Quantitative imaging of airway liquid absorption in cystic fibrosis

Abstract

New measures are needed to rapidly assess emerging treatments for cystic fibrosis (CF) lung disease. Using an imaging approach, we evaluated the absorptive clearance of the radiolabeled small molecule probe diethylene triamine penta-acetic acid (DTPA) as an in vivo indicator of changes in airway liquid absorption. DTPA absorption and mucociliary clearance rates were measured in 21 patients with CF (12 adults and nine children) and nine adult controls using nuclear imaging. The effect of hypertonic saline on DTPA absorption was also studied. In addition, in vitro studies were conducted to identify the determinants of transepithelial DTPA absorption. CF patients had significantly increased rates of DTPA absorption compared with control subjects but had similar mucociliary clearance rates. Treatment with hypertonic saline resulted in a decrease in DTPA absorption and an increase in mucociliary clearance in 11 out of 11 adult CF patients compared with treatment with isotonic saline. In vitro studies revealed that ∼ 50% of DTPA absorption can be attributed to transepithelial fluid transport. Apically applied mucus impedes liquid and DTPA absorption. However, mucus effects become negligible in the presence of an osmotic stimulus. Functional imaging of DTPA absorption provides a quantifiable marker of immediate response to treatments that promote airway surface liquid hydration.

Trial registration: ClinicalTrials.gov NCT01223183 NCT01486199.

FIGURE 1

Normalised right lung retention of technicium-99m–sulfur colloid (SC) and indium-111–diethylene triamine penta-acetic acid (DTPA) plotted against time for the a) adult cystic fibrosis (CF) group (n=12), b) paediatric CF group (n=9), c) adult CF group with hypertonic saline treatment (n=11) and d) control group (n=9). Data are presented as mean ± SEM.

FIGURE 2

Comparison of mean ± SEM normalised technetium-99m–sulfur colloid (SC) retention curves in healthy volunteers (n=9) and adult cystic fibrosis (CF) (n=12) subjects following nebulised isotonic saline treatment. The difference in per cent clearance at 80 min did not reach statistical significance (p=0.14).

FIGURE 3

a) Mucociliary clearance (MCC), b) total diethylene triamine penta-acetic acid (DTPA) clearance and c) DTPA absorption (ABS) in healthy volunteers (n=9), and adult (n=12) and paediatric cystic fibrosis (CF) (n=9) subjects following nebulised isotonic saline treatment. Horizontal lines represent the mean clearance rate for each study group. Open circles represent G551D subjects utilizing ivacaftor (not included in statistical analysis).

FIGURE 4

The effects of nebulised hypertonic saline (HS) treatment in individual cystic fibrosis adult subjects. a) Indium-111–diethylene triamine penta-acetic acid (DTPA) absorption (ABS) was decreased in every subject after inhalation of HS treatment. b) Technetium-99m–sulfur colloid clearance was increased by HS treatment in 11 out of 11 subjects. c) Total ¹¹¹In-DTPA clearance was increased by HS treatment in six out of 11 subjects (p=0.118). Paired t-test was used. Data are presented as mean ± SEM. IS: isotonic saline; MCC: mucociliary clearance. ***: p<0.001 versus IS; ^#: p=0.003 versus IS.

FIGURE 5

Comparison of airway surface liquid (ASL) volume and diethylene triamine penta-acetic acid (DTPA) absorption after addition of isotonic saline versus iso-osmolar solution of mannitol in water. a) ASL volume following apical volume expansion with either 10 μL isotonic saline (control) or 300 mM mannitol in water on differentiated cystic fibrosis human bronchial epithelial cell cultures. b) The per cent ASL absorption rate from 2 to 12 h after apical addition of isotonic saline or mannitol in water. c) The per cent ^99mTc-DTPA absorption rate over the same 10-h period after apical addition of isotonic saline or mannitol in water. Data are presented as mean ± SEM of 18 cultures from three different tissue donors. *: p<0.05 versus control; **: p<0.01 versus control.

FIGURE 6

The effect of accumulated mucus on airway surface liquid (ASL) and diethylene triamine penta-acetic acid (DTPA) absorption in cystic fibrosis (CF) human bronchial epithelial (HBE) cell cultures. a) ASL volume absorption represented as a per cent change at 12 h in the absence or presence of mucus (10 μL, sampled from non-CF HBE cells), hyperosmotic basolateral growth medium (normal medium plus 300 mM mannitol) or the combination of both. b) ^99mTc-DTPA absorption represented as a per cent change at 12 h in the absence or presence of mucus, hyperosmotic basolateral media or both. Data are presented as mean ± SEM of 12 cultures from two different tissue donors. ***: p<0.001 versus control.