You didn't try treating it with NAC?
High-Dose N-Acetylcysteine in Stable COPD: The 1-Year, Double-Blind, Randomized, Placebo-Controlled HIACE Study
https://www.sciencedirect.com/science/article/abs/pii/S0012369213604572
Results
Of 133 patients screened, 120 were eligible (93.2% men; mean age, 70.8 ± 0.74 years; %FEV1 53.9 ± 2.0%). Baseline characteristics were similar in the two groups. At 1 year, there was a significant improvement in forced expiratory flow 25% to 75% (P = .037) and forced oscillation technique, a significant reduction in exacerbation frequency (0.96 times/y vs 1.71 times/y, P = .019), and a tendency toward reduction in admission rate (0.5 times/y vs 0.8 times/y, P = .196) with NAC vs placebo. There were no significant between-group differences in mMRC dypsnea score, SGRQ score, and 6MWD. No major adverse effects were reported.
Conclusion
In this study, 1-year treatment with high-dose NAC resulted in significantly improved small airways function and decreased exacerbation frequency in patients with stable COPD.