Radioactivity In Organs Research Articles

The N-acetyltransferase 10 inhibitor [11C]remodelin: synthesis and preliminary positron emission tomography study in mice

Background4-(4-Cyanophenyl)-2-(2-cyclopentylidenehydrazinyl)thiazole (remodelin) is a potent N-acetyltransferase 10 (NAT10) inhibitor. This compound inhibits tumors and weakens tumor resistance to antitumor drugs. Moreover, remodelin has been found to enhance healthspan in an animal model of the human accelerated ageing syndrome. In this study, we synthesized C-11-labelled remodelin ([11C]remodelin) for the first time as a positron emission tomography (PET) probe and assessed its biodistribution in mice using PET.Results[11C]Remodelin was synthesized by the reaction of a boron ester precursor (1) with hydrogen [11C]cyanide, which was prepared from the cyclotron-produced [11C]carbon dioxide via [11C]methane. The decay-corrected radiochemical yield of [11C]remodelin was 6.2 ± 2.3% (n = 20, based on [11C]carbon dioxide) with a synthesis time of 45 min and radiochemical purity of > 90%. A PET study with [11C]remodelin showed high uptake of radioactivity in the heart, liver, and small intestine of mice. The metabolite analysis indicated moderate metabolism of [11C]remodelin in the heart.ConclusionsIn the present study, we successfully synthesized [11C]remodelin and assessed its biodistribution of radioactivity in the mouse organs and tissues with PET. We are planning to prepare tumor and inflammatory models in which overexpression of NAT10 is possibly induced and conduct PET imaging for these animal models with [11C]remodelin to elucidate the relationship between NAT10 and diseases.

Biodistribution and dosimetry of the PET radioligand [18F]CHDI-650 in mice for detection of mutant huntingtin aggregates

BackgroundHuntington’s disease (HD) is a rare neurodegenerative disorder caused by an expansion of the CAG trinucleotide repeat in the huntingtin gene which encodes the mutant huntingtin protein (mHTT) that is associated with HD-related neuropathophysiology. Noninvasive visualization of mHTT aggregates in the brain, with positron emission tomography (PET), will allow to reliably evaluate the efficacy of therapeutic interventions in HD. This study aimed to assess the radiation burden of [18F]CHDI-650, a novel fluorinated mHTT radioligand, in humans based on both in vivo and ex vivo biodistribution in mice and subsequent determination of dosimetry for dosing in humans.ResultsWild-type male and female CD-1 Swiss mice (n = 15/sex) were used to assess in vivo PET imaging-based and ex vivo biodistribution-based tracer distribution of [18F]CHDI-650 at 30-, 60-, 120-, 240- and 360-min post-injection. Three-dimensional volumes of interest of the organs were drawn on the co-registered PET/CT image and organs were collected after dissection. Organ radioactivity levels were determined using both modalities. The residence time was calculated and extrapolated to human phantoms. The absorbed and effective doses were computed with OLINDA/EXM 2.2 and IDAC-Dose2.1. Ex vivo and PET-imaging biodistribution of [18F]CHDI-650 showed rapid washout after 30 min in most of the organs with the highest uptake in the gallbladder and urine in mice. Extrapolation of the data to human phantoms with OLINDA showed a total mean in vivo based effective dose of 21.7 μSv/MBq with the highest equivalent organ dose in the urinary bladder wall (4.52 μSv/MBq). The total mean ex vivo based effective dose was calculated to be 20.6 μSv/MBq. The highest equivalent organ dose ex vivo in the urinary bladder wall was estimated to be 4.22 μSv/MBq. The predicted exposure in humans using IDAC-Dose correlated well to those obtained with OLINDA for both in vivo and ex vivo measurements (r = 0.9320 and r = 0.9368, respectively).ConclusionsDosimetry analysis indicated absorbed and effective doses of [18F]CHDI-650 are well below the recommended limits, suggesting that the radioligand is suitable for clinical assessment. Based on the highest effective dose estimates, an injection of 370 MBq in humans would result in a radiation dose of 8.03 mSv.

