Targeted molecular imaging in oncology

Preclinical evaluation of 68Ga-labeled folic acid conjugates for visualization of inflammatory foci

INTRODUCTION

Folate receptors (FR) have been considered a convenient target for different radiopharmaceuticals in recent years. Multifarious 68Ga-labeled folate conjugates have been proposed as promising agents for the PET imaging of FR-overexpressing malignant neoplasms. In addition, radiolabeled folate-based conjugates can be effective for imaging non-tumor pathological foci characterized by a pronounced cluster of activated macrophages. We previously reported that a conjugate of folic acid with the NODAGA-chelator, labeled with gallium-68 and containing a (His-Glu)2-tag ([68Ga]Ga-NODAGA-[Lys-(HE)2]-folic acid), is suitable for imaging tumor lesions characterized by an increased density of FR. Introduction of the (His-Glu)2-tag into the structure of the folate radioconjugate significantly reduced its accumulation in non-target tissues (e.g., kidneys), leaving the accumulation in tumors at least at the same level, and even increasing it. The present study assessed the suitability of the developed molecule (in comparison with the unmodified analog) for imaging foci of non-oncological etiology characterized by a pronounced macrophage response.

METHODS

Systemic juvenile idiopathic arthritis (JIA), reproduced in Wistar rats, was used as the pathology model. Acute inflammatory processes of soft tissues of septic and aseptic etiologies were selected as differential models.

RESULTS

The results obtained in this study showed a significantly elevated level of accumulation in the JIA focus compared to healthy rat paws and accumulation in the foci of differential models of the inflammatory process, which confirms the macrophage-mediated pathway of accumulation of the studied compounds. Simultaneously, the ratio of accumulation in pathology to accumulation in comparable healthy tissues in all studied pathologies was significantly high.

CONCLUSION

The data obtained allowed us to conclude the diagnostic potential of new radiolabeled folate-based conjugate with (His-Glu)2-tag for pharmacokinetic property optimization in the radionuclide diagnosis of rheumatoid and other diseases characterized by a pronounced macrophage immune response. The mathematical compartment model quantitatively confirmed that the additional (His-Glu)2 fragment introduced in the molecule acts in favor of potential radiopharmaceutical use to visualize inflammatory processes by positron emission tomography.

Microscopic investigation of" topically applied nanoparticles for molecular imaging of fresh tissue surfaces

Previous studies have shown that functionalized nanoparticles (NPs) topically applied on fresh tissues are able to rapidly target cell-surface protein biomarkers of cancer. Furthermore, studies have shown that a paired-agent approach, in which an untargeted NP is co-administered with a panel of targeted NPs, controls for the nonspecific behavior of the NPs, enabling quantitative imaging of biomarker expression. However, given the complexities in nonspecific accumulation, diffusion, and chemical binding of targeted NPs in tissues, studies are needed to better understand these processes at the microscopic scale. Here, fresh tissues were stained with a paired-agent approach, frozen, and sectioned to image the depth-dependent accumulation of targeted and untargeted NPs. The ratio of targeted-to-untargeted NP concentrations-a parameter used to distinguish between tumor and benign tissues-was found to diminish with increasing NP diffusion depths due to nonspecific accumulation and poor washout. It was then hypothesized and experimentally demonstrated that larger NPs would exhibit less diffusion below tissue surfaces, enabling higher targeted-to-untargeted NP ratios. In summary, these methods and investigations have enabled the design of NP agents with improved sensitivity and contrast for rapid molecular imaging of fresh tissues.

Hypoxia in cervical cancer: from biology to imaging

PURPOSE

Hypoxia imaging may improve identification of cervical cancer patients at risk of treatment failure and be utilized in treatment planning and monitoring, but its clinical potential is far from fully realized. Here, we briefly describe the biology of hypoxia in cervix tumors of relevance for imaging, and evaluate positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques that have shown promise for assessing hypoxia in a clinical setting. We further discuss emerging imaging approaches, and how imaging can play a role in future treatment strategies to target hypoxia.

METHODS

We performed a PubMed literature search, using keywords related to imaging and hypoxia in cervical cancer, with a particular emphasis on studies correlating imaging with other hypoxia measures and treatment outcome.

RESULTS

Only a few and rather small studies have utilized PET with tracers specific for hypoxia, and no firm conclusions regarding preferred tracer or clinical potential can be drawn so far. Most studies address indirect hypoxia imaging with dynamic contrast-enhanced techniques. Strong evidences for a role of these techniques in hypoxia imaging have been presented. Pre-treatment images have shown significant association to outcome in several studies, and images acquired during fractionated radiotherapy may further improve risk stratification. Multiparametric MRI and multimodality PET/MRI enable combined imaging of factors of relevance for tumor hypoxia and warrant further investigation.

CONCLUSIONS

Several imaging approaches have shown promise for hypoxia imaging in cervical cancer. Evaluation in large clinical trials is required to decide upon the optimal modality and approach.

