Direct in vivo measurement of targeted binding in a human tumor xenograft

In Silico Investigation of the Clinical Translatability of Competitive Clearance Glucose-Responsive Insulins

The glucose-responsive insulin (GRI) MK-2640 from Merck was a pioneer in its class to enter the clinical stage, having demonstrated promising responsiveness in in vitro and preclinical studies via a novel competitive clearance mechanism (CCM). The smaller pharmacokinetic response in humans motivates the development of new predictive, computational tools that can improve the design of therapeutics such as GRIs. Herein, we develop and use a new computational model, IM3PACT, based on the intersection of human and animal model glucoregulatory systems, to investigate the clinical translatability of CCM GRIs based on existing preclinical and clinical data of MK-2640 and regular human insulin (RHI). Simulated multi-glycemic clamps not only validated the earlier hypothesis of insufficient glucose-responsive clearance capacity in humans but also uncovered an equally important mismatch between the in vivo competitiveness profile and the physiological glycemic range, which was not observed in animals. Removing the inter-species gap increases the glucose-dependent GRI clearance from 13.0% to beyond 20% for humans and up to 33.3% when both factors were corrected. The intrinsic clearance rate, potency, and distribution volume did not apparently compromise the translation. The analysis also confirms a responsive pharmacokinetics local to the liver. By scanning a large design space for CCM GRIs, we found that the mannose receptor physiology in humans remains limiting even for the most optimally designed candidate. Overall, we show that this computational approach is able to extract quantitative and mechanistic information of value from a posteriori analysis of preclinical and clinical data to assist future therapeutic discovery and development.

Prognostic and immunological role of SERPINH1 in pan-cancer

Background: The SERPINH1 gene plays a vital part in tumorigenesis and development, whereas its potential as an immunotherapy target is still unknown. Hence, this research aimed to probe the roles of SERPINH1 in human tumors. Method: Using The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx) database, Oncomine, and SangerBox software, the pan-cancer expression of SERPINH1 and its correlation were systematically analyzed. SERPINH1 protein information was detected by the Human Protein Atlas (HPA) database and STRING database. The genomic alterations of SERPINH1 were studied using the c-BioPortal database. The influence of SERPINH1 on prognosis was analyzed using Kaplan-Meier plotter. The R package "clusterProfiler" was used for enrichment analysis to detect the role of SERPINH1. The TIMER2 database was used to further analyze the correlation between the immune cell infiltration score of TCGA samples and the expression of SERPINH1. Results: SERPINH1 overexpression was related to worse survival status in pan-cancer. In addition, high expression of SERPINH1 was positively associated with tumor stage and poor prognosis. Moreover, SERPINH1 played an important role in tumor microenvironment and immune regulation. Our study revealed that SERPINH1 expression has a strong correlation with immune cell filtration, immune regulation, chemokines, and immune checkpoints. Conclusion: Our research found that SERPINH1 was a risk factor and predictor of poor prognosis in various tumors. High expression of SERPINH1 may contribute to tumor immune-suppressive status. Also, SERPINH1 may become a potential immunotherapy target in pan-cancer.

Development and Evaluation of Competitive Inhibitors of Trastuzumab-HER2 Binding to Bypass the Binding-Site Barrier

Our group has developed and experimentally validated a strategy to increase antibody penetration in solid tumors through transient inhibition of antibody-antigen binding. In prior work, we demonstrated that 1HE, an anti-trastuzumab single domain antibody that transiently inhibits trastuzumab binding to HER2, increased the penetration of trastuzumab and increased the efficacy of ado-trastuzumab emtansine (T-DM1) in HER2+ xenograft bearing mice. In the present work, 1HE variants were developed using random mutagenesis and phage display to enable optimization of tumor penetration and efficacy of trastuzumab-based therapeutics. To guide the rational selection of a particular 1HE mutant for a specific trastuzumab-therapy, we developed a mechanistic pharmacokinetic (PK) model to predict within-tumor exposure of trastuzumab/T-DM1. A pharmacodynamic (PD) component was added to the model to predict the relationship between intratumor exposure to T-DM1 and the corresponding therapeutic effect in HER2+ xenografts. To demonstrate the utility of the competitive inhibition approach for immunotoxins, PK parameters specific for a recombinant immunotoxin were incorporated into the model structure. Dissociation half-lives for variants ranged from 1.1 h (for variant LG11) to 107.9 h (for variant HE10). Simulations predicted that 1HE co-administration can increase the tumor penetration of T-DM1, with inhibitors with longer trastuzumab binding half-lives relative to 1HE (15.5 h) further increasing T-DM1 penetration at the expense of total tumor uptake of T-DM1. The PK/PD model accurately predicted the response of NCI-N87 xenografts to treatment with T-DM1 or T-DM1 co-administered with 1HE. Model predictions indicate that the 1HE mutant HF9, with a trastuzumab binding half-life of 51.1 h, would be the optimal inhibitor for increasing T-DM1 efficacy with a modest extension in the median survival time relative to T-DM1 with 1HE. Model simulations predict that LG11 co-administration will dramatically increase immunotoxin penetration within all tumor regions. We expect that the mechanistic model structure and the wide range of inhibitors developed in this work will enable optimization of trastuzumab-cytotoxin penetration and efficacy in solid tumors.

The Use of Fluorescent Anti-CEA Antibodies to Label, Resect and Treat Cancers: A Review

A major barrier to the diagnosis and effective treatment of solid-tumor cancers is the difficulty in detection and visualization of tumor margins in primary and metastatic disease. The use of fluorescence can augment the surgeon's ability to detect cancer and aid in its resection. Several cancer types express carcinoembryonic antigen (CEA) including colorectal, pancreatic and gastric cancer. Antibodies to CEA have been developed and tagged with near-infrared fluorescent dyes. This review article surveyed the use of CEA antibodies conjugated to fluorescent probes for in vivo studies since 1990. PubMed and Google Scholar databases were queried, and 900 titles and abstracts were screened. Fifty-nine entries were identified as possibly meeting inclusion/exclusion criteria and were reviewed in full. Forty articles were included in the review and their citations were screened for additional entries. A total of 44 articles were included in the final review. The use of fluorescent anti-CEA antibodies has been shown to improve detection and resection of tumors in both murine models and clinically. The cumulative results indicate that fluorescent-conjugated anti-CEA antibodies have important potential to improve cancer diagnosis and surgery. In an emerging technology, anti-CEA fluorescent antibodies have also been successfully used for photoimmunotherapy treatment for cancer.

