Discrepancy between uptake and intracellular activity of moxifloxacin in a Staphylococcus aureus-human THP-1 monocytic cell model

In vitro and ex vivo activity of the fluoroquinolone DC-159a against mycobacteria

Antimicrobial resistance is a global health problem. In 2021, it was estimated almost half a million of multidrug-resistant tuberculosis (MDR-TB) cases. Besides, non-tuberculous mycobacteria (NTM) are highly resistant to several drugs and the emergence of fluoroquinolone (FQ) resistant M. tuberculosis (Mtb) is also a global concern making treatments difficult and with variable outcome. The aim of this study was to evaluate the activity of the FQ, DC-159a, against Mtb and NTM and to explore the cross-resistance with the currently used FQs.A total of 12 pre-extensively drug-resistant (XDR) Mtb, 2 XDR, 36 fully drug susceptible strains and 41 NTM isolates were included to estimate the in vitro activity of DC-159a, moxifloxacin (MOX) and levofloxacin (LX), using minimal inhibitory and bactericidal concentration (MIC and MBC). The activity inside the human macrophages and pulmonary epithelial cells were also determined.DC-159a was active in vitro and ex vivo against mycobacteria. Besides, it was more active than MOX/LX. Moreover, no cross-resistance was evidenced between DC-159a and LX/MOX as DC-159a could inhibit Mtb and MAC strains that were already resistant to LX/MOX.DC-159a could be a possible candidate in new therapeutic regimens for MDR/ XDR-TB and mycobacterioses cases.

Intracellular Accumulation of Novel and Clinically Used TB Drugs Potentiates Intracellular Synergy

The therapeutic repertoire for tuberculosis (TB) remains limited despite the existence of many TB drugs that are highly active in in vitro models and possess clinical utility. Underlying the lack of efficacy in vivo is the inability of TB drugs to penetrate microenvironments inhabited by the causative agent, Mycobacterium tuberculosis, including host alveolar macrophages. Here, we determined the ability of the phenoxazine PhX1 previously shown to be active against M. tuberculosis in vitro to differentially penetrate murine compartments, including plasma, epithelial lining fluid, and isolated epithelial lining fluid cells. We also investigated the extent of permeation into uninfected and M. tuberculosis-infected human macrophage-like Tamm-Horsfall protein 1 (THP-1) cells directly and by comparing to results obtained in vitro in synergy assays. Our data indicate that PhX1 (4,750 ± 127.2 ng/ml) penetrates more effectively into THP-1 cells than do the clinically used anti-TB agents, rifampin (3,050 ± 62.9 ng/ml), moxifloxacin (3,374 ± 48.7 ng/ml), bedaquiline (4,410 ± 190.9 ng/ml), and linezolid (770 ± 14.1 ng/ml). Compound efficacy in infected cells correlated with intracellular accumulation, reinforcing the perceived importance of intracellular penetration as a key drug property. Moreover, we detected synergies deriving from redox-stimulatory combinations of PhX1 or clofazimine with the novel prenylated amino-artemisinin WHN296. Finally, we used compound synergies to elucidate the relationship between compound intracellular accumulation and efficacy, with PhX1/WHN296 synergy levels shown to predict drug efficacy. Collectively, our data support the utility of the applied assays in identifying in vitro active compounds with the potential for clinical development. IMPORTANCE This study addresses the development of novel therapeutic compounds for the eventual treatment of drug-resistant tuberculosis. Tuberculosis continues to progress, with cases of Mycobacterium tuberculosis (M. tuberculosis) resistance to first-line medications increasing. We assess new combinations of drugs with both oxidant and redox properties coupled with a third partner drug, with the focus here being on the potentiation of M. tuberculosis-active combinations of compounds in the intracellular macrophage environment. Thus, we determined the ability of the phenoxazine PhX1, previously shown to be active against M. tuberculosis in vitro, to differentially penetrate murine compartments, including plasma, epithelial lining fluid, and isolated epithelial lining fluid cells. In addition, the extent of permeation into human macrophage-like THP-1 cells and H37Rv-infected THP-1 cells was measured via mass spectrometry and compared to in vitro two-dimensional synergy and subsequent intracellular efficacy. Collectively, our data indicate that development of new drugs will be facilitated using the methods described herein.

