Cellular Trafficking and Cytotoxicity of Anti-Cd19-Targeted Liposomal Doxorubicin in B Lymphoma Cells

Targeted Polymersome Delivery of a Stapled Peptide for Drugging the Tumor Protein p53:BCL-2-Family Axis in Diffuse Large B-Cell Lymphoma

Diffuse large B-cell lymphoma (DLBCL) remains a formidable diagnosis in need of new treatment paradigms. In this work, we elucidated an opportunity for therapeutic synergy in DLBCL by reactivating tumor protein p53 with a stapled peptide, ATSP-7041, thereby priming cells for apoptosis and enhancing their sensitivity to BCL-2 family modulation with a BH3-mimetic, ABT-263 (navitoclax). While this combination was highly effective at activating apoptosis in DLBCL in vitro, it was highly toxic in vivo, resulting in a prohibitively narrow therapeutic window. We, therefore, developed a targeted nanomedicine delivery platform to maintain the therapeutic potency of this combination while minimizing its toxicity via packaging and targeted delivery of a stapled peptide. We developed a CD19-targeted polymersome using block copolymers of poly(ethylene glycol) disulfide linked to poly(propylene sulfide) (PEG-SS-PPS) for ATSP-7041 delivery into DLBCL cells. Intracellular delivery was optimized in vitro and validated in vivo by using an aggressive human DLBCL xenograft model. Targeted delivery of ATSP-7041 unlocked the ability to systemically cotreat with ABT-263, resulting in delayed tumor growth, prolonged survival, and no overt toxicity. This work demonstrates a proof-of-concept for antigen-specific targeting of polymersome nanomedicines, targeted delivery of a stapled peptide in vivo, and synergistic dual intrinsic apoptotic therapy against DLBCL via direct p53 reactivation and BCL-2 family modulation.

Anti-HER2 Super Stealth Immunoliposomes for Targeted-Chemotherapy

Liposomes play an important role in the field of drug delivery by virtue of their biocompatibility and versatility as carriers. Stealth liposomes, obtained by surface decoration with hydrophilic polyethylene glycol (PEG) molecules, represent an important turning point in liposome technology, leading to significant improvements in the pharmacokinetic profile compared to naked liposomes. Nevertheless, the generation of effective targeted liposomes-a central issue for cancer therapy-has faced several difficulties and clinical phase failures. Active targeting remains a challenge for liposomes. In this direction, a new Super Stealth Immunoliposomes (SSIL2) composed of a PEG-bi-phospholipids derivative is designed that stabilizes the polymer shielding over the liposomes. Furthermore, its counterpart, conjugated to the fragment antigen-binding of trastuzumab (Fab'TRZ -PEG-bi-phospholipids), is firmly anchored on the liposomes surface and correctly orients outward the targeting moiety. Throughout this study, the performances of SSIL2 are evaluated and compared to classic stealth liposomes and stealth immunoliposomes in vitro in a panel of cell lines and in vivo studies in zebrafish larvae and rodent models. Overall, SSIL2 shows superior in vitro and in vivo outcomes, both in terms of safety and anticancer efficacy, thus representing a step forward in targeted cancer therapy, and valuable for future development.

Lipid-based nanoparticles as drug delivery systems for cancer immunotherapy

Immune checkpoint inhibitors (ICIs) have shown remarkable success in cancer treatment. However, in cancer patients without sufficient antitumor immunity, numerous data indicate that blocking the negative signals elicited by immune checkpoints is ineffective. Drugs that stimulate immune activation-related pathways are emerging as another route for improving immunotherapy. In addition, the development of nanotechnology presents a promising platform for tissue and cell type-specific delivery and improved uptake of immunomodulatory agents, ultimately leading to enhanced cancer immunotherapy and reduced side effects. In this review, we summarize and discuss the latest developments in nanoparticles (NPs) for cancer immuno-oncology therapy with a focus on lipid-based NPs (lipid-NPs), including the characteristics and advantages of various types. Using the agonists targeting stimulation of the interferon genes (STING) transmembrane protein as an exemplar, we review the potential of various lipid-NPs to augment STING agonist therapy. Furthermore, we present recent findings and underlying mechanisms on how STING pathway activation fosters antitumor immunity and regulates the tumor microenvironment and provide a summary of the distinct STING agonists in preclinical studies and clinical trials. Ultimately, we conduct a critical assessment of the obstacles and future directions in the utilization of lipid-NPs to enhance cancer immunotherapy.

DNA-Liposome Hybrid Carriers for Triggered Cargo Release

The design of simple and versatile synthetic routes to accomplish triggered-release properties in carriers is of particular interest for drug delivery purposes. In this context, the programmability and adaptability of DNA nanoarchitectures in combination with liposomes have great potential to render biocompatible hybrid carriers for triggered cargo release. We present an approach to form a DNA mesh on large unilamellar liposomes incorporating a stimuli-responsive DNA building block. Upon incubation with a single-stranded DNA trigger sequence, a hairpin closes, and the DNA building block is allowed to self-contract. We demonstrate the actuation of this building block by single-molecule Förster resonance energy transfer (FRET), fluorescence recovery after photobleaching, and fluorescence quenching measurements. By triggering this process, we demonstrate the elevated release of the dye calcein from the DNA–liposome hybrid carriers. Interestingly, the incubation of the doxorubicin-laden active hybrid carrier with HEK293T cells suggests increased cytotoxicity relative to a control carrier without the triggered-release mechanism. In the future, the trigger could be provided by peritumoral nucleic acid sequences and lead to site-selective release of encapsulated chemotherapeutics.