Multiplexed Molecular Imaging of Biomarker-Targeted SERS Nanoparticles on Fresh Tissue Specimens with Channel-Compressed Spectrometry

Biomarker-targeted surface-enhanced Raman scattering (SERS) nanoparticles (NPs) have been explored as a viable option for targeting and imaging multiple cell-surface protein biomarkers of cancer. While it has been demonstrated that this Raman-encoded molecular imaging (REMI) technology may potentially be used to guide tumor-resection procedures, the REMI strategy would benefit from further improvements in imaging speed. Previous implementations of REMI have utilized 1024 spectral channels (camera pixels) in a commercial spectroscopic CCD to detect the spectral signals from multiplexed SERS NPs, a strategy that enables accurate demultiplexing of the relative concentration of each NP "flavor" within a mixture. Here, we investigate the ability to significantly reduce the number of spectral-collection channels while maintaining accurate imaging and demultiplexing of up to five SERS NP flavors, a strategy that offers the potential for improved imaging speed and/or detection sensitivity in future systems. This strategy was optimized by analyzing the linearity of five multiplexed flavors of SERS NPs topically applied on tissues. The accuracy of this binning approach was then validated by staining tumor xenografts and human breast tumor specimens with a mixture of five NP flavors (four targeted NPs and one untargeted NP) and performing ratiometric imaging of specific vs. nonspecific NP accumulation. We demonstrate that with channel-compressed spectrometry using as few as 16 channels, it is possible to perform REMI with five NP flavors, with < 20% error, at low concentrations (< 10 pM) that are relevant for clinical applications.

Quantitative molecular phenotyping with topically applied SERS nanoparticles for intraoperative guidance of breast cancer lumpectomy

There is a need to image excised tissues during tumor-resection procedures in order to identify residual tumors at the margins and to guide their complete removal. The imaging of dysregulated cell-surface receptors is a potential means of identifying the presence of diseases with high sensitivity and specificity. However, due to heterogeneities in the expression of protein biomarkers in tumors, molecular-imaging technologies should ideally be capable of visualizing a multiplexed panel of cancer biomarkers. Here, we demonstrate that the topical application and quantification of a multiplexed cocktail of receptor-targeted surface-enhanced Raman scattering (SERS) nanoparticles (NPs) enables rapid quantitative molecular phenotyping (QMP) of the surface of freshly excised tissues to determine the presence of disease. In order to mitigate the ambiguity due to nonspecific sources of contrast such as off-target binding or uneven delivery, a ratiometric method is employed to quantify the specific vs. nonspecific binding of the multiplexed NPs. Validation experiments with human tumor cell lines, fresh human tumor xenografts in mice, and fresh human breast specimens demonstrate that QMP imaging of excised tissues agrees with flow cytometry and immunohistochemistry, and that this technique may be achieved in less than 15 minutes for potential intraoperative use in guiding breast-conserving surgeries.

Quantitative in vivo cell-surface receptor imaging in oncology: kinetic modeling and paired-agent principles from nuclear medicine and optical imaging

The development of methods to accurately quantify cell-surface receptors in living tissues would have a seminal impact in oncology. For example, accurate measures of receptor density in vivo could enhance early detection or surgical resection of tumors via protein-based contrast, allowing removal of cancer with high phenotype specificity. Alternatively, accurate receptor expression estimation could be used as a biomarker to guide patient-specific clinical oncology targeting of the same molecular pathway. Unfortunately, conventional molecular contrast-based imaging approaches are not well adapted to accurately estimating the nanomolar-level cell-surface receptor concentrations in tumors, as most images are dominated by nonspecific sources of contrast such as high vascular permeability and lymphatic inhibition. This article reviews approaches for overcoming these limitations based upon tracer kinetic modeling and the use of emerging protocols to estimate binding potential and the related receptor concentration. Methods such as using single time point imaging or a reference-tissue approach tend to have low accuracy in tumors, whereas paired-agent methods or advanced kinetic analyses are more promising to eliminate the dominance of interstitial space in the signals. Nuclear medicine and optical molecular imaging are the primary modalities used, as they have the nanomolar level sensitivity needed to quantify cell-surface receptor concentrations present in tissue, although each likely has a different clinical niche.

Comparison of estimated human dose of (68)Ga-MAA with (99m)Tc-MAA based on rat data

OBJECTIVE

(99m)Tc macroaggregated albumin ((99m)Tc-MAA) that had been used as a perfusion agent has been evaluated. In this study, we tried to estimate human absorbed dose of ⁶⁸Ga-MAA via commercially available kit from Pars-Isotopes, based on biodistribution data in wild-type rats, and compare our estimation with the available absorbed dose data from (99m)Tc-MAA.

METHODS

For biodistribution of ⁶⁸Ga-MAA, three rats were sacrificed at each selected times after injection (15, 30, 45, 60, and 120 min) and the percentage of injected dose per gram of each organ was measured by direct counting from rats data from 11 harvested organs. The medical internal radiation dose formulation was applied to extrapolate from rats to human and to project the absorbed radiation dose for various organs in humans.

