Research paperInhalation treatment of pulmonary fibrosis by liposomal prostaglandin E2
Graphical abstract
Introduction
Idiopathic pulmonary fibrosis (IPF), a chronic, progressive, and often fatal form of interstitial lung disease, is the most common form of idiopathic interstitial pneumonia [1]. IPF causes the loss of lung epithelial cells, accumulation of fibroblasts and myofibroblasts with replacement of normal functional tissue, extracellular matrix deposition, and alteration of lung architecture. These alterations and pulmonary hypertension lead to substantial impairment of respiration and gas exchange often resulting in patient morbidity and mortality [2], [3]. Treatment of IPF represents a major clinical challenge since this disorder does not have reliable therapeutic options and an effective therapy has yet to be identified and developed [2], [4], [5]. Patients may ultimately require supportive oxygen therapy or pulmonary transplantation. Consequently, the development of a novel effective treatment for this devastating disease is urgently needed.
Prostaglandin E2 (PGE2), a cyclooxygenase-derived lipid mediator, has attracted considerable attention for its role in the development and progression of IPF and as a possible therapeutic agent for this disease. A role for PGE2 in the treatment of IPF is based on the very specific and unique role that PGE2 plays in the lungs making “the lung as a privileged site for the beneficial actions of PGE2” [6]. In other organs and tissues, PGE2 often acts as a potent pro-inflammatory mediator and is involved in pathogenesis of many inflammatory diseases. In contrast, in the lungs, PGE2 limits the immune-inflammatory response and inhibits specific lung fibroblast functions, their proliferation, and synthesis of matrix proteins such as collagen. Consequently, PGE2 potentially can be used for the treatment of IPF [6], [7], [8], [9], [10]. Moreover, a synthetic analog of PGE2 (16,16-dimethyl-PGE2) recently was tested using a model of pulmonary fibrosis (intratracheal administration of bleomycin) with promising results for treatment of IPF [11].
Systemic delivery of PGE2 has several limitations including its short half-life in the blood stream, low accumulation in the lungs, and possible adverse side effects on other organs and tissues. In contrast, local inhalation delivery of PGE2 directly to the lungs has the potential to enhance the treatment of IPF by increasing its local pulmonary concentration and preventing (or at least limiting) its penetration into the bloodstream and distribution to other healthy organs. However, free native PGE2 cannot be delivered into the lungs by inhalation necessitating a special dosage form or delivery system that can be inhaled. We have shown that liposomes and some other nanoscale-based particles can be used for local inhalation delivery of drugs, antisense oligonucleotides and siRNA, vitamins, and imaging agents [12], [13], [14], [15]. It was observed that liposomes remain in the lungs after the inhalation delivery, limiting penetration of the payload into the blood stream and accumulation in other organs. Therefore, we hypothesize that the local pulmonary delivery of liposomes containing PGE2 can be used for the effective treatment of IPF and will limit its adverse side effects on other organs. To test this hypothesis, we used a standard bleomycin-induced murine model of IPF [16], [17], [18], [19], [20] to evaluate a liposomal drug delivery system, which delivers PGE2 topically to the lungs.
Section snippets
Materials
Egg phosphatidylcholine and cholesterol were purchased from Avanti Polar Lipids (Alabaster, AL, USA). PGE2 was obtained from Apichem Chemical Technology Co., Ltd. (Shanghai, China), and bleomycin was purchased from Sigma Aldrich (Ronkonkoma, NY, USA). Hairless SKH1 mice, 6–8 weeks old, were purchased from Charles River Laboratories (Wilmington, MA, USA).
Liposomal composition of PGE2
Liposomes were prepared as previously described [13], [14], [21], [22], [23]. Briefly, PGE2-loaded liposomes were prepared from egg
Selection of bleomycin dose
In order to select an appropriate dose of bleomycin, four doses (0.5; 1.0; 1.5; 2.0 U/kg) were tested. Bleomycin was instilled intratracheally, and mice were observed for 21 days after the instillation. The dose of 2.0 U/kg led to the death of 100% of animals within 21 days (Fig. 1A). The doses of 1.5 and 1.0 U/kg induced death of 50% and 25% of animals, respectively. The lowest tested dose (0.5 U/kg) did not induce animal death. Based on these results, 1.5 U/kg dose of bleomycin was selected for the
Discussion
The present study shows that intratracheal instillation of bleomycin at dose of 1.5 U/kg induces extensive lung fibrosis. As expected, the development of fibrosis was initiated by a marked pulmonary inflammation with subsequent transition into fibrosis. The sequence of the process was confirmed by morphological features of inflammation and overexpression of several genes involved in the development of inflammation. In fact, several chemokines, inflammatory cytokines, and interleukins were
Acknowledgements
The research was supported in part by NIH R01 CA111766 and P-30 ES-005022 Grants. We thank Valentin Starovoytov for his help with obtaining and providing analysis of transmission electron microscopy images.
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