In vivo imaging of insulin receptors by PET: preclinical evaluation of iodine-125 and iodine-124 labelled human insulin

The historical progression of positron emission tomography research in neuroendocrinology

The rapid and continual development of a number of radiopharmaceuticals targeting different receptor, enzyme and small molecule systems has fostered Positron Emission Tomography (PET) imaging of endocrine system actions in vivo in the human brain for several decades. PET radioligands have been developed to measure changes that are regulated by hormone action (e.g., glucose metabolism, cerebral blood flow, dopamine receptors) and actions within endocrine organs or glands such as steroids (e.g., glucocorticoids receptors), hormones (e.g., estrogen, insulin), and enzymes (e.g., aromatase). This systematic review is targeted to the neuroendocrinology community that may be interested in learning about positron emission tomography (PET) imaging for use in their research. Covering neuroendocrine PET research over the past half century, researchers and clinicians will be able to answer the question of where future research may benefit from the strengths of PET imaging.

Hot Spots for the Use of Intranasal Insulin: Cerebral Ischemia, Brain Injury, Diabetes Mellitus, Endocrine Disorders and Postoperative Delirium

A decrease in the activity of the insulin signaling system of the brain, due to both central insulin resistance and insulin deficiency, leads to neurodegeneration and impaired regulation of appetite, metabolism, endocrine functions. This is due to the neuroprotective properties of brain insulin and its leading role in maintaining glucose homeostasis in the brain, as well as in the regulation of the brain signaling network responsible for the functioning of the nervous, endocrine, and other systems. One of the approaches to restore the activity of the insulin system of the brain is the use of intranasally administered insulin (INI). Currently, INI is being considered as a promising drug to treat Alzheimer's disease and mild cognitive impairment. The clinical application of INI is being developed for the treatment of other neurodegenerative diseases and improve cognitive abilities in stress, overwork, and depression. At the same time, much attention has recently been paid to the prospects of using INI for the treatment of cerebral ischemia, traumatic brain injuries, and postoperative delirium (after anesthesia), as well as diabetes mellitus and its complications, including dysfunctions in the gonadal and thyroid axes. This review is devoted to the prospects and current trends in the use of INI for the treatment of these diseases, which, although differing in etiology and pathogenesis, are characterized by impaired insulin signaling in the brain.

Metal-ion coordinated self-assembly of human insulin directs kinetics of insulin release as determined by preclinical SPECT/CT imaging

Human insulin (HI) has fascinating metal-facilitated self-assembly properties that are essential for its biological function. HI has a natural Zn²⁺ binding site and we have previously shown that covalently attached abiotic ligands (e.g., bipyridine, terpyridine) can lead to the formation of nanosized oligomeric structures through the coordination of metal ions. Here we studied the hypothesis that metal ions can be used to directly control the pharmacokinetics of insulin after covalent attachment of an abiotic ligand that binds metal ions. We evaluated the pharmacokinetics (PK) and biodistribution of HI self-assemblies directed by metal ion coordination (i.e., Fe²⁺/Zn²⁺, Eu³⁺/Zn²⁺, Fe²⁺/Co³⁺) using preclinical SPECT/CT imaging and ex vivo gamma counting. HI was site-specifically modified with terpyridine (Tpy) at the Phe^B1 or Lys^B29 position to create conjugates that bind either Fe²⁺ or Eu³⁺, while its natural binding site (e.g., His^B10) preferentially coordinates with either Zn²⁺ or Co³⁺. HI was also functionalized with trans-cyclooctene (TCO) opposite to Tpy at Phe^B1 or Lys^B29, respectively, to allow for tetrazine-TCO coupling via a tetrazine-modified DTPA followed by ¹¹¹In-radiolabeling for SPECT/CT imaging. When the ¹¹¹In-B29Tpy-HI conjugate was coordinated with Fe²⁺/Zn²⁺, its retention at the injection site 6 h after injection was ~8-fold higher than the control without the metal ions, while its kidney accumulation was lower. ¹¹¹In-B1Tpy-HI showed comparable retention at the injection site 6 h after injection and slightly increased retention at 24 h. However, higher kidney accumulation and residence time of degraded ¹¹¹In-B1Tpy-HI was observed compared to that of ¹¹¹In-B29Tpy-HI. Quantitative PK analysis based on SPECT/CT images confirmed slower distribution from the injection site of the HI-metal ion assemblies compared to control HI conjugates. Our results show that the Tpy-binding site (i.e., Phe^B1 or Lys^B29) on HI and its coordination with the added metal ions (i.e., Fe²⁺/Zn²⁺ or Fe²⁺/Co³⁺) directed the distribution half-life of HI significantly. This clearly indicates that the PK of insulin can be controlled by complexation with different metal ions.

