Transport of molecules, particles, and cells in solid tumors

Mechanisms and strategies for effective delivery of antisense and siRNA oligonucleotides

The potential use of antisense and siRNA oligonucleotides as therapeutic agents has elicited a great deal of interest. However, a major issue for oligonucleotide-based therapeutics involves effective intracellular delivery of the active molecules. In this Survey and Summary, we review recent reports on delivery strategies, including conjugates of oligonucleotides with various ligands, as well as use of nanocarrier approaches. These are discussed in the context of intracellular trafficking pathways and issues regarding in vivo biodistribution of molecules and nanoparticles. Molecular-sized chemical conjugates and supramolecular nanocarriers each display advantages and disadvantages in terms of effective and nontoxic delivery. Thus, choice of an optimal delivery modality will likely depend on the therapeutic context.

Quantitative spatiotemporal analysis of antibody fragment diffusion and endocytic consumption in tumor spheroids

Antibody-based cancer treatment depends upon distribution of the targeting macromolecule throughout tumor tissue, and spatial heterogeneity could significantly limit efficacy in many cases. Antibody distribution in tumor tissue is a function of drug dosage, antigen concentration, binding affinity, antigen internalization, drug extravasation from blood vessels, diffusion in the tumor extracellular matrix, and systemic clearance rates. We have isolated the effects of a subset of these variables by live-cell microscopic imaging of single-chain antibody fragments against carcinoembryonic antigen in LS174T tumor spheroids. The measured rates of scFv penetration and retention were compared with theoretical predictions based on simple scaling criteria. The theory predicts that antibody dose must be large enough to drive a sufficient diffusive flux of antibody to overcome cellular internalization, and exposure time must be long enough to allow penetration to the spheroid center. The experimental results in spheroids are quantitatively consistent with these predictions. Therefore, simple scaling criteria can be applied to accurately predict antibody and antibody fragment penetration distance in tumor tissue.

Modulation of in vivo tumor radiation response via gold nanoshell-mediated vascular-focused hyperthermia: characterizing an integrated antihypoxic and localized vascular disrupting targeting strategy

We report noninvasive modulation of in vivo tumor radiation response using gold nanoshells. Mild-temperature hyperthermia generated by near-infrared illumination of gold nanoshell-laden tumors, noninvasively quantified by magnetic resonance temperature imaging, causes an early increase in tumor perfusion that reduces the hypoxic fraction of tumors. A subsequent radiation dose induces vascular disruption with extensive tumor necrosis. Gold nanoshells sequestered in the perivascular space mediate these two tumor vasculature-focused effects to improve radiation response of tumors. This novel integrated antihypoxic and localized vascular disrupting therapy can potentially be combined with other conventional antitumor therapies.

Polymer Implants for Intratumoral Drug Delivery and Cancer Therapy

To address the need for minimally invasive treatment of unresectable tumors, intratumoral polymer implants have been developed to release a variety of chemotherapeutic agents for the locoregional therapy of cancer. These implants, also termed "polymer millirods," were designed to provide optimal drug release kinetics to improve drug delivery efficiency and antitumor efficacy when treating unresectable tumors. Modeling of drug transport properties in different tissue environments has provided theoretical insights on rational implant design, and several imaging techniques have been established to monitor the local drug concentrations surrounding these implants both ex vivo and in vivo. Preliminary antitumor efficacy and drug distribution studies in a rabbit liver tumor model have shown that these implants can restrict tumor growth in small animal tumors (diameter < 1 cm). In the future, new approaches, such as three-dimensional (3-D) drug distribution modeling and the use of multiple drug-releasing implants, will be used to extend the efficacy of these implants in treating larger tumors more similar to intractable human tumors.

Antibody-based cancer therapy

BACKGROUND

The clinical efficacy of mAbs is generally ascribed to interference with signaling pathways leading to arrest of cell-cycle progression and inhibition of tumor growth. Furthermore, mAbs also have the capacity to activate effector functions of the innate immune system and facilitate the destruction of malignant cells.

OBJECTIVES

The induction of tumor-specific immunity is a desired outcome in cancer immunotherapy. The prevailing situation raises one major question that has to be addressed. This is the clear need for the induction of tumor-specific immunity by combining mAb treatment with other modalities of cancer immunotherapy.

METHODS

Through mAb treatment, recent efforts focus at initiating or enhancing active antitumor immune responses by i) potentiating co-stimulation and blocking co-inhibition; and ii) rendering tumors more immunogenic through increased tumor peptide expression.

RESULTS/CONCLUSIONS

In this review, the functional characteristics of mAbs, together with their mechanisms of action and clinical application, is summarized as is the potential of combination immunotherapies using mAbs for the augmentation of adaptive antitumor immunity. The results from preclinical and clinical studies demonstrate that mAbs can also promote tumor-specific active immunity.

Synthesis, characterization, and in vitro testing of superparamagnetic iron oxide nanoparticles targeted using folic Acid-conjugated dendrimers

Organic-coated superparamagnetic iron oxide nanoparticles (OC-SPIONs) were synthesized and characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. OC-SPIONs were transferred from organic media into water using poly(amidoamine) dendrimers modified with 6-TAMRA fluorescent dye and folic acid molecules. The saturation magnetization of the resulting dendrimer-coated SPIONs (DC-SPIONs) was determined, using a superconducting quantum interference device, to be 60 emu/g Fe versus 90 emu/g Fe for bulk magnetite. Selective targeting of the DC-SPIONs to KB cancer cells in vitro was demonstrated and quantified using two distinct and complementary imaging modalities: UV-visible and X-ray fluorescence; confocal microscopy confirmed internalization. The results were consistent between the uptake distribution quantified by flow cytometry using 6-TAMRA UV-visible fluorescence intensity and the cellular iron content determined using X-ray fluorescence microscopy.

Multifunctional quantum dots for cellular and molecular imaging

Multifunctional nanoparticle probes based on semiconductor quantum dots (QDs) have been developed for cancer targeting and imaging in living animals. The structural design involves encapsulating luminescent QDs with block copolymer, and linking this amphiphilic polymer to tumor-targeting ligands. In vivo targeting studies of human prostate cancer growing in nude mice indicate that the QD probes can be delivered to tumor sites by both enhanced permeation and retention and by antibody binding to cancer-specific cell surface biomarkers. Using both subcutaneous injection of QD-tagged cancer cells and systemic injection of multifunctional QD probes, we have achieved sensitive and multicolor fluorescence imaging of cancer cells under in vivo conditions. We have also integrated a whole-body macro-illumination system with wavelength-resolved spectral imaging for efficient background removal and precise delineation of weak spectral signatures. These results raise new possibilities for ultrasensitive and multiplexed imaging of molecular targets in vivo.