In vivo assessment of safety, biodistribution, and radiation dosimetry of the [18F]Me4FDG PET-radiotracer in adults

BackgroundApproaches targeting the sodium-glucose cotransporter (SGLT) could represent a promising future therapeutic strategy for numerous oncological and metabolic diseases. In this study, we evaluated the safety, biodistribution and radiation dosimetry of the glucose analogue positron emission tomography (PET) agent [18F] labeled alpha-methyl-4-deoxy-4-[18F]fluoro-D-glucopyranoside ([18F]Me4FDG) with high sodium-glucose cotransporter and low glucose transporter (GLUT) affinity. For this purpose, five healthy volunteers (1 man, 4 women) underwent multiple whole-body PET/computed tomography (CT) examinations starting with injection and up to 4 h after injection of averaged (2.4 ± 0.1) MBq/kg (range: 2.3–2.5 MBq/kg) administered activity. The PET/CT scans were conducted in 5 separate sessions, blood pressure and temperature were measured, and blood and urine samples were collected before the scans and one hour after injection to assess toxicity. Measurements of [18F]Me4FDG radioactivity in organs of interest were determined from the PET/CT scans at 5 time points. Internal dosimetry was performed on voxel level using a fast Monte Carlo approach.ResultsAll studied volunteers could well tolerate the [18F]Me4FDG and no adverse event was reported. The calculated effective dose was (0.013 ± 0.003) mSv/MBq. The organs with the highest absorbed dose were the kidneys with 0.05 mSv/MBq per kidney. The brain showed almost no uptake. After 60 min, (12 ± 15) % of the administered dose was excreted into the bladder.ConclusionFeaturing an effective dose of only 0.013 ± 0.003 mSv/MBq and no occurrence of side effects, the glucose analogue [18F]Me4FDG seems to be a safe radio-tracer with a favorable biodistribution for PET imaging and also within several consecutive scans.Trial registration numberNCT03557138, Registered 22 February 2017, https://ichgcp.net/clinical-trials-registry/NCT03557138.

Acetamidobenzoxazolone conjugated DOTA system for assessing 18 kDa translocator protein during pulmonary inflammation

Translocator protein (TSPO) is well known inflammatory target. We have designed a new molecule DOTA-MBP. In-silico studies have been performed to evaluate the binding affinity potential of MBP towards the TSPO. The stability of DOTA-MBP was assessed for Gd (III) for possible application as chelating vehicle or other MR contrast agent. For in vivo studies LPS induced lung inflammation model was develop in mice and after 24 h of LPS administration biodistribution and 99mTc-DOTA-MBP imaging have been performed to monitor the uptake and distribution of radioactivity in different peripheral organs with time. In conclusion we can say that DOTA-MBP as potential candidate to locate the lung inflammation as well as monitor the severity of disease through SPECT or MRI.

18F Site-Specific Labelling of a Single-Chain Antibody against Activated Platelets for the Detection of Acute Thrombosis in Positron Emission Tomography.

Positron emission tomography is the imaging modality of choice when it comes to the high sensitivity detection of key markers of thrombosis and inflammation, such as activated platelets. We, previously, generated a fluorine-18 labelled single-chain antibody (scFv) against ligand-induced binding sites (LIBS) on activated platelets, binding it to the highly abundant platelet glycoprotein integrin receptor IIb/IIIa. We used a non-site-specific bio conjugation approach with N-succinimidyl-4-[18F]fluorobenzoate (S[18F]FB), leading to a mixture of products with reduced antigen binding. In the present study, we have developed and characterised a novel fluorine-18 PET radiotracer, based on this antibody, using site-specific bio conjugation to engineer cysteine residues with N-[2-(4-[18F]fluorobenzamido)ethyl]maleimide ([18F]FBEM). ScFvanti-LIBS and control antibody mut-scFv, with engineered C-terminal cysteine, were reduced, and then, they reacted with N-[2-(4-[18F]fluorobenzamido)ethyl]maleimide ([18F]FBEM). Radiolabelled scFv was injected into mice with FeCl3-induced thrombus in the left carotid artery. Clots were imaged in a PET MR imaging system, and the amount of radioactivity in major organs was measured using an ionisation chamber and image analysis. Assessment of vessel injury, as well as the biodistribution of the radiolabelled scFv, was studied. In the in vivo experiments, we found uptake of the targeted tracer in the injured vessel, compared with the non-injured vessel, as well as a high uptake of both tracers in the kidney, lung, and muscle. As expected, both tracers cleared rapidly via the kidney. Surprisingly, a large quantity of both tracers was taken up by organs with a high glutathione content, such as the muscle and lung, due to the instability of the maleimide cysteine bond in vivo, which warrants further investigations. This limits the ability of the novel antibody radiotracer 18F-scFvanti-LIBS to bind to the target in vivo and, therefore, as a useful agent for the sensitive detection of activated platelets. We describe the first fluorine-18 variant of the scFvanti-LIBS against activated platelets using site-specific bio conjugation.

