**3.2. Aminoglycosides (kanamycin, amikacin and capreomycin)**

kanamycin or capreomycin. Hearing and renal function should be monitored in patients

Mutations associated with streptomycin resistance have been identified in the genes encoding 16S rRNA (*rrs*) [53] and ribosomal protein S12 (*rpsL*) [54-57]. Ribosomal protein S12 stabilizes the highly conserved pseudoknot structure formed by 16S rRNA [58]. Amino acid substitu‐ tions in RpsL affect the higher-order structure of 16S rRNA [51] and confer streptomycin resistance. Alterations in the 16S rRNA structure disrupt interactions between 16S rRNA and streptomycin, a process that results in resistance [59]. Mutations in *rps*L and *rrs* are the major mechanism of SM resistance [54, 56, 57], accounting for respectively about 50% and 20% of SM-resistant strains [54, 56, 57]. The most common mutation in *rps*L is a substitution in codon 43 from lysine to arginine [54, 56, 57], causing high-level resistance to SM. Mutation in codon 88 is also common [54, 56, 57]. Mutations of the *rrs* gene occur in the loops of the 16S rRNA and are clustered in two regions around nucleotides 530 and 915 [39, 54, 56, 57]. However, about 20–30% of SM-resistant strains with a low level of resistance (MIC < 32 μg/ml) do not have mutations in *rps*L or *rrs* [60], which indicates other mechanism(s) of resistance. A mutation in *gid*B, encoding a conserved 7-methylguanosine (m(7)G) methyltransferase specific for 16S rRNA, has been found to cause low-level SM resistance in 33% of resistant *M. tubercu‐ losis* isolates [61]. A subsequent study showed that while Leu16Arg change is a polymorphism not involved in SM resistance, other mutations in *gid*B appear to be involved in low-level SM resistance [62]. In addition, some low-level SM resistance seems to be caused by increased efflux as efflux pump inhibitors caused increased sensitivity to SM, although the exact

The fluoroquinolones (FQs) have broad-spectrum antimicrobial activity and so are widely used for the treatment of bacterial infections of the respiratory, gastrointestinal and urinary tracts, as well as sexually transmitted diseases and chronic osteomyelitis [63]. In contrast to many other antibiotics used to treat bacterial infections, the FQs have excellent in vitro and in vivo activity against *M. tuberculosis* [64, 65]. FQs include ciprofloxacin, ofloxacin, levofloxacin, and moxifloxacin. So, FQs are currently in use as second-line drugs in the treatment of TB. Adverse effects are relatively infrequent (0.5–10% of patients) and include gastrointestinal intolerance, rashes, dizziness, and headache. Most studies of fluoroquinolone side effects have been based on relatively short-term administration for bacterial infections, but trials have now shown the relative safety and tolerability of fluoroquinolones administered for months during

The cellular target of FQs in *M. tuberculosis* is DNA gyrase, a type II topoisomerase consist‐ ing of two A and two B subunits encoded by *gyrA* and *gyrB* genes, respectively [66]. Muta‐ tions in a small region of *gyrA*, called quinolone resistance-determining region (QRDR) and,

getting SM.

mechanism remains to be identified [62].

168 Tuberculosis - Current Issues in Diagnosis and Management

**3.1. Fluoroquinolones**

TB treatment in adults.

**3. Second-line antituberculosis drugs**

The aminoglycosides amikacin (AMK)/kanamycin (KAN) and the cyclic polypeptide cap‐ reomycin (CAP) are important injectable drugs in the treatment of multidrug-resistant tu‐ berculosis. Although belonging to two different antibiotic families, all exert their activity at the level of protein translation. Renal toxicity occurs from these drugs. Regular monitoring of hearing and renal function is recommended.

AMK and KAN are aminoglycosides that have a high level of cross-resistance between them [78-80]. The cyclic polypeptide CAP is structurally unrelated to the aminoglycosides and thus is a potential candidate to replace AMK or KAN if resistance to either of them is sus‐ pected [81, 82]. It has been demonstrated that the risk of treatment failure and mortality in‐ crease when CAP resistance emerges among MDR-TB cases [83]. However, cross-resistance in *M. tuberculosis* between AMK/KAN and CAP has been observed in both clinical isolates and laboratory-generated mutants [79, 80, 84, 85].

AMK/KAN and CAP primarily affect protein synthesis in *M. tuberculosis* and resistance to these drugs is associated with changes in the 16S rRNA (*rrs*) [78, 80, 81, 85, 86]. The *rrs* muta‐ tion A1401G causes high-level AMK/KAN and low-level CAP resistance. C1402T is associat‐ ed with CAP resistance and low-level KAN resistance. G1484T is linked to high-level AMK/KAN and CAP resistance [79, 80, 84, 86]. Low-level resistance to kanamycin has been correlated to mutations in the promoter region of the *eis* gene encoding aminoglycoside ace‐ tyltransferase, the enhanced intracellular survival protein, Eis [87].

Resistance to the cyclic peptide capreomycin has also been associated with mutations in *tlyA* [86]. The gene *tlyA* encodes a putative 2'-O-methyltransferase (TlyA) that has been suggested to methylate nucleotide C1402 in helix 44 of 16S rRNA and nucleotide C2158 in helix 69 of 23S rRNA in *M. tuberculosis* [81, 88]. Capreomycin binds to the 70S ribosome and inhibits mRNA–tRNA translocation [89]. It is believed that TlyA methylation enhan‐ ces the antimicrobial activity of capreomycin [81] and that disruption of *tlyA* leads to cap‐ reomycin resistance because the unmethylated ribosome is insensitive to the drug [81, 86, 88]. The identified mechanism of capreomycin resistance on the basis of in vitro selected mutants has found that *tlyA* mutations were common [80, 86] whereas infrequent in clini‐ cal isolates of *M. tuberculosis* [79, 80].