A computational diffusion model to study antibody transport within reconstructed tumor microenvironments

Background

Antibodies revolutionized cancer treatment over the past decades. Despite their successfully application, there are still challenges to overcome to improve efficacy, such as the heterogeneous distribution of antibodies within tumors. Tumor microenvironment features, such as the distribution of tumor and other cell types and the composition of the extracellular matrix may work together to hinder antibodies from reaching the target tumor cells. To understand these interactions, we propose a framework combining in vitro and in silico models. We took advantage of in vitro cancer models previously developed by our group, consisting of tumor cells and fibroblasts co-cultured in 3D within alginate capsules, for reconstruction of tumor microenvironment features.

Results

In this work, an experimental-computational framework of antibody transport within alginate capsules was established, assuming a purely diffusive transport, combined with an exponential saturation effect that mimics the saturation of binding sites on the cell surface. Our tumor microenvironment in vitro models were challenged with a fluorescent antibody and its transport recorded using light sheet fluorescence microscopy. Diffusion and saturation parameters of the computational model were adjusted to reproduce the experimental antibody distribution, with root mean square error under 5%. This computational framework is flexible and can simulate different random distributions of tumor microenvironment elements (fibroblasts, cancer cells and collagen fibers) within the capsule. The random distribution algorithm can be tuned to follow the general patterns observed in the experimental models.

Conclusions

We present a computational and microscopy framework to track and simulate antibody transport within the tumor microenvironment that complements the previously established in vitro models platform. This framework paves the way to the development of a valuable tool to study the influence of different components of the tumor microenvironment on antibody transport.

Bioinformatic Profiling of Prognosis-Related Genes in Malignant Glioma Microenvironment

BACKGROUND Gliomas are the most common primary tumors of the brain and spinal cord. The tumor microenvironment (TME) is the cellular environment in which tumors exist. This study aimed to identify the role of the TME and the effects of genes involved in the TME of malignant glioma. MATERIAL AND METHODS The ESTIMATE algorithms in the R package were used to calculate the immune and stromal scores of samples in the TCGA and GSE4290 datasets. The associations of stromal and immune scores with clinicopathological characteristics and overall survival of malignant glioma patients were assessed by analysis of variance and Kaplan-Meier analysis. Differentially expressed genes (DEGs) were obtained through the median immune and stromal score using the R package "limma". Functional enrichment analysis and the PPI network MCODE were used to analyze DEGs. RESULTS Increased immune and stromal scores were closely related with advanced glioma grade and poor prognosis (all P<0.01). In total, 558 DEGs were found and most were related to tumor prognosis. Functional enrichment analysis showed that DEGs were associated with cell-matrix regulation and immune response. Four hub modules related to tumor angiogenesis, collagen formation, and immune response were found and analyzed. Previously overlooked microenvironment-related genes such as LAMB1, FN1, ACTN1, TRIM, SERPINH1, CYBA, LAIR1, and LILRB2 showed prognostic values in malignant glioma patients. CONCLUSIONS The glioma stromal/immune scores are closely related to glioma grade, histology, and survival time. Some glioma microenvironment-related genes including LAMB1, FN1, ACTN1, TRIM6, SERPINH1, CYBA, LAIR1, and LILRB2 show prognostic values in malignant gliomas and serve as potential biomarkers.

Intratumoral distribution of YSNSG cyclopeptide in a mouse melanoma model using microdialysis

The YSNSG peptide is a synthetic cyclopeptide targeting α_vβ₃ integrin with antitumor activity. Previous study has determined main pharmacokinetic parameters in plasma and in tissue in healthy animals using microdialysis. First we aim to assess the impact of a 20 mg/kg dosage instead of 10 mg/kg in tumor growth inhibition. Secondly we aim to investigate the YSNSG peptide distribution in two different tumor regions in animals with melanoma. C57BL/6 mice were exposed at Days 8, 10 and 12 after melanoma cells implantation (B16F1) to different dosage of YSNSG peptide or control, respectively (n = 10 per group). Data analysis was performed at D16, 20 and 24 with a Nonlinear Mixed-Effects (NLME) approach. For pharmacokinetic study n = 8 mice (same disease condition) received YSNSG peptide by intravenous after insertion of two microdialysis probes in central peripheral region of tumor, respectively. Plasma and tissue samples were collected during 2 h. A non-compartmental analysis was performed to determine main pharmacokinetic parameters. There was a significant tumor growth inhibition in mice receiving 20 mg/kg vs Control (p < 0.02). Main plasma parameters were half-life elimination 25.8 ± 8.2 min, volume of distribution 11.9 ± 0.4 mL, clearance 19.8 ± 9.4 mL/h and area under the curve 1,173.6 µg.min/mL. Penetration rate of the YSNSG peptide from plasma to tumor tissue were 3.3 ± 2.1% and 3.4 ± 2.7% in central and peripheral, respectively. Contrary to subcutaneous distribution in healthy animals the distribution of the YSNSG peptide into tumoral tissue is low but seems non-heterogeneous between central and peripheral tumor region.

Physiologically Based Modeling of the Pharmacokinetics of "Catch-and-Release" Anti-Carcinoembryonic Antigen Monoclonal Antibodies in Colorectal Cancer Xenograft Mouse Models

Engineered monoclonal antibodies (mAbs) with pH-sensitive target release, or "catch-and-release" (CAR) binding, have shown promise in decreasing the extent of target-mediated mAb elimination, increasing mAb exposure, and increasing dose potency. This study developed a mechanistic physiologically based pharmacokinetic (PBPK) model to evaluate the effects of pH-sensitive CAR target binding on the disposition of anti-carcinoembryonic antigen (CEA) mAbs in mouse models of colorectal cancer. The PBPK model was qualified by comparing model-predicted plasma concentration-time data with data observed in tumor-bearing mice following the administration of T84.66, a "standard" anti-CEA mAb that demonstrates strong binding at pH 7.4 and 5.5. Further simulations evaluated the effects CAR pH-dependent binding, with decreasing CEA affinity with decreasing pH, on anti-CEA mAb plasma pharmacokinetics. Simulated data were compared with data observed for a novel CAR mAb, 10H6. The PBPK model provided precise parameter estimates, and excellent data characterization (median prediction error 18.4%) following fitting to T84.66 data. Simulations well predicted 10H6 data (median prediction error 21.4%). Sensitivity analyses demonstrated that key determinants of the disposition of CAR mAbs include the following: antigen binding affinity, the rate constant of mAb-CEA dissociation in acidified endosomes, antigen concentration, and the tumor vasculature reflection coefficient.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Noninvasive Interrogation of DLL3 Expression in Metastatic Small Cell Lung Cancer