Radiometal-Labeled Chitosan Microspheres as Transarterial Radioembolization Agents against Hepatocellular Carcinoma.

Transarterial radioembolization (TARE) is an emerging treatment for patients with unresectable hepatocellular carcinoma (HCC). This study successfully developed radiometal-labeled chitosan microspheres (111In/177Lu-DTPA-CMS) with a diameter of 36.5 ± 5.3 μm for TARE. The radiochemical yields of 111In/177Lu-DTPA-CMS were greater than 90% with high radiochemical purities (>98%). Most of the 111In/177Lu-DTPA-CMS were retained in the hepatoma and liver at 1 h after intraarterial (i.a.) administration. Except for liver accumulation, radioactivity in each normal organ was less than 1% of the injected radioactivity (%IA) at 72 h after injection. At 10 days after injection of 177Lu-DTPA-CMS (18.6 ± 1.3 MBq), the size of the hepatoma was significantly reduced by around 81%, while that of the rats in the control group continued to grow. This study demonstrated the effectiveness of 177Lu-DTPA-CMS in the treatment of N1-S1 hepatoma. 111In/177Lu-DTPA-CMS have the potential to be a superior theranostic pair for the treatment of clinical hepatoma.

The influence of temperature on biodistribution of N,N,N’,N’- ethylenediaminetetrakis(methylene phosphonic) acid labeled with gallium-68

Bone metastases are serious complication in the progression of various types of cancer. It determines the requirement of modern nuclear diagnostic tools, including positron emission tomography (PET). In this study the biodistribution of EDTMP labeled with gallium-68 (68Ga-EDTMP) prepared at different temperature (20, 50, and 95 °C) was investigated. All experimental studies were performed in healthy intact Wistar rats by measuring the radioactivity in organs and tissues with gamma counter. All 68Ga-EDTMP formulations accumulated predominantly in bones. Only in tibia the uptake of 68Ga-EDTMP prepared at 95 °C was higher (p < 0.05) than 68Ga-EDTMP prepared at 20 °C, but in other bones there weren’t any statistical differences in uptake of 68Ga-EDTMP formulations. The amounts of 68Ga-EDTMP formulations in soft organs and tissues were lower when compared with bones. In conclusion, a temperature of reaction mixture had an influence on the biodistribution of 68Ga-EDTMP in bones.

Selective Imaging of Matrix Metalloproteinase-13 to Detect Extracellular Matrix Remodeling in Atherosclerotic Lesions.

Overexpression and activation of matrix metalloproteinase-13 (MMP-13) within atheroma increases susceptibility to plaque rupture, a major cause of severe cardiovascular complications. In comparison to pan-MMP targeting [18F]BR-351, we evaluated the potential for [18F]FMBP, a selective PET radiotracer for MMP-13, to detect extracellular matrix (ECM) remodeling in vascular plaquespossessing markers of inflammation. [18F]FMBP and [18F]BR-351 were initially assessed in vitro by incubation with en face aortae from 8 month-old atherogenic ApoE-/- mice. Ex vivo biodistributions, plasma metabolite analyses, and ex vivo autoradiography were analogously performed 30 min after intravenous radiotracer administration in age-matched C57Bl/6 and ApoE-/- mice under baseline or homologous blocking conditions. En face aortae were subsequently stained with Oil Red O (ORO), sectioned, and subject to immunofluorescence staining for Mac-2 and MMP-13. High-resolution autoradiographic image analysis demonstrated target specificity and regional concordance to lipid-rich lesions. Biodistribution studies revealed hepatobiliary excretion, low accumulation of radioactivity in non-excretory organs, and few differences between strains and conditions in non-target organs. Plasma metabolite analyses uncovered that [18F]FMBP exhibited excellent in vivo stability (≥74% intact) while [18F]BR-351 was extensively metabolized (≤37% intact). Ex vivo autoradiography and histology of en face aortae revealed that [18F]FMBP, relative to [18F]BR-351, exhibited 2.9-fold greater lesion uptake, substantial specific binding (68%), and improved sensitivity to atherosclerotic tissue (2.9-fold vs 2.1-fold). Immunofluorescent staining of aortic en face cross sections demonstrated elevated Mac-2 and MMP-13-positive areas within atherosclerotic lesions identified by [18F]FMBP ex vivo autoradiography. While both radiotracers successfully identified atherosclerotic plaques, [18F]FMBP showed superior specificity and sensitivity for lesions possessing features of destructive plaque remodeling. The detection of ECM remodeling by selective targeting of MMP-13 may enable characterization of high-risk atherosclerosis featuring elevated collagenase activity.