**4. Conclusions**

**Author details**

do, Republic of Korea

344(17), 1294-303.

**References**

bility of drug-resistant *M. tuberculosis* strains.

Hum Nath Jnawali and Sungweon Ryoo\*

\*Address all correspondence to: scientist1@empal.com

Despite all the advances made in the treatment and management, TB still remains as one of the main public health problems that have plagued mankind for millennia. The challenges posed by *M. tuberculosis* infection, through its interaction with the immune system and its mechanisms for evasion, require many more breakthroughs to make a significant impact on the worldwide tuberculosis problem. The introduction of MDR and XDR strains of *M. tuberculosis* poses several problems in mycobacterial genetics and phthisiotherapy. Among the response priorities, rapid detection of anti-tuberculosis drug resistance, use of appropriate regimens for treatment, and new drug development are of paramount importance. However, regarding the dynamics of TB transmission, and also in view of rational development of new anti-TB drugs, it is extremely important to extend our knowledge on the molecular basis of drug resistance and all its complexity. It is necessary to clarify the association between specific mutations and the development of MDR-TB or the association between drug resistance and fitness. This would allow better evaluation of the transmission dynamics of resistant strains and more accurate prediction of a future disease scenario. Adequate monitoring of drug resistance, especially MDR/XDR-TB in new patients and its transmission, molecular charac‐ terization of the drug-resistant strains, and analysis of patients' immune status and genetic susceptibility are also needed to address the problem of the fitness, virulence and transmissi‐

First– and Second–Line Drugs and Drug Resistance

http://dx.doi.org/10.5772/54960

171

Korean Institute of Tuberculosis, Osong Saengmyeong, Cheongwon-gun, Chungcheongbuk-

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[2] Masjedi, M. R, Farnia, P, Sorooch, S, Pooramiri, M. V, Mansoori, S. D, Zarifi, A. Z, Velayati, A. A, & Hoffner, S. Extensively drug resistant tuberculosis: 2 years of sur‐

veillance in Iran. Clinical Infectious Diseases (2006). , 43(7), 840-7.

### **3.3. Ethionamide/prothionamide**

Ethionamide (ETH, 2-ethylisonicotinamide) is a derivative of isonicotinic acid and has been used as an antituberculosis agent since 1956. The MICs of ETH for *M. tuberculosis* are 0.5–2 μg/ ml in liquid medium, 2.5–10 μg/ml in 7H11 agar, and 5–20 μg/ml in LJ medium. Ethionamide and the similar drug prothionamide (PTH, 2-ethyl-4-pyridinecarbothioamide) act as prodrugs, like isoniazid. Which is activated by EtaA/EthA (a mono-oxygenase) [90, 91] and inhibits the same target as INH, the InhA of the mycolic acid synthesis pathway [92]. Once delivered into the bacterial cell, ethionamide undergoes several changes. Its sulfo group is oxidized by flavin monooxygenase, and the drug is then converted to 2-ethyl-4-aminopyridine. The intermediate products formed before 2-ethyl-4-aminopyridine seem to be toxic to mycobac‐ teria, but their structures are unknown (may be highly unstable compounds). Mutants resistant to ethionamide are cross-resistant to prothionamide. ETH frequently causes gastrointestinal side effects, such as abdominal pain, nausea, vomiting and anorexia. It can cause hypothyr‐ oidism, particularly if it is used with *para*-aminosalicyclic acid.

#### **3.4.** *p-***Amino salicylic acid**

*p-*Amino salicylic acid (PAS) was one of the first antibiotics to show anti-TB activity and was used to treat TB in combination with isoniazid and streptomycin [93]. Later, with the discovery of other more potent drugs including rifampicin, its use in first line regimens was discontinued. PAS is still useful as part of a treatment regimen for XDR TB although its benefit is limited and it is extremely toxic. Thymidylate synthase A, encoded by *thyA*, an enzyme involved in the biosynthesis of thymine, has been proposed recently as the target of PAS in *M. bovis* BCG [94]. Most common mutation in *thyA* was Thr202Ala, though few susceptible isolates also showed the same mutation [95]. However, its mechanism of action was never clearly elucidated. The most common adverse reactions associated with PAS are gastrointestinal disturbances.

#### **3.5. Cycloserine**

Cycloserine (CS) is an antibiotic that is used to treat TB. The exact mechanism of action of cycloserine is unknown, but it is thought to prevent the tuberculosis bacteria from making substances called peptidoglycans, which are needed to form the bacterial cell wall. This results in the weakening of bacteria's cell wall, which then kills the bacteria. Cycloserine possesses high gastric tolerance (compared with the other drugs) and lacks cross-resistance to other compounds. But it causes adverse psychiatric effects; [96, 97] which is its main drawback. So, psychiatric interrogation is necessary before prescribing cycloserine drug. Cycloserine is one of the cornerstones of treatment for MDR and XDR tuberculosis [96, 97, 98]. Terizidone (a combination of two molecules of cycloserine) might be less toxic [96, 97], although studies of this drug are scarce.
