Patients were recruited from the Chongqing Public Health Medical Treatment Center, one of the sites of the single-arm, multicenter, prospective, observational study, which was first launched in July 2020 across 26 centers in China (7). The present study included MDR/RR-TB patients aged 18 to 65 years who could only form an effective treatment regimen by adding delamanid on the basis of drug susceptibility testing results and previous treatment history, in accordance with the WHO and Chinese guidelines of 2019 (2, 3). The diagnosis of MDR-TB or RR-TB was based on microbiological (sputum culture/smear-positive, drug susceptibility testing) and chest radiography findings, in which MDR-TB refers to TB patients who were resistant to at least both isoniazid and rifampicin based on drug susceptibility testing, while RR-TB refers to those who were only resistant to rifampicin. Other inclusion criteria were: patients had not yet started treatment for MDR-TB, or although they had begun anti-TB treatment, they still needed further intensive treatment; the corrected QT interval (QTcF) recorded on the electrocardiogram was < 450 ms; and patients had no history of respiratory failure, heart failure or clinically significant arrhythmias. Detailed inclusion and exclusion criteria are shown in Supplementary Appendix 1.

The study was conducted in accordance with the Declaration of Helsinki and was approved by the ethics committee of Chongqing Public Health Medical Treatment Center (approval number 2020–044-01-KY). Written informed consent was obtained from all enrolled patients. The study was registered at clinicaltrials.gov (identifier NCT04421495) on June 9, 2020.

All patients included in the intensive phase were treated with delamanid plus the OBR for 24 weeks, in which the recommended dose of delamanid was 100 mg (i.e., 2 tablets) twice daily, postprandial. In the event of a missed dose of delamanid, the missed dose was immediately administered. If the time was approaching the next scheduled dose, there was no need to make up the missed dose. Instead, we proceeded directly to administer the next dose as per the regimen. After 24 weeks, the missed doses were replenished by oral administration at a dosage of 100 mg twice a day. The cumulative dosage of the delamanid was 672 tablets. In principle, delamanid should be combined with at least three drugs that were sensitive to the patient’s isolated strain to form a chemotherapy regimen, according to reliable drug susceptibility testing results. In the absence of reliable drug susceptibility testing results, delamanid was used in combination with at least four other drugs to which the patient’s isolated strain was likely to be sensitive, forming a chemotherapy regimen. Potentially sensitive drugs refer to those that have never been used or have been used for less than 3 months, or the main drugs in a treatment regimen that has been proven effective in previous treatments. Additionally, delamanid should not be added alone to a chemotherapy regimen that has been proven ineffective after clinical application. Therefore, according to WHO and Chinese guidelines (2, 3), an individualized OBR was developed by the investigators, based on prior anti-TB treatment history, baseline drug susceptibility testing results, and tolerance to drugs. The present study selected the following drugs in turn: levofloxacin, linezolid, clofazimine, cycloserine, pyrazinamide, ethambutol, prothionamide, amikacin or capreomycin. In principle, the dosage of OBR medication was determined based on the patient’s weight and detailed recommended dosages are presented in Supplementary Appendix 2. During the delamanid treatment period, only levofloxacin among the fluoroquinolone drugs was chosen; if clinically required, moxifloxacin could be used as a substitute for levofloxacin after the end of the intensive phase of treatment. All occurrences of QTcF prolongation > 60 ms were associated with the concomitant use of delamanid and fluoroquinolones. Thus, when the co-administration of these two classes of drugs was unavoidable for formulating an appropriate treatment regimen for MDR/RR-TB, close monitoring of the QTcF interval was initiated. When the QTcF interval was ≥ 500 ms, delamanid or other drugs (i.e., levofloxacin, clofazimine) that may cause QTcF prolongation were discontinued and the electrocardiography (ECG) was reviewed weekly until the QTcF interval was restored to <450 ms, but the use of delamanid was not resumed.

After 24 weeks of delamanid treatment, the OBR was generally continued. For patients with MDR-TB and pre-XDR-TB who had completed the intensive-phase treatment and whose sputum culture was negative at the end of intensive-phase treatment, cure was defined as the culture being negative for at least 5 consecutive measurements, with each sputum culture interval being ≥4 weeks and sputum smear results in the current month negative. Treatment completion was defined as after completion of the treatment regimen (intensive + continuous treatment phases), without evidence of treatment failure (i.e., chest radiography or clinical symptoms progression, serious drug reaction) or without inadequate bacteriological cure records, such as when there were no 5 consecutive sputum culture results (every 4 weeks) after the end of the intensive-phase treatment. Therefore, the treatment duration of the present study protocol was 11 to 18 months, including 6 months of the intensive phase and 5 to 12 months of the continuous phase.