Brain pharmacokinetics of two BBB penetrating bispecific antibodies of different size

BackgroundTransferrin receptor (TfR1) mediated enhanced brain delivery of antibodies have been studied extensively in preclinical settings. However, the brain pharmacokinetics, i.e. brain entry, distribution and elimination are still not fully understood for this class of antibodies. The overall aim of the study was to compare the brain pharmacokinetics of two BBB-penetrating bispecific antibodies of different size (210 vs 58 kDa). Specifically, we wanted to investigate if the faster systemic clearance of the smaller non-IgG antibody di-scFv3D6-8D3, in comparison with the IgG-based bispecific antibody mAb3D6-scFv8D3, was also reflected in the brain.MethodsWild-type (C57/Bl6) mice were injected with 125I-iodinated ([125I]) mAb3D6-scFv8D3 (n = 46) or [125I]di-scFv3D6-8D3 (n = 32) and euthanized 2, 4, 6, 8, 10, 12, 16, or 24 h post injection. Ex vivo radioactivity in whole blood, peripheral organs and brain was measured by γ-counting. Ex vivo autoradiography and nuclear track emulsion were performed on brain sections to investigate brain and parenchymal distribution. Capillary depletion was carried out at 2, 6, and 24 h after injection of [125I]mAb3D6-scFv8D3 (n = 12) or [125I]di-scFv3D6-8D3 (n = 12), to estimate the relative levels of radiolabelled antibody in brain capillaries versus brain parenchyma. In vitro binding kinetics for [125I]mAb3D6-scFv8D3 or [125I]di-scFv3D6-8D3 to murine TfR were determined by LigandTracer.Results[125I]di-scFv3D6-8D3 showed faster elimination from blood, lower brain Cmax, and Tmax, a larger parenchymal-to-capillary concentration ratio, and a net elimination from brain at an earlier time point after injection compared with the larger [125I]mAb3D6-scFv8D3. However, the elimination rate from brain did not differ between the antibodies. The study also indicated that [125I]di-scFv3D6-8D3 displayed lower avidity than [125I]mAb3D6-scFv8D3 towards TfR1 in vitro and potentially in vivo, at least at the BBB.ConclusionA smaller size and lower TfR1 avidity are likely important for fast parenchymal delivery, while elimination of brain-associated bispecific antibodies may not be dependent on these characteristics.

Abstract P768: Vascular Cellular Adhesion Molecule Targeting for the Treatment of Acute Neurovascular Inflammation

Targeted drug delivery is an attractive strategy for treatment of life-threatening inflammatory brain pathologies such as ischemic stroke. In these conditions, adhesion molecule expression (e.g. vascular cellular adhesion molecule-1, VCAM) is upregulated on endothelium. Hence, VCAM represents a relevant target for selective drug delivery. We have utilized αVCAM monoclonal antibodies to target therapeutic proteins and nanocarriers (liposomes and lipid nanoparticles, LNPs) to the pathologically-altered vasculature.To assess tissue biodistribution and therapeutic effects of VCAM-targeted agents, we used 2 mouse models: 1) acute inflammation induced by intra-striatal injection of tumor necrosis factor-α (TNF), and 2) ischemic stroke induced by transient middle cerebral artery occlusion (tMCAO). Biodistribution was assayed by measuring radioactivity in blood and organs 30 minutes after injection. Efficacy was determined by tracing radiolabeled albumin extravasation to assess edema.The results showed that VCAM targeting was increased by TNF injection, and that targeting to VCAM enhanced the cerebral accumulation of αVCAM conjugated thrombomodulin (TM) and nanocarriers by orders of magnitude (Fig A). It is noteworthy that αVCAM-TM and free αVCAM antibody had similar levels of brain uptake. Our data also demonstrated the protective effect of αVCAM-liposome loaded with dexamethasone on brain edema (Fig B), indicating the therapeutic potential of VCAM-targeting drug delivery strategies. Further, the results in tMCAO model recapitulated our findings in the TNF model that targeting to VCAM significantly improves delivery to the ischemic brain (Fig C). In summary, we have shown that targeting VCAM permits delivery of a diverse array of therapeutics to the inflamed vasculature in conditions such as ischemic stroke. Future work will focus on evaluating the therapeutic properties of VCAM-targeted drug delivery strategies.

