Decamethylcyclopentasiloxane (D(5)), a volatile cyclic methyl siloxane (VCMS), is used in industrial and consumer products. Inhalation pharmacokinetics of another VCMS, octamethylcyclotetrasiloxane (D(4)), have been extensively investigated and successfully modeled with a multispecies physiologically based pharmacokinetic (PBPK) model. Here, we develop an inhalation PBPK description for D(5), using the D(4) model structure as a starting point, with the objective of understanding factors that regulate free blood and tissue concentrations of this highly lipophilic vapor after inhalation in rats and humans. Compared with D(4), the more lipophilic D(5) required deep compartments in lung, liver, and plasma to account for slow release from tissues after cessation of exposures. Simulations of the kinetics of a stable D(5) metabolite, HO-D(5), required diffusion-limited uptake in fat, a deep tissue store in lung, and its elimination by fecal excretion and metabolism to linear silanols. The combined D(5)/HO-D(5) model described blood and tissue concentrations of parent D(5) and elimination of total radioactivity in single and repeat exposures in male and female rats at 7 and 160 ppm. In humans, D(5) kinetic data are more sparse and the model structure though much simplified, still required free and bound blood D(5) to simulate exhaled air and blood time courses from 1 h inhalation exposures at 10 ppm in five human volunteers. This multispecies PBPK model for D(5) highlights complications in interpreting kinetic studies where chemical in blood and tissues represents various pools with only a portion free. The ability to simulate free concentrations is essential for dosimetry based risk assessments for these VCMS.

Ultrafine particles (UFPs; aerodynamic diameter < 100 nm) may contribute to the respiratory and cardiovascular morbidity and mortality associated with particulate air pollution. We tested the hypothesis that inhalation of carbon UFPs has vascular effects in healthy and asthmatic subjects, detectable as alterations in blood leukocyte expression of adhesion molecules. Healthy subjects inhaled filtered air and freshly generated elemental carbon particles (count median diameter ~ 25 nm, geometric standard deviation ~ 1.6), for 2 hr, in three separate protocols: 10 μg/m3 at rest, 10 and 25 μg/m3 with exercise, and 50 μg/m3 with exercise. In a fourth protocol, subjects with asthma inhaled air and 10 μg/m3 UFPs with exercise. Peripheral venous blood was obtained before and at intervals after exposure, and leukocyte expression of surface markers was quantitated using multiparameter flow cytometry. In healthy subjects, particle exposure with exercise reduced expression of adhesion molecules CD54 and CD18 on monocytes and CD18 and CD49d on granulocytes. There were also concentration-related reductions in blood monocytes, basophils, and eosinophils and increased lymphocyte expression of the activation marker CD25. In subjects with asthma, exposure with exercise to 10 μg/m3 UFPs reduced expression of CD11b on monocytes and eosinophils and CD54 on granulocytes. Particle exposure also reduced the percentage of CD4+ T cells, basophils, and eosinophils. Inhalation of elemental carbon UFPs alters peripheral blood leukocyte distribution and expression of adhesion molecules, in a pattern consistent with increased retention of leukocytes in the pulmonary vascular bed.

Particulate air pollution is associated with asthma exacerbations and increased morbidity and mortality from respiratory causes. Ultrafine particles (particles less than 0.1 μ m in diameter) may contribute to these adverse effects because they have a higher predicted pulmonary deposition, greater potential to induce pulmonary inflammation, larger surface area, and enhanced oxidant capacity when compared with larger particles on a mass basis. We hypothesized that ultrafine particle exposure would induce airway inflammation in susceptible humans. This hypothesis was tested in a series of randomized, double-blind studies by exposing healthy subjects and mild asthmatic subjects to carbon ultrafine particles versus filtered air. Both exposures were delivered via a mouthpiece system during rest and moderate exercise. Healthy subjects were exposed to particle concentrations of 10, 25, and 50 μ g/m3, while asthmatics were exposed to 10 μ g/m3. Lung function and airway inflammation were assessed by symptom scores, pulmonary function tests, and airway nitric oxide parameters. Airway inflammatory cells were measured via induced sputum analysis in several of the protocols. There were no differences in any of these measurements in normal or asthmatic subjects when exposed to ultrafine particles at concentrations of 10 or 25 μ g/m3. However, exposing 16 normal subjects to the higher concentration of 50 μ g/m3 caused a reduction in maximal midexpiratory flow rate (−4.34 ± 1.78% [ultrafine particles] vs. +1.08 ± 1.86% [air], p =. 042) and carbon monoxide diffusing capacity (−1.76 ± 0.66 ml/min/mm Hg [ultrafine particles] vs. −0.18 ± 0.41 ml/min/mm Hg [air], p =. 040) at 21 h after exposure. There were no consistent differences in symptoms, induced sputum, or exhaled nitric oxide parameters in any of these studies. These results suggest that exposure to carbon ultrafine particles results in mild small-airways dysfunction together with impaired alveolar gas exchange in normal subjects. These effects do not appear related to airway inflammation. Additional studies are required to confirm these findings in normal subjects, compare them with additional susceptible patient populations, and determine their pathophysiologic mechanisms.