Cyclodextrins based delivery systems for macro biomolecules

Macro biomolecules are of vital importance in regulating the biofunctions in organisms, in which proteins (including peptides when mentioned below) and nucleic acids (NAs) are the most important. Therefore, these proteins and NAs can be applied as "drugs" to regulate the biofunctions from abnormal to normal. Either for proteins and NAs, the most challenging thing is to avoid the biodegradation or physicochemical degradation before they reach the targeted location, and then functions as complete functional structures. Hence, appropriate delivery systems are very important which can protect them from these degradations. Cyclodextrins (CDs) based delivery systems achieved mega successes due to their outstanding pharmaceutical properties and there have been several reviews on CDs based small molecule drug delivery systems recently. But for biomolecules, which are getting more and more important for modern therapies, however, there are very few reviews to systematically summarize and analyze the CDs-based macro biomolecules delivery systems, especially for proteins. In this review, there were some of the notable examples were summarized for the macro biomolecules (proteins and NAs) delivery based on CDs. For proteins, this review included insulin, lysozyme, bovine serum albumin (BSA), green fluorescent protein (GFP) and IgG's, etc. deliveries in slow release, stimulating responsive release or targeting release manners. For NAs, this review summarized cationic CD-polymers and CD-cluster monomers as NAs carriers, notably, including the multicomponents targeting CD-based carriers and the virus-like RNA assembly method siRNA carriers.

Liposomal doxorubicin as targeted delivery platform: Current trends in surface functionalization

Liposomal delivery systems have significantly enhanced the efficacy and safety of chemotherapeutic agents compared to free (non-liposomal) formulations. Liposomes are vesicles made up of lipophilic bilayer and a hydrophilic core which provides perfect opportunity for their application as transport vehicle for various therapeutic and diagnostic agents. Doxorubicin is the most exploited chemotherapeutic agent for evaluation of different liposomal applications, as its physicochemical properties permit high drug entrapment and easy remote loading in pre-formulated liposomes. Pegylated liposomal doxorubicin clinically approved and, on the market, Doxil®, exemplifies the benefits offered upon the surface modification of liposome with polyethylene glycol. This unique formulation prolonged the drug residence time in the circulation and increased accumulation of doxorubicin in tumor tissue via passive targeting (enhanced permeability and retention effect). However, there is ample scope for further improvement in the efficiency of targeting tumors by coupling biological active ligands onto the liposome surface to generate intelligent drug delivery systems. Small biomolecules such as peptides, fraction of antibodies and carbohydrates have the potential to target receptors present on the surface of the malignant cells. Hence, active targeting of malignant cells using functionalised nanocarrier (liposomes encapsulated with doxorubicin) have been attempted which is reviewed in this article.

Peptide and protein nanoparticle conjugates: versatile platforms for biomedical applications

Peptide- and protein-nanoparticle conjugates have emerged as powerful tools for biomedical applications, enabling the treatment, diagnosis, and prevention of disease. In this review, we focus on the key roles played by peptides and proteins in improving, controlling, and defining the performance of nanotechnologies. Within this framework, we provide a comprehensive overview of the key sequences and structures utilised to provide biological and physical stability to nano-constructs, direct particles to their target and influence their cellular and tissue distribution, induce and control biological responses, and form polypeptide self-assembled nanoparticles. In doing so, we highlight the great advances made by the field, as well as the challenges still faced in achieving the clinical translation of peptide- and protein-functionalised nano-drug delivery vehicles, imaging species, and active therapeutics.

Biodistribution, Clearance, and Long-Term Fate of Clinically Relevant Nanomaterials

Realization of the immense potential of nanomaterials for biomedical applications will require a thorough understanding of how they interact with cells, tissues, and organs. There is evidence that, depending on their physicochemical properties and subsequent interactions, nanomaterials are indeed taken up by cells. However, the subsequent release and/or intracellular degradation of the materials, transfer to other cells, and/or translocation across tissue barriers are still poorly understood. The involvement of these cellular clearance mechanisms strongly influences the long-term fate of used nanomaterials, especially if one also considers repeated exposure. Several nanomaterials, such as liposomes and iron oxide, gold, or silica nanoparticles, are already approved by the American Food and Drug Administration for clinical trials; however, there is still a huge gap of knowledge concerning their fate in the body. Herein, clinically relevant nanomaterials, their possible modes of exposure, as well as the biological barriers they must overcome to be effective are reviewed. Furthermore, the biodistribution and kinetics of nanomaterials and their modes of clearance are discussed, knowledge of the long-term fates of a selection of nanomaterials is summarized, and the critical points that must be considered for future research are addressed.

Intracellular trafficking of a pH-responsive drug metal complex

We previously developed a pH-responsive copper-doxorubicin (CuDox) cargo in lysolipid-based temperature-sensitive liposomes (LTSLs). The CuDox complex is released from the particle by elevated temperature; however, full release of doxorubicin from CuDox requires a reduced pH, such as that expected in lysosomes. The primary goal of this study is to evaluate the cellular uptake and intracellular trafficking of the drug-metal complex in comparison with intact liposomes and free drug. We found that the CuDox complex was efficiently internalized by mammary carcinoma cells after release from LTSLs. Intracellular doxorubicin and copper were 6-fold and 5-fold greater, respectively, after a 0.5h incubation with the released CuDox complex, as compared to incubation with intact liposomes containing the complex. Total cellular doxorubicin fluorescence was similar following CuDox and free doxorubicin incubation. Imaging and mass spectrometry assays indicated that the CuDox complex was initially internalized intact but breaks down over time within cells, with intracellular copper decreasing more rapidly than intracellular doxorubicin. Doxorubicin fluorescence was reduced when complexed with copper, and nuclear fluorescence was reduced when cells were incubated with the CuDox complex as compared with free doxorubicin. Therapeutic efficacy, which typically results from intercalation of doxorubicin with DNA, was equivalent for the CuDox complex and free doxorubicin and was superior to that of liposomal doxorubicin formulations. Taken together, the results suggest that quenched CuDox reaches the nucleus and remains efficacious. In order to design protocols for the use of these temperature-sensitive particles in cancer treatment, the timing of hyperthermia relative to drug administration must be examined. When cells were heated to 42°C prior to the addition of free doxorubicin, nuclear drug accumulation increased by 1.8-fold in cancer cells after 5h, and cytotoxicity increased 1.4-fold in both cancer and endothelial cells. Endothelial cytotoxicity was similarly augmented with mild hyperthermia applied prior to treatment with released CuDox. In summary, we find that the drug-metal complex formed in temperature-sensitive particles can be internalized by cancer and endothelial cells resulting in therapeutic efficacy that is similar to free doxorubicin, and this efficacy can be enhanced by elevated temperature.

