Background: Bronchial hyperresponsiveness (BHR) is a key feature of asthma. Compound D, an active compound in Phlai, can bind cysteinyl leukotrienes receptors that play a role for asthma treatment.

Objective: To determine the effect of Phlai capsules on BHR measured by the methacholine challenge test.

Materials and Methods: A randomized, double-blind, placebo-controlled, crossover study was conducted in adult asthmatic patients with partly controlled symptoms. Each patient received four weeks of treatment with either Phlai or placebo separated by a 2-week washout period. The main outcome was provocative concentration of methacholine causing a 20% drop in FEV₁ (PC₂₀). Asthma control test (ACT) scores, and fractional exhaled nitric oxide (FeNO) levels were secondary end points.

Results: Thirty patients were randomly allocated to either the Phlai or the placebo group. All patients had allergic rhinitis and received inhaled corticosteroid and long-acting beta2-agonist (ICS/LABA) or ICS alone. Four weeks after treatment, mean PC₂₀ in the Phlai group was higher than in the placebo group at 9.76±1.56 mg/mL versus 6.05±1.65 mg/mL (p=0.151). The improvement of ACT scores in the Phlai group was significantly higher than in the placebo group. FeNO levels decreased after treatment in the Phlai group. All patients tolerated the treatment well and had no side effects.

Conclusion: In adult asthmatic patients with partly controlled symptoms, concomitant treatment with Phlai capsules tended to decrease BHR and significantly improve symptom scores.