5 курс / Пульмонология и фтизиатрия / Clinical_Tuberculosis_Friedman_Lloyd_N_,_Dedicoat

The development of faropenem, a structurally similar compound that has been modified for oral administration, has displayed bactericidal activity against M. tuberculosis in vitro even in the absence of clavulanic acid,168 and offers a more practical option than other drugs in the class that require parenteral administration.

Interestingly there was greater variability in the MIC for both amoxicillin and carbapenems amongst drug-resistant clinical isolates, with some strains having MICs for amoxicillin−clavulinic acid of less than 1 µg/mL.162,163 This suggests some drug-resistant strains, including XDR-TB strains, might be hyper-susceptible to β-lactams relative to drug-sensitive M. tuberculosis, and therefore more amenable to therapy with them.

The widespread use of β-lactam antibiotics means they have relatively well-established safety profiles and can be used for pediatric MDR-TB, where drug options are far more limited than for adults. However, due to their broad spectrum of action they are at risk of affecting the host microbiome and their common use for other indications means that, as for the fluoroquinolones, there is a risk that undiagnosed patients may be exposed to them as monotherapy, promoting the development of resistance.

Clofazimine

Clofazimine, a riminophenazine, was initially developed in the 1950s for the treatment of M. tuberculosis164 but found its place in the treatment of leprosy. Its mode of action has only recently been partly elucidated and remains incomplete. Redox cycling is likely to be important, whereby clofazimine undergoes cycles of enzymatic reduction, generating reactive oxygen species that are toxic to the cell.169 In addition, destabilization and dysfunction of the mycobacterial membrane, perhaps as a consequence of the accumulation of lysophospholipids and interference with potassium ion uptake may be of importance.170,171 As for bedaquiline, resistance is conferred by mutations in Rv0678 (which lead to drug efflux) and potentially pepQ.170,171 Additionally, mutations in Rv1979c, which encodes a putative permase, may confer clofazimine monoresistance, although similarly to pepQ, the significance of these mutations in clinical isolates remains undetermined.196 It is active against persister organisms in vitro172 and has been successfully combined with bedaquiline and pyrazinamide15 to effectively treat mice in a relapse-free short-course regimen. Perhaps most remarkable are its lipophilic properties that result in massive tissue accumulation in spleen, liver, and other organs.173 In the lungs of mice chronically treated with clofazimine it can reach tissue concentrations of >100-fold the MIC of 0.5 µg/mL. Part of this accumulation is the formation of crystal-like drug inclusion bodies174 and accumulation has also been reported in human lung samples.

Interest refocused on its potential as an anti-TB agent after reports of a highly successful short-course treatment for MDR-TB in a cohort of patients from Bangladesh.5 Using a 9-month treatment protocol a relapse-free cure was obtained in 87% of patients. In combination with clofazimine the regimen included high-dose isoniazid and gatifloxacin alongside other standard MDR-TB drugs. The regimen’s success may have been dependent on a high proportion of patients with pyrazinamide-susceptible strains or strains with low-level isoniazid resistance, rather than the activity

of clofazimine alone. However, it did show that even with currently available drugs in select patient groups high success rates can be achieved with short courses of treatment. Subsequently, a meta-analysis of 12 030 patients from 25 countries, used to inform WHO guidelines, identified an adjusted risk difference of 0.06 (95% CI 0.01–0.10) for treatment success versus failure or relapse with the use of clofazimine.23 This has led to the re-positioning of clofazimine as a priority drug in the treatment of MDR TB.

HOST-DIRECT THERAPY

The challenge to identify and develop sufficient new drugs that are both active against M. tuberculosis and have an acceptable human safety profile while attempting to prevent acquisition and spread of resistance has led to increased interest in modulation of the human immune response through host-directed therapies to improve the efficacy of antibiotics and potentially shorten treatment, reduce TB-related immunopathology and enhance immunological memory to prevent relapse.

