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Nebulized amphotericin B (AMB) combined with intravenous AMB was studied in persistently leukopenic rats with invasive pulmonary aspergillosis. Pulmonary concentrations of AMB after aerosol treatment were substantially higher than after intravenous liposomal AMB. Nebulized liposomal AMB in addition to intravenous AMB resulted in significantly prolonged survival compared to controls.
Amphotericin B (AMB) deoxycholate (AMB-DOC) and liposomal AMB remain important drugs for the treatment of invasive fungal infections despite the fact that new nonpolyene antifungal agents have been developed and are now frequently used (1, 5, 6). Treatment of an established pulmonary Aspergillus infection in patients with prolonged persistent leukopenia is, despite the use of potent antifungal drugs, correlated with high failure rates (1). Therefore, there is still a critical need to optimize treatment of invasive aspergillosis. Failure of treatment of invasive pulmonary aspergillosis with intravenous AMB is probably, in part, the result of inadequate pulmonary deposition of AMB after intravenous administration. It has been shown that only a small percentage of intravenously administered AMB is actually delivered to the lungs (7, 14). As shown in a rat model of invasive pulmonary aspergillosis (IPA), with administration of drugs via inhalation, the lungs are directly targeted, which results in high, long-lasting pulmonary drug concentrations (10, 11). The hypothesis of the present study is that aerosol treatment in addition to intravenous treatment can result in higher success rates in terms of improvement of survival in experimental IPA.
Aerosol generation and treatment of animals with nebulized liposomal AMB (L-AMB) was previously described (12, 13). The experimental Aspergillus fumigatus infection was previously described (2, 9). For biodistribution experiments, groups of 3 animals were euthanized with intravenous pentobarbital (100 mg/kg of body weight) at 30 min after aerosol treatment. AMB in blood and tissues was determined by high-performance liquid chromatography (11). The lower limit of quantification of this assay was 0.2 mg/ml. The concentration of AMB in lung tissue at 30 min after nebulization of liposomal AMB was 46.7 ± 10.5 μg/g. This concentration exceeds by far the MIC for Aspergillus fumigatus, which is 0.4 to 0.8 mg/liter for sensitive strains. It also exceeds that found after intravenous administration of a single dose of 10 mg/kg liposomal AMB (16.4 ± 2.4 μg/g). In infected lung tissue, the concentration of AMB at 30 min after nebulization was 36.7 ± 5.90 μg/g and did not differ statistically significantly from that in uninfected lungs. No AMB was detected in blood or organs other than lungs after aerosol administration. From the point of toxicity, this is a major advantage. However, Aspergillus usually disseminates from the lungs to other organs, and pulmonary administered AMB will not be able to prevent this. It is our opinion that optimal therapy of an established infection of invasive pulmonary aspergillosis therefore combines the pulmonary route with intravenous administration of adequate dosages of AMB-DOC or liposomal AMB.
Colloidal gold-labeled liposomes were utilized as histochemical markers of liposome tissue deposition and cellular uptake (Fig. (Fig.1).1). Colloidal gold-labeled liposomes were prepared as described previously by Daemen et al. (4). The lipid film consisted of the same lipids as the liposomal membrane of liposomal AMB in the same molar ratio (2:1:0.8 hydrogenated soy phosphatidyl choline/distearoyl phosphatidyl glycerol/cholesterol). This method yielded liposomes with an average particle size of 100 nm. Colloidal gold-labeled liposomes were nebulized at 30 h after fungal inoculation. Directly after nebulization, lung lobes were excised, washed, and fixed as described by Schiffelers (13). With this method, intact liposomes and uptake of liposomes by macrophages could be visualized. The black clusters are silver-enhanced colloidal gold-labeled liposomes. The liposomes are deposited not only at the pulmonary epithelia of bronchioli but also in the alveolar regions of the lungs (Fig. (Fig.1a).1a). Liposomes deposited in the alveolar sacs and ducts are partially internalized by alveolar macrophages (Fig. (Fig.1b).1b). Grocott methenamine staining of a section of infected left lung lobes at 30 h after inoculation of 1.5 × 105 Aspergillus conidia shows that radially grown hyphae are formed out of conidia (Fig. (Fig.1c).1c). Combined hematoxylin-eosin staining and silver enhancement of the adjacent section shows that nebulized liposomes are deposited close to the Aspergillus hyphae (Fig. (Fig.1d).1d). The deposition pattern of colloidal gold-labeled liposomes indicated that nebulized liposomal AMB was able to penetrate deep in the lower respiratory tract, since the membrane characteristics and particle sizes of both liposomal products were similar. Our data show that the liposomal formulation reached the infected regions of the lung, was deposited close to the mycelium of the fungus, and was furthermore partially incorporated by alveolar macrophages. Uptake of liposomes by alveolar macrophages is believed to be succeeded by release of the drug (8).
For efficacy experiments, treatment was started at the start of mycelial outgrowth, at 16 h after fungal inoculation. Groups of 15 animals received monotherapy or combination therapy (intravenous plus aerosol treatment). The intravenous regimens previously showed optimal efficacy in the treatment of IPA in rats (3, 9). The efficacy endpoints were survival (Fig. (Fig.2)2) and dissemination of infection to the liver. The control group did not receive any treatment. Treatment with a high dose of intravenous liposomal AMB (10 mg/kg daily) or intravenous AMB-DOC (1 mg/kg daily) resulted in a significantly prolonged survival compared to controls (P = 0.0001). Addition of a single dose of nebulized liposomal AMB to this regimen improved survival compared to the intravenous regimen. Treatment of animals with monotherapy of nebulized liposomal AMB was as effective as the intravenous treatments. The effect of the different treatment regimens on dissemination (expressed as percentage of culture-positive organs) is shown in Table Table1.1. Cultures revealed that, at the time of death, the infection had disseminated to the liver of all untreated rats. Monotherapy with intravenous AMB-DOC, liposomal AMB, or a combination of intravenous with aerosol treatment resulted in a reduction of dissemination to the liver. Monotherapy with nebulized liposomal AMB did not prevent dissemination to the liver.
The combination of systemic and local administration of AMB was superior to that of either treatment alone. Intravenous treatment was crucial for preventing dissemination of the infection from the lungs. This finding is clinically relevant, as current treatment strategies for aspergillosis in leukopenic patients are still disappointing and there is a critical need for new antifungal therapies which are both effective and have little toxicity. We suggest that liposomal AMB delivered via the pulmonary route, in addition to the current standard of care (intravenous administration of AMB), can be a major step forward in the optimization of treatment of invasive pulmonary aspergillosis.