The Notch ligand DLL3 has emerged as a novel therapeutic target expressed in small cell lung cancer (SCLC) and high-grade neuroendocrine carcinomas. Rovalpituzumab teserine (Rova-T; SC16LD6.5) is a first-in-class DLL3-targeted antibody-drug conjugate with encouraging initial safety and efficacy profiles in SCLC in the clinic. Here we demonstrate that tumor expression of DLL3, although orders of magnitude lower in surface protein expression than typical oncology targets of immunoPET, can serve as an imaging biomarker for SCLC. We developed ⁸⁹Zr-labeled SC16 antibody as a companion diagnostic agent to facilitate selection of patients for treatment with Rova-T based on a noninvasive interrogation of the in vivo status of DLL3 expression using PET imaging. Despite low cell-surface abundance of DLL3, immunoPET imaging with ⁸⁹Zr-labeled SC16 antibody enabled delineation of subcutaneous and orthotopic SCLC tumor xenografts as well as distant organ metastases with high sensitivity. Uptake of the radiotracer in tumors was concordant with levels of DLL3 expression and, most notably, DLL3 immunoPET yielded rank-order correlation for response to SC16LD6.5 therapy in SCLC patient-derived xenograft models. Cancer Res; 77(14); 3931-41. ©2017 AACR.

Kinetics of Reactive Modules Adds Discriminative Dimensions for Selective Cell Imaging

Living cells are chemical mixtures of exceptional interest and significance, whose investigation requires the development of powerful analytical tools fulfilling the demanding constraints resulting from their singular features. In particular, multiplexed observation of a large number of molecular targets with high spatiotemporal resolution appears highly desirable. One attractive road to address this analytical challenge relies on engaging the targets in reactions and exploiting the rich kinetic signature of the resulting reactive module, which originates from its topology and its rate constants. This review explores the various facets of this promising strategy. We first emphasize the singularity of the content of a living cell as a chemical mixture and suggest that its multiplexed observation is significant and timely. Then, we show that exploiting the kinetics of analytical processes is relevant to selectively detect a given analyte: upon perturbing the system, the kinetic window associated to response read-out has to be matched with that of the targeted reactive module. Eventually, we introduce the state-of-the-art of cell imaging exploiting protocols based on reaction kinetics and draw some promising perspectives.

Tumor microenvironment and therapeutic response

The tumor microenvironment significantly influences therapeutic response and clinical outcome. Microenvironment-mediated drug resistance can be induced by soluble factors secreted by tumor or stromal cells. The adhesion of tumor cells to stromal fibroblasts or to components of the extracellular matrix can also blunt therapeutic response. Microenvironment-targeted therapy strategies include inhibition of the extracellular ligand-receptor interactions and downstream pathways. Immune cells can both improve and obstruct therapeutic efficacy and may vary in their activation status within the tumor microenvironment; thus, re-programme of the immune response would be substantially more beneficial. The development of rational drug combinations that can simultaneously target tumor cells and the microenvironment may represent a solution to overcome therapeutic resistance.

Spatial Modeling of Drug Delivery Routes for Treatment of Disseminated Ovarian Cancer

In ovarian cancer, metastasis is typically confined to the peritoneum. Surgical removal of the primary tumor and macroscopic secondary tumors is a common practice, but more effective strategies are needed to target microscopic spheroids persisting in the peritoneal fluid after debulking surgery. To treat this residual disease, therapeutic agents can be administered by either intravenous or intraperitoneal infusion. Here, we describe the use of a cellular Potts model to compare tumor penetration of two classes of drugs (cisplatin and pertuzumab) when delivered by these two alternative routes. The model considers the primary route when the drug is administered either intravenously or intraperitoneally, as well as the subsequent exchange into the other delivery volume as a secondary route. By accounting for these dynamics, the model revealed that intraperitoneal infusion is the markedly superior route for delivery of both small-molecule and antibody therapies into microscopic, avascular tumors typical of patients with ascites. Small tumors attached to peritoneal organs, with vascularity ranging from 2% to 10%, also show enhanced drug delivery via the intraperitoneal route, even though tumor vessels can act as sinks during the dissemination of small molecules. Furthermore, we assessed the ability of the antibody to enter the tumor by in silico and in vivo methods and suggest that optimization of antibody delivery is an important criterion underlying the efficacy of these and other biologics. The use of both delivery routes may provide the best total coverage of tumors, depending on their size and vascularity.

A mechanistic tumor penetration model to guide antibody drug conjugate design

Antibody drug conjugates (ADCs) represent novel anti-cancer modalities engineered to specifically target and kill tumor cells expressing corresponding antigens. Due to their large size and their complex kinetics, these therapeutic agents often face heterogeneous distributions in tumors, leading to large untargeted regions that escape therapy. We present a modeling framework which includes the systemic distribution, vascular permeability, interstitial transport, as well as binding and payload release kinetics of ADC-therapeutic agents in mouse xenografts. We focused, in particular, on receptor dynamics such as endocytic trafficking mechanisms within cancer cells, to simulate their impact on tumor mass shrinkage upon ADC administration. Our model identified undesirable tumor properties that can impair ADC tissue homogeneity, further compromising ADC success, and explored ADC design optimization scenarios to counteract upon such unfavorable intrinsic tumor tissue attributes. We further demonstrated the profound impact of cytotoxic payload release mechanisms and the role of bystander killing effects on tumor shrinkage. This model platform affords a customizable simulation environment which can aid with experimental data interpretation and the design of ADC therapeutic treatments.