Octamethylcyclotetrasiloxane (D4) has been used for more than 40 years in industrial applications and consumer products, including the personal care industry. D4 possesses many properties suitable for personal care products, such as low surface tension, water repellency, and thermal and chemical stability. The skin is a major route of exposure to D4 for humans. The main objective of this study was to evaluate the percutaneous absorption of neat D4 in human skin using the human skin/nude mouse model. This information is needed to aid in assessing potential risks associated with the intended use of D4. To determine whether D4 accumulates in adipose tissue of the skin, the distribution of D4 in human skin layers following application of neat D4 was also evaluated. In this study, a mean of 1.09 ± 0.46% of the applied dose was absorbed by the animal under semioccluded conditions. Only about 0.02% of the applied dose remained in the skin after 24 h of exposure (or 72 h after application). The majority (94.59 ± 12.28%) of the dose evaporated from the site. Excretion in the volatile trap (or expired volatiles) accounted for 42% of the radioactivity that was absorbed, while 49% were excreted in the urine and feces. Despite the lipophilic properties of D4, a significant accumulation of D4 in adipose tissue of the skin was not observed 24 h following application to the surface of the skin. The small amount of D4 detected in the skin was distributed mainly in the epidermis (61%), with lower amounts in the dermis (29%) and subcutaneous adipose tissue (10%). Dermal absorption studies using human skin transplanted onto nude mice showed that this model could be successfully applied for in vivo percutaneous absorption studies of D4, and presumably of other cyclic siloxanes. The fraction of the percutaneous dose of D4 absorbed in this model was found to be consistent with results reported by others using different experimental approaches.

*Department of Environmental Medicine, University of Rochester, Rochester, New York 14642; ^NIEHS, Statistics and Biomathematics Branch, Research Triangle Park, North Carolina 12233; ^.Inhalation Toxicology Research Institute, Albuquerque, New Mexico 87185; §The Procter and Gamble Company, Miami Valley Labs., Cincinnati, Ohio 45253; and ^Environmental Protection Agency, Office of Pollution Prevention and Toxics, Washington, DC 20460

This paper presents the description of a revised, physiology-oriented compartmental kinetics (''POCK'') model of alveolar clearance and retention of biologically insoluble, respirable particles. By postulating a deposit-activated maximum macrophage recruitment rate leading to a quasi-steady state of the alveolar macrophage population on the alveolar epithelial surface, the model uses a theoretical derivation of an exposure-dependent distribution of particles in the alveolar macrophage population to determine the total load in mobile and immobilized macrophages. For this, the model assumes an invariant maximum volume capacity of the macrophages for particle uptake and a material-dependent critical load of the macrophages that causes total loss of their inherent mobility. Prior to a gradual onset of mobility decrease, there is a material-dependent range of low macrophage burdens without mobility impairment. Using independently determined physiological data for classical clearance rate coefficients, as well as for the lifetime of the alveolar macrophages and their particle turnover by phagocytosis, the model seems to be applicable to experimental results obtained for rats. A constant set of model parameters and a minimum of three material-dependent, physiologically meaningful model variables were sufficient to simulate the alveolar lung burden and available lymph node load data of 15 different subchronic or chronic exposures of Fischer 344 rats to diesel soot, carbon black or xerographic toner. For constant deposition rates, the model predicts the establishment of quasi-steady states for the total load of the alveolar macrophage pool. The final load would increase with increasing deposition rate and, particularly under overload conditions, i.e. at high deposition rates, the number of immobilized macrophages would grow significantly. According to the model, overload does not cause an excessive growth of the total burden of the macrophage pool, but leads to a tremendous increase of the particulate burden of the interstitial space. This compartmental burden is not available for macrophage-mediated classical clearance. Except for partial removal to the lymph nodes, the interstitial burden will persist even when exposures are discontinued and the alveolar macrophage population recovers to full mobility. Subchronic exposure studies seem to bear this out, but due to lack of experimental data for the burdens in most of the alveolar subcompartments of the model, the corresponding predictions cannot be validated at the present time.