Pharmacokinetics, tissue distribution and excretion of a novel long-acting human insulin analogue – recombinant insulin LysArg in rats

As a novel long-acting recombinant human insulin analogue, it is necessary to carry out the preclinical research for insulin LysArg. The purpose of this study was to characterise the pharmacokinetic, tissue distribution and excretion of insulin LysArg and provide a reference for its development. Three methods were used to measure the content of insulin LysArg in biological samples after a single subcutaneous administration in rats, including radioassay, radioassay after precipitation with TCA and separation by HPLC. After Subcutaneous administration of recombinant insulin LysArg 1, 2, 4 U/kg in rats, it showed both Cmax and AUC0–t were positively correlated with the dose. In the meanwhile, after a single subcutaneous administration of recombinant insulin LysArg at 2 U/kg in rats, the amount of radioactivity in most organs was highest at 1.5 h and then decreased gradually, no accumulation was found. The highest level of insulin LysArg was observed in the kidney. Like other macromolecules, insulin LysArg was mainly excreted from urine. The study fully illustrated the pharmacokinetic pattern of insulin LysArg, provided valuable informations to support its further development about safety and toxicology.

Development of a CD19 PET tracer for detecting B cells in a mouse model of multiple sclerosis

BackgroundB cells play a central role in multiple sclerosis (MS) through production of injurious antibodies, secretion of pro-inflammatory cytokines, and antigen presentation. The therapeutic success of monoclonal antibodies (mAbs) targeting B cells in some but not all individuals suffering from MS highlights the need for a method to stratify patients and monitor response to treatments in real-time. Herein, we describe the development of the first CD19 positron emission tomography (PET) tracer, and its evaluation in a rodent model of MS, experimental autoimmune encephalomyelitis (EAE).MethodsFemale C57BL/6 J mice were induced with EAE through immunization with myelin oligodendrocyte glycoprotein (MOG1–125). PET imaging of naïve and EAE mice was performed 19 h after administration of [64Cu]CD19-mAb. Thereafter, radioactivity in organs of interest was determined by gamma counting, followed by ex vivo autoradiography of central nervous system (CNS) tissues. Anti-CD45R (B220) immunostaining of brain tissue from EAE and naïve mice was also conducted.ResultsRadiolabelling of DOTA-conjugated CD19-mAb with 64Cu was achieved with a radiochemical purity of 99% and molar activity of 2 GBq/μmol. Quantitation of CD19 PET images revealed significantly higher tracer binding in whole brain of EAE compared to naïve mice (2.02 ± 0.092 vs. 1.68 ± 0.06 percentage of injected dose per gram, % ID/g, p = 0.0173). PET findings were confirmed by ex vivo gamma counting of perfused brain tissue (0.22 ± 0.020 vs. 0.12 ± 0.003 % ID/g, p = 0.0010). Moreover, ex vivo autoradiography of brain sections corresponded with PET imaging results and the spatial distribution of B cells observed in B220 immunohistochemistry—providing further evidence that [64Cu]CD19-mAb enables visualization of B cell infiltration into the CNS of EAE mice.ConclusionCD19-PET imaging can be used to detect elevated levels of B cells in the CNS of EAE mice, and has the potential to impact the way we study, monitor, and treat clinical MS.

Preliminary Evaluation of Astatine-211-Labeled Bombesin Derivatives for Targeted Alpha Therapy.

There are various diagnostic and therapeutic agents for prostate cancer using bombesin (BBN) derivatives, but astatine-211 (211At)-labeled BBN derivatives have yet to be studied. This study presented a preliminary evaluation of 211At-labeled BBN derivative. Several nonradioactive iodine-introduced BBN derivatives (IB-BBNs) with different linkers were synthesized and their binding affinities measured. Because IB-3 exhibited a comparable affinity to native BBN, [211At]AB-3 was synthesized and the radiochemical yields of [211At]AB-3 was 28.2 ± 2.4%, with a radiochemical purity of >90%. The stability studies and cell internalization/externalization experiments were performed. [211At]AB-3 was taken up by cells and internalized; however, radioactivity effluxed from cells over time. In addition, the biodistribution of [211At]AB-3, with and without excess amounts of BBN, were evaluated in PC-3 tumor-bearing mice. Despite poor stability in murine plasma, [211At]AB-3 accumulated in tumor tissue (4.05 ± 0.73%ID/g) in PC-3 tumor-bearing mice, which was inhibited by excess native BBN (2.56 ± 0.24%ID/g). Accumulated radioactivity in various organs is probably due to free 211At. Peptide degradation in murine plasma and radioactivity efflux from cells are areas of improvement. The development of 211At-labeled BBN derivatives requires modifying the BBN sequence and preventing deastatination.

