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240 Medicina (Kaunas) 2010;46(4):240-8 Tigecycline – how powerful is it in the fight against -resistant bacteria?

Vaida Šeputienė1, Justas Povilonis1, Julija Armalytė1, 2, Kęstutis Sužiedėlis1, 2, Alvydas Pavilonis3, Edita Sužiedėlienė1 1Department of Biochemistry and Biophysics, Faculty of Natural Sciences, Vilnius University, 2Institute of Oncology, Vilnius University, 3Department of Microbiology, Kaunas University of Medicine, Lithuania

Key words: tigecycline; mechanism of action; in vitro and in vivo activity; resistance.

Summary. Tigecycline is a semisynthetic analogue of earlier and represents the first member of a novel class of antimicrobials – – recently approved for clinical use. It is active against a broad range of gram-negative and gram-positive bacterial species including clinically important multidrug-resistant nosocomial and community-acquired bacterial pathogens. The exact molecular basis of tigecycline action is not clear at present, although similarly to the tetra- cyclines, it has been shown to inhibit the translation elongation step by binding to the ribosome 30S subunit and preventing aminoacylated tRNAs to accommodate in the ribosomal A site. Importantly, tigecycline overcomes the action of ribosomal protection proteins and is not a substrate for tetracy- cline efflux pumps of most bacteria – well-known and prevalent cellular mechanisms of microbial resistance. The present review summarizes current knowledge on the molecular mecha- nism of the tigecycline action, antibacterial activity against various bacteria, clinical application, development of resistance to glycylcyclines.

Introduction intra-abdominal infections (cIAIs), and communi- Use of antimicrobial drugs over the last 60 years ty-acquired bacterial pneumonia (CABP) (1–2) and in medicine and veterinary has triggered a devel- introduced during the past two decades into clinical opment of very effi cient genetic and biochemi- practice (1). cal mechanisms within bacteria enabling them to Clinical and pharmacological studies of tigecy- live in the antibiotic-containing environments. In cline are summarized recently in several compre- recent years, the emergence and spread of multi- hensive reviews including data on the large in vitro drug-resistant (MDR) pathogenic microorganisms, and clinical trials performed in number of countries which possess exceptional risk to human health, worldwide (3–5). In the present work, we focused have become a problem of special signifi cance. The our attention on molecular basis of tigecycline ac- list of available for effi cient treatment tion and on the issue of microbial resistance to gly- of community-acquired and nosocomial infections cylcyclines. caused by methicillin-resistant Staphylococcus au- reus (MRSA), -resistant Enterococcus, structure and molecular extended-spectrum β-lactamase (ESBL)-producing basis of action , , From the chemical point of view, glycylcyclines aeruginosa, carbapenemase-producing Acinetobacter are related to tetracyclines, albeit constitute a new baumannii became dramatically short. The avail- class of antibacterials (Fig. 1). Tigecycline (formerly ability of the effective antibacterials, which are ac- GAR-936, Trademark: Tygacil®; Wyeth Pharma- tive against a broad spectrum of antibiotic resistant ceuticals, Inc., Philadelphia, PA, USA), the fi rst gram-positive, gram-negative bacteria and anaero- member of the class, is a structural derivative of bic species, becomes of critical importance in the semisynthetic drug , differing from it cases of serious bacterial infections, when empiri- by the long side chain at the 9 position of carbon cal therapy often must be applied to avoid patient atom (9-tert-butyl-glycylamido moiety) of the D morbidity and mortality, and reduce the costs of ring of tetracyclic nucleus (Fig. 1). Search for more healthcare. microbiologically active structural analogs of tetra- Tigecycline is the fi rst antimicrobial of glycylcy- cycline in 1990s led to discovery of some derivatives cline class, recently approved for use in the clinical of minocycline such as N, N-dimethylglycylamido practice for the treatment of adult complicated skin (DMG) substituent at 9 position (DMG-MINO), and skin-structure infections (cSSSIs), complicated which differently from other developed analogs was Correspondence to E. Sužiedėlienė, Department of Biochem- Adresas susirašinėti: E. Sužiedėlienė, VU Gamtos mokslų fa- istry and Biophysics, Faculty of Natural Sciences, Vilnius Uni- kulteto Biochemijos ir biofi zikos katedra, M. K. Čiurlionio 21, versity, M. K. Čiurlionio 21, 03101 Vilnius, Lithuania 03101 Vilnius. El. paštas: [email protected] E-mail: [email protected] Medicina (Kaunas) 2010; 46(4) Tigecycline – how powerful is it in the fi ght against antibiotic-resistant bacteria? 241

