This study is a multicenter real-world study conducted in 16 centers in Henan Province. The study was approved by the Research and Clinical trial Ethics Committee of the First Affiliated Hospital of Zhengzhou University (Ethical Review No. 2020-KY-257).

The recruitment time was from July 2020 to August 2021, and the eligible patients were 40–85 years old. IPF was diagnosed based on the diagnostic guidelines for IPF by the ATS/ERS/JRS/ALAT in 2018 (Raghu et al., 2018); patients with image findings suggesting probable UIP were diagnosed by transbronchial lung biopsy or surgical lung biopsy. All the high-resolution computed tomographic images and lung histopathology specimens were reviewed uniformly by at least one expert chest radiologist and one pathologist. The range dose of pirfenidone was 1,200–1,800 mg/d, taken orally ter in die (t.i.d., i.e., three times a day). Other eligibility criteria were as follows: 1) An FVC % predicted of 50% or more. 2) A diffusion capacity of the lung for carbon monoxide (DLco) between 30 and 90% predicted. 3) A ratio of the forced expiratory volume in 1 s to the FVC that was equal to or greater than 0.80. 4) A 6-min walk distance (6 MWD) of over 150 m at baseline. Written informed consent was obtained from all patients.

Exclusion criteria were as follows: 1) Use of other antifibrotic drugs, including nintedanib, high-dose prednisone (>10 mg), immunosuppressants, rituximab, and N- acetylcysteine. 2) Participation in any study of IPF drugs during the preceding one month. 3) History of malignant tumors during the preceding 5 years. 4) Patients receiving antineoplastic therapy. 5) Patients suffering from active or latent tuberculosis during the preceding six months. If an acute exacerbation was reported at any time during the trial, the researcher had the choice to either start any other treatment or increase the dose as required.

All patients had been treated with an oral pirfenidone capsule with a dose of ≥1,200 mg/d and a maximum dose of 1,800 mg/d. The drug was administered in three equal doses and taken with three meals. Physical examination, clinical laboratory tests, lung function test, 6 MWD test, and the analysis of total St George’s Respiratory Questionnaire (SGRQ) score were performed at baseline and at week 52. Telephone follow-up was carried out on the 2nd, 4th, 8th, 13th, 26th, 39th, and 52nd weeks to evaluate exercise tolerance and dyspnea, acute aggravation of symptoms, and hospitalization due to illness. Adverse drug reactions and treatment measures were documented meticulously. We assessed the adequacy and repeatability of all lung function results according to the American Thoracic Society standards (Culver et al., 2017). Safety outcomes were determined from clinical and laboratory evaluations and the records of adverse events that occurred within the 52 weeks and were coded using the Common Toxicity Criteria (CTC) version 2.0 of the National Cancer Institute.

Serial blood samples were collected at the baseline. Peripheral venous blood samples (2 ml) were drawn from each patient prior to dosing and at 1.5, 2, 4, 6, and 8 h post-dosing. All blood samples were collected in EDTA tubes and centrifuged at × 3,500 g for 10 min to isolate the plasma. Ultra-performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) was used to analyze the plasma concentrations of pirfenidone. The calibration curve covered the range of 0.2–20.0 mg/L. An analysis of quality control samples indicated good precision (coefficients of variation ≤4.2%) and accuracy (measured concentrations ≤4.7% from target concentrations) (Wang et al., 2021). The area under the curve (AUC) at baseline was evaluated using the linear trapezoidal method, whereas the AUC 8 h was multiplied by 3 to obtain the AUC 24 h.

The primary endpoint was the annual rate of decline in the FVC over the 52-week period. According to the international guidelines (Culver et al., 2017), pulmonary function was measured at the baseline and at the 52nd week. The secondary end points were the assessment of absolute changes in the percentage of predicted FVC, DLco, 6 MWD reduction, and the time to the first confirmed or suspected acute exacerbation of IPF or death from baseline to week 52.