Sensors of ionizing radiation effects on the immunological microenvironment of cancer

PURPOSE

When cancer develops in an immunocompetent host it represents the result of a successful deception of the immune system as to the nature of the danger and the type of response needed to reject the neoplastic tissue. We will briefly review some of the recently emerged evidence that irradiation of the tumor and its microenvironment can induce essential molecular signals required for an effective response of the immune system to the tumor.

CONCLUSIONS

The subversion of a highly organized tissue architecture is a hallmark of cancer, and results in uneven distribution of oxygen and nutrients, interstitial pressure gradients and areas of patchy necrosis and inflammation. In this microenvironment, cancer cells that carry mutations favoring survival rather than cell death in response to stress find a selection advantage. Importantly, the signals derived from the disruption of orderly physiology within tissues are also what the immune system has evolved to respond to. The type of response is tuned to be adequate to the cause of the disruption. An infectious organism will carry or elicit from the involved tissue a number of 'danger signals' leading to development of cell mediated and humoral responses to both eliminating the invader and preventing future infections. In contrast, a simple wound will call for a repair response. The sensors of the type of damage are complex molecular interactions between the damaged organ and cells of the innate and adaptive immune system. Progress in the identification of these interactions elucidates which pathways are specifically altered in cancer. It also provides a novel understanding of the radiation-induced effects on tumor immunogenicity. We propose that specific radiation-induced effects could be successfully exploited to improve the effectiveness of immunotherapy.

Metabolic mechanisms of tumor resistance to T cell effector function

Established tumors develop ways to elude destruction by the host immune system. Recent work has revealed that tumors can take advantage of the generation of metabolic dysregulation to inhibit immune responses. Effector T-cell functions are particularly sensitive to nutrient availability in the tumor microenvironment. In this review, we highlight experimental data supporting the importance of glucose, oxygen, tryptophan, and arginine for optimal T-cell function, and the mechanisms by which these nutrients may become depleted in the tumor microenvironment. These observations provide a conceptual framework for modulating metabolic features of the T cell-tumor interaction, toward the end of promoting more effective immune-mediated tumor destruction in vivo.

Impact of tumor-specific targeting and dosing schedule on tumor growth inhibition after intravenous administration of siRNA-containing nanoparticles

This study addresses issues of relevance for siRNA nanoparticle delivery by investigating the functional impact of tumor-specific targeting and dosing schedule. The investigations are performed using an experimental system involving a syngeneic mouse cancer model and a theoretical system based on our previously described mathematical model of siRNA delivery and function. A/J mice bearing subcutaneous Neuro2A tumors approximately 100 mm(3) in size were treated by intravenous injection with siRNA-containing nanoparticles formed with cyclodextrin-containing polycations (CDP). Three consecutive daily doses of transferrin (Tf)-targeted nanoparticles carrying 2.5 mg/kg of two different siRNA sequences targeting ribonucleotide reductase subunit M2 (RRM2) slowed tumor growth, whereas non-targeted nanoparticles were significantly less effective when given at the same dose. Furthermore, administration of the three doses on consecutive days or every 3 days did not lead to statistically significant differences in tumor growth delay. Mathematical model calculations of siRNA-mediated target protein knockdown and tumor growth inhibition are used to elucidate possible mechanisms to explain the observed effects and to provide guidelines for designing more effective siRNA-based treatment regimens regardless of delivery methodology and tumor type.

Factors determining antibody distribution in tumors

The development of antibody therapies for cancer is increasing rapidly, primarily owing to their specificity. Antibody distribution in tumors is often extremely uneven, however, leading to some malignant cells being exposed to saturating concentrations of antibody, whereas others are completely untargeted. This is detrimental because large regions of cells escape therapy, whereas other regions might be exposed to suboptimal concentrations that promote a selection of resistant mutants. The distribution of antibody depends on a variety of factors, including dose, affinity, antigens per cell and molecular size. Because these parameters are often known or easily estimated, a quick calculation based on simple modeling considerations can predict the uniformity of targeting within a tumor. Such analyses should enable experimental researchers to identify in a straightforward way the limitations in achieving evenly distributed antibody, and design and test improved antibody therapeutics more rationally.

Peptide-based Biopolymers in Biomedicine and Biotechnology

Peptides are emerging as a new class of biomaterials due to their unique chemical, physical, and biological properties. The development of peptide-based biomaterials is driven by the convergence of protein engineering and macromolecular self-assembly. This review covers the basic principles, applications, and prospects of peptide-based biomaterials. We focus on both chemically synthesized and genetically encoded peptides, including poly-amino acids, elastin-like polypeptides, silk-like polymers and other biopolymers based on repetitive peptide motifs. Applications of these engineered biomolecules in protein purification, controlled drug delivery, tissue engineering, and biosurface engineering are discussed.

Synergistic antitumor activity of taxol and immunotoxin SS1P in tumor-bearing mice

PURPOSE

To investigate the combined antitumor activity in mice of immunotoxin SS1P and Taxol.

METHODS

Immunodeficient mice were implanted with A431/K5 tumors expressing mesothelin. Established tumors were treated i.v. with immunotoxin SS1P alone, i.p. with Taxol alone, or with the two agents together. SS1P was radiolabeled with (111)In and used to study the effect of Taxol on its uptake by A431/K5 tumors.

RESULTS

Using doses at which either agent alone caused stabilization of tumor growth, the combination was synergistic causing long-lasting complete remissions in many animals. In contrast, synergy was not observed when the same cells were treated with these agents in vitro. Tumor uptake of (111)In-SS1P was not affected by treatment with Taxol.

CONCLUSION

The combination of Taxol and SS1P exerts a synergistic antitumor effect in animals but not in cell culture. This effect is not secondary to increased tumor uptake of the immunotoxin. Synergy could be due to improved immunotoxin distribution within the tumor or could involve factors released by other cell types in the tumors.

