Intra-tumor distribution of PEGylated liposome upon repeated injection: No possession by prior dose.

Liposomes have proven to be a viable means for the delivery of chemotherapeutic agents to solid tumors. However, significant variability has been detected in their intra-tumor accumulation and distribution, resulting in compromised therapeutic outcomes. We recently examined the intra-tumor accumulation and distribution of weekly sequentially administered oxaliplatin (l-OHP)-containing PEGylated liposomes. In that study, the first and second doses of l-OHP-containing PEGylated liposomes were distributed diversely and broadly within tumor tissues, resulting in a potent anti-tumor efficacy. However, little is known about the mechanism underlying such a diverse and broad liposome distribution. Therefore, in the present study, we investigated the influence of dosage interval on the intra-tumor accumulation and distribution of "empty" PEGylated liposomes. Intra-tumor distribution of sequentially administered "empty" PEGylated liposomes was altered in a dosing interval-dependent manner. In addition, the intra-tumor distribution pattern was closely related to the chronological alteration of tumor blood flow as well as vascular permeability in the growing tumor tissue. These results suggest that the sequential administrations of PEGylated liposomes in well-spaced intervals might allow the distribution to different areas and enhance the total bulk accumulation within tumor tissue, resulting in better therapeutic efficacy of the encapsulated payload. This study may provide useful information for a better design of therapeutic regimens involving multiple administrations of nanocarrier drug delivery systems.

Anti-PEG IgM and complement system are required for the association of second doses of PEGylated liposomes with splenic marginal zone B cells.

The accelerated blood clearance (ABC) phenomenon makes it crucial to use PEGylated liposomes and micelles to deliver drugs. The ABC phenomenon is an immune response against an initial dose of PEGylated liposome, which causes subsequent doses to be rapidly cleared by macrophages in the liver. We recently found that in the early phase of the ABC phenomenon, subsequent doses of PEGylated liposomes were associated with splenic marginal zone (MZ)-B cells and were transported from the MZ to the follicle (FO). In this study, we investigated the underlying mechanisms behind the association of subsequent doses of PEGylated liposomes with MZ-B cells in the spleen. Serum factors, anti-PEG IgM and complement system, were crucial to the association of PEGylated liposomes with MZ-B cells, while the sensitization of MZ-B cells by the first dose of PEGylated liposomes was not significant. It was the complement receptors (CRs) on the MZ-B cells, rather than either the PEG-specific B-cell receptors or the IgM Fc receptors, that were the main contributors to the association between PEGylated liposomes and MZ-B cells. It appeared that anti-PEG IgM would bind to PEGylated liposomes and causes subsequent complement activation, resulting in the formation of immune complexes of PEGylated liposome-anti-PEG IgM-complement. The MZ-B cells then recognized these immune complexes via their CRs. Such an association via CRs might have triggered the transport of the immune complex by MZ-B cells to the FO in the spleen. The information obtained in this study might be useful in the development of an efficient antigen delivery system to usher PEGylated nanoparticles into FO dendritic cells.

Anti-PEG IgM Is a Major Contributor to the Accelerated Blood Clearance of Polyethylene Glycol-Conjugated Protein.

Limited therapeutic efficacy of polyethylene glycol-conjugated (PEGylated) protein drugs has been recently reported in animals and human following repeat injections. Since there are reports that an accelerated blood clearance (ABC) phenomenon is caused by repeated injection of PEGylated liposome, there is an assumption that PEGylated proteins lose their long circulating property when they are injected repeatedly due to the induction of anti-PEG antibody. Although induction of anti-PEG antibody by PEGylated protein has been reported, there is little evidence of accelerated blood clearance of PEGylated protein upon repeated injection. Herein, we investigated the blood concentration of PEGylated ovalbumin (PEG-OVA), a model PEGylated protein, upon its repeated injection. A single intravenous administration of PEG-OVA elicited an anti-PEG IgM response but not anti-PEG IgG response, while the administration did not elicit antibody against OVA. At 24 h postinjection of test PEG-OVA, although control mice showed 41.6% dose of PEG-OVA in blood, the mice pretreated with PEG-OVA showed rapid clearance of test PEG-OVA from blood and undetectable level of PEG-OVA. Interestingly, the anti-PEG IgM induced by PEGylated liposome did not affect the blood concentration of subsequent dose of PEG-OVA. Our result suggests that anti-PEG IgM is a major contributor to the accelerated blood clearance of PEG-conjugated protein, but the presence of anti-PEG IgM in blood circulation does not necessarily affect circulating property of entire PEGylated materials.

