# Mycobacterial Cell Wall: A Source of Successful Targets for Old and New Drugs
## 4 New Generation of Cell Wall Inhibitors
### Target: Peptidoglycan  
#### 4.3.1 \(\upbeta\)-Lactams and Clavulanic Acid  
_Discovery._ The first class of synthetic drugs that successfully treated bacterial infections were the sulfa drugs in the early 1930s [163]; however, by the early 1940s, penicillin had quickly eclipsed the sulfa drugs. Penicillin, a \(\upbeta\)-lactam compound, was active against Gram-positive and Gram-negative, yet penicillin, like the sulfa drugs, had no activity against _M. tuberculosis_. For penicillin, the lack of activity was due to the presence in _M. tuberculosis_ of a potent \(\upbeta\)-lactamase BlaC. The innate resistance of _M. tuberculosis_ to \(\upbeta\)-lactams decreased when blaC was deleted from _M. tuberculosis_ or when the \(\upbeta\)-lactamase irreversible inhibitor clavulanic acid [248] was used in combination with \(\upbeta\)-lactams [249]. [PERSON] and colleagues used this knowledge to validate the combination of meropenem, a carbapenem with poor affinity for BlaC, and clavulanic acid as an efficacious _M. tuberculosis_ inhibitor [250]. _Activity._ The combination meropenem plus clavulanate is part of the group D3 (add-on agents) of second-line TB drugs. Meropenem is a potent inhibitor of drug-susceptible and XDR _M. tuberculosis_ strains (MIC range from 0.6 \(\upmu\)m to 3.3 \(\upmu\)m) when combined with clavulanate [250]. Interestingly, this combination also inhibits non-replicating _M. tuberculosis_[250]. In a chronic model of _M. tuberculosis_ infection in mice, meropenem had a modest activity in reducing lung burdens [251]. Surprisingly, the addition of clavulanate to the meropenem treatment did not increase meropenem activity in mice. In mice intravenously infected with _M. tuberculosis_, the lung burden had increased by almost a 1 log\({}_{10}\) after 4 weeks of treatment with meropenem and clavulanate showing no inhibitory activity of thiscombination in vivo [252]. However, studies in mice are problematic since the mice dehydropropeptidase (DHP) rapidly cleaves the \(\beta\)-lactam ring. Meropenem is administered via intravenous injection and new regimens are steering away from injectables. Therefore, [PERSON] and colleagues tested raropenem, an _oral_\(\beta\)-lactam, and demonstrated activity against _M. tuberculosis_ (MIC 4.6 \(\upmu\)m) without clavulanate addition (meropenem MIC is 8-fold higher without clavulanate) [253]. Faropenem is also more soluble in water than meropenem (solubility 14.7 g/L vs. 8 g/L for meropenem). Using the dehydropropeptidase inhibitor, probenecid, [PERSON] and colleagues demonstrated that _M. tuberculosis_-infected mice had a small but significant reduction in lung burden after 9 days of treatment with a combination faropenem/clavulanate/probenecid. In a study using DHP-I knockout mice [254], mice were infected intratracheally with _M. tuberculosis_ H37 Rv and treated for 8 consecutive days, 10 to 12 days post infection. Meropenem was given subcutaneously three times a day in combination with amoxicillin and clavulanate. Another group of mice received faropenem orally with amoxicillin and clavulanate. A control group was treated with moxifloxacin. Treatment also consisted of a dose of probenecid prior to drug administration. While the lung burden was 7.7 log10 CFU/mouse at the beginning of the treatment, the lung burdens at the end of the treatment were 7.8 and 7.5 log10 CFU/mouse for the meropenem and faropenem groups, respectively. Only the mice receiving moxifloxacin had a 2-log10 reduction in lung burden [254]. Experiments performed in other animal models (rabbits and monkeys) revealed that intravenous injections of meropenem with or without clavulanate and dehydroproptidae inhibitor led to serious adverse effects (diarrhea, weight loss and death) [251]. _Mode of action._ Meropenem binds to and inhibits the L,D-transpeptidase LdtM1 (Rv0116c), which catalyzes the 3\(\rightarrow\)3 cross-linkage in the peptidoglycan [255,256]. Faropenem is 14-fold more potent in inactivating LdtM1 than meropenem, but this did not correlate with increasing killing against intracellular _M.