Indium-111 oxine labeled erythrocytes: cellular distribution and efflux kinetics of the label

Unexpected transformation of n.c.a. [111In]InCl3 in stock solutions into an unreactive [111In]In-species

While performing multiple indium-111 labeling of DOTA-modified peptides from a single batch of [¹¹¹In]InCl₃, inconsistent radiochemical yields were observed. We found that the formation of a radioactive impurity in the [¹¹¹In]InCl₃ stock solution hampered the reactivity of the indium-111 during radiolabeling reactions. The formation of this unknown ¹¹¹In-species could be successfully suppressed by increasing the concentration of chloride ions in the stock solution and [¹¹¹In]InCl₃ was "recovered". Radiolabeling of DOTA-peptides with the stabilized [¹¹¹In]InCl₃ resulted again in acceptable radiochemical yields. In addition, we report convenient iTLC systems that allow distinguishing between [¹¹¹In]InCl₃, the formed unknown ¹¹¹In-species, radiocolloids, and radiolabeled peptides (DOTANOC).

Long-Circulating Sterically Stabilized Liposomes in the Treatment of Infections

The administration of antimicrobial agents encapsulated in long-circulating sterically stabilized liposomes results in a considerable enhancement of therapeutic efficacy compared with the agents in the free form. After liposomal encapsulation, the pharmacokinetics of the antimicrobial agents is significantly changed. An increase in circulation time and reduction in toxic side effects of the agents are observed. In contrast to other types of long-circulating liposomes, an important characteristic of these sterically stabilized liposomes is that their prolonged blood circulation time is, to a high degree, independent of liposome characteristics such as liposome particle size, charge and lipid composition (rigidity) of the bilayer, and lipid dose. This provides the opportunity to manipulate antibiotic release from these liposomes at the site of infection, which is important in view of the differences in pharmacodynamics of different antibiotics and can be done without compromising blood circulation time and degree of target localization of these liposomes. Depending on the liposome characteristics and the agent encapsulated, antibiotic delivery to the infected site is achieved, or the liposomes act as a micro-reservoir function for the antibiotic. In experimental models of localized or disseminated bacterial and fungal infections, the sterically stabilized liposomes have successfully been used to improve antibiotic treatment using representative agents of various classes of antibacterial agents such as the beta-lactams, the aminoglycosides, and the quinolones or the antifungal agent amphotericin B. Extensive biodistribution studies have been performed. Critical factors that contribute to liposome target localization in infected tissue have been elucidated. Liposome-related factors that were investigated were poly(ethylene glycol) density, particle size, bilayer fluidity, negative surface charge, and circulation kinetics. Host-related factors focused on the components of the inflammatory response.

Host factors influencing the preferential localization of sterically stabilized liposomes in Klebsiella pneumoniae-infected rat lung tissue

PURPOSE

To gain insight into the host factors influencing liposome localization at sites of bacterial infection.

METHODS

In a unilateral Klebsiella pneumoniae pneumonia rat model, capillary permeability and number of circulating leukocytes was quantified and related to the degree of liposome target localization.

RESULTS

Liposome localization was highest in the hemorrhagic zone of infection, a zone characterized by markedly increased capillary permeability and high bacterial numbers. Both liposome localization and capillary permeability correlated positively with severity of infection. Lung instillation of other inflammatory stimuli, such as lipopolysaccharide or 0.1 M HCl inducing increased capillary permeability, also promoted liposome localization. As liposomal target localization in leukopenic rats was similar to that in immunocompetent rats, contribution of circulating leukocytes seems limited. Intrapulmonary distribution of liposomes shows that leukocytes at the target site are involved in liposome uptake after extravasation.

CONCLUSIONS

Increased capillary permeability plays a crucial role in liposome localization at the infected site, whereas contribution of leukocytes is limited. These results suggest inflammatory conditions that could benefit from liposomal drug delivery. The involvement of leukocytes in liposome uptake at the target site could be important information in the selection of appropriate drugs.

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.

Localization of sterically stabilized liposomes in experimental rat Klebsiella pneumoniae pneumonia: dependence on circulation kinetics and presence of poly(ethylene)glycol coating

Preferential localization of liposomes at sites of infection or inflammation has been demonstrated in a variety of experimental models. Most studies report enhanced localization at the target site of poly(ethyelene) glycol (PEG)-coated liposomes as compared to conventional non-coated liposomes. It is generally accepted that the prolonged circulation time of PEG-coated liposomes increases target site exposure, which results in increased target localization. A quantitative relationship between circulation kinetics and localization at the pathological site has not been defined as yet. Besides, an effect of the PEG coating itself has been suggested, as theoretically the PEG coating may facilitate liposome extravasation. In the present study, in a rat model of an acute unilateral Klebsiella pneumoniae pneumonia, circulation kinetics of PEG-coated liposomes were manipulated by incorporation of different amounts of phosphatidylserine (PS) and variation of lipid dose, additionally allowing evaluation of the saturability of the localization process. In addition, this paper addresses the effect of the PEG coating, by comparing the circulation kinetics and target localization of long-circulating 'PEG-free' and PEG-coated liposomes. It is shown that the degree of liposome localization at the target site is positively linearly related to the area under the blood concentration time curve (AUC) of the liposome formulations, irrespective of PEG coating. This finding is discussed in relation to the equation of Kedem and Katchalsky, which describes protein influx into sites of infection or inflammation.

Localization of sterically stabilized liposomes in Klebsiella pneumoniae-infected rat lung tissue: influence of liposome characteristics

Sterically stabilized liposomes are able to localize at sites of infection and could serve as carriers of antimicrobial agents. For a rational optimization of liposome localization, the blood clearance kinetics and biodistribution of liposomes differing in poly(ethylene glycol) (PEG) density, particle size, bilayer fluidity or surface charge were studied in a rat model of a unilateral pneumonia caused by Klebsiella pneumoniae. It is shown that all liposome preparations studied localize preferentially in the infected lung compared to the contralateral non-infected lung. A reduction of the PEG density or rise in particle size resulted in a higher uptake by the mononuclear phagocyte system, lower blood circulation time and lower infected lung localization. Differences in bilayer fluidity did not affect blood clearance kinetics or localization in the infected lung. Increasing the amount of negatively charged phospholipids in the liposome bilayer did not affect blood clearance kinetics, but did reduce localization of this liposome preparation at the site of lung infection. In conclusion, the degree of localization at the infected site is remarkably independent of the physicochemical characteristics of the PEG liposomes. Substantial selective liposome localization can be achieved provided that certain criteria regarding PEG density, size and inclusion of charged phospholipids are met. These properties seem to be a direct consequence of the presence of the polymer coating operating as a repulsive steric barrier opposing interactions with biological components.