A physiology-oriented compartmental kinetics model of alveolar retention of inhaled insoluble particulate matter in rat lungs was proposed in a recent paper, (W. Stöber, P.E. Morrow, and M.D. However, 1989, Fundam. App. Toxicol. 13, 823-843), and the retention patterns obtained with the model for a hypothetical set of input data appeared to simulate phenomena which were observed in inhalation studies with Fischer 344 rats. The present paper represents the results of applying the new model for simulations of the actual experimental retention data of five different inhalation studies with Fischer 344 rats exposed to three different materials. The experimental data showed that model adjustments had to be made in order to account for clearance effects that appeared to be influenced by the age of the animals. After these adjustments were made and an appropriate set of values for the model parameters describing the respective exposure conditions was used, the model was constrained to represent the empirical data of all of the studies by one unique set of parameter values. Changes in particular values of this set were considered to be acceptable only if they reflected changes of relevant properties of the inhaled particulate matter. The final simulations did not completely comply with this self-imposed criterion. However, the degree of compliance and the simulation quality achieved with a minimum of parameter variations seem to be unprecedented in retention modeling. The results of the study encourage attempts for further refining the present model.

The rate of clearance of inhaled 99mTc-pentetate aerosols has been used as an indicator of pulmonary epithelial "permeability" in human and animal studies. In order to evaluate this technique further, groups of eight male ferrets (Mustela putorius furo) were given acute exposures to aerosols of CdCl2 or NaCl via endotracheal tube. Serial evaluations of the thoracic clearance rate of inhaled 99mTc-pentetate aerosols (MMAD = 0.6 microns, sigma g = 1.6) were made before exposure and at fixed time points after toxicant exposure (3 or 6, 24, 48 h, and 5 days after CdCl2). These serial evaluations of 99mTc-pentetate thoracic clearance were conducted at three cadmium intake levels (3, 10, and 30 min exposure to 10 mg/m3 CdCl2) in order to evaluate possible dose-related response relationships. The rate of thoracic clearance of Tc-pentetate was observed to be slowed at 3 h after exposure to CdCl2 aerosol and subsequently increased to well above control rates. The time sequence observed strongly suggests that increased Tc-pentetate clearance rates are indicative of a tissue response or repair process, rather than acute lung injury. Tracheal epithelial penetration of macromolecular tracers was not observed to increase in histological evaluations following serial sacrifice.

Certain viruses, e.g. measles, influenza and varicella are transmitted by aerosols and hence aerosolized vaccine against the respective virus has potential immunologic merit. For measles vaccine the rationale is to circumvent the passively transmitted maternal antibody and therefore to be able to immunize at an early age before natural disease can occur. For influenza vaccine the rationale is to be able to provide immunity at the site of natural infection, and thus abort the very short incubation period infection. Studies with aerosolized measles vaccine have not convincingly demonstrated that aerosol vaccine is superior to subcutaneous vaccine but no studies of stability of aerosolized measles have been conducted nor has it been documented that the methods of aerosolization deliver vaccine to the deep respiratory tract. For influenza, no aerosolized vaccine studies have been conducted. Initial studies to test stability of virus under aerosol conditions which will deliver an appropriate sized particle have been carried out. The next step must now be taken.

To determine if respiratory ammonia (NH3) alters airway responses to sulfuric acid (H2SO4) aerosols, 15 asthmatics inhaled a H2SO4 aerosol at high and low respiratory NH3 levels. A D30 generator produced droplets with an MMAD=0.8 μm at a concentration of 350 μg/m3. All asthmatics inhaled the H2SO4 via a mouthpiece for 20 minutes at rest followed by 10 minutes of exercise on a bicycle ergometer. Low oral NH3 levels averaged 63 μg/m3 while high ammonia levels averaged 340 μg/m3. Following exercise, sulfuric acid at low ammonia levels compared to high ammonia levels produced significantly greater reductions in FEV1 (19% vs 8%, p<0.001) and maximum expiratory flow rates at 60% total lung capacity (47% vs 23%, p<0.001). These data support the hypothesis that endogenous respiratory ammonia can neutralize inhaled acid aerosols and mitigate their airway toxicity in asthmatics. Failure to eliminate oral ammonia may result in an underestimation of the effect of acid aerosols on airway function.