Design maps for nanoparticles targeting the diseased microvasculature

Systemically administered ligand-coated nanoparticles have been proved to recognize biological targets in-vivo. This can provide breakthrough solutions for the early detection, imaging and cure of diseases. In cardiovascular applications, nanoparticles have been targeted directly to the diseased vasculature, and such delivery approach is becoming increasingly popular even in cancer research, supported by the growing body of evidences on the biological differences between normal and tumor vasculature. This work focuses on the optimal design of nanoparticles for vascular targeting throughout mathematical modeling. Such nanoparticles should be engineered so as to recognize specifically and adhere firmly to the diseased vessel walls withstanding the hydrodynamic dislodging forces and control uptake by the endothelial cells. A stochastic approach for predicting the adhesion strength of nanoparticles to a cell layer under flow has been coupled to a mathematical model for the receptor-mediated endocytosis of nanoparticles. The main geometrical, biophysical and biological parameters governing both events have been identified and their relative importance highlighted. Three different states for the particle/cell system have been predicted, namely no adhesion, adhesion with no endocytosis and adhesion with endocytosis, based upon the geometrical and biophysical properties of the particle and the biological conditions at the site of adhesion. Design maps have been generated to be used as a preliminary reference for choosing the properties of the nanoparticle as a function of physiological parameters, as the wall shear stress and the receptors surface density, at the site of desired adhesion within the target vasculature.

Nanotechnology applications in cancer

Cancer nanotechnology is an interdisciplinary area of research in science, engineering, and medicine with broad applications for molecular imaging, molecular diagnosis, and targeted therapy. The basic rationale is that nanometer-sized particles, such as semiconductor quantum dots and iron oxide nanocrystals, have optical, magnetic, or structural properties that are not available from molecules or bulk solids. When linked with tumor targeting ligands such as monoclonal antibodies, peptides, or small molecules, these nanoparticles can be used to target tumor antigens (biomarkers) as well as tumor vasculatures with high affinity and specificity. In the mesoscopic size range of 5-100 nm diameter, nanoparticles also have large surface areas and functional groups for conjugating to multiple diagnostic (e.g., optical, radioisotopic, or magnetic) and therapeutic (e.g., anticancer) agents. Recent advances have led to bioaffinity nanoparticle probes for molecular and cellular imaging, targeted nanoparticle drugs for cancer therapy, and integrated nanodevices for early cancer detection and screening. These developments raise exciting opportunities for personalized oncology in which genetic and protein biomarkers are used to diagnose and treat cancer based on the molecular profiles of individual patients.

Immunotoxin and Taxol synergy results from a decrease in shed mesothelin levels in the extracellular space of tumors

Recombinant immunotoxins are chimeric proteins composed of the Fv portion of a tumor-specific antibody fused to a toxin. SS1P (CAT-5001) is an immunotoxin composed of an antimesothelin Fv fused to a 38-kDa portion of Pseudomonas exotoxin A. Immunotoxins have been shown to be active in lymphomas and leukemias, but are much less active against solid tumors. We recently reported that Taxol and other chemotherapeutic agents show striking synergistic antitumor activity in mice when immunotoxin SS1P, which targets the mesothelin antigen on solid tumors, is given with Taxol. Using a pair of Taxol-sensitive and Taxol-resistant KB tumors equally sensitive to immunotoxin SS1P, we examined the mechanism of synergy. We show that synergy is only observed with Taxol-sensitive tumors, ruling out an effect of Taxol on endothelial cells. We also show that the KB tumors have high levels of shed mesothelin in their extracellular space; these levels increase with tumor size and, after Taxol treatment, dramatically fall in the drug-sensitive but not the drug-resistant tumors. Because the mesothelin levels in the tumor exceed the levels of SS1P in the tumor, and because shed mesothelin is being continuously released into the circulation at a high rate, we propose that synergy is due to the Taxol-induced fall in shed antigen levels.

Antibiological barrier nanovector technology for cancer applications

The advent of sophisticated drug delivery strategies for cancer applications has inundated the scientific and clinical community with new tactics and approaches such as molecular targeting, nanotechnology-based methods and personalized therapies. Unfortunately, the clinical impact has been moderate at best, falling significantly short from revolutionizing existing chemotherapeutic methodologies. To this day, a cancer patient has a higher probability of receiving traditional systemically administered drugs than a more sophisticated targeted or nanotechnology-based therapeutic. This is not a reflection upon the novelty or quality of the technologies, but an indication of opportunity for a new approach that offers the realisation of the full potential of these scientific advances. This approach acknowledges the significance of the numerous biological barriers presented in the human body and their sequential nature. It is then recommended that computational mathematical tools are used to predict which nanovectors, surface modifications, therapeutic agents and penetration enhancers to use for a multi-stage drug delivery strategy. An approach where several stages of micro-/nano-vectors are nested within each other and delivered to overcome specific biological barriers to ultimately release a concentrated dose of a therapeutic payload at the intended lesion site. This novel, multi-stage strategy enables efficient localised delivery of chemotoxic drugs that may lead to significant improvements in therapy efficacy, reduced systemic toxicity and decreased total amount of injected drugs.

Treatment of solid tumors by interstitial release of recoiling short-lived alpha emitters

A new method utilizing alpha particles to treat solid tumors is presented. Tumors are treated with interstitial radioactive sources which continually release short-lived alpha emitting atoms from their surface. The atoms disperse inside the tumor, delivering a high dose through their alpha decays. We implement this scheme using thin wire sources impregnated with (224)Ra, which release by recoil (220)Rn, (216)Po and (212)Pb atoms. This work aims to demonstrate the feasibility of our method by measuring the activity patterns of the released radionuclides in experimental tumors. Sources carrying (224)Ra activities in the range 10-130 kBq were used in experiments on murine squamous cell carcinoma tumors. These included gamma spectroscopy of the dissected tumors and major organs, Fuji-plate autoradiography of histological tumor sections and tissue damage detection by Hematoxylin-Eosin staining. The measurements focused on (212)Pb and (212)Bi. The (220)Rn/(216)Po distribution was treated theoretically using a simple diffusion model. A simplified scheme was used to convert measured (212)Pb activities to absorbed dose estimates. Both physical and histological measurements confirmed the formation of a 5-7 mm diameter necrotic region receiving a therapeutic alpha-particle dose around the source. The necrotic regions shape closely corresponded to the measured activity patterns. (212)Pb was found to leave the tumor through the blood at a rate which decreased with tumor mass. Our results suggest that the proposed method, termed DART (diffusing alpha-emitters radiation therapy), may potentially be useful for the treatment of human patients.