Time-dependent pretreatment with bevacuzimab increases tumor specific uptake of cetuximab in preclinical oral cavity cancer studies

Inadequate delivery of therapeutics into tumors has been suggested as a reason for poor response. We hypothesize that bevacizumab, an antibody to vascular endothelial growth factor (VEGF), can improve cetuximab uptake in squamous cell carcinoma tumors. Athymic nude mice were implanted with OSC19 and SCC1 human cancer lines in a subcutaneous flank model. Mice were imaged daily for 14 days after intravenous tail vein injections of the following groups: IgG-IRDye800 (Control), cetuximab-IRDye800 (CTX800 Only), bevacizumab-IRDye800 (BVZ800 Only), cetuximab-IRDye800 + bevacuzimuab-IRDye800 (Simultaneous), and unlabeled bevacizumab followed by cetuximab-IRDye800 3 days later (Neoadjuvant). Within single-agent groups, the CTX800 Only tumor-specific uptake (TSU) was significantly higher than BVZ800 Only at Day 13 (TSU 8.6 vs 2.8, P < 0.001). The Simultaneous treatment with BVZ800 and CTX800 demonstrated no increase in antibody delivery. However, administration of unlabeled bevacizumab 3 days prior to CTX800 (Neoadjuvant group) resulted in significantly higher tumor specific delivery than administration of both antibodies at the same time (11.8 vs Simultaneous 5.0, P < 0.001). This difference can be attributed to a slower decline in tumor fluorescence intensity (-6.8% vs. Simultaneous -11.5% per day, respectively). Structural changes in pericyte coverage and functional vessel changes demonstrating decreased proliferation and tumor growth corroborate these fluorescence results. Although simultaneous administration of bevacizumab with cetuximab failed to increase antibody delivery to the tumor, pretreatment with bevacizumab improved TSU reflecting an increase in tumor-specific uptake of cetuximab as a result of vessel normalization.

Toward Localized In Vivo Biomarker Concentration Measurements

We know a great deal about the biochemistry of cells because they can be isolated and studied. The biochemistry of the much more complex in vivo environment is more difficult to study because the only ways to quantitate concentrations is to sacrifice the animal or biopsy the tissue. Either method disrupts the environment profoundly and neither method allows longitudinal studies on the same individual. Methods of measuring chemical concentrations in vivo are very valuable alternatives to sacrificing groups of animals. We are developing microscopic magnetic nanoparticle (mNP) probes to measure the concentration of a selected molecule in vivo. The mNPs are targeted to bind the selected molecule and the resulting reduction in rotational freedom can be quantified remotely using magnetic spectroscopy. The mNPs must be contained in micrometer sized porous shells to keep them from migrating and to protect them from clearance by the immune system. There are two key issues in the development of the probes. First, we demonstrate the ability to measure concentrations in the porous walled alginate probes both in phosphate buffered saline and in blood, which is an excellent surrogate for the complex and challenging in vivo environment. Second, sensitivity is critical because it allows microscopic probes to measure very small concentrations very far away. We report sensitivity measurements on recently introduced technology that has allowed us to improve the sensitivity by two orders of magnitude, a factor of 200 so far.

Toward Localized In Vivo Biomarker Concentration Measurements

We know a great deal about the biochemistry of cells because they can be isolated and studied. The biochemistry of the much more complex in vivo environment is more difficult to study because the only ways to quantitate concentrations is to sacrifice the animal or biopsy the tissue. Either method disrupts the environment profoundly and neither method allows longitudinal studies on the same individual. Methods of measuring chemical concentrations in vivo are very valuable alternatives to sacrificing groups of animals. We are developing microscopic magnetic nanoparticle (mNP) probes to measure the concentration of a selected molecule in vivo. The mNPs are targeted to bind the selected molecule and the resulting reduction in rotational freedom can be quantified remotely using magnetic spectroscopy. The mNPs must be contained in micrometer sized porous shells to keep them from migrating and to protect them from clearance by the immune system. There are two key issues in the development of the probes. First, we demonstrate the ability to measure concentrations in the porous walled alginate probes both in phosphate buffered saline and in blood, which is an excellent surrogate for the complex and challenging in vivo environment. Second, sensitivity is critical because it allows microscopic probes to measure very small concentrations very far away. We report sensitivity measurements on recently introduced technology that has allowed us to improve the sensitivity by two orders of magnitude, a factor of 200 so far.

Single- and two-photon fluorescence recovery after photobleaching

Fluorescence recovery after photobleaching (FRAP) is a microscopy technique for measuring the kinetics of fluorescently labeled molecules and can be applied both in vitro and in vivo for two- and three-dimensional systems. This introduction discusses the three basic FRAP methods: traditional FRAP, multiphoton FRAP (MPFRAP), and FRAP with spatial Fourier analysis (SFA-FRAP). Each discussion is accompanied by a description of the mathematical analysis appropriate for situations in which the recovery kinetics is dictated by free diffusion. In some experiments, the recovery kinetics is dictated by the boundary conditions of the system, and FRAP is then used to quantify the connectivity of various compartments. Because the appropriate mathematical analysis is independent of the bleaching method, the analysis of compartmental connectivity is discussed last, in a separate section.

Quantification of blood-brain barrier solute permeability and brain transport by multiphoton microscopy

Development of an optimal systemic drug delivery strategy to the brain will require noninvasive or minimally invasive methods to quantify the permeability of the cerebral microvessel wall or blood-brain barrier (BBB) to various therapeutic agents and to measure their transport in the brain tissue. To address this problem, we used laser-scanning multiphoton microscopy to determine BBB permeability to solutes (P) and effective solute diffusion coefficients (Deff) in rat brain tissue 100-250 μm below the pia mater. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a microgrinder. Sodium fluorescein, fluorescein isothiocyanate (FITC)-dextrans, or Alexa Fluor 488-immunoglobulin G (IgG) in 1% bovine serum albumin (BSA) mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery by a syringe pump at a constant rate of ∼3 ml/min. P and Deff were determined from the rate of tissue solute accumulation and the radial concentration gradient around individual microvessels in the brain tissue. The mean apparent permeability P values for sodium fluorescein (molecular weight (MW) 376 Da), dextran-4k, -20k, -40k, -70k, and IgG (MW ∼160 kDa) were 14.6, 6.2, 1.8, 1.4, 1.3, and 0.54 × 10-7 cm/s, respectively. These P values were not significantly different from those of rat pial microvessels for the same-sized solutes (Yuan et al., 2009, "Non-Invasive Measurement of Solute Permeability in Cerebral Microvessels of the Rat," Microvasc. Res., 77(2), pp. 166-73), except for the small solute sodium fluorescein, suggesting that pial microvessels can be a good model for studying BBB transport of relatively large solutes. The mean Deff values were 33.2, 4.4, 1.3, 0.89, 0.59, and 0.47 × 10-7 cm2/s, respectively, for sodium fluorescein, dextran-4k, -20k, -40k, -70k, and IgG. The corresponding mean ratio of Deff to the free diffusion coefficient Dfree, Deff/Dfree, were 0.46, 0.19, 0.12, 0.12, 0.11, and 0.11 for these solutes. While there is a significant difference in Deff/Dfree between small (e.g., sodium fluorescein) and larger solutes, there is no significant difference in Deff/Dfree between solutes with molecular weights from 20,000 to 160,000 Da, suggesting that the relative resistance of the brain tissue to macromolecular solutes is similar over a wide size range. The quantitative transport parameters measured from this study can be used to develop better strategies for brain drug delivery.

