We demonstrate for the first time in the current study that inhaled doxycycline, in an adequate formulation with HP-gamma-CD, inhibits the allergen-induced airway reactivity, eosinophilic infiltration in BAL compartment and in bronchial walls, and peribronchial inflammation. Furthermore, we provide evidence for the first time that inhaled doxycycline prevents allergen-induced bronchial morphological changes and thus inhibits glandular hyperplasia, airway wall and smooth muscle layer thickening and peribronchial collagen deposition. Effects of doxycycline on tissue remodeling were comparable to those obtained after exposure to fluticasone, an inhaled steroid on the market used in the current study as a reference therapy. Mechanisms of pharmacological activity of doxycycline in the context of bronchial inflammation were investigated and the main findings are a doxycycline-induced MMP inhibition and impairment of Th2 inflammation. Indeed, doxycycline exposure leads to a significant increase in the anti-inflammatory cytokine IL-10 and a decrease of IL-5 and IL-13 levels in the lung parenchyma. Doxycycline also reduced MMP-9 activation in the lungs from mice exposed to allergens.
In line with our previous works demonstrating that MMPs and more specifically MMP-9 play a critical role in allergen-induced airway inflammation [7], [12], [13], [15] and [31], we designed the present work as an attempt to control airway inflammation and remodeling by a molecule known to be a potent MMP inhibitor. Doxycycline was selected because its MMP inhibitory activity was confirmed by numerous studies and because this molecule was shown to have a very low toxicity in human adults. To our knowledge, this is the first demonstration that an inhaled non-steroidal drug (doxycycline) exerts an effect on both airway inflammation and airway remodeling features such as Goblet cells hyperplasia, collagen deposition and basement membrane and smooth muscle cells layer thickness. Other researchers have published an interesting report on the inhibition of toluene diisocyanate-induced airway inflammation by doxycycline [32]. In that paper, however, the drug was given orally at high dosages, higher than those currently recommended for antimicrobial therapy and remodeling parameters were not assessed. In the present study, mice were challenged with the allergen by an exposure to a new formulation given by aerosols and not by intratracheal instillation. Our model may therefore mimic more closely the clinical situation by the use of a well-characterized model of allergen-induced inflammation and airway remodeling in mice. By using a topical therapeutical approach allowed by an original galenic formulation, our study clearly shows that an inhaled doxycycline-based therapy for asthma is of potential interest and is worth to be confirmed in other animal models and possibly in humans.
Doxycycline in solution without excipient has a very low pH and accidental tetracycline inhalation has been reported to be responsible for toxic pneumonia [33]. In addition, doxycycline is only poorly soluble at pH 7.4 and is not stable with a tendency to precipitate rapidly. We therefore used cyclodextrins (HP-gamma-CD) for increasing the solubility and stability of doxycycline at physiological pH. We did previously describe that inhaled HP-gamma-CD was non-toxic and did not provoke by itself some modifications of the airway architecture or induce inflammation [29]. The selection of this specific cyclodextrin is based on its higher potency to increase doxycycline solubility as compared to different related compounds (data not shown), and mainly on its safety profile [29]. In the present study, we demonstrate that HP-gamma-CD does not interfere with the inflammatory response and does not bear per se any pharmacological activity when used in a topical lung application in an inflammatory airway disease. Furthermore, we demonstrated previously that HP-gamma-CD does not modify the aerosol characteristics in terms of droplet size and nebulization speed in our experimental conditions [29].
To date, there are still controversies about the exact mechanism for MMP inhibition by tetracyclines. In the present study, we investigated whether the topical exposure by inhalation to doxycycline modulated the expression of key Th2 and anti-inflammatory cytokines and we found IL-5 and IL-13 production being decreased and, interestingly, a large increase in the anti-inflammatory cytokine IL-10 production. To our knowledge, this is the first report of a decreased production of the Th2 cytokines IL-5 and IL-13 after tetracycline exposure. Due to the properties of those two cytokines, this observation could explain at least in part the decreased airway responsiveness, reduced eosinophilic inflammation and airway remodeling prevention [5], [6] and [34]. To date, the precise mechanism of action of the inhaled steroid fluticasone leading to control of inflammation is also not really well understood. However, our result suggests different mechanism of action between fluticasone and Doxycycline regarding the differences found in their respective effects on IL-10 production. The finding of an increased IL-10 production is of great interest since a relative IL-10 deficiency could favour asthma. Indeed, the number of IL-10-producing T cells has been reported to be lower in severe asthmatics when compared to milder patients [35] and IL-10 production is increased in the BAL cells of mice exposed to an immunotherapy protocol [36]. IL-10 has also been shown to decrease eosinophil survival and cytokine secretion [37] and could even mediate a part of the anti-inflammatory effects of steroid [38]. Finally, IL-10 interacts with the MMP metabolism since it inhibits MMP-2 and -14 transcription in some cancer cell lines [39] and [40]. Taken together, these data suggest that doxycycline-induced IL-10 production could inhibit the allergen-induced airway inflammation and airway remodeling (collagen deposition and membrane thickening) both indirectly by diminishing the venue of inflammatory cells and directly by inhibiting MMP transcription. Altogether, our data about cytokine measurements suggest an inhibition of Th2 inflammation and a possible stimulation of T regulatory lymphocytes (Treg) by doxycycline inhalation [32].
Doxycycline decreases the inducible nitric oxide synthase (NOS) activity as demonstrated in vitro [41] and in an animal model of LPS exposure [42]. As a correlation has been found between nitric oxide production and the extent of airway remodeling in humans [43], another possible explanation of our findings could reside in an inhibition of nitric oxide synthesis by tetracyclines.
Nevertheless, direct MMP inhibition by doxycycline is thought to be the main cause of many pharmacological effects of tetracyclines. Indeed, doxycycline has been proposed as a therapy to reduce MMP activation and expression (particularly MMP-2 and -9) in abdominal aortic aneurysms [44] and has been reported to decrease pathological remodeling after myocardial infarction in rat [45]. Doxycycline has the ability to inhibit MMPs belonging to the gelatinase subfamily [17] and [46] but this compound also inhibit other MMPs as MMP-3 and MMP-7 [47]. Moreover, doxycycline has been described to have putative inhibitory effects on members of collagenase subfamily of MMPs. Indeed, doxycycline is quite effective on MMP-8 and MMP-13 inhibition and, in a lesser extent, on MMP-1 [19] and [48]. In the present study, we can not rule out, however, that an inhibition of non-gelatinase members of the MMP family could cause some side effects. In the present work, mice exposed to doxycycline for up to 96 days did not display any sign of exaggerated inflammation or any visible toxicity. In the present study we demonstrate that, after doxycycline exposure, global MMP activity in lung tissue is decreased and that the proportion of activated MMP-9 is lower than measured in vehicle-exposed mice. This strongly suggests that, in our study, a significant part of the pharmacological effect obtained with doxycycline is linked to a direct MMP inhibition as the central role of MMP-9 in the allergen-induced inflammation has been described elsewhere using the same protocol of allergen immunisation and exposure.
In conclusion, the administration of doxycycline by aerosols in a complex with HP-gamma-CD prevents the allergen-induced airway inflammation and remodeling by modulation of both MMPs and interleukins. Doxycycline should be considered for other studies designed to confirm its disease modifying agent role in asthma. Our results suggest that inhaled doxycycline should be investigated as a potential new therapy for asthma.