Anticancer carrier-linked prodrugs in clinical trials

Coupling of low molecular weight anticancer drugs to antibodies, serum proteins or polymers through a cleavable linker has been an effective method for improving the therapeutic index of cytotoxic established agents. Modern drug-antibody conjugates that have recently entered clinical trials have primarily used highly potent drugs such as calicheamicin or maytansins. Gemtuzumab ozogamicin, a conjugate of calicheamicin and an anti-CD33 humanized antibody, is the first drug-antibody conjugate to receive market approval. Drug conjugates that have undergone clinical assessment include N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugates with doxorubicin, camptothecin, paclitaxel and Pt(II) complexes, poly(ethylene glycol) conjugates with camptothecin and paclitaxel, polyglutamate conjugates with paclitaxel and camptothecin, a methotrexate-albumin conjugate and an albumin-binding doxorubicin prodrug. This review summarizes the Phase I-III studies that have been performed with these macromolecular prodrugs.

Imatinib mesylate for targeting the platelet-derived growth factor β receptor in combination with fluorouracil and leucovorin in patients with refractory pancreatic, bile duct, colorectal, or gastric cancer—A dose-escalation Phase I trial

BACKGROUND

In previous experimental models, because of its ability to inhibit the activity of platelet-derived growth factor beta receptor, imatinib decreased the interstitial fluid pressure and improved the delivery and efficacy of anticancer drugs, including fluorouracil. The objective of this Phase I study was to define the dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of imatinib in combination with fluorouracil and leucovorin in patients with chemotherapy-refractory gastrointestinal cancer.

METHODS

A 3-patient cohort dose-escalating study design was used. Patients received leucovorin 200 mg/m2 followed by fluorouracil 2000 mg/m2 as a 24-hour infusion on Days 1 and 2 combined with imatinib on Days -4, -3, -2, -1, 1, 2, 3, and 4. Cycles were repeated every 2 weeks, and the imatinib dose was escalated from 300 mg daily to 700 mg daily in 100-mg steps.

RESULTS

Thirty patients were enrolled at 5 dose levels. Frequent and dose-dependant National Cancer Institute Common Toxicity Criteria grade 1-4 adverse events with suspected relation to the treatment were anemia (43%), nausea (33%), fluid retention (27%), elevated serum gamma-glutamyl-transpeptidase (20%), and diarrhea. DLTs were severe neutropenia, central fluid retention, and severe nausea observed in 1 patient each, resulting in an MTD for imatinib of 600 mg per day. There were no differences in imatinib pharmacokinetics before or during chemotherapy. A minor response was observed; and signs of clinical activity, including the resolution of ascites and improvement in performance status, were noted in some patients.

CONCLUSIONS

The combination of biweekly fluorouracil/leucovorin and imatinib 600 mg daily given in a week-on/week-off schedule was feasible and safe. Nausea and fluid retention represented the DLTs.

Interstitial flow and its effects in soft tissues

Interstitial flow plays important roles in the morphogenesis, function, and pathogenesis of tissues. To investigate these roles and exploit them for tissue engineering or to overcome barriers to drug delivery, a comprehensive consideration of the interstitial space and how it controls and affects such processes is critical. Here we attempt to review the many physical and mathematical correlations that describe fluid and mass transport in the tissue interstitium; the factors that control and affect them; and the importance of interstitial transport on cell biology, tissue morphogenesis, and tissue engineering. Finally, we end with some discussion of interstitial transport issues in drug delivery, cell mechanobiology, and cell homing toward draining lymphatics.

Functional neoangiogenesis imaging of genetically engineered mouse prostate cancer using three-dimensional power Doppler ultrasound

We report the first application of high-frequency three-dimensional power Doppler ultrasound imaging in a genetically engineered mouse (GEM) prostate cancer model. We show that the technology sensitively and specifically depicts functional neoangiogenic blood flow because little or no flow is measurable in normal prostate tissue or tumors smaller than 2-3 mm diameter, the neoangiogenesis "switch-on" size. Vascular structures depicted by power Doppler were verified using Microfil-enhanced micro-computed tomography (micro-CT) and by correlation with microvessel distributions measured by immunohistochemistry and enhanced vascularity visualized by confocal microscopy in two GEM models [transgenic adenocarcinoma of the mouse prostate (TRAMP) and PSP94 gene-directed transgenic mouse adenocarcinoma of the prostate (PSP-TGMAP)]. Four distinct phases of neoangiogenesis in cancer development were observed, specifically, (a) an early latent phase; (b) establishment of a peripheral capsular vascular structure as a neoangiogenesis initiation site; (c) a peak in tumor vascularity that occurs before aggressive tumor growth; and (d) rapid tumor growth accompanied by decreasing vascularity. Microsurgical interventions mimicking local delivery of antiangiogenesis drugs were done by ligating arteries upstream from feeder vessels branching to the prostate. Microsurgery produced an immediate reduction of tumor blood flow, and flow remained low from 1 h to 2 weeks or longer after treatment. Power Doppler, in conjunction with micro-CT, showed that the tumors recruit secondary blood supplies from nearby vessels, which likely accounts for the continued growth of the tumors after surgery. The microsurgical model represents an advanced angiogenic prostate cancer stage in GEM mice corresponding to clinically defined hormone-refractory prostate cancer. Three-dimensional power Doppler imaging is completely noninvasive and will facilitate basic and preclinical research on neoangiogenesis in live animal models.

Antitumor efficacy and local distribution of doxorubicin via intratumoral delivery from polymer millirods

The purpose of this study was to evaluate the antitumor efficacy and local drug distribution from doxorubicin-containing poly(D,L-lactide-co-glycolide) (PLGA) implants for intratumoral treatment of liver cancer in a rabbit model. Cylindrical polymer millirods (length 8 mm, diameter 1.5 mm) were produced using 65% PLGA, 21.5% NaCl, and 13.5% doxorubicin. These implants were placed in the center of VX2 liver tumors (n = 16, 8 mm in diameter) in rabbits. Tumors were removed 4 and 8 days after millirod implantation, and antitumor efficacy was assessed using tumor size measurements, tumor histology, and fluorescent measurement of drug distribution. The treated tumors were smaller than the untreated controls on both day 4 (0.17 +/- 0.06 vs. 0.31 +/- 0.08 cm(2), p = 0.048) and day 8 (0.14 +/- 0.04 vs. 1.8 +/- 0.8 cm(2), p = 0.025). Drug distribution profiles demonstrated high doxorubicin concentrations (>1000 microg/g) at the tumor core at both time points and drug penetration distances of 2.8 and 1.3 mm on day 4 and 8, respectively. Histological examination confirmed necrosis throughout the tumor tissue. Biodegradable polymer millirods successfully treated the primary tumor mass by providing high doxorubicin concentrations to the tumor tissue over an eight day period.