Sputum smears (direct or concentrated methods), clinical symptoms and weight assessment were performed within 1 week before the first dose of delamanid, at weeks 2 and 4 during treatment and then every 4 weeks until the end of treatment. Sputum samples were cultured using Lowenstein-Jenden medium (solid culture) or the mycobacterial growth indicator tube (MGIT) system (liquid culture) at baseline (i.e., within 1 week or day 1 before the first dose of delamanid), at weeks 2 and 4 during treatment, and then every 4 weeks until the end of treatment. Drug susceptibility tests on Mycobacterium tuberculosis-positive isolates were performed at baseline (within 1 week or day 1 before the first dose of delamanid) and again at the end of the intensive phase of treatment when the sputum culture was positive. In the course of subsequent treatment, if there was a positive culture and the interval from the last drug sensitivity test was ≥ 24 weeks, drug susceptibility testing was performed again. For patients with treatment failure, a drug susceptibility test was added on the basis of a positive sample from the last treatment failure. Phenotypic drug susceptibility tests (solid or liquid drug sensitivity, microplate assay) or molecular drug susceptibility tests were used at baseline before treatment, while phenotypic drug susceptibility tests were employed during treatment and the follow-up periods. Chest CT examinations were carried out within 1 week before the first dose of delamanid and every 12 weeks during delamanid treatment until the end of treatment, with additional chest CT examinations carried out when delamanid was discontinued. ECG measurements were made within 1 week before the first dose of delamanid (baseline), once at 2-, 4-, 8-, 12-, 16-, 20-, and 24-weeks during delamanid treatment, with an additional ECG examination every 12 weeks after the discontinuation of delamanid. ECG follow-up was performed weekly when the QTcF interval was ≥ 450 ms and < 500 ms or with QTcF intervals ≥ 60 ms from baseline during delamanid treatment until the QTcF interval returned to < 30 ms from baseline, or the QTcF interval was < 450 ms.

The efficacy outcomes included a favorable outcome, time to sputum culture conversion and the time to sputum smear conversion over 24 weeks. Treatment outcomes included cure, treatment completed, treatment failure, death, loss to follow-up and non-evaluation (Supplementary Appendix 3), where a favorable outcome was defined as the proportion of patients evaluated as cured and treatment completion (intensive + continuation phases). Initial sputum culture/smear conversion was defined as 2 or more consecutive negative culture/smear results at least 4 weeks apart over 24 weeks treatment of delamanid in patients with a positive sputum culture/smear at baseline. The time of sputum culture/smear conversion was the specimen collection date when the negative sputum culture/smear result was first obtained.

Safety outcomes included the incidence of treatment-emergent adverse events (TEAEs), drug-related TEAEs and serious AEs (SAE). The severity of TEAEs was graded according to the Division of AIDS (DAIDS) Table for Grading the Severity of Adult and Pediatric Adverse Events Corrected (ver. 2.1) (8). In addition, clinically significant QT prolongation was defined as a QTcF ≥500 ms or a QTcF change from baseline ≥ 60 ms during the intensive-phase treatment as an event of concern. The related definitions of AEs are presented in Supplementary Appendix 4.

All statistical analyses were performed using SAS software (SPSS Inc., Chicago, Ill., USA). Continuous variables are expressed as the median with range (minimum-maximum), and categorical variables as numbers with percentages. All patients who received at least one dose of delamanid were included in the safety set, which was used for analysis of safety outcomes during the 24-week delamanid treatment period. Efficacy outcomes were conducted on the full analysis set (FAS) of patients who received at least one dose of delamanid, where a favorable outcome refers to completion of the entire course of treatment (intensive + continuation phases) or earlier cure, while sputum culture/smear conversion was assessed during the 24-week delamanid treatment period. Two-sided 95% confidence intervals (CIs) for favorable outcomes were calculated using the Clopper-Pearson method. For the times to sputum culture/smear conversion, the Kaplan–Meier nonparametric maximum likelihood method was used for point estimation of the survival function over a predictable time period, and the Greenwood formula was combined with the Delta method to construct a two-sided Wald asymptotic 95% CI on a log (−log) scale. Moreover, the corresponding 95% CIs for the median, 25th percentile and 75th percentile were calculated using the Brookmeyer-Crowley method. Furthermore, a descriptive subgroup analysis of efficacy outcomes was conducted stratified by fluoroquinolone resistance (or not) or the numbers of background drugs administered (4 vs. 5 vs. 6 drugs).