RESULTS

The biodistribution data for ⁶⁸Ga-MAA showed that the most of the activity was taken up by the lung (more than 97 %) in no time. Our dose prediction shows that a 185-MBq injection of ⁶⁸Ga-MAA into humans might result in an estimated absorbed dose of 4.31 mGy in the whole body. The highest absorbed doses are observed in the adrenals, spleen, pancreas, and red marrow with 0.36, 0.34, 0.26, and 0.19 mGy, respectively.

CONCLUSION

Since the (99m)Tc-MAA remains longer than ⁶⁸Ga-MAA in the lung and ⁶⁸Ga-MAA has good image qualities and results in lower amounts of dose delivery to the critical organs such as gonads, red marrow, and adrenals, the use of ⁶⁸Ga-MAA is recommended.

Assessment of early changes in 3H-fluorothymidine uptake after treatment with gefitinib in human tumor xenograft in comparison with Ki-67 and phospho-EGFR expression

Background

The purpose of this study was to evaluate whether early changes in 3′-deoxy-3′-³H-fluorothymidine (³H-FLT) uptake can reflect the antiproliferative effect of gefitinib in a human tumor xenograft, in comparison with the histopathological markers, Ki-67 and phosphorylated EGFR (phospho-EGFR).

Methods

An EGFR-dependent human tumor xenograft model (A431) was established in female BALB/c athymic mice, which were divided into three groups: one control group and two treatment groups. Mice in the treatment groups were orally administered a partial regression dose (100 mg/kg/day) or the maximum tolerated dose of gefitinib (200 mg/kg/day), once daily for 2 days. Mice in the control group were administered the vehicle (0.1% Tween 80). Tumor size was measured before and 3 days after the start of treatment. Biodistribution of ³H-FLT and ¹⁸F-FDG (%ID/g/kg) was examined 3 days after the start of the treatment. Tumor cell proliferative activity with Ki-67 was determined. Immunohistochemical staining of EGFR and measurement of phospho-EGFR were also performed.

Results

High expression levels of EGFR and Ki-67 were observed in the A431 tumor. After the treatment with 100 and 200 mg/kg gefitinib, the uptake levels of ³H-FLT in the tumor were significantly reduced to 67% and 61% of the control value, respectively (0.39 ± 0.09, 0.36 ± 0.06, 0.59 ± 0.11%ID/g/kg for 100 mg/kg, 200 mg/kg, and control groups, respectively; p < 0.01 vs. control), but those of ¹⁸F-FDG were not. After the treatment with 100 and 200 mg/kg gefitinib, the expression levels of Ki-67 in the tumor were markedly decreased (4.6 ± 2.4%, 6.2 ± 1.8%, and 10.4 ± 5.7% for 100 mg/kg, 200 mg/kg, and control groups, respectively, p < 0.01 vs. control). The expression levels of the phospho-EGFR protein also significantly decreased (29% and 21% of the control value for 100, and 200 mg/kg, respectively p < 0.01 vs. control). There was no statistically significant difference in tumor size between pre- and post-treatments in each group.

Conclusion

In our animal model, ³H-FLT uptake levels significantly decreased after the treatment with two different doses of gefitinib before a significant change in tumor size was observed. These results were confirmed by the immunohistochemical staining of Ki-67 and phospho-EGFR protein immunoassay. Thus, it was indicated that early changes in ³H-FLT uptake may reflect the antiproliferative effect of gefitinib in a mouse model of a human epidermoid cancer.

Novel Tc-99m labeled ELR-containing 6-mer peptides for tumor imaging in epidermoid carcinoma xenografts model: a pilot study

OBJECTIVE

ELR-containing peptides targeting CXCR2 could be the excellent candidate for targeting ligand of molecular tumor imaging. In this study, we had developed two ELR-containing 6-mer peptides and evaluated the diagnostic performance of Tc-99m labeled 6-mer peptides as a molecular imaging agent in murine models bearing KB epidermoid carcinoma.

METHODS

Peptides were synthesized using Fmoc solid phase peptide synthesis. Radiolabeling efficiency with Tc-99m was evaluated using instant thin-layer chromatography. In KB epidermoid cancer-bearing mice, gamma images had acquired and tumor-to-muscle uptake ratio was calculated. Competition and biodistribution studies had performed.

RESULTS

Two 6-mer peptides, ELR-ECG and ECG-ELR were successfully synthesized. After radiolabeling procedures with Tc-99m, the complex Tc-99m ELR-ECG and Tc-99m ECG-ELR were prepared in high yield. In the gamma camera imaging of murine model, Tc-99m ELR-ECG was substantially accumulated in the subcutaneously engrafted tumor and tumor uptake had been suppressed by the free ELR co-injection. However, Tc-99m ECG-ELR was minimally accumulated in the tumor.