A selective decoy-doxorubicin complex for targeted co-delivery, STAT3 probing and synergistic anti-cancer effect

A novel selective decoy oligodeoxynucleotide (dODN)-doxorubicin (DOX) complex is reported for cancer theranostics. It eliminates the use of a ligand or carrier for targeted delivery and disassembles into therapeutic dODN and DOX upon encountering over-activated STAT3 in cancer cells. Hence, in situ STAT3 probing and synergistic anti-cancer effect are attained at the same time.

Improved drug delivery and therapeutic efficacy of PEgylated liposomal doxorubicin by targeting anti-HER2 peptide in murine breast tumor model

Targeted cancer therapy is a powerful therapeutic strategy to management of cancer. HER2 as an anticancer target has long been studied. Its overexpression plays an important role in the pathogenesis and progressiveness of breast and other cancers. To establish efficient and reliable drug delivery to HER2-overexpressing cells, the authors of this study have developed anti-HER2 (ErbB2) peptide-liposomal formulations of doxorubicin (DOX) by an engineered breast tumor-targeting peptide ligand, AHNP, Anti-HER2/neu peptide, (FCDGFYACYADV) with three glycine amino acids as spacer before its original sequencing. Towards this goal, PEGylated liposome doxorubicin (PLD) bearing different ligand densities of AHNP was prepared and characterized for their size, zeta potential and peptide conjugation. The AHNP functionalization and density effects on breast tumor cell uptake, selective cytotoxicity, prevention of tumor growth and the tissue biodistribution of encapsulated DOX were studied in mice bearing TUBO breast cancer tumor model. The findings demonstrated that increasing the ligand density of AHNP increases cytotoxicity and cell-uptake in SKBR3 and TUBO cells which overexpress HER2 but not in MDA-MB-231with low HER2 expression profile. The anticancer activity was also superior for targeted liposomal DOX with more AHNP densities. Overall, the results showed that optimum AHNP density functionalization of PLD can significantly improve selectivity and the therapeutic index of liposomal DOX in the treatment of HER2 positive breast cancer and merits further investigation.

Targeted liposomal drug delivery systems for the treatment of B cell malignancies

Nanoparticulate systems have demonstrated significant potential for overcoming the limitations of non-specific adverse effects related to chemotherapy. The treatment of blood malignancies employing targeted particulate drug delivery systems presents unique challenges and considerable research has been focused towards the development of targeted liposomal formulations for B cell malignancies. These formulations are aimed at achieving selectivity towards the malignant cells by targeting several cell surface markers which are over-expressed in that specific malignancy. CD19, CD20, CD22 and CD74 are few of such markers of which CD19, CD22 and CD74 are internalizing and CD20 is non-internalizing. Systems which have been developed to target both types of these cell surface markers are discussed. Specifically, the efficacy and development of targeted liposomes is considered. A number of studies have demonstrated the advantages of targeted liposomal systems encapsulating doxorubicin or vincristine. However, liposomal encapsulation of newer anti-neoplastic agents such as AD 198 which are superior to doxorubicin should be considered.

Monoclonal antibody-based therapies in cancer: advances and challenges

Conventional anticancer therapeutics often suffer from lack of specificity, resulting in toxicities to normal healthy tissues and poor therapeutic index. Antibody-mediated delivery of anticancer drugs or toxins to tumor cells through tumor selective or overexpressed antigens is progressively being recognized as an effective strategy for increasing the therapeutic index of anticancer drugs. In this review we focus on three therapeutic modalities in the field of antibody-mediated targeting, including antibody-drug conjugates (ADCs), immunotoxins (ITs) and immunoliposomes (ILs). Design considerations for development of each of the above therapeutic modalities are discussed. Furthermore, an overview of ADCs, ITs or ILs approved for use in clinical oncology and those currently in clinical development is provided. Challenges encountered by the field of antibody-based targeting are discussed and concepts around development of the next generation of antibody therapeutics are presented.

Enhanced growth inhibition of osteosarcoma by cytotoxic polymerized liposomal nanoparticles targeting the alcam cell surface receptor

Osteosarcoma is the most common primary malignancy of bone in children, adolescents, and adults. Despite extensive surgery and adjuvant aggressive high-dose systemic chemotherapy with potentially severe bystander side effects, cure is attainable in about 70% of patients with localized disease and only 20%-30% of those patients with metastatic disease. Targeted therapies clearly are warranted in improving our treatment of this adolescent killer. However, a lack of osteosarcoma-associated/specific markers has hindered development of targeted therapeutics. We describe a novel osteosarcoma-associated cell surface antigen, ALCAM. We, then, create an engineered anti-ALCAM-hybrid polymerized liposomal nanoparticle immunoconjugate (α-AL-HPLN) to specifically target osteosarcoma cells and deliver a cytotoxic chemotherapeutic agent, doxorubicin. We have demonstrated that α-AL-HPLNs have significantly enhanced cytotoxicity over untargeted HPLNs and over a conventional liposomal doxorubicin formulation. In this way, α-AL-HPLNs are a promising new strategy to specifically deliver cytotoxic agents in osteosarcoma.

Challenges in development of targeted liposomal therapeutics

Liposomes, phospholipid vesicles with a bilayered membrane structure, have been widely used as pharmaceutical carriers for drugs and genes, in particular for treatment of cancer. To enhance the efficacy of the liposomal drugs, drug-loaded liposomes are targeted to the tumors by means of passive (enhanced permeability and retention mediated) targeting, based on the longevity of liposomes in blood and its accumulation in pathological sites with compromised vasculature, and active targeting, based on the attachment of specific ligands to the liposomal surface to bind certain antigens on the target cells. Antibody-targeted liposomes loaded with anticancer drugs demonstrate high potential for clinical applications. This review highlights evolution of liposomes for both passive and active targeting and challenges in development of targeted liposomal therapeutics specifically antibody-targeted liposomes.

Immunoliposomal drug-delivery system targeting lectin-like oxidized low-density lipoprotein receptor–1 for carotid plaque lesions in rats

Object

Targeted drug delivery with immunoliposomes has been applied to various in vivo animal models and is newly focused as a novel therapeutic target. Lectin-like oxidized low-density lipoprotein receptor–1 (LOX1) is a potent regulator of systemic atherosclerosis, and the authors focused on its effect on carotid plaques. The authors developed a LOX1-targeted liposomal rho-kinase inhibitor and examined the therapeutic effect on carotid intimal hypertrophy in rats.