Efficacy, Pharmacokinetics, Biodistribution and Excretion of a Novel Acylated Long-Acting Insulin Analogue INS061 in Rats

Purpose

Long-acting insulin analogues are known to be a major player in the management of glucose levels in type I diabetic patients. However, highly frequent hypo- and hyperglycemic incidences of current long-acting insulins are the important factor to limit stable management of glucose level for clinical benefits. To further optimize the properties for steadily controlling glucose level, a novel long-acting insulin INS061 was designed and its efficacy, pharmacokinetics, biodistribution and excretion profiles were investigated in rats.

Methods

The glucose-lowering effects were evaluated in a streptozocin-induced diabetic rats compared to commercial insulins via subcutaneous administration. The pharmacokinetics, biodistribution, and excretion were examined by validated analytical methods including radioactivity assay and radioactivity assay after the precipitation with TCA and the separation by HPLC.

Results

INS061 exhibited favorable blood glucose lowering effects up to 24 h compared to Degludec. Pharmacokinetic study revealed that the concentration-time curves of INS061 between two administration routes were remarkably different. Following intravenous administration, INS061 was quickly distributed to various organs and tissues and slowly eliminated over time with urinary excretion being the major route for elimination, and the maximum plasma concentrations (Cmax) and systemic exposures (AUC) increased in a linear manner.

Conclusion

The present structural modifications of human insulin possessed a long-acting profile and glucose-lowering function along with favorable in vivo properties in rats, which establish a foundation for further preclinical and clinical evaluation.

Brain insulin action in schizophrenia: Something borrowed and something new

Insulin signaling in the central nervous system is at the intersection of brain and body interactions, and represents a fundamental link between metabolic and cognitive disorders. Abnormalities in brain insulin action could underlie the development of comorbid schizophrenia and type 2 diabetes. Among its functions, central nervous system insulin is involved in regulation of striatal dopamine levels, peripheral glucose homeostasis, and feeding regulation. In this review, we discuss the role and importance of central nervous system insulin in schizophrenia and diabetes pathogenesis from a historical and mechanistic perspective. We describe central nervous system insulin sites and pathways of action, with special emphasis on glucose metabolism, cognitive functioning, inflammation, and food preferences. Finally, we suggest possible mechanisms that may explain the actions of central nervous system insulin in relation to schizophrenia and diabetes, focusing on glutamate and dopamine signaling, intracellular signal transduction pathways, and brain energetics. Understanding the interplay between central nervous system insulin and schizophrenia is essential to disentangling this comorbid relationship and may provide novel treatment approaches for both neuropsychiatric and metabolic dysfunction. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Probing insulin sensitivity in diabetic kidney disease: is there a stronger role for functional imaging?

Clinical and experimental evidence support a cause-effect relationship between altered insulin signaling and development of kidney disease of metabolic and non-metabolic origin. However, the current criteria to measure and/or estimate the insulin resistance (IR) are available as research tool but are very difficult to implement in the clinical practice. Therefore, a better understanding of the key players contributing to IR may lead to the development of new non-invasive tools to assess organ-specific insulin sensitivity (IS). We will therefore first introduce the concept that IR and kidney disease may be causally linked as suggested by clinical and experimental studies. We will then, expand on the potential mechanisms leading to altered renal insulin signaling. After reviewing the limitation of currently available strategies to determine IR, this review article will focus on imaging techniques that could be utilized to determine renal IR and that could be tested to predict kidney disease development and progression.

(125)I-Tetrazines and Inverse-Electron-Demand Diels-Alder Chemistry: A Convenient Radioiodination Strategy for Biomolecule Labeling, Screening, and Biodistribution Studies

A convenient method to prepare radioiodinated tetrazines was developed, such that a bioorthogonal inverse electron demand Diels-Alder reaction can be used to label biomolecules with iodine-125 for in vitro screening and in vivo biodistribution studies. The tetrazine was prepared by employing a high-yielding oxidative halo destannylation reaction that concomitantly oxidized the dihydrotetrazine precursor. The product reacts quickly and efficiently with trans-cyclooctene derivatives. Utility was demonstrated through antibody and hormone labeling experiments and by evaluating products using standard analytical methods, in vitro assays, and quantitative biodistribution studies where the latter was performed in direct comparison to Bolton-Hunter and direct iodination methods. The approach described provides a convenient and advantageous alternative to conventional protein iodination methods that can expedite preclinical development and evaluation of biotherapeutics.