CONCLUSIONS

Two ELR-containing 6-mer peptides, ELR-ECG and ECG-ELR, were developed as a molecular imaging agent to target CXCR2 of epidermoid carcinoma. Tc-99m ELR-ECG had showed significant uptake in tumor and it was good candidate for a tumor imaging.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Orthogonally protected artificial amino acid as tripod ligand for automated peptide synthesis and labeling with [(99m)Tc(OH(2))(3)(CO)(3)](+)

1,2-Diamino-propionic acid (Dap) is a very strong chelator for the [(99m)Tc(CO)(3)](+) core, yielding small and hydrophilic complexes. We prepared the lysine based Dap derivative l-Lys(Dap) in which the ε-NH(2) group was replaced by the tripod through conjugation to its α-carbon. The synthetic strategy produced an orthogonally protected bifunctional chelator (BFC). The -NH(2) group of the α-amino acid portion is Fmoc- and the -NH(2) of Dap are Boc-protected. Fmoc-l-Lys(Dap(Boc)) was either conjugated to the N- and C-terminus of bombesin BBN(7-14) or integrated into the sequence using solid-phase peptide synthesis (SPPS). We also replaced the native lysine in a cyclic RGD peptide with l-Lys(Dap). For all peptides, quantitative labeling with the [(99m)Tc(CO)(3)](+) core at a 10 μM concentration in PBS buffer (pH = 7.4) was achieved. For comparison, the rhenium homologues were prepared from [Re(OH(2))(3)(CO)(3)](+) and Lys(Dap)-BBN(7-14) or cyclo-(RGDyK(Dap)), respectively. Determination of integrin receptor binding showed low to medium nanomolar affinities for various receptor subtypes. The IC(50) of cyclo-(RGDyK(Dap[Re(CO)(3)])) for α(v)β(3) is 7.1 nM as compared to 3.1 nM for nonligated RGD derivative. Biodistribution studies in M21 melanoma bearing nude mice showed reasonable α(v)β(3)-integrin specific tumor uptake. Altogether, orthogonally protected l-Lys(Dap) represents a highly versatile building block for integration in any peptide sequence. Lys(Dap)-precursors allow high-yield (99m)Tc-labeling with [(99m)Tc(OH(2))(3)(CO)(3)](+), forming small and hydrophilic complexes, which in turn leads to peptide radiopharmaceuticals with excellent in vivo characteristics.

Synthesis and evaluation of 64Cu-labeled monomeric and dimeric NGR peptides for MicroPET imaging of CD13 receptor expression

The NGR-containing peptides have been shown to bind specifically to CD13/aminopeptidase N (APN) receptor, one of the attractive tumor vasculature biomarkers. In this study, we evaluated (64)Cu-labeled monomeric and dimeric NGR peptides for microPET imaging of CD13 receptor expression in vivo. Western blot analysis and immunofluorescence staining were performed to identify CD13-positive and CD13-negative cell lines. NGR-containing peptides were conjugated with 1,4,7,10-tetraazadodecane-N,N',N″,N‴-tetraacetic acid (DOTA) and labeled with (64)Cu (t(1/2) = 12.7 h) in ammonium acetate buffer. The resulting monomeric ((64)Cu-DOTA-NGR1) and dimeric ((64)Cu-DOTA-NGR2) peptides were then subjected to in vitro stability, cell uptake and efflux, small animal micorPET, and biodistribution studies. In vitro studies demonstrated that CD13 receptors are overexpressed in human fibrosarcoma HT-1080 cells and negative in human colon adenocarcinoma HT-29 cells. The binding affinity of (64)Cu-DOTA-NGR2 to HT-1080 cells was measured to be within low nanomolar range and about 2-fold higher than that of (64)Cu-DOTA-NGR1. For small animal microPET studies, (64)Cu-DOTA-NGR2 displayed more favorable in vivo performance in terms of higher tumor uptake and slower tumor washout in CD13-positive HT-1080 tumor xenografts as compared to (64)Cu-DOTA-NGR1. As expected, significantly lower tumor uptake and poorer tumor/normal organ contrast were observed for both (64)Cu-DOTA-NGR1 and (64)Cu-DOTA-NGR2 in CD13-negative HT-29 tumor xenografts in comparison with those in the HT-1080 tumor xenografts. The CD13-specific tumor activity accumulation of both (64)Cu-DOTA-NGR1 and (64)Cu-DOTA-NGR2 was further demonstrated by significant reduction of tumor uptake in HT-1080 tumor xenografts with a coinjected blocking dose of cyclic NGR peptide [c(CNGRC)]. The biodistribution results were consistent with the quantitative analysis of microPET imaging. We concluded that both (64)Cu-DOTA-NGR1 and (64)Cu-DOTA-NGR2 have good and specific tumor uptake in CD13-positive HT-1080 tumor xenografts. (64)Cu-DOTA-NGR2 showed higher tumor uptake and better tumor retention than (64)Cu-DOTA-NGR1, presumably due to bivalency effect and increase in apparent molecular size. (64)Cu-DOTA-NGR2 is a promising PET probe for noninvasive detection of CD13 receptor expression in vivo.

