Antitumor effects of a monoclonal antibody that binds anionic phospholipids on the surface of tumor blood vessels in mice

Trial watch: Monoclonal antibodies in cancer therapy

During the past 20 years, dozens-if not hundreds-of monoclonal antibodies have been developed and characterized for their capacity to mediate antineoplastic effects, either as they activate/enhance tumor-specific immune responses, either as they interrupt cancer cell-intrinsic signal transduction cascades, either as they specifically delivery toxins to malignant cells or as they block the tumor-stroma interaction. Such an intense research effort has lead to the approval by FDA of no less than 14 distinct molecules for use in humans affected by hematological or solid malignancies. In the inaugural issue of OncoImmunology, we briefly described the scientific rationale behind the use of monoclonal antibodies in cancer therapy and discussed recent, ongoing clinical studies investigating the safety and efficacy of this approach in patients. Here, we summarize the latest developments in this exciting area of clinical research, focusing on high impact studies that have been published during the last 15 months and clinical trials launched in the same period to investigate the therapeutic profile of promising, yet hitherto investigational, monoclonal antibodies.

Profile of bavituximab and its potential in the treatment of non-small-cell lung cancer

Bavituximab is a an unconjugated, chimeric immunoglobulin G1 (IgG1) monoclonal antibody directed against the phosphatidylserine (PS) expressed on tumor endothelium, with a specific mechanism of action. PS is an anionic membrane phospholipid, physiologically restricted to the internal membrane leaflet; various pathophysiologic processes cause the exposure of PS on the external membrane leaflet. Bavituximab, once bound, starts up host effector activities, such as antibody dependent cellular cytotoxicity, causing vessel destruction and enhancing antitumor immunity. Phase I clinical trials of bavituximab administered as monotherapy or in combination with other chemotherapeutic agents in adults with pretreated solid tumors have been accomplished, indicating that bavituximab can be safely dispensed weekly, with a recommended dose of 1 and 3 mg/kg. A Phase II randomized, placebo-controlled trial of bavituximab plus docetaxel, in the second-line therapy setting of locally advanced or metastatic non-small-cell lung cancer, has been conducted and recently presented, suggesting a clinical benefit of the combination, with an overall response rate of 17% and median overall survival of more than 11 months. A Phase III trial is currently ongoing. Bavituximab has been studied in combination with platinum-based doublets with promising results. In the present paper we summarize the preclinical development and clinical experience with bavituximab in non-small-cell lung cancer.

Bavituximab plus paclitaxel and carboplatin for the treatment of advanced non-small-cell lung cancer

OBJECTIVE

Bavituximab is a phosphatidylserine (PS)-targeting monoclonal antibody with immune-modulating and tumor-specific vascular targeting properties. Preclinical studies have shown activity against numerous solid tumors and at least an additive effect in combination with chemotherapy. This study evaluated bavituximab in combination with paclitaxel and carboplatin in patients with previously untreated, locally advanced or metastatic non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

This phase II, open-label study (NCT00687817) was conducted in 49 patients with stage IIIB/IV NSCLC utilizing a Simon two-stage design. Patients were treated with up to six cycles of carboplatin area under the concentration-time curve (AUC) 5 plus paclitaxel 175 mg/m2 every 21 days with weekly bavituximab 3 mg/kg followed by bavituximab monotherapy until progression or unacceptable toxicity.

RESULTS

The primary efficacy endpoint of overall response rate (ORR) was 40.8% (complete response [CR] 2.0%, partial response [PR] was 38.8%). Median progression-free survival (PFS) and overall survival (OS) were 6.0 and 12.4 months, respectively. Treatment-related adverse events (AEs) occurred in 40.8% of patients. The most common treatment-related AEs were anemia (10.2%), asthenia, vomiting, paresthesia, anorexia, and fatigue (6.1% each). One patient with a central, cavitating squamous tumor developed fatal hemoptysis and aspiration.

CONCLUSION

Bavituximab in combination with paclitaxel-carboplatin as first-line therapy demonstrated a tolerable safety profile and potential efficacy in this single-arm phase II trial in patients with advanced local or metastatic NSCLC. Randomized trials with this regimen are in progress.

CLINICALTRIALSGOV IDENTIFIER

NCT00687817.

Phosphatidylserine-targeted bimodal liposomal nanoparticles for in vivo imaging of breast cancer in mice

Phosphatidylserine (PS) that is normally constrained to the inner plasma membrane becomes exposed on the surface of endothelial cells (ECs) in tumor vasculature. In the present study, we report the development of a novel tumor vasculature-targeted liposomal nanoprobe by conjugating a human monoclonal antibody, PGN635 that specifically targets PS to polyethylene glycol-coated liposomes. MR contrast, superparamagnetic iron oxide nanoparticles (SPIO) were packed into the core of liposomes, while near-infrared dye, DiR was incorporated into the lipophilic bilayer. The liposomal nanoprobe PGN-L-IO/DiR was fully characterized, and its binding specificity and subsequent internalization into PS-exposed vascular ECs was confirmed by in vitro MRI and histological staining. In vivo longitudinal MRI and optical imaging were performed after i.v. injection of the liposomal nanoprobes into mice bearing breast MDA-MB231 tumors. At 9.4T, T2-weighted MRI detected drastic reduction on signal intensity and T2 values of tumors at 24h. Ionizing radiation significantly increased PS exposure on tumor vascular ECs, resulting in a further MRI signal loss of tumors. Concurrent with MRI, optical imaging revealed a clear tumor contrast at 24h. Intriguingly, PGN-L-IO/DiR exhibited distinct pharmacokinetics and biodistribution with significantly reduced accumulations in liver or spleen. Localization of PGN-L-IO/DiR to tumor was antigen specific, since a control probe of irrelevant specificity showed minimal accumulation in the tumors. Our studies indicate that PS-targeted liposomes may provide a useful platform for tumor-targeted delivery of imaging contrast agents or potentially anti-cancer drugs for cancer theranostics.

