Fighting resistant tuberculosis with old compounds: the carbapenem paradigm

Abstract

Despite intensive efforts to improve tuberculosis (TB) care and control, the global burden of TB is falling only slowly, and the evolution of drug resistance remains uncertain. According to the WHO, the estimated incidence in 2009 was 9.4 million new cases [1World Health Organization Global tuberculosis control 2010. WHO report 2010.Available at: http://www.who.int/tb/en/Google Scholar]. In recent years, TB treatment has become more complicated because of the emergence of multidrug-resistant (MDR) TB, i.e. TB caused by strains resistant to isoniazid and rifampin. In 2009, an estimated 250 000 patients had MDR TB, representing 3.3% of new TB cases and up to 28% in certain countries of the former Soviet Union [1World Health Organization Global tuberculosis control 2010. WHO report 2010.Available at: http://www.who.int/tb/en/Google Scholar]. In 2008, the number of deaths caused by MDR TB was estimated to be 150 000. The emergence of extensively drug-resistant (XDR) TB, defined by resistance to isoniazid and rifampin combined with additional resistance to a fluoroquinolone and at least 1 s line injectable agent (amikacin, kanamycin, or capreomycin), has been associated with increasing mortality (65–100%), as efficient therapy is not available [2Kliiman K Altraja A Predictors of poor treatment outcome in multi‐ and extensively drug‐resistant pulmonary TB.Eur Respir J. 2009; 33: 1085-1094Crossref PubMed Scopus (111) Google Scholar]. By July 2010, 58 countries had reported at least one case of XDR TB [1World Health Organization Global tuberculosis control 2010. WHO report 2010.Available at: http://www.who.int/tb/en/Google Scholar]. These alarming data highlight the urgent need for new anti-TB drugs. Except for fluroquinolones, no novel anti-TB antibiotic has been introduced in the last 45 years. β-Lactams have not been considered to be useful drugs for the treatment of TB, because Mycobacterium tuberculosis naturally produces BlaC, an extended-spectrum class A β-lactamase [3Hugonnet JE Blanchard JS Irreversible inhibition of theMycobacterium tuberculosis beta‐lactamase by clavulanate.Biochemistry. 2007; 46: 11998-12004Crossref PubMed Scopus (175) Google Scholar]. However, older reports have shown that the combination of amoxycillin and clavulanic acid is bactericidal in vitro [4Cynamon MH Palmer GS In vitro activity of amoxicillin in combination with clavulanic acid againstMycobacterium tuberculosis.Antimicrob Agents Chemother. 1983; 24: 429-431Crossref PubMed Scopus (84) Google Scholar], an efficacy that can be accounted for by the recent demonstration that BlaC is irreversibly inactivated by clavulanic acid [3Hugonnet JE Blanchard JS Irreversible inhibition of theMycobacterium tuberculosis beta‐lactamase by clavulanate.Biochemistry. 2007; 46: 11998-12004Crossref PubMed Scopus (175) Google Scholar]. Reduction of the burden of M. tuberculosis in the sputum of patients treated with amoxycillin and clavulanic acid has been reported [5Chambers HF Kocagöz T Sipit T Turner J Hopewell PC Actvity of amoxicillin/clavulanate in patients with tuberculosis.Clin Infect Dis. 1998; 26: 874-877Crossref PubMed Scopus (116) Google Scholar], but clinical assessments for the treatment of MDR TB are limited to anecdotal cases in which amoxycillin–clavulanic acid was used in association with second-line drugs [6Nadler JP Berger J Nord JA Cofsky R Saxena M Amoxicillin–clavulanic acid for treating drug‐resistantMycobacterium tuberculosis.Chest. 1991; 99: 1025-1026Crossref PubMed Scopus (103) Google Scholar]. Among the different classes of β-lactams, very recent data concerning carbapenems have opened new avenues towards the potential use of this class in the treatment of TB, especially for MDR and XDR TB. Until recently, all β-lactams were thought to exclusively act on the active-site serine d,d-transpeptidases, the classic penicillin-binding proteins that catalyse the final cross-linking step of peptidoglycan synthesis. We have recently shown that these d,d-transpeptidases are bypassed by a novel family of polymerases, the active-site cysteine l,d-transpeptidases (Ldts), in β-lactam-resistant enterococci and in M. tuberculosis [7Mainardi JL Villet R Bugg TD Mayer C Arthur M Evolution of peptidoglycan biosynthesis under the selective pressure of antibiotics in Gram‐positive bacteria.FEMS Microbiol Rev. 2008; 32: 386-408Crossref PubMed Scopus (140) Google Scholar, 8Lavollay M Arthur M Fourgeaud M et al.The peptidoglycan of stationary phaseMycobacterium tuberculosis predominantly contains cross‐links generated by L,D‐transpeptidation.J Bacteriol. 2008; 190: 4360-4366Crossref PubMed Scopus (265) Google Scholar]. These enzymes are promising targets for the use of carbapenems, or for the development of anti-TB drugs belonging to this class, because the Ltds are responsible for the formation of 80% of the cross-links in M. tuberculosis, and these enzymes are irreversibly inactivated by carbapenems by the formation of covalent adducts [8Lavollay M Arthur M Fourgeaud M et al.The peptidoglycan of stationary phaseMycobacterium tuberculosis predominantly contains cross‐links generated by L,D‐transpeptidation.J Bacteriol. 