Antifibrotic treatment response and prognostic predictors in patients with idiopathic pulmonary fibrosis and exposed to occupational dust

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, fibrosing interstitial pneumonia of unknown cause defined by the histopathologic and/or radiologic pattern of usual interstitial pneumonia (UIP) [ 1]. It appears mainly in older adults and is associated with unrelenting decline in lung function, advancing respiratory failure and high mortality rate.

The possible involvement of occupational exposure (past and/or still going) has been considered and investigated with regards both to the pathogenesis and the progression of the disease. Previous observational studies have implicated occupational exposures to metal, stone and wood dust, chemical fumes and organic dust in the pathogenesis of IPF and subjects exposed to these have an increased risk of developing the condition [ 2, 3]. Although a causal relationship between occupational exposure and IPF has not been recognized yet, the epidemiological evidence of occupations proved to be related to IPF is continuously growing [ 4]. Notably, males with a history of heavy smoking and occupational exposure to harmful agents, specifically birds and wood dust, have been identified as a particular group at increased risk of developing severe pulmonary fibrosis [ 5].

While the involvement of occupational and environmental factors in the pathogenesis of IPF have been explored to a considerable extent, studies investigating possible influence of job activity on the clinical features and prognosis of this condition are few. Indeed, only one report evaluated the clinical, physiologic and radiologic characteristics and the prognosis in patients with IPF according to their occupation. The main finding was that dust-exposure occupation was significantly associated with mortality after adjusting for major confounders such as age, sex, lung function parameters and radiologic aspects [ 6]. The little data available on the possible impact of environmental factors on IPF prognosis shows that both long-term exposure to and average concentration of ambient particulate (PM ₁₀ and PM _2.5) are associated with overall mortality and an increase in the rate of decline of forced vital capacity (FVC) in patients with IPF [ 7, 8].

Investigations taking into account occupational and environmental exposure in the assessment of the response to antifibrotic therapy (pirfenidone and nintedanib) in IPF patients are lacking.

Therefore, in this study we evaluated clinical and functional characteristics at diagnosis and after 12 month treatment in patients with IPF according to their occupational dust exposure. In addition, we investigated if such occupational exposure might influence the prognosis of IPF in terms of mortality and initiation to long-term oxygen therapy (LTOT). In fact, although the latter can vary from one physician to another, it indicates an undoubted important worsening of the condition.

In this study, we have shown that 62 of the 115 patients (54%) with a diagnosis of IPF had an occupational dust exposure, defined as lasting 10 or more years. IPF patients with occupational dust exposure exhibited a lower educational level and a higher frequency of reported asbestos exposure. Age, gender, smoking history, dyspnea, comorbidities, lung function parameters and prognostic indices (GAP, CPI) were similar in patients with and without occupational exposure to dusts, both at diagnosis and after 12 month therapy with pirfenidone or nintedanib. Occupational dust exposure did not seem to influence the timing of initiation of LTOT in the follow-up period.

The finding of a lower educational level in the dust-exposed group was expected and corresponds to the results of other studies comparing the educational level between workers exposed to various dusts and administration office workers [ 16].

Also the higher percentage of IPF patients exposed to asbestos in the dust-exposed group was not surprising. Indeed, these two occupational exposures are often associated with a number of work activities as carpentry, engineering, construction and quarrying. In our study population, asbestos exposure did not seem to affect both clinical characteristics and lung function measurements either before and after 12 months of IPF treatment, indicating a careful exclusion of an asbestos etiology in the diagnostic workup [ 17]. It is noteworthy that the differential diagnosis between asbestosis and IPF is challenging, and an accurate patient recall of historic asbestos exposure is an essential factor in discriminating the two conditions. In our study, of almost 50% of patients in the exposed group who had exposure to asbestos, 17 had an occupational exposure, while the others were exposed non-occupationally (household and neighbourhood); of the non-exposed group only 3 had been exposed in occupational settings. The estimated non-occupational exposure was not sufficient to support a diagnosis of asbestosis. Particular attention was paid in investigating not only the timing but also the intensity of asbestos exposure as a heavy exposure for few years may be equivalent to a small exposure for many years. Considering time and intensity of exposure, the latter calculated approximately by job title and working history, only a few patients in the exposed group had an asbestos exposure that could support a diagnosis of asbestosis. As patients suffering from asbestosis experience a better survival rate than the general IPF population, we took specific care to ascertain a correct differential diagnosis. In subjects with a significant asbestos exposure, IPF were diagnosed during a multidisciplinary dynamic discussion after formal work-up, including extensive patient history, chest radiograph and CT scan (also looking for benign asbestos-associated pleural disease such as pleural plaques, diffuse pleural fibrosis and benign asbestos pleural effusion), but rarely searching for asbestos bodies in BAL and lung tissue.

At the diagnosis, age, age at symptoms onset and lung function parameters, particularly FVC % and DLCO % predicted, were found similar in patients exposed and not exposed. These findings seem to disagree with those of Lee et al. [ 6] who found an early onset of IPF and a reduced DLCO in dust-exposed workers. However, they evaluated occupational exposure differently: 1) medical and occupation records were collected retrospectively from a web-based registry; 2) patients were categorized into five groups according to occupations (unemployed or homemakers; farmers, fishers or ranchers; sales or service personnel; clerical or professional personnel; and workers exposed to dusts, but excluding organic dusts); 3) an early onset of IPF and a reduced DLCO were found significant when dust-exposed workers were compared to unemployed or homemakers but to none of the other group, making this result rather weak.

