Why are new TB drugs needed?
New TB drugs are needed because of the complexity and toxicity of the current TB drug regimes. There is also the major problem of TB drug resistance. This together with the problem of the interactions of the current TB drugs with the ARVs taken by HIV positive people, means that there is an urgent need for new TB drugs. However, what is required of these new TB drugs is considerable as:
New TB drugs need to provide:
- Shorter and simpler, but still affordable, multi drug regimes for drug sensitive TB
- Shorter, more effective , less toxic, and less expensive regimes for drug resistant TB
- Short, simple, easily tolerable and safe regimes for latent TB
- Drugs with few drug drug interactions, so they can be safely provided for people with HIV.
Before a new drug is made available for general use it needs to go through a series of clinical trials. This is to make sure that among other things, it is effective and doesn’t have too many side effects. The main trials for any drug are referred to as phases 1, 2 and 3. In addition there are now trials referred to as phase 2a and phase 2b.1“Phases of clinical research”, Wikipedia
https://en.wikipedia.org/wiki/Phases_of_clinical_research Those drugs going through a phase 3 trial are the closest to being made available for general use, provided of course that the phase 3 trial is successful.
A phase three trial for any drug takes a long time. A phase 3 trial for a TB drug requires a lengthy TB drug treatment period. This has to then be followed by a one to two year period of patient follow up to determine treatment failure and relapse. Only then is the effectiveness of the TB drug known.
In any TB drug trial the proposed new TB drug must not be taken on its own. It must be taken with at least two other existing TB drugs or with other new TB drugs.
The Nix-TB trial is the first TB clinical trial to test a new drug combination which has the possibility of being a shorter, all oral, and affordable treatment for XDR-TB. This combination does not require injections and has far fewer pills. The Nix-TB drug combination is pretomanid, bedaquiline and linezolid. This drug combination is predicted to be able to cure XDR-TB in six to nine months.
In February 2017 the results became available for the first 72 people to be treated in the study. Thirty one people had completed treatment and 6 months of post treatment follow up. Of these 31 people, 29 people cleared XDR-TB. This means that TB bacteria could no longer be cultured from their sputum. Twenty people stopped taking the drug after six months of follow up and only one relapsed.
New TB drugs under development
A drugs “pipeline” in drug development terms, refers to all the drugs at different stages of development. It also shows how the drugs are progressing down the “pipeline” through the various clinical trial stages etc. At the point at which they exit the “pipeline” they are available for general use.
The pipeline of new drugs for TB under clinical development is not considered to be a very extensive drugs “pipeline” for such a major disease as TB.2Zhenkun, Ma. “Global tuberculosis drug development pipeline: the need and the reality”, The Lancet, Vol 375(9731), 12 June 2010, 2011-2109
The Stop TB Partnership has a Working Group on New TB Drugs, which helps to coordinate, guide, and accelerate the speed of worldwide development of new TB therapies.3“Accelerating Discovery”, www.newtbdrugs.org
TB drug bedaquiline
Bedaquiline is the active substance in the new TB drug Sirturo. Sirturo is available for the treatment of drug resistant TB, when there are an insufficient numbers of other TB drugs available. There is much more about bedaquiline.
The Nitroimidazoles are an existing class of drugs known to have antimicrobial activity. Two “next generation” or derivatives of this class of drugs, OPC-67683 (now also known as delamanid) and PA-824 are under development as potential TB drugs.4“OPC-67683”,
www.tbonline.info/ There is much more about delamanid.
TB drug Pretomanid (PA-824)
PA-824 is another nitroimidazo-oxazole currently being developed by the TB Alliance. It also can potentially be used for the treatment of both drug sensitive and drug resistant TB, and it has also shown activity against both latent and active TB.
TB drug TBA-354
In March 2016 it was announced that the TB Alliance had voluntarily halted the clinical development program for TBA-354.5“Phase 1 Clinical Trial of TB Drug Candidate TBA-354 Discontinued”,
AZD5847 is a potential new TB drug being developed by AstraZeneca. In December 2012 it was announced that the first patient had been enrolled in a Phase 2a trial of the drug in South Africa, to assess the effectiveness of the drug for patients with TB, including patients with HIV and TB coinfection.6“AstraZeneca, Case Western Reserve initiate AZD5847 Phase 2a trial in TB”, 12 December 2012,
www.news-medical.net/news/20121212/AstraZeneca-Case-Western-Reserve-initiate-AZD5847-Phase-2a-trial-in-TB The study is sponsored by the National Institute of Allergy and Infectious Diseases (NIAID), which is part of the U.S. National Institutes of Health.
Several members of the fluoroquinoles class of drugs are currently already used as second line TB drugs for the treatment of multi drug resistant TB. However, the older drugs such as ofloxacin and levofloxacin are often used rather than the newer fluoroquinolones moxifloxacin and gatifloxacin. Moxifloxacin and gatifloxacin are currently being developed for the treatment of drug sensitive TB.
Each of these potential TB drugs is currently undergoing evaluation in a phase 3 trial, to see how effective it is when substituted for either ethambutol or isoniazid, to shorten the treatment of drug sensitive TB from the standard six months to four months. The gatifloxacin trial is being conducted by the Oflotub Consortium and moxifloxacin is being developed by Bayer and the TB Alliance.
A recent meta analysis found that fluoroquinolone substitution for isoniazid or ethambutol in short course regimens might result in more frequent unfavourable treatment outcomes compared with the standard regimen, in particular an increased incidence of relapse.7Qiaoling, R. "Moxifloxacin and gatifloxacin for initial therapy of tuberculosis: a meta-analysis of randomized clinical trials", Emerging Microbes & Infections, Feb 2016 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4777926/
The Rifamycins are potent inhibitors of mycobacterial activity. Three semi synthetic rifamycins - rifampicin, rifapentine, and rifabutin - have been used for the treatment of various microbial infections. Rifampicin is a key component of first line drug treatment for TB. Rifapentine is attractive as a possible TB drug for shortening treatment, and for intermittent TB drug treatment, and clinical trials are under way to further assess this.8Zhenkun, Ma. “Global tuberculosis drug development pipeline: the need and the reality”, The Lancet, Vol 375(9731), 12 June 2010, 2011-2109
TB drug combinations
What is ideally needed is not just one new TB drug but several which can be used together. In July 2012 it was announced that a phase 2A study had been carried out investigating a number of drug combinations including PA-824 over a 14 day period to assess their suitability for further development. The combination of PA-824 moxifloxacin and pyrazinamide had the greatest early bactericidal activity (EBA).
One advantage of this drug combination is that it does not involve either isoniazid or rifampicin. It is therefore suitable for use with patients who are resistant to these drugs. A regimen without rifampicin would greatly simplify the provision of TB treatment alongside HIV antiretroviral therapy.
This three drug combination now needs to be further investigated over a longer period of time, taking into account that PA-824 and moxifloxacin all have at least some potential to cause cardiac side effects.9Diacon, A. “14-day bactericidal activity of PA-824, bedaquiline, pyrazinamide, and moxifloxacin combinations: a randomised trial”, The Lancet, 23 July 2012,