Tatsuhiro Ishida

A combinatorial approach to producing sterically stabilized (Stealth) immunoliposomal drugs

We have developed a method for producing sterically stabilized immunoliposomal drugs (SIL) readily applicable to a ‘mix and match’ combinatorial approach for the simple manufacture of a variety of ligand‐targeted liposomal drugs. Ligands coupled to the terminus of polyethylene glycol (PEG) in micelles formed from PEG‐lipid derivatives (mPEG2000‐DSPE) could be transferred into preformed, drug‐containing liposomes from the micelles in a temperature‐ and time‐dependent manner. Antibody densities up to 100 μg antibody/μmol of phospholipid, and up to 3 mol% of mPEG2000‐DSPE, could be simultaneously transferred from the ligand‐coupled micelles into the liposomal outer monolayer with negligible drug leakage from liposomes during transfer and good stability in human plasma. Transfer of anti‐CD19 into SIL resulted in a three‐fold increase in binding of these liposomes to CD19+ human B cell lymphoma cells.

Accelerated clearance of PEGylated liposomes in rats after repeated injections

Polyethylene glycol-modified liposomes (PEGylated liposomes) represent promising carrier systems for therapeutic agents. Herein, we report on a study of the effect of repeated injection of PEGylated liposomes on their pharmacokinetics in rats. The first dose resulted in a reduction in the circulation time and an increase in hepatic accumulation of the second dose in a time-interval of injection-dependent manner. No significant increases in the number of Kupffer cells were detectable, although the liver most likely played an important role in the accelerated clearance. Interestingly, the acceleration in clearance became less pronounced, when the third dose was injected at 4, 7 or 14 days after the second injection (the second dose was given 5 weeks after the first injection). An accelerated clearance was evoked in normal rats by the transfusion of serum from rats that had received PEGylated liposomes 5 days earlier, indicating that humoral serum factor(s) are also involved in causing the accelerated clearance. A complement consumption assay indicated that the complement system is not the factor. In summary, multiple injections of PEGylated liposomes clearly altered their pharmacokinetic behavior in rats. It is likely that cellular immunity (Kupffer cells) and humoral immunity are required to cause the phenomenon. The results reported here have a considerable impact in and important implications on the clinical application, design and engineering of PEGylated liposomes for use in repeated intravenous administration.

Targeted delivery and triggered release of liposomal doxorubicin enhances cytotoxicity against human B lymphoma cells

Dioleoylphosphatidylethanolamine (DOPE)-containing liposomes that demonstrated pH-dependent release of their contents were stabilized in the bilayer form through the addition of a cleavable lipid derivative of polyethylene glycol (PEG) in which the PEG was attached to a lipid anchor via a disulfide linkage (mPEG-S-S-DSPE). Liposomes stabilized with either a non-cleavable PEG (mPEG-DSPE) or mPEG-S-S-DSPE retained an encapsulated dye at pH 5.5, but treatment at pH 5.5 of liposomes stabilized with mPEG-S-S-DSPE with either dithiothreitol or cell-free extracts caused contents release due to cleavage of the PEG chains and concomitant destabilization of the DOPE liposomes. While formulations loaded with doxorubicin (DXR) were stable in culture media, DXR was rapidly released in human plasma. pH-Sensitive liposomes, targeted to the CD19 epitope on B-lymphoma cells, showed enhanced DXR delivery into the nuclei of the target cells and increased cytotoxicity compared to non-pH-sensitive liposomes. Pharmacokinetic studies suggested that mPEG-S-S-DSPE was rapidly cleaved in circulation. In a murine model of B-cell lymphoma, the therapeutic efficacy of an anti-CD19-targeted pH-sensitive formulation was superior to that of a stable long-circulating formulation of targeted liposomes despite the more rapid drug release and clearance of the pH-sensitive formulation. These results suggest that targeted pH-sensitive formulations of drugs may be able to increase the therapeutic efficacy of entrapped drugs.

Use of the Post-Insertion Technique to Insert Peptide Ligands into Pre-Formed Stealth Liposomes with Retention of Binding Activity and Cytotoxicity

AbstractPurpose: Simple methods for the large-scale manufacture of ligand-targeted liposomes will be needed if clinical trials are to proceed. We tested a recently developed technology for inserting peptide ligands into preformed Stealth liposomes. Antagonist G-targeted liposomes (PLG) were prepared and loaded with doxorubicin and their cellular association and cytotoxicity were evaluated using the human small cell lung cancer H69 cell line. Methods: The hexapeptide antagonist G was covalently coupled via a thioether bond to the terminus of polyethylene glycol (PEG) in micelles formed from maleimide-derivatized poly(ethylene glycol) (Mr 2000) distearoylphosphatidylethanolamine followed by transfer into preformed liposomes during a one-step incubation. For cellular association, we used radiolabeled liposomes. Cytotoxicity was evaluated using the MTT in vitro proliferation assay. Results: The postinsertion approach to the formation of peptide-targeted liposomes led to the production of PLG bearing a maximum of approximately 0.3 μg antagonist G/μmol phospholipid. These liposomes had increased cellular association to H69 cells relative to nontargeted liposomes and, when loaded with doxorubicin, they resulted in similar levels of cytotoxicity to those obtained by conventional coupling techniques. Conclusions: The postinsertion technique is a simple, effective means for the production of biologically active peptide-targeted liposomes.