Advances in molecular imaging for diagnosis of digestive tract cancers

Digestive tract cancers are common cancer types and have high incidence and mortality. Currently available diagnostic methods have some limitations that make an early and accurate diagnosis and prompt treatment difficult. Molecular imaging, which has been formally defined as visualization, characterization and measurement at the molecular level instead of the anatomic level, significantly increases the sensitivity and specificity of cancer detection. Several modalities have been utilized for molecular imaging in digestive tract cancers, such as endoscopy, scintigraphy (PET/SPECT), magnetic resonance imaging (MRI), and ultrasound (US). Antibodies, peptides, and aptamers are classes of molecular probes that have been extensively used as affinity ligands. After being conjugated with various labels such as radioisotopes, fluorophore, supermagnetic or paramagnetic metals and microbubbles, the probes can specifically target tumor cells and stroma and are used with imaging modalities to detect cancers. Molecular imaging is a methodology for not only the early detection of cancer, but also the judgment of tumor staging and the guidance of therapy. With the development of new instrument and probes, as well as multi-modal platforms, molecular imaging has been gradually perfected and taken from bench to bedside, bringing opportunities for early, accurate and comprehensive diagnosis of digestive tract cancers.

fac-[TcO3(tacn)]+: a versatile precursor for the labelling of pharmacophores, amino acids and carbohydrates through a new ligand-centred labelling strategy

Herein, we report a protocol for the synthesis of [(99m)TcO(3)(tacn)](+) ([1](+)) (tacn = 1,4,7-triazacyclononane) that is suitable for clinical translation. Bioconjugates containing pharmacophores ([TcO(NO(2)-Imi)(tacn)](+); [3](+)), artificial amino acids ([TcO(Fmoc-allyl-His)(tacn)](+); [5](+)), and glucose derivatives ([TcO(allyl-tetraacetylglucose)(tacn)](+); [7](+)) were synthesized by cycloaddition strategies and fully characterized ((99)Tc and (99m)Tc). These new technetium complexes are stable at neutral pH and demonstrate the potential and flexibility of the [3+2] cycloaddition labelling concept. In addition to the synthetic work, the first biodistribution studies of [1](+) and the small [3+2] cycloadduct [(99m)TcO(NO(2)-Imi)(tacn)](+) ([3](+)) were completed. The biodistribution studies suggest the stability of these complexes in vivo. Furthermore, it was demonstrated that the high hydrophilicity of the [(99m)TcO(3)(tacn)](+) building block is a complement to the complexes of the fac-{Tc(CO)(3)}(+) core.

Molecular and Functional Imaging of Breast Cancer

Background

Significant efforts have been directed toward developing and enhancing imaging methods for the early detection, diagnosis, and characterization of small breast tumors. Molecular and functional imaging sets the stage for enhancement of current methodology.

Methods

Current imaging modalities are described based on the molecular characteristics of normal and malignant tissue. New molecular imaging methods that have the potential for clinical use are also discussed.

Results:

Dynamic contrast-enhanced magnetic resonance imaging is more sensitive than mammography in BRCA1 carriers. It is used in screening and in the early evaluation of neoadjuvant therapy. Positron emission mammography is 91% sensitive and 93% specific in detecting primary breast cancers. Sentinel node scintigraphy is a key component of axillary lymph node evaluation. Other imaging modalities being studied include Tc99m sestamibi, radiolabeled thymidine or uridine, estrogen receptor imaging, magnetic resonance spectroscopy, and diffusion magnetic resonance imaging.

Conclusions

Molecular and functional imaging of the breast will likely alter clinical practice in diagnosing and staging primary breast cancer and assessing response to therapy since it will provide earlier information regarding the underlying biology of individual breast cancers, tumor stage, potential treatment strategies, and biomarkers for early evaluation of treatment effects.

Immuno-positron emission tomography in cancer models

Positron emission tomography (PET) is playing an increasingly important role in the diagnosis, staging, and monitoring response to treatment in a variety of cancers. Recent efforts have focused on immuno-PET, which uses antibody-based radiotracers, to image tumors based on expression of tumor-associated antigens. It is postulated that the specificity afforded by antibody targeting should both improve tumor detection and provide phenotypic information related to primary and metastatic lesions that will guide therapy decisions. Advances in antibody-engineering are providing the tools to develop antibody-based molecules with pharmacokinetic properties optimized for use as immuno-PET radiotracers. Coupled with technical advances in the design of PET scanners, immuno-PET holds promise to improve diagnostic imaging and to guide the use of targeted therapies. An overview of the preclinical immuno-PET studies in cancer models is reviewed here.

Syntheses of bifunctional 2,3-diamino propionic acid-based chelators as small and strong tripod ligands for the labelling of biomolecules with (99m)Tc

The labelling of targeting biomolecules requires small and hydrophilic complexes in order to not affect the binding properties of the vectors. 2,3-Diamino propionic acid (dap) is a small and strong, albeit scarcely used, tripod ligand for the fac-[(99m)Tc(CO)(3)](+) moiety. We have introduced at the alpha-carbon atom in the basic dap structure various second functionalities such as carboxylato, amino and alpha-amino acid groups via various spacers in order to yield bifunctional chelators. These dap derivatives can be coupled to targeting molecules for application in molecular imaging. Full characterizations of the bifunctional chelators, X-ray structures of intermediates and of one rhenium complex, as well as labelling studies with (99m)Tc, are presented.