A combination of radiosurgery and soluble tissue factor enhances vascular targeting for experimental glioblastoma

Radiosurgery for glioblastoma is limited to the development of resistance, allowing tumor cells to survive and initiate tumor recurrence. Based on our previous work that coadministration of tissue factor and lipopolysaccharide following radiosurgery selectively induced thrombosis in cerebral arteriovenous malformations, achieving thrombosis of 69% of the capillaries and 39% of medium sized vessels, we hypothesized that a rapid and selective shutdown of the capillaries in glioblastoma vasculature would decrease the delivery of oxygen and nutrients, reducing tumor growth, preventing intracranial hypertension, and improving life expectancy. Glioblastoma was formed by implantation of GL261 cells into C57Bl/6 mouse brain. Mice were intravenously injected tissue factor, lipopolysaccharide, a combination of both, or placebo 24 hours after radiosurgery. Control mice received both agents after sham irradiation. Coadministration of tissue factor and lipopolysaccharide led to the formation of thrombi in up to 87 ± 8% of the capillaries and 46 ± 4% of medium sized vessels within glioblastoma. The survival rate of mice in this group was 80% versus no survivor in placebo controls 30 days after irradiation. Animal body weight increased with time in this group (r = 0.88, P = 0.0001). Thus, radiosurgery enhanced treatment with tissue factor, and lipopolysaccharide selectively induces thrombosis in glioblastoma vasculature, improving life expectancy.

Purine nucleoside phosphorylase targeted by annexin v to breast cancer vasculature for enzyme prodrug therapy

BACKGROUND AND PURPOSE

The targeting of therapeutics is a promising approach for the development of new cancer treatments that seek to reduce the devastating side effects caused by the systemic administration of current drugs. This study evaluates a fusion protein developed as an enzyme prodrug therapy targeted to the tumor vasculature. Cytotoxicity would be localized to the site of the tumor using a protein fusion of purine nucleoside phosphorylase (PNP) and annexin V. Annexin V acts as the tumor-targeting component of the fusion protein as it has been shown to bind to phosphatidylserine expressed externally on cancer cells and the endothelial cells of the tumor vasculature, but not normal vascular endothelial cells. The enzymatic component of the fusion, PNP, converts the FDA-approved cancer therapeutic, fludarabine, into a more cytotoxic form. The purpose of this study is to determine if this system has a good potential as a targeted therapy for breast cancer.

METHODS

A fusion of E. coli purine nucleoside phosphorylase and human annexin V was produced in E. coli and purified. Using human breast cancer cell lines MCF-7 and MDA-MB-231 and non-confluent human endothelial cells grown in vitro, the binding strength of the fusion protein and the cytotoxicity of the enzyme prodrug system were determined. Endothelial cells that are not confluent expose phosphatidylserine and therefore mimic the tumor vasculature.

RESULTS

The purified recombinant fusion protein had good enzymatic activity and strong binding to the three cell lines. There was significant cell killing (p<0.001) by the enzyme prodrug treatment for all three cell lines, with greater than 80% cytotoxicity obtained after 6 days of treatment.

CONCLUSION

These results suggest that this treatment could be useful as a targeted therapy for breast cancer.

Inhibition of nitric oxide and antiphospholipid antibody-mediated thrombosis

The antiphospholipid syndrome (APS) is characterized by recurrent vascular thrombosis, thrombocytopenia, and fetal loss occurring in the presence of antiphospholipid antibodies (aPL). Along with arterial and venous thrombosis and pregnancy complications, patients with APS have an increased risk of myocardial infarction, stroke, and coronary artery disease, resulting from vascular cell dysfunction induced by aPL. Accumulating evidence to date indicates that interactions between circulating aPL and cell surface molecules of target cells, primarily endothelial cells and platelets, underlie the vascular disease phenotypes of APS. However, the molecular basis of APS is poorly understood. Nitric oxide produced by endothelial cells is a key determinant of vascular health that regulates several physiologic processes, including thrombosis, endothelial-leukocyte interaction, vascular cell migration, and the modulation of vascular tone. This review will discuss recent findings that indicate a novel mechanism by which aPL antagonize endothelial cell production of nitric oxide and thereby promote thrombosis.

Phosphatidylserine-targeting antibody induces M1 macrophage polarization and promotes myeloid-derived suppressor cell differentiation

Multiple tumor-derived factors are responsible for the accumulation and expansion of immune-suppressing myeloid-derived suppressor cells (MDSC) and M2-like tumor-associated macrophages (TAM) in tumors. Here, we show that treatment of tumor-bearing mice with docetaxel in combination with the phosphatidylserine-targeting antibody 2aG4 potently suppressed the growth and progression of prostate tumors, depleted M2-like TAMs, and MDSCs, and increased the presence of M1-like TAMs and mature dendritic cells in the tumors. In addition, the antibody markedly altered the cytokine balance in the tumor microenvironment from immunosuppressive to immunostimulatory. In vitro studies confirmed that 2aG4 repolarized TAMs from an M2- to an M1-like phenotype and drove the differentiation of MDSCs into M1-like TAMs and functional dendritic cells. These data suggest that phosphatidylserine is responsible for the expansion of MDSCs and M2-like TAMs in tumors, and that bavituximab, a phosphatidylserine-targeting antibody currently in clinical trials for cancer, could reverse this process and reactivate antitumor immunity.

Systemic delivery of SapC-DOPS has antiangiogenic and antitumor effects against glioblastoma

Saposin C-dioleoylphosphatidylserine (SapC-DOPS) nanovesicles are a nanotherapeutic which effectively target and destroy cancer cells. Here, we explore the systemic use of SapC-DOPS in several models of brain cancer, including glioblastoma multiforme (GBM), and the molecular mechanism behind its tumor-selective targeting specificity. Using two validated spontaneous brain tumor models, we demonstrate the ability of SapC-DOPS to selectively and effectively cross the blood-brain tumor barrier (BBTB) to target brain tumors in vivo and reveal the targeting to be contingent on the exposure of the anionic phospholipid phosphatidylserine (PtdSer). Increased cell surface expression of PtdSer levels was found to correlate with SapC-DOPS-induced killing efficacy, and tumor targeting in vivo was inhibited by blocking PtdSer exposed on cells. Apart from cancer cell killing, SapC-DOPS also exerted a strong antiangiogenic activity in vitro and in vivo. Interestingly, unlike traditional chemotherapy, hypoxic cells were sensitized to SapC-DOPS-mediated killing. This study emphasizes the importance of PtdSer exposure for SapC-DOPS targeting and supports the further development of SapC-DOPS as a novel antitumor and antiangiogenic agent for brain tumors.

