The rate of spontaneous mutations that confer resistance to an individual drug are well known: 1 mutation for every 107 cell divisions for EMB, 1 for every 108 divisions for STM and INH, and 1 for every 1010 divisions for RMP.Patients with extensive pulmonary TB have approximately 1012 bacteria in their body, and therefore will probably be harboring approximately 105 EMB-resistant bacteria, 104 STM-resistant bacteria, 104 INH-resistant bacteria and 10² RMP-resistant bacteria. Resistance mutations appear spontaneously and independently, so the chances of them harbouring a bacterium that is spontaneously resistant to both INH and RMP is
1 in 108 × 1 in 1010 = 1 in 1018, and the chances of them harbouring a bacterium that is spontaneously resistant to all four drugs is 1 in 1033. This is, of course, an oversimplification, but it is a useful way of explaining combination therapy.There are other theoretical reasons for supporting combination therapy. The different drugs in the regimen have different modes of action. INH are bacteriocidal against replicating bacteria. EMB is bacteriostatic at low doses, but is used in TB treatment at higher, bactericidal doses. RMP is bacteriocidal and has a sterilizing effect. PZA is only weakly bactericidal, but is very effective against bacteria located in acidic environments, inside macrophages, or in areas of acute inflammation.All TB regimens in use were 18 months or longer until the appearance of rifampicin. In 1953, the standard UK regimen was 3SPH/15PH or 3SPH/15SH2. Between 1965 and 1970, EMB replaced PAS.
Another meta-analysis came to a similar conclusion, namely that rifamycin-containing regimens taken for 3 months or longer had a better profile in preventing TB reactivation. Research
There is some evidence from animal and clinical studies that suggests that moxifloxacin-containing regimens as short as four months may be as effective as six months of conventional therapy.Bayer is currently running a phase II clinical trial in collaboration with the TB Alliance to evaluate shorter treatment regimens for TB; encouragingly, Bayer have also promised that if the trials are successful, Bayer will make moxifloxacin affordable and accessible in countries that need it. Another approach for anti-TB drug development, which does not rely on antibiotics, consists of targeting NAD+ synthase, an essential enzyme in tuberculosis bacteria but not in humans. Low level laser therapy for treating tuberculosis is not supported by reliable evidence. History
Streptomycin and para-aminosalicylic acid were developed by the mid-1940s. In 1960, Edinburgh City Hospital physician Sir John Crofton, addressed the Royal College of Physicians in London with a lecture titled "Tuberculosis Undefeated", and proposed that "the disease could be conquered, once and for all". With his colleagues at Edinburgh, he recognised that germs that developed only a mild resistance to one drug was significant. His team showed that when treating new cases of TB, strict compliance to a combination of three therapies, or the triple therapy, (streptomycin, para-aminosalicylic acid and isoniazid) could provide a complete cure. It became known as the Edinburgh method and became standard treatment for at least 15 years.