On-board SPECT for localizing functional targets: a simulation study

Single photon emission computed tomography (SPECT) was investigated for imaging on-board radiation therapy machines in order to localize functional and molecular targets. A computer-simulated female NCAT phantom was positioned supine on a flat-top treatment couch. Twenty tumor locations were defined in the upper torso. The eight lung tumors were subject to the effects of respiratory motion. Tumor diameters of 10.8, 14.4, and 21.6 mm were simulated for tumor-to-background ratios of 3:1 and 6:1 that are characteristic of the radiotracer 99mTc-sestamibi. Projection images representing scan times of 4, 8, and 20 min were simulated for an anterior, half-circular trajectory. Images were reconstructed with attenuation correction by ordered-subsets expectation maximization (OSEM) using six subsets and five iterations. Contrast-to-noise ratios (CNRs) were calculated from ensembles of 25 images. Cross correlation with a noise-free tumor template was used to select the most suspicious tumor location within a 14.4-mm-radius search volume surrounding each tumor, with only that one tumor in each search volume. Localization accuracy was assessed by calculating average distances between measured and true tumor locations. Localization accuracy and CNRs were strongly affected by tumor location relative to the detector trajectory. For example, CNR values near the chest wall were greater by a factor of 3.5 than for tumors near the spine and posterior ribs, a much greater effect than the factor of 1.6 difference in CNR between 6:1 and 3:1 tumor uptakes. Typically, tumors of 6:1 uptake were localized as accurately with 4 min of scan time as tumors of 3:1 uptake that had been imaged for 20 min. Using 4 min scans, 14.4 and 21.6 mm anterior tumors of 6:1 uptake were localized within 2 mm. These results suggest that SPECT, on-board radiation therapy machines, may be a viable modality for localizing certain functional and molecular targets using relatively short scan times.

Magnetic Nanoparticles for Early Detection of Cancer by Magnetic Resonance Imaging

This article provides a brief overview of recent progress in the synthesis and functionalization of magnetic nanoparticles and their applications in the early detection of malignant tumors by magnetic resonance imaging (MRI). The intrinsic low sensitivity of MRI necessitates the use of large quantities of exogenous contrast agents in many imaging studies. Magnetic nanoparticles have recently emerged as highly efficient MRI contrast agents because these nanometer-scale materials can carry high payloads while maintaining the ability to move through physiological systems. Superparamagnetic ferrite nanoparticles (such as iron oxide) provide excellent negative contrast enhancement. Recent refinement of synthetic methodologies has led to ferrite nanoparticles with narrow size distributions and high crystallinity. Target-specific tumor imaging becomes possible through functionalization of ferrite nanoparticles with targeting agents to allow for site-specific accumulation. Nanoparticulate contrast agents capable of positive contrast enhancement have recently been developed in order to overcome the drawbacks of negative contrast enhancement afforded by ferrite nanoparticles. These newly developed magnetic nanoparticles have the potential to enable physicians to diagnose cancer at the earliest stage possible and thus can have an enormous impact on more effective cancer treatment.

Use of radionuclides in cancer research and treatment

Cancer occurs as a result of misregulation of cell growth, which appears to be a consequence of alteration in the function of oncogenes and tumour suppressor genes. Ionising radiation has been used, since the discovery of X-rays in 1896 by Roentgen, both in cancer research and treatment of the disease. The main purpose of cancer research is to understand the molecular alterations involved in the development and progression of the disease in order to improve diagnosis and develop personalised therapies, by focusing on the features of the tumoral cell and the biological events associated to carcinogenesis. Radioisotopic techniques have been used routinely for in vitro research in the molecular and cellular biology of cancer for more than 20 years and are in the process of being substituted by alternative non-radioactive techniques. However in vivo techniques such as irradiation of cells in culture and/or experimental animal models and radioactive labelling are in development, due in part to advances in molecular imaging technologies. The objective of this review is to analyse in an integrative way the applications of ionising radiation in cancer research and therapy. It had been divided into two parts. The first one will approach the techniques applied to cancer research and the second will summarise how ionising radiation is applied to the treatment of neoplastic disease.

Syntheses of high-valent fac-[99mTcO3]+ complexes and [3+2] cycloadditions with alkenes in water as a direct labelling strategy

Reported herein is a new concept for the labelling of biomolecules with small [(99m)TcO(3)](+) complexes through a [3+2] cycloaddition with alkenes for radiopharmaceutical applications. We developed convenient reactions for the synthesis of small, water stable fac-[TcO(3)(tacn-R)](+) complexes ((99)Tc and (99m)Tc, tacn = 1,4,7-triazacyclononane, R = H, -CH(2)-C(6)H(5), -CH(2)-C(6)H(4)COOH). With alkenes, these high valent [(99m)TcO(3)](+) complexes undergo [3+2] cycloaddition with formation of the corresponding Tc(V)-glycolato complexes. The (99m)Tc(V) and (99m)Tc(VII) complexes are stable at 37 degrees C in water and in the presence of serum proteins. Therefore, new opportunities in technetium chemistry are enabled with a high potential for medicinal and biological applications. In contrast to classical labelling, the presented strategy is ligand and not metal-centred.

