Examining sputum from patients with chronic obstructive pulmonary disease may help predict the course of the disease.
A mass spectrometric panel of biomarkers related to mucus hydration and inflammation examined in sputa showed elevated levels of metabolites from multiple pathways in patients with COPD. These correlated with sputum neutrophil counts and COPD exacerbations. In particular, sialic acid and hypoxanthine concentrations were strongly associated with disease severity, according to a study reported in the journal CHEST® authored by Charles R. Esther Jr. MD, PhD, and colleagues.
Given that an improved understanding of the pathways associated with airway pathophysiology in COPD will identify new predictive biomarkers and novel therapeutic targets, Dr. Esther and colleagues posed the question: Which physiologic pathways are altered and predict exacerbations in the airways of subjects with COPD?
They noted that in persons with COPD – characterized by dominant small airway obstruction associated with airway inflammation – multiple inflammatory pathways, as well as indices of oxidative stress (including oxidized glutathione and 8-isoprostane), are elevated in sputum. Because inflammation is a challenging therapeutic target, identification of other biologic pathways involved in COPD pathogenesis could point to novel biomarkers and therapeutic targets.
Using this approach in cystic fibrosis (CF), the authors have previously identified small molecule metabolites correlated with airway inflammation. Findings from that research supported development of a mass spectrometric biomarker panel for simultaneous measurement of inflammatory markers coupled to biomarkers of mucus hydration. The researchers applied this technology to sputum supernatants collected through the Subpopulations and Intermediate Outcome Measures in COPD Study (SPIROMICS), which included subjects with COPD, as well as relevant smoking and nonsmoking controls.
Addressing inflammation
“Inhaled steroids are really more effective for allergic inflammation as in asthma and less so for the neutrophilic inflammation that dominates in COPD. The challenge is that neutrophilic inflammation is also a key response to infection, and it’s really hard to find an anti-inflammatory that suppresses neutrophilic inflammation well enough to get clinical benefit but not so much that the patient becomes vulnerable to infection. Lots of clinical trials of anti-inflammatories in cystic fibrosis or COPD have been stopped because treated subjects had more trouble with infection,” Dr. Esther stated in an interview,
The investigators analyzed cell-free sputum supernatants from 980 subjects, including samples from 77 healthy nonsmokers (NS), 341 ever-smokers with preserved spirometry (SPS), and 562 subjects with COPD (178 GOLD [Global Initiative for Chronic Obstructive Lung Disease]1, 303 GOLD 2, and 81 GOLD 3). Among the subjects with COPD, elevated biomarkers from multiple pathways correlated with sputum neutrophil counts.
The most significant analytes (at FDR [False Discovery Rate] 0.1) were sialic acid (a mucin marker), hypoxanthine, xanthine, methylthioadenosine, adenine, and glutathione, with sialic acid and hypoxanthine strongly associated with measures of disease severity. Elevation of sialic acid and hypoxanthine were associated with shorter time to exacerbation and improved prediction models of future exacerbations.
Study results
Sialic acid was elevated in all GOLD groups relative to NS healthy controls, with a 2.8-fold (0.44 log) increase in GOLD 2 and 3.7 fold (0.56 log) increase in GOLD 3 relative to NS. Sialic acid was also elevated in the most severe disease cohorts (GOLD 2 and GOLD 3) relative to smokers with preserved spirometry (SPS) and those with less severe disease (GOLD 1).
Because mucin secretion and inflammation are also related to the pathophysiology of pulmonary exacerbations, Dr. Esther and colleagues had hypothesized that sputum biomarkers would be predictive of future exacerbations. Within the full cohort, both sialic acid and hypoxanthine were significantly elevated in those who had multiple (two or more) pulmonary exacerbations relative to those who had none (P = .001). Similar, though less significant findings were observed for xanthine (P = .01), methylthioadenosine (P = .01), adenine (P = .01), and glutathione (P = .01).
Sputum tests needed
While tests still need to be developed, Dr. Esther noted in an interview that they would be based on well-established technologies commonly utilized in clinical laboratories. “Sputum biomarkers of mucus hydration and adenosine metabolism could help clinicians predict which patients with COPD are likely to experience multiple pulmonary exacerbations. Tests would be applied to patients with COPD at higher risk for exacerbations; for example, those who have low lung function or a history of prior exacerbations.”
Dr. Esther noted that these biomarkers could be helpful in developing novel therapies. “Using sialic acid to assess mucus concentrations is much easier than other methods, so it could help in developing mucolytic treatments. Also, adenosine metabolism represents a novel therapeutic target in COPD. Drugs that modify adenosine metabolism that have been approved for other diseases such as gout could be tested in COPD. As with mucus hydration, the biomarkers we identified (particularly hypoxanthine) could be utilized to make sure that novel therapies are having the intended impact on airway adenosine metabolism.”
The research was supported by SPIROMICS (funded by NIH and the COPD Foundation). Dr. Esther reported having no relevant disclosures.