Kinetic analysis of tissue distribution of doxorubicin incorporated in liposomes in rats: I

Pharmacokinetic Analysis of the Tissue Distribution of Octaarginine Modified Liposomes in Mice

We recently found that octaarginine modified liposomes (R8-Lip) can be efficiently internalized by cultured cells. The purpose of the present study was to quantitatively determine the effect of R8-density on the tissue distribution of R8-Lip in mice, using their clearance as an index. R8 was introduced in the form of stearylated R8 (STR-R8). The liposomes were composed of cholesterol and egg phosphatidylcholine and were labeled with [(3)H]cholesteryl hexadecyl ether. Various densities of R8 (3%, 10% and 30%) containing liposomes were prepared with a diameter of approximately 70-80 nm. The tissue distribution of R8-Lip was determined after their i.v. administration into mice and the effect of R8-density on tissue distribution was compared with uptake clearance, the calculated tissue distribution divided by the area under the blood concentration-time course. As results, R8-Lip were more rapidly eliminated from circulating blood and distributed to many tissues, especially liver depending on the R8-density. However, the tissue uptake clearance represented similar value to that of positively charge liposomes. Based on these results, we conclude that the R8-dependent increase in R8-Lip in various tissues tested indicates that positive charge, but not PTD function derived from R8 predominantly responsible for the enhancement of tissue distribution. Therefore, it is suggested that topology control of R8 is important to exhibit the PTD function.

Factors governing the in vivo tissue uptake of transferrin-coupled polyethylene glycol liposomes in vivo

Liposomes, coated with transferrin (Tf)-coupled polyethylene glycol are considered to be potent carriers for drug delivery to various organs via receptor-mediated endocytosis. Since Tf receptors were ubiquitously expressed in various organs, additional perturbation of the liposomes such as regulation of the size may be required to exhibit the tissue selectivity. In the present study, the effect of size on the uptake of transferrin-coupled polyethylene glycol liposomes (Tf-PEG-L) to various organs was investigated. In liver and brain, Tf-dependent uptake was found to be dependent on the size of the liposomes used. In small liposomes with a diameter of 60-80 nm, Tf-PEG-L was taken up to these organs more efficiently than PEG-L. This Tf-dependent uptake for small liposomes decreased by the high dose administration, suggested that Tf-PEG-L is taken up via Tf receptor-mediated endocytosis even under the physiological condition, in which plasma concentration of endogenous Tf remains high. On the other hand, Tf receptor-mediated uptake was also observed in the heart, but size-dependency was not observed in this case. Collectively, these results indicate that size dependency in the uptake of Tf-PEG-L is tissue-dependent and therefore, controlling the size of Tf-PEG-L may be useful for the success of tissue targeting.

Liposomal doxorubicin

Doxorubicin is a potent antineoplastic agent with activity against numerous human cancers. Encapsulation of doxorubicin inside a liposome alters bioavailability, biodistribution and thus its biological activity significantly. The physical properties of the liposome (size, lipid components and lipid dose) play a major role in determining drug retention and pharmacokinetics. The therapeutic benefits of liposomal doxorubicin will therefore depend on these physical characteristics. Here we review the toxicity and efficacy of liposomal doxorubicin determined for various liposome compositions (size, lipid composition and drug-to-lipid ratio). These physical properties can be independently varied using the transmembrane pH gradient-dependent drug encapsulation procedure. The results show that the toxicity of the formulation is related to drug retention in the circulation. The antitumor activity is more sensitive to the size of the liposomes. By optimizing these parameters, liposomal doxorubicin formulations can be optimized for improved therapeutic activity.