Tetracycline Minocycline

N, N-dimethylglycyclamido minocycline (DMG-MINO)

Tigecycline

Fig. 1. Chemical structures of tetracycline derivatives and tigecycline active in vitro not only against a wide range of tet- is thought to be achieved, similarly to tetracyclines, racycline-susceptible bacteria but against those pos- through interfering with accommodation of ami- sessing known determinants of resistance to early noacyl-tRNA in the ribosomal A site, which pre- tetracyclines (6). Further development of more ef- cedes the peptidyl-transfer reaction (Fig. 3) (8–10). fi cient compounds led to discovery of GAR-963 in Notably, tigecycline binds 5 times stronger to the 1993, presently known as tigecycline (7) (Fig. 2). N- same ribosomal high-affi nity site as compared to alkyl glycylamido side chain at the carbon 9 position tetracycline, albeit in different orientation (8–10). equips tigecycline by several features, responsible Structural data of the tigecycline/ribosome com- for its biological activity: 1) increases lipid solubil- plex are still lacking, although three dimensional ity of the drug; 2) creates a steric hindrance, which structures of T. thermophilus ribosome 30S subunit prevents tigecycline from the effl ux out the cell by with bound tetracycline, determined by x-ray crys- most membrane-bound effl ux proteins; 3) increases tallography, have shown that the high-affi nity bind- affi nity to the binding site of its cellular target – ri- ing site 1 (Tet-1) is located between the head and bosome. the shoulder of the 30S subunit proximal to 16S Tigecycline enters bacterial cell either through rRNA helix 34 and A site (11). The oxygen atoms passive diffusion or active transport routes (Fig. 3). of the 16S rRNA phosphate backbone interact with In the cytosol, similarly to tetracyclines and their the hydrophilic part of tetracycline through many structural analogs, it reversibly binds to ribosome hydrogen-bonding interactions and are additionally 30S subunit and blocks bacterial growth by inhibi- coordinated by Mg2+, bound to tetracycline mol- tion of protein biosynthesis (8–9). The effect is bac- ecule. From structural data, tetracycline binding is teriostatic rather than bactericidal in most bacteria. proposed to create a steric hindrance, which pre- The inhibition of tigecycline-mediated translation vents aminoacyl-tRNA from positioning (rotation)

Medicina (Kaunas) 2010; 46(4) 242 Vaida Šeputienė, Justas Povilonis, Julija Armalytė, et al.

1993 2005 2006 2007 2008 2009

Fig. 2. Timetable of tigecycline (Tygacil) discovery, approval for clinical use, and reports on resistance

