BackgroundIdiopathic pulmonary fibrosis is an irreversible fibrosing lung disorder with a poor prognosis. Current treatments slow the rate of decline in lung function and may influence survival, but they have a significant side-effect profile and so additional therapeutic options are required. People with idiopathic pulmonary fibrosis have altered innate immunity and altered lung microbiota, with the bacterial burden relating to mortality. Two randomised controlled trials have demonstrated beneficial effects with co-trimoxazole (SEPTRIN^®; Essential Generics Ltd, Egham, UK; Chemidex Generics Ltd, Egham, UK), with the suggestion of an improvement in rates of survival.ObjectivesTo determine the clinical efficacy of co-trimoxazole in people with moderate to severe idiopathic pulmonary fibrosis.DesignA Phase II, double-blind, placebo-controlled, parallel-group, randomised multicentre study.SettingUK specialist interstitial lung disease centres.ParticipantsPatients who were randomised had idiopathic pulmonary fibrosis diagnosed by a multidisciplinary team. In addition, patients had significant breathlessness (i.e. a Medical Research Council Dyspnoea Scale score of > 1) and impaired lung function (i.e. a forced vital capacity of < 75% predicted). Patients could be taking licensed medication for idiopathic pulmonary fibrosis, but were excluded if they had significant comorbidities, including airflow obstruction.InterventionOral co-trimoxazole, 960 mg twice per day (two 480-mg tablets twice per day), compared with placebo tablets (two tablets twice per day) for a median of 27 months (range 12–42 months). Otherwise, both trial groups had standard care.Main outcome measuresThe primary outcome was the time to death (all causes), transplant or first non-elective hospital admission. Secondary outcomes were the individual components of the primary end point and the number of respiratory-related events. Questionnaires (the King’s Brief Interstitial Lung Disease questionnaire; the Medical Research Council Dyspnoea Scale; EuroQol-5 Dimensions, five-level version; the Leicester Cough Questionnaire; and the Cough Symptom Score) and lung function tests (forced vital capacity and diffusing capacity for carbon monoxide) were undertaken at baseline and at 12 months.ResultsThe trial randomised a total of 342 (295 male) patients (active treatment group,n = 170; placebo group,n = 172), using minimisation for hospital and receipt of licensed antifibrotic medication, from 39 UK hospitals. The patients had a mean (standard deviation) age of 71.3 years (7.47 years) and a mean forced vital capacity of 2.25 l (0.56 l). A total of 137 (40%) patients were taking pirfenidone (Esbriet, Roche Holding AG, Basel, Switzerland) and 116 (34%) were taking nintedanib (Ofev^®, Boehringer Ingelheim, Brackness, UK). There was one post-randomisation exclusion from the co-trimoxazole group, but no withdrawals. There was no difference in the time to event for the composite primary end point (co-trimoxazole: hazard ratio 1.2, 95% confidence interval 0.9 to 1.6;p = 0.319). Likewise, there was no difference in other event outcomes, lung function measurements or patient-reported outcomes, other than a beneficial effect on the total Leicester Cough Questionnaire score, the social domain of the Leicester Cough Questionnaire score and the chest domain of the King’s Brief Interstitial Lung Disease questionnaire in the adjusted analysis. The repeated-measures analysis showed a significant overall difference in Cough Symptom Score. There were significantly more reports of nausea, but fewer reports of diarrhoea, with co-trimoxazole; however, differences in frequency of hyperkalaemia, rash and headache were not significant. The limitations of the trial were that it was not possible to evaluate the lung microbiota, there were missing data for secondary end points and there was no health economic analysis.ConclusionThese results suggest that co-trimoxazole does not reduce the likelihood of death or number of hospitalisations among people with idiopathic pulmonary fibrosis with moderate to severe idiopathic pulmonary fibrosis. Further work is required to evaluate the effect in subgroups of individuals with idiopathic pulmonary fibrosis or the effect of antibiotics with different antibacterial properties.Trial registrationCurrent Controlled Trials ISRCTN17464641.FundingThis project was funded by the Efficacy and Mechanism Evaluation programme, a Medical Research Council and National Institute for Health Research (NIHR) partnership. This will be published in full inEfficacy and Mechanism Evaluation; Vol. 8, No. 9. See the NIHR Journals Library for further project information.