Anticancer therapy by tumor vessel infarction with polyethylene glycol conjugated retargeted tissue factor

tTF-NGR consists of the extracellular domain of tissue factor and the peptide GNGRAHA, a ligand of the surface protein aminopeptidase N and of integrin αvβ3. Both surface proteins are upregulated on endothelial cells of tumor vessels. tTF-NGR shows antitumor activity in xenografts and inhibition of tumor blood flow in cancer patients. We performed random TMS(PEG)12 PEGylation of tTF-NGR to improve the antitumor profile of the molecule. PEGylation resulted in an approximately 2-log step decreased procoagulatory activity of the molecule. Pharmacokinetic studies in mice showed a more than 1-log step higher mean area under the curve. Comparison of the LD10 values for both compounds and their lowest effective antitumor dose against human tumor xenografts showed an improved therapeutic range (active/toxic dose in mg/kg body weight) of 1/5 mg/kg for tTF-NGR and 3/>160 mg/kg for TMS(PEG)12 tTF-NGR. Results demonstrate that PEGylation can significantly improve the therapeutic range of tTF-NGR.

Phosphatidylserine synthase 2: high efficiency for synthesizing phosphatidylserine containing docosahexaenoic acid

Phosphatidylserine (PS), the major anionic phospholipid in eukaryotic cell membranes, is synthesized by the integral membrane enzymes PS synthase 1 (PSS1) and 2 (PSS2). PSS2 is highly expressed in specific tissues, such as brain and testis, where docosahexaenoic acid (DHA, 22:6n-3) is also highly enriched. The purpose of this work was to characterize the hydrocarbon-chain preference of PSS2 to gain insight on the specialized role of PSS2 in PS accumulation in the DHA-abundant tissues. Flag-tagged PSS2 was expressed in HEK cells and immunopurified in a functionally active form. Purified PSS2 utilized both PE plasmalogen and diacyl PE as substrates. Nevertheless, the latter was six times better utilized, indicating the importance of an ester linkage at the sn-1 position. Although no sn-1 fatty acyl preference was noted, PSS2 exhibited significant preference toward DHA compared with 18:1 or 20:4 at the sn-2 position. Preferential production of DHA-containing PS (DHA-PS) was consistently observed with PSS2 purified from a variety of cell lines as well as with microsomes from mutant cells in which PS synthesis relies primarily on PSS2. These findings suggest that PSS2 may play a key role in PS accumulation in brain and testis through high activity toward DHA-containing substrates that are abundant in these tissues.

Dying cell clearance and its impact on the outcome of tumor radiotherapy

The induction of tumor cell death is one of the major goals of radiotherapy and has been considered to be the central determinant of its therapeutic outcome for a long time. However, accumulating evidence suggests that the success of radiotherapy does not only derive from direct cytotoxic effects on the tumor cells alone, but instead might also depend – at least in part – on innate as well as adaptive immune responses, which can particularly target tumor cells that survive local irradiation. The clearance of dying tumor cells by phagocytic cells of the innate immune system represents a crucial step in this scenario. Dendritic cells and macrophages, which engulf, process and present dying tumor cell material to adaptive immune cells, can trigger, skew, or inhibit adaptive immune responses, respectively. In this review we summarize the current knowledge of different forms of cell death induced by ionizing radiation, the multi-step process of dying cell clearance, and its immunological consequences with special regard toward the potential exploitation of these mechanisms for the improvement of tumor radiotherapy.

Vascular endothelial-targeted therapy combined with cytotoxic chemotherapy induces inflammatory intratumoral infiltrates and inhibits tumor relapses after surgery

Surgery is the most effective therapy for cancer in the United States, but disease still recurs in more than 40% of patients within 5 years after resection. Chemotherapy is given postoperatively to prevent relapses; however, this approach has had marginal success. After surgery, recurrent tumors depend on rapid neovascular proliferation to deliver nutrients and oxygen. Phosphatidylserine (PS) is exposed on the vascular endothelial cells in the tumor microenvironment but is notably absent on blood vessels in normal tissues. Thus, PS is an attractive target for cancer therapy after surgery. Syngeneic mice bearing TC1 lung cancer tumors were treated with mch1N11 (a novel mouse chimeric monoclonal antibody that targets PS), cisplatin (cis), or combination after surgery. Tumor relapses and disease progression were decreased 90% by combination therapy compared with a 50% response rate for cis alone (P = .02). Mice receiving postoperative mch1N11 had no wound-related complications or added systemic toxicity in comparison to control animals. Mechanistic studies demonstrated that the effects of mch1N11 were associated with a dense infiltration of inflammatory cells, particularly granulocytes. This strategy was independent of the adaptive immune system. Together, these data suggest that vascular-targeted strategies directed against exposed PS may be a powerful adjunct to postoperative chemotherapy in preventing relapses after cancer surgery.

Development of anti-infectives using phage display: biological agents against bacteria, viruses, and parasites

The vast majority of anti-infective therapeutics on the market or in development are small molecules; however, there is now a nascent pipeline of biological agents in development. Until recently, phage display technologies were used mainly to produce monoclonal antibodies (MAbs) targeted against cancer or inflammatory disease targets. Patent disputes impeded broad use of these methods and contributed to the dearth of candidates in the clinic during the 1990s. Today, however, phage display is recognized as a powerful tool for selecting novel peptides and antibodies that can bind to a wide range of antigens, ranging from whole cells to proteins and lipid targets. In this review, we highlight research that exploits phage display technology as a means of discovering novel therapeutics against infectious diseases, with a focus on antimicrobial peptides and antibodies in clinical or preclinical development. We discuss the different strategies and methods used to derive, select, and develop anti-infectives from phage display libraries and then highlight case studies of drug candidates in the process of development and commercialization. Advances in screening, manufacturing, and humanization technologies now mean that phage display can make a significant contribution in the fight against clinically important pathogens.

High resolution tumor targeting in living mice by means of multispectral optoacoustic tomography

Background

Tumor targeting is of high clinical and biological relevance, and major efforts have been made to develop molecular imaging technologies for visualization of the disease markers in tissue. Of particular interest is apoptosis which has a profound role within tumor development and has significant effect on cancer malignancy.

Methods

Herein, we report on targeting of phosphatidylserine-exposing cells within live tumor allograft models using a synthetic near infrared zinc(II)-dipicolylamine probe. Visualization of the probe biodistribution is performed with whole body multispectral optoacoustic tomography (MSOT) system and subsequently compared to results attained by planar and tomographic fluorescence imaging systems.