Fig. 3. Molecular mechanism of tigecycline action Ribosome A (aminoacyl), P (peptidyl) and E (exit) sites, ribosome 30S and 50S subunits, messenger RNA (mRNA) aminoacyl-tRNA/EF-Tu complexes are shown. in A site for peptidyl-transfer reaction (11). Similar is, indeed, a predominant tetracycline binding site mode of action has been proposed for tigecycline and thus steric interference is a major mechanism of (Fig. 3). Notably, tetracycline binding appears do tetracycline action (13). In a similar study, neither not interfere with earlier events of elongation cy- tetracycline nor tigecycline binding to EF-Tu has cle – the binding of aminoacyl-tRNA to the ribos- been shown to have a signifi cant role in translation ome in a form of ternary complex with EF-Tu/GTP inhibition (14). and the codon/anticodon (decoding) recognition- As it has been mentioned above, tigecycline is dependent ribosome-mediated GTP hydrolysis. active against tetracycline-resistant bacterial strains Therefore, bound tetracycline prevents accommo- (6). Tetracycline resistance in various bacteria is dation of aminoacyl-tRNA in the A site just after very common and is mediated through two most decoding and release of EF-Tu/GDP. The second prevailing protein-based mechanisms: export of tet- tetracycline-binding site, Tet-5, is located in 30S racyclines from the cell by the Tet effl ux proteins subunit body, close to the 16S rRNA helix 44 and is (TetA-E, TetK) and protection of the ribosome proposed to participate in the mechanism of inhibi- from tetracycline binding by ribosomal protection tion (11). Additionally, tetracycline has been crys- proteins (TetO, TetM) (8). Large diversity of tet tallized in complex (1:1) with a modifi ed form of genes coding for both groups of proteins are known E. coli EF-Tu-Mg-GDP suggesting another possible to function in tetracycline-resistant gram-negative mode of action through its binding to EF-Tu (12). and gram-positive bacteria either alone or in com- However, recent molecular dynamics simulations of binations (8). It is speculated that long side chain of ribosome 30S subunit have shown that Tet-1 site tigecycline, most probably, prevents it from binding Medicina (Kaunas) 2010; 46(4) Tigecycline – how powerful is it in the fi ght against antibiotic-resistant bacteria? 243 to most effl ux proteins and transport from the cell atypical organisms. Tigecycline was demonstrated (15) (Fig. 3). to be potent antibacterial drug against vancomycin- Ribosomal protection proteins show high simi- intermediate and vancomycin-resistant enterococci larity to translation elongation factors such as EF-G (VRE), MRSA, extended-spectrum β-lactamase and EF-Tu and are members of GTPase protein (ESBL)-producing Escherichia coli, Klebsiella pneu- superfamily (16). They bind to the binding site moniae, multidrug-resistant Acinetobacter bauman- of elongation factors EF-G and EF-Tu in the 50S nii, and penicillin-resistant Streptococcus pneumo- subunit, induce conformational changes of the ri- niae (2–4, 20–22). bosome, and release tetracycline from its inhibitory It must be noted that interpretive criteria for site in 30S subunit, thus eliminating a steric bar- tigecycline susceptibility established by the U.S. rier for accommodation of aminoacyl-tRNA dur- Food and Drug Administration (FDA) and by the ing elongation step of translation (16). It has been European Committee on Antimicrobial Suscepti- proposed that either higher affi nity of tigecycline to bility testing (EUCAST) differ for some clinically Tet-1 binding site compared to tetracycline and its important bacteria, FDA breakpoints being higher derivatives or its unique orientation, when bound to than those estimated by EUCAST (1, 23). In addi- ribosome, could prevent tigecycline from dissocia- tion, there are no defi nitive breakpoints for some tion via the action of ribosome protection proteins nonfermenting pathogens yet. in tetracycline-resistant bacteria (10). Most studies use the broth microdilution meth- Importantly, tigecycline appears to overcome not od for the determination of microbiological activ- only tetracycline resistance, but also most of other ity of tigecycline according to the Clinical Labora- antibiotic resistance mechanisms known in bacteria, tory Standards Institute (CLSI) recommendations such as drug target modifi cation, enzymatic degra- (24). Some investigations have reported that several dation, DNA gyrase mutations. This makes tigecy- methodological aspects must be taken into consider- cline a promising antibacterial agent against a broad ation when performing susceptibility testing by this spectrum of clinically important antibiotic-resistant method. Petersen et al. (25) reported that use of the pathogens. media, older than 12 h, might lead to the oxidation There are, however, some exceptions. Certain of tigecycline. Thus, partial loss of tigecycline ac- bacteria with clinical signifi cance, such as Pseu- tivity could give falsely higher minimum inhibitory domonas aeruginosa, Proteus spp., Providencia spp., concentrations (MICs). Another recent study has and Morganella spp., have been reported to show in- shown that tigecycline MICs for Enterobacter spe- trinsic lowered susceptibility to tigecycline (17–19). cies determined by the Etest are signifi cantly higher Bacterial effl ux pumps belonging to so-called resist- that those determined by the broth microdilution ance-nodulation-division (RND) family and con- method, and therefore strains determined as tigecy- ferring multidrug resistance phenotype appear to be cline nonsusceptible according to the Etest should responsible, at least in part, for the reduced tigecy- be confi rmed by the broth microdilution method cline susceptibility of bacteria listed above (Fig. 3). (26). These observations imply the most likely pathways Tigecycline MICs for gram-positive species are for the emergence of effl ux-mediated tigecycline generally lower as compared to gram-negative bac- resistance through mutations in transport proteins teria. Thus, the MIC90 (MIC at which 90% of the and/or their transcriptional regulators. The possible isolates tested were inhibited) for most Enterococcus trends for development of resistance to tigecycline fecalis and clinical isolates ob- will be discussed below. tained in a series of studies was in a range of 0.12–

