Course of murine tuberculosis and response to first-line therapy depends on route of infection and inoculum size

BACKGROUND

In the search for new anti-tuberculosis drugs, numerous potential drugs are being screened in vitro. In animal models, promising new anti-tuberculosis drugs are assessed in terms of toxic side effects and comparative therapeutic efficacy. Mice are frequently used and experimental infections are established in different ways.

OBJECTIVE

To investigate to what extent the route of Mycobacterium tuberculosis inoculation is a determinant in the pathogenesis of tuberculosis (TB) and the therapeutic outcome. Results will contribute to insight into the translational value of TB models used for preclinical studies.

DESIGN

TB in mice was established through intratracheal or intravenous mycobacterial inoculation. The efficacy of a 26-week treatment regimen was evaluated, including assessment of relapse of infection 13 weeks post-treatment.

RESULTS

It was shown that the course of TB and the therapeutic response, in terms of histopathological characteristics and mycobacterial load, in lungs and extra- pulmonary organs is substantially different and dependent on the route of infection applied and the inoculum size used.

CONCLUSION

When evaluating the comparative therapeutic potential of novel anti-tuberculosis drugs or drug treatment schedules investigated in different studies, it should be noted that the route of infection applied and the inoculum size used influence the course of murine TB and the therapeutic response to the standard first- line anti-tuberculosis drug regimen.

Dynamics of interferon-gamma release assay and cytokine profiles in blood and respiratory tract specimens from mice with tuberculosis and the effect of therapy

There are limitations on diagnostic methods to differentiate between active and latent tuberculosis (TB), and the prediction of latent progression to TB disease is yet complex. Traditionally, tuberculosis-specific host immune response was visualized using the tuberculin skin test. Nowadays, IFN-γ release assays (IGRA) provide a more specific and sensitive tool, by which exposure to Mtb could be determined. However, the merit of IGRA aids in diagnosing active TB is yet unclear. We adapted IGRA for use in mice, and quantifying bead-based flow cytometry techniques were used to assess cytokine profiles during the course of untreated infection and to investigate the value of IGRA and cytokines as biomarkers for therapy response. High variability of IGRA results during progression of active TB infection related to various phases of infection was obtained. However, a significant decrease in IGRA results and in levels of IFN-γ, IL-17, IP-10 or MIG was observed and appeared to be associated with successful therapy. This outcome does not support the value of IGRA to accurately diagnose active TB or to monitor infection progression. However, IGRA proved to be a useful biomarker to monitor therapy success. In addition, different cytokines might serve as biomarkers.

Role of ceftazidime dose regimen on the selection of resistant Enterobacter cloacae in the intestinal flora of rats treated for an experimental pulmonary infection

OBJECTIVES

The effect of ceftazidime dosing increments and frequency of dosing on the selection of ceftazidime-resistant Enterobacter cloacae in the intestine was studied in rats, during treatment of a pulmonary infection caused by Klebsiella pneumoniae.

METHODS

Rats with pulmonary infection (n = 10 per group) received therapy with doses of ceftazidime at 3.1 to 400 mg/kg per day at a frequency of every 6,12 or 24 h for 18 days, starting 24 h after bacterial inoculation of the lung. Emergence of resistance in intestinal E. cloacae was monitored by culturing fresh stool specimens at days 0, 8, 15, 22, 29, 36 and 43 on agar plates with (6.4 mg/L) and without ceftazidime. Pharmacodynamic indices and time within the mutant selection window (MSW) were assessed in infected rats for each regimen. Ceftazidime-resistant E. cloacae mutants were characterized by determination of the beta-lactamase activity under cefoxitin-induced and non-induced conditions.

RESULTS

A reduction of intestinal ceftazidime-susceptible E. cloacae was observed and showed a significant correlation with the fAUC/MIC at days 8, 15 and 22 and with the fCmax on days 8, 15, 22, 29 and 36. More rats treated with 12-25 and 50-100 mg/kg per day every 6 h were found colonized with ceftazidime-resistant E. cloacae mutants than animals treated every 12 h or every 24 h. The proportion of rats colonized with ceftazidime-resistant E. cloacae mutants at days 15, 36 and 43 correlated with the time during which ceftazidime plasma concentrations were within the boundaries of the MSW. Only at day 15 was a correlation demonstrated between the fCmax and significantly fewer rats colonized with ceftazidime-resistant E. cloacae. Ceftazidime-resistant E. cloacae mutants (MIC >or= 128 mg/L) were characterized as stable derepressed mutants.