The implementation of the 6-min walk test was defined and controlled according to the standards defined and validated by du (Bois et al., 2011); additional control measures were taken according to the ERS/ATS guidelines for field walking testing (Holland et al., 2014). Disease progression was defined as a decline of greater than 10% in the absolute FVC % predicted, hospitalization due to respiratory diseases, or a drop of 50 m or more in the 6 MWD compared with the baseline measurement. Acute exacerbation of IPF was defined as the acute clinically significant deterioration of respiratory functions characterized by new evidence of extensive alveolar abnormalities, which met the following diagnostic criteria (Collard et al., 2016): 1) The previous or current diagnosis was IPF. 2) Acute deterioration or dyspnea, usually lasting <30 days. 3) Computed tomography showed bilateral ground glass shadows and/or overlapping consolidations of background types consistent with the common type of interstitial pneumonia. 4) A deterioration that does not account completely for heart failure or humoral overload. Cases that are of unknown cause but do not fulfill the criteria listed due to missing computed tomography data and have been termed “suspected acute exacerbations of IPF.” Additional secondary endpoints included the change in the total SGRQ score measured over the entire treatment period. The SGRQ, which is used to evaluate the quality of life, is a self-administered questionnaire that consists of three different domains (symptoms, activity, and impact). The score of each domain ranges from 0 to 100, and the total score represents the weighted average of the three sub-scores, with higher scores corresponding to a poorer quality of life (Jones et al., 1991; Furukawa et al., 2017; Prior et al., 2019).

The research data were statistically analyzed using the SPSS25.0 software (IBM Corporation, Armonk, NY, United States). To analyze the mean change, the missing values due to death were assigned the worst result (e.g., FVC = 0). The normal distribution, skewed distribution, and counting data were expressed as average ±standard deviation, median (lower quartile, upper quartile), and frequency (constituent ratio), respectively. The T, χ2, and Mann-Whitney U tests were performed for comparisons among groups. The data were all double-tailed statistics, with an alpha value of 0.05. For time-to-event analyses, the high-concentration group was compared with the low-concentration group using the log-rank test.

From July 2020 to August 2021, 405 blood samples were collected from 371 patients treated with pirfenidone (≥1,200 mg/d), of which 31 collected two blood samples and 1 collected four blood samples while adjusting the drug dose. After excluding the patients with inadequate clinical features, test results, pulmonary functions, 6 MWD and SGRQ scores at baseline, and follow-ups within 52 weeks, along with the patients who could not be diagnosed as having IPF by imaging or pathology, 99 patients who completed the visits were finally enrolled in the study (Figure 1).

A UPLC-MS/MS-based method for the simultaneous measurement of pirfenidone and its main metabolites in the plasma of the patients was established in our center. Blood samples were collected for drug concentration determination after repeated administration. The plasma concentration of pirfenidone in patients with IPF was 112.8 ± 65.5 mg h/L. Upon viewing the AUC of the receiver operating characteristic curve (Figure 2A), we found that pirfenidone exposure was strongly correlated with the progression of the disease, and the cut-off value was 104.483 mg h/L; the patients were assigned into two different plasma concentration groups: the low-concentration group (AUC lower than 100 mg h/L) and the high concentration group (AUC ≥100 mg h/L). The distribution of pirfenidone plasma concentrations showed considerable differences between the two groups (Figure 2B).

Overall, the two study groups were similar in terms of demographic and baseline characteristics (Table 1). Most enrolled patients in the low- and high-concentration groups were males (80.4%and 76.7%, respectively), with a median age and smoking history of 64 and 67 years and 66.1 and 58.1%, respectively. Most patients were diagnosed within the preceding 3 years, with 80.4% in the low-concentration group and 79.1% in the high-concentration group. The mean (±SD) baseline FVC of the predicted value was 78.6% ± 16.7% in the low-concentration group and 74.4% ± 18.5% in the high-concentration group. The baseline values of the 6 MWD were 452.5 (388.5–505.0) m in the low-concentration group and 415.0 (385.0–465.0) m in the high-concentration group; the percentage of predicted DLco was 53.6% ± 12.2% and 53.7% ± 14.9%, respectively.