Relationship between the concentration of anti-polyethylene glycol (PEG) immunoglobulin M (IgM) and the intensity of the accelerated blood clearance (ABC) phenomenon against PEGylated liposomes in mice.

PEGylation, which is the surface modification of nanocarriers with polyethylene glycol (PEG), has increased the circulation time and reduced the immunogenic responses to nanocarriers. However, many reports have demonstrated that the intravenous injection of sterically stabilized PEGylated liposome (SL) causes an accelerated blood clearance (ABC) of subsequent doses via anti-PEG immunoglobulin M (IgM)-mediated complement activation. In the present study, the relationships between serum anti-PEG IgM concentration, the intensity of complement activation and the hepatic clearance of SL were quantitatively investigated for their role in the ABC phenomenon. Interestingly, with increasing serum anti-PEG IgM concentrations, the intensity of complement activation increased linearly, while the intensity of the hepatic clearance of SL was increased and then saturated. In addition, only 15-17% of anti-PEG IgM in blood circulation induced by SL at different doses was associated with a second dose SL. The present results indicate that it is the hepatic uptake of SL that is the limiting step in the ABC phenomenon, rather than the association of anti-PEG IgM to the SL and a subsequent complement activation.

Comprehensive analysis of PEGylated liposome-associated proteins relating to the accelerated blood clearance phenomenon by combination with shotgun analysis and conventional methods.

PEGylated liposome, sterically stabilized by polyethylene glycol (PEG), results in reduced recognition of the liposome by the mononuclear phagocyte system. Recently, we reported regarding the accelerated blood clearance (ABC) phenomenon that PEGylated liposome is cleared very rapidly from blood circulation upon repeated injection. Anti-PEG IgM production and subsequent complement activation were crucial in causing the ABC phenomenon. However, there still remains the possibility that unknown plasma factors might affect the fate of PEGylated liposome that is subjected to the ABC phenomenon. A label-free approach to shotgun analysis is a great tool for characterizing proteins in a biological system. In this study, therefore, a shotgun analysis was employed to identify plasma protein bound on PEGylated liposome after the ABC phenomenon was induced in the mouse model. The analysis revealed that immunoglobulin and complement components (C1 and C3) are the major proteins. Subsequent analysis with enzyme-linked immunosorbent assay and Western blotting showed that the immunoglobulin was IgM and that the complement system was mainly activated via an anti-PEG IgM-mediated classical pathway. These results support our earlier assumptions-anti-PEG IgM and complement activation were the major causes of the ABC phenomenon. Our proposed analytical strategy would be expected to provide useful information for the development and design of the nanocarrier drug delivery system.

Influence of dose and animal species on accelerated blood clearance of PEGylated liposomal doxorubicin.

We recently demonstrated that Doxil loses its long-circulating properties when injected repeatedly at doses below 2 mg/m(2) in dogs. In studies using other animal species, PEGylated liposomal doxorubicin has been reported not to induce the accelerated blood clearance (ABC) phenomenon. We investigated the issue of whether Doxil can elicit the ABC phenomenon in several species. In minipigs, the ABC phenomenon was induced at 2 mg/m(2). In other animal species, the ABC phenomenon was not observed at higher doses (>2 mg/m(2)), but was observed at much lower doses (0.2 mg/m(2)). The pharmacokinetic profile of a second dose of Doxil reflected the circulating anti-PEG IgM level induced by the first dose. The ABC phenomenon was not observed at the clinically recommended DXR dose (20 mg/m(2)) in any animal species. These results indicate that Doxil can cause the ABC phenomenon in all animals tested, the extent of induction was dependent on the first dose of Doxil, and a higher Doxil dose lessened the ABC phenomenon. The current study results suggest that a careful study design including selection of animal species is important for preclinical studies using PEGylated liposomal formulations even if they contain anticancer drugs that suppress the host immune response.