Results

Compared to whole body optical visualization methods, MSOT attains remarkably better imaging capacity by delivering high-resolution scans of both disease morphology and molecular function in real time. Enhanced resolution of MSOT clearly showed that the probe mainly localizes in the vessels surrounding the tumor, suggesting that its tumor selectivity is gained by targeting the phosphatidylserine exposed on the surface of tumor vessels.

Conclusions

The current study demonstrates the high potential of MSOT to broadly impact the fields of tumor diagnostics and preclinical drug development.

Anticancer β-hairpin peptides: membrane-induced folding triggers activity

Several cationic antimicrobial peptides (AMPs) have recently been shown to display anticancer activity via a mechanism that usually entails the disruption of cancer cell membranes. In this work, we designed an 18-residue anticancer peptide, SVS-1, whose mechanism of action is designed to take advantage of the aberrant lipid composition presented on the outer leaflet of cancer cell membranes, which makes the surface of these cells electronegative relative to the surface of noncancerous cells. SVS-1 is designed to remain unfolded and inactive in aqueous solution but to preferentially fold at the surface of cancer cells, adopting an amphiphilic β-hairpin structure capable of membrane disruption. Membrane-induced folding is driven by electrostatic interaction between the peptide and the negatively charged membrane surface of cancer cells. SVS-1 is active against a variety of cancer cell lines such as A549 (lung carcinoma), KB (epidermal carcinoma), MCF-7 (breast carcinoma), and MDA-MB-436 (breast carcinoma). However, the cytotoxicity toward noncancerous cells having typical membrane compositions, such as HUVEC and erythrocytes, is low. CD spectroscopy, appropriately designed peptide controls, cell-based studies, liposome leakage assays, and electron microscopy support the intended mechanism of action, which leads to preferential killing of cancerous cells.

Development of bavituximab, a vascular targeting agent with immune-modulating properties, for lung cancer treatment

Bavituximab is a chimeric monoclonal antibody directed against the membrane phospholipid phosphatidylserine. Phosphatidylserine exposure is increased on endothelial cells and apoptotic cancer cells in solid tumors, allowing tumor-specific targeting of bavituximab. Bavituximab binding results in tumor vessel occlusion and enhanced antitumor immunity. Preclinical investigations have demonstrated efficacy as monotherapy and in combination with other modalities against multiple cancer types. Phase I clinical trials of bavituximab monotherapy and in combination with chemotherapy in adults with refractory solid tumors have been completed. Phase II trials of bavituximab in combination with chemotherapy for the first- and second-line treatment of advanced non-small-cell lung cancer are currently ongoing. This article summarizes the preclinical development and clinical experience with bavituximab in non-small-cell lung cancer.

Near-infrared Optical Imaging of Exposed Phosphatidylserine in a Mouse Glioma Model

Phosphatidylserine (PS) is normally intracellular but becomes exposed on the luminal surface of vascular endothelial cells in tumors. It also becomes exposed on tumors cells responding to therapy. In the present study, we optically imaged exposed PS in vivo using PGN635, a novel monoclonal antibody that binds PS. The F(ab')(2) fragment of PGN635 was labeled with the near-infrared (NIR) dye, IRDye800CW. In vivo dynamic NIR imaging was performed after injection of 800CW-PGN635 into mice bearing radiation-treated or untreated U87 glioma xenografts growing subcutaneously or orthotopically. NIR optical imaging revealed a clear tumor contrast in nonirradiated subcutaneous U87 gliomas after injection of 800CW-PGN635. The tumor contrast was visible as early as 4 hours later and was maximal 24 hours later (tumor-to-normal tissue ratio [TNR] = 2.8 ± 0.7). Irradiation enhanced the tumor contrast at 24 hours (TNR = 4.0 ± 0.3). Similar results were observed for orthotopic gliomas. Localization of 800CW-PGN635 to tumors was antigen specific because 800CW-Aurexis, a control probe of irrelevant specificity, did not localize to the tumors, and preadministration of unlabeled PGN635 blocked the uptake of 800CW-PGN635. Fluorescence microscopy confirmed that 800CW-PGN635 was binding to PS-positive vascular endothelial cells in nonirradiated gliomas. Irradiation of the gliomas increased PS exposure on both tumor vascular endothelial cells and tumor cells and gave rise to an increase in tumor contrast with 800CW-PGN635 that was predictive of the reduction in tumor growth. 800CW-PGN635 may be a useful new imaging probe for detection of exposed PS in tumors responding to therapy.

In vivo targeting of cell death using a synthetic fluorescent molecular probe

A synthetic, near-infrared, fluorescent probe, named PSS-794 was assessed for its ability to detect cell death in two animal models. The molecular probe contains a zinc(II)-dipicolylamine (Zn(2+)-DPA) affinity ligand that selectively targets exposed phosphatidylserine on the surface of dead and dying cells. The first animal model used rats that were treated with dexamethasone to induce thymic atrophy. Ex vivo fluorescence imaging and histological analysis of excised organs showed thymus uptake of PSS-794 was four times higher than a control fluorophore that lacked the Zn(2+)-DPA affinity ligand. In addition, the presence of PSS-794 produced a delayed and higher build up of dead and dying cells in the rat thymus. The second animal model employed focal beam radiation to induce cell death in tumor-bearing rats. Whole-body and ex vivo imaging showed that the amount of PSS-794 in a radiation-treated tumor was almost twice that in a non-treated tumor. The results indicate that PSS-794 may be useful for preclinical optical detection of tumor cell death due to therapy.

Transbilayer phospholipids molecular imaging

Nuclear medicine has become a key part of molecular imaging. In the present review article, we focus on the transbilayer phospholipids as exquisite targets for radiolabelled probes in molecular imaging. Asymmetry of phospholipid distribution is a characteristic of mammalian cell membranes. Phosphatidylcholine and sphyngomyelin cholinophospholipids are primarily located within the external leaflet of the cell membrane. Phosphatidylserine and phosphatidylethanolamine aminophospholipids, and also phosphatidylinositol are primarily located within the internal leaflet of the cell membrane. New radiolabelled tracers have been designed in preclinical and clinical research for PET-CT and SPECT-CT molecular imaging of transbilayer phospholipids.

