Kimmo Jokivarsi

Cerebral PET imaging and histological evidence of transglutaminase inhibitor cystamine induced neuroprotection in transgenic R6/2 mouse model of Huntington's disease.

To investigate efficacy of cystamine induced neuroprotection, we conducted PET imaging studies of cerebral glucose metabolism with [(18)F]FDG (2-deoxy-2-[(18)F]fluoro-d-glucose) and striatal dopamine D2 receptor function with [(11)C]raclopride in R6/2 transgenic Huntington mice. In the control mice, exponentially decreasing glucose utilization was observed in the striatum N(str) [SUV]=(41.75+/-11.80)(58,str)*exp(-(0.041+/-0.007)*t [days]); cortex N(cort) [SUV]=24.14+/-3.66)(58,cort)*exp(-(0.043+/-0.007)*t [days]); and cerebellum N(cer) [SUV]=(34.97+/-10.58)(58,cer)*exp(-(0.037+/-0.008)*t [days]) as a function of age starting at 58 days. Given that the underlying degeneration rate in the cystamine treated mice is similar to that observed in control animals, the protection coefficient (beta) calculated from the equation N(t)=N(58)*exp(-(1-beta)*k*t) was 0.133+/-0.035 for the striatum; 0.122+/-0.028 for the cortex and 0.224+/-00.042 for the cerebellum with a dose of 100 mg/kg. The 50 mg/kg cystamine dose provided significant protection only for the striatum and only minor protection was obtained using lower doses. Striatal binding potential of [(11)C]raclopride was 1.059+/-0.030 in the control mice, and enhanced in the cystamine treated animals in a dose dependent manner up to 1.245+/-0.063 using the 100 mg/kg dose. Histological analysis confirmed cystamine induced neuroprotection of striatal and cortical neurons and Nissl staining revealed that formation of cellular inclusions was reversed in a dose dependent manner. Cerebral imaging and histological evidence support the use of cystamine as a neuroprotective agent for Huntingtons disease (HD) pathology.

Anatomical and Functional Assessment of Brown Adipose Tissue by Magnetic Resonance Imaging

Brown adipose tissue (BAT) is the primary tissue responsible for nonshivering thermogenesis in mammals. The amount of BAT and its level of activation help regulate the utilization of excessive calories for thermogenesis as opposed to storage in white adipose tissue (WAT) which would lead to weight gain. Over the past several years, BAT activity in vivo has been primarily assessed by positron emission tomography‐computed tomography (PET‐CT) scan using 2‐[18F]‐fluoro‐2‐deoxy‐D‐glucose (18F‐FDG) to measure glucose utilization associated with BAT mitochondrial respiration. In this study, we demonstrate the feasibility of mapping and estimating BAT volume and metabolic function in vivo in rats at a 9.4T magnetic resonance imaging (MRI) scanner using sequences available from clinical MR scanners. Based on the morphological characteristics of BAT, we measured the volume distribution of BAT with MRI sequences that have strong fat‐water contrast. We also investigated BAT volume by utilizing spin‐echo MRI sequences. The in vivo MRI‐estimated BAT volumes were correlated with direct measurement of BAT mass from dissected samples. Using MRI, we also were able to map hemodynamic responses to changes in BAT metabolism induced pharmacologically by β3‐adrenergic receptor agonist, CL‐316,243 and compare this to BAT activity in response to CL‐316,243 assessed by PET 18F‐FDG. In conclusion, we demonstrate the feasibility of measuring BAT volume and function in vivo using routine MRI sequences. The MRI measurement of BAT volume is consistent with quantitative measurement of the tissue ex vivo.