Encapsulation of nucleic acids and opportunities for cancer treatment

The development of nucleic acid drugs for the treatment of various cancers has shown great promise in recent years. However, efficient delivery of these drugs to target cells remains a significant challenge towards the successful development of such therapies. This review provides a comprehensive overview of encapsulation technologies being developed for the delivery of nucleic acid-based anti-cancer agents. Both micro and nanoparticles systems are discussed along with their use in delivering plasmid DNA as well as oligonucleotides. The majority of the systems discussed have used DNA immunotherapy as the potential mode of anticancer therapy, which requires targeting to antigen presenting cells. Other applications, including those with oligonucleotides, focus on targeting tumor cells directly. The results obtained so far show the excellent promise of encapsulation as an efficient means of delivering therapeutic nucleic acids.

Inhibition of carcinoma cell-derived VEGF reduces inflammatory characteristics in xenograft carcinoma

The stroma of carcinomas shares several characteristics with inflamed tissues including a distorted vasculature, active angiogenesis and macrophage infiltration. In addition, the tumor interstitial fluid pressure (P(IF)) of the stroma is pathologically elevated. We show here that bevacizumab [rhuMab vascular endothelial growth factor (VEGF), Avastin], a monoclonal antibody to VEGF, at a dose of 5 mg/kg modulated inflammation in KAT-4 xenograft human anaplastic thyroid carcinoma tissue. At this dose, bevacizumab reduced the density of macrophages, MHC class II antigen expression by macrophages and IL-1beta mRNA expression. Furthermore, bevacizumab lowered tumor extracellular fluid volume, plasma protein leakage from tumor vessels, the number of CD31-positive structures and tumor P(IF). The tumor plasma volume and the number of alpha-smooth muscle actin-positive vessels, however, remained unchanged. Our data suggest that carcinoma cell-derived VEGF either directly or indirectly participates in maintaining an inflammatory microenvironment in experimental KAT-4 carcinoma. Furthermore, our data indicate that the reduction of inflammation resulting in reduced vascular permeability and decrease in the tumor extracellular fluid volume by bevacizumab contributes to reduced tumor P(IF).

A driving force for change: interstitial flow as a morphoregulator

Dynamic stresses that are present in all living tissues drive small fluid flows, called interstitial flows, through the extracellular matrix. Interstitial flow not only helps to transport nutrients throughout the tissue, but also has important roles in tissue maintenance and pathobiology that have been, until recently, largely overlooked. Here, we present evidence for the various effects of interstitial flow on cell biology, including its roles in embryonic development, tissue morphogenesis and remodeling, inflammation and lymphedema, tumor biology and immune cell trafficking. We also discuss possible mechanisms by which interstitial flow can induce morphoregulation, including direct shear stress, matrix-cell transduction (as has been proposed in the endothelial glycocalyx) and the newly emerging concept of autologous gradient formation.

Micellar nanocarriers: pharmaceutical perspectives

Micelles, self-assembling nanosized colloidal particles with a hydrophobic core and hydrophilic shell are currently successfully used as pharmaceutical carriers for water-insoluble drugs and demonstrate a series of attractive properties as drug carriers. Among the micelle-forming compounds, amphiphilic copolymers, i.e., polymers consisting of hydrophobic block and hydrophilic block, are gaining an increasing attention. Polymeric micelles possess high stability both in vitro and in vivo and good biocompatibility, and can solubilize a broad variety of poorly soluble pharmaceuticals many of these drug-loaded micelles are currently at different stages of preclinical and clinical trials. Among polymeric micelles, a special group is formed by lipid-core micelles, i.e., micelles formed by conjugates of soluble copolymers with lipids (such as polyethylene glycol-phosphatidyl ethanolamine conjugate, PEG-PE). Polymeric micelles, including lipid-core micelles, carrying various reporter (contrast) groups may become the imaging agents of choice in different imaging modalities. All these micelles can also be used as targeted drug delivery systems. The targeting can be achieved via the enhanced permeability and retention (EPR) effect (into the areas with the compromised vasculature), by making micelles of stimuli-responsive amphiphilic block-copolymers, or by attaching specific targeting ligand molecules to the micelle surface. Immunomicelles prepared by coupling monoclonal antibody molecules to p-nitrophenylcarbonyl groups on the water-exposed termini of the micelle corona-forming blocks demonstrate high binding specificity and targetability. This review will discuss some recent trends in using micelles as pharmaceutical carriers.

Effect of target dynamics on pharmacokinetics of a novel therapeutic antibody against the epidermal growth factor receptor: implications for the mechanisms of action

The epidermal growth factor receptor (EGFR) is overexpressed on many solid tumors and represents an attractive target for antibody therapy. Here, we describe the effect of receptor-mediated antibody internalization on the pharmacokinetics and dose-effect relationship of a therapeutic monoclonal antibody (mAb) against EGFR (2F8). This mAb was previously found therapeutically active in mouse tumor models by two dose-dependent mechanisms of action: blockade of ligand binding and induction of antibody-dependent cell-mediated cytotoxicity. In vitro studies showed 2F8 to be rapidly internalized by EGFR-overexpressing cells. In vivo, accelerated 2F8 clearance was observed in cynomolgus monkeys at low doses but not at high doses. This enhanced clearance seemed to be receptor dependent and was included in a pharmacokinetic model designed to explain its nonlinearity. Receptor-mediated clearance was also found to affect in situ antibody concentrations in tumor tissue. Ex vivo analyses of xenograft tumors of 2F8-treated nude mice revealed that relatively high antibody plasma concentrations were required for maximum EGFR saturation in high-EGFR-expressing human A431 tumors, in contrast to lower-EGFR-expressing human xenograft tumors. In summary, receptor-mediated antibody internalization and degradation provides a saturable route of clearance that significantly affects pharmacokinetics, particularly at low antibody doses. EGFR saturation in normal tissues does not predict saturation in tumor tissue as local antibody concentrations in EGFR-overexpressing tumors may be more rapidly reduced by antibody internalization. Consequently, antibody saturation of the receptor may be affected, thereby affecting the local mechanism of action.