Vitamin D

Vitamin D was first identified as a potential treatment for TB through the use of cod liver oil in 1849175 and has more recently been shown to enhance mycobacterial killing by macrophages and resolution of inflammatory responses during TB treatment.176 Deficiency has been associated with an increased risk of TB in two meta-anal- yses,177,178 although, in one, this relationship was not found in subgroup analyses on African patients and HIV-infected patients.

Despite these associations, no evidence was found to indicate that adding vitamin D to standard TB treatment improves outcomes in a meta-analysis of five studies,179 a subsequent doubleblind placebo-controlled trial180 or Cochrane review of eight studies.181 A more recent individual patient meta-analysis suggested that vitamin D supplementation did reduce time to sputum culture conversion in patients with MDR-TB but not drug-suscep- tible TB, although numbers with MDR-TB were small and followup was limited to 8 weeks.182 At present there is little evidence to suggest that any benefit is of clinical relevance.

Metformin

Metformin, a synthetically derived biguanide from the Galega officinalis plant, was introduced as a glucose-lowering treatment for type 2 diabetes mellitus (T2DM) in the 1950s. In addition to other proposed mechanisms of action, it is known to inhibit the mitochondrial respiratory chain in the human liver by activating AMP-activated protein kinase (AMPK), which switches on catabolic pathways generating cellular ATP and stops those involved in gluconeogenesis, leading to a reduction in glucose. It is of particular interest given that T2DM is associated with both an increased risk of developing active TB183 and worse outcomes once on treatment.184

Metformin-induced AMPK activation has an anti-inflamma- tory effect through promoting formation of anti-inflammatory M2 macrophages and T-regulatory and CD8 memory T cells and

regulation of cellular autophagy, a key mechanism in the control of intracellular pathogens like M. tuberculosis.185 Recent work showed that macrophages exposed to metformin in vitro had greater antimycobacterial activity through increased production of reactive oxygen species and promotion of phago-lysosome fusion. In the same study, TB-infected mice treated with metformin had reduced lung pathology and chronic inflammation and enhanced mycobacterial clearance when used in combination with isoniazid or ethambutol.186 However, another study showed that when metformin was combined with the full four-drug first-line regimen there was no statistically significant increase in bactericidal activity.187

Retrospective cohort data from patients with T2DM taking TB treatment has shown that those taking metformin had lower mortality than those not in a mechanism independent of improved blood glucose control.188 However, these retrospective data may be prone to confounding as metformin is commonly prescribed as a first-line treatment for T2DM, with other treatments such as insulin often added to rather than replacing metformin, and because patients prescribed other medications often have other comorbidities such as renal failure.

In summary, for patients with T2DM and TB co-infection, while retrospective data suggest a benefit to metformin use, pending further prospective clinical studies there remain insufficient data to make any specific recommendations. In patients without diabetes mellitus, it is possible that metformin may have some additive effect to TB treatment through immune modulation, but translation to clinical use will first require a more comprehensive understanding of its action in vitro and in the mouse model.185

NEW REGIMENS

To prevent resistance development during prolonged therapy and to target differentially metabolically active mycobacterial subpopulations, antituberculous therapy is delivered in multi-drug regimens. However, new drugs are usually either added to, or used to replace, a single drug in existing multi-drug regimens for both drug-sensitive and MDR TB. The characteristics of an “ideal” regimen (or “target regimen profiles”, TRPs) for drug-sensitive TB, MDR TB and a “universal” regimen have been outlined by the WHO. Increasingly, potential regimens are identified on the basis of their possible advantages, e.g., all oral treatment, high activity, synergism or lack of use in existing regimens, and then assessed in a systematic fashion using in vitro, murine, 14-day phase IIa (of monotherapy or combinations) and 8-week phase IIb combination studies in humans. However, this process is hampered by the lack of a clearly defined optimal pathway whereby these pre-clinical and early-phase clinical studies (and the various measures of anti-mycobacterial activity that come with them) are able to predict clinically important outcomes. Several solutions have been proposed to reduce the risk of phase III trial failures and more efficiently shepherd efficacious regimens into clinical use. There is increasing interest in phase IIc trials in which the same outcomes are measured, with a similar sample size to that in other phase II studies, but the experimental regimen is given for the full duration planned for the phase III trial, and 12-month post-treatment completion outcomes are assessed. These studies