CONCLUSIONS

Colonization with stable derepressed ceftazidime-resistant E. cloacae mutants particularly occurred when rats were exposed to moderate doses of ceftazidime (12-25 or 50-100 mg/kg per day) administered every 6 h. Emergence of resistance was correlated with time within the MSW.

In vivo synergistic interaction of liposome-coencapsulated gentamicin and ceftazidime

Antimicrobial agents may interact synergistically. But to ensure synergy in vivo, the drugs should both be present at the site of infection at sufficiently high concentrations for an adequate period of time. Coencapsulation of the drugs in a drug carrier may ensure parallel tissue distributions. Since liposomes localize preferentially at sites of infection, this mode of drug delivery could, in addition, increase drug concentrations at the focus of infection. The therapeutic efficacy of gentamicin and ceftazidime coencapsulated into liposomes was examined by monitoring survival in a rat model of an acute unilateral pneumonia caused by antibiotic-susceptible and antibiotic-resistant Klebsiella pneumoniae strains. It is shown that administration of gentamicin in combination with ceftazidime in the free form either as single dose or as 5-day treatment resulted in an additive effect on rat survival in both models. In contrast, targeted delivery of liposome-coencapsulated gentamicin and ceftazidime resulted in a synergistic interaction of the antibiotics in both models. Consequently, liposome coencapsulation of gentamicin and ceftazidime allowed both a shorter course of treatment at lower cumulative doses compared with administration of the antibiotics in the free form to obtain complete survival of rats. Liposomal coencapsulation of synergistic antibiotics may open new perspectives in the treatment of severe infections.

Improved efficacy of ciprofloxacin administered in polyethylene glycol-coated liposomes for treatment of Klebsiella pneumoniae pneumonia in rats

Animal and clinical data show that high ratios of the area under the concentration-time curve and the peak concentration in blood to the MIC of fluoroquinolones for a given pathogen are associated with a favorable outcome. The present study investigated whether improvement of the therapeutic potential of ciprofloxacin could be achieved by encapsulation in polyethylene glycol (PEG)-coated long-circulating sustained-release liposomes. In a rat model of unilateral Klebsiella pneumoniae pneumonia (MIC = 0.1 microg/ml), antibiotic was administered at 12- or 24-h intervals at twofold-increasing doses. A treatment period of 3 days was started 24 h after inoculation of the left lung, when the bacterial count had increased 1,000-fold and some rats had positive blood cultures. The infection was fatal within 5 days in untreated rats. Administration of ciprofloxacin in the liposomal form resulted in delayed ciprofloxacin clearance and increased and prolonged ciprofloxacin concentrations in blood and tissues. The ED(50) (dosage that results in 50% survival) of liposomal ciprofloxacin was 3.3 mg/kg of body weight/day given once daily, and that of free ciprofloxacin was 18.9 mg/kg/day once daily or 5.1 mg/kg/day twice daily. The ED(90) of liposomal ciprofloxacin was 15.0 mg/kg/day once daily compared with 36.0 mg/kg/day twice daily for free ciprofloxacin; 90% survival could not be achieved with free ciprofloxacin given once daily. In summary, the therapeutic efficacy of liposomal ciprofloxacin was superior to that of ciprofloxacin in the free form. PEG-coated liposomal ciprofloxacin was well tolerated in relatively high doses, permitting once daily administration with relatively low ciprofloxacin clearance and without compromising therapeutic efficacy.