Addition of low-dose tumor necrosis factor-alpha to systemic treatment with STEALTH liposomal doxorubicin (Doxil) improved anti-tumor activity in osteosarcoma-bearing rats

Improved efficacy of Doxil (STEALTH liposomal doxorubicin) compared to free doxorubicin has been demonstrated in the treatment of several tumor types. We have shown that addition of low-dose tumor necrosis factor (TNF) to systemic Doxil administration dramatically improved tumor response in the highly vascularized rat soft tissue sarcoma BN175. Whether a similar enhanced efficacy can be achieved in less vascularized tumors is uncertain. We therefore examined the effect of systemic administration of Doxil in combination with low-dose TNF in intermediate vascularized osteosarcoma-bearing rats (ROS-1). Small fragments of the osteosarcoma were implanted s.c. in the lower limb. Treatment was started when the tumors reached an average diameter of 1 cm. Rats were treated with five i.v. injections at 4-day intervals with Doxil or doxorubicin and TNF. Systemic treatment with Doxil resulted in a better tumor growth delay than free doxorubicin, but with progressive diseases in all animals. The 3.5-fold augmented accumulation of Doxil compared to free doxorubicin presumably explains the enhanced tumor regression. Addition of low-dose TNF augmented the anti-tumor activity of Doxil, although no increased drug uptake was found compared to Doxil alone. In vitro studies showed that ROS-1 is sensitive to TNF, but systemic treatment with TNF alone did not result in a tumor growth delay. Furthermore, we demonstrated that treatment with Doxil alone or with TNF resulted in massive coagulative necrosis of tumor tissue. In conclusion, combination therapy of Doxil and low-dose TNF seems attractive for the treatment of highly vascularized tumors, but also of intermediate vascularized tumors like the osteosarcoma.

Autologous morphogen gradients by subtle interstitial flow and matrix interactions

Cell response to extracellular cues is often driven by gradients of morphogenetic and chemotactic proteins, and therefore descriptions of how such gradients arise are critical to understanding and manipulating these processes. Many of these proteins are secreted in matrix-binding form to be subsequently released proteolytically, and here we explore how this feature, along with small dynamic forces that are present in all tissues, can affect pericellular protein gradients. We demonstrate that 1), pericellular gradients of cell-secreted proteins can be greatly amplified when secreted by the cell in matrix-binding form as compared to a nonmatrix-interacting form; and 2), subtle flows can drive significant asymmetry in pericellular protein concentrations and create transcellular gradients that increase in the direction of flow. This study thus demonstrates how convection and matrix-binding, both physiological characteristics, combine to allow cells to create their own autologous chemotactic gradients that may drive, for example, tumor cells and immune cells into draining lymphatic capillaries.

The effective dispersion of nanovectors within the tumor microvasculature

The effective longitudinal diffusion of nanovectors along non-permeable and permeable capillaries has been studied considering the contribution of molecular and convective diffusion based on the Taylor's theory of shear dispersion. The problem is of importance in the transport of nanovectors used for the intravascular delivery of drugs and contrast agents. It has been shown that for a given capillary size and hemodynamic conditions a critical radius acr exists for which the effective longitudinal diffusion along the capillary has a minimum: Nanovectors with a < acr diffuse mainly by Brownian diffusion whereas nanovectors with a < acr diffuse mainly by convection and the effective diffusion coefficient grows with a. In permeable conduits, the effective diffusion reduces significantly compared to normal non-leaky vessels and it has been derived that acr grows almost linearly with the hydraulic permeability Lp of blood vessels. It has been shown that the blood conduits with the largest effective longitudinal diffusivity could be preferentially targeted by the circulating vectors. Based on these findings, the following strategies are proposed to increase the number of nanovectors targeting the tumor vessels: (i) The use of nanovectors with a critical radius for normal vessels, (ii) the injecting of bolus of nanovectors with different radii, and (iii) the normalization of the tumor vasculature. Finally, it has been emphasized that the size of the vector should be selected depending on the body district where the tumoral mass is developing and on the type, malignancy, and state of the tumor.

Mechanobiology in the third dimension

Cells are mechanically coupled to their extracellular environments, which play critical roles in both communicating the state of the mechanical environment to the cell as well as in mediating cellular response to a variety of stimuli. Along with the molecular composition and mechanical properties of the extracellular matrix (ECM), recent work has demonstrated the importance of dimensionality in cell-ECM associations for controlling the sensitive communication between cells and the ECM. Matrix forces are generally transmitted to cells differently when the cells are on two-dimensional (2D) vs. within three-dimensional (3D) matrices, and cells in 3D environments may experience mechanical signaling that is unique vis-à-vis cells in 2D environments, such as the recently described 3D-matrix adhesion assemblies. This review examines how the dimensionality of the extracellular environment can affect in vitro cell mechanobiology, focusing on collagen and fibrin systems.

The Penetration of Anticancer Drugs through Tumor Tissue as a Function of Cellular Adhesion and Packing Density of Tumor Cells

To reach cancer cells in optimal quantities, therapeutic agents must be delivered to tumors through their imperfect blood vascular system, cross vessel walls into the interstitium, and penetrate multiple layers of tissue. Strategies to enhance drug penetration have potential to improve therapeutic outcome. The development of multicellular layers (MCLs), in which tumor cells are grown on a semipermeable Teflon support membrane, has facilitated quantification of drug penetration through solid tissue. The goals of the present study were to quantify the penetration of anticancer drugs as a function of cellular adhesion and packing density and to determine the effects of variable penetration on therapeutic efficacy in this model system. We compared the properties of MCLs grown from two epithelioid and round subclones of a colon carcinoma cell line. One pair of epithelioid and round sublines differed in expression of alpha-E-catenin, and both pairs generated MCLs with different packing density. The penetration of commonly used anticancer agents (paclitaxel, doxorubicin, methotrexate, and 5-fluorouracil) through MCLs derived from these cell lines was significantly greater through the round (loosely packed) than through the epithelioid (tightly packed) sublines. In MCLs treated with doxorubicin, we observed greater survival in the tightly packed cell lines than in the loosely packed cell lines. Impaired penetration of anticancer agents through MCLs derived from the tightly packed cell lines and relative resistance to killing of cells within them by doxorubicin treatment strengthen the role of tumor cell adhesion and packing density as contributing to drug resistance.