might best be combined with a Bayesian statistical approach in order to provide actionable outcomes, predicting the chances of success if a given regimen was moved into phase III.189 An alternative or potentially complementary approach is the multi-arm multi-stage (MAMS) trial design where multiple experimental arms are evaluated simultaneously, with poorly performing arms dropped early, minimizing time, expense and sample size, thus advancing the most promising regimens.

CONCLUSIONS

The US FDA approval of Bedaquiline is a milestone in the development of new anti-TB drugs.10 Bedaquiline is highly selective for mycobacteria, but its target, ATP synthase, is an essential energy-generating mechanism, found throughout all kingdoms of life. Modification of bedaquiline has led to compounds active against other clinically important bacteria,190,191 so it also represents the discovery of a novel class of antibiotics. It is reminiscent of the discovery of streptomycin in 1943 as part of a search for anti-TB agents, that not only led to the first effective regimen for the treatment of TB in combination with isoniazid and paraaminosalicylic acid, but also spawned a class of antibiotics, aminoglycosides,192 that have found wide therapeutic applications in infectious diseases. It is plausible that bedaquiline, in combination with other new or repurposed compounds, may herald the beginning of a similar era in the advancement of TB treatment last seen in the 1950s and 1960s.

Even the introduction of a single new drug, such as bedaquiline, into MDR-TB regimens can have a significant impact. It will be particularly useful to patients who have developed treatmentlimiting toxicity or have additional resistance, and it is increasingly taking the place of injectable agents in long MDR and XDR-TB treatment regimens around the world, as seen in the recent World Health Organization guidelines. Some countries such as South Africa are already experimenting with it in place of an injectable agent in the short-course MDR-TB regimen, with apparent success, although the effectiveness of this remains to be confirmed by stage 2 of the STREAM trial. The development of bedaquiline, combined with the availability of new and repurposed drugs such as pretomanid, linezolid, and clofazimine, has sparked a series of trials aiming to shorten treatment and improve outcomes for drug-resistant TB, such as ZeNix (NCT03086486) and TB-PRACTECAL (NCT02589782).

Scrutiny of safety will be particularly important where evaluating new regimens in drug-susceptible patients who have a high chance of a favorable outcome with existing SCC. Some regimens with new anti-TB drugs have performed particularly well in the mouse model compared with SCC, and appear to be able to effect relapse-free cure with only 3 months of therapy.15,193 These need to be tested in clinical trials, but the experience of rifapentine and moxifloxacin suggests that the mouse model may not always be predictive of short-course efficacy in humans. Nevertheless, there are now a sufficient number of drug classes to configure a potentially “universal” regimen, without any cross-resistance to existing agents, that could be evaluated for treatment of all TB patients, regardless of drug susceptibility.

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103.Fujiwara M, Kawasaki M, Hariguchi N, Liu Y, and Matsumoto M. Mechanisms of resistance to delamanid, a drug for Mycobacterium tuberculosis. Tuberculosis (Edinb). 2018;108:186–94.

104.Haver HL et al. Mutations in genes for the F420 biosynthetic pathway and a nitroreductase enzyme are the primary resistance determinants in spontaneous in vitro-selected PA-824-resistant mutants of Mycobacterium tuberculosis. Antimicrob Agents Chemother. 2015;59(9):5316–23.

105.Hurdle JG et al. A microbiological assessment of novel nitrofuranylamides as anti-tuberculosis agents. J Antimicrob Chemother. 2008;62(5):1037–45.

106.Acquired Resistance to Bedaquiline and Delamanid in Therapy for Tuberculosis. N Engl J Med. 2015;373(25):e29.