Increased exposure of phosphatidylethanolamine on the surface of tumor vascular endothelium

We have previously shown that oxidative stress within the tumor microenvironment causes phosphatidylserine (PS) to redistribute from the inner to the outer membrane leaflet of the endothelial cells (EC) creating a highly specific marker for the tumor vasculature. Because the distribution of phosphatidylethanolamine (PE) and PS within the membrane is coregulated, we reasoned that PE would also be localized in the outer membrane leaflet of tumor EC. To demonstrate this, the PE-binding peptide duramycin was biotinylated and used to determine the distribution of PE on EC in vitro and in vivo. Exposure of cultured EC to hypoxia, acidity, reactive oxygen species, or irradiation resulted in the formation of membrane blebs that were intensely PE-positive. When biotinylated duramycin was intravenously injected into tumor-bearing mice, it preferentially localized to the luminal surface of the vascular endothelium. Depending on tumor type, 13% to 56% of the tumor vessels stained positive for PE. PE-positive vessels were observed in and around hypoxic regions of the tumor. With the exception of intertubular vessels of the kidney, normal vessels remained unstained. To test the potential of PE as a biomarker for imaging, duramycin was conjugated to the near-infrared fluorophore 800CW and used for optical imaging of RM-9 prostate carcinomas. The near-infrared probe was easily detected within tumors in live animals. These results show that PE, like PS, becomes exposed on tumor vascular endothelium of multiple types of tumors and holds promise as a biomarker for noninvasive imaging and drug targeting.

Enhancing the potency of a whole-cell breast cancer vaccine in mice with an antibody-IL-2 immunocytokine that targets exposed phosphatidylserine

Phosphatidylserine (PS), an anionic phospholipid normally restricted to the inner leaflet of the plasma membrane, is immunosuppressive when externalized on the outside of cell membranes. Exposed PS inhibits the maturation and function of dendritic cells (DCs), and induces the production of multiple immunosuppressive mediators. In the present study, we determined whether blocking these effects of PS while simultaneously introducing interleukin-2 (IL-2) could improve the immunogenicity of a whole-cell cancer vaccine. An immunocytokine (2aG4-IL2) was made by genetically linking IL-2 with a PS targeting antibody, 2aG4, that can block the immunosuppressive effects of PS. The 2aG4-IL2/4T1 vaccine was generated by coating the PS exposed on irradiated 4T1 cells with 2aG4-IL2. Tumor growth, spontaneous metastasis, and survival of vaccinated mice challenged with live 4T1 tumor cells were assessed. Eighty percent of mice inoculated with 2aG4-IL2/4T1 vaccine survived free of tumor, as compared with 20% in the 2aG4/4T1 group, 20% in the C44-IL2/4T1 group, and none in the C44/4T1 control group (P=0.001 for 2aG4-IL2/4T1 versus all others groups). The incidence, number of spontaneous lung metastases was significantly lower in the 2aG4-IL2/4T1 vaccinated group than in the other groups. Splenocytes from 2aG4-IL2/4T1 vaccinated mice had significantly higher 4T1 specific cytotoxicity and ability to secrete interferon-gamma (IFNγ) than did splenocytes from mice in the other groups. These results demonstrate that a potent whole-cell vaccine can be created by coating irradiated tumor cells with 2aG4-IL2. Such vaccine could potentially be an effective treatment modality for patients with residual disease or at "high-risk" for recurrence.

Retargeting adenoviral vectors to improve gene transfer into tumors

Gene targeting to tumors using adenoviral vectors holds great potential for cancer imaging and therapy, but the limited efficacy of current methods used to improve delivery to target tissues and reduce unwanted interactions remain substantial barriers to further development. Progress in characterizing the set of molecular interactions used by adenoviral vectors to infect particular tissues has aided the development of novel strategies for retargeting vectors to tumor cells. One method is chemical retargeting of adenovirus using bispecific antibodies against both viral capsid proteins and tumor-specific cell surface molecules. This approach can be combined either with competitive inhibitors designed to reduce viral tropism in undesired tissues, or with traditional therapeutics to increase the expression of surface molecules for improved tumor targeting. Ablating liver cell-specific interactions through mutation of capsid proteins or chemical means are promising strategies for reducing adenovirus-induced liver toxicity. The nature of tumor neovasculature also influences adenoviral delivery, and the use of vascular disrupting agents such as combretastatin can help elucidate these contributions. In this investigation, we evaluate a variety of these methods for retargeting adenoviral vectors to tumor cells in vitro and in vivo, and assess the contributions of specific molecular and tissue interactions that affect adenoviral transgene delivery.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Antiphospholipid antibodies promote leukocyte-endothelial cell adhesion and thrombosis in mice by antagonizing eNOS via β2GPI and apoER2

In antiphospholipid syndrome (APS), antiphospholipid antibodies (aPL) binding to β2 glycoprotein I (β2GPI) induce endothelial cell-leukocyte adhesion and thrombus formation via unknown mechanisms. Here we show that in mice both of these processes are caused by the inhibition of eNOS. In studies of cultured human, bovine, and mouse endothelial cells, the promotion of monocyte adhesion by aPL entailed decreased bioavailable NO, and aPL fully antagonized eNOS activation by diverse agonists. Similarly, NO-dependent, acetylcholine-induced increases in carotid vascular conductance were impaired in aPL-treated mice. The inhibition of eNOS was caused by antibody recognition of domain I of β2GPI and β2GPI dimerization, and it was due to attenuated eNOS S1179 phosphorylation mediated by protein phosphatase 2A (PP2A). Furthermore, LDL receptor family member antagonism with receptor-associated protein (RAP) prevented aPL inhibition of eNOS in cell culture, and ApoER2-/- mice were protected from aPL inhibition of eNOS in vivo. Moreover, both aPL-induced increases in leukocyte-endothelial cell adhesion and thrombus formation were absent in eNOS-/- and in ApoER2-/- mice. Thus, aPL-induced leukocyte-endothelial cell adhesion and thrombosis are caused by eNOS antagonism, which is due to impaired S1179 phosphorylation mediated by β2GPI, apoER2, and PP2A. Our results suggest that novel therapies for APS can now be developed targeting these mechanisms.