Methods

LOX1-targeted rho-kinase inhibitor fasudil-containing liposomes, composed of hydrogenated soy phosphatidylcholine/cholesterol/PEG2000-DSPE, were prepared by conjugating anti-LOX1 antibodies on the surface and by remote loading of fasudil. Carotid intimal hypertrophy was induced by balloon injury, and the drugs were intravenously administered on Day 3 postinjury. The rats were divided into 4 groups: nontreatment, treatment with intravenous fasudil (2 mg), treatment with liposomal fasudil (2 mg), and treatment with LOX1-targeted liposomal fasudil (2 mg). The authors compared intimal hypertrophy, atherosclerotic factor, and matrix metalloproteinase-9 expression among groups.

Results

DiI–labeled LOX1-targeted liposomes were prominently observed in the lesions on Day 7 after the surgery. The intimal thickness was significantly reduced in the LOX1-targeted liposomal fasudil–treated group (mean 81.6 ± 13.9 μm) compared with the other groups (no treatment 105.4 ± 16.8 μm; fasudil treatment 102.4 ± 20.0 μm; and liposomal fasudil treatment 102.8 ± 22.2 μm; p = 0.046). Matrix metalloproteinase-9 expression was also significantly reduced in the LOX1-targeted liposomal fasudil group.

Conclusions

Liposomes conjugated with anti-LOX1 antibody effectively reached carotid artery lesions, and liposomal rho-kinase significantly inhibited intimal hypertrophy. The new liposomal drug delivery system targeting LOX1 may become a therapeutic strategy for atherosclerotic diseases.

Targeting and internalization of sterically stabilized liposome modified with ZCH-4-2E8

Sterically stabilized liposome (SL) modified with 2E8 monoclonal antibody (2E8-SL) was prepared in order to evaluate its targeting ability and internalization efficiency against tumor cells with high expression of CD19. 2E8 was coupled to the surface of SL using post-insertion technique. The shape of liposomes was observed under a transmission electronic microscope. The average size of liposomes was determined by using the Zetasizer instrument. The binding and internalization of 2E8-SL against tumor cells with higher expression of CD19 were tested by flow cytometry and confocal microscopy. The mean diameter of 2E8-SL was 121.25 nm. 2E8-SL was stable after up to 24 days in various buffers. 2E8-SL showed specific binding to tumor cells with high expression of CD19, including B cells in peripheral blood mononuclear cells (PBMCs). 2E8-SL could efficiently internalize into Nalm6 cells with CD19 high expression. It is suggested that 2E8-SL may serve as useful delivery carrier of anti-cancer drugs targeting to hematological malignant tumors with CD19 high expression.

Cellular Association and Cytotoxicity of Doxorubicin-Loaded Immunoliposomes Targeted via Fab' Fragments of an Anti-CD74 Antibody

The purpose of our research was to evaluate in vitro therapeutic efficacy of doxorubicin (DXR)-loaded immunoliposomes with Fab' fragments of the anti-CD74 antibody LL1 attached to the surface. LL1 is well suited for targeting purposes because it is internalized very fast by B-lymphoma cells. However, at in vivo application whole antibodies show fast clearance in circulation. Taking this fact into consideration, this study was initiated to elucidate the prospects of using Fab' fragments of LL1 in stead of the whole antibody for future targeting in vivo of DXR-loaded liposomes. The Fab' fragments were covalently attached to the surface of sterically stabilized liposomes by use of a PEG-based heterobifunctinal coupling agent. LL1 Fab' conjugated sterically stabilized DXR liposomes showed approximately six times faster accumulation of the drug in Raji human B-lymphoma cells than nontargeted liposomes. In vitro cytotoxicity, quantitated by a tetrazolium assay, against Raji cells gave IC(50) values of 0.13, 0.45, and 0.11 microM for DXR-loaded immunoliposomes, DXR-loaded liposomes and free drug, respectively. The results from this study suggest that DXR-loaded immunoliposomes targeted with Fab' fragments from the anti-CD74 antibody LL1 could be a useful system for future in vivo experiments.

Rapid high throughput assay for fluorimetric detection of doxorubicin--application of nucleic acid-dye bioprobe

A double stranded DNA based fluorescence bioprobe for anticancer agent (doxorubicin) detection is described. This method provides a new way for sensitive DNA/drug interaction study by a homogeneous assay. The probe employs the long-wavelength intercalating fluorophore TOTO-3 (TT3). The anticancer agent, doxorubicin, which interacts with the DNA-TT3 complex, was indirectly measured by the decrease in the fluorescence intensity. Various oligonucleotides with different sequences were examined. Doxorubicin has preference for the oligonucleotide 5'AGCACG3'. Enhanced fluorescence observed for the TT3 intercalation with this oligonucleotide makes the DNA-dye complex a suitable bioprobe for doxorubicin detection by competitive assay. A home-built CCD camera setup was applied along with 384 well plate assay format for high throughput fluorescence imaging. The detection limit can be as low as 25 ng mL(-1) with an upper limit of 100 microg mL(-1). The recovery test with spiked serum sample shows that this method can be a potential routine method for therapeutic drug monitoring (TDM).

Expression and purification of two anti-CD19 single chain Fv fragments for targeting of liposomes to CD19-expressing cells

Antibody-targeted liposomal anticancer drugs combine the specificity of antibodies with large payloads of entrapped drugs. We previously showed that liposomal doxorubicin (DXR) targeted via anti-CD19 monoclonal antibodies (mAb) or their Fab' fragments against the B-cell antigen CD19 led to improved therapeutic effects in murine B-cell lymphoma models relative to non-targeted liposomal DXR. We now are examining the use of anti-CD19 single chain fragments of the antibody variable region (scFv) as a targeting moiety, to test the hypothesis that scFv have advantages over full-sized mAb or Fab' fragments. We expressed two different anti-CD19 scFv constructs, HD37-C and HD37-CCH in E. coli, and purified the scFvs using two different methods. The HD37-CCH construct was selected for coupling studies due to its relative stability and activity in comparison to HD37-C. When coupled to liposomes, the HD37-CCH scFv showed increased binding in vitro to CD19-positive Raji cells, compared to non-targeted liposomes. Cytotoxicity data showed that HD37-CCH scFv-targeted liposomes loaded with DXR were more cytotoxic than non-targeted liposomal DXR. Our results suggest that anti-CD19 scFv constructs should be explored further for their potential in treating B-lymphoid leukemias and lymphomas.