107.Hoffmann H et al. Delamanid and bedaquiline resistance in Mycobacterium tuberculosis ancestral Beijing genotype causing extensively drug-resistant tuberculosis in a Tibetan refugee. Am J Respir Crit Care Med. 2016;193(3):337–40.

108.Diacon AH et al. Early bactericidal activity of delamanid (OPC67683) in smear-positive pulmonary tuberculosis patients. Int J Tuberc Lung Dis. 2011;15(7):949–54.

109.Jindani A, Dore CJ, and Mitchison DA. Bactericidal and sterilizing activities of antituberculosis drugs during the first 14 days. Am J Respir Crit Care Med. 2003;167(10):1348–54.

110.Gler MT et al. Delamanid for multidrug-resistant pulmonary tuberculosis. N Engl J Med. 2012;366(23):2151–60.

111.von Groote-Bidlingmaier F et al. Efficacy and safety of delamanid in combination with an optimised background regimen for treatment of multidrug-resistant tuberculosis: A multicentre, randomised, double-blind, placebo-controlled, parallel group phase 3 trial. Lancet Respir Med. 2019;7(3):249–59.

112.Dooley KE et al. QT effects of bedaquiline, delamanid or both in MDR-TB patients: The deliberate trial. Conference on Retroviruses and Opportunistic Infections, Seattle, Washington, USA, 2019.

113.Tyagi S et al. Bactericidal activity of the nitroimidazopyran PA-824 in a murine model of tuberculosis. Antimicrob Agents Chemother. 2005;49(6):2289–93.

114.Nuermberger E et al. Combination chemotherapy with the nitroimidazopyran PA-824 and first-line drugs in a murine model of tuberculosis. Antimicrob Agents Chemother. 2006;50(8):2621–5.

115.Nuermberger E et al. Powerful bactericidal and sterilizing activity of a regimen containing PA-824, moxifloxacin, and pyrazinamide in a murine model of tuberculosis. Antimicrob Agents Chemother. 2008;52(4):1522–4.

116.Li SY et al. Bactericidal and sterilizing activity of a novel regimen with bedaquiline, pretomanid, moxifloxacin, and pyrazinamide in a murine model of tuberculosis. Antimicrob Agents Chemother. 2017;61(9).

117.Tasneen R et al. Sterilizing activity of novel TMC207and PA-824- containing regimens in a murine model of tuberculosis. Antimicrob Agents Chemother. 2011;55(12):5485–92.

118.Tasneen R et al. Contribution of oxazolidinones to the efficacy

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119.Xu J et al. Contribution of pretomanid to novel regimens containing bedaquiline with either linezolid or moxifloxacin and pyrazinamide in murine models of tuberculosis. Antimicrob Agents Chemother. 2019;63(5).

120.Diacon AH et al. Early bactericidal activity and pharmacokinetics of PA-824 in smear-positive tuberculosis patients. Antimicrob Agents Chemother. 2010;54(8):3402–7.

121.Diacon AH et al. Phase II dose-ranging trial of the early bactericidal activity of PA-824. Antimicrob Agents Chemother. 2012;56(6):3027–31.

122.Diacon AH et al. 14-day bactericidal activity of PA-824, bedaquiline, pyrazinamide, and moxifloxacin combinations: A randomised trial. Lancet. 2012;380(9846):986–93.

123.Dawson R et al. Efficiency and safety of the combination of moxifloxacin, pretomanid (PA-824), and pyrazinamide during the first 8 weeks of antituberculosis treatment: A phase 2b, open-label, partly randomised trial in patients with drugsusceptible or drug-resistant pulmonary tuberculosis. Lancet. 2015;385(9979):1738–47.

124.Dawson R et al. Efficacy of bedaquiline, pretomanid, moxifloxacin & PZA (BPAMZ) against DS- & MDR-TB. Conference on Restroviruses and Opportunistic Infections, Seattle, Washington, USA, 2017.