Accelerated clearance of a second injection of PEGylated liposomes in mice.

We recently reported that the firstly injected PEGylated liposomes dramatically affected the rate of blood clearance of secondly injected PEGylated liposomes in rats in a time interval of injection dependent manner [J. Control. Release (2003)]. Mice are frequently used in evaluations of the therapeutic efficacy of PEGylated liposomal formulations, but the pharmacokinetics of repeatedly injected PEGylated liposomes in mice is not fully understood. In this study, therefore, we examined in mice the effect of the repeated injection of PEGylated liposomes on their pharmacokinetics. An intravenous pretreatment with PEGylated liposomes produced a striking change in the biodistribution of the second dose which was given several days after the first injection. The first dose resulted in a reduction in the circulation half-life of the second dose. The degree of alteration was dependent on the time interval between the injections. The rapid clearance of the second dose was strongly related to hepatic clearance (CLh). This finding suggests that a considerable increase in hepatic accumulation accounts for this phenomenon. But, no liver injury or an increase in the number of Kupffer cells were detected in histopathological evaluations. Collectively, although the multiple injections of the PEGylated liposomes had no obvious physical effects, such as inflammation, their pharmacokinetic behavior was clearly altered in mice. The results obtained here have important implications not only with respect to the design and engineering of liposomes for human use, but for evaluating the therapeutic efficacy of liposomal formulations in experimental animal models as well.

Particle size-dependent triggering of accelerated blood clearance phenomenon.

A repeat-injection of polyethylene glycol-modified liposomes (PEGylated liposomes) causes a rapid clearance of them from the blood circulation in certain cases that is referred to as the accelerated blood clearance (ABC) phenomenon. In the present study, we examined whether polymeric micelles trigger ABC phenomenon or not. As a preconditioning treatment, polymeric micelles (9.7, 31.5, or 50.2 nm in diameter) or PEGylated liposomes (119, 261 or 795 nm) were preadministered into BALB/c mice. Three days after the preadministration [(3)H]-labeled PEGylated liposomes (127 nm) as a test dose were administered into the mice to determine the biodistribution of PEGylated liposomes. At 24h after the test dose was given, accelerated clearance of PEGylated liposomes from the bloodstream and significant accumulation in the liver was observed in the mice preadministered with 50.2-795 nm nanoassemblies (PEGylated liposomes or polymeric micelles). In contrast, such phenomenon was not observed with 9.7-31.5 nm polymeric micelles. The enhanced blood clearance and hepatic uptake of the test dose (ABC phenomenon) were related to the size of triggering nanoassemblies. Our study provides important information for developing both drug and gene delivery systems by means of nanocarriers.

T cell-independent B cell response is responsible for ABC phenomenon induced by repeated injection of PEGylated liposomes

Repeated injection of polyethyleneglycol-modified (PEGylated) liposomes causes a rapid clearance of them from the bloodstream, this phenomenon is called accelerated blood clearance (ABC). In the present study, we focused on the immune system responsible for the ABC phenomenon. PEGylated liposomes were preadministered to BALB/c mice and [(3)H]-labeled ones were then administered to them 3 days after the preadministration. Consistent with our previous results, the preadministration with PEGylated liposomes triggered the rapid clearance of [(3)H]-labeled PEGylated liposomes from the bloodstream, but that with PEGylated liposomes encapsulating doxorubicin (Dox) did not. In addition, we found that the ABC phenomenon was observed when a mixture of free Dox and PEGylated liposomes was preadministered. These data indicate that immune cells responsible for the ABC phenomenon might be selectively damaged by the Dox encapsulated in PEGylated liposomes. The ABC phenomenon was also observed in BALB/c nu/nu mice, but not in BALB/c SCID mice. The amount of anti-PEG IgM antibody induced by the stimulation with the PEGylated liposomes was significantly increased in the BALB/c nu/nu mice, but not in the BALB/c SCID ones. These data indicate that a T cell-independent B cell response would play a significant role in the ABC phenomenon. Furthermore, the present study suggests that PEGylated liposomes might be recognized by B cells as a thymus-independent type 2 (TI-2) antigen. The present study provides important information for the future development of liposomal medicines.

Anti-MUC-1 immunoliposomal doxorubicin in the treatment of murine models of metastatic breast cancer.