Influence of the protein kinase C activator phorbol myristate acetate on the intracellular activity of antibiotics against hemin- and menadione-auxotrophic small-colony variant mutants of Staphylococcus aureus and their wild-type parental strain in human THP-1 cells

In a previous study (L. G. Garcia et al., Antimicrob. Agents Chemother. 56:3700-3711, 2012), we evaluated the intracellular fate of menD and hemB mutants (corresponding to menadione- and hemin-dependent small-colony variants, respectively) of the parental COL methicillin-resistant Staphylococcus aureus strain and the pharmacodynamic profile of the intracellular activity of a series of antibiotics in human THP-1 monocytes. We have now examined the phagocytosis and intracellular persistence of the same strains in THP-1 cells activated by phorbol 12-myristate 13-acetate (PMA) and measured the intracellular activity of gentamicin, moxifloxacin, and oritavancin in these cells. Postphagocytosis intracellular counts and intracellular survival were lower in PMA-activated cells, probably due to their higher killing capacities. Gentamicin and moxifloxacin showed a 5- to 7-fold higher potency (lower static concentrations) against the parental strain, its hemB mutant, and the genetically complemented strain in PMA-activated cells and against the menD strain in both activated and nonactivated cells. This effect was inhibited when cells were incubated with N-acetylcysteine (a scavenger of oxidant species). In parallel, we observed that the MICs of these drugs were markedly reduced if bacteria had been preexposed to H(2)O(2). In contrast, the intracellular potency of oritavancin was not different in activated and nonactivated cells and was not decreased by the addition of N-acetylcysteine, regardless of the phenotype of the strains. The oritavancin MIC was also unaffected by preincubation of the bacteria with H(2)O(2). Thus, activation of THP-1 cells by PMA may increase the intracellular potency of certain antibiotics (probably due to synergy with reactive oxygen species), but this effect cannot be generalized to all antibiotics.

Pharmacokinetic/pharmacodynamic-based treatment of disseminated Mycobacterium avium

Disseminated Mycobacterium avium complex (MAC) is treated with a macrolide and ethambutol. However, the kill rates are extremely slow so that therapy takes many months to years to achieve and even then more than 40% of patients are not completely cured. Recent studies have demonstrated that assays that detect extracellular MAC have a limited predictive value. Antibiotics kill at a much slower and more disappointing rate against bacilli within macrophages. Use of pharmacodynamic/pharmacokinetic models has resulted in design of new doses and dosing schedules for disseminated MAC, as well as new susceptibility breakpoints for ethambutol and moxifloxacin.

Internalization of Staphylococcus aureus in lymphocytes induces oxidative stress and DNA fragmentation: possible ameliorative role of nanoconjugated vancomycin

Staphylococcus aureus is the most frequently isolated pathogen causing bloodstream infections, skin and soft tissue infections and pneumonia. Lymphocyte is an important immune cell. The aim of the present paper was to test the ameliorative role of nanoconjugated vancomycin against Vancomycin-sensitive Staphylococcus aureus (VSSA) and vancomycin-resistant Staphylococcus aureus (VRSA) infection-induced oxidative stress in lymphocytes. VSSA and VRSA infections were developed in Swiss mice by intraperitoneal injection of 5 × 10⁶ CFU/mL bacterial solutions. Nanoconjugated vancomycin was adminstrated to VSSA- and VRSA-infected mice at its effective dose for 10 days. Vancomycin was adminstrated to VSSA- and VRSA-infected mice at a similar dose, respectively, for 10 days. Vancomycin and nanoconjugated vancomycin were adminstrated to normal mice at their effective doses for 10 days. The result of this study reveals that in vivo VSSA and VRSA infection significantly increases the level of lipid peroxidation, protein oxidation, oxidized glutathione level, nitrite generation, nitrite release, and DNA damage and decreases the level of reduced glutathione, antioxidant enzyme status, and glutathione-dependent enzymes as compared to control group, which were increased or decreased significantly near to normal in nanoconjugated vancomycin-treated group. These findings suggest the potential use and beneficial role of nanoconjugated vancomycin against VSSA and VRSA infection-induced oxidative stress in lymphocytes.