After treatment with pirfenidone capsules, the adjusted annual rate of decline in the FVC (the primary end point) was −90.0 ml per year in the high-concentration group as compared with −260.0 ml per year in the low concentration group, with a difference of 190.3 ml per year (95% CI, 109.2–280.0; p < 0.001) (Table 2; Figure 3B).

The FVC % predicted changed significantly during the 52-week treatment period between the two different plasma concentration groups (p < 0.001) (Table 2). The proportion of patients with a decline of ≥10% in the absolute FVC % predicted or who had died at week 52 was less in the high-concentration group than in the low concentration group (9.3% vs. 33.9%, p = 0.004) (Figure 3A). The high-concentration group showed a significantly lower decline over the 52-week period in the 6 MWD than the low concentration group (−15.0 m and −40.0 m, respectively, between-group difference, 23 m; 95% CI, 5.0–35.0; p = 0.006) (Table 2). At week 52, five patients (11.63%) in the high-concentration group and 21 patients (37.5%) in the low-concentration group had a 6 MWD reduction of more than 50 m or died, for a relative decrease of 28.6% in the high concentration group (Figure 3C).

During the assessment of changes in the percentage of predicted DLco from the baseline to week 52, the high- and low-concentration groups showed a decrease of −5.3 and −7.9% in DLco, respectively (between-group difference, 2.5%; 95% CI, −0.3 to 5.6; p = 0.078) (Table 2). At week 52, no significant difference in the average change in the total SGRQ score was observed between the two groups (2.7 and 2.9 in the high- and low-concentration groups, respectively; between-group difference, −0.3; 95% CI, −2.1–1.6; p = 0.764) (Table 2).

In the analyses of the time to the first acute exacerbation, a significant increase was observed in the high-concentration group compared with that in the low-concentration group (hazard ratio, 0.30; 95% CI, 0.11–0.82; p = 0.046) (Figure 3D); the percentage of patients with an acute exacerbation of IPF was lower in the high-concentration group than in the low-concentration group (7.0% vs. 21.4%) (Table 3). The all-cause mortality analysis showed two deaths (4.7%) over the 52-week period in the high-concentration group and three deaths (5.4%) in the low-concentration group; no significant difference was observed between the two groups (hazard ratio, 0.85; 95% CI, 0.14–5.09; p = 0.860) (Table 3). Similarly, the percentage of patients who died from IPF in the high-concentration group was lower than that in the low-concentration group, although the difference was not significant (p = 0.461) (Table 3).

Table 4 summarizes the adverse events associated with oral administration of pirfenidone in patients within the 52 weeks. In the overall population, the most common adverse events associated with oral pirfenidone were digestive tract-related adverse reactions such as nausea, vomiting, abdominal pain, diarrhea, dyspepsia, gastroesophageal reflux, weight loss, and rash. The total number of adverse events was 37 (86.0%) in the high-concentration group and 35 (62.5%) in the low-concentration group. There was a significant difference in the total incidence of adverse effects between the different concentration groups (p = 0.009).

In the high-concentration group, adverse events related to nausea, dyspepsia, weight loss, arthritis, and rash were more common compared with those in the low-concentration group. In the low-concentration group, one patient (1.8%) had grade 3 gastrointestinal adverse events, and two patients (3.6%) had grade 3 weight loss adverse events. One patient (2.3%) in the high-concentration group showed grade 4 skin-related adverse effects, while another (1.8%) in the low-concentration group showed grade 3 skin-related adverse effects. The adverse events were mostly mildly to moderately severe, easy to occur in the early stage of oral drug increment, and could be gradually tolerated or significantly relieved by drug treatment (such as proton pump inhibitors) or adjustment of living habits. In this study, liver-related adverse reactions were very rare. Only one patient (1.0%) had alanine or aspartate aminotransferase levels at least three times higher than the upper normal limit in the total population; the total bilirubin level of this patient increased to more than double the upper normal limit. However, the liver injury was reversible, and liver functions were restored after treatment with hepatoprotective drugs.