0.25 mg/L. The MIC90 value for vancomycin-resis- Tigecycline antibacterial activity in vitro tant strains of both species was 0.12 mg/L (20–22, Recent reports on the global testing such as 27–28). the Tigecycline Evaluation and Surveillance Trial Susceptibility ranges estimated for S. aureus were (TEST Program) and other tigecycline studies con- similar to those for Enterococcus spp. and varied from ducted in a number of countries have shown in vitro ≤0.125 to 0.5 mg/L (MIC90) (20–22, 27–28). Simi- susceptibility of large collection of isolates to tige- lar tigecycline activity was estimated against MRSA cycline from community and nosocomial infections isolates (MIC90, 0.25–0.5 mg/L) (20–22, 27–28). (20–22). In addition, cumulative data on the tigecy- Borbone et al. (27) showed that tigecycline was cline activity against bacterial isolates in comparison more active against MRSA and enterococci (MIC90, with other antimicrobials have been recently pre- 0.25 and 0.12 mg/L, respectively) as compared to sented (22). These studies have shown that tigecy- (MIC90, 2 mg/L) and quinupristin/dalfo- cline is active in vitro against various gram-positive pristin (MIC90, 0.5 and 2–4 mg/L, respectively). and gram-negative bacteria including multidrug- Similarly, tigecycline MIC90 values for Streptococ- resistant strains, aerobic, anaerobic bacteria, and cus pneumoniae ranged from 0.06 mg/L to 0.5 mg/L.

Medicina (Kaunas) 2010; 46(4) 244 Vaida Šeputienė, Justas Povilonis, Julija Armalytė, et al.

The penicillin-resistant S. pneumoniae isolates also MIC90 values of 16 to ≥32 mg/L; P. mirabilis and were highly susceptible to tigecycline (MIC90 range indole-positive Proteeae (Proteus spp., Morganella of 0.06–0.5 mg/L) (20–22, 27–28). Representa- spp., Providencia spp.) show MIC90 of 2–8 mg/L (4, tive study published by Darabi et al. (29) showed 20–22, 27–30, 35). that 98.4% of S. pneumoniae isolates were inhibited by tigecycline at the concentration of <0.12 mg/L, Tigecycline antibacterial activity in vivo, whereas 100% of penicillin-resistant S. pneumoniae clinical use, pharmacodynamic and were inhibited by this concentration. pharmacokinetic aspects Tigecycline susceptibility breakpoints for Enter- Clinical effi ciency of tigecycline in the treatment obacteriaceae, defi ned by FDA and EUCAST, are 2 of adult complicated skin and skin-structure infec- mg/L and 1 mg/L, respectively (1, 23). Numerous tions and complicated intra-abdominal infections studies identifi ed clinical Escherichia coli isolates has been evaluated in several Phase 3, multicentered, as being the most tigecycline-susceptible among randomized, double-blind studies (36–39). The

Enterobacteriaceae. Thus, tigecycline MIC90 for E. clinical cure rates for tigecycline monotherapy were coli ranged from 0.25 mg/L to 0.5 mg/L includ- found to be similar to that of vancomycin- ing ESBL-producing strains (20–22, 28). Of ESBL- combination for the treatment of cSSSIs (most of producers, 98.3% to 100% were found to be tige- them were extensive cellulitis or surgery-requiring cycline-susceptible according to numerous studies soft tissue infection), where methicillin-sensitive (20–22, 28). (MSSA) (cure rates, 88.8% Tigecycline susceptibility breakpoint for most of and 90.8%, respectively) and MRSA were dominat- Klebsiella spp., Enterobacter spp., Serratia spp. was ing pathogens (cure rates, 78.1% and 75.8%, respec-

MIC90 ≤2 mg/L including ESBL-positive K. pneu- tively) (38). Similarly, tigecycline (585 patients) has moniae (20–22, 28). been demonstrated to be noninferior to the imi- Recent studies show, however, that some E. clo- penem-cilastatin combination (607 patients) for the acae, K. pneumoniae, S. marcescens clinical isolates treatment of cIAIs, 50% of which were complicated exhibit reduced susceptibility to tigecycline show- by appendicitis and 14% complicated by cholecys- ing MIC90 of 4–8 mg/L (30). Emergence of Enter- titis (39). obacteriaceae strains with lowered susceptibility to Tigecycline has been also evaluated in Phase 3, tigecycline appears to be related to the constitutive open-label, noncomparative study in the treatment expression of multidrug effl ux systems (31–32). of patients with selected serious infections caused Among gram-negative non-, by resistant gram-negative microbes (Enterobacter