Selective antibody-mediated targeting of class I MHC to EGFR-expressing tumor cells induces potent antitumor CTL activityin vitro andin vivo

Epidermal growth factor receptor (EGFR) is highly overexpressed in many tumor types. We present a new fusion molecule that can target solid tumors that express EGFR. The fusion molecule combines the advantage(s) of the well-established tumor targeting capabilities of high affinity recombinant fragments of antibodies with the known efficient, specific and potent killing ability of CD8 T lymphocytes directed against highly antigenic MHC/peptide complexes. A recombinant chimeric molecule was created by the genetic fusion of the scFv antibody fragment derived from the anti-EGFR monoclonal antibody C225, to monomeric single-chain HLA-A2 complexes containing immunodominant tumor or viral-specific peptides. The fusion protein can induce very efficiently CTL-dependent lysis of EGFR-expressing tumor cells regardless of the expression of self peptide-MHC complexes. Moreover, the molecule exhibited very potent antitumor activity in vivo in nude mice bearing preestablished human tumor xenografts. These in vitro and in vivo results indicate that recombinant scFv-MHC-peptide fusion molecules might represent a novel and powerful approach to immunotherapy of solid tumors, bridging antibody and T lymphocyte attack on cancer cells.

Salmonella typhimurium specifically chemotax and proliferate in heterogeneous tumor tissue in vitro

Multi-drug resistance greatly limits the efficacy of conventional blood-born chemotherapeutics, which have limited ability to penetrate tumor tissue and are ineffective at killing quiescent cells far from tumor vasculature. Nonpathogenic, motile bacteria can overcome both of theses limitations. We hypothesize that the accumulation of S. typhimurium in tumors is controlled by two mechanisms: (1) chemotaxis towards compounds produced by quiescent cancer cells and (2) preferential growth within tumor tissue. We tested this hypothesis by quantifying the relative contributions of these mechanisms using the tumor cylindroid model, which mimics the microenvironments of in vivo tumors. Time-lapse fluorescence microscopy was used to measure the accumulation of GFP-labeled S. typhimurium into cylindroids of different size. Cylindroids larger than 500 microm in diameter contain quiescent cells, whereas cylindroids smaller than 500 microm do not. Spatio-temporal profiles of bacterial concentration were fit to a mathematical model to calculate two parameters that describe bacterial interaction with tumors: intratumoral bacterial growth, M, and intratumoral bacterial chemoattraction, K. It was observed that S. typhimurium is attracted to cylindroids and accumulate at long time points in the central region of large cylindroids. Both intratumoral bacterial growth and chemotaxis were significantly greater in large cylindroids, suggesting that quiescent cells secrete bacterial chemoattractants and the presence of necrotic and quiescent cells enable S. typhimurium to replicate in tumor tissue. In this study, several mechanisms of S. typhimurium accumulation in solid tumors have been quantified, which we believe is an important step in the development of bacterial-based therapeutics to target tumor quiescence.

Expression of the 5T4 oncofoetal antigen in renal cell carcinoma: a potential target for T-cell-based immunotherapy

The 5T4 oncofoetal antigen is a heavily glycosylated cell surface protein found on human placental trophoblast and on diverse types of human cancer but is not expressed at significant levels on adult human tissues in health. It therefore satisfies the criteria for a tumour-associated antigen and is an ideal target for the immunotherapy of cancer. We report here that 5T4 is strongly expressed on the majority of renal cell carcinomas and therefore this population of patients is suitable for trials of 5T4-targeted therapies. In particular, we have shown that T cells from renal cell carcinoma patients can be genetically modified to kill 5T4 expressing renal cancer cell lines by introduction of a chimeric-signalling protein. This protein consists of a single chain antibody fragment capable of binding antigen directly at the cell surface and then activating the T cell by virtue of a CD3zeta-signalling domain. This is a powerful tool that bypasses a number of mechanisms that allow tumours to escape T-cell killing and can be readily scaled up for clinical use.

Magnetic resonance imaging reveals functional diversity of the vasculature in benign and malignant breast lesions

BACKGROUND

Tumor perfusion through the microvascular network can be imaged noninvasively by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). The objective of the current study was to quantify the microvascular perfusion parameters in various human breast lesions and to determine whether they varied between benign lesions and malignancy and whether they were altered with increased invasiveness.

METHODS

Perfusion parameters in 22 benign fibrocystic changes, 15 ductal carcinomas in situ (DCIS), 30 infiltrating ductal carcinomas (IDC), and 22 fibroadenomas were measured using high-resolution DCE-MRI. Pixel-by-pixel image analysis yielded parametric images of two perfusion indicators: the influx transcapillary transfer constant (k(trans)) and the efflux transcapillary rate constant (k(ep)). Correlations of lesion type and perfusion parameters were calculated using Spearman correlation. Logistic regression analysis evaluated the best predictors of the kinetic parameters that differentiate between IDC and benign lesions.

RESULTS

The perfusion parameters exhibited a progressive increase from benign fibrocystic changes to DCIS and IDC, with a significant correlation between lesion type and the parameters' values (range of correlation coefficients, 0.56-0.76; P < 0.0001). In addition, k(trans) increased from low-grade DCIS to high-grade DCIS. Fibroadenomas were characterized uniquely by high k(trans) but low k(ep). Stepwise logistic regression selected k(trans) as the best predictor for distinguishing benign fibrocystic changes from IDC, yielding 93% sensitivity and 96% specificity.

CONCLUSIONS

The microvascular perfusion parameters in breast lesions were elevated with invasiveness. Quantification of these parameters using high-resolution DCE-MRI was helpful for differentiating between breast lesions and should improve breast carcinoma diagnosis.

111In-labelled somatostatin analogues in a rat tumour model: somatostatin receptor status and effects of peptide receptor radionuclide therapy

PURPOSE

Peptide receptor scintigraphy with the radioactive somatostatin analogue 111In-DTPA-octreotide is a sensitive and specific technique to show in vivo the presence of somatostatin receptors on various tumours. Since 111In emits not only gamma rays but also therapeutic Auger and internal conversion electrons with a medium to short tissue penetration (0.02-10 microm and 200-550 microm, respectively), 111In-DTPA-octreotide is also being used for peptide receptor radionuclide therapy (PRRT). In this study we investigated the therapeutic effects of 111In-DTPA-octreotide in tumours of various sizes. Regrowth of a tumour despite PRRT with 111In-DTPA-octreotide may be due to the lack of crossfire from 111In, whereby any possible receptor-negative tumour cell can multiply. We therefore also investigated the somatostatin receptor status of the tumour before and after PRRT.