Criteria for the selection of aerosol concentrations to be used in inhalation studies assessing the toxicity and carcinogenicity of chemical substances were discussed by the authors in a meeting sponsored by the National Toxicology Program. Concepts in the design of aerosol inhalation studies emerged from that meeting and are being communicated through this publication. Inhalation studies assessing the toxicity and carcinogenicity of aerosols have often used maximum exposure levels on the basis of technological feasibility. Evidence has now accumulated that the amount of pulmonary burden of deposited particles impacts on particle clearance above some as yet not well-defined exposure concentration. The sequelae are such that lung clearance decreases with increased particulate burden to the point of approaching complete cessation. This paper focuses on the major determinants in establishing maximal aerosol concentrations for use in inhalation toxicity studies with special emphasis on experimental design features to assess lung retention. The subject matter of this paper is a rapidly developing area in terms of knowledge. Accordingly, the contents of this article are intended as guidelines and not as absolute rules for the conduct and interpretation of inhalation exposure studies.

The state of the art for modeling the retention of inhaled insoluble particles deposited in the alveolar region of the lung is briefly reviewed, and a new compartmental model of long-term retention is proposed. Wherever possible, this new model favors the replacement of simple first-order kinetics of particle transport processes in the lung by quantified mechanisms derived from or suggested by experimental data of published studies in lung physiology and histopathology. In particular, all macrophage-mediated transport processes, including classical alveolar clearance onto the mucociliary escalator, are modeled as dependent on actual macrophage mobility and are assumed to be influenced by the finite macrophage life time. The mobility is predicted to decrease with increasing particle burden of the macrophage, and there is a limit to the macrophage capacity for accumulating burdens of insoluble particles by phagocytosis. Furthermore, at high particle burdens, macrophages will be progressivity sequestered by irreversible aggregation and immobilization. Using published data on Fischer 344 rats for a quantitative demonstration of the patterns of the new model under chronic exposures, a basic set of model parameters predicts that, at moderate particle deposition rates, retention is limiting itself by establishing a steady state, and the alveolar burden is almost completely eliminated during the postexposure period. However, at high particle deposition rates, the alveolar particle burden increases continuously during the exposure period, and only a small fraction of the deposit is subject to clearance after termination of exposure. In qualitative terms, these are typical features of the "overload" effect which has been observed in a number of recent chronic aerosol inhalation exposure studies with animals.

Epidemiologic studies have reported an increased incidence of respiratory infections and illness in association with elevated indoor levels of nitrogen dioxide (NO2). Animal exposure studies have found that brief exposures to peak levels of NO2 produce greater morbidity than continuous lower level exposure. In order to examine the effect of NO2 inhalation on human alveolar macrophages, normal volunteers were exposed sequentially to air or NO2, by double-blind randomization, in an environmental chamber. Two exposure protocols with comparable concentration x time products were used: (a) continuous 0.60 ppm NO2 (n = 9), and (b) background 0.05 ppm NO2 with three 15-min peaks of 2.0 ppm (n = 15). Inhalation of NO2 caused no significant changes in pulmonary function or airway reactivity in either exposure protocol. Alveolar macrophages obtained by bronchoalveolar lavage 3 1/2 hr after exposure to continuous 0.60 ppm NO2 tended to inactivate influenza virus in vitro less effectively than cells collected after air exposure (1.96 vs 1.25 log10 plaque-forming units on Day 2 of incubation, P less than 0.07). Four of nine subjects accounted for the observed impairment in virus inactivation; cells from these four subjects demonstrated an increase in interleukin-1 (IL-1) production after NO2 vs air, whereas the five remaining subjects decreased IL-1 production after NO2. In contrast, intermittent peak exposure did not alter the rate of viral inactivation or IL-1 production. This methodology has the potential to identify pollutant effects on mechanisms of respiratory defense in humans.

