In vitro and in vivo efficacy of β-lactams against replicating and slowly growing/nonreplicating Mycobacterium tuberculosis

Abstract

Beta-lactams, in combination with beta-lactamase inhibitors, are reported to have activity against Mycobacterium tuberculosis bacteria growing in broth, as well as inside the human macrophage. We tested representative beta-lactams belonging to 3 different classes for activity against replicating M. tuberculosis in broth and nonreplicating M. tuberculosis under hypoxia, as well as against streptomycin-starved M. tuberculosis strain 18b (ss18b) in the presence or absence of clavulanate. Most of the combinations showed bactericidal activity against replicating M. tuberculosis, with up to 200-fold improvement in potency in the presence of clavulanate. None of the combinations, including those containing meropenem, imipenem, and faropenem, killed M. tuberculosis under hypoxia. However, faropenem- and meropenem-containing combinations killed strain ss18b moderately. We tested the bactericidal activities of meropenem-clavulanate and amoxicillin-clavulanate combinations in the acute and chronic aerosol infection models of tuberculosis in BALB/c mice. Based on pharmacokinetic/pharmacodynamic indexes reported for beta-lactams against other bacterial pathogens, a cumulative percentage of a 24-h period that the drug concentration exceeds the MIC under steady-state pharmacokinetic conditions (%TMIC) of 20 to 40% was achieved in mice using a suitable dosing regimen. Both combinations showed marginal reduction in lung CFU compared to the late controls in the acute model, whereas both were inactive in the chronic model.

Fig 1

Isobologram indicating synergy between amoxicillin and clavulanate against replicating M. tuberculosis H37Rv. The dashed line joins the fractional MICs of individual agents. Clavulanate was inactive on its own. Its MIC was assumed to be 80 μg/ml.

Fig 2

Pharmacokinetic profile for amoxicillin (200 mg/kg) (●) plus clavulanate (50 mg/kg) (■) and meropenem (300 mg/kg) (▲) plus clavulanate (75 mg/kg) (⧫) in infected mice.

Fig 3

Efficacies of meropenem-clavulanate and amoxicillin-clavulanate in the acute (A) and chronic (B) mouse models for TB. The bacterial load at the beginning of treatment (early control) is indicated by the dashed lines. Solid horizontal lines indicate the mean values.