B cell-intrinsic toll-like receptor 7 is responsible for the enhanced anti-PEG IgM production following injection of siRNA-containing PEGylated lipoplex in mice.

Recently, we reported that immunostimulatory siRNA-containing PEGylated lipoplex (PEGylated siRNA-lipoplex) activates the immune system, resulting in the enhanced production of anti-PEG IgM. However, the enhancing mechanism upon anti-PEG IgM production has not been fully elucidated. In this study, we employed toll-like receptor 7 knock out (TLR7 KO) mice, and showed how PEGylated siRNA-lipoplex activates the innate immune system through TLR7 and consequently enhances anti-PEG IgM production. In addition, we showed that SCID mice reconstituted with TLR7-deficient B cells failed to enhance anti-PEG IgM production following the injection of PEGylated siRNA-lipoplex, but that SCID mice reconstituted with wild type B cells did enhance anti-PEG IgM production. These results suggest that immune activation via B cell-intrinsic TLR7, but not other TLR7-expressing cells, contributes predominantly to an enhanced anti-PEG IgM production in response to the intravenous injection of PEGylated siRNA-lipoplexes. A strategy to evade B cell-intrinsic TLR7 activation by siRNA, such as chemical modification, may overcome immunological barriers to PEGylated liposome-based siRNA therapeutics.

Generation, characterization and in vivo biological activity of two distinct monoclonal anti-PEG IgMs.

PEGylation, the attachment of polyethylene glycol (PEG) to nanocarriers and proteins, is a widely accepted approach to improving the in vivo efficacy of the non-PEGylated products. However, both PEGylated liposomes and PEGylated proteins reportedly trigger the production of specific antibodies, mainly IgM, against the PEG moiety, which possibly leads to a reduction in safety and therapeutic efficacy of the PEGylated products. In the present study, two monoclonal anti-PEG IgMs--HIK-M09 via immunization with an intravenous injection of PEGylated liposomes (SLs) and HIK-M11 via immunization with a subcutaneous administration of PEGylated ovalbumin (PEG-OVA) were successfully generated. The generated IgMs showed efficient reactivity to mPEG2000 conjugated to 1,2-distearoyl-sn-glycero-3-phospho-ethanolamine (DSPE), PEGylated liposome (SL) and PEG-OVA. It appears that HIK-M09 recognizes ethoxy (OCH2CH2) repeat units along with a terminal motif of PEG, while HIK-M11 recognizes only ethoxy repeat units of PEG. Such unique properties allow HIK-M09 to bind with dense PEG. In addition, their impact on the in vivo clearance of the PEGylated products was investigated. It was found that the generated ant-PEG IgMs induced a clearance of SL as they were intravenously administered with SL. Interestingly, the HIK-M11, generated by PEG-OVA, induced the clearance of both SL and PEG-OVA, while the HIK-M09, generated by SL, induced the clearance of SL only. We here revealed that the presence of serum anti-PEG IgM and the subsequent binding of anti-PEG IgM to the PEGylated products are not necessarily related to the enhanced clearance of the products. It appears that subsequent complement activation following anti-PEG IgM binding is the most important step in dictating the in vivo fate of PEGylated products. This study may have implications for the design, development and clinical application of PEGylated products and therapeutics.

Selective delivery of oxaliplatin to tumor tissue by nanocarrier system enhances overall therapeutic efficacy of the encapsulated oxaliplatin.