From September 26, 2020 to October 30, 2021, 33 patients were enrolled in the study to receive delamanid treatment, of whom 26 completed the entire treatment course (intensive + continuation phases). Among the 7 patients who withdrew from the study, 2 had delamanid-related SAEs, 2 had a QTcF interval ≥ 500 ms or clinically significant ventricular arrhythmias, and 3 withdrew their informed consent (Figure 1). Finally, all enrolled 33 patients who received delamanid were included in the FAS and safety set.

The median age of patients was 41 years (range: 22–63), and most were male (20, 60.6%). All enrolled patients had been diagnosed with pulmonary TB, 23 (82.1%) of 28 patients had bilateral involvement, with 19 (67.9%) exhibiting cavities on chest radiographs. There were 8 (24.2%) cases of RR-TB, 15 (45.5%) cases of MDR-TB, and 10 (30.3%) cases of pre-XDR-TB (referred to as tuberculosis caused by Mycobacterium tuberculosis that met the definition of MDR/RR-TB and was resistant to any fluoroquinolone) (9). At baseline, 93.5% (29/31) of patients had a positive culture of Mycobacterium tuberculosis and 41.4% (12/29) had positive smears. Most of the patients (24, 72.7%) were treated with 5 background drugs, 6 (18.2%) were treated with 4 background drugs and 3 (9.1%) were treated with 6 background drugs. The most commonly used drugs in OBR were cycloserine (97.0%), linezolid (93.9%), clofazimine (93.9%) and levofloxacin (66.7%) (Table 1). The majority of patients (23/29, 79.3%) received delamanid treatment for ≥ 24 weeks, with a median treatment duration of 168 days (range: 28–197).

A total of 26 patients completed the entire treatment course (intensive + continuous phases), with a treatment success rate of 78.8% (95% CI, 61.1, 91.0) (Table 2). There were 19 fluoroquinolone-sensitive patients and 7 fluoroquinolone-resistant patients who had favorable outcomes, with relatively low treatment success rates in patients who exhibited fluoroquinolone resistance (70.0% vs. 82.6%). In addition, the treatment success rates for patients receiving 4, 5 and 6 background drugs were 83.3% (95% CI, 35.9, 99.6), 75.0% (95% CI, 53.3, 90.2), and 100% (95% CI, 29.2, 100), respectively (Supplementary Table 2).

Two MDR-TB patients had baseline negative culture results that remained negative within 24 weeks. Of 2 RR-TB patients without sputum sample testing at baseline, 1 had a negative culture at 24 weeks, while the other had negative cultures after baseline but turned positive at 24 weeks, and remained negative at subsequent time points and was therefore considered to be a contaminated sample. Among the 29 patients with a baseline culture-positive, the sputum culture conversion was observed in 25 (86.2%) patients within 24 weeks of delamanid treatment, with a median time to sputum culture conversion of 38 days (IQR: 24–77) (Figure 2A). Fluoroquinolone-sensitive patients had a shorter median time to sputum culture conversion (29 vs. 68 days) than fluoroquinolone-resistant patients over 24 weeks of delamanid treatment (Supplementary Table 2). Furthermore, compared with patients receiving 5 or 6 background drugs, those receiving 4 background drugs had the shortest median time to sputum culture conversion (31 vs. 38 vs. 56 days), but the proportion of patients achieving culture conversion within 24 weeks was also the lowest (60.0% vs. 90.5% vs. 100%). At 24 weeks, 12 out of 13 patients (92.3%) had sputum culture conversions, including all 4 fluoroquinolone-resistant patients (100%) and 8 out of 9 fluoroquinolone-sensitive patients (88.9%) (Table 2 and Supplementary Table 2).

Of the 4 patients who did not convert culturally within 24 weeks of delamanid treatment, except for 1 RR-TB patient who withdrew from the study due to QTcF interval prolongation, the remaining 3 patients completed the entire treatment course and achieved favorable outcomes. Two of them had positive culture results at week 4 but no sputum samples were available until week 24; in which, 1 pre-XDR-TB patient converted at week 28, while the other MDR-TB patient still could not produce sputum samples was therefore presumed not to have converted. The remaining 1 patient with MDR-TB had positive sputum cultures at weeks 16, 24 and 32 when sufficient sputum samples were available for testing. However, adequate sputum samples could not be obtained at other time points, and no evidence of treatment failure (clinical symptoms or chest radiography findings progression, or adverse drug reactions) was observed until the end of OBR treatment.