M. catarrhalis (MIC90, 0.5 mg/L), S. maltophilla spp., , Klebsiella pneumoni- (MIC90, 2mg/L) (4), and A. baumannii (MIC90, 1–2 ae). The clinical indications included cSSSIs, cIAIs, mg/L) isolates demonstrated the highest suscepti- CABP, health care-associated pneumonia (HCAP) bility to tigecycline (20–22, 28). Tigecycline MIC90 including ventilator-associated pneumonia (VAP), values for A. baumannii were reported to be lowest and bacteremia including catheter-related pneumo- of all antimicrobials tested including carbapenems nia (40). (22, 28). Demonstration of the effectiveness and safety of Tigecycline exhibited good activity in vitro tigecycline led to its approval by the FDA in 2005 against anaerobic species isolated from clinical sam- for treatment of adult cSSSIs and cIAIs (Fig. 2). One ples. Clostridium spp., Eubacterium lentum, Fuso- year later, tigecycline was approved by the European bacterium nucleatum, micros, Por- Medicines Evaluation Agency (EMEA) for the same phyromonas spp., Prevotella spp., Propionibacterium indications (2). acnes, Veillonella spp. showed MIC90 ranging from Several Phase 3 studies, in which tigecycline was 0.12 to 0.5 mg/L (33). Among anaerobes, Bacteroi- compared with levofl oxacin, as well as noncompara- des fragilis and fragilis species exhibited tive clinical studies involving patients with CABP the highest MIC90 values of 1–8 and 2–16 mg/L, have also demonstrated drug effi cacy (41–43). In respectively (34). 2009, tigecycline was approved by the U.S. FDA for Tigecycline was shown to be potent antimicro- treatment of CABP (Fig. 2) (44). bial in vitro against atypical pathogens such as My- Based on the results of clinical studies, tigecy- coplasma pneumoniae, Legionella spp., Mycoplasma cline is suggested as a suitable antimicrobial for the hominis, Chlamydia pneumoniae, Chlamydia tracho- empirical monotherapy in the case of polymicrobial matis, non-tuberculosis strains of Mycobacterium infections and where multidrug-resistant pathogens (reviewed in 3). are involved (1). There is a group of pathogens, however, which Tigecycline (Tygacil) is administered intrave- exhibit reduced in vitro susceptibility to tigecycline. nously (1). The recommended adult dose of tige- Thus, P. aeruginosa clinical isolates display high cycline is 100 mg as a loading dose, then followed