Oxaliplatin (trans-l-diaminocyclohexane oxalatoplatinum; l-OHP), a third-generation platinum antitumor drug, is currently approved in combination with 5-flurouracil (5-FU)/leucovorin (FOLFOX) for standard first- and second-line treatment of metastatic or advanced-stage colorectal cancer. Despite l-OHP's better tolerability in comparison with other platinum compounds such as cisplatin and carboplatin, its clinical efficiency is limited by the dose-limiting side effects including cumulative neurotoxicity and acute dysesthesias. In addition, like other platinum chemotherapeutic agents, l-OHP therapy is limited by reduced accumulation levels in tumor tissues, nonselective accumulation in healthy organs and/or tissues, inactivation by conjugation with glutathione, and the development of drug resistance. Accordingly, successful outcome of cancer treatment using l-OHP requires selective delivery of a relatively high concentration of the drug to tumor tissues. In this review we focus on utilization of different drug-delivery vehicles such as liposomes, polymeric nanocarriers, and carbon nanotubes in enhancing selective delivery of l-OHP to tumor tissues and consequently improving overall efficacy of l-OHP-containing drug-delivery systems.

Application of polyglycerol coating to plasmid DNA lipoplex for the evasion of the accelerated blood clearance phenomenon in nucleic acid delivery.

Cationic liposomes (CLs) have shown promise as nonviral delivery systems. To achieve in vivo stability and long circulation, most liposomes are modified with hydrophilic polymer polyethylene glycol (PEG). However, we have reported that repeated administration of PEG-coated CLs containing plasmid DNA (pDNA; PEGylated lipoplexes) induces what is referred to as "the accelerated blood clearance (ABC) phenomenon" and, consequently, subsequently administered lipoplexes lose their prolonged circulation characteristics. Anti-PEG IgM produced in response to the first dose of PEG-coated pDNA-lipoplexes (PEG-DCL) has proven to be a major cause of the ABC phenomenon. In this study, to evade and/or attenuate this unexpected immune response, we modified the surface of a lipoplex with polyglycerol (PG)-derived lipid. The PG-coated pDNA-lipoplex (PG-DCL) attenuated the production of anti-polymer IgM, whereas PEG-coated pDNA-lipoplex (PEG-DCL) did not. In addition, a second dose of PG-DCL maintained the accumulation level in the tumor tissue of a tumor-bearing mouse model, comparable to that of the first dose, whereas the tumor accumulation level of a second dose of PEG-DCL was significantly compromised, compared with the first dose of PEG-DCL. Our results indicate that surface modification of lipoplex with PG represents a viable means for the attenuation, and/or evasion, of the ABC phenomenon that is encountered upon repeated administrations of nucleic acids containing PEG-coated nanocarriers.

Sequential treatment of oxaliplatin-containing PEGylated liposome together with S-1 improves intratumor distribution of subsequent doses of oxaliplatin-containing PEGylated liposome.

We recently reported that combination therapy with metronomic S-1 dosing and oxaliplatin (l-OHP)-containing PEGylated liposomes improved antitumor activity in a murine colorectal tumor model. However, little is known about the mechanism underlying such improved therapeutic efficacy. Here we investigated the impact of combined treatment on biodistribution, tumor accumulation and intratumor distribution of test PEGylated liposomes and on the structure of tumor vasculature in a solid tumor. The combined treatment clearly enhanced tumor accumulation and intratumor distribution of a subsequent test dose of PEGylated liposome as a result of on the one hand prolonging blood circulation of test liposome and on the other hand the alteration in tumor microenvironment. The l-OHP-containing PEGylated liposomes contributed predominantly to the enhanced tumor accumulation and altered tumor distribution of test liposome. On the other hand, metronomic S-1 dosing contributed to the altered tumor distribution but not the tumor accumulation of test liposome. The antitumor effect of the combined treatment, reflected by the proportion of apoptotic cells in the tumor, was approximately equally accounted for by each of the two treatments, leading to a roughly additive effect. In conclusion, 1-OHP-containing PEGylated liposome together with S-1 enhanced intratumor influx, leading to improved antitumor activity of subsequently injected 1-OHP-containing PEGylated liposomes and/or S-1. This strategy we propose, which is clinically applicable, may overcome the problems related to the use of EPR effect-based nanocarrier systems.