Tissue distribution of sprifermin (recombinant human fibroblast growth factor 18) in the rat following intravenous and intra-articular injection

Fibroblast growth factor 18 (FGF18) is involved in chondrogenesis and articular cartilage repair. We investigated tissue distribution and pharmacokinetics of radioactive [3H]sprifermin, a recombinant human FGF18, in rats after a single intravenous (i.v.) or intra-articular (i.a.) injection. In two studies (48-96-h [n=23] and 28-day [n=12]), 35 male albino (Sprague Dawley) rats received single i.v. or i.a. dose [3H]sprifermin (0.24mg/kg). Radioactivity was measured in blood, serum, and (in animals receiving i.a. administration) in the knee joint by liquid scintillation counting. Radioactivity in organs, tissues, and distribution in the whole body were measured with whole-body autoradiography. After i.v. injection, radioactivity peaked in serum and whole blood after 4 and 24h, respectively, with greater total radioactivity in serum. After i.a. injection, radioactivity peaked in serum and whole blood after 24 and 48h, respectively; intact [3H]sprifermin was not detected in vena caval serum and systemic exposure was low, approximately 20% of that with i.v. injection. Following i.v. injection, radioactivity was mainly found in the liver, adrenal glands, kidney, and spleen; following i.a. injection, radioactivity was preferentially concentrated in articular cartilage after initial distribution in the joint capsule, and still evident in the joint after 28 days. After i.a. injection of [3H]sprifermin in rats, radioactivity was concentrated in the knee joint, particularly articular cartilage, with low levels in other investigated tissues. Systemic exposure to sprifermin was greater with i.v. than i.a. injection. Subsequent clinical investigation in patients with osteoarthritis has reported consistent results.

Radiosynthesis and evaluation of acetamidobenzoxazolone based radioligand [11C]N′-MPB for visualization of 18 kDa TSPO in brain

Modified acetamidobenzoxazolone radioligand [11C]N′-MPB for visualization of 18 kDa TSPO.

Therapeutic Efficacy and Dosimetry of Targeted Alpha Therapy using 225Ac-PSMA-617 in a Murine Model of Prostate Cancer

Targeted alpha therapy shows promise as a treatment in metastatic castration-resistant prostate cancer (mCRPC), owing to the high dose deposition and short range characteristics of alpha particle radiation. Ac-225 decays via multiple alpha particles – each capable of inducing irreparable DNA double-strand breaks in cell nuclei. PSMA-617 is a targeting ligand with specific affinity for prostate-specific membrane antigen (PSMA), a protein that is overexpressed in mCRPC but has low expression in normal tissues. Coupled with the lethal effects of alpha radiation, 225Ac-PSMA-617 is potentially highly cytotoxic to mCRPC while sparing normal tissues. The objective of this study was to evaluate the efficacy of alpha-particle radioligand therapy in a mouse model of mCRPC. Radiation dosimetry analysis was carried out to determine tumor dose as well as dose-limiting organs. NSG mice bearing subcutaneous PSMA-expressing C4-2 tumors were injected with escalating activities of 225Ac-PSMA-617: 20, 40, and 100 kBq/mouse (n=8/group). The tumor volumes were assessed over time by weekly low-dose microCT and compared with an untreated control group. In parallel, twenty-five NSG mice were used in a biodistribution study at five time points – 1, 4, 24, 48, and 168 hours post-administration of 40 kBq of the radiopharmaceutical. After sacrifice, percent tumor and organ radioactivity uptake was measured using gamma spectroscopy. Biexponential curve-fitting was used to fit the time-activity curves for the tumor and each organ, and integrated according to standard medical internal radiation dose methods to estimate the tumor and organ doses. Significant tumor growth retardation was observed in all treatment groups compared with the untreated group. Mice treated with 100 kBq exhibited some weight loss, while the mice treated with lower activities experienced only transient weight loss. There was a significant survival benefit conferred on tumor-bearing mice treated with 225Ac-PSMA617. The biodistribution over five time points showed high uptake and slow tumor activity clearance, and low uptake with fast clearance in non-target organs. The salivary glands, a dose-limiting organ in humans, did not show high uptake in mice, possibly due to lower PSMA expression. Estimates of the absorbed dose to the tumor and organs are reported. An administered activity of 40 kBq per mouse of the peptidomimetic 225AcPSMA-617 is well-tolerated and results in significant tumor growth retardation and improved survival. This work provides a preclinical foundation for further studies towards a more effective treatment option for advanced castration-resistant prostate cancer.