As reported above, our patients exposed to occupational dust did not exhibit reduction in lung function parameters at the IPF diagnosis compared to those non exposed. In population based studies, the issue of a possible effect of dust exposure at work on long-term excess decline in lung function is still controversial. Indeed, the results of previous reports are inconsistent and the differences in study design, qualitative and quantitative dust exposure and the lung function indices chosen as outcome (FEV ₁, FVC, and/or FEV ₁/FVC) make them even less conclusive. Lastly, smoking is a major injury for lung function and, therefore, a crucial bias in the evaluation of the lung effects of occupational dust exposure [ 18].

Both pirfenidone and nintendanib have been shown to lessen the decline in pulmonary function in patients with IPF [ 19]. In particular, they reduce the number of patients experiencing a decline in FVC of 10% or greater with the result of slowing the disease progression, as compared to no therapy [ 19]. Although we could not compare the rate of disease progression between patients with and without pharmacological treatment, we found that exposure to dust at work does not seem to impact on the beneficial effect of 1 year pirfenidone and nintendanib. In fact, the stability of lung function after 1 year of therapy was present in patients with and without occupational dust exposure, and dust exposure was not associated with post-treatment FVC % pred. and DLCO % pred. Furthermore, the percentage of patients with occupational dust exposure did not differ between those who exhibited a decrease ≥10%, a decrease < 10% and a stability or increase in pre-post therapy FVC % pred. To our knowledge, this is the first study investigating the possible influence of occupational exposure to dusts on IPF therapy, and it suggests that the beneficial effect of pirfenidone and nintendanib on lung function is not influenced by the exposure to occupational dust.

In the present IPF patients, no difference was found in GAP index score for IPF mortality, CPI and the percentage of those who initiated LTOT based on dust exposure at work. Whether occupational dust exposure could have an impact on the prognosis of IPF patients is still unknown. Lee et al. reported that exposure to dust was associated with an increased risk of mortality in patients with IPF in a survival analysis where occupation was adjusted for age, sex, pulmonary function, arterial partial pressure of oxygen (PaO ₂), and honeycombing on the HRCT scan, but not in a survival analysis where occupation was adjusted for GAP stage [ 6], indicating that the question of a possible association between occupational dust exposure and IPF prognosis remains unanswered.

In the present study also CPI was not related to dust exposure, strengthening the result of GAP index score. Interestingly, the relationship between CPI and occupational exposure in patients with IPF has not been previously investigated.

IPF patients who started LTOT during follow-up showed lower levels of respiratory function (FEV ₁; FVC; DLCO; TLC), worse prognostic indices (CPI; GAP) and a higher level of disability at diagnosis compared to patients who did not start LTOT. Increased GAP score and CPI at diagnosis were potential predictors of long-term oxygen therapy initiation, whereas a higher level of FEV ₁ was protective. Occupational exposure did not affect LTOT. With regards to respiratory function parameters, our data confirm the results of previous studies that found an association between LTOT and low levels of FEV ₁, FVC and DLCO in patients with pulmonary fibrosis and other chronic lung diseases [ 20, 21].

This observational study has some limitations. First, occupational history was self-reported and, therefore, potentially involving a recall bias. Data on measurements of intensity of dust exposure at work, as well as the latency between exposure cessation and disease diagnosis, were not available. However, years of exposure were gathered and the average of 36.74 years proves a considerable exposure. Furthermore, as the intensity of dust exposure generally differs between different occupations, we divided our dust exposed group in two categories based on data collected on job title: highest/heaviest dust exposure and moderate dust exposure. Of the 46 exposed, 50% had probable intense exposure (for example: turners, construction workers, carpenters, textile workers) and the remaining 50% moderate exposure (example: cleaners, farmers, breeders). We did not find any significant difference in spirometric parameters at diagnosis and at 12 months as well as in the number of deaths and in the timing of initiation of LTOT between the two categories of the exposed and between the highest/heaviest dust exposed compared with the non-exposed (data not shown).

Second, complete data on lung function at diagnosis and after 12 months were available for only 89 patients, all under pharmacological treatment. Thus, given the absence of follow-up data for the group of untreated patients, it was not possible to comment on whether anti-fibrotic treatment is effective in slowing disease progression/reduce the number of patients experiencing decline in FVC > 10% regardless of dust exposure. Similarly, there are no data to demonstrate that dust exposure does not impact on the beneficial effect of anti-fibrotic therapy at 1 year.

Third, the sample size of the study population was relatively small to evaluate the exposure effect of each occupational dust, i.e. organic dust, stone, sand, metal and wood dust. Although this might have implicated an underestimation of a specific kind of dust, we believe that the investigation on occupational dust exposure in its whole is particularly informative of the possible impact of occupations on IPF prognostic predictors and antifibrotic treatment. The limited sample size was also responsible, at least in part, for the small number of deaths we registered during the follow-up period and, therefore, for the missed opportunity to perform a mortality analysis. Nevertheless, we examined two well-recognized prognostic indices (GAP and CPI) and the initiation of LTOT, another robust prognostic predictor, contributing to address the question of the relationship, if any, between occupational dust exposure and IPF prognosis. It should be noted that available literature on this issue at present is scarce and controversial [ 6, 22]. Finally, it has to be mentioned that GAP has been validated at diagnosis and not at 1 year. Nevertheless, we thought we could calculate and report it since it has been found that GAP models performed similarly in diagnosis and in pooled follow-up visits [ 12].

IPF patients with occupational dust exposure have clinical and functional characteristics similar to non-exposed IPF patients at diagnosis. Also the effect of 12 month antifibrotic therapy was found analogous in the two groups, irrespective of dust exposure. The prognostic indices, GAP and CPI, and the timing of LTOT initiation did not appear to be affected by such occupational exposure. Although the results of our study were mainly negative, we believe that it is worthy to explore each possible factor which may influence the natural history and the response to therapy of IPF, a disease associated to rapid progression, fast worsening quality of life and high mortality.