Clearance of apoptotic cells: implications in health and disease

Recent advances in defining the molecular signaling pathways that regulate the phagocytosis of apoptotic cells have improved our understanding of this complex and evolutionarily conserved process. Studies in mice and humans suggest that the prompt removal of dying cells is crucial for immune tolerance and tissue homeostasis. Failed or defective clearance has emerged as an important contributing factor to a range of disease processes. This review addresses how specific molecular alterations of engulfment pathways are linked to pathogenic states. A better understanding of the apoptotic cell clearance process in healthy and diseased states could offer new therapeutic strategies.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Anti-phospholipid human monoclonal antibodies inhibit CCR5-tropic HIV-1 and induce beta-chemokines

Traditional antibody-mediated neutralization of HIV-1 infection is thought to result from the binding of antibodies to virions, thus preventing virus entry. However, antibodies that broadly neutralize HIV-1 are rare and are not induced by current vaccines. We report that four human anti-phospholipid monoclonal antibodies (mAbs) (PGN632, P1, IS4, and CL1) inhibit HIV-1 CCR5-tropic (R5) primary isolate infection of peripheral blood mononuclear cells (PBMCs) with 80% inhibitory concentrations of <0.02 to ∼10 µg/ml. Anti-phospholipid mAbs inhibited PBMC HIV-1 infection in vitro by mechanisms involving binding to monocytes and triggering the release of MIP-1α and MIP-1β. The release of these β-chemokines explains both the specificity for R5 HIV-1 and the activity of these mAbs in PBMC cultures containing both primary lymphocytes and monocytes.

Targeting mutant p53 protein and the tumor vasculature: an effective combination therapy for advanced breast tumors

Breast cancer progression depends upon the elaboration of a vasculature sufficient for the nourishment of the developing tumor. Breast tumor cells frequently contain a mutant form of p53 (mtp53), a protein which promotes their survival. The aim of this study was to determine whether combination therapy targeting mtp53 and anionic phospholipids (AP) on tumor blood vessels might be an effective therapeutic strategy for suppressing advanced breast cancer. We examined the therapeutic effects, singly, or in combination, of p53 reactivation and induction of massive apoptosis (PRIMA-1), which reactivates mtp53 and induces tumor cell apoptosis, and 2aG4, a monoclonal antibody that disrupts tumor vasculature by targeting AP on the surface of tumor endothelial cells and causes antibody-dependent destruction of tumor blood vessels, leading to ischemia and tumor cell death. Xenografts from two tumor cell lines containing mtp53, BT-474 and HCC-1428, were grown in nude mice to provide models of advanced breast tumors. After treatment with PRIMA-1 and/or 2aG4, regressing tumors were analyzed for vascular endothelial growth factor (VEGF) expression, blood vessel loss, and apoptotic markers. Individual drug treatment led to partial suppression of breast cancer progression. In contrast, combined treatment with PRIMA-1 and 2aG4 was extremely effective in suppressing tumor growth in both models and completely eradicated approximately 30% of tumors in the BT-474 model. Importantly, no toxic effects were observed in any treatment group. Mechanistic studies determined that PRIMA-1 reactivated mtp53 and also exposed AP on the surface of tumor cells as determined by enhanced 2aG4 binding. Combination treatment led to significant induction of tumor cell apoptosis, loss of VEGF expression, as well as destruction of tumor blood vessels. Furthermore, combination treatment severely disrupted tumor blood vessel perfusion in both tumor models. The observed in vitro PRIMA-1-induced exposure of tumor epithelial cell AP might provide a target for 2aG4 and contribute to the increased effectiveness of such combination therapy in vivo. We conclude that the combined targeting of mtp53 and the tumor vasculature is a novel effective strategy for combating advanced breast tumors.

Optical imaging of mammary and prostate tumors in living animals using a synthetic near infrared zinc(II)-dipicolylamine probe for anionic cell surfaces

In vivo optical imaging shows that a fluorescent imaging probe, comprised of a near-infrared fluorophore attached to an affinity group containing two zinc(II)-dipicolylamine (Zn-DPA) units, targets prostate and mammary tumors in two different xenograft animal models. The tumor selectivity is absent with control fluorophores whose structures do not have appended Zn-DPA targeting ligands. Ex vivo biodistribution and histological analyses indicate that the probe is targeting the necrotic regions of the tumors, which is consistent with in vitro microscopy showing selective targeting of the anionic membrane surfaces of dead and dying cells.

Antibody-based vascular tumor targeting

The inhibition of angiogenesis represents a major step toward a more selective and better-tolerated therapy of cancer. An alternative way to take advantage of a tumor's absolute dependence on a functional neovasculature is illustrated by the strategy of "antibody-based vascular tumor targeting." This technology aims at the selective delivery of bioactive molecules to the tumor site by their conjugation to a carrier antibody reactive with a tumor-associated vascular antigen. A number of high-affinity monoclonal antibodies are nowadays available which have demonstrated a remarkable ability to selectively localize to the tumor vasculature. Indeed, some of them have already progressed from preclinical animal experiments to clinical studies in patients with cancer, acting as vehicles for the site-specific pharmacodelivery of proinflammatory cytokines or radionuclides.In this chapter, we present a selection of well-characterized markers of angiogenesis which have proven to be suitable targets for antibody-based vascular targeting approaches. Furthermore, different transcriptomic and proteomic methodologies for the discovery of novel vascular tumor markers are described. In the last two sections, we focus on the discussion of antibody-based vascular tumor targeting strategies for imaging and therapy applications in oncology.

Antiphosphatidylserine antibody combined with irradiation damages tumor blood vessels and induces tumor immunity in a rat model of glioblastoma

PURPOSE

The vascular targeting antibody bavituximab is being combined with chemotherapy in clinical trials in cancer patients. Bavituximab targets the membrane phospholipid, phosphatidylserine, complexed with beta2-glycoprotein I. Phosphatidylserine is normally intracellular but becomes exposed on the luminal surface of vascular endothelium in tumors. Phosphatidylserine exposure on tumor vessels is increased by chemotherapy and irradiation. Here, we determined whether treatment with the murine equivalent of bavituximab, 2aG4, combined with irradiation can suppress tumor growth in a rat model of glioblastoma.

EXPERIMENTAL DESIGN

F98 glioma cells were injected into the brains of syngeneic rats where they grow initially as a solid tumor and then infiltrate throughout the brain. Rats with established tumors were treated with 10 Gy whole brain irradiation and 2aG4.