Quantity and accessibility for specific targeting of receptors in tumours

Synaphic (ligand-directed) targeting of drugs is an important potential new approach to drug delivery, particularly in oncology. Considerable success with this approach has been achieved in the treatment of blood-borne cancers, but the advances with solid tumours have been modest. Here, we have studied the number and availability for ligand binding of the receptors for two targeting ligands. The results show that both paucity of total receptors and their poor availability are major bottlenecks in drug targeting. A tumour-penetrating peptide greatly increases the availability of receptors by promoting transport of the drug to the extravascular tumour tissue, but the number of available receptors still remains low, severely limiting the utility of the approach. Our results emphasize the importance of using drugs with high specific activity to avoid exceeding receptor capacity because any excess drug conjugate would lose the targeting advantage. The mathematical models we describe make it possible to focus on those aspects of the targeting mechanism that are most likely to have a substantial effect on the overall efficacy of the targeting.

Modeling tumor microenvironments in vitro

Tumor progression depends critically upon the interactions between the tumor cells and their microenvironment. The tumor microenvironment is heterogeneous and dynamic; it consists of extracellular matrix, stromal cells, immune cells, progenitor cells, and blood and lymphatic vessels. The emerging fields of tissue engineering and microtechnologies have opened up new possibilities for engineering physiologically relevant and spatially well-defined microenvironments. These in vitro models allow specific manipulation of biophysical and biochemical parameters, such as chemical gradients, biomatrix stiffness, metabolic stress, and fluid flows; thus providing a means to study their roles in certain aspects of tumor progression such as cell proliferation, invasion, and crosstalk with other cell types. Challenges and perspectives for deconvolving the complexity of tumor microenvironments will be discussed. Emphasis will be given to in vitro models of tumor cell migration and invasion.

Strategies for advancing cancer nanomedicine

Cancer nanomedicines approved so far minimize toxicity, but their efficacy is often limited by physiological barriers posed by the tumour microenvironment. Here, we discuss how these barriers can be overcome through innovative nanomedicine design and through creative manipulation of the tumour microenvironment.

Measuring interstitial diffusion, convection, and binding parameters in mouse tumors

Noninvasive techniques have been developed for the assessment of various parameters in normal and diseased tissues of mice. This protocol describes the measurement of extravascular parameters, including interstitial diffusion, convection, and binding parameters, in mouse tumors. A fluorescently labeled molecule of interest is infused into the tumor interstitium, followed by imaging using single-photon microscopy or multiphoton laser-scanning microscopy (MPLSM). Fluorescence recovery after photobleaching (FRAP) with spatial Fourier analysis is performed. To measure interstitial diffusion coefficients, multiphoton FRAP is performed.

Measuring leukocyte-endothelial interactions in mice

Intravital imaging techniques are powerful tools in the study of tumor pathophysiology. These techniques can be used to measure vascular parameters (e.g., angiogenesis, hemodynamics, and leukocyte-endothelial interactions) in tumors and normal vascular networks in mice. This protocol describes the use of single-photon microscopy and multiphoton laser-scanning microscopy to follow the migration of Rhodamine-6G, a fluorescently labeled tracer, following injection into the tail vein of a mouse. The Rhodamine-6G enters the circulatory system and labels leukocytes. It is possible to monitor changes in the interactions between leukocytes and the endothelium by determining the numbers of rolling and adhering leukocytes as well as the total flux of these cells.

Measuring vascular permeability in mice

Noninvasive techniques have been developed for the assessment of vascular parameters, including vascular permeability, in normal and diseased tissues of mice. In this protocol, mice are injected with an appropriate fluorescent tracer. The effective average vascular permeability (P) of a region of vessels is measured by single-photon microscopy. Additionally, the vascular permeability of individual vessel segments within ~600 µm of a tumor/window interface is measured by multiphoton laser-scanning microscopy (MPLSM).

Measuring angiogenesis and hemodynamics in mice

This protocol outlines methods to measure vascular parameters, including angiogenesis (e.g., vascular density, length, diameter) and hemodynamics (e.g., erythrocyte velocity), in tumors and normal vascular networks in mice. Fluorescein-isothiocyanate (FITC)-dextran is injected into the tail vein of mice to visualize microvessels within 150 μm (using single-photon microscopy) or ~600 μm (using multiphoton laser-scanning microscopy [MPLSM]) of a tumor/window interface. Randomly selected areas (three to six locations/tumor or animal) are investigated using long-working-distance objectives with appropriate magnification. During each observation period, FITC-fluorescence images are recorded for 60 sec, and the videotapes are analyzed off-line for single-photon microscopy; or a three-dimensional (3D) image stack of the vessel network is generated, and vessel properties are measured for MPLSM. If desired, red blood cell (RBC) flux can be measured on a vessel-by-vessel basis using fluorescent tracer RBCs.

Delivery of molecular and cellular medicine to solid tumors

To reach cancer cells in a tumor, a blood-borne therapeutic molecule or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically, and then integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of physiological barriers in solid tumors, and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

Application of PBPK modeling to predict monoclonal antibody disposition in plasma and tissues in mouse models of human colorectal cancer

This investigation evaluated the utility of a physiologically based pharmacokinetic (PBPK) model, which incorporates model parameters representing key determinants of monoclonal antibody (mAb) target-mediated disposition, to predict, a priori, mAb disposition in plasma and in tissues, including tumors that express target antigens. Monte Carlo simulation techniques were employed to predict the disposition of two mAbs, 8C2 (as a non-binding control mouse IgG1 mAb) and T84.66 (a high-affinity murine IgG1 anti-carcinoembryonic antigen mAb), in mice bearing no tumors, or bearing colorectal HT29 or LS174T xenografts. Model parameters were obtained or derived from the literature. (125)I-T84.66 and (125)I-8C2 were administered to groups of SCID mice, and plasma and tissue concentrations were determined via gamma counting. The PBPK model well-predicted the experimental data. Comparisons of the population predicted versus observed areas under the plasma concentration versus time curve (AUC) for T84.66 were 95.4 ± 67.8 versus 84.0 ± 3.0, 1,859 ± 682 versus 2,370 ± 154, and 5,930 ± 1,375 versus 5,960 ± 317 (nM × day) at 1, 10, and 25 mg/kg in LS174T xenograft-bearing SCID mice; and 215 ± 72 versus 233 ± 30, 3,070 ± 346 versus 3,120 ± 180, and 7,884 ± 714 versus 7,440 ± 626 in HT29 xenograft-bearing mice. Model predicted versus observed 8C2 plasma AUCs were 312.4 ± 30 versus 182 ± 7.6 and 7,619 ± 738 versus 7,840 ± 24.3 (nM × day) at 1 and 25 mg/kg. High correlations were observed between the predicted median plasma concentrations and observed median plasma concentrations (r (2) = 0.927, for all combinations of treatment, dose, and tumor model), highlighting the utility of the PBPK model for the a priori prediction of in vivo data.