Pharmaceutical and biological properties of doxorubicin encapsulated in liposomes (L-ADM): the effect of repeated administration on the systemic phagocytic activity and pharmacokinetics

We investigated the biodistribution and antitumour activity of doxorubicin (ADM) encapsulated in liposomes (L-ADM) after two administrations in tumour bearing mice. The effect of the first administration on phagocytic activity was also examined. The biodistribution of L-ADM after the second dosing at an interval of 4d was remarkably different from that after the first. The concentration of ADM in plasma and tumour after the second injection was higher, but that in the liver was lower than after the first administration. This decrease in distribution to the liver is thought to have contributed to the difference in the biodistribution characteristics of L-ADM. With regard to antitumour effect, the activity was similar between L-ADM and a solution of ADM (F-ADM). To investigate the effect of the first administration of L-ADM on biodistribution, systemic phagocytic activity was measured after the injection of F-ADM, L-ADM, or 'empty' liposomes not containing ADM. F-ADM and L-ADM (7.5 mg ADM/kg body weight) reduced phagocytic activity to approximately 50% and 30% of control, respectively. This finding suggests that entrapment of ADM in liposomes enhances both the distribution of the drug to the reticuloendothelial system (RES) and its suppressive effect on RES activity. These results indicate that the decrease in RES activity by L-ADM must be considered in estimation of the pharmacokinetics, antitumour activity, and toxicity of L-ADM in clinical use when given by repeat administration or used in combination with other antitumour agents.

Size dependent liposome degradation in blood: in vivo/in vitro correlation by kinetic modeling

The degradation of liposomes in blood circulation is important in regulating the releasing rate of encapsulated compounds. In this study, the effect of liposome size--one of the principal determining factors in liposome disposition--on their degradation in serum/blood was evaluated quantitatively both in vitro and in vivo. In the in vitro study, the time courses of the degradation of liposomes in fresh rat serum were measured continuously using 5(6)-carboxyfluorescein (CF) as an aqueous phase marker and were described by the kinetic model with the lag time (tau), first order degradation rate constant (k), and the maximum degradation (alpha). Both k and alpha increased with the increase of liposome size, which indicated a higher affinity of larger liposomes for complement activation. In the in vivo study, the degradation of liposomes was evaluated sensitively by a first order degradation rate constant (kd) in blood circulation. The kd was obtained by kinetically modeling the liposome degradation in vivo using 3H-inulin as an aqueous phase marker. The size dependent kd correlated well with the hepatic uptake clearance, which suggests an underlying complement activation mechanism common to both degradation and hepatic uptake of liposomes. There was a good correlation in the degradation rate constant between in vitro and in vivo trials. These kinetic analyses validate the quantitative evaluation of liposome degradation in blood circulation and provide a useful way to predict the degradation of liposomes in vivo from in vitro experiments.

Drug distribution and a pulmonary adverse effect of intraperitoneally administered doxorubicin niosomes in the mouse

Niosomes (non-ionic surfactant vesicles) prepared from C16G2 (a hexadecyl-diglycerol ether), and loaded with doxorubicin, were administered intraperitoneally to male AKR mice at dose levels of 0, 2.5, 5.0, and 10.0 mg kg-1. Free drug was given at 10.0 mg kg-1 by the intraperitoneal route. At a dose level of 10.0 mg kg-1, peak doxorubicin levels in the central compartment were attained faster with the free drug than with the niosome formulation. However, the peak plasma levels were similar for the free drug and the niosome preparation at the 10 mg kg-1 dose level. With doxorubicin administered as the niosome preparation by the intraperitoneal route at 2.5, 5.0, and 10.0 mg kg-1, mean peak plasma concentrations of the drug showed a tendency to be dose-related although the differences were not significant. Over the 24 h period of the experiment, with doxorubicin at 10 mg kg-1, the niosome formulation delivered significantly more drug to the plasma compartment than the free drug (p < 0.05). When doxorubicin was given in niosomes at 2.5, 5.0 and 10.0 mg kg-1 by the intraperitoneal route, the resulting levels of doxorubicin in cardiac tissue were not dose related and the differences not significant and, although the mean peak cardiac-tissue concentration was higher in animals receiving the free drug at 10.0 mg kg-1 intraperitoneally than in mice given intraperitoneal doxorubicin niosomes at this dose level, the differences were again not significant. There were clinical signs of toxicity in mice given doxorubicin-containing niosomes intraperitoneally at 5.0 and 10.0 mg kg-1, and at post-mortem an accumulation of fluid in the pleural cavity was evident. These changes were not seen in mice dosed intraperitoneally with free drug at 10 mg kg-1, or in animals given doxorubicin niosomes intraperitoneally at 2.5 mg kg-1. In mice dosed intraperitoneally with doxorubicin niosomes at 12.0 mg kg-1 and at a dose volume of 0.2-0.4 mL, histological examination of the lungs demonstrated a congestion of the alveolar capillaries, and an increased number of acute inflammatory cells in the alveolar walls. There was no histological evidence of lung toxicity in mice dosed with doxorubicin niosomes at 12.0 mg kg-1 when the formulation was administered with the higher dose volume of 1.8-2.0 mL. Importantly there was no histological evidence of lung toxicity in mice dosed with empty niosomes intraperitoneally or with doxorubicin niosomes given intravenously at 12.0 mg kg-1.

