Treatment

From DrugPedia: A Wikipedia for Drug discovery

Active tuberculosis will kill about two of every three people affected if left untreated. Treated tuberculosis has a mortality rate of less than 5% (or less in developed countries where intensive supportive measures are available).

The standard "short" course treatment for tuberculosis (TB), if it is active, is isoniazid, rifampicin, pyrazinamide, and ethambutol for two months, then isoniazid and rifampicin alone for a further four months. The patient is considered cured at six months (although there is still a relapse rate of 2 to 3%). For latent tuberculosis, the standard treatment is six to nine months of isoniazid alone.

If the organism is known to be fully sensitive, then treatment is with isoniazid, rifampicin, and pyrazinamide for two months, followed by isoniazid and rifampicin for four months. Ethambutol need not be used.

Drugs

All first-line anti-tuberculous drug names have a standard three-letter and a single-letter abbreviation:

• ethambutol is EMB or E,
• isoniazid is INH or H,
• pyrazinamide is PZA or Z,
• rifampicin is RMP or R,
• streptomycin is STM or S.

The US commonly uses abbreviations and names that are not internationally recognised: rifampicin is called rifampin and abbreviated RIF; streptomycin is commonly abbreviated SM.

Drug regimens are similarly abbreviated in a standardised manner. The drugs are listed using their single letter abbreviations (in the order given above, which is roughly the order of introduction into clinical practice). A prefix denotes the number of months the treatment should be given for; a subscript denotes intermittent dosing (so ₃ means three times a week) and no subscript means daily dosing. Most regimens have an initial high-intensity phase, followed by a continuation phase (also called a consolidation phase or eradication phase): the high-intensity phase is given first, then the continuation phase, the two phases divided by a slash.

So,

2HREZ/4HR₃

means isoniazid, rifampicin, ethambutol, pyrazinamide daily for two months, followed by four months of isoniazid and rifampicin given three times a week.

These standard abbreviations are used in the rest of this article.

There are six classes of second-line drugs (SLDs) used for the treatment of TB. A drug may be classed as second-line instead of first-line for one of two possible reasons: it may be less effective than the first-line drugs (e.g., p-aminosalicylic acid); or, it may have toxic side-effects (e.g., cycloserine); or it may be unavailable in many developing countries (e.g., fluoroquinolones):

• aminoglycosides: e.g., amikacin (AK), kanamycin;
• polypeptides: e.g., capreomycin, viomycin, enviomycin;
• fluoroquinolones: e.g., ciprofloxacin (CIP), levofloxacin, moxifloxacin (MXF);
• thioamides: e.g. ethionamide, prothionamide
• cycloserine (the only antibiotic in its class);
• p-aminosalicylic acid (PAS or P).

Other drugs that may be useful, but are not on the WHO list of SLDs:

• rifabutin
• macrolides: e.g., clarithromycin (CLR);
• linezolid (LZD);
• thioacetazone (T);
• thioridazine;
• arginine;
• vitamin D;
• R207910.

These drugs may be considered "third-line drugs" and are listed here either because they are not very effective (e.g., clarithromycin) or because their efficacy has not been proven (e.g., linezolid, R207910). Rifabutin is effective, but is not included on the WHO list because for most developing countries, it is impractically expensive.

The standard regimen

Rationale and evidence for the standard regimen

Tuberculosis has been treated with combination therapy for over fifty years. Drugs are not used singly (except in latent TB or chemoprophylaxis), and regimens that use only single drugs result in the rapid development of resistance and treatment failure.<ref name="MRC1948">Medical Research Council Streptomycin in Tuberculosis Trials Committee (1948). "Streptomycin treatment for pulmonary tuberculosis". Brit Med J ii: 769–82. </ref><ref name="Wang2006">Wang J-Y, Hsueh P-R, Jan I-S, et al. (2006). "Empirical treatment with a fluoroquinolone delays the treatment for tuberculosis and is associated with a poor prognosis in endemic areas". Thorax 61: 903–8. doi:10.1136/thx.2005.056887. PMID 16809417. </ref> The rationale for using multiple drugs to treat TB are based on simple probability. The frequency of spontaneous mutations that confer resistance to an individual drug are well known: 1 in 10⁷ for EMB, 1 in 10⁸ for STM and INH, and 1 in 10¹⁰ for RMP.<ref name="David1970">David H. L. (1970). "Probability Distribution of Drug-Resistant Mutants in Unselected Populations of Mycobacterium tuberculosis". Appl Microbiol 20 (5): 810–4. PMID 4991927. </ref> A patient with extensive pulmonary TB has approximately 10¹² bacteria in his 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 him harbouring a bacterium that is spontaneously resistant to both INH and RMP is 1 in 10⁶, and the chances of him harbouring a bacterium that is spontaneously resistant to all four drugs is 1 in 10¹¹. 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. RMP began to be used to treat TB in 1968 and the BTS study in the 1970s showed that 2HRE/7HR was efficacious. In 1984, a BTS study showed that 2HRZ/4HR was efficacious,<ref name="BTS1984">British Thoracic Society (1984). "A controlled trial fo six months' chemotherapy in pulmonary tuberculosis. Final report: results during the 36 months after the end of chemotherapy and beyond". Brit J Diseases Chest 78 (4): 330–36. PMID 6386028. </ref> with a relapse rate of less than 3% after two years.<ref name="Ormerod1987">Ormerod LP, Horsfield N (1987). "Short-course antituberculous chemotherapy for pulmonary and pleural disease: five years' experience in clinical practice". Brit J Diseases Chest 81 (3): 268–71. doi:10.1016/0007-0971(87)90160-4. </ref> In 1995, with the recognition that INH resistance was increasing, the BTS recommended adding EMB or STM to the regimen: 2HREZ/4HR or 2SHRZ/4HR, which are the regimens currently recommended. The WHO also recommend a six month continuation phase of HR if the patient is still culture positive after 2 months of treatment (approximately 15% of patients with fully-sensitive TB) and for those patients who have extensive bilateral cavitation at the start of treatment.