Ex-vivo/in-vitro anti-polyethylene glycol (PEG) immunoglobulin M production from murine splenic B cells stimulated by PEGylated liposome.

We have reported that PEGylated liposomes lose their long-circulating properties when injected twice into the same animal within a certain interval (the accelerated blood clearance (ABC) phenomenon). We assumed that this phenomenon was triggered via the abundant secretion of anti-polyethylene glycol (PEG) immunoglobulin M (IgM) in response to the first dose of PEGylated liposomes and that the spleen played an important role in the production of anti-PEG IgM. However, no direct evidence has yet confirmed this suspicion. In the current study, we verified, both in vitro and ex vivo, that spleen cells are indeed responsible for the production of anti-PEG IgM in response to PEGylated liposomes. In this study, spleen cells obtained from either naïve mice or mice pre-treated with PEGylated liposomes induced the production of anti-PEG IgM in a dose- and time-dependent manner, upon incubation with PEGylated liposomes. In addition, we confirmed that among the different fractions of splenic B cells, IgM-positive B cells, rather than CD45R-positive or CD19-positive splenic B cells, which are presumed to be the marginal zone B (MZB) cells, are the major cells producing anti-PEG IgM in the response to stimulation by PEGylated liposomes. These results may provide new insights into the mechanisms underlying the anti-PEG IgM production in response to the stimulation by PEGylated liposomes.

The accelerated blood clearance (ABC) phenomenon: clinical challenge and approaches to manage.

Despite the clinical introduction of an increasing number of polyethylene glycol (PEG)-conjugated substances, PEG has been named as the cause of an unexpected immunogenic response known as the "accelerated blood clearance (ABC) phenomenon." This phenomenon has been extensively observed during the repeated administration of PEG-conjugated substances and PEGylated nanocarriers including PEGylated liposomes, PEGylated nanoparticles, PEGylated micelles, etc., resulting in the increased clearance and reduced efficacy of PEG-conjugated substances/PEGylated nanocarriers. In this review, therefore, we focused on the possible mechanisms underlying the induction of such a phenomenon and emphasized the factors affecting its magnitude. In addition, the clinical implications of the ABC phenomenon on the therapeutic efficacy of PEG-conjugated substances/PEGylated nanocarriers, along with the new approaches that can be applied to manage and/or abrogate the induction of the ABC phenomenon, are also discussed.

Use of polyglycerol (PG), instead of polyethylene glycol (PEG), prevents induction of the accelerated blood clearance phenomenon against long-circulating liposomes upon repeated administration.

The accelerated blood clearance (ABC) phenomenon accounts for the rapid systemic clearance of PEGylated nanocarriers upon repeated administrations. IgM production against the polyethylene glycol (PEG) coating in PEGylated liposomes is now known to be responsible for such unexpected pharmacokinetical alterations. The ABC phenomenon poses a remarkable clinical challenge by reducing the therapeutic efficacy of encapsulated drugs and causing harmful effects due to the altered tissue distribution pattern of the drugs. In this study, we investigated the in vivo performance of liposomes modified with polyglycerol (PG) upon repeated injection, and the in vivo therapeutic efficacy of such liposomes when they encapsulated a cytotoxic agent, doxorubicin (DXR). Repeated injection of PEG-coated liposomes in rats induced the ABC phenomenon, while repeated injection of PG-coated liposomes did not. In addition, DXR-containing PG-coated liposomes showed antitumor activity that was superior to that of free DXR and similar to that of DXR-containing PEG-coated liposomes upon repeated administration. These results indicate that polyglycerol (PG) might represent a promising alternative to PEG via enhancing the in vivo performance of liposomes by not eliciting the ABC phenomenon upon repeated administration.