Cellular accumulation of fluoroquinolones is not predictive of their intracellular activity: studies with gemifloxacin, moxifloxacin and ciprofloxacin in a pharmacokinetic/pharmacodynamic model of uninfected and infected macrophages

Fluoroquinolones enter eukaryotic cells but the correlation between cellular accumulation and activity remains poorly established. Gemifloxacin is known to accumulate to a larger extent than most other fluoroquinolones in tissues. Using murine J774 macrophages and human THP-1 monocytes, we show that gemifloxacin accumulates more than ciprofloxacin and even moxifloxacin. Whilst showing indistinguishable kinetics of accumulation in J774 macrophages, gemifloxacin was released at an approximately two-fold slower rate than ciprofloxacin and its release was only partial. Gemifloxacin was also a weaker substrate than ciprofloxacin for the efflux transporter Mrp4 active in J774 macrophages. In cells infected with Listeria monocytogenes or Staphylococcus aureus (typical cytoplasmic and phagolysosomal organisms, respectively), gemifloxacin was equipotent to moxifloxacin and ciprofloxacin in concentration-dependent experiments if data are normalised based on the minimum inhibitory concentration (MIC) in broth. Thus, larger cellular concentrations of gemifloxacin than of moxifloxacin or ciprofloxacin were needed to obtain a similar target effect. Fractionation studies showed a similar subcellular distribution for all three fluoroquinolones, with approximately two-thirds of the cell-associated drug recovered in the soluble fraction (cytosol). These data suggest that cellular accumulation of fluoroquinolones is largely a self-defeating process as far as activity is concerned, with the intracellular drug made inactive in proportion to its accumulation level. Whilst these observations do not decrease the intrinsic value of fluoroquinolones for the treatment of intracellular infections, they indicate that ranking fluoroquinolones based on cell accumulation data without measuring the corresponding intracellular activity may lead to incorrect conclusions regarding their real potential.

Toward a new age of cellular pharmacokinetics in drug discovery

Pharmacokinetics, pharmacology, and toxicology are the major determinants of the success or failure of candidates during drug development. Because inappropriate pharmacokinetics often leads to inefficacy, even toxicity, pharmacokinetics studies have been regarded as crucial components in drug preclinical and clinical research. However, new data increasingly reveal that drug concentrations in plasma or tissues cannot totally explain the efficacy of drug on the target organ. For most drugs that interact with targets localized in cells, intracellular penetration, accumulation, distribution, and elimination are important parameters governing the efficacy in the target cells. So, there is a pressing need to clarify the cellular pharmacokinetics and thus evaluate the efficacy of drugs in the target cells. This review provides a general overview regarding current knowledge about cellular pharmacokinetics in some specific cells and also summarizes the factors that can influence cellular pharmacokinetics. It concludes by discussing potential strategies for optimizing cellular pharmacokinetics and advocating that global cellular pharmacokinetics studies be conducted in future research toward improving drug efficacy.

Contribution of Catalase and Superoxide Dismutase to the Intracellular Survival of Clinical Isolates of Staphylococcus aureus in Murine Macrophages

The present study was performed in order to carefully investigate the interaction of Staphylococcus aureus with murine macrophages and the contribution of catalase and superoxide dismutase in intracellular persistence of Staphylococcus aureus within murine macrophages during in vitro infection. We have reported that Staphylococcus aureus internalized by murine macrophages did not appear to be rapidly killed. Data indicating the contribution of a single catalase and superoxide dismutase in intracellular survival of Staphylococcus aureus were provided using established biochemical assays. The results of the present experiment suggest that the survival of Staphylococcus aureus within phagocytic cells is facilitated by its ability to resist oxidative products. Organisms in the log phase of growth clearly demonstrate a resistance to oxidative products.

