The goal of this clinical trial is to determine if doxofylline and procaterol are effective in treating patients with stable chronic obstructive pulmonary disease (COPD). It will also assess the safety of both drugs.

The main questions it aims to answer are:

• Does doxofylline demonstrate a comparable bronchodilator effect to procaterol in COPD participants?
• What medical problems do participants experience when taking doxofylline and procaterol?"

This study is a randomized crossover study. COPD patients aged 40 years or older with stable disease were included. Each patient received 4 weeks of treatment with either doxofylline or procaterol followed by a 2-week washout period, and then 4 weeks of treatment with the other drug. Respiratory symptoms were assessed by modified Medical Research Council (mMRC) dyspnea scale and COPD Assessment Test (CAT). Pulmonary functions were assessed by spirometry with bronchodilator testing, and adverse events were recorded. Spirometry data including forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and forced expiratory flow at 25-75% of FVC (FEF25-75) were also collected.

Inclusion Criteria:

• COPD diagnosis confirmed by spirometry (post-bronchodilator FEV1/FVC < 70%)
• Aged 40 years or older
• Smoking 10 pack-years or more
• Postbronchodilator FEV1 < 80%

Exclusion Criteria:

• History of COPD exacerbation within 3 months
• Oral or intravenous corticosteroid treatment within 6 weeks
• Oral bronchodilators e.g. doxofylline, theophylline, procaterol, or salbutamol within 1 week before randomization
• Asthma
• Tracheostomy, invasive or noninvasive mechanical ventilation
• Inability to perform spirometry or 6-minute walk test
• Pregnant or breastfeeding women

Individual participant data (IPD) and documents will be available for sharing immediately after publication for a period of 2 years.