Anti-polyethyleneglycol antibody response to PEGylated substances.

In contrast to the general assumption that polyethyleneglycol (PEG)-conjugated substances lack immunogenicity and antigenic, it has been reported that they can elicit antibodies against PEG (mainly anti-PEG immunoglobulin M (IgM)). In patients, the presence of anti-PEG antibodies may limit therapeutic efficacy of PEGylated substances as a consequence of inducing rapid clearance of and neutralizing biological activity of the substances. Here, we introduce specific examples of PEGylated substances including several PEGylated proteins and PEGylated particles (PEGylated nanocarriers) which induce anti-PEG antibody responses. Finally, we emphasize that the immunogenicity of PEGylated substances should be tested in the development stage and that the titer of anti-PEG antibodies in patients should be pre-screened and monitored prior to and throughout a course of treatment with a PEGylated substance.

Alteration of tumor microenvironment for improved delivery and intratumor distribution of nanocarriers.

Nanocarrier-based cancer chemotherapeutics are thought to increase therapeutic efficiency and reduce the side effects of associated chemotherapeutic agents by altering the agents' pharmacokinetics and tissue distribution following intravenous administration. In spite of these favorable properties, nanocarrier-based cancer chemotherapeutics are not always effective because of their heterogeneous intratumoral localization. Homogeneous distribution of nanocarriers in a tumor would improve the efficacy of nanocarrier-based cancer chemotherapeutics. In this article, we describe and discuss some trials that attempt to manipulate the barriers in the tumor microenvironment that hinder extravasation through the tumor vasculature and penetration of nanocarriers in solid tumors. Alterations of the tumor microenvironment that relate directly to the intratumoral distribution of nanocarriers may be potential strategies to improve the delivery of nanocarrier-based cancer chemotherapeutics.

The co-delivery of oxaliplatin abrogates the immunogenic response to PEGylated siRNA-lipoplex.

PURPOSE: In vivo application of siRNA/PEGylated cationic liposome complex (lipoplex) is impeded by two main obstacles: cytokine responses and anti-PEG IgM responses to PEGylated siRNA-lipoplex. Here, we investigated whether co-administration of oxaliplatin (l-OHP) abrogates the cytokine release and anti-PEG IgM production by PEGylated siRNA-lipoplex. METHODS: Free l-OHP was administered either simultaneously or 30 min prior to PEGylated siRNA-lipoplex administration, and cytokine response and anti-PEG IgM production were evaluated. In addition, the effect of the liposomal encapsulation of l-OHP on the immunogenic response of PEGylated siRNA-lipoplex was investigated. RESULTS: Simultaneous co-administration of free l-OHP with PEGylated siRNA-lipoplex caused a significant reduction in anti-PEG IgM production, along with an increase in the cytokine response. Free l-OHP injected prior to the lipoplex injection, however, successfully reduced cytokine release and anti-PEG IgM response. Platination of siRNA by simultaneously administered free l-OHP might facilitate the dissociation of double-stranded siRNA to single-stranded siRNA, resulting in the inducement of a potent immuno-stimulation of siRNA via endosomal toll-like receptors (TLRs). On the other hand, encapsulation of l-OHP into the siRNA-lipoplex resulted in a reduction of both anti-PEG IgM production and cytokine responses. CONCLUSIONS: Our results suggest that, besides the expected therapeutic efficacy of co-administration, encapsulation of l-OHP into the PEGylated siRNA-lipoplex has great potential for minimizing the immunostimulation of PEGylated siRNA-lipoplex, resulting in a safe, applicable, and compliant treatment regimen for sequential clinical administration.

[Development of siRNA delivery strategy by active control of tumor microenvironment].