Moxifloxacin pharmacokinetics/pharmacodynamics and optimal dose and susceptibility breakpoint identification for treatment of disseminated Mycobacterium avium infection

Organisms of the Mycobacterium avium-intracellulare complex (MAC) have been demonstrated to be susceptible to moxifloxacin. However, clinical data on how to utilize moxifloxacin to treat disseminated MAC are scanty. In addition, there have been no moxifloxacin pharmacokinetic-pharmacodynamic (PK/PD) studies performed for MAC infection. We utilized an in vitro PK/PD model of intracellular MAC to study moxifloxacin PK/PD for disseminated disease. Moxifloxacin doses, based on a serum half-life of 12 h, were administered, and the 0- to 24-h area under the concentration-time curve (AUC(0-24)) to MIC ratios associated with 1.0 log(10) CFU/ml per week kill and 90% of maximal kill (EC(90)) were identified. The AUC(0-24)/MIC ratio associated with 1.0 log(10) CFU/ml kill was 17.12, and that with EC(90) was 391.56 (r(2) = 0.97). Next, the moxifloxacin MIC distribution in 102 clinical isolates of MAC was identified. The median MIC was 1 to 2 mg/liter. Monte Carlo simulations of 10,000 patients with disseminated MAC were performed to determine the probability that daily moxifloxacin doses of 400 and 800 mg/day would achieve or exceed 1.0 log(10) CFU/ml per week kill or EC(90). Doses of 400 and 800 mg/day achieved the AUC(0-24)/MIC ratio of 17.12 in 64% and 92% of patients, respectively. The critical concentration of moxifloxacin against MAC was identified as 0.25 mg/liter in Middlebrook media. The proposed susceptibility breakpoint means that a larger proportion of clinical isolates is resistant to moxifloxacin prior to therapy. For patients infected with susceptible isolates, however, 800 mg a day should be examined for safety and efficacy for disseminated M. avium disease.

Plectasin shows intracellular activity against Staphylococcus aureus in human THP-1 monocytes and in a mouse peritonitis model

Antimicrobial therapy of infections with Staphylococcus aureus can pose a challenge due to slow response to therapy and recurrence of infection. These treatment difficulties can partly be explained by intracellular survival of staphylococci, which is why the intracellular activity of antistaphylococcal compounds has received increased attention within recent years. The intracellular activity of plectasin, an antimicrobial peptide, against S. aureus was determined both in vitro and in vivo. In vitro studies using THP-1 monocytes showed that some intracellular antibacterial activity of plectasin was maintained (maximal relative efficacy [E(max)], 1.0- to 1.3-log reduction in CFU) even though efficacy was inferior to that of extracellular killing (E(max), >4.5-log CFU reduction). Animal studies included a novel use of the mouse peritonitis model, exploiting extra- and intracellular differentiation assays, and assessment of the correlations between activity and pharmacokinetic (PK) parameters. The intracellular activity of plectasin was in accordance with the in vitro studies, with an E(max) of a 1.1-log CFU reduction. The parameter most important for activity was fC(peak)/MIC, where fC(peak) is the free peak concentration. These findings stress the importance of performing studies of extra- and intracellular activity since these features cannot be predicted from traditional MIC and killing kinetic studies. Application of both the THP-1 and the mouse peritonitis models showed that the in vitro results were similar to findings in the in vivo model with respect to demonstration of intracellular activity. Therefore the in vitro model was a good screening model for intracellular activity. However, animal models should be applied if further information on activity, PK/pharmacodynamic parameters, and optimal dosing regimens is required.

Contrasting effects of human THP-1 cell differentiation on levofloxacin and moxifloxacin intracellular accumulation and activity against Staphylococcus aureus and Listeria monocytogenes

BACKGROUND AND AIMS

Listeria monocytogenes and Staphylococcus aureus invade and multiply in THP-1 monocytes. Fluoroquinolones accumulate in these cells, but are less active against intracellular than extracellular forms of L. monocytogenes and S. aureus. We examined whether differentiation of THP-1 monocytes into adherent, macrophage-like cells increases fluoroquinolone uptake and activity.

METHODS

THP-1 monocytes were differentiated with phorbol myristate acetate (PMA) and compared with unstimulated cells for: (i) moxifloxacin and levofloxacin accumulation; and (ii) activity against phagocytosed L. monocytogenes and S. aureus (5 h contact).

RESULTS

The differentiation of THP-1 monocytes caused: (i) a 3- to 4-fold increase in moxifloxacin uptake and a significant increase in its activity against intracellular L. monocytogenes (from 1.3 log(10) to 2.1 log(10) cfu decrease compared with the post-phagocytosis inoculum), but not against S. aureus (1.0-1.2 log(10) cfu decrease throughout); and (ii) no change in levofloxacin accumulation and intracellular activity against either L. monocytogenes or S. aureus.

