% of the pre-bronchodilator value. Although in the latest GINA guidelines this issue is no longer addressed, the same criteria have been used for evaluation of the response to corticosteroids. A corticosteroid trial compared spirometric tests before and at the end of oral prednisolone (e.g. 30 mg/d) taken for two weeks or a course of inhaled steroid (e.g. beclomethasone 500 pg twice daily or equivalent) taken for six weeks. A positive response to corticosteroids justified prescription of regular inhaled steroid. Subjective improvement as a single efficacy parameter is not considered to be a satisfactory end point. Objective improvement is seen in 10-20% of patients with COPD. 

Reproductive Effects. No studies were found regarding the reproductive toxicity of chlorobenzene in humans. In a two-generation inhalation study, chlorobenzene did not adversely affect various reproductive parameters in rats (Nair et al. 1987). While results of this study suggest reproductive toxicity may not be an area of concern to humans, other considerations are warranted before firm conclusions can be made regarding risk to humans. The slight increase in the occurrence of degeneration of the germinal epithelium of the testes provides some evidence for further... 

Shortly before exposure to the test chemical, the animals are weighed and exposed to different test chemical concentrations in the designated chamber for 4 hours. The temperature at which the test is performed should be maintained at 22 °C ( 2°C). Ideally, the relative humidity should be maintained between 30% and 70%, but in certain cases (e.g., aerosols), even this may not be practicable. Food should be withheld during exposure. Water also may be withheld in certain cases. The observation period for acute inhalation toxicity in animals should be at least 14 days. For clinical examination and pathology, the parameters listed for acute oral toxicity may be followed. The following conditions should be monitored during the experiment using a standard inhalation chamber. 

CRITICAL ASSESSEMENT OF THE METHOD In general pharmacological studies during anesthesia should be assessed appropriately due to the possible interaction between the test compound and the used anesthetic as well as due to the reduced tone of the autonomic nervous system. Enteral administration of the candidate compound should be avoided, because enteral absorption of the test compound might be reduced due to the impaired intestinal motility during anesthesia. With respect to the effect of the aesthetic compound itself on intermediary metabolism the barbiturate pentobarbital sodium is the most inert anesthetic and does not cause alterations of metabolic blood and tissue parameters. In contrast, e.g. urethane as well as isoflurane (inhalation aesthetic) influences by itself substantially metabolic parameters over time (hours). 

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