Based on results from preclinical and clinical studies, a five-drug combination of isoniazid, rifapentine, pyrazinamide, ethambutol, and clofazimine was identified with treatment shortening potential for drug-susceptible tuberculosis; the Clo-Fast trial aimed to determine the efficacy and safety of this regimen. We compared 3 months of isoniazid, rifapentine, pyrazinamide, ethambutol, and clofazimine, administered with a clofazimine loading dose, to the standard 6 month regimen of isoniazid, rifampicin, pyrazinamide, and ethambutol in drug-susceptible tuberculosis.

Clo-Fast was a phase 2c open-label trial recruiting participants at six sites in five countries. Participants aged 18 years or older with pulmonary tuberculosis who were sputum smear positive for acid-fast bacilli or molecular tuberculosis assay positive (with Mycobacterium tuberculosis with sensitivity to rifampicin and isoniazid) were eligible for enrolment. Individuals with HIV infection with a CD4 ⁺ cell count ≥100 cells per mm ³ could participate. Participants were randomly assigned in a 2:1 ratio (group 1: group 2) or a 2:1:1 ratio (group 1: group 2: group 3), depending on consent to participate in the intensive pharmacokinetic visits required in group 3, using a central web-based system with permuted blocks. The group 1 regimen included 8 weeks of rifapentine–isoniazid–pyrazinamide–ethambutol–clofazimine, with a 2-week 300 mg clofazimine loading dose, followed by 5 weeks of rifapentine–isoniazid–pyrazinamide–clofazimine (13 weeks total). The group 2 control regimen included 8 weeks of isoniazid–rifampicin–pyrazinamide–ethambutol followed by 18 weeks of rifampicin–isoniazid. Group 3 was identical to group 1 over the first 4 weeks of treatment, except that the regimen was administered without a clofazimine loading dose (100 mg daily); after 4 weeks of group 3 treatment, participants transitioned to local standard of care to complete treatment. Group 3 was designed to assess the effect of a 2-week loading dose on clofazimine pharmacokinetics. Randomisation was stratified by HIV status and advanced disease on chest radiograph. The primary efficacy endpoint was time to sputum culture-negative status by 12 weeks. The primary safety endpoint was the proportion of participants experiencing any grade 3 or worse adverse event over 65 weeks. The key secondary endpoint was unfavourable clinical or bacteriological outcomes by week 65. The efficacy analysis population contained participants assigned to groups 1 and 2 who were not late exclusions (no positive culture at screening, entry, or week 1, or if rifampicin resistance or isoniazid resistance was detected at screening or entry); the safety analysis population contained all randomly assigned participants who took at least one dose of treatment. The trial was registered with ClinicalTrials.gov ID: NCT04311502 .

104 participants were randomly assigned to group 1 (n=58), group 2 (n=31), and group 3 (n=15). 82 (79%) were male and 74 (71%) had radiographically advanced disease; 30 (29%) were people with HIV. The trial was stopped early for lack of clinical efficacy. For the primary efficacy outcome, 49 (89%) of 55 group 1 participants and 28 (90%) of 31 group 2 participants had stable sputum culture conversion by week 12 (adjusted hazard ratio 1·21 [90% CI 0·82–1·79]; p=0·2089). Adverse events grade 3 or worse occurred in 26 (45%) of 58 group 1 participants and five (16%) of 31 group 2 participants (difference 30%, 90% CI 14–45; p=0·002). The cumulative probability of a week 65 unfavourable outcome was 52% (95% CI 37–69) in group 1 versus 27% (14–50) in group 2 (p=0·049).

Although the trial was stopped early, we found that a 3-month regimen containing clofazimine and rifapentine had 12-week culture conversion rates that did not differ statistically from the standard of care. The regimen was associated with an unacceptably high proportion of participants with unfavourable composite clinical outcomes and grade 3 or worse adverse events.

US National Institutes of Health Advancing Clinical Therapeutics Globally for HIV/AIDS and Other Infections (ACTG) and the National Institute of Allergy and Infectious Diseases.