CONCLUSIONS

Although differentiation of monocytes enhances the uptake and activity of moxifloxacin against L. monocytogenes, this cannot be extended to other intracellular bacteria and to levofloxacin. These results further demonstrate that antibiotic intracellular accumulation and activity are not necessarily linked and suggest that intracellular drug and pathogen combinations must be studied individually.

Replication of Colonic Crohn's Disease Mucosal Escherichia coli Isolates within Macrophages and Their Susceptibility to Antibiotics

There is increasing evidence that Escherichia coli organisms are important in Crohn's disease (CD) pathogenesis. In CD tissue they are found within macrophages, and the adherent-invasive CD ileal E. coli isolate LF82 can replicate inside macrophage phagolysosomes. This study investigates replication and antibiotic susceptibility of CD colonic E. coli isolates inside macrophages. Replication of CD colonic E. coli within J774-A1 murine macrophages and human monocyte-derived macrophages (HMDM) was assessed by culture and lysis after gentamicin killing of noninternalized bacteria and verified by electron microscopy (EM). All seven CD colonic isolates tested replicated within J774-A1 macrophages by 3 h (6.36-fold +/- 0.7-fold increase; n = 7 isolates) to a similar extent to CD ileal E. coli LF82 (6.8-fold +/- 0.8-fold) but significantly more than control patient isolates (5.2-fold +/- 0.25-fold; n = 6; P = 0.006) and E. coli K-12 (1.0-fold +/- 0.1-fold; P < 0.0001). Replication of CD E. coli HM605 within HMDM (3.9-fold +/- 0.7-fold) exceeded that for K-12 (1.4-fold +/- 0.2-fold; P = 0.03). EM showed replicating E. coli within macrophage vacuoles. Killing of HM605 within J774-A1 macrophages following a 3-h incubation with antibiotics at published peak serum concentrations (C(max)) was as follows: for ciprofloxacin, 99.5% +/- 0.2%; rifampin, 85.1% +/- 6.6%; tetracycline, 62.8% +/- 6.1%; clarithromycin, 62.1% +/- 5.6% (all P < 0.0001); sulfamethoxazole, 61.3% +/- 7.0% (P = 0.0007); trimethoprim, 56.3% +/- 3.4% (P < 0.0001); and azithromycin, 41.0% +/- 10.5% (P = 0.03). Ampicillin was not effective against intracellular E. coli. Triple antibiotic combinations were assessed at 10% C(max), with ciprofloxacin, tetracycline, and trimethoprim causing 97% +/- 0.0% killing versus 86% +/- 2.0% for ciprofloxacin alone. Colonic mucosa-associated E. coli, particularly CD isolates, replicate within macrophages. Clinical trials are indicated to assess the efficacy of a combination antibiotic therapy targeting intramacrophage E. coli.

Factors compromising the activity of moxifloxacin against intracellular Staphylococcus aureus

OBJECTIVES

The aim of this study was to determine the intracellular activity of moxifloxacin against a reference strain and a clinical strain and to study the factors compromising the intracellular activity of moxifloxacin.

METHODS

The bactericidal activity of moxifloxacin at therapeutic concentrations was studied against extracellular (broth) and intracellular (infected THP-1 monocytes) forms of Staphylococcus aureus and compared with that of levofloxacin. The activity of moxifloxacin was also evaluated in the presence of alkalinizing agents, in intracellular salt medium mimicking the phagolysosomal environment and in cell lysate.

RESULTS

Moxifloxacin, bactericidal against two S. aureus strains (ATCC 25923 and a clinical isolate, Sa2669) in broth, accumulated over 6-fold in monocytes. Against intracellular bacteria, moxifloxacin displayed a markedly reduced activity, not better than levofloxacin, with a maximal reduction of 1 log(10) cfu at 5 h. Cellular accumulation of moxifloxacin was not modified by the addition of efflux pump inhibitors or lysosomal alkalinizing agents. Alkalinization of phagolysosomes significantly enhanced intracellular killing by moxifloxacin. The bactericidal activity of moxifloxacin, abolished in the intracellular salt medium, was partially restored when the pH was raised from 5.0 to 7.4. The binding to intracellular components (35%) did not influence the activity of moxifloxacin. In all cases, surviving bacteria remained fully susceptible to the antibiotic.