124 Iodine: a longer-life positron emitter isotope-new opportunities in molecular imaging

(124)Iodine ((124)I) with its 4.2 d half-life is particularly attractive for in vivo detection and quantification of longer-term biological and physiological processes; the long half-life of (124)I is especially suited for prolonged time in vivo studies of high molecular weight compounds uptake. Numerous small molecules and larger compounds like proteins and antibodies have been successfully labeled with (124)I. Advances in radionuclide production allow the effective availability of sufficient quantities of (124)I on small biomedical cyclotrons for molecular imaging purposes. Radioiodination chemistry with (124)I relies on well-established radioiodine labeling methods, which consists mainly in nucleophilic and electrophilic substitution reactions. The physical characteristics of (124)I permit taking advantages of the higher PET image quality. The availability of new molecules that may be targeted with (124)I represents one of the more interesting reasons for the attention in nuclear medicine. We aim to discuss all iodine radioisotopes application focusing on (124)I, which seems to be the most promising for its half-life, radiation emissions, and stability, allowing several applications in oncological and nononcological fields.

Production of iodine-124 and its applications in nuclear medicine

Until recently, iodine-124 was not considered to be an attractive isotope for medical applications owing to its complex radioactive decay scheme, which includes several high-energy gamma rays. However, its unique chemical properties, and convenient half-life of 4.2 days indicated it would be only a matter of time for its frequent application to become a reality. The development of new medical imaging techniques, especially improvements in the technology of positron emission tomography (PET), such as the development of new detectors and signal processing electronics, has opened up new prospects for its application. With the increasing use of PET in medical oncology, pharmacokinetics, and drug metabolism, (124)I-labeled radiopharmaceuticals are now becoming one of the most useful tools for PET imaging, and owing to the convenient half-life of I-124, they can be used in PET scanners far away from the radionuclide production site. Thus far, the limited availability of this radionuclide has been an impediment to its wider application in clinical use. For example, sodium [(124)I]-iodide is potentially useful for diagnosis and dosimetry in thyroid disease and [(124)I]-M-iodobenzylguanidine ([(124)I]-MIBG) has enormous potential for use in cardiovascular imaging, diagnosis, and dosimetry of malignant diseases such as neuroblastoma, paraganglioma, pheochromocytoma, and carcinoids. However, despite that potential, both are still not widely used. This is a typical scenario of a rising new star among the new PET tracers.

Insulin, IGF-1 and GLP-1 signaling in neurodegenerative disorders: targets for disease modification?

Insulin and Insulin Growth Factor-1 (IGF-1) play a major role in body homeostasis and glucose regulation. They also have paracrine/autocrine functions in the brain. The Insulin/IGF-1 signaling pathway contributes to the control of neuronal excitability, nerve cell metabolism and cell survival. Glucagon like peptide-1 (GLP-1), known as an insulinotropic hormone has similar functions and growth like properties as insulin/IGF-1. Growing evidence suggests that dysfunction of these pathways contribute to the progressive loss of neurons in Alzheimer's disease (AD) and Parkinson's disease (PD), the two most frequent neurodegenerative disorders. These findings have led to numerous studies in preclinical models of neurodegenerative disorders targeting insulin/IGF-1 and GLP-1 signaling with currently available anti-diabetics. These studies have shown that administration of insulin, IGF-1 and GLP-1 agonists reverses signaling abnormalities and has positive effects on surrogate markers of neurodegeneration and behavioral outcomes. Several proof-of-concept studies are underway that attempt to translate the encouraging preclinical results to patients suffering from AD and PD. In the first part of this review, we discuss physiological functions of insulin/IGF-1 and GLP-1 signaling pathways including downstream targets and receptors distribution within the brain. In the second part, we undertake a comprehensive overview of preclinical studies targeting insulin/IGF-1 or GLP-1 signaling for treating AD and PD. We then detail the design of clinical trials that have used anti-diabetics for treating AD and PD patients. We close with future considerations that treat relevant issues for successful translation of these encouraging preclinical results into treatments for patients with AD and PD.