A dual-ligand approach for enhancing targeting selectivity of therapeutic nanocarriers

Conjugation of ligands to nano-scale drug carriers targeting over-expressed cell surface receptors is a promising approach for delivery of therapeutic agents to tumor cells. However, most commonly utilized ligands are directed at receptors expressed not only on target cells but also on other cells in the body, leading to unintended uptake in these off-target cells. In this study, a novel, dual-ligand approach is reported, which targets tumor cells while sparing off-target cells by exploiting the fact that tumor cells typically over-express multiple types of surface receptors. This approach was tested in the human KB cell line, which over-expresses both folate receptor (FR) and the epidermal growth factor receptor (EGFR). Liposomal nanocarriers loaded with doxorubicin and bearing controlled numbers of both folic acid and a monoclonal antibody against the EGFR were designed. Cytotoxicity was used to determine targeting selectivity of the designed carriers in vitro by utilizing KB cells expressing both FR and EGFR and off-target control cells in which one or both receptors were blocked. The data demonstrates that nanocarriers can be designed to achieve toxicity only when all targeted receptors are available, providing an approach to improve selectivity over current single-ligand approaches.

Determination of doxorubicin levels in whole tumor and tumor nuclei in murine breast cancer tumors

PURPOSE

Pharmacokinetic studies on liposomal drugs have previously measured total drug levels in tumors, which include non-bioavailable drug. However, drugs must be released from liposomes to have activity. We have developed a method for measuring levels of bioavailable (released) doxorubicin in vivo in tumors that will allow therapeutic activity to be correlated with bioavailable drug levels.

EXPERIMENTAL DESIGN

Mice orthotopically implanted with mammary carcinoma (4T1) were injected i.v. 10 days after implantation with free doxorubicin or formulations of liposomal doxorubicin with different drug release rates. Tumors were excised at various times after injection, and total tumor doxorubicin levels were determined by acidified isopropanol extraction of whole tumor homogenates. Bioavailable doxorubicin levels were determined by extraction of doxorubicin from isolated tumor nuclei.

RESULTS

Free doxorubicin had high levels of bioavailability in tumor tissue; 95% of the total doxorubicin in tumors was bound to nuclear DNA by 24 hours after injection. Administration of Doxil, a slow release liposomal formulation of doxorubicin, gave an area under the time-versus-concentration curve (AUC) for total doxorubicin 7 days after injection that was 87-fold higher than that obtained for free doxorubicin, and 49% of the liposomal doxorubicin was bioavailable. For liposomes with a more rapid doxorubicin release rate, by 7 days after injection, the AUC(0-7 days) for total doxorubicin was only 14-fold higher than that for free doxorubicin and only 27% of liposomal doxorubicin was bioavailable.

CONCLUSIONS

This technique allows correlations to be made between drug bioavailability and therapeutic activity and will help in the rational design of drug carriers.

Development of pH-sensitive liposomes that efficiently retain encapsulated doxorubicin (DXR) in blood

We have reported that targeted, pH-sensitive sterically stabilized liposomes are able to increase the cytotoxicity of DXR in vitro against B lymphoma cells, but the rate of release of DXR in plasma was too rapid to permit the results to be extended to in vivo applications. The purpose of the study reported here is two-fold. First, to understand the mechanism of the rapid release of DXR from pH-sensitive sterically stabilized liposomes (PSL) in human plasma. Second, to reformulate the above liposomes to improve their drug retention, while retaining their pH sensitivity. The stability of the PSL formulations in human plasma was evaluated by comparing the rate of release of encapsulated DXR with that of HPTS, a water-soluble fluorescent marker. Since DXR, but not HPTS, a water soluble-less membrane permeable fluorescence marker, was rapidly released from liposomes in the presence of plasma, the rapid release of DXR is likely caused by the diffusion of DXR molecules through the lipid bilayer, not by the disruption of the membrane. In order to develop more stable PSL formulations, various molar ratios of the membrane rigidifying lipid, hydrogenated soy HSPC and/or CHOL, were added to the lipid composition and the rate of release of encapsulated solutes and pH-sensitivity were evaluated. The compositions that showed the best drug retention and pH-sensitivity were a mixture of DOPE/HSPC/CHEMS/CHOL/mPEG(2000)-DSPE at a molar ratio of 4:2:2:2:0.3 and DOPE/HSPC/CHEMS/CHOL at a molar ratio of 4:2:2:2. Our formulations, if targeted to internalizing antigens on cancer cells, may increase intracellular drug release rates within acidic compartment, resulting in a further increase in the therapeutic efficacy of targeted anticancer drug-containing liposomes.

In Vitro Cytotoxicity of Stealth Liposomes Co-encapsulating Doxorubicin and Verapamil on Doxorubicin-Resistant Tumor Cells

Multidrug resistance (MDR) is a major obstacle to successful clinical cancer chemotherapy. A novel doxorubicin anti-resistant Stealth liposomes (DARSLs), prepared by co-encapsulating doxorubicin (DOX) and verapamil (VER) into stealth liposomes, has been developed. The average particle size of DARSLs was 118.1+/-22.3 nm. Encapsulation efficiencies of DOX and VER in DARSLs were greater than 95% and 70%, respectively. The IC(50) of DARSLs as measured by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay in multidrug resistant rat prostate cancer Mat-LyLu-B2 (MLLB2) cells was 0.079+/-0.017 microM, 13 fold less than that for liposomal DOX with free VER (LDFV 0.96+/-0.46 microM) but only about 2 times less than FDFV. The IC(50) cytotoxicity on MLLB2 cells of the various formulations was as follows: DARSLs approximately LDLV<FDFV<FDLV<LDFV<LD<FD, (LD: liposomal DOX; LV: liposomal VER; FD: free DOX; FV: free VER). Similar cytotoxicities were shown between DARSLs and FDFV in DOX-resistant human uterus sarcoma MES-SA/DX5 cells, reversing DOX-resistance to that shown by FD on DOX-sensitive MES-SA cells. For MLLB2 cells, DARSLs was the most cytotoxic, but its intracellular concentration of DOX, measured as mean cellular fluorescence with flow cytometry was lower (p<0.01) than that observed with the FDFV formulation. In conclusion, DARSLs was an effective DOX formulation which could overcome drug resistance in DOX-resistant tumor cells, but its mechanisms of action may be complex.