Efficient systemic siRNA delivery to cells in the target tissue is a current critical challenge in the drug delivery field. Several studies have demonstrated that nanoparticles such as polyethylene glycol (PEG)-coated siRNA-lipoplexes may enhance the systemic delivery of siRNA to tumor. However, the disordered tumor microenvironment still poses a potential impediment with respect to the efficient delivery of PEG-coated siRNA-lipoplexes. We recently showed that metronomic S-1 dosing (daily oral administration) enhanced the accumulation of PEG-coated liposome containing anticancer drug in solid tumor tissue and thereby increased therapeutic efficacy in tumor-bearing mouse model. To extend this work, we tried to investigate the effect of metronomic S-1 dosing on the intratumoral accumulation of PEG-coated siRNA-lipoplex and, thereby, their therapeutic efficacy in solid tumor-bearing mouse model. Results showed that metronomic S-1 dosing improved systemic delivery of intravenously injected PEG-coated siRNA-lipoplexes into solid tumor tissue. In addition, the combined therapy of S-1 and PEG-coated siRNA-lipoplexes showed potent tumor growth suppressive effect. Our proposed strategy may pose a promising therapeutic one to conquer cancer progression with siRNA.

Suppression of immune response by antigen-modified liposomes encapsulating model agents: a novel strategy for the treatment of allergy.

A specific antigen-sensitized animal has antigen-specific immune cells that recognize the antigen. Therefore, an antigen-modified drug carrier would be recognized by the immune cells. When such a carrier encapsulates certain drugs, these drugs should be specifically delivered to the immune cells. To examine this strategy, ovalbumin (OVA) was used as model antigen, and mice were presensitized with 100 µg of OVA with Alum. For preparing OVA-modified liposomes (OVA-lipo), OVA was incubated with DSPE-PEG-NHS and resulting DSPE-PEG-OVA was inserted into liposomes. OVA-specific IgG was produced 6-fold higher by intravenous injection of OVA-lipo thrice (10 µg as OVA in each injection) in OVA-sensitized mice, than that by the injection of control liposomes, suggesting that OVA-lipo was recognized by the antigen-specific immune cells. Moreover, intra-splenic accumulation of OVA-lipo was observed in OVA-sensitized mice, but not in naive mice. To achieve the delivery of a drug to specific immune cells, OVA-lipo encapsulated low dose of doxorubicin (DOX) as a model drug (20 µg DOX/mouse, Ca. 1 mg/kg) was injected in the sensitized mice. The injection of OVA-lipo encapsulating DOX suppressed the production of IgE against OVA, suggesting that the specific delivery of the drug to immune cells responsible for OVA recognition was achieved and that these immune cells were removed by the drug treatment. This strategy would be useful for the fundamental treatment of allergy by the use of immunosuppressing agents.

Transport of PEGylated liposomes from the splenic marginal zone to the follicle in the induction phase of the accelerated blood clearance phenomenon.

The accelerated blood clearance (ABC) phenomenon has been reported to enhance the clearance of PEGylated liposomes from the blood circulation when the liposomes are injected into the same animal repeatedly. We have shown that anti-PEG IgM production from splenic B cells is crucial in the ABC phenomenon. In this study, we describe the crucial role of marginal zone (MZ) B cells in the anti-PEG IgM production and recognition of PEGylated liposomes in the induction phase of ABC phenomenon. Suppression of the anti-PEG IgM production was correlated with the disappearance of IgM(high) cells in the MZ, particularly MZ-B cells, following cyclophosphamide (CPA)-treatment, confirming that splenic MZ-B cells are responsible for anti-PEG IgM production. The MZ-B cells stimulated by a first dose of PEGylated liposomes internalized the second dose of PEGylated liposomes in a PEG modification-dependent manner and transported the liposomes into the follicle (FO) region. To the best of our knowledge, this is the first report showing that PEGylated liposome is recognized by MZ-B cells and transported to the FO region like blood-borne antigens or immune complexes. It is likely that PEGylated liposomes are recognized as a TI-2 antigen by the first line of defense against life-threatening infections by blood-borne organisms. Our study may have implications for immunogenicity of synthesized polymer-grafted therapeutics including nanocarriers, nucleic acids and proteins.