125.Tweed CD et al. Bedaquiline, moxifloxacin, pretomanid, and pyrazinamide during the first 8 weeks of treatment of patients with drug-susceptible or drug-resistant pulmonary tuberculosis: A multicentre, open-label, partially randomised, phase 2b trial. Lancet Respir Med. 2019;7(12):1048–58.

126.FDA approves new drug for treatment-resistant forms of tuberculosis that affects the lungs: Food and Drug Administration; [Available from: https://www.fda.gov/news-events/press-announcements/ fda-approves-new-drug-treatment-resistant-forms-tuberculosis- affects-lungs.

127.Lesher GY, Froelich EJ, Gruett MD, Bailey JH, and Brundage RP. 1,8-Naphthyridine derivatives. A new class of chemotherapeutic agents. J Med Pharm Chem. 1962;91:1063–5.

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129.Aubry A, Veziris N, Cambau E, Truffot-Pernot C, Jarlier V, and Fisher LM. Novel gyrase mutations in quinolone-resistant and -hypersusceptible clinical isolates of Mycobacterium tuberculosis: Functional analysis of mutant enzymes. Antimicrob Agents Chemother. 2006;50(1):104–12.

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133.Nuermberger EL et al. Moxifloxacin-containing regimen greatly reduces time to culture conversion in murine tuberculosis. Am J Respir Crit Care Med. 2004;169(3):421–6.

134.Nuermberger EL et al. Moxifloxacin-containing regimens of reduced duration produce a stable cure in murine tuberculosis. Am J Respir Crit Care Med. 2004;170(10):1131–4.

135.Jindani A et al. High-dose rifapentine with moxifloxacin for pulmonary tuberculosis. N Engl J Med. 2014;371(17):1599–608.

136.Merle CS et al. A four-month gatifloxacin-containing regimen for treating tuberculosis. N Engl J Med. 2014;371(17):1588–98.

137.Gillespie SH et al. Four-month moxifloxacin-based regimens for drugsensitive tuberculosis. N Engl J Med. 2014;371(17):1577–87.

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144.Hu Y, Liu A, Ortega-Muro F, Alameda-Martin L, Mitchison D, and Coates A. High-dose rifampicin kills persisters, shortens treatment duration, and reduces relapse rate in vitro and in vivo. Front Microbiol. 2015;6:641.

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Introduction

Bacille Calmette–Guérin BCG efficacy

Protective immunity against Mycobacterium tuberculosis Target populations for an improved TB vaccine

New vaccine approaches Challenges in vaccine development Conclusions

References

INTRODUCTION

Prophylactic vaccination is the most effective strategy to control any infectious disease epidemic. It has served to eradicate smallpox and significantly reduce morbidity and mortality due to countless other childhood diseases including polio, pertussis, and measles. Infectious pathogens which induce a latent phase, such as

Mycobacterium tuberculosis (M.tb), will be considerably more difficult to eradicate. All currently licensed vaccines, with the exception of Bacille Calmette–Guérin (BCG), are based on the induction of humoral immunity and protective antibodies. However, for diseases such as tuberculosis (TB), malaria, and human immunodeficiency virus (HIV) where cell-mediated immunity is also important for protection, a different strategy may be required. In TB, the only currently available vaccine, BCG, is effective in some populations some of the time, but overall has failed to control the global TB epidemic. A more effective vaccine regimen is urgently needed. We will review what is known about BCG and the leading approaches currently being developed in an attempt to improve on it in this chapter.

BACILLE CALMETTE–GUÉRIN

Development of BCG

BCG is a live attenuated vaccine that was developed at the Institut Pasteur in Lille, France by Albert Calmette and Camille Guérin. Continuous subculture of Mycobacterium bovis, the causative

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agent of bovine TB, led to considerable attenuation. Safety and protective efficacy was demonstrated in animal models followed by the first human immunization in 1921.1 The initial strain of BCG was widely distributed from Europe to meet growing international demand prior to the introduction of standardized seed stocks. Further culture resulted in the establishment of distinct local strains with sometimes major differences in genetic and antigenic compositions.2 There are now approximately 13 daughter strains collectively known by the generic term BCG. Initially given by the oral route, BCG is currently administered parenterally throughout most of the world, at or soon after birth.