Antagonist G-mediated targeting and cytotoxicity of liposomal doxorubicin in NCI-H82 variant small cell lung cancer

The aim of the present study was to characterize the interactions of antagonist G (H-Arg-D-Trp-N(me)Phe-D-Trp-Leu-Met-NH 2)-targeted sterically stabilized liposomes with the human variant small cell lung cancer (SCLC) H82 cell line and to evaluate the antiproliferative activity of encapsulated doxorubicin against this cell line. Variant SCLC tumors are known to be more resistant to chemotherapy than classic SCLC tumors. The cellular association of antagonist G-targeted (radiolabeled) liposomes was 20-30-fold higher than that of non-targeted liposomes. Our data suggest that a maximum of 12,000 antagonist G-targeted liposomes were internalized/cell during 1-h incubation at 37 masculine C. Confocal microscopy experiments using pyranine-containing liposomes further confirmed that receptor-mediated endocytosis occurred, specifically in the case of targeted liposomes. In any of the previously mentioned experiments, the binding and endocytosis of non-targeted liposomes have revealed to be negligible. The improved cellular association of antagonist G-targeted liposomes, relative to non-targeted liposomes, resulted in an enhanced nuclear delivery (evaluated by fluorimetry) and cytotoxicity of encapsulated doxorubicin for incubation periods as short as 2 h. For an incubation of 2 h, we report IC50 values for targeted and non-targeted liposomes containing doxorubicin of 5.7 +/- 3.7 and higher than 200 micro M doxorubicin, respectively. Based on the present data, we may infer that receptors for antagonist G were present in H82 tumor cells and could mediate the internalization of antagonist G-targeted liposomes and the intracellular delivery of their content. Antagonist G covalently coupled to liposomal drugs may be promising for the treatment of this aggressive and highly heterogeneous disease.

Improved outcome when B-cell lymphoma is treated with combinations of immunoliposomal anticancer drugs targeted to both the CD19 and CD20 epitopes

PURPOSE

We have reported previously that successful immunoliposomal drug therapy with liposomal doxorubicin (DXR) against xenograft B-lymphoma models required targeting against an internalizing B-cell antigen, CD19 (P. Sapra and T. M. Allen. Cancer Res 2002;62:7190-4.). Here we compare targeting of immunoliposomal formulations of DXR with vincristine (VCR) targeted against CD19 versus a noninternalizing (CD20) epitope. We also examine the effect of targeting immunoliposomes with antibody combinations in an attempt to increase the total number of binding sites (apparent antigen density) at the target cell surface.

EXPERIMENTAL DESIGN

Cell association of immunoliposomes (CD19-targeted, CD20-targeted, or combinations of the two) with human B-cell lymphoma (Namalwa) cells were studied using radiolabeled liposomes. Therapeutic efficacy of the same formulations was determined in a severe combined immunodeficient murine model.

RESULTS

Therapeutic results in severe combined immunodeficient mice bearing Namalwa cells administered anti-CD20-targeted liposomal DXR were barely improved over those found for nontargeted liposomal DXR or free DXR but, surprisingly, administration of anti-CD20-targeted liposomal VCR resulted in a significantly improved therapeutic outcome compared with nontargeted liposomal VCR, free VCR, or anti-CD20-targeted liposomal DXR. Treatment of murine B lymphoma with single injections of combinations of anti-CD19- and anti-CD20-targeted liposomal VCR led to cures in 70% of mice. However, mice injected with similar combinations of liposomal DXR did not have improved survival rates over anti-CD19-targeted liposomal DXR by itself.

CONCLUSIONS

The success of immunoliposomal therapy in combination regimens varies with the type of encapsulated drug and the nature of the target epitopes.

Improved Therapeutic Responses in a Xenograft Model of Human B Lymphoma (Namalwa) for Liposomal VincristineversusLiposomal Doxorubicin Targeted via Anti-CD19 IgG2a or Fab′ Fragments

PURPOSE

Monoclonal antibody-mediated targeting of liposomal anticancer drugs to surface antigens expressed on malignant B cells can be an effective strategy for treating B-cell malignancies. In a murine model of human B-cell lymphoma, we have made in vitro and in vivo comparisons of long-circulating sterically stabilized (Stealth) immunoliposome (SIL) formulations of two anticancer drugs, vincristine (VCR) and doxorubicin (DXR), with different mechanisms of action and drug release rates.

EXPERIMENTAL DESIGN

SIL formulations of VCR or DXR were conjugated to the monoclonal antibody anti-CD19 (SIL[alphaCD19]) or its Fab' fragments (SIL[Fab']). Specific binding of SILs to Namalwa cells was studied using radiolabeled liposomes, and cytotoxicities of DXR- or VCR-loaded SILs were quantitated by a tetrazolium assay. Pharmacokinetic and drug leakage experiments were performed in mice using dual-labeled liposomes, and the therapeutic responses of SILs were evaluated in a Namalwa (human B lymphoma) cell xenograft model.

RESULTS

SIL[alphaCD19] or SIL[Fab'] had higher association with and cytotoxicity against Namalwa cells than nontargeted liposomes. SIL[Fab'] had longer circulation times than SIL[alphaCD19], and VCR had faster release rates from the liposomes than DXR. SIL formulations of either VCR or DXR had significantly better therapeutic outcomes than nontargeted liposomes or free drugs. SILs loaded with VCR were superior to those loaded with DXR. SIL[Fab'] had better therapeutic outcomes than SIL[alphaCD19] for the drug DXR but were equally efficacious for the drug VCR.

CONCLUSIONS

Treatment of a B lymphoma model with single injections of anti-CD19-targeted liposomal formulations of VCR resulted in high levels of response and long-term survivors. Responses to anti-CD19-targeted liposomal DXR were more modest, although the longer circulation times of SIL[Fab'] versus SIL[alphaCD19] led to superior therapeutics for DXR-loaded immunoliposomes.