MRI in ocular drug delivery

Conventional pharmacokinetic methods for studying ocular drug delivery are invasive and cannot be conveniently applied to humans. The advancement of MRI technology has provided new opportunities in ocular drug-delivery research. MRI provides a means to non-invasively and continuously monitor ocular drug-delivery systems with a contrast agent or compound labeled with a contrast agent. It is a useful technique in pharmacokinetic studies, evaluation of drug-delivery methods, and drug-delivery device testing. Although the current status of the technology presents some major challenges to pharmaceutical research using MRI, it has a lot of potential. In the past decade, MRI has been used to examine ocular drug delivery via the subconjunctival route, intravitreal injection, intrascleral injection to the suprachoroidal space, episcleral and intravitreal implants, periocular injections, and ocular iontophoresis. In this review, the advantages and limitations of MRI in the study of ocular drug delivery are discussed. Different MR contrast agents and MRI techniques for ocular drug-delivery research are compared. Ocular drug-delivery studies using MRI are reviewed.

Evaluation of human major histocompatibility complex class I chain-related A as a potential target for tumor imaging

To investigate the feasibility of using human major histocompatibility complex class I chain-related A (MICA) as a target for tumor imaging diagnosis, 10C6, a monoclonal antibody (mAb) that specifically recognizes MICA in vitro, was labeled with (99m)Tc (technetium) and administered into mice bearing MICA-positive human ovarian epithelial carcinoma line SKOV3. Measurement of organ-specific radioactivity showed that tumor accumulated radioactivity continuously, while the uptake in the other organs decreased over time. Scintigram showed that the tumor became clearly visible at 24h post-injection of radio-labeled 10C6 mAb. These results suggest that MICA is a promising target for tumor imaging and mAb 10C6 may be used clinically for early tumor diagnosis.

Metastatic human colonic carcinoma: molecular imaging with pretargeted SPECT and PET in a mouse model

PURPOSE

To prospectively determine if a bispecific monoclonal antibody (MoAb) pretargeting method with a radiolabeled hapten peptide can depict small (<0.3 mm in diameter) microdisseminated human colon cancer colonies in the lungs of nude mice.

MATERIALS AND METHODS

Animal studies were approved in advance by animal care and use committees. Animals injected intravenously with a human colon cancer cell line to establish microdisseminated colonies in the lungs were pretargeted with TF2--a recombinant, humanized, anti-carcinoembryonic antigen (CEA) and anti-histamine-succinyl-glycine (HSG) bispecific MoAb; 21 hours later, a radiolabeled HSG peptide was given. Imaging and necropsy data for tumor-bearing animals given the anti-CEA bispecific MoAb (n = 38, all studies) were compared with those of animals given fluorine 18 ((18)F) fluorodeoxyglucose (FDG) (n = 15, all studies), peptide alone (n = 20, all studies), or an irrelevant anti-CD22 bispecific MoAb (n = 12, all studies). Uptake of these agents in the lungs of non-tumor-bearing animals enabled assessment of specificity (n = 15, 4, and 6 for TF2 pretarget, hapten peptide alone, and (18)F-FDG, respectively).

RESULTS

TF2-pretargeting helped localize tumors in the lungs within 1.5 hours of the radiolabeled HSG peptide injection, while the peptide alone, irrelevant bispecific MoAb pretargeted peptide, and (18)F-FDG failed. Necropsy data indicated that the signal in tumor-bearing lungs was five times higher than in blood within 1.5 hours, increasing to 50 times higher by 24 hours. Peptide uptake in tumor-bearing lungs pretargeted with TF2 was nine times higher than in non-tumor-bearing lungs, while it was only 1.5-fold higher with (18)F-FDG or the peptide alone. Micro-positron emission tomographic (PET) images showed discrete uptake in individual metastatic tumor colonies; autoradiographic data demonstrated selective targeting within the lungs, including metastases less than 0.3 mm in diameter.

CONCLUSION

Bispecific antibody pretargeting is highly specific for imaging micrometastatic disease and may thus provide a complementary method to (18)F-FDG at clinical examination.

Study of Apoptosis Induced by 188Re-DTPA-DG in MCF-7 Breast Carcinoma and A549 Pulmonary Carcinoma Cells

OBJECTIVE

To evaluate apoptosis induced by rehenium-188-labeled diethylenetriamine pentaacetic acid-glucosamine (188Re-DTPA-DG) in MCF-7 breast carcinoma cells and A549 pulmonary carcinoma cells.

METHODS

Through the use of flow cytometry (FCM) with CBA software to detect apoptosis, cells of both the MCF-7 and A549 cell lines were divided into groups exposed to 188Re-DTPA-DG, 188Re-perrhenate (188ReO4-), and saline, respectively. The first two groups were further divided into subgroups on the basis of their exposure to radioactivity at 37, 55.5, or 74 kBq/mL, with the saline-exposed group divided into three corresponding subgroups. Each subgroup was introduced into 5 replicate wells of a culture plate, and the morphology of the cells in each well was determined by flow cytometry at 6-hour intervals for 18 hours. In order to determine the affinity of 188Re-DTPA-DG for tumor tissue, the biodistribution of the radiolabeled agent was assessed in breast tumor-bearing nude mice.