Lung imaging - two dimensional gamma scintigraphy, SPECT, CT and PET

This review will cover the principles of imaging the deposition of inhaled drugs and some of the state-of-the art imaging techniques being used today. Aerosol deposition can be imaged and quantified by the addition of a radiolabel to the aerosol formulation. The subsequent imaging of the inhaled deposition pattern can be acquired by different imaging techniques. Specifically, this review will focus on the use of two-dimensional planar, gamma scintigraphy, SPECT, CT and PET. This review will look at how these imaging techniques are used to investigate the mechanisms of drug delivery in the lung and how the lung anatomy and physiology have the potential to alter therapeutic outcomes.

Bringing physiology into PET of the liver

Several physiologic features make interpretation of PET studies of liver physiology an exciting challenge. As with other organs, hepatic tracer kinetics using PET is quantified by dynamic recording of the liver after the administration of a radioactive tracer, with measurements of time-activity curves in the blood supply. However, the liver receives blood from both the portal vein and the hepatic artery, with the peak of the portal vein time-activity curve being delayed and dispersed compared with that of the hepatic artery. The use of a flow-weighted dual-input time-activity curve is of importance for the estimation of hepatic blood perfusion through initial dynamic PET recording. The portal vein is inaccessible in humans, and methods of estimating the dual-input time-activity curve without portal vein measurements are being developed. Such methods are used to estimate regional hepatic blood perfusion, for example, by means of the initial part of a dynamic (18)F-FDG PET/CT recording. Later, steady-state hepatic metabolism can be assessed using only the arterial input, provided that neither the tracer nor its metabolites are irreversibly trapped in the prehepatic splanchnic area within the acquisition period. This is used in studies of regulation of hepatic metabolism of, for example, (18)F-FDG and (11)C-palmitate.

Iodine-124: a promising positron emitter for organic PET chemistry

The use of radiopharmaceuticals for molecular imaging of biochemical and physiological processes in vivo has evolved into an important diagnostic tool in modern nuclear medicine and medical research. Positron emission tomography (PET) is currently the most sophisticated molecular imaging methodology, mainly due to the unrivalled high sensitivity which allows for the studying of biochemistry in vivo on the molecular level. The most frequently used radionuclides for PET have relatively short half-lives (e.g. ¹¹C: 20.4 min; ¹⁸F: 109.8 min) which may limit both the synthesis procedures and the time frame of PET studies. Iodine-124 (¹²⁴I, t_1/2 = 4.2 d) is an alternative long-lived PET radionuclide attracting increasing interest for long term clinical and small animal PET studies. The present review gives a survey on the use of ¹²⁴I as promising PET radionuclide for molecular imaging. The first part describes the production of ¹²⁴I. The second part covers basic radiochemistry with ¹²⁴I focused on the synthesis of ¹²⁴I-labeled compounds for molecular imaging purposes. The review concludes with a summary and an outlook on the future prospective of using the long-lived positron emitter ¹²⁴I in the field of organic PET chemistry and molecular imaging.

Ultrasound-guided intratumoral administration of collagenase-2 improved liposome drug accumulation in solid tumor xenografts

PURPOSE

To investigate the effect of intratumoral administration of collagenase-2 on liposomal drug accumulation and diffusion in solid tumor xenografts.

METHODS

Correlation between tumor interstitial fluid pressure (IFP) and tumor physiological properties (size and vessel fraction by B-mode and Doppler ultrasound, respectively) was determined. IFP response to intravenous or intratumoral collagenase-2 (0.1%) treatment was compared with intratumoral deactivated collagenase-2. To evaluate drug accumulation and diffusion, technetium-99 m-((99m)Tc)-liposomal doxorubicin (Doxil) was intravenously injected after collagenase-2 (0.1 and 0.5%, respectively) treatment, and planar scintigraphic images acquired and percentage of the injected dose per gram tissue calculated. Subsequently, tumors were subjected to autoradiography and histopathology.

RESULTS

IFP in two-week-old head and neck squamous cell carcinoma xenografts was 18 ± 3.7 mmHg and not correlated to the tumor size but had reverse correlation with the vessel fraction (r = -0.91, P < 0.01). Intravenous and intratumoral collagenase-2 use reduced IFP by a maximum of 35-40%. Compared to the control, the low IFP level achieved through intratumoral route remained for a long period (24 vs. 2 h, P < 0.05). SPECT images and autoradiography showed significantly higher (99m)Tc-Doxil accumulation in tumors with intratumoral collagenase-2 treatment, confirmed by %ID/g in tumors (P < 0.05), and pathological findings showed extensive distribution of Doxil in tumors.