Cellular association and cytotoxicity of anti-CD74-targeted lipid drug-carriers in B lymphoma cells

The ability to selectively target anti-cancer drugs via specific ligands against antigens expressed on malignant cells could greatly improve the therapeutic indices of the drugs. In this paper an anti-CD74 antibody (Ab), LL1, was covalently attached to the surface of sterically stabilized lipid drug-carriers (emulsions and liposomes) by use of a PEG-based heterobifunctional coupling agent. Target cells internalize LL1 very fast and that was found to be true for the LL1-lipid drug-carrier complexes as well. During a 24 h in vitro incubation with the target Raji B-lymphoma cells about 30% of the added complexes were associated with the cells. The corresponding value for drug-carrier without targeting ligand was near 0.6%. Displacement experiments showed that free LL1 competed well with LL1-complexes indicating preserved immunoreactivity. Non-target cells showed only unspecific association of LL1-complexes. A dioleoylated derivative of the anti-cancer drug 3',5'-O-dioleoyl-FUdR (FUdR) (FUdR-dO) loaded into LL1-lipid drug-carriers showed good cytotoxic activity. In vitro 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cytotoxicity tests against neoplastic B-cells gave IC30 values of 0.45, 1.25, 5.30 and 7.30 microM for the prodrug FUdR-dO in LL1-emulsions, LL1-liposomes, emulsions and liposomes, respectively. The value for the parent drug FUdR was calculated to 4.35 microM. In the light of the extensive and specific delivery of LL1-lipid drug-carriers to B-cells and the selective cytotoxicity of the incorporated drug, we infer that the complexes may be useful in the selective elimination of circulating malignant B-cells in vivo.

Ligand-targeted liposomal anticancer drugs

Antibody or ligand-mediated targeting of liposomal anticancer drugs to antigens expressed selectively or over-expressed on tumor cells is increasingly being recognized as an effective strategy for increasing the therapeutic indices of anticancer drugs. This review summarizes some recent advances in the field of ligand-targeted liposomes (LTLs) for the delivery of anticancer drugs. New approaches used in the design and optimization of LTLs is discussed and the advantages and potential problems associated with their therapeutic applications are described. New technologies are widening the spectrum of ligands available for targeting and are allowing choices to be made regarding affinity, internalization and size. The time is rapidly approaching where we will see translation of anticancer drugs entrapped in LTLs to the clinic.

Ligand-targeted therapeutics in anticancer therapy

Cytotoxic chemotherapy or radiotherapy of cancer is limited by serious, sometimes life-threatening, side effects that arise from toxicities to sensitive normal cells because the therapies are not selective for malignant cells. So how can selectivity be improved? One strategy is to couple the therapeutics to antibodies or other ligands that recognize tumour-associated antigens. This increases the exposure of the malignant cells, and reduces the exposure of normal cells, to the ligand-targeted therapeutics.

Adventures in targeting

An overview of our experiences in the field of immunoliposomal anticancer drugs is provided with respect to choice of ligand, and choice of model system, in order to provide some guidance as to the rational use of this new technology. Liposomes targeted by either peptide or monoclonal antibodies showed significantly higher binding to their respective target cells in vitro compared to non-targeted liposomes in all model systems examined. This higher binding led to higher cytotoxicities relative to non-targeted liposomes. For the immunoliposomes to deliver their entrapped drug to target cell in vivo, long circulations half-lives are required. We have evaluated the pharmacokinetics of liposomes prepared by several different coupling techniques, and have found significant differences in the clearance of these immunoliposomes from the circulation. Immunoliposomes prepared with whole anti-CD19 IgG coupled by the Mal-PEG-DSPE method demonstrated a short plasma half-life, which may reflect the random orientation of the MAb on the liposome surface. Coupling methods that mask or eliminate the Fc region result in immunoliposomes that have clearance rates more similar to untargeted liposomes. Insertion of peptides or antibodies into pre-formed liposomes through incubation with ligand-coupled PEG micelles resulted in immunoliposomes, termed post-insertion liposomes, that demonstrated comparable in vitro binding, pharmacokinetic profiles and in vivo therapeutic efficacy to liposomes made by conventional coupling methods. The therapeutic efficacy of liposomes, prepared by various coupling methods and targeted by different ligands, was compared in several different animal models of either haematological malignancies, pseudometastatic disease or solid tumours. In our hands, successful in vivo targeting has been obtained when the target is either small or readily accessible from the vasculature, where the liposomes have longer circulating half-lives and/or where a ligand against an internalizing epitope has been chosen. These results should aid in the rational design of applications for immunoliposomal drugs in the future.

Targeted delivery of indinavir to HIV-1 primary reservoirs with immunoliposomes

The tissue distribution of indinavir, free or incorporated into sterically stabilized anti-HLA-DR immunoliposomes, has been evaluated after a single subcutaneous injection to C3H mice. Administration of free indinavir resulted in low drug levels in lymphoid organs. In contrast, sterically stabilized anti-HLA-DR immunoliposomes were very efficient in delivering high concentrations of indinavir to lymphoid tissues for at least 15 days post-injection increasing by up to 126 times the drug accumulation in lymph nodes. The efficacy of free and immunoliposomal indinavir has been evaluated in vitro. Results showed that immunoliposomal indinavir was as efficient as the free agent to inhibit HIV-1 replication in cultured cells. The toxicity and immunogenicity of repeated administrations of liposomal formulations have also been investigated in rodents. No significant differences in the levels of hepatic enzymes of mice treated with free or liposomal indinavir were observed when compared to baseline and control untreated mice. Furthermore, histopathological studies revealed no significant damage to liver and spleen when compared to the control group. Liposomes bearing Fab' fragments were 2.3-fold less immunogenic than liposomes bearing the entire IgG. Incorporation of antiviral agents into sterically stabilized immunoliposomes could represent a novel therapeutic strategy to target specifically HIV reservoirs and treat more efficiently this retroviral infection.

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.

