Isoniazid (INH) is a drug commonly used to treat tuberculosis (TB) worldwide. Sometimes, the bacteria that cause TB can become resistant to INH. Resistance means that bacteria have adapted to a drug and are able to live in the presence of the drug. When TB becomes resistant to INH, INH does not work as well at fighting the bacteria. This study treated people with INH-resistant TB with different doses of INH to see if INH can still fight the bacteria if the dose is increased. We evaluated how well the drug works at higher doses for participants who have resistant TB as well as how well the drug works at regular doses for participants who have TB that is not resistant. The study also evaluated the safety and tolerability of the different doses of INH. Tolerability is how well people can put up with the side effects of a drug. Using increased doses of INH to treat TB that is resistant to INH is experimental and has not been approved by regulatory authorities. While there is some evidence that this approach will work, this has not yet been proven.

A5312 was a two-stage, two-step, phase IIa, open-label, randomized clinical trial among adult participants with sputum smear positive pulmonary TB evaluating the early bacterial activity (EBA).

No study drug was administered under Step 1. Data collected in Step 1: (a) determined eligibility to Step 2, and (b) allowed characterization of INH MICs in three groups. Groups 1, 2, and 3 consist of participants infected with TB with inhA mutations, with drug susceptible TB (DS-TB), and with TB with katG resistance-conferring mutations, respectively.

Participants enrolled to Step 2 received the study drug, INH, which was given with vitamin B6 >=25 mg daily, by mouth. During both stages, participants in Group 1 who met Step 2 entry criteria were randomized to receive 5, 10, or 15 mg/kg of INH daily for 7 days. During Stage 2, participants in Group 2 who met Step 2 entry criteria received 5 mg/kg of INH daily for 7 days. Under protocol version 3.0 during Stage 2, participants in Group 3 who met Step 2 entry criteria were randomized to receive 15 or 20 mg/kg of INH daily for 7 days. After completion of 7 days of INH alone, participants were referred to begin standard anti-TB chemotherapy according to local guidelines.

In Step 2, prior to initiation of treatment, sputum was collected for quantitative culture on solid medium (for colony forming units (CFU) for Groups 1 and 2 only) and liquid medium (for determination of time to positivity (TTP) for all groups). Sixteen-hour sputum collections were performed daily during INH treatment, as per standard early bacterial activity (EBA) methodology. Sampling for PK analysis was performed at steady state on Day 6 (±1). Safety and tolerability were monitored via clinical evaluations throughout the study and through scheduled laboratory evaluations.

The study consisted of two stages, as follows:

Stage 1-Pilot study to ensure feasibility:

The goal of Stage 1 was to demonstrate feasibility, not treatment efficacy.

Participants were recruited at a single clinical site. All eligible participants entered Step 1 of the study (determination of INH resistance, measurement of INH minimum inhibitory concentrations (MIC)). Among Group 1 participants who met the Step 2 entry criteria, 15 participants were randomized 1:1:1 to receive 5, 10, or 15 mg/kg daily of INH for 7 days with evaluations performed as described above.

Stage 1 completed March 26, 2015. A total of 15 Group 1, 44 Group 2, and 12 Group 3 participants were enrolled in Step 1 only during Stage 1. These participants did not receive study treatment. They provided sputum samples for MIC determination.

Stage 2-Main study:

During Stage 2, Group 1 participants who met Step 2 entry criteria were randomized 1:1:1 to receive 5, 10, or 15 mg/kg of INH daily for 7 days. Group 2 participants who met Step 2 entry criteria were enrolled and received INH at a dose of 5 mg/kg daily. Group 3 participants who met Step 2 entry criteria were enrolled and randomized 1:1 to receive INH at a dose of 15 or 20 mg/kg daily.

In Stage 1, Group 1 participants who did not meet Step 2 entry criteria, all Group 2 participants and all Group 3 participants were referred to a local TB program for treatment. In Stage 2, Group 1, 2 and 3 participants who did not meet Step 2 entry criteria, were referred to a local TB program for treatment.

Protocol Versions:

Key differences in protocol versions include the following:

• Study entry criteria were changed from protocol versions 1.0 to 2.0

  • No longer excluded individuals for antiretroviral therapy use
  • Exclusion of individuals with any MDR-TB treatment with second-line anti-TB drugs was relaxed to exclude only those with more than 7 cumulative days of use.

• Protocol version 2.0 allowed additional sites to enroll participants.

• Under protocol versions 1.0 and 2.0, eligible individuals in Group 1 and Group 2 could enroll in Step 1 and Step 2, and eligible individuals in Group 3 could be enrolled in Step 1 only. Under protocol version 3.0, eligible individuals in Group 3 could enroll in Step 1 and Step 2.

• Early bactericidal activity was described using both solid culture CFU and liquid culture TTP under protocol versions 1.0 and 2.0. Under protocol version 3.0, early bactericidal activity was measured by liquid culture TTP only.

• There were technical difficulties with measuring the isoniazid minimum inhibitory concentration using 1% agar solution for participants enrolled under protocols versions 1.0 and 2.0. For protocol version 3.0, the Thermo Fisher MYCOTB Sensititre plate was substituted to overcome the technical difficulties.

Inclusion Criteria for Step 1:

• New or recurrent pulmonary TB with sputum positive for acid-fast bacilli on direct microscopy of at least grade 1+ (International Union Against Tuberculosis and Lung Disease [IUATLD] scale) at the study laboratory on at least one pre-treatment sputum sample within 14 days prior to entry.