CONCLUSIONS

The defeat of intracellular activity of moxifloxacin against S. aureus appeared to be more substantially related to cellular parameters (acidic pH and composition of the phagolysosomes) than to the intrinsic activity of the drug and to pharmacokinetic properties.

Pharmacodynamic evaluation of the intracellular activities of antibiotics against Staphylococcus aureus in a model of THP-1 macrophages

The pharmacodynamic properties governing the activities of antibiotics against intracellular Staphylococcus aureus are still largely undetermined. Sixteen antibiotics of seven different pharmacological classes (azithromycin and telithromycin [macrolides]; gentamicin [an aminoglycoside]; linezolid [an oxazolidinone]; penicillin V, nafcillin, ampicillin, and oxacillin [beta-lactams]; teicoplanin, vancomycin, and oritavancin [glycopeptides]; rifampin [an ansamycin]; and ciprofloxacin, levofloxacin, garenoxacin, and moxifloxacin [quinolones]) have been examined for their activities against S. aureus (ATCC 25923) in human THP-1 macrophages (intracellular) versus that in culture medium (extracellular) by using a 0- to 24-h exposure time and a wide range of extracellular concentrations (including the range of the MIC to the maximum concentration in serum [C(max); total drug] of humans). All molecules except the macrolides caused a net reduction in bacterial counts that was time and concentration/MIC ratio dependent (four molecules tested in detail [gentamicin, oxacillin, moxifloxacin, and oritavancin] showed typical sigmoidal dose-response curves at 24 h). Maximal intracellular activities remained consistently lower than extracellular activities, irrespective of the level of drug accumulation and of the pharmacological class. Relative potencies (50% effective concentration or at a fixed extracellular concentration/MIC ratio) were also decreased, but to different extents. At an extracellular concentration corresponding to their C(max)s (total drug) in humans, only oxacillin, levofloxacin, garenoxacin, moxifloxacin, and oritavancin had truly intracellular bactericidal effects (2-log decrease or more, as defined by the Clinical and Laboratory Standards Institute guidelines). The intracellular activities of antibiotics against S. aureus (i) are critically dependent upon their extracellular concentrations and the duration of cell exposure (within the 0- to 24-h time frame) to antibiotics and (ii) are always lower than those that can be observed extracellularly. This model may help in rationalizing the choice of antibiotic for the treatment of S. aureus intracellular infections.

Factors influencing the intracellular activity of fluoroquinolones: a study using levofloxacin in a Staphylococcus aureus THP-1 monocyte model

OBJECTIVES

Recent studies have raised the question of whether the intracellular activity of quinolones is optimal with respect to their cellular accumulation. The aim of this study was to compare the intracellular and extracellular activities of a commonly used quinolone, levofloxacin, and to examine the causes of the possible inconsistency between intracellular and extracellular effects.

METHODS

The bactericidal activity of levofloxacin at therapeutic levels, alone or in combination with various efflux-pump inhibitors or alkalinizing agents, was studied against Staphylococcus aureus ATCC 25923 in Mueller-Hinton (MH) broth and in a THP-1 monocytic cell model, using intracellular salt medium (ISM) mimicking the phagolysosomal environment, and in cell lysate.

RESULTS

Levofloxacin accumulation was 2-fold higher in uninfected than in infected cells. Intracellular activity was significantly lower than extracellular activity (decrease in the inoculum of < or = 1 log10 cfu/mL at 4 or 8 mg/L versus > or = 2 log10 units at > or = 1 mg/L in MH broth over 5 h). Persisters remained fully susceptible to the drug. The efflux pump inhibitors verapamil and gemfibrozil did not affect killing of intracellular bacteria, although gemfibrozil increased cellular accumulation of levofloxacin 1.7-fold. The lysosomotropic alkalinizing agents chloroquine and ammonium chloride significantly enhanced intracellular killing by levofloxacin. The bactericidal activity of levofloxacin, abolished in ISM, was partially restored when the pH was neutralized from 5.0 to 7.4. Binding to intracellular components (20%) substantially decreased the efficiency of levofloxacin.

