Introduction
Human airway surface liquid (ASL) contains many antimicrobial substances, including lysozyme, lactoferrin, and β defensins that are salt-sensitive. An increase in salt concentration inhibits the antibacterial activity of these substances. Conversely, they are more potent at lower salt concentrations [
1‐
4].
Xylitol is a five-carbon sugar that is used as a nutritive sweetener. When added to the apical surface of airway epithelia, it can lower the ASL salt concentration, resulting in enhanced antimicrobial properties. Using a radiotracer method, we found that xylitol has low permeability across an
in vitro model of well-differentiated human airway epithelia. Following addition to the apical surface, the amount of xylitol in the ASL decreased progressively; after 8–12 hours, only 50% of the applied sugar had diffused to the basolateral surface [
5]. We recently tested the safety of aerosolized xylitol in normal volunteers. All subjects tolerated the exposures well without any significant change in Forced Expiratory Volume (FEV) 1, or laboratory parameters [
6].
The main aim of this study is to assess the rate at which xylitol disappears from the ASL. It is difficult to measure the actual salt concentration in the ASL because collecting the fluid induces instantaneous changes in its composition [
7]. Currently, the most widely used method for sampling ASL is bronchoalveolar lavage (BAL); however, BAL has limitations. First, it requires instillation of normal saline into lung segments, resulting in enormous dilution of the ASL. Second, there is a highly variable return of the instilled liquid. Third, the relative contribution of airway surface is insignificant compared to the alveolar surface sampled by BAL. This results in underrepresentation of the airway component when sampling ASL. Recently, a new method for sampling human airway surface liquid using a bronchoscopic microsampling (BMS) probe was reported by Ishizaka et al [
8]. This method has been used to determine the ASL concentration of Levaquin after oral administration [
9]. We describe the results of xylitol concentration in ASL obtained using a microsampling probe after aerosolization and compare it with the traditional BAL sampling.
Discussion
In this study, we evaluated the airway retention time of aerosolized xylitol using two methods. We previously reported the safety of aerosolized xylitol in normal volunteers. We now show that xylitol can be deposited in conducting airways after nebulization. After a single dose nebulization, xylitol was detected in ASL for at least 3 hours. This retention time is shorter than our in vitro data using the same detection method; however, the prolonged nebulization time (mean of 48 minutes) and the constant airway exposure during that time have to be considered when interpreting these data. Further, we found that the concentration was higher in the airways as sampled by the probe compared to alveoli plus airways as sampled by BAL.
ASL collection using a microsampling probe may prove valuable as a research tool and possibly aid in patient care. We found it safe and easy to use. BAL mostly samples alveolar fluid and, hence, significantly underrepresents concentration of products in the airway. The probe, in contrast, collects fluid directly from the airways without much dilution by alveolar liquid. A limitation of the probe is that it most likely collects volume of liquid in excess of the actual ASL by capillary action, drawing liquid from the mucosa and submucosa [
16]. It may also stimulate submucosal gland secretion. Currently, however, there is no accurate method of ASL collection without altering its composition.
Not surprisingly, xylitol concentration was higher using BMS probe sampling compared to BAL, which is expected given the inhaled route of administration and specific sampling of ASL without any dilution by alveolar compartment. This is in contrast to the previous study using a microsampling probe and BAL, where Levaquin concentrations obtained from BAL were twice as high as that from the airway probe after oral dosing [
9].
As to mechansims of clearance of xylitol from the airways, there are several possibilities, including mucociliary clearance, exhalation during tidal breathing, diffusion across the airway epithelia, and drug metabolism. Our data does not favor a particular mechanism. The airway retention time was significantly longer in vitro than in vivo. In our in vitro experiments, there are only airway epithelial cells without mucociliary clearance and relatively large volumes of xylitol were added to the apical surface of respiratory epithelim, which may have prolonged the retention time by adding to the distance of diffusion.
It was interesting to note that the ASL volume retrieved by the probes was higher after 3 hours compared to earlier time points. One possible explanation is the learning curve with the use of the microsampling probe to collect ASL. In this study, however, we recruited and completed the study on subjects assigned to the 3-hour time point before the 1.5-hour group. One of the possibilities for the increase in ASL volume at 3 hours is an osmotic effect of xylitol, resulting in dilution of ASL.
In the past, xylitol was used as an intravenous nutrition in doses as high as 0.25 gm/kg/hr [
17]. After intravenous use, the half-life of xylitol is about 20 minutes in humans [
18]. Exogenous xylitol is rapidly oxidized in the liver by NAD-linked polyol dehydrogenase into xylulose, which is then is phosphorylated and eventually metabolized by glycolysis or gluconeogenesis [
19]. Therefore, to be effective in the lower respiratory tract, xylitol must be administered via aerosol route.
To our knowledge, this is the first study to assess airway deposition and retention time of aerosolized xylitol. A few limitations must be acknowledged. First, this study was done in healthy volunteers. In patients with lung disease, such as cystic fibrosis and those who are critically ill, the airway retention time may be different due to difference in epithelial integrity and permeability. Second, we did not study time points beyond 3 hours after nebulization. Third, because of the prolonged nebulization time and the preparation time for bronchoscopy, we were unable to assess the early pharmacokinetics of aerosolized xylitol. Fourth, we did not study clearance in proximal airway segments such as trachea or main stem bronchi. Finally, for safety and comfort reasons, ASL samples were collected from different sets of volunteers at different time points, which may have contributed to intersubject variability of the clearance estimates.
In conclusion, the MRT of aerosolized xylitol was greater than 1 hour. Aerosolized xylitol may be effective by transiently enhancing the innate immunity of the ASL and maintaining a sterile lung compartment, and, thus, prevent colonization in patients who are ventilated and in subjects with cystic fibrosis.
Acknowledgements
We thank Thomas Recker and Abby Fessler for assistance with laboratory procedures, the staff of the General Clinical Research Center (RR00059) and Bronchoscopy Lab for help with the human volunteer study, the volunteers, Jamie Kesselring for assistance with manuscript preparation, Philip Karp, Tamara Nesselhauf, Pamella Hughes, and Tom Moninger from the In Vitro Models and Cell Culture Core [supported by the National Heart, Lung and Blood Institute, the Cystic Fibrosis Foundation, and the National Institutes of Diabetes and Digestive and Kidney Diseases (DK54759)], funded in part by the RDP (R458), and the SCOR grant from the NIH (HL61234), and the support of the NIH K12 RR017700.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
All authors read and approved the final manuscript.