The fate of breast cancer patients is dependent upon elimination or control of metastases. We studied the effect of antibody-targeted liposomes containing entrapped doxorubicin (DXR) on development of tumours in two models of breast cancer, pseudometastatic and metastatic, in mice. The former used the mouse mammary carcinoma cell line GZHI, which expresses the human MUC-1 gene (L. Ding, E.N. Lalani, M. Reddish, R. Koganty, T. Wong, J. Samuel, M.B. Yacyshyn, A. Meikle, P.Y.S. Fung, J. Taylor-Papadimitriou, B.M. Longenecker, Cancer Immunol. Immunother. 36 (1993) 9--17). GZHI cells seed into the lungs of Balb/c mice following intravenous injection. The latter used the 4T1-MUC1 cell line, a MUC-1 transfectant of the mouse mammary carcinoma cell line 4T1, which metastasizes from a primary mammary fatpad (mfp) implant to the lungs (C.J. Aslakson, F.R. Miller, Cancer Res. 52 (1992) 1399--1405). B27.29, a monoclonal antibody against the MUC-1 antigen, was used to target sterically stabilized immunoliposomes (SIL[B27.29]) to tumour cells. In vitro, SIL[B27.29] showed high specific binding to both GZHI and 4T1-MUC1 cells. The IC(50) of DXR-loaded SIL[B27.29] was similar to that of free drug for GZHI cells. In the pseudometastatic model, mice treated with a single injection of 6 mg DXR/kg in DXR-SIL[B27.29] at 24 h after cell implantation had longer survival times than those injected with non-targeted liposomal drug. In the metastatic model, severe combined immune deficiency mice given weekly injectionsx3 of 2.5 mg DXR/kg encapsulated in either targeted or non-targeted liposomes were almost equally effective in slowing growth of the primary tumour and reducing development of lung tumours. Surgical removal of the primary tumour from mfp, followed by various chemotherapy regimens, was attempted, but removal of the primary tumour was generally incomplete; tumour regrowth occurred and metastases developed in the lungs in all treatment groups. DXR-SL reduced the occurrence of regrowth of the primary tumour, whereas neither targeted liposomal drug or free drug prevented regrowth. We conclude that monoclonal antibody-targeted liposomal DXR is effective in treating early lesions in both the pseudometastatic and metastatic models, but limitations to the access of the targeted liposomes to tumour cells in the primary tumour compromised their therapeutic efficacy in treating the more advanced lesions.

Evasion of the Accelerated Blood Clearance Phenomenon by Coating of Nanoparticles with Various Hydrophilic Polymers

The accelerated blood clearance (ABC) phenomenon is induced upon repeated injections of poly(ethylene glycol) (PEG)-coated colloidal carriers. It is essential to suppress this phenomenon in a clinical setting because the pharmacokinetics must be reproducible. In this study, we evaluated the induction of the ABC phenomenon using nanoparticles coated with various hydrophilic polymers instead of PEG. Nanoparticles encapsulating prostaglandin E1 were prepared by the solvent diffusion method from a blend of poly(lactic acid) (PLA) and block copolymers consisting of various hydrophilic polymers and PLA. Coating of nanoparticles with poly(N-vinyl-2-pyrrolidone) (PVP), poly(4-acryloylmorpholine), or poly(N,N-dimethylacrylamide) led to extended residence of the nanoparticles in blood circulation in rats, although they had a shorter half-life than the PEG-coated nanoparticles. The ABC phenomenon was not induced upon repeated injection of PVP-coated nanoparticles at various time intervals, dosages, or frequencies, whereas it was elicited by PEG-coated nanoparticles. In addition, anti-PVP IgM antibody, which is estimated to be one of the crucial factors for induction of the ABC phenomenon, was not produced after injection of PVP-coated nanoparticles. These results suggest that the use of PVP, instead of PEG, as a coating material for colloidal carriers can evade the ABC phenomenon.

Oxaliplatin encapsulated in PEG-coated cationic liposomes induces significant tumor growth suppression via a dual-targeting approach in a murine solid tumor model

We recently designed a PEG-coated cationic liposome targeted to angiogenic vessels and showed, in a murine dorsal air sac model, potent anti-angiogenic activity of an oxaliplatin (l-OHP) formulation of this liposome. In the present study, we extended the l-OHP formulation to a murine tumor-xenograft model. Following three injections, l-OHP containing PEG-coated cationic liposomes showed substantial tumor growth suppression and increased survival time of tumor-bearing mice without apparent side effects, compared with other l-OHP containing PEG-coated neutral liposomes and free l-OHP. In vivo imaging showed a preferential tumor accumulation and a broader distribution of PEG-coated cationic liposomes, compared with PEG-coated neutral liposomes. In addition, PEG-coated cationic liposomes delivered larger amounts of l-OHP into the tumor tissue than other l-OHP formulations, correlating with its antitumor efficiency. In vitro studies indicated that PEG-coated cationic liposomes were internalized not only by tumor cells but also by endothelial cells, and consequently its l-OHP formulation displayed higher cytotoxicity towards both cell types as compared with l-OHP containing PEG-coated neutral liposomes. In summary, l-OHP containing PEG-coated cationic liposomes induced significant tumor growth suppression, presumably by delivering encapsulated l-OHP into both tumor endothelial cells and tumor cells. Such dual targeting approach, i.e. vascular-targeting and tumor-targeting with a single liposomal l-OHP formulation, may have great potential for overcoming some major limitations in conventional chemotherapy.