CONCLUSIONS

Levofloxacin exhibited substantially lower intracellular activity than extracellular activity. Cellular compartmentalization of the drug, phagolysosomal environment and antibiotic binding to cellular components most likely contribute to this failure.

Activity of three β-lactams (ertapenem, meropenem and ampicillin) against intraphagocytic Listeria monocytogenes and Staphylococcus aureus

OBJECTIVES

Assessment of the activity of three beta-lactams [ertapenem (a carbapenem with a prolonged half-life), meropenem and ampicillin] against intraphagocytic Listeria monocytogenes and Staphylococcus aureus.

METHODS

Quantitative measurements of cfu changes in broth and in THP-1 macrophages (post-phagocytosis) over time (5 and 24 h) at concentrations spanning from sub-MICs to C(max) (maximal concentration typically observed in patients' serum upon administration of conventional doses); morphological studies using an electron microscope; evaluation of drug stability (HPLC), protein binding (equilibrium dialysis) and measurement of drug cellular accumulation (microbiological assay).

RESULTS

Ertapenem was unable to control L. monocytogenes growth in THP-1 macrophages at all concentrations and times tested, even under conditions where ampicillin and meropenem were bactericidal. This behaviour could not be ascribed to drug instability, protein binding or lack of cell accumulation in comparison with ampicillin or meropenem. Ertapenem, ampicillin and meropenem were equally effective at reducing the post-phagocytosis inoculum of S. aureus ( approximately 1 log cfu), and caused conspicuous changes in the morphology of intracellular bacteria consistent with their lysis. These effects were obtained, however, only at large multiples (100-fold or more) of the MIC maintained over 24 h. Because of the high intrinsic antimicrobial potency of the beta-lactams studied, these concentrations were below the C(max).

CONCLUSIONS

Ertapenem will probably be ineffective against intraphagocytic forms of L. monocytogenes for reasons that remain to be discovered. Conversely, ertapenem could be an alternative to ampicillin and meropenem against intraphagocytic S. aureus since its longer half-life may allow high concentrations to be maintained for more prolonged times.

Intracellular pharmacodynamics of antibiotics

This article establishes the pharmacokinetic-pharmacodynamic parameters that are important when considering the intracellular activity of antibiotics. Generally speaking, the main classes of antibiotics seem to share globally the same properties against extracellular and intracellular organisms. The specific cellular pharmacokinetic properties may modulate those parameters so as to let other ones to become critical. Simple rules, such as equating accumulation and activity, are certainly incorrect, and other determinants need to be added to the equation. Finally, this article emphasizes the fact that much remains to be done in this area before rational therapeutic choices can be made.

Human THP-1 monocyte uptake and cellular disposition of 14C-grepafloxacin

Uptake of (14)C-grepafloxacin into human mononuclear (THP-1) cells was determined at pH 7.4, 6.8, or 5.0 over a 4-log antibiotic concentration. Grepafloxacin was taken up by THP-1 monocytes rapidly by both a passive and an active transport mechanism at pH 7.4. Its uptake was initially linear, with equilibrium being reached after approximately 1 h. Efflux followed first-order clearance and was complete within 1 h, suggesting no longterm sequestering of the antibiotic occurred. Neither cell number nor serum protein binding appeared to have any effect on antibiotic uptake. High intracellular concentrations were achieved and the ratios of cellular to extracellular antibiotic concentration (IC/EC) were between 529 and 644 at 0.04 micro g/ml at pH 7.4 and 6.8, suggesting that monocytes may contain sufficient levels of grepafloxacin for affecting bacteriostatic killing. Grepafloxacin disposition within the THP-1 monocytes showed large amounts present in the nucleus and cell sap in stimulated and unstimulated cells, and its presence was evenly distributed throughout the cytosol, nuclei, lysosomes, mitochondria, and ribosomes. After stimulation by zymogen A, Staphylococcus aureus, or Streptococcus pneumoniae, increased amounts of grepafloxacin were found within THP-1 monocytes and isolated phagosome vacuoles. No antibiotic sequestration occurred inside stimulated monocytes, although a sufficient intracellular grepafloxacin concentration was available to kill phagocytized bacteria. Metabolic inhibitors, suppressors of K(+)/Cl(-) and Cl(-) transporters, inhibitors of the phagocytic process, low temperature, and low pH inhibited grepafloxacin uptake by THP-1 monocytes.