CONCLUSIONS

Intratumoral injection of collagenase-2 could effectively reduce IFP in HNSCC xenografts for a longer period than using intravenous approach, which allowed for more efficient accumulation and homogeneous diffusion of the Doxil within the tumor interstitium.

Standardized high current solid tellurium-124 target for cyclotron production of the radionuclides iodine-123,124

The production yield of iodine radionuclides (123/124I) by solid target irradiation strongly depends on the quality of the tellurium (Te) target. To avoid poor quality 124Te targets frequently obtained using the classical method, a new technology for high quality electroplated elemental 124Te was developed. In this method, simultaneous preparation of four smooth, dense and homogeneous 124Te targets was achieved within 1 h period. The prepared targets were capable of withstanding high beam currents without burning or losing the enriched material during bombardment. As a result, in comparison to the classical method, the overall yield of 123I increased by nearly 25%.

Optimization and assessment of quantitative 124I imaging on a Philips Gemini dual GS PET/CT system

PURPOSE

Quantitative (124)I PET imaging is challenging as (124)I has a complex decay scheme. In this study the performance of a Philips Gemini dual GS PET/CT system was optimized and assessed for (124)I.

METHODS

The energy window giving the maximum noise equivalent count rate (NECR) and NEMA 2001-NU2 image quality were measured. The activity concentration (AC) accuracy of images calibrated using factors from (18)F and (124)I decaying source measurements were investigated.

RESULTS

The energy window 455-588 keV gave the maximum NECR of 9.67 kcps for 233 MBq. (124)I and (18)F image quality was comparable, although (124)I background variability was increased. The average underestimation in AC in (124)I images was 17.9 +/- 2.9% for nonuniform background and 14.7 +/- 2.9% for single scatter simulation (SSS) subtraction scatter correction. At 224 MBq the underestimation was 10.8 +/- 11.3%, which is comparable to 7.7 +/- 5.3% for (18)F, but increased with decreasing activity.

CONCLUSIONS

The best (124)I PET quantitative accuracy was achieved for the optimized energy window, using SSS scatter correction and calibration factors from decaying (124)I source measurements. The quantitative accuracy for (124)I was comparable to that for (18)F at high activities of 224 MBq but diminishing with decreasing activity. Specific corrections for prompt gamma-photons may further improve the quantitative accuracy.

Overview of image-guided radiation therapy

Radiation therapy has gone through a series of revolutions in the last few decades and it is now possible to produce highly conformal radiation dose distribution by using techniques such as intensity-modulated radiation therapy (IMRT). The improved dose conformity and steep dose gradients have necessitated enhanced patient localization and beam targeting techniques for radiotherapy treatments. Components affecting the reproducibility of target position during and between subsequent fractions of radiation therapy include the displacement of internal organs between fractions and internal organ motion within a fraction. Image-guided radiation therapy (IGRT) uses advanced imaging technology to better define the tumor target and is the key to reducing and ultimately eliminating the uncertainties. The purpose of this article is to summarize recent advancements in IGRT and discussed various practical issues related to the implementation of the new imaging techniques available to radiation oncology community. We introduce various new IGRT concepts and approaches, and hope to provide the reader with a comprehensive understanding of the emerging clinical IGRT technologies. Some important research topics will also be addressed.

Synthesis and in Vitro Evaluation of 18F- and 19F-Labeled Insulin: A New Radiotracer for PET-based Molecular Imaging Studies

A new and regioselective strategy was developed for the preparation of fluorine-18-labeled insulin as a novel positron emission tomography (PET) tracer. [18F]-4-Fluorobenzoic acid (4-18FBA), which was produced in 83 +/- 8% yield (n = 10), through the use of succinimidyl [18F]-4-fluorobenzoate (4-(18)FSB), was conjugated through a short spacer (6-aminohexanoic acid, AHx) to the PheB1 residue of a protected form of insulin. 18FB-AHx-insulin (8b) was repeatedly prepared in practical quantities (10-20 mCi, 370-740 MBq) in good radiochemical yield (9 +/- 5%, n = 9) and in a specific activity of 7.8 mCi/micromol. The final product was characterized by comparing the radioHPLC and radioTLC of 8b with that of the 19F-analogue (19FB-AHx-insulin, 8a) and by analyzing a carrier-added synthesis by mass spectrometry. Dithiothreitol and endoproteinase Glu-C digestion experiments on 8a confirmed that the prosthetic group was in fact conjugated to the PheB1 residue. An insulin receptor (IR) phosphorylation assay using CHO-hIR cells overexpressing recombinant human insulin receptors indicated no statistical difference in the extent of autophosphorylation stimulated by 8a as compared to that for human insulin (EC50 values of 0.82 nM and 1.0 nM, respectively). The stimulation of 2-deoxyglucose uptake in 3T3-L1 mouse adipocytes utilizing 8a versus unmodified human insulin gave similar EC50 values of 0.68 nM and 0.41 nM, respectively. The IC50 values for 8a versus native insulin for the displacement of 125I-insulin from HEK-293 cells were also the same within experimental error (2.6 nM for 8a versus 2.4 nM for unmodified human insulin). These results support the use of the 18F-insulin analogue as a PET tracer for imaging the distribution of insulin in vivo.