METHODS

The radiotherapeutic effects of different doses of 111In-DTPA-octreotide in vivo were investigated in Lewis rats bearing small (< or = 1 cm2) or large (> or = 8 cm2) somatostatin receptor-positive rat pancreatic CA20948 tumours expressing the somatostatin receptor subtype 2 (sst2). In addition, the somatostatin receptor density on the tumour after injection of a therapeutic labelled somatostatin analogue was investigated when the tumour was either declining in size or regrowing after an initial reduction in size. To initiate a partial response of the tumour (so that regrowth would follow) and not a complete response, a relatively low dose was administered.

RESULTS

Impressive radiotherapeutic effects of 111In-labelled octreotide were observed in this rat tumour model. Complete responses (up to 50%) were found in the animals bearing small (< or 1 cm2) tumours after at least three injections of 111 MBq or a single injection of 370 MBq 111In-DTPA-octreotide, leading to a dose of 6.3-7.8 mGy/MBq (1-10 g tumour). In the rats bearing the larger (> or = 8 cm2) tumours, the effects were much less pronounced and only partial responses were achieved in these groups. Clear sst2 expression was found in the control as well as in the treated tumours. A significantly higher tumour receptor density (p<0.001) was found when the tumours regrew after an initial decline in size after low-dose PRRT in comparison with the untreated tumours.

CONCLUSION

Therapy with 111In-labelled somatostatin analogues is feasible but should preferably start as early as possible during tumour development. One might also consider the use of radiolabelled somatostatin analogues in an adjuvant setting after surgery of somatostatin receptor-positive tumours in order to eradicate occult metastases. We showed that PRRT led to an increase in the density of somatostatin receptors when the tumours regrew after an initial decline in size because of PRRT. Upregulation of the somatostatin receptor may lead to higher uptake of radiolabelled peptides in therapeutic applications, which would probably make repeated injections of radiolabelled peptides more effective.

In vivo molecular and cellular imaging with quantum dots

Quantum dots (QDs), tiny light-emitting particles on the nanometer scale, are emerging as a new class of fluorescent probe for in vivo biomolecular and cellular imaging. In comparison with organic dyes and fluorescent proteins, QDs have unique optical and electronic properties: size-tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous excitation of multiple fluorescence colors. Recent advances have led to the development of multifunctional nanoparticle probes that are very bright and stable under complex in vivo conditions. A new structural design involves encapsulating luminescent QDs with amphiphilic block copolymers and linking the polymer coating to tumor-targeting ligands and drug delivery functionalities. Polymer-encapsulated QDs are essentially nontoxic to cells and animals, but their long-term in vivo toxicity and degradation need more careful study. Bioconjugated QDs have raised new possibilities for ultrasensitive and multiplexed imaging of molecular targets in living cells, animal models and possibly in humans.

Radiolabeled Small-Molecule Ligands for Prostate-Specific Membrane Antigen: In vivo Imaging in Experimental Models of Prostate Cancer

PURPOSE

Prostate-specific membrane antigen (PSMA) is a cell surface protein that is overexpressed in prostate cancer, including hormone-refractory and metastatic disease. Our goal in this study was to develop a series of PSMA-based imaging agents for clinical use.

EXPERIMENTAL DESIGN

We have synthesized and evaluated the in vivo biodistribution of two radiolabeled urea derivatives that have high affinity for PSMA in severe combined immunodeficient mice harboring MCF-7 (breast, PSMA-negative), PC-3 (prostate, PSMA-negative), and LNCaP (prostate, PSMA-positive) xenografts. Radiopharmaceutical binding selectivity and tumor uptake were also evaluated in vivo using dedicated small animal positron emission tomography, single photon emission computed tomography, and gamma scintigraphic imaging devices. N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-S-[(11)C]methyl-L-cysteine ([(11)C]DCMC K(i), 3.1 nmol/L) and N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-S-3-[(125)I]iodo-L-tyrosine ([(125)C]DCIT K(i), 1.5 nmol/L) were synthesized using [(11)C]CH(3)I and with [(125)I]NaI/Iodogen, respectively.

RESULTS

At 30 minutes postinjection, [(11)C]DCMC and [(125)I]DCIT showed tumor/muscle ratios of 10.8 and 4.7, respectively, with clear delineation of LNCaP-derived tumors on imaging. MCF-7- and PC-3-derived tumors showed significantly less uptake of [(11)C]DCMC or [(125)I]DCIT.

CONCLUSION

These results show the feasibility of imaging PSMA-positive prostate cancer using low molecular weight agents.

Codon-Optimized Gaussia Luciferase cDNA for Mammalian Gene Expression in Culture and in Vivo

Photoproteins have played a major role in advancing our understanding of biological processes. A broader array of biocompatible, nontoxic, and novel reporters can serve to expand this potential. Here we describe the properties of a luciferase from the copepod marine organism Gaussia princeps. It is a monomeric protein composed of 185 aa (19.9 kDa) with a short coding sequence (555 bp) making it suitable for viral vectors. The humanized form of Gaussia luciferase (hGLuc) was efficiently expressed in mammalian cells following delivery by HSV-1 amplicon vectors. It was found to be nontoxic and naturally secreted, with flash bioluminescence characteristics similar to those of other coelenterazine luciferases. hGLuc generated over 1000-fold higher bioluminescent signal intensity from live cells together with their immediate environment and over 100-fold higher intensity from viable cells alone (not including secreted luciferase) or cell lysates, compared to humanized forms of firefly (hFLuc) and Renilla (hRLuc) luciferases expressed under similar conditions. Furthermore, hGLuc showed 200-fold higher signal intensity than hRLuc and intensity comparable to that of hFLuc in vivo under standard imaging conditions. Gaussia luciferase provides a sensitive means of imaging gene delivery and other events in living cells in culture and in vivo, with a unique combination of features including high signal intensity, secretion, and ATP independence, thus being able to report from the cells and their environment in real time.