Targeting murine heart and brain: visualisation conditions for multi-pinhole SPECT with (99m)Tc- and (123)I-labelled probes

Development of 11C-Labeled ω-sulfhydryl fatty acid tracer for myocardial imaging with PET

[¹¹C]-S-methyl-16-thiopalmitic acid (a) was developed with excellent heart-to-background uptake ratios and higher retention in heart. Myocardial uptake and metabolism of the tracer is markedly higher CPT I dependent. When compared to [¹¹C]-S-methyl-14-thiomyristic acid (b), [¹¹C]-S-methyl-12-thiododecanoic acid (c) and [¹¹C]-palmitate, a showed an early high uptake and a significantly slower late clearance in heart and a prolonged myocardial elimination half-life (30 min). Analysis of heart tissue and urine samples showed that a was metabolized via beta-oxidation in myocardium. Small animal PET images of the accumulation of a in the rat myocardium were clearly superior to [¹¹C]-palmitate. These initial studies suggest that a could be a potentially useful clinical PET tracer to assess myocardial fatty acid metabolism.

Longitudinal imaging of the ageing mouse

Several non-invasive imaging techniques are used to investigate the effect of pathologies and treatments over time in mouse models. Each preclinical in vivo technique provides longitudinal and quantitative measurements of changes in tissues and organs, which are fundamental for the evaluation of alterations in phenotype due to pathologies, interventions and treatments. However, it is still unclear how these imaging modalities can be used to study ageing with mice models. Almost all age related pathologies in mice such as osteoporosis, arthritis, diabetes, cancer, thrombi, dementia, to name a few, can be imaged in vivo by at least one longitudinal imaging modality. These measurements are the basis for quantification of treatment effects in the development phase of a novel treatment prior to its clinical testing. Furthermore, the non-invasive nature of such investigations allows the assessment of different tissue and organ phenotypes in the same animal and over time, providing the opportunity to study the dysfunction of multiple tissues associated with the ageing process. This review paper aims to provide an overview of the applications of the most commonly used in vivo imaging modalities used in mouse studies: micro-computed-tomography, preclinical magnetic-resonance-imaging, preclinical positron-emission-tomography, preclinical single photon emission computed tomography, ultrasound, intravital microscopy, and whole body optical imaging.

Pharmacological challenge and synaptic response - assessing dopaminergic function in the rat striatum with small animal single-photon emission computed tomography (SPECT) and positron emission tomography (PET)

Disturbances of dopaminergic neurotransmission may be caused by changes in concentrations of synaptic dopamine (DA) and/or availabilities of pre- and post-synaptic transporter and receptor binding sites. We present a series of experiments which focus on the regulatory mechanisms of the dopamin(DA)ergic synapse in the rat striatum. In these studies, DA transporter (DAT) and/or D(2) receptor binding were assessed with either small animal single-photon emission computed tomography (SPECT) or positron emission tomography (PET) after pharmacological challenge with haloperidol, L-DOPA and methylphenidate, and after nigrostriatal 6-hydroxydopamine lesion. Investigations of DAT binding were performed with [(123)I]N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane ([(123)I]FP-CIT). D(2) receptor bindingd was assessed with either [(123)I](S)-2-hydroxy-3-iodo-6-methoxy-N-[(1-ethyl-2-pyrrolidinyl)methyl]benzamide ([(123)I]IBZM) or [(18)F]1[3-(4'fluorobenzoyl)propyl]-4-(2-keto-3-methyl-1-benzimidazolinyl)piperidine ([(18)F]FMB). Findings demonstrate that in vivo investigations of transporter and/or receptor binding are feasible with small animal SPECT and PET. Therefore, tracers that are radiolabeled with isotopes of comparatively long half-lives such as (123)I may be employed. Our approach to quantify DAT and/or D(2) receptor binding at baseline and after pharmacological interventions inducing DAT blockade, D(2) receptor blockade, and increases or decreases of endogenous DA concentrations holds promise for the in vivo assessment of synaptic function. This pertains to animal models of diseases associated with pre- or postsynaptic DAergic deficiencies such as Parkinson's disease, Huntington's disease, attention-deficit/hyperactivity disorder, schizophrenia or drug abuse.