A mechanistic compartmental model for total antibody uptake in tumors

Antibodies are under development to treat a variety of cancers, such as lymphomas, colon, and breast cancer. A major limitation to greater efficacy for this class of drugs is poor distribution in vivo. Localization of antibodies occurs slowly, often in insufficient therapeutic amounts, and distributes heterogeneously throughout the tumor. While the microdistribution around individual vessels is important for many therapies, the total amount of antibody localized in the tumor is paramount for many applications such as imaging, determining the therapeutic index with antibody drug conjugates, and dosing in radioimmunotherapy. With imaging and pretargeted therapeutic strategies, the time course of uptake is critical in determining when to take an image or deliver a secondary reagent. We present here a simple mechanistic model of antibody uptake and retention that captures the major rates that determine the time course of antibody concentration within a tumor including dose, affinity, plasma clearance, target expression, internalization, permeability, and vascularization. Since many of the parameters are known or can be estimated in vitro, this model can approximate the time course of antibody concentration in tumors to aid in experimental design, data interpretation, and strategies to improve localization.

Measurement of magnetic nanoparticle relaxation time

PURPOSE

Nanoparticle relaxation time measurements have many applications including characterizing molecular binding, viscosity, heating, and local matrix stiffness. The methods capable of in vivo application are extremely limited. The hypothesis investigated by the authors was that the relaxation time could be measured quantitatively using magnetic spectroscopy of nanoparticle Brownian motion (MSB).

METHODS

The MSB signal (1) reflects the nanoparticle rotational Brownian motion, (2) can be measured from very low nanoparticle concentrations, and (3) is a function of the product of the drive frequency and the relaxation time characterizing Brownian motion. To estimate the relaxation time, the MSB signal was measured at several frequencies. The MSB signal for nanoparticles with altered relaxation time is a scaled version of that for reference nanoparticles with a known relaxation time. The scaling factor linking the altered and reference MSB measurements is the same factor linking the altered and reference relaxation times. The method was tested using glycerol solutions of varying viscosities to obtain continuously variable relaxation times.

RESULTS

The measured relaxation time increased with increasing viscosity of the solution in which the nanoparticles resided. The MSB estimated relaxation time matched the calculated relaxation times based on viscosity with 2% average error.

CONCLUSIONS

MSB can be used to monitor the nanoparticle relaxation time quantitatively through a scale space correlation of the MSB signal as a function of frequency.

A microfluidic system for investigation of extravascular transport and cellular uptake of drugs in tumors

Three-dimensional (3D) tumor models have been established in various microfluidic systems for drug delivery and resistance studies in vitro. However, one of the main drawbacks of these models is non-uniform distribution of cells, leaving regions with very low cell density within the 3D structures. As a result, molecular diffusion in the cell compartments is faster than that observed in solid tumors. To solve this problem, we developed a new technique for preparation of 3D tumor models in vitro. It was based on a microfluidic device containing three parallel channels separated by narrowly spaced posts. Tumor cells were loaded into the central channel at high density. To test the system, B16.F10 melanoma cells were perfusion-cultured overnight and the resulting 3D structure was characterized in terms of viability, density, and morphology of cells as well as transport properties of small fluorescent molecules. Immediately upon loading of tumor cells, the cell density was comparable to those observed in B16.F10 tumor tissues in vivo; and the viability of tumor cells was maintained through the overnight culture. The tumor model displayed low extracellular space and high resistance to diffusion of small molecules. For membrane-permeant molecules (e.g., Hoechst 33342), the rate of interstitial penetration was extremely slow, compared to membrane-impermeant molecules (e.g., sodium fluorescein). This versatile tumor model could be applied to in vitro studies of transport and cellular uptake of drugs and genes.

Temporal changes in microvessel leakiness during wound healing discriminated by in vivo fluorescence recovery after photobleaching

Regeneration of injured tissue is a dynamic process, critically dependent on the formation of new blood vessels and restructuring of the nascent plexus. Endothelial barrier function, a functional correlate of vascular restructuring and maturation, was quantified via intravital microscopic analysis of 150 kDa FITC-dextran-perfused blood vessels within discrete wounds created in the panniculus carnosus (PC) muscle of dorsal skinfold chamber (DSC) preparations in mice. Time to recovery of half-peak fluorescence intensity (t(1/2)) within individual vessel segments in three functional regions of the wound (pre-existing vessels, angiogenic plexus and blind-ended vessels (BEVs)) was quantified using in vivo fluorescence recovery after photobleaching (FRAP) and linear regression analysis of recovery profiles. Plasma flux across the walls of new vessel segments, particularly BEVs, was greater than that of pre-existing vessels at days 5-7 after injury (P < 0.05). TNP-470 reduced the permeability of BEVs at the leading edge of the advancing vascular plexus as measured by the decrease in luminal t(1/2) (P < 0.05), confirming the utility of FRAP as a quantitative measure of endothelial barrier function. Furthermore, these data are suggestive of a role for TNP-470 in selection for less leaky vascular segments within healing wounds. Increased FITC-dextran leakage was observed from pre-existing vessels after treatment with TNP-470 (P < 0.05), consistent with induction of transient vascular damage, although the significance of this finding is unclear. Using in vivo FRAP this study demonstrates the relationship between temporal changes in microvascular macromolecular flux and the morphology of maturing vascular segments. This combination of techniques may be useful to assess the therapeutic potential of angiogenic agents in restoring pre-injury levels of endothelial barrier function, following the establishment of a functional vascular plexus such as in models of wounding or tumour development.