Insulin disposition in the lung following oral inhalation in humans : a meta-analysis of its pharmacokinetics

BACKGROUND

Oral inhalation of insulin potentially offers non-invasive treatment and better glycaemic control in diabetes by virtue of its apparently faster absorption into the systemic circulation compared with subcutaneous injection. Nevertheless, the lung kinetics of inhaled insulin in humans have yet to be fully clarified because of the complexity of insulin-glucose (patho)physiology and the difficulty in approximating the inhaled dose. As a result, there remains considerable debate on the mechanisms of absorption and metabolism of insulin in the lung.

OBJECTIVES

To develop and apply a physiologically realistic insulin-glucose kinetic model to a meta-analysis of insulin-glucose profiles from well-controlled clinical studies of inhaled insulin published in the literature, and thereby, to derive the kinetic descriptors of insulin in the lung following inhalation through curve fitting.

MODEL DEVELOPMENT

The model assumed first-order absorption (k(a,L)) and parallel non-absorptive loss (k(mm,L)), the latter primarily occurring via metabolism and mucociliary clearance in the lung, alongside two systemic compartments. Where necessary, glucose-dependent endogenous pancreatic insulin secretion was also taken into account by using blood glucose data as the second independent variable.

RESULTS

Despite the model's simplicity and the use of mean data, 16 insulin-glucose profiles from ten clinical studies were successfully fitted to the model, yielding values for the rate constants k(a,L) and k(mm,L). Whole serum insulin profiles were rate-determined by k(a,L) and k(mm,L) combined, representing 'flip-flop' pharmacokinetics. The best estimate for k(a,L) was found to be 0.020-0.032 h(-1), effectively unchanged across doses (0.3-1.8 IU/kg), formulations (powder vs liquid) and subjects (healthy vs diabetic), suggesting passive diffusive absorption of insulin from the lung. In contrast, the values for k(mm,L) were much larger (0.5-1.6 h(-1)) and decreased with increasing inhaled dose. Therefore, it is likely that dose-dependent saturable lung metabolism controls the value of k(mm,L), alongside mucociliary clearance. As a result, the absolute bioavailability ranged from 1.5% to 4.8%. The modelling analysis also enabled derivation of increased values for both k(a,L) and k(mm,L) as a possible cause of faster absorption for deep inspiratory manoeuvres and increased absorption in smokers, and faster and increased absorption for insulin lispro.

CONCLUSIONS

Although some of these results need to be substantiated experimentally, it appears that this modelling analysis has enabled unification of the literature information associated with the kinetics and mechanisms of insulin disposition in the lung following inhalation in humans.

Real time non-invasive imaging of receptor-ligand interactions in vivo

Non-invasive longitudinal detection and evaluation of gene expression in living animals can provide investigators with an understanding of the ontogeny of a gene's biological function(s). Currently, mouse model systems are used to optimize magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and optical imaging modalities to detect gene expression and protein function. These molecular imaging strategies are being developed to assess tumor growth and the tumor microenvironment. In addition, pre-labeling of progenitor cells can provide invaluable information about the developmental lineage of stem cells both in organogenesis and tumorigenesis. The feasibility of this approach has been extensively tested by targeting of endogenous tumor cell receptors with labeled ligand (or ligand analog) reporters and targeting enzymes with labeled substrate (or substrate analog). We will primarily discuss MRI, PET, and SPECT imaging of cell surface receptors and the feasibility of non-invasive imaging of gene expression using the tumor microenvironment (e.g., hypoxia) as a conditional regulator of gene expression.