Medicina (Kaunas) 2010; 46(4) Tigecycline – how powerful is it in the fi ght against antibiotic-resistant bacteria? 245 by the doses of 50 mg every 12 hours. The recom- As it has been mentioned above, the most com- mended duration of the therapy is 5–14 days. Tige- mon basis of intrinsic tigecycline resistance ob- cycline has long terminal half life (t1/2) yielding 42 served in , Proteus mirabilis, hours and 27 hours after infusion of multiple 50- Morganella morganii species is drug recognition and mg doses every 12 hours and a single 100-mg drug effl ux by RND family transporters, which typically dose, respectively (45). Tigecycline exhibits high confer multidrug-resistant phenotype. Thus, up-reg- volume distribution ranging from 7.2 to 8.6 L/kg ulation of the family members, effl ux pumps AcrAB either after administration of a single dose or at and MexAB-OprM, was demonstrated in clinical multiple doses. It has excellent tissue penetration, Morganella morganii and Proteus mirabilis strains with and it was found to be distributed in various tissues reduced tigecycline susceptibility (17–18). Overex- (lungs, liver, heart, skin, meninges, bone) and body pression of RND family members, acrAB and acrEF, fl uids of animals and humans. While highly potent has been already reported in clinical E. coli, K. pneu- for treatment of deeply infected tissues, tigecycline moniae, E. cloacae isolates with decreased suscepti- exhibits relatively low maximum plasma concentra- bility to tigecycline (31–32, 46, 47). In addition to tions (Cmax) of 0.8–1 mg/L (45). This raises some this list, there are two tigecycline-resistant blood- concerns about drug use for treatment of blood- stream A. baumannii isolates (MICs of 4 and 16 stream infections caused by microorganisms with mg/L for tigecycline, respectively) recovered from MIC values of ≥1 mg/L for tigecycline. patients who received tigecycline (Fig. 2). AdeABC Tigecycline does not undergo extensive metabo- transporter has been proposed to mediate drug re- lism, and its major route of elimination is via biliary sistance in these isolates (48). Notably, several other or fecal as an unchanged drug. The sec- cases of tigecycline-resistant A. baumannii clinical ondary, albeit minor routes of elimination, are renal isolates recovered from patients receiving treatment excretion and liver glucuronidation (45). with tigecycline have been reported (49–50). In According to the reports from clinical trials, some cases, however, a rapid development of tige- tigecycline was generally good tolerated. The ma- cycline resistance of this highly adaptable pathogen jor adverse effects reported were reversible nausea, has been monitored. These observations, along with diarrhea, vomiting at the initial phase (days 1–2) of several earlier studies starting 2006, where the isola- administration (1). tion of tigecycline-nonsusceptible MDR (including carbapenem-resistant) A. baumannii clinical strains Development of resistance to tigecycline is reported (see for review 50), alert the activity of Recent reports on the results of surveillance trials tigecycline against A. baumannii might be limited at show that tigecycline remained active against gram- least for some clinical indications, e.g., bloodstream positive and gram-negative bacteria since its intro- duction in 2005, whereas the proportion of strains infections, where subtherapeutic levels of tigecy- resistant to its comparators, such as vancomycin- cline might rapidly select resistance. resistant E. faecium, fl uoroquinolone and broad- The most likely pathway of up-regulation of spectrum β-lactam-resistant Enterobacteriaceae, has transporters in glycylcycline-susceptible bacteria increased during the same period (46). is via mutations in genes coding either for protein Nevertheless, the cases of emergence of tigecy- regulators or protein components of effl ux pumps. cline resistance in usually susceptible clinical bacte- This was early demonstrated in two veterinary Sal- rial isolates must be carefully examined as well as monella spp. as well as in vitro generated laboratory potential molecular mechanisms of such resistance strains with reduced susceptibility to glycylcyclines, investigated. Of concern is the recent report on the which harbored mutations in effl ux pump compo- isolation of clinical tigecycline-resistant E. faecalis nents TetA and TetB, respectively (51). As the rate of strain from the catheter urine sample of the patient mutations was low and they only slightly decreased who underwent intra-abdominal surgery and had susceptibility of mutant strains to glycylcyclines, the treatment course with several antibiotics includ- minor role of such mutations in the development of ing tigecycline (44) (Fig. 2). The E. faecalis isolate resistance in clinical bacterial strains has been pro- showed MICs of 2 mg/L and 1 mg/L as determined posed. Nevertheless, more recently, mutations in by the Etest and microdilution methods, whereas marA and ramA genes, coding for transcription reg- MIC values for susceptible E. faecalis strain were ulators and resulting in the overexpression of RND 0.047 mg/L and 0.125 mg/L, respectively. Search transporters, have been found to be responsible for for the mechanism, responsible for the reduced decreased susceptibility of E. coli and E. cloacae tigecycline susceptibility of E. faecalis isolate in- clinical isolates to tigecycline (32, 47). Overexpres- cluding altered expression of effl ux pumps, tigecy- sion of ramA gene coding for a positive regulator cline-modifying tetX gene (see below), 16S rRNA of AcrAB in clinical K. pneumoniae isolates with re- mutations, however, did not reveal possible target duced susceptibility to tigecycline has been recently sites (44). linked to inactivating mutations in another gene,