Monitoring and DOTS

DOTS stands for "Directly Observed Therapy, Short-course" and is a major plank in the WHO global TB eradication programme. The WHO advises that all TB patients should have at least the first two months of their therapy observed (and preferably the whole of it observed): this means an independent observer watching tuberculosis patients swallow their anti-TB therapy. The independent observer is often not a healthcare worker and may be a shopkeeper or a tribal elder or similar senior person within that society. DOTS is used with intermittent dosing (thrice weekly or 2HREZ/4HR₃). Twice weekly dosing is effective<ref> "A 62-dose, 6 month therapy for pulmonary and extrapulmonary tuberculosis: A twice-weekly, directly observed, and cost-effective regimen" (1990). Ann Intern Med 112 (6): 407–415. PMID 2106816. </ref> but not recommended by the WHO, because there is no margin for error (accidentally omitting one dose per week results in once weekly dosing, which is ineffective).

Treatment with properly implemented DOTS has a success rate exceeding 95% and prevents the emergence of further multi-drug resistant strains of tuberculosis.

Some people recommend monthly surveillance until cultures convert to negative; this does not form any part of the UK or WHO recommendations for TB. If cultures are positive or symptoms do not resolve after three months of treatment, it is necessary to re-evaluate the patient for drug-resistant disease or nonadherence to drug regimen. If cultures do not convert to negative despite three months of therapy, consider initiating directly observed therapy.

Extra-pulmonary tuberculosis

Tuberculosis not affecting the lungs is called extra-pulmonary tuberculosis. Disease of the central nervous system is specifically excluded from this classification.

The UK and WHO recommendation is 2HREZ/4HR; the US recommendation is 2HREZ/7HR. There is good evidence from randomised-controlled trials to say that in tuberculous lymphadenitis<ref>Campbell IA, Ormerod LP, Friend JA, Jenkins PA, Prescott RJ. (1993). "Six months versus nine months chemotherapy for tuberculosis of lymph nodes: final results". Respir Med. 87 (8): 621–3. doi:10.1016/S0954-6111(05)80265-3. PMID 8290746. </ref> and in TB of the spine,<ref>Upadhyay SS, Saji MJ, Yau AC. (1996). "Duration of antituberculosis chemotherapy in conjunction with radical surgery in the management of spinal tuberculosis". Spine 21 (16): 1898–1903. doi:10.1097/00007632-199608150-00014. </ref><ref>Medical Research Council Working Party on tuberculosis of the spine.. "Five-year assessment of controlled trials of chort-course chemotherapy regimens of 6, 9 or 18 months' duration for spinal tuberculosis in patients ambulatory from the start or undergoing radical surgery". Int Orthopaed 23 (2): 73–81. </ref><ref>Parthasarathy R, Sriram K, Santha T, et al. (1999). "Short-course chemotherapy for tuberculosis of the spine: a comparison between ambulant treatment and radical surgery—ten-year report". J Bone Joint Surg Brit Vol 81B (3): 464–71. doi:10.1302/0301-620X.81B3.9043. PMID 10872368. </ref> the six month regimen is equivalent to the nine month regimen; the US recommendation is therefore not supported by the evidence.

Up to 25% of patients with TB of the lymph nodes (TB lymphadenitis) will get worse on treatment before they get better and this usually happens in the first few months of treatment. A few weeks after starting treatment, lymph nodes often start to enlarge, and previously solid lymph nodes may become fluctuant. This should not be interpreted as failure of therapy and is a common reason for patients (and their physicians) to panic unnecessarily. With patience, two to three months into treatment the lymph nodes start to shrink again and re-aspiration or re-biopsy of the lymph nodes is unnecessary: if repeat microbiological studies are ordered, they will show the continued presence of viable bacteria with the same sensitivity pattern, which further adds to the confusion: physicians inexperienced in the treatment of TB will then often add second-line drugs in the belief that the treatment is not working. In these situations, all that is required is re-assurance. Steroids may be useful in resolving the swelling, especially if it is painful, but they are unnecessary. Additional antibiotics are unnecessary and the treatment regimen does not need to be lengthened.