Abstract 1141: Preclinical analysis of biodistribution and PET imaging of a zirconium-89 labeled PSMA-targeted antibody-chelator conjugate

Abstract The PSMA targeted thorium-227 conjugate PSMA-TTC (BAY 2315497) is a targeted alpha therapy approach for metastatic castration resistant prostate cancer that has shown strong anti-tumor activity in PSMA-positive prostate cancer models (Hammer et al. AACR 2017/2018/2019). PSMA-TTC consists of a fully human IgG1 PSMA-targeted antibody covalently linked to a 3,2-HOPO chelator moiety, stably complexing the alpha emitter thorium-227. As this chelator can also complex zirconium-89, positron emission tomography (PET) imaging using the zirconium-89 labeled variant of the PSMA antibody-chelator conjugate could support clinical development of PSMA-TTC by enabling monitoring of organ distribution. Herein, we describe radiolabeling of the PSMA antibody-chelator conjugate with zirconium-89 and its quality control using bioanalytical assays. In addition, the zirconium-89 labeled PSMA antibody-chelator conjugate was injected into mice bearing LNCaP xenograft tumors and biodistribution was assessed by measurement of radioactivity in tumor and organs as well as PET imaging at different time points after injection. The PSMA antibody-chelator conjugate was successfully labeled with zirconium-89 at high yield, purity and specific activity. Target binding was confirmed by determination of the immunoreactive fraction. Biodistribution experiments revealed high, long lasting uptake of zirconium-89 in PSMA-positive LNCaP tumors after single injection of the zirconium-89 labeled conjugate. An uptake of 40-50% of the injected dose per gram of tumor was measured at 24, 48, 72, 168 and 240h after injection. The PET images showed that the zirconium-89 labeled PSMA antibody-chelator conjugate provided an excellent image contrast for the delineation of LNCaP xenografts between 48 and 168 h after administration. Other organs showed only very little background and tumor-to-heart ratio was greater than 10 when PET imaging was performed between 48 and 168 h after injection. These results indicate that the zirconium-89 labeled PSMA antibody-chelator conjugate shows promise for PET imaging studies. These properties may allow its use to support clinical development of PSMA-TTC. Citation Format: Stefanie Hammer, Sabine Zitzmann-Kolbe, Felix Oden, Joerg Jannsen, Eva Bickel, Andre Mueller, Lothar Everz, Urs B. Hagemann, Jenny Karlsson, Olav B. Ryan, Hartwig Hennekes, Patricia E. Cole, Dominik Mumberg. Preclinical analysis of biodistribution and PET imaging of a zirconium-89 labeled PSMA-targeted antibody-chelator conjugate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1141.

Radiosynthesis and evaluation of a novel monoacylglycerol lipase radiotracer: 1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-benzyl-1H-pyrazol-3-yl)azetidine-1-[11C]carboxylate

Monoacylglycerol lipase (MAGL) is a major serine hydrolase that hydrolyses 2-arachidonoylglycerol (2-AG) into arachidonic acid (AA) and glycerol in the brain. Because 2-AG and AA are endogenous biologically active ligands in the brain, the inhibition of MAGL is an attractive therapeutic target for neurodegenerative diseases. In this study, to visualize MAGL via positron emission tomography (PET), we report a new carbon-11-labeled radiotracer, namely 1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-benzyl-1H-pyrazol-3-yl)azetidine-1-[11C]carboxylate ([11C]6). Compound 6 exhibited high in vitro binding affinity (IC50 = 0.41 nM) to MAGL in the brain with a suitable lipophilicity (cLogD = 3.29). [11C]6 was synthesized by reacting 1,1,1,3,3,3-hexafluoropropanol (7) with [11C]phosgene ([11C]COCl2), followed by a reaction with 3-(1-benzyl-1H-pyrazol-3-yl)azetidine hydrochloride (8), which resulted in a 15.0 ± 6.8% radiochemical yield (decay-corrected, n = 7) based on [11C]CO2 and a 45 min synthesis time from the end of bombardment. A biodistribution study in mice showed high uptake of radioactivity in MAGL-rich organs, including the lungs, heart, and kidneys. More than 90% of the total radioactivity was irreversibly bound in the brain homogenate of rats 5 min and 30 min after the radiotracer injection. PET summation images of rat brains showed high radioactivity in all brain regions. Pretreatment with 6 or MAGL-selective inhibitor JW642 significantly reduced the uptake of radioactivity in the brain. [11C]6 is a promising PET tracer which offers in vivo specific binding and selectivity for MAGL in rodent brains.