Therapeutic efficacy of liposome-encapsulated gentamicin in rat Klebsiella pneumoniae pneumonia in relation to impaired host defense and low bacterial susceptibility to gentamicin

Long-circulating liposomes (LCL) may be used as targeted antimicrobial drug carriers as they localize at sites of infection. As a result, LCL-encapsulated gentamicin (LE-GEN) has demonstrated superior antibacterial activity over the free drug in a single-dose study of immunocompetent rats with Klebsiella pneumoniae pneumonia. In the present study, the therapeutic efficacy of LE-GEN was evaluated by monitoring rat survival and bacterial counts in blood and lung tissue in clinically relevant models, addressing the issue of impaired host defense and low bacterial antibiotic susceptibility. The results show that in immunocompetent rats infected with the high-GEN-susceptibility K. pneumoniae strain, a single dose of LE-GEN is clearly superior to an equivalent dose of free GEN. Yet complete survival can also be obtained with multiple doses of free GEN. In leukopenic rats infected with the high-GEN-susceptible K. pneumoniae strain, free GEN at the maximum tolerated dose (MTD) was needed to obtain survival. However, with the addition of a single dose of LE-GEN to free-GEN treatment, complete survival can be obtained using a sevenfold-lower cumulative amount of GEN than with free-GEN treatment alone. In leukopenic rats infected with low-GEN-susceptible K. pneumoniae cells, free GEN at the MTD did not result in survival. The use of LE-GEN is needed for therapeutic success. Increasing LE-GEN bilayer fluidity resulted in an increased GEN release from the liposomes and hence improved rat survival, thus showing the importance of the liposome lipid composition for therapeutic efficacy. These results warrant further clinical studies of liposomal formulations of aminoglycosides in immunocompromised patients with severe infections.

Administration of liposomal agents and blood clearance capacity of the mononuclear phagocyte system

As liposomes are cleared from the circulation to a substantial extent by the phagocytic cells of the mononuclear phagocyte system (MPS), there is a question whether administration of liposome-based therapeutic agents interferes with clearance of infectious organisms by the MPS from blood. In the present study, at first the effect of administration of three types of empty liposomes (devoid of drug), differing in blood residence time, on carbon clearance and bacterial clearance from blood was studied with mice. Classical liposomes (LIP A) and placebo liposomes with lipid composition as in AmBisome (LIP B) or as in Doxil (LIP C) were used. Liposomes were administered intravenously as a single dose. Second, the effect of multiple-dose administration of AmBisome on bacterial blood clearance was studied with rats. AmBisome was administered with two different dosage schedules. The blood clearance capacity of the MPS was monitored at different time points after the last liposome injection. It was shown that the carbon blood clearance capacity of the MPS was impaired only at a high lipid dose of empty classical liposomes. The bacterial blood clearance capacity was never impaired, not even after prolonged treatment with AmBisome administered in a clinically relevant regimen.

Doxorubicin entrapped in sterically stabilized liposomes: effects on bacterial blood clearance capacity of the mononuclear phagocyte system

The introduction of long-circulating liposomes sterically stabilized by surface coating with polyethylene glycol has expanded the potential for drug targeting to tumors. In recent clinical studies, evidence of significant antitumor activity has been obtained with the industrially prepared formulation of long-circulating polyethylene glycol-coated liposomes containing doxorubicin, referred to as DOXIL. Previous studies performed in rats showed that doxorubicin-containing liposomes can exert major toxic effects on the liver macrophage population for a considerable period of time; a strong impairment of phagocytic function and even a substantial depletion of the liver macrophage populations were observed. In the present study, the phagocytic function of the mononuclear phagocyte system (MPS) after administration of DOXIL at a clinically relevant dosage schedule was evaluated in rats. Phagocytic function of the MPS was assessed by determining bacterial blood clearance capacity. The observations reported herein show that DOXIL is fairly well tolerated regarding bacterial blood clearance capacity of the MPS when administered in a regimen that resembles the clinical setting closely. This outcome has important implications with regard to the clinical utility of the liposomal drug, especially in the restricted context of immunocompromised cancer patients who easily develop systemic infections and should not be confronted with a therapy-induced reduction of the bacterial blood clearance capacity of the MPS.