Safety of BCG

SAFETY PROFILE OF BCG

BCG remains one of the most widely used vaccines globally, having been administered to over 4 billion people. It has a very wellestablished safety profile with correct intradermal administration on the upper arm generally resulting in a local reaction of a small pustule followed by a small scar. Other possible adverse events include regional lymphadenitis and systemic disease, although the latter is associated almost exclusively with immunosuppression.3

As a live vaccine, BCG has always been contraindicated in the immunosuppressed including HIV-infected adults, but until recently has been considered safe for administration in HIVinfected infants. However, recent data from studies in South Africa indicated an increased risk of BCG-osis, leading to a change in the World Health Organization (WHO) recommendations.4,5 These

state that routine BCG vaccination should be withheld in HIVprevalent regions until the HIV status of the infant is confirmed. As this is not possible until approximately 6 weeks of age, there is a significant delay in the administration of BCG during which time the infant is at risk of acquiring M.tb infection and potentially TB disease. Given the considerable overlap in the HIV and TB epidemics, a vaccine that can be safely administered to HIV-infected infants is urgently needed.

When instilled intravesically as immunotherapy for bladder cancer, BCG is tolerated without significant morbidity in 95% of patients.6 Symptoms associated with the immune stimulation include urinary frequency and burning, mild malaise, and low-grade fever. Pulmonary involvement is a rare complication occurring in 0.3%–0.7% of patients, presenting as interstitial pneumonitis or miliary dissemination.7,8 Systemic septic and/ or hypersensitivity reaction occurs in approximately 1 in 15,000 treated patients.8 Numerous BCG strains are currently in use, but no differences in side effects were noted in a meta-analysis of 24 trials.9

BCG DISEASE

BCG lymphadenitis is characterized by enlarged ipsilateral axillary nodes (although supraclavicular, nuchal, and cervical nodes can also be involved), and is most frequently observed in children under the age of 6 months.10,11 Non-suppurative lymphadenitis may be considered part of the normal course of BCG vaccination and managed conservatively, usually resolving spontaneously over weeks to months.11 In cases of suppurative lymphadenitis, nodes can rupture leading to sinus and fistula formation, and may result in residual complications. Treatment approaches include antituberculous therapy, node aspiration, and surgical excision.12

Systemic complications can present as osteitis/osteomyelitis or disseminated BCG disease, but are extremely rare and most commonly observed in BCG-vaccinated children with underlying primary immunodeficiency. Osteomyelitis occurs in approximately 1 in 100,000 cases and usually involves epiphysis of long tubular bones.13 The incidence of disseminated BCG disease in vaccinated children is estimated at 2–3.4 per million with a fatality rate of 80%–85%,12 whereas 0.4% of bladder cancer patients were reported to develop severe disseminated BCG sepsis following intravesical therapy.14 The spectrum of symptoms of disseminated BCG disease is similar to that of TB disease, including persistent fever, night sweats, and weight loss. Negative sputum, blood, tissue, or urine cultures and polymerase chain reaction analysis for M. bovis are not uncommon. In such cases, a chest radiograph or bone marrow biopsy may permit diagnosis.15–17

There are no consensus guidelines for the treatment of systemic BCG disease, although approaches typically include the administration of corticosteroids together with a combination of antituberculous drugs excluding pyrazinamide, to which M. bovis is intrinsically resistant.18 BCG-vaccine strains differ in their drug susceptibilities,19 and acquired resistances to rifampicin, isoniazid, and ethambutol have been described.20,21 Higher dosages and prolonged treatment are likely necessary for the management of BCG disease.10 Immune reconstitution inflammatory syndrome can occur following the initiation of antiretroviral therapy (ART) in HIV-infected BCG-vaccinated children, or following

hematopoietic stem cell transplantation in BCG-infected patients with severe combined immunodeficiency.22,23