Therapeutic Efficacy and Dosimetry of Targeted Alpha Therapy using 225Ac-PSMA-617 in a Murine Model of Prostate Cancer

Targeted alpha therapy shows promise as a treatment in metastatic castration-resistant prostate cancer (mCRPC), owing to the high dose deposition and short range characteristics of alpha particle radiation. Ac-225 decays via multiple alpha particles – each capable of inducing irreparable DNA double-strand breaks in cell nuclei. PSMA-617 is a targeting ligand with specific affinity for prostate-specific membrane antigen (PSMA), a protein that is overexpressed in mCRPC but has low expression in normal tissues. Coupled with the lethal effects of alpha radiation, 225Ac-PSMA-617 is potentially highly cytotoxic to mCRPC while sparing normal tissues. The objective of this study was to evaluate the efficacy of alpha-particle radioligand therapy in a mouse model of mCRPC. Radiation dosimetry analysis was carried out to determine tumor dose as well as dose-limiting organs. NSG mice bearing subcutaneous PSMA-expressing C4-2 tumors were injected with escalating activities of 225Ac-PSMA-617: 20, 40, and 100 kBq/mouse (n=8/group). The tumor volumes were assessed over time by weekly low-dose microCT and compared with an untreated control group. In parallel, twenty-five NSG mice were used in a biodistribution study at five time points – 1, 4, 24, 48, and 168 hours post-administration of 40 kBq of the radiopharmaceutical. After sacrifice, percent tumor and organ radioactivity uptake was measured using gamma spectroscopy. Biexponential curve-fitting was used to fit the time-activity curves for the tumor and each organ, and integrated according to standard medical internal radiation dose methods to estimate the tumor and organ doses. Significant tumor growth retardation was observed in all treatment groups compared with the untreated group. Mice treated with 100 kBq exhibited some weight loss, while the mice treated with lower activities experienced only transient weight loss. There was a significant survival benefit conferred on tumor-bearing mice treated with 225Ac-PSMA617. The biodistribution over five time points showed high uptake and slow tumor activity clearance, and low uptake with fast clearance in non-target organs. The salivary glands, a dose-limiting organ in humans, did not show high uptake in mice, possibly due to lower PSMA expression. Estimates of the absorbed dose to the tumor and organs are reported. An administered activity of 40 kBq per mouse of the peptidomimetic 225AcPSMA-617 is well-tolerated and results in significant tumor growth retardation and improved survival. This work provides a preclinical foundation for further studies towards a more effective treatment option for advanced castration-resistant prostate cancer.

Internal Radiation Dose Assessment in Nuclear Medicine Practices by Using Locally Developed IRDE Software

In nuclear medicine practices, internal radiation dosimetry offers methods for calculation of radiation absorbed dose and risks from radionuclides incorporated inside the body. To manually perform internal radiation dosimetry is time-consuming and errors can occur in each step leading to developing software tools to ease users. There are many software packages available; however, many of them have limited functions. Locally developed IRDE software has been used to calculate the absorbed dose per unit of radioactivity in the target organ. The dose calculation methodology in nuclear medicine practices is described in this study along with a preliminary result on dose calculation for Bangladeshi population due to ingestion of 131I radioisotope in nuclear medicine practices. IRDE is user-friendly, graphic user interface-based software. It can be performed all steps of internal dosimetry within single environment lead to reducing calculation time and reducing possibility of error. IRDE also provides fast and accurate results which may be useful for a routine work in nuclear medicine facilities.&#x0D; Bangladesh J. Nuclear Med. 21(1): 26-30, January 2018