Liposomal amphotericin B (AmBisome) reduces dissemination of infection as compared with amphotericin B deoxycholate (Fungizone) in a rate model of pulmonary aspergillosis

The efficacy of AmBisome, a liposomal formulation of amphotericin B, was compared with that of Fungizone (amphotericin B desoxycholate), in a rat model of unilateral, pulmonary aspergillosis. Repeated administration of cyclophosphamide resulted in persistent, severe granulocytopenia. The left lung was inoculated with a conidial suspension of Aspergillus fumigatus, thus establishing an unilateral infection. Antifungal treatment was started 40 h after fungal inoculation, at which time mycelial disease was confirmed by histological examination. Both Fungizone 1 mg/kg and AmBisome 10 mg/kg resulted in increased survival in terms of delayed as well as reduced mortality. Quantitative cultures of lung tissue showed that only AmBisome 10 mg/kg resulted in reduction of the number of fungal cfus in the inoculated left lung. Compared with Fungizone, both AmBisome 1 mg/kg/day and AmBisome 10 mg/kg/day significantly prevented dissemination from the infected left lung to the right lung. In addition, both AmBisome regimens reduced hepatosplenic dissemination, and the 10 m/kg dosage fully prevented this complication. In conclusion, when compared with Fungizone, in this model AmBisome is more effective in reducing dissemination of unilateral, pulmonary aspergillosis, even when given in relatively low dosage. Such low dosages may have a place in prophylactic settings.

Amphotericin B liposomes with prolonged circulation in blood: in vitro antifungal activity, toxicity, and efficacy in systemic candidiasis in leukopenic mice

Pegylated amphotericin B (AmB) liposomes (PEG-AmB-LIP) were compared with laboratory-prepared nonpegylated AmB liposomes (AmB-LIP), a formulation with a lipid composition the same as that in AmBisome, as well as with industrially prepared AmBisome regarding their in vitro antifungal activities, toxicities, blood residence times, and therapeutic efficacies. Killing of Candida albicans (> 99.9%) during short-term (6-h) incubation was observed at 0.2 mg of AmB per liter for AmB desoxycholate, 0.4 mg of AmB per liter for PEG-AmB-LIP, 0.8 mg of AmB per liter for AmB-LIP, and 12.8 mg of AmB per liter for AmBisome. The maximum tolerated doses of PEG-AmB-LIP, AmB-LIP, and AmBisome were 15, 19, and > 31 mg of AmB per kg of body weight, respectively. In contrast to AmB-LIP, the blood residence time of PEG-AmB-LIP was prolonged and dose independent. In a model of systemic candidiasis in leukopenic mice at a dose of 5 mg of AmB per kg, PEG-AmB-LIP was completely effective and AmB-LIP was partially effective, whereas AmBisome was not effective. AmB-LIP at 11 mg of AmB per kg was partially effective. AmBisome at 29 mg of AmB per kg was completely effective. In conclusion, the therapeutic efficacies of AmB liposomes can be improved by preparing AmB liposomes in which a substantial reduction in toxicity is achieved while antifungal activity is retained. In addition, therapeutic efficacy is favored by a prolonged residence time of AmB liposomes in blood.

Efficacy of gentamicin or ceftazidime entrapped in liposomes with prolonged blood circulation and enhanced localization in Klebsiella pneumoniae-infected lung tissue

Polymer (PEG-PE)-coated liposomes exhibit prolonged circulation time in blood and substantial localization in Klebsiella pneumoniae-infected lung tissue in rats. Therefore, to determine the therapeutic effect, gentamicin and ceftazidime were entrapped in these liposomes and administered to rats experimentally infected with pneumonia: Relatively high and sustained concentrations of liposome-associated antibiotic in blood were observed. Compared with antibiotics alone, one dose of liposome-entrapped gentamicin or ceftazidime increased the therapeutic effect of the drugs, survival of rats, and bacterial killing in lungs. One dose of liposome-entrapped ceftazidime was as effective as a continuous 2-day infusion of nonentrapped ceftazidime. Since antibiotic-containing liposomes are stable during circulation and liposome-entrapped ceftazidime and gentamicin have low bactericidal activity in vitro, the superior therapeutic effect of the liposome-encapsulated antibiotics results from localization and subsequent degradation of liposomes and the resulting release of entrapped antibiotic at the infection site.