RESULTS

Combination treatment doubled the median survival time of the rats, and 13% of animals were rendered disease free. Neither treatment given individually was as effective. We identified two mechanisms. First, irradiation induced phosphatidylserine exposure on tumor blood vessels and enhanced antibody-mediated destruction of tumor vasculature by monocytes/macrophages. Second, the antibody treatment induced immunity to F98 tumor cells, which are normally weakly immunogenic. Surviving rats were immune to rechallenge with F98 tumor cells. In vitro, 2aG4 enhanced the ability of dendritic cells (DCs) to generate F98-specific cytotoxic T cells. Phosphatidylserine exposure, which is induced on tumor cells by irradiation, likely suppresses tumor antigen presentation, and 2aG4 blocks this tolerogenic effect.

CONCLUSION

Bavituximab combined with radiotherapy holds promise as a vascular targeting and immune enhancement strategy for the treatment of human glioblastoma.

HIV-1, lipid rafts, and antibodies to liposomes: implications for anti-viral-neutralizing antibodies (Review)

The human immunodeficiency virus type 1 (HIV-1) is an enveloped virus with a lipid bilayer that contains several glycoproteins that are anchored in, or closely associated with, the membrane surface. The envelope proteins have complex interactions with the lipids both on the host cells and on the target cells. The processes of budding from host cells and entry into target cells occur at sites on the plasma membrane, known as lipid rafts, that represent specialized regions that are rich in cholesterol and sphingolipids. Although the envelope glycoproteins are antigenic molecules that potentially might be used for development of broadly neutralizing antibodies in a vaccine to HIV-1, the development of such antibodies that have broad specificities against primary field isolates of virus has been largely thwarted to date by the ability of the envelope proteins to evade the immune system through various mechanisms. In this review, the interactions of HIV-1 with membrane lipids are summarized. Liposomes are commonly used as models for understanding interactions of proteins with membrane lipids; and liposomes have also been used both as carriers for vaccines, and as antigens for induction of antibodies to liposomal lipids. The possibility is proposed that liposomal lipids, or liposome-protein combinations, could be useful as antigens for inducing broadly neutralizing antibodies to HIV-1.

Flipping the wound that doesn't heal: the upside of coagulation in cancer

Two mechanisms for targeting tumor vasculature are well recognized: antiangiogenesis and vascular disruption. In this issue of Blood, Bieker and colleagues take therapeutic advantage of coagulation and add a third mechanism, vascular infarction, to this group by demonstrating the effect through first-in-man administration of tTF-NGR.

Effect of molecular structure of cationic surfactants on biophysical interactions of surfactant-modified nanoparticles with a model membrane and cellular uptake

The aim of this study was to test the hypothesis that the molecular structure of cationic surfactants at the nanoparticle (NP) interface influences the biophysical interactions of NPs with a model membrane and cellular uptake of NPs. Polystyrene NPs (surfactant-free, 130 nm) were modified with cationic surfactants. These surfactants were of either dichained (didodecyldimethylammonium bromide [DMAB]) or single-chained (cetyltrimethylammonium bromide [CTAB] and dodecyltrimethylammonium bromide [DTAB]) forms, with the latter two having different hydrophobic chain lengths. Biophysical interactions of these surfactant-modified NPs with an endothelial cell model membrane (EMM) were studied using a Langmuir film balance. Changes in surface pressure (SP) of EMM as a function of time following interaction with NPs and in the compression isotherm (pi-A) of the lipid mixture of EMM in the presence of NPs were analyzed. Langmuir-Schaeffer (LS) films, which are EMMs that have been transferred onto a suitable substrate, were imaged by atomic force microscopy (AFM), and the images were analyzed to determine the mechanisms of the NP-EMM interaction. DMAB-modified NPs showed a greater increase in SP and a shift toward higher mean molecular area (mmA) than CTAB- and DTAB-modified NPs, indicating stronger interactions of DMAB-modified NPs with the EMM. However, analysis of the AFM phase and height images of the LS films revealed that both DMAB- and CTAB-modified NPs interacted with the EMM but via different mechanisms: DMAB-modified NPs penetrated the EMM, thus explaining the increase in SP, whereas CTAB-modified NPs anchored onto the EMM's condensed lipid domains and hence did not cause any significant change in SP. Human umbilical vein endothelial cells showed greater uptake of DMAB- and CTAB-modified NPs than of DTAB-modified or unmodified NPs. We conclude that (i) the dichained and single-chained cationic surfactants on NPs have different mechanisms of interaction with the model membrane and that (ii) NPs that demonstrate greater biophysical interactions with the membrane also show greater cellular uptake. Biophysical interactions of NPs with a model membrane thus could be effectively used for developing nanocarriers with optimized surface properties for drug delivery and imaging applications.

Targeting inside-out phosphatidylserine as a therapeutic strategy for viral diseases

Targeting exposed anionic phospholipids on a spectrum of virus-infected cells can protect against lethal virus infections in vivo.

Ligand-based vascular targeting of disease

This review illustrates the basic principles of ligand-based vascular targeting and presents some of the most advanced results obtained in this field, not only in terms of biopharmaceuticals, which are currently being investigated in clinical and preclinical studies, but also in terms of enabling technologies that facilitate target and ligand discovery. Whereas most of the vascular targeting research activities have so far concentrated on tumoral angiogenesis, the development of non-oncological applications has recently gained momentum and is likely to become an important area of modern pharmaceutical research.

Effects of apoptosis and lipid peroxidation on T-lymphoblastoid phospholipid-dependent procoagulant activity

BACKGROUND

Coagulation has an absolute requirement for macromolecular complexes to be assembled on a negatively charged phospholipid (PL) surface. Previously, we reported that malignant T-lymphoblastoid cells have the ability to support procoagulant activity (PCA) independently of tissue factor by providing such a surface.

OBJECTIVE

To explore the effect of two pathophysiologic processes, apoptosis and lipid peroxidation (LP), on this PL-dependent PCA.

METHODS

Three different assays for PL-dependent PCA (factor IXa-initiated FXa and thrombin generation and prothrombinase activity) were used to investigate this PCA after exposing three T-lymphoblastoid cell lines to either apoptotic stimuli (1 microM staurosporine) or oxidative stress (4 mm H(2)O(2) and 40 microM CuSO(4)). Surface exposure of anionic PL was measured by flow cytometry using annexin A5(FITC) and an antibody (3G4) specific for native, but not oxidized, phosphatidylserine (PS).