Medicina (Kaunas) 2010; 46(4) 246 Vaida Šeputienė, Justas Povilonis, Julija Armalytė, et al. homologous to Salmonella regulatory gene ramR, (Fig. 1), and an equivalent modifi cation has been which is observed in tigecycline-resistant Salmo- demonstrated for tigecycline (54). It has been pro- nella strains (52). Authors propose that acquisition posed that modifi cation results in a weakened ability of mutations in ramR could lead to development of of tetracycline to coordinate magnesium, which is tigecycline-resistant phenotype through intermedi- critical for drug binding to the ribosome. The in- ate phenotypes (52). troduction of tetX copy into E. coli resulted in much Mutations in genes coding for transporters oth- smaller increase in tigecycline MICs as compared er than RND type, at least in part, were found to to other tetracyclines indicating that the effect of be responsible for elevated tigecycline MICs as it TetX-mediated cellular tigecycline modifi cation is has been demonstrated by the selection of resistant possibly still outcompeted by higher ribosome af- MRSA mutants through the serial passage on the in- fi nity of tigecycline leading to translation inhibi- creasing concentrations of tigecycline. Subsequent tion (54). So far, no tetracycline-resistant clinical transcription profi ling has detected overexpression isolates, harboring tetX gene, have been reported. of mepA gene, coding for a novel single protein ef- Nevertheless, the shown susceptibility of tigecycline fl ux pump belonging to multidrug and toxin extru- to enzyme-based modifi cation extends the spec- sion (MATE) family (53). trum of possible mechanisms of resistance to gly- At present, there is no evidence on the mutations cylcycline, which might occur in vivo. of genes, coding for ribosome protection proteins Knowing the enormous adaptability of microor- of tet family (TetM, TetO, TetS), to be responsible ganisms including the potency to develop antibiotic for the reduced susceptibility to tigecycline. Over- resistance, it is unlikely to expect it to be escaped expression of TetM protein, commonly observed in with tigecycline, e.g. through further development tetracycline-resistant strains, was not suffi cient to pre-existing mechanisms of tetracycline resistance render laboratory S. aureus strains to become tige- and/or acquiring new ones. The prudent use of cycline resistant (53). tigecycline for therapeutic application hopefully will Another known tetracycline-resistance mecha- preserve its effectiveness against clinically important nism based on the drug inactivation by modifi ca- multidrug-resistant pathogens. tion enzyme TetX has been shown to be active in vitro using tigecycline as a substrate (54). Flavin- Acknowledgments dependent monooxygenase TetX effi ciently hydrox- Lithuanian State Science and Study Foundation ylates tetracycline and its derivatives at carbon 11 supported this work.

Tigeciklinas. Kokio veiksmingumo ginklas kovoje su antibiotikams atspariomis bakterijomis?

Vaida Šeputienė1, Justas Povilonis1, Julija Armalytė1, 2, Kęstutis Sužiedėlis1, 2, Alvydas Pavilonis3, Edita Sužiedėlienė1 1Vilniaus universiteto Gamtos mokslų fakulteto Biochemijos ir biofizikos katedra, 2Vilniaus universiteto Onkologijos institutas, 3Kauno medicinos universiteto Mikrobiologijos katedra

Raktažodžiai: tigeciklinas, veikimo mechanizmas, aktyvumas in vitro ir in vivo, atsparumas.

Santrauka. Tigeciklinas yra pusiau sintetinis tetraciklinų analogas, priskiriamas naujai glicilciklinų antibiotikų klasei, neseniai patvirtintas tam tikrų indikacijų infekcijoms gydyti JAV ir Europos Sąjungoje. Antibiotikas yra veiksmingas prieš daugelį gramneigiamų ir gramteigiamų bakterijų, tarp jų – prieš plintančius visuomenės ir hospitalinių infekcijų sukėlėjus, pasižyminčius dauginiu atsparumu antibiotikams. Tikslus tigeciklino molekulinis veikimo mechanizmas nepakankamai ištirtas. Manoma, kad tigeciklinas prisijungia prie bakterijų ribosomų 30S subvieneto toje pačioje prisijungimo vietoje kaip tetraciklinas ir slopina baltymų biosintezės (transliacijos) eigą, neleisdamas reakcijos substratui – aminoacil-tRNR tiksliai išsidėstyti ribosomos A centre peptidiniam ryšiui sudaryti. Pabrėžtina, kad tigeciklinas pasižymi gebėjimu prisijungti prie ribosomų ir yra aktyvus bakterijose, turinčiose gerai žinomus ir plačiai paplitusius atsparu- mo tetraciklinui molekulinius mechanizmus: apsauginius ribosomos baltymus, pašalinančius tetracikliną iš ribosomų, bei baltyminius siurblius, šalinančius tetracikliną iš ląstelių. Apžvalgoje apibendrinama naujausia informacija apie tigeciklino veikimo mechanizmą, vartojimą klinikoje, diskutuojama apie galimus atsparu- mo tigeciklinui atsiradimo kelius.

Medicina (Kaunas) 2010; 46(4) Tigecycline – how powerful is it in the fi ght against antibiotic-resistant bacteria? 247

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Received 28 May 2009, accepted 6 April 2010 Straipsnis gautas 2009 05 28, priimtas 2010 04 06

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