High resolution SPECT imaging for visualization of intratumoral heterogeneity using a SPECT/CT scanner dedicated for small animal imaging

OBJECTIVES

Tumor interiors are never homogeneous and in vivo visualization of intratumoral heterogeneity would be an innovation that contributes to improved cancer therapy. But, conventional nuclear medicine tests have failed to visualize heterogeneity in vivo because of limited spatial resolution. Recently developed single photon emission computed tomographic (SPECT) scanners dedicated for small animal imaging are of interest due to their excellent spatial resolution of <1 mm, but few studies have focused on the evaluation of intratumoral heterogeneity. We investigated the optimal conditions related to high resolution imaging of heterogeneous tumor interiors using a small animal SPECT scanner.

METHODS

The conditions related to SPECT/CT visualization of heterogeneous tumor interiors were investigated using phantoms with (111)In and simulations of actual small animal imaging. The optimal conditions obtained were validated by in vivo imaging of sarcoma 180-bearing mice.

RESULTS

Larger number of counts must be obtained within limited acquisition time to visualize tumor heterogeneity in vivo in animal imaging, compared to cases that simply detect tumors. At an acquisition time of 30 min, better image quality was obtained with pinhole apertures diameter of 1.4 mm than of 1.0 mm. The obtained best spatial resolution was 1.3 mm, it was acceptable for our purpose, though a little worse than the best possible performance of the scanner (1.0 mm). Additionally, the reconstruction parameters, such as noise suppression, voxel size, and iteration/subset number, needed to be optimized under the limited conditions and were different from those found under the ideal condition. The minimal radioactivity concentration for visualization of heterogeneous tumor interiors was estimated to be as high as 0.2-0.5 MBq/mL. Liposomes containing (111)In met this requirement and were administered to tumor-bearing mice. SPECT imaging successfully showed heterogeneous (111)In distribution within the tumors in vivo with good spatial resolution. A threshold of 0.2 MBq/g for clear visualization of tumor heterogeneity was validated. Autoradiograms obtained ex vivo of excised tumors confirmed that the in vivo SPECT images accurately depicted the heterogeneous intratumoral accumulation of liposomes.

CONCLUSION

Intratumoral heterogeneity was successfully visualized under the optimized conditions using a SPECT/CT scanner.

Evaluation of [125I]IPOS as a molecular imaging probe for hypoxia-inducible factor-1-active regions in a tumor: comparison among single-photon emission computed tomography/X-ray computed tomography imaging, autoradiography, and immunohistochemistry

To image hypoxia-inducible factor-1 (HIF-1)-active tumors, we previously developed a chimeric protein probe ([(123/125) I]IPOS) that is degraded in the same manner as HIF-1α under normoxic conditions. In the present study, we aim to show that the accumulation of radioiodinated POS reflects the expression of HIF-1. In vivo single-photon emission computed tomography (SPECT)/X-ray CT (CT) imaging, autoradiography, and double-fluorescent immunostaining for HIF-1α and pimonidazole (PIMO) were carried out 24 h after the injection of [(125) I]IPOS. Tumor metabolite analysis was also carried out. A tumor was clearly visualized by multi-pinhole, high-resolution SPECT/CT imaging with [(125) I]IPOS. The obtained images were in accordance with the corresponding autoradiograms and with the results of ex vivo biodistribution. A metabolite analysis revealed that 77% of the radioactivity was eluted in the macromolecular fraction, suggesting that the radioactivity mainly existed as [(125) I]IPOS in the tumors. Immunohistochemistry revealed that the HIF-1α-positive areas and PIMO-positive areas were not always identical, only some of the regions were positive for both markers. The areas showing [(125) I]IPOS accumulation were positively and significantly correlated with the HIF-1α-positive areas (R = 0.75, P < 0.0001). The correlation coefficient between [(125) I]IPOS-accumulated areas and HIF-1α-positive areas was significantly greater than that between the [(125) I]IPOS-accumulated areas and the PIMO-positive areas (P < 0.01). These findings indicate that [(125) I]IPOS accumulation reflects HIF-1 expression. Thus, [(123/125) I]IPOS can serve as a useful probe for the molecular imaging of HIF-1-active tumors.