RESULTS AND CONCLUSIONS

Both apoptosis and LP significantly enhanced the PCA of cells, to a level that was greater than that observed following calcium ionophore treatment, suggesting that the increased activity was not solely due to anionic PL exposure. Whereas cells undergoing apoptosis bound both annexin A5(FITC) and 3G4, only annexin A5(FITC) bound to cells undergoing LP. This implies that apoptosis increases PCA by causing the translocation of oxidized/native PS to the outer membrane, whereas LP appears to increase the PCA, possibly due to malondialdehyde adducts altering the net charge on the cell surface, which allows PLs other than PS to participate in thrombin generation.

Coincident exposure of phosphatidylethanolamine and anionic phospholipids on the surface of irradiated cells

The major anionic phospholipid, phosphatidylserine (PS), and the neutral phospholipid, phosphatidylethanolamine (PE), are largely confined to the inner leaflet of the plasma membrane bilayer in mammalian cells under normal conditions. This asymmetry is lost when cells undergo apoptosis, become activated, or are exposed to irradiation, reactive oxygen species or certain drugs. It is not known whether exposure of anionic phospholipids (APLs) and PE occurs simultaneously or in the same region of the plasma membrane. Here we examined the coincidence of exposure of APLs and PE on the surface of bovine aortic endothelial cells and NS0 myeloma cells after irradiation. The cells were irradiated (5 Gy) and stained for APLs and PE using liposomes coated with either an Fab' fragment of a PS-binding antibody (bavituximab) or a PE-binding peptide (duramycin). Using live cell imaging and flow cytometry, we showed that irradiation leads to synchronous externalization of APLs and PE. The time course of appearance of APLs and PE on the cell surface was the same and the two phospholipid types remained colocalized over time. Distinct patches double positive for APLs and PE were visible. Larger areas of APLs and PE appeared to have detached from the cytoskeleton to form membrane blebs which protruded and drifted on the cell surface. We conclude that APLs and PE coincidently appear on the external leaflet of the plasma membrane of cells after irradiation. Probably, this is because PE and the major APL, PS, share common regulatory mechanisms of translocation.

Vascular imaging of solid tumors in rats with a radioactive arsenic-labeled antibody that binds exposed phosphatidylserine

PURPOSE

We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of vascular endothelial cells in tumors, probably in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a chimeric monoclonal antibody that binds phosphatidylserine could be labeled with radioactive arsenic isotopes and used for molecular imaging of solid tumors in rats.

EXPERIMENTAL DESIGN

Bavituximab was labeled with (74)As (beta(+), T(1/2) 17.8 days) or (77)As (beta(-), T(1/2) 1.6 days) using a novel procedure. The radionuclides of arsenic were selected because their long half-lives are consistent with the long biological half lives of antibodies in vivo and because their chemistry permits stable attachment to antibodies. The radiolabeled antibodies were tested for the ability to image subcutaneous Dunning prostate R3227-AT1 tumors in rats.

RESULTS

Clear images of the tumors were obtained using planar gamma-scintigraphy and positron emission tomography. Biodistribution studies confirmed the specific localization of bavituximab to the tumors. The tumor-to-liver ratio 72 h after injection was 22 for bavituximab compared with 1.5 for an isotype-matched control chimeric antibody of irrelevant specificity. Immunohistochemical studies showed that the bavituximab was labeling the tumor vascular endothelium.

CONCLUSIONS

These results show that radioarsenic-labeled bavituximab has potential as a new tool for imaging the vasculature of solid tumors.

Radiation improves gene delivery by a novel transferrin-lipoplex nanoparticle selectively in cancer cells

Selective gene transfer to tumor is critical in cancer gene therapy. We previously used ionizing radiation to improve adenovirus uptake in intrahepatic tumors but liver cytotoxicity associated with the viral administration still occurred. Here, we explore the potential of radiation for improving gene delivery by a virus-mimicking nanoparticle, transferrin (Tf)-cationic liposome-DNA complex (Tf-lipoplex). Transduction by Tf-lipoplex was highly efficient in various cell lines and further increased by radiation in a dose- and time-dependent manner. This radiation induction, which was associated with an increase in Tf-lipoplex uptake (3- to 4-folds in hepatocytes WB and lung cancer cells, LLC1), was absent when a Tf-deficient complex was used or abolished by the presence of free Tf, suggesting that Tf receptor (TfR) interaction is required for radiation induction. Radiation (10-20 Gy) markedly induced transgene (LacZ) expression in LLC1 xenografts (3.5- to 7.4-folds), correlating with increased plasmid content and TfR expression in irradiated tumors. Moreover, Tf-lipoplex-mediated gene expression was not observed in the liver or other normal tissues regardless of radiation treatment. We conclude that radiation improves Tf-lipoplex gene delivery selectively to tumor cells both in vitro and in vivo. Our findings may provide insight in developing ligand-specific lipoplex for molecularly targeted cancer gene therapy.

Discovery of a phosphatidylserine-recognizing peptide and its utility in molecular imaging of tumour apoptosis

The exposure of phosphatidylserine (PS) molecules from the inner to the outer leaflet of the plasma membrane has been recognized as a well-defined molecular epitope of cells undergoing apoptosis. Examination and monitoring of PS exposure is an extensively used molecular marker in non-invasive apoptosis imaging under a variety of clinical conditions, including the assessment of therapeutic anti-cancer agents and myocardial infarction. Herein, we report the identification of a PS-recognizing peptide which was identified by the screening of an M13 phage display peptide library onto PS-coated ELISA plates. Repeated biopanning for a total of four rounds revealed a predominant enrichment of the phage clone displaying peptide sequence, CLSYYPSYC (46%). The identified phage clone evidenced enhanced binding to a number of apoptotic cells over non-apoptotic cells, and this binding was inhibited by both annexin V and synthesized peptide displayed on the phage. The binding of the fluorescein-labelled CLSYYPSYC peptide to apoptotic versus normal cells was assessed by both FACS analysis and fluorescence microscopy. Optical imaging after the systemic administration of fluorescein-labelled CLSYYPSYC peptide to tumour-bearing nude mice (H460 cells xenograft model) treated with a single dose of an anticancer drug (camp-tothecin) indicated peptide homing to the tumour. The histological examination of tumour tissues showed intense staining of the tumour vasculature and apoptotic tumour cells. With these results, the CLSYYPSYC peptide is recognized as a novel PS-recognizing moiety which may possibly be developed into a molecular probe for the imaging of apoptosis in vivo. This application would clearly be relevant to assessments of the efficacy of anticancer therapy in tumours.