Among the 12 patients with baseline smear-positive results, 11 (91.7%) achieved sputum smear conversion over 24 weeks delamanid treatment, with a median time to sputum smear conversion of 18.5 days (IQR: 12–62.5) (Figure 2B). The median time to sputum smear conversion was similar between patients with fluoroquinolones resistance and those who were sensitive to fluoroquinolones (19 vs. 18 days) (Supplementary Table 2). In addition, the median times to sputum smear conversion for patients receiving 4, 5 and 6 background drugs were 12 (IQR: 12–12), 33 (IQR: 15–62.5) and 16.5 (IQR: 14–19), respectively. At 24 weeks, 5 out of 6 patients (83.3%) had sputum smear conversion, including all 2 fluoroquinolone-resistant patients (100%) and 3 out of 4 fluoroquinolone-sensitive patients (75.0%) (Table 2 and Supplementary Table 2).

A total of 18 (54.6%) of 33 patients experienced 32 TEAEs, most of which were of grade 1 or 2 in severity, with 10 patients (30.3%) experiencing grade 3 or 4 AEs (Table 3). The most commonly occurring TEAEs were liver toxicity (6, 18.2%), hematological abnormalities (5, 15.2%), QT interval prolongation (4, 12.1%) and gastrointestinal reactions (4, 12.1%). A total of 6 (18.2%) patients had 7 delamanid-related TEAEs, with the most common being QT interval prolongation (3, 9.1%) and gastrointestinal reactions (2, 6.1%) (Supplementary Table 3). The incidences of SAEs and delamanid-related SAEs were 9.1% and 3.0%, respectively. No patients died during the treatment period and no patients had neuropsychiatric events or mental disorders during the study.

Patients treated with delamanid-containing regimens exhibited a significant prolongation of the QTcF interval from baseline for the 24 weeks of treatment (p = 0.008; Supplementary Figure 1). The maximum changes from baseline in the QTcF interval was observed at week 16, with a maximum value of 24.1 ms. The mean changes from baseline between weeks 4 and 24 were basically stable, ranging from 20.1 ms to 24.1 ms. Four patients had a QTcF interval ≥ 60 ms relative to baseline or a QTcF interval ≥ 500 ms. Of note, all 4 patients who had QTcF interval prolongation were treated with delamanid, levofloxacin and clofazimine in combination. One case of grade 2 QTcF interval prolongation occurred after 51 days of clofazimine treatment, thought to be related to clofazimine treatment, and the patient recovered after 29 days without dose adjustment or discontinuation. Another case of grade 3 QTcF interval prolongation occurred after 23 days of treatment, which was considered to be related to delamanid, levofloxacin and clofazimine treatment, but the patient recovered after 13 days without dose adjustment or discontinuation. The other 2 cases of QTcF interval prolongation occurred after 28 and 84 days of delamanid treatment, and were considered to be related to the delamanid treatment, resulting in permanent discontinuation of delamanid. Among these 2 patients, 1 case of grade 4 QTcF interval prolongation resulted in hospitalization, which improved after 3 days; another case of grade 1 QTcF interval prolongation improved after 6 days. Furthermore, 2 patients experienced delamanid-related gastrointestinal reactions, which also improved after the permanent discontinuation of delamanid, while 1 patient who presented with atrial premature beats did not improve after the permanent discontinuation of delamanid.

In addition to delamanid, the background regimens also produced TEAEs (Supplementary Table 3). Five patients experienced linezolid-related hematological abnormalities, with the majority (4, 80.0%) being grade 3 or 4 in severity. One patient who experienced 3 episodes of anemia had improved symptoms after 3 dose reductions of linezolid, while the other patient did not improve after dose reductions. One patient had abnormal hemoglobin levels and improved after the discontinuation of linezolid, while the other patient developed anemia and the outcome was unknown after the discontinuation of linezolid. The remaining patient developed 2 episodes of abnormal hemoglobin after 55 days and 68 days of linezolid treatment, respectively, and achieved improvement after the discontinuation of linezolid. Gastrointestinal reactions occurred in 3 patients who were treated with prothionamide, and all patients improved after discontinuation or permanent discontinuation of prothionamide. There were 6 patients with liver toxicity, with 1 case related to the combination of prothionamide and pyrazinamide, 2 cases related to pyrazinamide treatment, and 1 case each associated with levofloxacin or prothionamide, respectively; the other case was associated with chronic hepatitis B.