Effect of BCG on the tuberculin skin test

Despite recent development of the more specific interferon gamma release assays (IGRAs), the tuberculin skin test (TST) is still widely used in the diagnosis of M.tb infection; particularly in resource-poor settings. BCG immunization can result in a positive TST, which may persist for up to 25 years and confound diagnosis.24,25 BCG interference with TST reactivity was part of the rationale for not introducing BCG into routine clinical practice in the United States. Ideally, a new TB vaccine would not interfere with the TST.

BCG EFFICACY

Efficacy against TB

EFFICACY AGAINST M.TB INFECTION

It was long assumed that BCG was not protective against the establishment of M.tb infection, based largely on animal challenge models and autopsy studies showing no difference in incidence of pulmonary foci between the vaccinated and unvaccinated subjects.26 Furthermore, the incidence of latent M.tb infection (LTBI) in endemic countries is very high despite good BCG coverage.27,28 Opportunities to study this were previously limited by the TST which, due to cross-reactivity, is unable to distinguish between a positive response caused by M.tb or BCG vaccination. This is now circumvented by the availability of IGRAs, and several studies have since reported the efficacy of BCG against infection.29–32 In a recent meta-analysis of 14 retrospective case–control studies in which participants had recent exposure to M.tb and had been screened for infection using IGRAs, BCG was significantly associated with protection from infection (overall risk ratio, 0.81; 95% confidence interval [CI] 0.71–0.92).33

EFFICACY AGAINST TB DISEASE

When administered at birth, BCG confers consistent, reliable, and cost-effective protection against TB meningitis and disseminated disease.34–36 Withdrawal of BCG vaccination from Sweden and the former Czechoslovakia was associated with a concomitant increase in the cases of TB meningitis and mycobacterial glandular disease.37,38 In a meta-analysis of randomized-controlled trials, the summary protective effect against miliary or meningeal TB was 86% (95% CI 65–95).35 However, the efficacy of BCG against pulmonary disease, the most common form of TB, varies greatly by geographical location.39 High levels of protection (>70% efficacy) have been reported in schoolchildren in the UK and Denmark, the general population in Norway, and Native Americans in Alaska.40–43 However, protection has been negligible (<20% efficacy) in studies conducted in Southern India, Malawi, Georgia (US), and Colombia.44–47 A meta-analysis of 14 prospective trials and 12 case–control studies estimated an overall BCG efficacy of 50%.36

DURATION OF PROTECTION

The MRC trial in UK adolescents demonstrated high BCG efficacy for up to 15 years following vaccination.41 This durability was confirmed to last at least 10 years in observational studies.48 Although a meta-analysis of 10 randomized trials estimated an average efficacy >10 years post-vaccination of just 14% (95% CI 9–32),49 studies in Brazil and Norway suggest that protection may extend beyond this period.50,51 A long-term follow-up study of American Indians and Alaska Natives reported protection for up to 60 years.52 A recent case–control study in children given BCG at 12–13 years of age reported 57% protection (95% CI 33–72) at 15–20 years post-vaccination which waned between 20 and 29 years.53

BCG REVACCINATION

Several studies have indicated that revaccination with BCG confers no additional protection to neonatal vaccination.54–56 However, a recent phase II placebo-controlled prevention-of-infection trial conducted in South African adolescents found that although BCG revaccination did not demonstrate efficacy in preventing initial infection, it did result in significantly reduced rates of sustained M.tb infection.57