A growth factor antagonist as a targeting agent for sterically stabilized liposomes in human small cell lung cancer

The ability of a growth factor antagonist, [D-Arg(6),D-Trp(7,9)-N(me)Phe(8)]-substance P(6-11), named antagonist G, to selectively target polyethylene glycol-grafted liposomes (known as sterically stabilized liposomes) to a human classical small cell lung cancer (SCLC) cell line, H69, was examined. Our results showed that radiolabeled antagonist G-targeted sterically stabilized liposomes (SLG) bound to H69 cells with higher avidity than free antagonist G and were internalized (reaching a maximum of 13000 SLG/cell), mainly through a receptor-mediated process, likely involving clathrin-coated pits. This interaction was confirmed by confocal microscopy to be peptide- and cell-specific. Moreover, it was shown that SLG significantly improved the nuclear delivery of encapsulated doxorubicin to the target cells, increasing the cytotoxic activity of the drug over non-targeted liposomes. In mice, [(125)I]tyraminylinulin-containing SLG were long circulating, with a half-life of 13 h. Use of peptides like antagonist G to promote binding and internalization of sterically stabilized liposomes, with their accompanying drug loads, i.e., anticancer drugs, genes or antisense oligonucleotides, into target cells has the potential to improve therapy of SCLC.

In vitro and in vivo comparison of immunoliposomes made by conventional coupling techniques with those made by a new post-insertion approach

Ligand-targeted liposomes have the potential to increase the therapeutic efficacy of antineoplastic agents. Recently, a combinatorial approach to the preparation of ligand-targeted liposomes has been developed, termed the post-insertion technique, which will facilitate the production of targeted liposomes. In this paper, Stealth immunoliposomes (SIL) coupled to anti-CD19 made by either a conventional coupling technique (SIL[anti-CD19]), or by the post-insertion technique (PIL[anti-CD19], were compared with respect to their in vitro binding and cytotoxicity and their ability to improve in vivo survival in tumor-bearing mice. The in vitro binding and uptake of PIL[anti-CD19] by CD19-expressing, B-cell lymphoma (Namalwa) cells was similar to that of SIL[anti-CD19] and both were significantly higher than binding of non-targeted liposomes (SL). In addition, no significant differences were found between the respective in vitro cytotoxicities of doxorubicin-loaded PIL[anti-CD19] or SIL[anti-CD19], or in their in vivo therapeutic efficacy in a murine model of human B-lymphoma. Overall, the results demonstrate that the post-insertion technique is a simple, flexible and effective means for preparing targeted liposomal drugs for clinical applications.

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.

Specific binding of sterically stabilized anti-B-cell immunoliposomes and cytotoxicity of entrapped doxorubicin

Administration of doxorubicin (DXR) formulated in sterically stabilized liposomes, (SL) containing engrafted poly(ethylene glycol)-modified phosphatidylethanolamine (PEG-PE) on their surface, has been shown to increase the therapeutic index of the drug. A further improvement could be achieved through targeting of liposome-entrapped drug selectively to cancer cells. This paper describes the conjugation of the anti-B-cell lymphoma monoclonal antibody LL2 to the surface of DXR-loaded liposomes by use of a PEG-based heterobifunctional coupling agent. Competitive-binding ELISA of the resulting immunoliposomes (SIL) against the monoclonal anti-idiotype antibody, WN, indicated preserved immunological activity. The pH-sensitive probe, HPTS was used to study the binding of liposomes with target cells. The results showed a 3.8-fold increased cellular association of SIL compared to that of SL and an apparent internalization of SIL into low pH compartments. Addition of an excess of unconjugated free LL2 displaced about 72% of the HPTS-SIL association with cells. Experiments with 125I-labeled free and SIL-bound LL2 showed approximately 50% degradation for both preparations. In vitro MTT cytotoxicity tests against neoplastic B cells gave IC(50) values of 1.6, 2.9 and 0.35 microM for DXR-SIL, DXR-SL and free DXR, respectively. Leakage of drug from the liposomes apparently reduced the specificity of the cytotoxic action of DXR-SIL.

Internalizing antibodies and targeted cancer therapy: direct selection from phage display libraries

Antibody internalization is required for the success of many targeted therapeutics, such as immunotoxins, immunoliposomes, antibody-drug conjugates and for the targeted delivery of genes or viral DNA into cells. Recently, it has become possible to directly select antibody fragments from phage display libraries for internalization into mammalian cells. Here we review the therapeutic applications of internalized antibodies and describe how phage display enables the isolation of internalizing antibodies to novel or known targets.

Selective targeting of immunoliposomal doxorubicin against human multiple myeloma in vitro and ex vivo

Circulating malignant CD19(+) B cells have been implicated in the pathogenesis and relapse of multiple myeloma (MM). This study investigated the therapeutic applicability of using long-circulating liposome-encapsulated doxorubicin (DXR) targeted against the internalizing CD19 antigens present on human MM cells. In vitro binding studies using the CD19(+) MM cell line ARH77 demonstrated that CD19-directed immunoliposomes (SIL[anti-CD19]) specifically attached to these cells. Formulations of immunoliposomal doxorubicin (DXR-SIL[anti-CD19]) showed a higher association with, and higher cytotoxicity against, ARH77 cells than did non-targeted liposomal doxorubicin (DXR-SL) or isotype-matched controls (DXR-NSIL[IgG2a]). By using the pH-sensitive fluorophore, 1-hydroxypyrene-3,6, 8-trisulfonic acid, binding of SIL[anti-CD19] to CD19 antigens was shown to trigger receptor-mediated internalization of the antibody-antigen complexes into endosomes. Targeting of SIL[anti-CD19] to CD19(+) B cells was also demonstrated in a heterogeneous mixture of peripheral blood mononuclear cells (PBMC) from MM patients. A decrease in cellular DNA (which is an indicator of apoptosis) caused by the cytotoxicity of DXR-SIL[anti-CD19] to myeloma PBMC was determined by using flow cytometry. While PBMC treatment with free DXR resulted in non-specific cytotoxicity to both B and T cells, DXR-SL were only minimally cytotoxic to either. In contrast, DXR-SIL[anti-CD19] were selectively cytotoxic for B cells in PBMC, indicating that this treatment may be effective in eliminating circulating malignant B cells in MM patients.

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 microg antibody/micromol 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.