Biodistribution of liposomal amphotericin B (AmBisome) and amphotericin B-desoxycholate (Fungizone) in uninfected immunocompetent mice and leucopenic mice infected with Candida albicans

The biodistribution of liposomal amphotericin B (L-AmB; AmBisome) and amphotericin B-desoxycholate were compared after a single injection of drug in uninfected immunocompetent mice and in leucopenic mice 6 h after inoculation with Candida albicans. Amphotericin B-desoxycholate was administered at the maximum tolerated dose (MTD) of 0.3 mg/kg whereas L-AmB was given at either 0.3 mg/kg or the MTD of 7 mg/kg. Amphotericin B (AmB) concentrations in the blood, liver, spleen, lungs and kidneys were determined by HPLC analysis at various intervals during the 48 h after administration. The biodistribution of both preparations of AmB followed similar patterns in both uninfected immunocompetent mice as well as those that were leucopenic and infected with C. albicans. Administration of L-AmB resulted in increased concentrations of drug in the blood, liver, and spleen but decreased concentrations in the kidney and lung. Hepatosplenic uptake of L-AmB was highly dose dependent with 7 mg/kg resulting in a relatively prolonged blood circulation. Blood and tissues retained high AmB concentrations after administration of L-AmB at the MTD. By using radiolabelled L-AmB, it was found that the high AmB concentrations in blood represented liposome-associated AmB and that during circulation in blood slow release of AmB occurred.

Liposomes with prolonged blood circulation and selective localization in Klebsiella pneumoniae-infected lung tissue

Studies of two types of small liposomes, differing with respect to their lipid composition in terms of bilayer fluidity, charge, and hydrophilicity of the liposomal surface, were done to evaluate their usefulness for delivery of encapsulated therapeutic agents to sites of infection. The liposomes showed substantial localization in infected lung tissue after intravenous administration. This was demonstrated in a model of unilateral Klebsiella pneumoniae pneumonia in rats, in which the left lung was infected but the right lung of the same animal developed no infection. The degree of localization in the infected left lung was different for the two types of liposomes. For the liposome type with the longer blood residence time, containing a surface coating of polyethylene glycol, localization in the infected left lung was dependent on the liposomal dose, correlated with the intensity of infection, and reached 9% of the injected liposomal dose in severely infected rats.

Liposomes as carriers of antimicrobial agents or immunomodulatory agents in the treatment of infections

Targeting of antimicrobial agents by means of liposomes is under investigation and may be of importance in the treatment of infections that prove refractory to conventional forms of antimicrobial treatment. The ability to achieve a significantly longer residence time of liposomes in plasma and limited uptake of liposomes by the mononuclear phagocyte system opens up new areas of investigation and potential therapeutic application. By manipulating the liposomal composition, rates of uptake and intracellular degradation can be influenced and thereby the rates at which liposome-encapsulated agents are released and become available to exert their therapeutic action. With respect to the targeting of macrophage modulators at the mononuclear phagocyte system by means of liposomes for maximal stimulation of the nonspecific antimicrobial resistance, experimental evidence is now available of the potential usefulness of liposomes as carriers of these agents. This approach may also be of importance for the potentiation of treatment of severe infections.

Enhanced localization of liposomes with prolonged blood circulation time in infected lung tissue

In an experimental model of unilateral pneumonia caused by Klebsiella pneumoniae in rats we investigated whether intravenous administration of liposomes with prolonged blood circulation time resulted in significant localization of liposomes in infected lung tissue. Liposomes (100 nm) composed of hydrogenated phosphatidylinositol:hydrogenated phosphatidylcholine:cholesterol (molar ratio, 1:10:5) radiolabeled with gallium-67-deferoxamine showed relatively long blood circulation time. The degree of localization of these long circulating liposomes in the infected left lung was significantly higher compared to that of localization of 110 nm egg phosphatidylglycerol:egg phosphatidylcholine:cholesterol (molar ratio, 1:10:5) liposomes which exhibited relatively short blood circulation time. At 16 h after administration of the long circulating liposomes (when 10% of the injected dose was still present in the bloodstream) localization of liposomes in the infected left lung was increased up to 10-fold compared to the left lung of uninfected rats, and appeared to be highly correlated with the intensity of the infection. In the uninfected right lung the localization of long circulating liposomes was not increased. The degree of localization of liposomes in the infected tissue is dependent on the residence time of liposomes in the blood compartment. The extent of localization of long circulating liposomes in infected tissue appeared to be dependent on the liposomal dose administered.