Kinetic modelling of liposome degradation in serum: effect of size and concentration of liposomes in vitro

The purpose of this study is to propose a new method for quantitative evaluation of liposome degradation in serum. The time course of liposome degradation in rat serum was monitored continuously, using 6(5)-carboxyfluorescein as an aqueous phase marker. The degradation curves exhibited three characteristic phases: lag time, degradation, and plateau. This curve was described by a kinetic model with three parameters: lag time (tau), first-order degradation rate constant (k), and maximum degradation (alpha). The rate and extent of the degradation of liposomes were evaluated separately in terms of k and alpha, respectively. The effects of size and concentration of liposomes on their degradation kinetics were examined using this method. Both k and alpha increased with increasing liposomal size. The increased affinity of larger liposomes for complement was suggested to increase both k and alpha. On the other hand, alpha decreased with increasing liposomal concentration without altering k. The decreased extent of degradation was considered to result from the depletion of complement components. There was no significant effect of size and concentration of liposomes on tau. Quantitative evaluation of the rate and extent of degradation of liposomes will provide deeper insights into the interaction between liposomes and serum components, and basic information on liposomes as potential drug carriers.

Kinetic analysis of tissue distribution of doxorubicin incorporated in liposomes in rats (II)

The objective of this study is to perform kinetic modelling of the tissue distribution of doxorubicin encapsulated into liposomes (L-DXR), especially to the heart and liver. The release process of doxorubicin (DXR) from liposomes in blood was quantified by a release clearance. This parameter defines a release rate of DXR based on the concentration of L-DXR in blood and was estimated from kinetic modelling of DXR distribution to the heart after L-DXR administration. The distribution of free DXR to the heart was modelled separately. The experimental data for this modelling were reported previously (Harashima et al., Biopharm. Drug. Disposit., 13, 155-170 (1992)). This analysis provided a free DXR concentration profile as well as a release clearance of DXR after L-DXR administration. There was a remarkable difference in the free DXR concentration in blood between free and liposomal administration. The area under the DXR curve in the heart was reduced by approximately one third from that for the first two hours after DXR administration by liposomal encapsulation, which could be the reason for reduced cardiac toxicity. In our previous report, the distribution of L-DXR to the liver was shown to be explained by a sequentially linked two-compartment model with efflux process. The validity of this efflux model was examined in this study by a repeated dose study. The apparent uptake clearance decreased with time and showed a second peak after the repeated dose, which justified the efflux model. These kinetic analyses give quantitative understanding of the effect of liposomal encapsulation on the tissue distribution of DXR.

Kinetic modeling of liposome degradation in blood circulation

The aim of this study is to develop a kinetic model for the quantitative evaluation of, and to examine dose dependency in liposome degradation in blood circulation in vivo. Multilamellar liposomes labeled with 3H-inulin were administered intravenously into rats and the time courses of blood concentration and urinary excretion of 3H-inulin were measured. The dosages of liposomes were fixed at 1, 5, and 100 mumolPCkg-1. Remarkable saturation was found in the time courses of both blood concentration and urinary excretion. Then a kinetic model for the degradation of liposomes in blood was developed, assuming that the degradation follows the first order rate process for each dose. The model fitted the observed time courses of excreted 3H-inulin well, and dose dependency could be observed in the rate constants for liposome degradation, which are more sensitive than urinary excretion of 3H-inulin. The degradation rate constant correlated well with the uptake rate constant, which suggests the same underlying mechanism for both uptake and degradation. These results indicate the usefulness of kinetic modeling in the quantitative evaluation of liposome degradation in blood circulation in vivo.