This was a multi-centered, prospective, open-label, randomized-controlled study conducted at 5 centers in China from November 2021 to August 2022. Patients were randomized using the Interactive Web Response System after written informed consents were obtained from the parents. Patients were stratified based on the severity of the disease. Patients were randomized 1:1 to receive the ambroxol spray or the ambroxol solution. The treatment group received spray 2 inhalations (15 mg) three times a day and the control group received the ambroxol solution 2.5 ml (7.5 mg) three times a day if the patient was aged 2–5-year-old or 5 ml (15 mg) three times a day if the patient was 6-year-old. Patients’ baseline characteristics are obtained upon initiation of therapy. Patients are followed for 3 to 7 days post-initiation depending on time of discharge.

The study protocol was approved by independent ethics committees or institutional review boards at every treatment center and was conducted in accordance with the ethical principles of the Declaration of Helsinki.

Patients are included if they are (1) aged 2–6 years old, (2) have acute respiratory infections (e.g., acute bronchitis, acute upper respiratory tract infection, pneumonia) causing sputum production and coughing, and (3) can be monitored daily. Key exclusion criteria include, (1) recent treatment with expectorant medications within 24 h of randomization, (2) chronic lung conditions such as bronchial asthma, interstitial lung disease, or bronchiectasis, (3) significant chronic heart, brain, gastrointestinal, renal, hematologic, endocrine, or other systemic disease, (4) AST or ALT >1.5 times upper limit of normal or total bilirubin or serum creatinine > the upper limit of normal, and (5) poor adherence to drug regimen.

The primary endpoint was the changes in cough symptom scores. Cough symptom scoring system was adopted from previously validated scoring systems (15, 16). The scores range from 0 to 5 with 0 being no cough and 5 being distressing cough that affects most daily activities. The full cough evaluation score is included in Supplementary Material 1. Day time coughing and nocturnal coughing were scored at baseline, during treatment day 1 and 4, and at the end of treatment. Patients’ guardians were provided with instructions on the scoring system and guardians scored the patients based on the provided descriptions.

Visual analog scale (VAS) was used to assess severity of cough. Patients’ guardians were asked to mark on a 0–100 mm VAS strip regarding their subjective evaluation of their children's cough severity with a score of 10 representing the most severe cough and a score of 0 representing absence of cough.

Quality of life assessment using the Parent-proxy Children's Acute Cough-specific Quality of Life (PAC-QoL) questionnaire was also done at baseline and the end of treatment. This 12-question questionnaire aims to assess the quality of life for parents of children with chronic cough. The score is used and validated in previous studies. However, one question was modified to better suit our patient population. Supplementary Material 2 includes the questions assessed by the PAC-QoL (17, 18).

Adherence to medication is assessed using the medication diary provided to the parents. Adherence assessment is based on the time of administration of medication and the amount of medication taken. Acceptable adherence is deemed to be administration within 2 min with little to no residual medication. Supplementary Material 3 include the full criteria adopted from previous literature. Poor adherence is deemed to be administration requiring more than 2 min and patients spitting out or refusing the medication or choking on the medication. Medication adherence observations are categorized into swallowing medication whole, taking medication with small amounts of residue, spitting out medication, choking on medication, and refusing to take medication (19, 20).

Safety outcome is recorded as adverse events (AEs) that occurred from day 1 of treatment to the end of treatment. All AEs were evaluated for the likelihood of association with ambroxol treatment.

All data analyses were performed using SAS (Statistical Analysis Software 9.4, SAS Institute Inc, Cary, North Carolina, USA). Descriptive statistics such as mean, standard deviation, and frequency by treatment arm will be used to summarize the results. All analyses are done using recorded data. No missing values were imputed. All patients randomized and received at least 1 treatment dose was included in the analysis.

The study was designed to show non-inferiority for the primary endpoint in the modified intention-to-treat population. Based on past literature, the expected mean difference post treatment is 2 ± 0.5. With a one-sided significance level of 0·025, 73 patients in each group would be needed to reject a null hypothesis of inferiority with 85% power. After factoring in 20% patients loss to follow up, we estimated that we would need to follow approximately 88 patients in each arm (total 176 patients) to achieve power.

Between November 2021 and August 2022, a total of 162 patients were screened for study eligibility. Of these patients, 7 patients did not fulfill the criteria for randomization, and 1 patient was excluded after randomization due to non-adherence to the study regimen. Accordingly, 75 patients were randomly assigned to receive the ambroxol spray and 79 patients to receive the ambroxol solution for the modified intention-to-treat analysis (Figure 1). Demographic and baseline clinical characteristics were similar for the 2 groups (Table 1). Mean age of participants is 4 years old. Approximately 49% of participants had acute pneumonia with 57% of participants have one or more comorbidities. Most common concomitant medication class used is antibiotic medications with majority of patients receiving either azithromycin or cefotaxime.

At the end of treatment, the cough symptom score compared to baseline was significantly improved in the ambroxol spray arm compared to the ambroxol solution arm (Table 2). During the daytime, the mean value of the change from the baseline in the spray group was −2.4 (0.86), and that in the solution group was −2.1 (0.94) (P = 0.0004). The average change in nighttime score compared to baseline in the spray group was −2.3 (0.90), compared with −2.1 (0.96) in the solution group (P = 0.02). The total average change from the baseline in the spray group was −4.7 (1.54), an 81.26% decrease, compared with −4.2 (1.62), a 73.66% decrease, in the solution group (P = 0.0008).

At the end of the study, all secondary outcomes are statistically different comparing the spray and the solution arms (Table 3). At the end of treatment, the change from baseline regarding cough severity measured by VAS was −5.7 in the spray group and −5.2 in the solution group (P = 0.0064). Compared to baseline, the VAS decreased by 82.62% in the spray group compared to a 76.25% decrease in the solution group (P = 0.0064). The change from baseline regarding quality-of-life assessment measured using the PAC-QoL questionnaire was −16.4 (decreased by 69.09%) in the spray group and −12.4 (decreased by 51.08%) in the solution group (P < 0.0001). All parents in the spray group reported good adherence regarding easy administration of medication (within 2 min with little to no residual medication) compared to only 49% in the solution group (P < 0.0001). Good adherence, defined as swallowing all of the medications or taking medications with small amounts of residue, was reported in 100% of patients in the spray group compared to 81% in the solution group (P < 0.0001). A total of 97% of patients in the spray group was able to take all the medication on first attempt compared to 38% in the solution group (P < 0.0001).

A total of 1 (1.33%) adverse event was reported in the spray group and 9 (6.33%) was reported in the solution group (Table 4). Out of the 9 adverse events reported in the solution group, 2 (2.53%) of the treatment-emergent adverse events were determined to be grade 3. Some patients had multiple adverse events reported. No serious adverse event was reported in the spray group.