18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography Enables the Detection of Recurrent Same-Site Deep Vein Thrombosis by Illuminating Recently Formed, Neutrophil-Rich Thrombus

Background— Accurate detection of recurrent same-site deep vein thrombosis (DVT) is a challenging clinical problem. Because DVT formation and resolution are associated with a preponderance of inflammatory cells, we investigated whether noninvasive 18F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) imaging could identify inflamed, recently formed thrombi and thereby improve the diagnosis of recurrent DVT. Methods and Results— We established a stasis-induced DVT model in murine jugular veins and also a novel model of recurrent stasis DVT in mice. C57BL/6 mice (n=35) underwent ligation of the jugular vein to induce stasis DVT. FDG-PET/computed tomography (CT) was performed at DVT time points of day 2, 4, 7, 14, or 2+16 (same-site recurrent DVT at day 2 overlying a primary DVT at day 16). Antibody-based neutrophil depletion was performed in a subset of mice before DVT formation and FDG-PET/CT. In a clinical study, 38 patients with lower extremity DVT or controls undergoing FDG-PET were analyzed. Stasis DVT demonstrated that the highest FDG signal occurred at day 2, followed by a time-dependent decrease (P<0.05). Histological analyses demonstrated that thrombus neutrophils (P<0.01), but not macrophages, correlated with thrombus PET signal intensity. Neutrophil depletion decreased FDG signals in day 2 DVT in comparison with controls (P=0.03). Recurrent DVT demonstrated significantly higher FDG uptake than organized day 14 DVT (P=0.03). The FDG DVT signal in patients also exhibited a time-dependent decrease (P<0.01). Conclusions— Noninvasive FDG-PET/CT identifies neutrophil-dependent thrombus inflammation in murine DVT, and demonstrates a time-dependent signal decrease in both murine and clinical DVT. FDG-PET/CT may offer a molecular imaging strategy to accurately diagnose recurrent DVT.

Radiosynthesis of N-(4-chloro-3-[11C]methoxyphenyl)-2-picolinamide ([11C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4)

N-(Chloro-3-methoxyphenyl)-2-picolinamide (3, ML128, VU0361737) is an mGlu4 positive allosteric modulator (PAM), which is potent and centrally penetrating. 3 is also the first mGlu4 PAM to show efficacy in a preclinical Parkinson disease model upon systemic dosing. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers a possibility to investigate mGlu4 expression in vivo under physiologic and pathological conditions. We synthesized a carbon-11 labeled ML128 ([(11)C]3) as a PET radiotracer for mGlu4, and characterized its biological properties in Sprague Dawley rats. [(11)C]3 was synthesized from N-(4-chloro-3-hydroxyphenyl)-2-picolinamide (2) using [(11)C]CH3I. Total synthesis time was 38±2.2min (n=7) from the end of bombardment to the formulation. The radioligand [(11)C]3 was obtained in 27.7±5.3% (n=5) decay corrected radiochemical yield based on the radioactivity of [(11)C]CO2. The radiochemical purity of [(11)C]3 was >99%. Specific activity was 188.7±88.8GBq/mol (n=4) at the end of synthesis (EOS). PET images were conducted in 20 normal male Sprague Dawley rats including 11 control studies, 6 studies blocking with an mGlu4 modulator (4) to investigate specificity and 3 studies blocking with an mGlu5 modulator (MTEP) to investigate selectivity. These studies showed fast accumulation of [(11)C]3 (peak activity between 1-3min) in several brain areas including striatum, thalamus, hippocampus, cerebellum, and olfactory bulb following with fast washout. Blocking studies with the mGlu4 modulator 4 showed 22-28% decrease of [(11)C]3 accumulation while studies of selectivity showed only minor decrease supporting good selectivity over mGlu5. Biodistribution studies and blood analyses support fast metabolism. Altogether this is the first PET imaging ligand for mGlu4, in which the labeled ML128 was used for imaging its in vivo distribution and pharmacokinetics in brain.

High Efficiency Diffusion Molecular Retention Tumor Targeting

Here we introduce diffusion molecular retention (DMR) tumor targeting, a technique that employs PEG-fluorochrome shielded probes that, after a peritumoral (PT) injection, undergo slow vascular uptake and extensive interstitial diffusion, with tumor retention only through integrin molecular recognition. To demonstrate DMR, RGD (integrin binding) and RAD (control) probes were synthesized bearing DOTA (for 111 In3+), a NIR fluorochrome, and 5 kDa PEG that endows probes with a protein-like volume of 25 kDa and decreases non-specific interactions. With a GFP-BT-20 breast carcinoma model, tumor targeting by the DMR or IV methods was assessed by surface fluorescence, biodistribution of [111In] RGD and [111In] RAD probes, and whole animal SPECT. After a PT injection, both probes rapidly diffused through the normal and tumor interstitium, with retention of the RGD probe due to integrin interactions. With PT injection and the [111In] RGD probe, SPECT indicated a highly tumor specific uptake at 24 h post injection, with 352%ID/g tumor obtained by DMR (vs 4.14%ID/g by IV). The high efficiency molecular targeting of DMR employed low probe doses (e.g. 25 ng as RGD peptide), which minimizes toxicity risks and facilitates clinical translation. DMR applications include the delivery of fluorochromes for intraoperative tumor margin delineation, the delivery of radioisotopes (e.g. toxic, short range alpha emitters) for radiotherapy, or the delivery of photosensitizers to tumors accessible to light.