Diffusion anisotropy in collagen gels and tumors: the effect of fiber network orientation

The interstitial matrix is comprised of cross-linked collagen fibers, generally arranged in nonisotropic orientations. Spatial alignment of matrix components within the tissue can affect diffusion patterns of drugs. In this study, we developed a methodology for the calculation of diffusion coefficients of macromolecules and nanoparticles in collagenous tissues. The tissues are modeled as three-dimensional, stochastic, fiber networks with varying degrees of alignment. We employed a random walk approach to simulate diffusion and a Stokesian dynamics method to account for hydrodynamic hindrance. We performed our analysis for four different structures ranging from nearly isotropic to perfectly aligned. We showed that the overall diffusion coefficient is not affected by the orientation of the network. However, structural anisotropy results in diffusion anisotropy, which becomes more significant with increase in the degree of alignment, the size of the diffusing particle, and the fiber volume fraction. To test our model predictions we performed diffusion measurements in reconstituted collagen gels and tumor xenografts. We measured fiber alignment and diffusion with second harmonic generation and multiphoton fluorescent recovery after photobleaching techniques, respectively. The results showed for the first time in tumors that the structure and orientation of collagen fibers in the extracellular space leads to diffusion anisotropy.

Effective electrophoretic mobilities and charges of anti-VEGF proteins determined by capillary zone electrophoresis

Macromolecules such as therapeutic proteins currently serve an important role in the treatment of eye diseases such as wet age-related macular degeneration and diabetic retinopathy. Particularly, bevacizumab and ranibizumab have been shown to be effective in the treatment of these diseases. Iontophoresis can be employed to enhance ocular delivery of these macromolecules, but the lack of information on the properties of these macromolecules has hindered its development. The objectives of the present study were to determine the effective electrophoretic mobilities and charges of bevacizumab, ranibizumab, and model compound polystyrene sulfonate (PSS) using capillary zone electrophoresis. Salicylate, lidocaine, and bovine serum albumin (BSA), which have known electrophoretic mobilities in the literature, were also studied to validate the present technique. The hydrodynamic radii and diffusion coefficients of BSA, bevacizumab, ranibizumab, and PSS were measured by dynamic light scattering. The effective charges were calculated using the Einstein relation between diffusion coefficient and electrophoretic mobility and the Henry equation. The results show that bevacizumab and ranibizumab have low electrophoretic mobilities and are net negatively charged in phosphate buffered saline (PBS) of pH 7.4 and 0.16M ionic strength. PSS has high negative charge but the electrophoretic mobility in PBS is lower than that expected from the polymer structure. The present study demonstrated that capillary electrophoresis could be used to characterize the mobility and charge properties of drug candidates in the development of iontophoretic drug delivery.

Tumor microvasculature and microenvironment: novel insights through intravital imaging in pre-clinical models

Intravital imaging techniques have provided unprecedented insight into tumor microcirculation and microenvironment. For example, these techniques allowed quantitative evaluations of tumor blood vasculature to uncover its abnormal organization, structure and function (e.g., hyper-permeability, heterogeneous and compromised blood flow). Similarly, imaging of functional lymphatics has documented their absence inside tumors. These abnormalities result in elevated interstitial fluid pressure and hinder the delivery of therapeutic agents to tumors. In addition, they induce a hostile microenvironment characterized by hypoxia and acidosis, as documented by intravital imaging. The abnormal microenvironment further lowers the effectiveness of anti-tumor treatments such as radiation therapy and chemotherapy. In addition to these mechanistic insights, intravital imaging may also offer new opportunities to improve therapy. For example, tumor angiogenesis results in immature, dysfunctional vessels--primarily caused by an imbalance in production of pro- and anti-angiogenic factors by the tumors. Restoring the balance of pro- and anti-angiogenic signaling in tumors can "normalize" tumor vasculature and thus, improve its function, as demonstrated by intravital imaging studies in preclinical models and in cancer patients. Administration of cytotoxic therapy during periods of vascular normalization has the potential to enhance treatment efficacy.

ImmunoPET imaging of B-cell lymphoma using 124I-anti-CD20 scFv dimers (diabodies)

Rapid clearing engineered antibody fragments for immunoPET promise high sensitivity at early time points. Here, tumor targeting of anti-CD20 diabodies (scFv dimers) for detection of low-grade B-cell lymphomas were evaluated. In addition, the effect of linker length on oligomerization of the diabody was investigated. Four rituximab scFv variants in the V(L)-V(H) orientation with different linker lengths between the V domains (scFv-1, scFv-3, scFv-5, scFv-8), plus the scFv-5 with a C-terminal cysteine (Cys-Db) for site-specific modification were generated. The scFv-8 and Cys-Db were radioiodinated with (124)I for PET imaging, and biodistribution of (131)I-Cys-Db was carried out at 2, 4 10 and 20 h. The five anti-CD20 scFv variants were expressed as fully functional dimers. Shortening the linker to three or one residue did not produce higher order of multimers. Both (124)I-labeled scFv-8 and Cys-Db exhibited similar tumor targeting at 8 h post injection, with significantly higher uptakes than in control tumors (P < 0.05). At 20 h, less than 1% ID/g of (131)I-labeled Cys-Db was present in tumors and tissues. Specific tumor targeting and high contrast images were achieved with the anti-CD20 diabodies. These agents extend the repertoire of reagents that can potentially be used to improve detection of low-grade lymphomas.

Intravital imaging of stromal cell dynamics in tumors

Tumor stroma, consisting of the extracellular matrix and multiple cell types such as immune cells, fibroblasts and vascular cells, contributes to the malignancy of solid tumors by a variety of mechanisms. Intravital imaging by different microscopy techniques, especially by confocal and multi-photon microscopy, has proven to be a powerful method for analyzing the cell-cell and cell-matrix interactions in the dynamic tumor microenvironments. Intravital imaging has fostered the acquisition of data on parameters such as motility of different cell types in distinct tumor regions or manipulated with defined challenges, kinetics of tumor cell killing by T cells or macrophage-assisted tumor cell extravasation, functionality of the vasculature, protease activity and metabolic state. Achieving the direct observation of intact tumors offered by intravital imaging provides unique insights into tumor biology that will continue to deepen our understanding of the processes leading to malignancy and of the ways they can be targeted.

