Pulmonary diseases, such as chronic obstructive pulmonary disease (COPD) and asthma, are complex human airway diseases, which affect millions of people worldwide. Despite their complexity, it is well understood that human airway diseases are often associated with local (lung) inflammation. The incidence of pulmonary diseases appears to be growing worldwide. For example, according to a report from the US Centers for Disease Control and Prevention (CDC), greater than 6% of total American population suffered from asthma in 2004, up from a little over 3% in 1980. For patients with asthma, inhaled corticosteroids (ICS) are often prescribed as first-line therapy to control symptoms, improve lung function, and reduce morbidity and mortality . Among these patients with asthma, 5–10% are characterized as having severe disease that do not adequately respond to current therapeutic options, in part because of side effects associated with elevated doses and/or a plateau in dose response. Treatment options for severe asthma are oral steroids (e.g., prednisone) or a high dose of an ICS. However, long-term use of oral steroids or high-dose ICS therapy has the potential to cause a number of severe side effects, including impaired growth in children, decreased bone mineral density, cataracts, skin thinning and bruising, altered glucose metabolism, and hypothalamic–pituitary–adrenal (HPA) axis suppression [1–5]. Numerous studies have demonstrated that the side effects of glucocorticoid therapy for human airway diseases are related to systemic exposure. Most importantly, side effects are mediated by the glucocorticoid receptor in both the lung and systemic tissues [6–9]. Because of this, pulmonary targeting, such as inhaled delivery, is believed to provide an advantage over systemically administered compound (IV or oral) because the same degree of efficacy may be achieved using a lower dose of inhaled drug. However, despite the success of using an inhaler for pulmonary administration, similar side effects still remain for ICS, especially when the doses are escalated. This raises a question with regard to what portion of the efficacy observed with inhaled ICS is related to local pulmonary exposure and what portion of the efficacy is from systemic exposure. Thus, improved discernment of pulmonary vs. systemic efficacy remains a key element to the development of new drugs with better safety profiles.
One key concept for reducing systemic side effects via pulmonary drug delivery is to select drug candidates with prolonged pulmonary efficacy and minimal systemic exposure [7, 9]. It is believed that a drug with durable, pulmonary-targeted activity, and low systemic exposure would have a theoretical advantage over currently marketed therapies. Pharmacokinetic/pharmacodynamic (PK/PD) modeling suggested that pulmonary targeting might be achievable via modification of the pharmacokinetic profile . Pharmacokinetic (PK) parameters such as long lung retention, high lung deposition, high receptor binding, and high lipophilicity have been sought to improve or maintain the pulmonary-targeted efficacy . In addition, appropriate physicochemical properties (i.e., dissolution rate, solid state form), particle size, and formulation can be utilized to further optimize the PK profile. Drugs with the aforementioned profile should provide the benefit of greater pulmonary exposures with reduced systemic exposure, ultimately resulting in an improved therapeutic index, assuming that efficacy is not driven by systemic drug exposure [10, 12, 13].
Despite an understanding of what is needed, a major hurdle for pulmonary drug discovery is to assess therapeutic index (topic effects vs. systemic effects) with an appropriate preclinical animal model(s). To date, appropriate animal models have not been fully characterized in the literature. In an attempt to characterize pulmonary vs. systemic side effects preclinically, we chose the acute lipopolysaccharide (LPS)-induced inflammation model in rats as an efficacy model and also to set doses for multiple-dose side effect studies. This model utilizes the recruitment and activation of neutrophils into bronchial alveolar lavage fluid (BALF) as the efficacy endpoint. This model was selected for the study because it provides two distinctive advantages. First, this acute animal model has been well studied by researchers and used to mimic human pulmonary inflammation [14–18]. Secondly, compared with other animal models such as the mouse ovalbumin model, the rat LPS model offers the advantage of serial blood sampling and more precise delivery of drug into the lung via intratracheal dosing. We chose two ICS compounds to evaluate in this rat LPS model—fluticasone propionate and ciclesonide.
Fluticasone is a highly potent anti-inflammatory drug that is the most commonly prescribed inhaled glucocorticoid. It is one of the available ICS with a good combination of PK and PD properties. It has high receptor binding affinity, high clearance (~liver blood flow), poor bioavailability (<1%), high protein binding, and it has been used effectively at low to medium doses to treat patients with mild and moderate asthma. However, fluticasone is associated with adverse systemic effects at high doses and is therefore administered twice daily.
Ciclesonide has been reported to have similar efficacy to fluticasone but fewer side effects due to its special drug design. Ciclesonide is a prodrug that is converted to an active metabolite, desisobutyryl-ciclesonide (des-CIC), in pulmonary airways by endogenous esterases. This onsite activation reduces oropharyngeal exposure and subsequent side effects. In radioligand binding assays, des-CIC and fluticasone exhibited similar high-affinity binding to the glucocorticoid receptor, whereas ciclesonide exhibited 100-fold less binding affinity than fluticasone . Furthermore, des-CIC undergoes reversible esterification to fatty acid conjugates in the lung. These conjugates slowly re-release des-CIC and act to greatly enhance lung retention which should provide more topical efficacy and less systemic side effects. Once in the systemic circulation, des-CIC is rapidly metabolized by P-450 enzymes, mainly CYP3A4 . It has been claimed that ciclesonide has a better safety profile compared to other ICS. For example, Belvisi et al.  showed that in preclinical models of antigen-induced airway eosinophilia and Sephadex-induced lung edema using Brown Norway rats, ciclesonide showed comparable efficacy with fluticasone, although ciclesonide was 7–9-fold less potent in terms of ED50. In a subsequent 7-day side effect study with Sprague–Dawley rats, ciclesonide was 44-fold less potent at inducing adrenal involution, sixfold less potent at inducing thymus involution, and 22-fold less potent at decreasing bone growth than fluticasone .
The goal of the present study is to evaluate whether efficacy and side effect profiles can be differentiated preclinically in conjunction with systemic exposure by utilizing nanosuspension formulation. These findings will help to determine whether a simple and robust preclinical model can be established that is useful for screening of new ICS drug candidates. A nanosuspension drug delivery formulation was used to administer fluticasone and ciclesonide intratracheally (IT) to rats. Recently, utilization of nano drug delivery for both efficacy and safety evaluation has drawn lots of attentions from researchers, and its advantages were widely accepted by industry [20–26]. In our study, the acute LPS rat model was used to establish the dose–response curves for efficacy. Based on these data, doses were picked for 6-day repeat dose studies for the evaluation of the side effects. Adrenal and thymus involution as well as lung and heart tissue receptor occupancy was measured to assess side effects. Corticosterone levels in whole blood were measured for biomarker evaluation .