Radiosynthesis and evaluation of an 18F-labeled positron emission tomography (PET) radioligand for metabotropic glutamate receptor subtype 4 (mGlu4).

Four 4-phthalimide derivatives of N-(3-chlorophenyl)-2-picolinamide were synthesized as potential ligands for the PET imaging of mGlu4 in the brain. Of these compounds, N-(3-chloro-4-(4-fluoro-1,3-dioxoisoindolin-2-yl)phenyl)-2-picolinamide (3, KALB001) exhibited improved binding affinity (IC50 = 5.1 nM) compared with ML128 (1) and was subsequently labeled with 18F. When finally formulated in 0.1 M citrate buffer (pH 4) with 10% ethanol, the specific activity of [18F]3 at the end of synthesis (EOS) was 233.5 ± 177.8 GBq/μmol (n = 4). The radiochemical yield of [18F]3 was 16.4 ± 4.8% (n = 4), and the purity was over 98%. In vivo imaging studies in a monkey showed that the radiotracer quickly penetrated the brain with the highest accumulation in the brain areas known to express mGlu4. Despite some unfavorable radiotracer properties like fast washout in rodent studies, [18F]3 is the first 18F-labeled mGlu4 radioligand, which can be further modified to improve pharmacokinetics and brain penetrability for future human studies.

Response to Letter Regarding Article, “18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography Enables the Detection of Recurrent Same-Site Deep Vein Thrombosis by Illuminating Recently Formed, Neutrophil-Rich Thrombus”

We thank Dr Klok and colleagues for their comments on our article.1 Deep venous thrombosis (DVT) is a relatively common disorder that is associated with potentially deadly complications, such as pulmonary embolism, and carries a substantial risk for the postthrombotic syndrome and recurrent DVT. Novel strategies are needed to predict and ultimately to reduce the development of such DVT-related complications. It is in this context that our group recently presented a novel application of 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) imaging of inflammation to detect recurrent DVT, a vexing clinical problem insufficiently addressed by duplex ultrasound.1 In their letter, Klok et al posed questions about aspects of the clinical substudy reported in our article. We wish to provide some answers to their inquiries. One …

Radiolabeling and biodistribution of methyl 2-(methoxycarbonyl)-2-(methylamino) bicyclo [2.1.1] - hexane -5-carboxylate, a potential neuroprotective drug

Methyl 2-(methoxycarbonyl) -2-(methylamino) bicyclo[2.1.1] -hexane -5-carboxylate (MMMHC) is developed as a potential neuroprotective drug. It was labeled with C-11 from the desmethyl precursor methyl 2-(methoxycarbonyl)-2-amino bicyclo[2.1.1]-hexane-5-carboxylate with [11C]methyl triflate in acetone solution at 60 degrees C with labeling yield of 69% and with radiochemical purity of >99%. Positron Emission Tomography (PET) studies in a normal rat showed that Methyl 2-(methoxycarbonyl)-2-([11C]methylamino)bicyclo[2.1.1]-hexane-5-carboxylate ([11C] MMMHC) accumulated mainly in the cortical brain areas after iv administration. Frontal cortex/cerebellum ratios in a rat brain were 8.0/6.0, 6.8/4.2, 6.3/4.3, 5.5/4.2 and 5.2/4.5 percent of the injected dose in 100 ml at 2 min, 5 min, 10 min, 20 min and 40 min respectively after i.v. injection. During 20-40 min, 2.9+/-0.4% of the total activity stayed in the brain. These results showed that MMMHC could be labeled with C-11 with high yield, and it passed the brain-blood barrier and accumulated in several brain regions.