Feasibility of experimental BT4C glioma models for somatostatin receptor 2-targeted therapies

<div><p></p><p>Somatostatin receptor subtype 2 (sstr₂) is regarded as a potential target in malignant gliomas for new therapeutic approaches. Therefore, visualizing and quantifying tumor sstr₂ expression in vivo would be highly relevant for the future development of sstr₂-targeted therapies. The purpose of this study was to evaluate sstr₂ status in experimental BT4C malignant gliomas.</p><p><b>Methods.</b> Rat BT4C malignant glioma cells were injected into BDIX rat brain or subcutaneously into nude mice. Tumor uptake of [⁶⁸Ga]DOTA-(Tyr³)-Octreotide ([⁶⁸Ga]DOTATOC), a somatostatin analog binding to sstr₂, was studied by positron emission tomography/computed tomography (PET/CT). Additionally, subcutaneous tumor-bearing mice underwent PET imaging with 5-deoxy-5-[¹⁸F]fluororibose-NOC ([¹⁸F]FDR-NOC), a novel glycosylated peptide tracer also targeting sstr₂. Ex vivo tissue radioactivity measurements, autoradiography and immunohistochemistry were performed to study sstr₂ expression.</p><p><b>Results.</b> Increased tumor uptake of [⁶⁸Ga]DOTATOC was detected at autoradiography with mean tumor-to-brain ratio of 68 ± 30 and tumor-to-muscle ratio of 9.2 ± 3.8 for rat glioma. High tumor-to-muscle ratios were also observed in subcutaneous tumor-bearing mice after injection with [⁶⁸Ga]DOTATOC and [¹⁸F]FDR-NOC with both autoradiography (6.7 ± 1.5 and 4.3 ± 0.8, respectively) and tissue radioactivity measurements (6.5 ± 0.8 and 4.8 ± 0.6, respectively). Furthermore, sstr₂ immunohistochemistry showed positive staining in both tumor models. However, surprisingly low tumor signal compromised PET imaging. Mean SUV_max for rat gliomas was 0.64 ± 0.28 from 30 to 60 min after [⁶⁸Ga]DOTATOC injection. The majority of subcutaneous tumors were not visualized by [⁶⁸Ga]DOTATOC or [¹⁸F]FDR-NOC PET.</p><p><b>Conclusions.</b> Experimental BT4C gliomas show high expression of sstr₂. Weak signal in PET imaging, however, suggests only limited benefit of [⁶⁸Ga]DOTATOC or [¹⁸F]FDR-NOC PET/CT in this tumor model for in vivo imaging of sstr₂ status.</p></div

A Novel Positron Emission Tomography (PET) Approach to Monitor Cardiac Metabolic Pathway Remodeling in Response to Sunitinib Malate

<div><p>Sunitinib is a tyrosine kinase inhibitor approved for the treatment of multiple solid tumors. However, cardiotoxicity is of increasing concern, with a need to develop rational mechanism driven approaches for the early detection of cardiac dysfunction. We sought to interrogate changes in cardiac energy substrate usage during sunitinib treatment, hypothesising that these changes could represent a strategy for the early detection of cardiotoxicity. Balb/CJ mice or Sprague-Dawley rats were treated orally for 4 weeks with 40 or 20 mg/kg/day sunitinib. Cardiac positron emission tomography (PET) was implemented to investigate alterations in myocardial glucose and oxidative metabolism. Following treatment, blood pressure increased, and left ventricular ejection fraction decreased. Cardiac [¹⁸F]-fluorodeoxyglucose (FDG)-PET revealed increased glucose uptake after 48 hours. [¹¹C]Acetate-PET showed decreased myocardial perfusion following treatment. Electron microscopy revealed significant lipid accumulation in the myocardium. Proteomic analyses indicated that oxidative metabolism, fatty acid β-oxidation and mitochondrial dysfunction were among the top myocardial signalling pathways perturbed. Sunitinib treatment results in an increased reliance on glycolysis, increased myocardial lipid deposition and perturbed mitochondrial function, indicative of a fundamental energy crisis resulting in compromised myocardial energy metabolism and function. Our findings suggest that a cardiac PET strategy may represent a rational approach to non-invasively monitor metabolic pathway remodeling following sunitinib treatment.</p></div

Physiological effects of sunitinib in two rodent models.

<p>Effects of sunitinib treatment on MAP (A, n = 9/group) and LVEF (B, n = 13/group) in female Balb/CJ mice. D-E) Effect of sunitinib treatment on systolic blood pressure (C), LVEF (D) and heart rate (E) in female Sprague-Dawley rats (n = 6/group, Significant difference between groups (unpaired t-test, *p<0.05 **p<0.01 ***p<0.001), Significant change from pre-treatment values (paired t-test, †p<0.05 ††p<0.01 †††p<0.001). Error bars = SEM)</p

Proteomic analyses of sunitinib effects in myocardial tissue.

<p>The most significantly perturbed canonical pathways from (A) crude membrane and (B) cytosolic fractions from sunitinib treated and control mouse myocardial tissue (n = 4/group) C) Proteins perturbed in the three most significantly enriched canonical pathways. D) Ingenuity Pathway Analysis (IPA) to determine the most significantly enriched network which is centred on Mitochondrial complex 1. Colour intensity indicates the degree of up- (red) or down- (green) regulation. IPA predictions are shown in blue (predicted inhibition) and orange (predicted activation). Black indicates no predicted effect. Continuous lines indicate a direct relationship between two proteins; a discontinuous line indicates indirect association.</p

Effect of sunitinib treatment on myocardial metabolism and perfusion measured by PET.

<p>A) Myocardial metabolic rate of glucose (MMRG) (mice, n = 10/group), B) K_mono (rats, n = 6/group). C-E) The effects of sunitinib treatment on overall perfusion of the myocardium in the rat model: (C) Representative images of the myocardium as seen in Carimas software (version 2.7) which is used to select region of interest (ROI), subsequently used to construct 17 segment heat maps for the 11C-Acetate data (rats) (D) which are used to determine global perfusion values. D) Representative pre- and 5 days post-treatment in one treated and one vehicle rat E) The effects of sunitinib treatment on overall perfusion of the myocardium (rats, n = 6/group, * = significant difference between groups (unpaired t-test, p<0.05), Significant change from pre-treatment (paired t-test, †p<0.05 ††p<0.01), bar* indicates significantly different overall on-treatment values (two-way ANOVA p < 0.05) Error bars = SEM).</p

Confirmation of proteomic findings in mouse myocardial tissue.

<p>A) ELISA to measure Mitochondrial complex 1 levels in myocardial tissue lysates. B) Immunoblot analysis of lysates from the cytosolic fraction of myocardial tissue from vehicle or sunitinib treated animals (n = 4/group) as described. C-G) Densitometry assessment of the presented western blots as described (* p < 0.05, error bars represent SEM).</p