• Infected with an M. tuberculosis strain for which Hain GenoType MTBDRplus genotype, performed at the study laboratory within 14 days prior to study entry, reveals one of the following results for INH susceptibility testing:

  • inhA promoter or functional mutation only (Group 1 participants, eligible for Steps 1 and 2)
  • No mutations in the inhA or katG genes (Group 2 participants, eligible for Step 1 and, during Stage 2 of the study, also eligible for Step 2)
  • katG mutation with or without an inhA mutation (Group 3 participants, eligible for Step 1 and, during Stage 2 of the study, also eligible for Step 2)

• Ability and willingness of the participant or legal guardian/representative to provide informed consent.

Inclusion Criteria for Step 2:

• Entry into Step 1.

• During Stage 1 of the protocol: inhA promoter or functional mutation only (Group 1).

• During Stage 2 of the protocol: inhA promoter or functional mutation only (Group 1) OR mutations in neither inhA nor katG genes (Group 2) or mutation in the katG gene, with or without mutations in inhA promoter or functional genes (Group 3).

• Body weight: 40 kg to 90 kg, inclusive.

• Laboratory values obtained within 30 days prior to entry:

  • Absolute neutrophil count (ANC) >=750 cells/mm^3
  • Hemoglobin >= 7.4 g/dL
  • Platelet count >= 50,000/mm^3
  • Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) <=3 X upper limit of normal (ULN)
  • Total bilirubin <=2.5 X ULN

• HIV infection status must be documented as either absent or present, as defined below:

Absence of HIV-1 infection within 30 days prior to Step 2 entry OR HIV-1 infection at any time prior to Step 2 entry.

• For HIV-positive candidates only: CD4+ cell count of >=50 cells/mm^3, performed within 7 days prior to entry at a DAIDS-approved laboratory
• For females of reproductive potential, negative serum or urine pregnancy test within 7 days prior to entry. Female participants who are participating in sexual activity that could lead to pregnancy must agree to use one reliable non-hormonal method of contraception (condoms or an IUD), or another method (diaphragm or cervical cap) if it is approved by the national regulatory authority and used according to package insert, while receiving study medications.
• Willingness to be hospitalized for a minimum of 9 consecutive days.
• Ability to produce an overnight sputum sample of sufficient quality and quantity.

Exclusion Criteria for Step 1:

-There are no exclusion criteria for Step 1.

Exclusion Criteria for Step 2:

-Current treatment with INH or receipt of INH during the 7 days prior to Step 2 entry.

NOTE: Participants who have been started on INH-containing anti-TB treatment and have received this treatment for less than or equal to 2 weeks, but for whom TB drugs have been discontinued because of resistance to INH (with or without resistance to RIF), can participate in the study, but may need to be hospitalized, at the discretion of the investigator, while these drugs wash out; the minimum washout period for these drugs is 7 days.

• Protocol versions 1.0 and 2.0 only: Any prior history of treatment for MDR-TB with second-line anti-TB drugs. This includes all drugs with anti-TB activity, except INH, RIF, ethambutol, pyrazinamide, and streptomycin.

• Protocol versions 1.0 and 2.0 only: Receipt of more than 7 cumulative days of antibiotics intended for bacterial treatment that may have anti-TB activity, including amoxicillin/clavulanate (Augmentin), linezolid, metronidazole, or drugs from the quinolone class, with any of those days falling within the 14 days prior to Step 1 screening sputum collection.

• Protocol version 3.0 only: Receipt of more than 7 cumulative days of second-line anti-TB drugs (including all drugs with anti-TB activity, except INH, RIF, ethambutol, pyrazinamide, and streptomycin) and/or antibiotics intended for bacterial treatment that may have anti-TB activity, including amoxicillin/clavulanate (Augmentin), linezolid, metronidazole, or drugs from the quinolone class, with any of those days falling within the 14 days prior to Step 1 screening sputum collection. The minimum washout period for these drugs is 7 days prior to Step 2 pre-entry sputum collection.

• Known exposure to a person diagnosed with extensively drug-resistant (XDR)-TB or known personal diagnosis of XDR-TB in the past.

• Protocol version 1.0: For HIV+ participants only: Current treatment, or treatment within 30 days prior to entry, with antiretroviral therapy (ART) or expected to initiate ART within 8 days after Step 2 entry. Prior receipt of ART for the prevention of mother-to-child-transmission is not exclusionary.

• Breastfeeding.

• Known allergy/sensitivity to INH.

• Karnofsky score <60 or poor general condition where any delay in full TB treatment cannot be tolerated in the opinion of the investigator (at screening).

• Any of the following co-morbidities, complications, or underlying medical conditions:

  • Known current neurological TB (eg, TB of the spine, TB meningitis)
  • Peripheral neuropathy >=Grade 2 within 14 days prior to entry
  • Current or history of epilepsy, defined as seizure disorder requiring current treatment with an antiepileptic medicine or history of any seizures within the prior year

• Any condition as determined by physical examination, medical history, laboratory data, or chest x-ray which, in the opinion of the investigator, would interfere with participation in the study.

All participants

Measured from baseline through Day 21

For participants who took at least one dose of study treatment in Step 2, all new diagnoses (per ACTG criteria for clinical events and other diseases), signs/symptoms (S/S) of grade 2 or higher or any S/S that led to a change in treatment regardless of grade, and all laboratory events regardless of grade. The DAIDS AE Grading Table (V1.0) and EAE Manual (V2.0) were used. Adverse events were only collected on participants who started study treatment. No treatment was administered in Step 1.