RESULTS

Change in morphology of the cell nucleus was more evident in the 188Re-DTPA-DG-treated than in the 188ReO4--treated group, and no change in nuclear morphology was seen in the saline-exposed group. The study data suggested that there was a greater ratio of apoptotic to nonapoptotic cells among the 188Re-DTPA-DG-treated than among the 188ReO4--treated or saline-exposed cells (p<0.01), and a greater change in cell-nuclear morphology in the 188Re-DTPA-DG-treated than in the 188ReO4--treated cells. Furthermore, 188Re-DTPA-DG had a more significant apoptosis-inducing effect on both MCF-7 and A549 cells than did 188ReO4-. The biodistribution study in tumor-bearing nude mice showed that the concentration of 188Re-DTPA-DG in tumor tissue was much higher than in normal tissue, that 188Re-DTPA-DG was rapidly cleared from the blood, and that the main route of its clearance was via the kidneys.

CONCLUSIONS

188Re-DTPA-DG has a significant apoptotic effect on carcinoma cells. 188Re-DTPA-DG is an effective radiopharmaceutical for intratumoral radiation therapy.

Noninvasive Scintigraphic Detection of Tumor with 99mTc-DTPA-Deoxyglucose: An Experimental Study

OBJECTIVES

Glucosamine is a highly attractive scaffold for a glucosyl ligand, and shows activity with glucose transporters and hexokinase. In the study reported here, diethylenetriamine-pentaacetic acid-D-glucosamine (DTPA-DG) was synthesized by conjugating D-glucosamine to DTPA, and was labeled with technetium-99m ((99m)Tc). We investigated (99m)Tc-DTPA-DG for tumor detection.

METHODS

The biodistribution and imaging of (99m)Tc-DTPA-DG in mammary tumor-bearing mice were compared to those in a control group of mice with oleum terebinthinae (turpentine oil)-induced inflammation. Both groups of mice were given an intravenous injection of 3.7 MBq/0.1 mL of (99m)Tc-DTPA-DG through the tail vein.

RESULTS

(99m)Tc-DTPA-DG accumulated in the tumor tissue to a percentage of 2.10 +/- 0.02% of the injected dose per gram of tissue (%ID/g) at 2 hours after injection, versus an accumulation of 0.81 +/- 0.03%ID/g in the inflamed tissue. The tumor-to-contralateral muscle tissue ratio of (99m)Tc-DTPA-DG was 5.01 +/- 1.02, while the inflamed tissue-to-contralateral muscle tissue ratio was 1.2 +/- 0.08. Gamma-camera imaging revealed the tumor tissue at 2 hours after injection of (99m)Tc-DTPA-DG. The tumor-to-background ratio of (99m)Tc-DTPA-DG (3.8 +/- 0.95) at 2 hours was significantly (p < 0.05) higher in mammary tumor-bearing mice than was the inflamed tissue-to-background ratio (1.2 +/- 0.62) in the mice with inflammation.

CONCLUSIONS

(99m)Tc-DTPA-DG showed excellent tumor targeting and has promise as an imaging agent for clinical tumor targeting.

Pre vivo, ex vivo and in vivo evaluations of [68Ga]-EDTMP

INTRODUCTION

The objectives of this study were to develop a simple preparation method for [68Ga]-EDTMP and to evaluate the applicability of [68Ga]-EDTMP as a potential positron emission tomography (PET) bone imaging agent using pre vivo, ex vivo and in vivo models.

METHODS

[68Ga]-EDTMP was prepared using 68Ga]-gallium chloride eluted from the 68Ge/68Ga generator and commercially available Multibone kits. Binding affinity to bone compartments was evaluated using a recently established pre vivo model. In vivo (microPET) and ex vivo experiments were performed in mice, and the results of which were compared with those obtained with [18F]-fluoride.

RESULTS

[68Ga]-EDTMP was accessible via simple kit preparation and predominantly accumulated in bone tissue in vivo, ex vivo and pre vivo. Binding to mineral bone was irreversible, and low binding was observed in organic bone. In vivo microPET evaluation revealed predominant uptake in bone with renal excretion. Compared with [18F]-fluoride, the uptake was lower and the PET image quality was reduced.

CONCLUSIONS

From the present evaluation, apart from the autonomy for PET centers without an onsite cyclotron, the advantage of [68Ga]-EDTMP over [18F]-fluoride is not apparent and the future clinical prospect of [68Ga]-EDTMP remains speculative.

Preclinical imaging of mammary intraepithelial neoplasia with positron emission tomography

Small-animal imaging with positron emission tomography (PET) has become a valuable tool for evaluating preclinical models of breast cancer and other diseases. In this review, we examine a number of issues related to preclinical imaging studies with PET, using transgenic models of ductal carcinoma in situ and metastasis as specific examples. We discuss imaging components such as reconstruction, normalization, and extraction of quantitative parameters. We also analyze the effect of longitudinal correlations on cohort size and present some simple statistical techniques for determining cohort sizes that may be helpful in designing preclinical imaging studies. We describe studies that are greatly facilitated by access to non-invasive imaging data including a study involving multiple endpoints and another investigating metastasis. We conclude with a brief survey of emerging approaches in small-animal PET imaging.