Quantitative analysis of gentamicin, azithromycin, telithromycin, ciprofloxacin, moxifloxacin, and oritavancin (LY333328) activities against intracellular Staphylococcus aureus in mouse J774 macrophages

Using J774 macrophages, the intracellular activities of gentamicin, azithromycin, telithromycin, ciprofloxacin, moxifloxacin, and oritavancin (LY333328) against Staphylococcus aureus (strain ATCC 25923) have been quantitatively assessed in a 24-h model. S. aureus was positively localized in phagolysosomes by confocal and electron microscopy, and extracellular growth was prevented with 0.5 mg of gentamicin/liter (1x MIC) in controls. When tested at extracellular concentrations equivalent to their maximum concentrations in human serum, all antibiotics except azithromycin caused a significant reduction of the postphagocytosis inoculum within 24 h, albeit to markedly different extents (telithromycin [2 mg/liter], 0.60 log; ciprofloxacin [4.3 mg/liter], 0.81 log; gentamicin [18 mg/liter], 1.21 log; moxifloxacin [4 mg/liter], 1.51 log; oritavancin [25 mg/liter], 3.49 log). Intracellular activities were not systematically related to drug accumulation (apparent cellular-to-extracellular concentration ratios in infected cells: ciprofloxacin, 3.2; gentamicin, 6.8; telithromycin, 8.7; moxifloxacin, 13.4; azithromycin, 50; oritavancin, 348). Intracellular activity was not directly correlated to extracellular activity as measured in broth. Conditions of pH 5 (i.e., mimicking that of phagolysosomes) markedly reduced the activity of gentamicin, azithromycin, and telithromycin (>or=32 x) and fairly extensively reduced that of ciprofloxacin and moxifloxacin (>or=4 x) but did not affect oritavancin activity. We conclude that the cellular accumulation of antibiotics is not the only parameter to take into account for intracellular activity but that local environmental conditions (such as pH) and other factors can also prove critical.

Comparative intracellular (THP-1 macrophage) and extracellular activities of beta-lactams, azithromycin, gentamicin, and fluoroquinolones against Listeria monocytogenes at clinically relevant concentrations

The activities of ampicillin, meropenem, azithromycin, gentamicin, ciprofloxacin, and moxifloxacin against intracellular hemolysin-positive Listeria monocytogenes were measured in human THP-1 macrophages and were compared with the extracellular activities observed in broth. All extracellular concentrations were adjusted to explore ranges that are clinically achievable in human serum upon conventional therapy. In broth, ampicillin, meropenem, and azithromycin were only bacteriostatic, whereas gentamicin, ciprofloxacin, and moxifloxacin were strongly bactericidal in a concentration-dependent manner. In cells, ampicillin, meropenem, azithromycin, and ciprofloxacin were slightly bactericidal (0.3- to 0.8-log CFU reductions), moxifloxacin was strongly bactericidal (2.1-log CFU reduction), and gentamicin was virtually inactive. The difference in the efficacies of moxifloxacin and ciprofloxacin in cells did not result from a difference in levels of accumulation in cells (6.96 +/- 1.05 versus 7.75 +/- 1.03) and was only partially explainable by the difference in the MICs (0.58 +/- 0.04 versus 1.40 +/- 0.17 mg/liter). Further analysis showed that intracellular moxifloxacin expressed only approximately 1/7 of the activity demonstrated against extracellular bacteria and ciprofloxacin expressed only 1/15 of the activity demonstrated against extracellular bacteria. Gentamicin did not increase the intracellular activities of the other antibiotics tested. The data suggest (i) that moxifloxacin could be of potential interest for eradication of the intracellular forms of L. monocytogenes, (ii) that the cellular accumulation of an antibiotic is not the only determinant of its intracellular activity (for fluoroquinolones, it is actually a self-defeating process as far as activity is concerned), and (iii) that pharmacodynamics (activity-to-concentration relationships) need to be considered for the establishment of efficacy against intracellular bacteria, just as they are for the establishment of efficacy against extracellular infections.