2008; 190: 4360-4366Crossref PubMed Scopus (265) Google Scholar, 9Gupta R Lavollay M Mainardi JL Arthur M Bishai WR Lamichhane G TheMycobacterium tuberculosis protein LdtMt2 is a nonclassical transpeptidase required for virulence and resistance to amoxicillin.Nat Med. 2010; 16: 466-469Crossref PubMed Scopus (217) Google Scholar]. Moreover, one of the five Ltds of M. tuberculosis (LdtMt2) is essential for virulence in a mouse model of acute infection [9Gupta R Lavollay M Mainardi JL Arthur M Bishai WR Lamichhane G TheMycobacterium tuberculosis protein LdtMt2 is a nonclassical transpeptidase required for virulence and resistance to amoxicillin.Nat Med. 2010; 16: 466-469Crossref PubMed Scopus (217) Google Scholar]. Carbapenems are slowly hydrolysed by the β-lactamase BlaC [3Hugonnet JE Blanchard JS Irreversible inhibition of theMycobacterium tuberculosis beta‐lactamase by clavulanate.Biochemistry. 2007; 46: 11998-12004Crossref PubMed Scopus (175) Google Scholar], and one member of the carbapenem family, meropenem, has been reported to be uniformly active in vitro in association with clavulanic acid against a panel of XDR strains (meropenem MIC of <1 mg/L in the presence of 2.5 mg/L clavulanic acid [10Hugonnet JE Tremblay LW Boshoff HI Barry 3rd, CE Blanchard JS Meropenem–clavulanate is effective against extensively drug‐resistantMycobacterium tuberculosis.Science. 2009; 323: 1215-1218Crossref PubMed Scopus (399) Google Scholar]. Carbapenems may also have potential applications for the treatment of susceptible TB, because meropenem–clavulanic acid is active against non-replicative forms (‘dormant’ forms) of M. tuberculosis [10Hugonnet JE Tremblay LW Boshoff HI Barry 3rd, CE Blanchard JS Meropenem–clavulanate is effective against extensively drug‐resistantMycobacterium tuberculosis.Science. 2009; 323: 1215-1218Crossref PubMed Scopus (399) Google Scholar], which are difficult to eradicate even with isoniazid and rifampin. The adaptive response of M. tuberculosis during the transition from aerobic growth to stationary phase results in the activation of a ‘dormancy’ regulon, which includes genes that are likely to play an essential role in the long-term survival of the bacteria, and therefore encode potential targets for the development of sterilizing drugs. Among these genes, Rv0116c, which encodes LdtMt1, is upregulated 17-fold [11Betts JC Lukey PT Robb LC McAdam RA Duncan K Evaluation of a nutrient starvation model ofMycobacterium tuberculosis persistence by gene and protein expression profiling.Mol Microbiol. 2002; 43: 717-731Crossref PubMed Scopus (1129) Google Scholar]. Thus, LdtMt1 could be an essential target accounting for the bactericidal activity of meropenem–clavulanic acid against dormant forms of M. tuberculosis in vitro [10Hugonnet JE Tremblay LW Boshoff HI Barry 3rd, CE Blanchard JS Meropenem–clavulanate is effective against extensively drug‐resistantMycobacterium tuberculosis.Science. 2009; 323: 1215-1218Crossref PubMed Scopus (399) Google Scholar]. In a mouse model, imipenem alone had antimycobacterial activity, although this carbapenem was less effective than isoniazid [12Chambers HF Turner J Schecter GF Kawamura M Hopewell PC Imipenem for treatment of tuberculosis in mice and humans.Antimicrob Agents Chemother. 2005; 49: 2816-2821Crossref PubMed Scopus (97) Google Scholar]. More recently, imipenem or meropenem associated with clavulanic acid was shown to improve the survival of mice infected with M. tuberculosis, although the combinations were less effective than isoniazid alone [13Veziris N Truffot C Mainardi JL Jarlier V Activity of carbapenems combined with clavulanate against murine tuberculosis.Antimicrob Agents Chemother. 2011; 55: 2597-2600Crossref PubMed Scopus (53) Google Scholar]. To date, the clinical experience with carbapenems is positive but very limited, and the β-lactams have only been used in combination with first-line and second-line drugs [12Chambers HF Turner J Schecter GF Kawamura M Hopewell PC Imipenem for treatment of tuberculosis in mice and humans.Antimicrob Agents Chemother. 2005; 49: 2816-2821Crossref PubMed Scopus (97) Google Scholar]. Despite very encouraging in vitro data highlighting carbapenems as promising agents for the treatment of resistant TB, there are several limitations, including the intravenous route of administration of all approved carbapenems (except for faropenem in Japan), the risks inherent in long-term vascular access, high costs, and very limited clinical experience. However, in vitro efficacy and the evolution of resistance justify the design of clinical studies to assess the efficacy of carbapenems in association with clavulanic acid in the treatment of MDR and XDR TB. The development of carbapenemrelated compounds may also lead to drugs that are not hydrolysed by BlaC and are appropriate for TB treatment. Mainardi JL has received funds for speaking, consultancy, advisory board membership, travel from Pfizer, Novartis, Aventis, Astrazeneca, GSK, Janssen. L. Gutmann has received funds from Biomerieux and Cepheid. Mainardi JL has received research funding from Novartis and Merck Shape and Dohme (MSD).