The aim of the present study was to determine whether the nephrotoxicity of the uranium-containing compound uranyl fluoride (UO2F2) is enhanced after unilateral nephrectomy. Unilaterally nephrectomized (NPX) and sham-operated (SO) rats were given single intravenous injections of UO2F2 at doses delivering 100 or 250 μg U/kg 16 days after surgery. Between the second and third day after the administration of either dose of UO2F2, the urinary excretion of the cellular enzymes lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) and the plasma solute albumin began to increase significantly in both the NPX and SO rats. The urinary excretion of the plasma solute glucose did not begin to increase significantly in the NPX and SO rats until 4 days after the administration of either dose of UO2F2. During the fifth day following the administration of either dose of UO2F2 (which was also the last day that urinary data were collected) the urinary excretion of LDH, AST, and glucose in the NPX and SO rats was greater than that during any previous day. The urinary excretion of these three compounds during this fifth day was greater in the SO rats than in the NPX rats. Also during the fifth day following the injection of either dose of UO2F2, the fractional excretion of glucose was higher in the SO rats than in the NPX rats. By the end of the fifth day, the level of histologically demonstrable cellular necrosis in the pars recta of proximal tubules in the renal cortex and outer medulla of the NPX and SO rats was statistically similar. Therefore, the nephropathy in rats induced by UO2F2 is not made more severe as a result of unilateral nephrectomy.

This study deals with the hypothesis that the lymphatic uptake of particles from the lung parenchyma increases when phagocytosis by pulmonary macrophages is inhibited. Cadmium chloride was chosen as the toxicant to inhibit phagocytosis and was administered as an aerosol to rats at concentrations of 1.5 mg Cd/m3 (mass median aerodynamic diameter = 0.4 micron, sigma g = 1.4) and 5.0 mg Cd/m3 (MMAD = 0.4 micron, sigma g = 1.6), each for 30 min. Control animals were exposed to a saline aerosol. Lung clearance and lymphatic uptake were assayed after exposing the cadmium-exposed rats to titanium dioxide (TiO2) dust at concentrations of 12-15 mg/m3 (MMAD = 1.0 micron, sigma g = 2.3) for 6 h. Preexposure to 5 mg Cd/m3 decreased the initial deposition of TiO2 by 40% compared to a saline preexposure. Although the overall clearance of TiO2 from the lungs was not different in the cadmium-exposed animals, the lymph node burden was 2.7 times higher in the CdCl2-exposed animals than in the controls. Exposures to 1.5 mg Cd/m3 had no effect on lung clearance or lymphatic uptake of TiO2. When TiO2 exposure preceded a 5.0 mg Cd/m3 exposure, the results were similar; i.e., more TiO2 was found in the lymph nodes of the animals. This study supports the concept that lymphatic uptake of dust particles increases when phagocytosis by alveolar macrophages is decreased.

Long-Evans hooded rats were exposed for 2 h to aerosols of hydrated, radiolabeled iron (59Fe) oxide (MMAD = 1.6 micron; sigma g = 3.0) in order to produce a low mass burden of particles (approximately equal to 30 micrograms) in the lung. The kinetics of particle clearance and the association of the particles with alveolar macrophages (AM) were measured. Two to four hours after exposure, lavaged particles were linearly related to AM numbers harvested, and 60% of the 59Fe activity was physically associated with AM. By 24 h, greater than 90% of the lavaged particles were associated with AM. Such an association was found for at least 75% of the particulate burdens in the lungs. If all the 59Fe is assumed to be AM associated, the 59Fe per AM predicts the total AM population size to be 2.14 X 10(7) cells. This number, in conjunction with the alveolar clearance rate of the particles, suggested the number of AM leaving the lung daily was 2.8 X 10(5) cells.

The diagnostic utility of radioaerosols for lung ventilation and lung permeability procedures requires a strong, stable coordinate covalent bond between a radionuclide and a ligand. The stability of the radiopharmaceutical before and after nebulization and hence its molecular size, are an absolute prerequisite for the method to be reproducible. To examine the possible effects of aerosolization on radiopharmaceuticals used for pulmonary imaging, the authors examined the radiochemical purity of Tc-99m DTPA following ultrasonic nebulization, jet nebulization (Dautrebande D-31), and inhalation with subsequent appearance in plasma and urine of dogs. Paper and liquid chromatographic methods were applied to determine radiochemical purity. Chromatographic assays showed a binding efficiency of less than 50% for ultrasonicly aerosolized Tc-99m DTPA. Cooling of the ultrasonic coupling fluid increased the binding efficiency to greater than 95% following nebulization and inhalation. Jet nebulization did not affect the radiochemical purity of the radiopharmaceutical. Ultrasonic nebulization will partially destroy the Tc-99m DTPA complex; cooling the ultrasonic connecting fluid and constant aerosol monitoring are required for consistent, reproducible results.