Intravital microscopy of tumor angiogenesis and regression in the dorsal skin fold chamber: mechanistic insights and preclinical testing of therapeutic strategies

Tumor angiogenesis is a major step in tumor progression to clinically symptomatic cancer and thus a potential target for cancer therapy. It is essential to understand the fundamental mechanisms of the angiogenic processes to provide a rational for testing inhibitory strategies for cancer treatment. The dorsal skin fold chamber provides a suitable (chronic) model for intravital microscopy to monitor the same tumor in time-lapse imaging series and in real-time functional analysis e.g., of blood flow. Adaptation of this model to several rodent species and tumor types has led to numerous physical and drug based therapy options. With modification of implantation techniques, motility and invasion of individual cells can be visualized, in addition to angiogenesis and microcirculation. Modern fluorescent techniques such as ex vivo labelling of specific cell populations and the introduction of stably fluorescent protein expressing cell lines further enhance the suitability of this technique. In addition, laser scanning and multiphoton microscopy in combination with genetically altered mouse strains and cell lines are making the DCSF even more attractive for mechanistic and interventional studies in cancer research. Here we review the preparation as well as the applications of the DCSF in tumor angiogenesis.

Imaging angiogenesis and the microenvironment

Intravital microscopy has provided unprecedented insights into tumor pathophysiology, including angiogenesis and the microenvironment. Tumor vasculature shows an abnormal organization, structure, and function. Tumor vessels are leaky, blood flow is heterogeneous and often compromised. Vascular hyperpermeability and the lack of functional lymphatic vessels inside tumors causes elevation of interstitial fluid pressure in solid tumors. These abnormalities form physiological barriers to the delivery of therapeutic agents to tumors and also lead to a hostile microenvironment characterized by hypoxia and acidosis, which hinders the effectiveness of anti-tumor treatments such as radiation therapy and chemotherapy. In addition, host-tumor interactions regulate expression of pro- and anti-angiogenic factors, resulting in pathophysiological characteristics of the tumor. On the other hand, in a physiological setting, angiogenic vessels become mature and form long-lasting functional units. Restoring the balance of pro- and anti-angiogenic factors in tumors may "normalize" tumor vasculature and thus improve its function. Administration of cytotoxic therapy during the vascular normalization would enhance its efficacy.

Diffusion in brain extracellular space

Diffusion in the extracellular space (ECS) of the brain is constrained by the volume fraction and the tortuosity and a modified diffusion equation represents the transport behavior of many molecules in the brain. Deviations from the equation reveal loss of molecules across the blood-brain barrier, through cellular uptake, binding, or other mechanisms. Early diffusion measurements used radiolabeled sucrose and other tracers. Presently, the real-time iontophoresis (RTI) method is employed for small ions and the integrative optical imaging (IOI) method for fluorescent macromolecules, including dextrans or proteins. Theoretical models and simulations of the ECS have explored the influence of ECS geometry, effects of dead-space microdomains, extracellular matrix, and interaction of macromolecules with ECS channels. Extensive experimental studies with the RTI method employing the cation tetramethylammonium (TMA) in normal brain tissue show that the volume fraction of the ECS typically is approximately 20% and the tortuosity is approximately 1.6 (i.e., free diffusion coefficient of TMA is reduced by 2.6), although there are regional variations. These parameters change during development and aging. Diffusion properties have been characterized in several interventions, including brain stimulation, osmotic challenge, and knockout of extracellular matrix components. Measurements have also been made during ischemia, in models of Alzheimer's and Parkinson's diseases, and in human gliomas. Overall, these studies improve our conception of ECS structure and the roles of glia and extracellular matrix in modulating the ECS microenvironment. Knowledge of ECS diffusion properties is valuable in contexts ranging from understanding extrasynaptic volume transmission to the development of paradigms for drug delivery to the brain.

Antibody tumor penetration: transport opposed by systemic and antigen-mediated clearance

Antibodies have proven to be effective agents in cancer imaging and therapy. One of the major challenges still facing the field is the heterogeneous distribution of these agents in tumors when administered systemically. Large regions of untargeted cells can therefore escape therapy and potentially select for more resistant cells. We present here a summary of theoretical and experimental approaches to analyze and improve antibody penetration in tumor tissue.

An experimental approach to measure mass diffusion in rat tumor tissue

The objective of this research is to evaluate the usefulness of a macroscopic, fluorescent, imaging technique to quantify spatiotemporal mass transport parameters in in vitro solid tumor tissues taken from rat models. Fluorescent images captured during the experiments are digitally analyzed to determine the concentration of a fluorescent marker dye as it diffuses into tissue specimens taken from rat tumors. The collected concentration data are used to estimate local diffusion coefficients. An analysis of the distribution of the local diffusion data indicates that the local diffusion coefficient is spatially dependent within the tumor tissue. When mass transfer is restricted to one dimension, the current technique can be used to determine the concentration distribution of fluorescent molecules on the tissue surface and to estimate the mass transfer parameters within the heterogeneous tumor tissue.

Recombinant carcinoembryonic antigen as a reporter gene for molecular imaging

PURPOSE

Reporter genes can provide a way of noninvasively assessing gene activity in vivo. However, current reporter gene strategies may be limited by the immunogenicity of foreign reporter proteins, endogenous expression, or unwanted biological activity. We have developed a reporter gene based on carcinoembryonic antigen (CEA), a human protein with limited normal tissue expression.

METHODS

To construct a CEA reporter gene for PET, a CEA minigene (N-A3) was fused to the extracellular and transmembrane domains of the human Fc gamma RIIb receptor. The NA3-Fc gamma RIIb recombinant gene, driven by a CMV promoter, was transfected in Jurkat (human T cell leukemia) cells. Expression was analyzed by flow cytometry, immunohistochemistry (IHC), and microPET imaging.

RESULTS

Flow cytometry identified Jurkat clones stably expressing NA3-Fc gamma RIIb at low, medium, and high levels. High and medium NA3-Fc gamma RIIb expression could also be detected by Western blot. Reporter gene positive and negative Jurkat cells were used to establish xenografts in athymic mice. IHC showed staining of the tumor with high reporter gene expression; medium and low N-A3 expression was not detected. MicroPET imaging, using an anti-CEA (124)I-labeled single-chain Fv-Fc antibody fragment, demonstrated that only high N-A3 expression could be detected. Specific accumulation of activity was visualized at the N-A3 positive tumor as early as 4 h. MicroPET image quantitation showed tumor activity of 1.8 +/- 0.2, 15.2 +/- 1.3, and 4.6 +/- 1.2 percent injected dose per gram (%ID/g) at 4, 20, and 48 h, respectively. Biodistribution at 48 h demonstrated tumor uptake of 4.8 +/- 0.8%ID/g.

CONCLUSION

The CEA N-A3 minigene has the potential to be used as a reporter gene for imaging cells in vivo.