Efficacy against other mycobacterial diseases

Vaccination with BCG may confer protection against non-tubercu- lous mycobacterial species due to cross-reactivity with conserved, often immunodominant, antigens.58 The estimates of protective efficacy against leprosy, caused by Mycobacterium leprae, vary from 20% to 90%.45,59 Although one meta-analysis determined an overall BCG-vaccine protective effect of 41% (95% CI 16–66) for trials and 60% (95% CI 51–70) for observational studies,60 another analysis reported just 26% (95% CI 14–37) and suggested that protection had been overestimated in observational studies.61 Crossprotection of BCG against Buruli ulcer disease has been reported in some studies62,63 but not in others.64,65 Murine studies have demonstrated a protective effect of BCG against infection with

Mycobacterium avium and Mycobacterium kansasii.66 A study of neonates in the Czech Republic found that M. avium intracellulare complex-associated lymphadenitis was lower in BCG-vaccinated compared with unvaccinated children.38

Efficacy against non-mycobacterial diseases

ALL-CAUSE MORTALITY IN INFANTS

There is evidence, predominantly from observational studies in West Africa, to suggest that BCG may have a non-specific effect on all-cause infant mortality67–71 which is most pronounced among girls.72,73 Protection beyond the target pathogen could be promoted by heterologous lymphocyte activation and innate immune memory,74 as supported by recent in vitro studies.75,76 However, studies in Greenland and Denmark did not identify reduced hospitalization rates due to infectious diseases other than TB in children BCG-vaccinated at birth.77,78 Such discrepancies may

be associated with geographical differences thought to influence immunity and BCG-vaccine efficacy, as described later. A systematic review found that the receipt of BCG vaccine was associated with reduction in all-cause mortality, but reported a high risk of bias in a number of the published studies and stated uncertainty toward evidence.79 If a non-specific effect is found to be reproducible and widely applicable in high-quality randomized-controlled trials, this must be taken into consideration when evaluating noninferiority of BCG replacement vaccine candidates.

BLADDER CANCER THERAPY

The relationship between mycobacteria and cancer was first recognized almost a century ago.80 Early animal studies demonstrated that BCG-infected mice were resistant to transplantation of tumor cells, leading to the discovery of tumor necrosis factor (TNF).81,82 Intravesical BCG therapy is now the standard of care for high-risk, non-muscle-invasive bladder cancer,83,84 demonstrating superiority over other intravesical agents.85,86 The mechanism of action remains unclear but it has been suggested that BCG may be internalized by bladder cancer cells due to oncogenic aberrations that activate micropinocytosis, leading to recruitment of immune cells to the site and subsequent cytotoxicity and targeted killing of cancer cells.87

INFLAMMATORY AND AUTOIMMUNE DISEASES

As a strong inducer of Th1 immunity, BCG may confer efficacy against inflammatory and autoimmune diseases. This hypothesis has been supported by preclinical studies demonstrating protection against allergic asthma, multiple sclerosis, and insulindependent diabetes. Although some observational or intervention studies in humans have also indicated a beneficial effect, robust controlled prospective studies are required.88 In a recent randomized 8-year prospective study of type 1 diabetic subjects with longterm disease, two doses of BCG resulted in stable and long-term reductions in blood sugar in a small number of diabetic subjects.89

Reasons for variability in BCG efficacy

The variability in BCG efficacy against TB disease poses the greatest challenge to new TB vaccine development. Understanding the underlying causes will be essential if we are to avoid a new generation of vaccines being subject to the same pitfall. Several hypotheses, which are not mutually exclusive, have been proposed as described next and summarized in Table 12.1.

DIFFERENCES IN BCG STRAINS

Distinct local strains of BCG show considerable variation in genetic and antigenic compositions,2 and in vitro T-cell responses vary when peripheral blood mononuclear cells (PBMCs) are taken from individuals vaccinated with different strains.90 However, the impact of these changes on protective efficacy remains unclear. Although some preclinical studies indicate a divergence of protective immunity conferred by different BCG strains,91 others suggest comparable potency.92 In humans, similar levels of protection were observed with BCG or an attenuated strain of Mycobacterium microti, suggesting that strain differences may not play a major