Development of radioiodinated acridine derivatives for in vivo imaging of prion deposits in the brain.

Prion diseases are caused by deposition of abnormal prion protein aggregates (PrPSc) in the central nervous system. This study aimed to develop in vivo imaging probes that can detect cerebral PrPSc deposits. We synthesized several quinacrine-based acridine (AC) derivatives with 2,9-substitution and radioiodinated them. The AC derivatives were evaluated as prion-imaging probes using recombinant mouse prion protein (rMoPrP) aggregates and brain sections of mouse-adapted bovine spongiform encephalopathy (mBSE)-infected mice. The distribution of these compounds in mice was also evaluated. The 2-methoxy derivative [125I]2 exhibited the highest binding affinity for rMoPrP aggregates with an equilibrium dissociation constant (Kd) value of 43.4nM. Fluorescence imaging with 2 showed clear signals at the thioflavin T (ThT)-positive amyloid deposits in the mBSE-infected mouse brain. Although a discrepancy was observed between the in vitro binding of AC derivatives to the aggregates and in vivo distribution of these compounds in the brain and we failed to identify prospective prion-imaging probes in this study, the AC derivatives may be considered a useful scaffold for the development of in vivo imaging probes. Further chemical modification of these AC derivatives may discover clinically applicable prion imaging probes.

Radiation dosimetry and efficacy of an 89Zr/225Ac-labeled humanized anti-MUC5AC antibody.

INTRODUCTION: Theranostic applications are currently difficult to achieve owing to the limited evaluation of suitable chelators for therapeutic nuclides, such as 225Ac and 227Th. With a focus on targeted α therapy and theranostics using human IgG as a drug-delivery system (i.e., combining highly cytotoxic α-particle emitter radiation with efficient tumor targeting), we developed a recombinant humanized Nd2 (hNd2) as an anti-MUC5AC antibody since MUC5AC is highly expressed in patients with pancreatic cancer. Therefore, we aimed to evaluate the performance of 89Zr- (for diagnosis) and 225Ac- (for therapy) labeling of these antibodies using well-controlled radioisotope (RI)-labeling technology in pancreatic cancer mouse models. METHODS: 89Zr-labeled hNd2 (NMK89) and 225Ac-labeled hNd2 (NMT25) were manufactured by chemical conjugation using affinity peptides. A binding assay and the evaluation of plasma stability were performed in vitro to confirm the properties of NMK89 and NMT25. In vivo, we evaluated biodistribution, positron emission tomography (PET)/computed tomography (CT) imaging, antitumor effects, and toxicity. Moreover, the exposure dose in humans was estimated based on the biodistribution evaluation in normal mice. RESULTS: NMK89 and NMT25 showed binding specificity to MUC5AC and stability with radiochemical purity ≥90% in mice and human plasma following incubation for 168 h. NMK89 showed high accumulation in tumors and low non-specific accumulation in normal tissues. The antitumor effect of NMT25 was dose-dependent and significantly suppressed tumor growth in the NMT25 treatment groups compared with the control group (p < 0.05). NMK89 and NMT25 showed similar pharmacokinetics and biodistribution characteristics. Additionally, the human estimated exposure dose of NMK89 and NMT25 was confirmed, and the effective dose of NMK89 and NMT25 was 0.33 mSv/MBq and 177.5 mSv/MBq, respectively. CONCLUSION: NMK89 showed specific accumulation in the MUC5AC-expressing tumors, while NMT25 showed strong antitumor effects. These results suggest NMK89 and NMT25 as promising theranostic agents for pancreatic cancer.

Synthesis and characterization of novel phenylindoles as potential probes for imaging of ß-amyloid plaques in the brain.

We synthesized a novel series of phenylindole (PI) derivatives and evaluated their biological activities as probes for imaging Aß plaques in vivo. The affinity for Aß plaques was assessed by an in vitro-binding assay using pre-formed synthetic Aß aggregates. 2-phenyl-1H-indole (2-PI) derivatives showed high affinity for Aß42 aggregates with K(i) values ranging from 4 to 32 nM. 2-PI derivatives clearly stained Aß plaques in an animal model of AD. In biodistribution experiments using normal mice, 2-PI derivatives displayed sufficient uptake for imaging, ranging from 1.1% to 2.6% ID/g. Although additional modifications are necessary to improve uptake by and clearance from the brain, 2-PI derivatives may be useful as a backbone structure to develop novel Aß imaging agents.

The Thymidine Phosphorylase Imaging Agent 123I-IIMU Predicts the Efficacy of Capecitabine.

UNLABELLED: Recently, companion diagnostics with nuclear medicine techniques have been anticipated as more suitable means than biopsy for predicting treatment efficacy. The anticancer effect of capecitabine, an orally administered chemotherapeutic agent activated by thymidine phosphorylase (TP), is positively associated with tumor TP expression levels. This study aimed to assess whether TP imaging using a radiolabeled uracil derivative, (123)I-5-iodo-6-[(2-iminoimidazolidinyl)methyl]uracil ((123)I-IIMU), could predict the efficacy of capecitabine treatment. METHODS: Sensitivity to doxifluridine, a metabolite of capecitabine and direct substrate for TP, was assessed by water-soluble tetrazolium salt assays in vitro for 3 human colon cancer cell lines with different TP expression profiles. The intracellular uptake and retention of (123)I-IIMU were evaluated. Mice inoculated with each cell line were treated with capecitabine for 2 wk, and tumor growth was compared. In vivo distribution studies and SPECT/CT imaging of (123)I-IIMU were performed in inoculated mice. RESULTS: In vitro experiments showed a positive relation between TP expression levels and doxifluridine sensitivity. In vitro studies revealed that intracellular uptake and retention of (123)I-IIMU were dependent on TP expression levels. In vivo experiments in inoculated mice showed that (123)I-IIMU accumulation in tumor tissue was in line with TP expression levels and susceptibility to capecitabine treatment. Moreover, SPECT/CT imaging of (123)I-IIMU in tumor-inoculated mice showed that (123)I-IIMU reflects TP expression levels in tumor tissues. CONCLUSION: (123)I-IIMU could be used as an in vivo companion diagnostic for predicting the efficacy of capecitabine treatment.

A Novel CYP11B2-Specific Imaging Agent for Detection of Unilateral Subtypes of Primary Aldosteronism.

CONTEXT: Although adrenal vein sampling is the standard method to distinguish unilateral from bilateral forms of primary aldosteronism, it is an invasive and technically difficult procedure. (11)C-metomidate (MTO)-positron emission tomography was reported as a potential replacement for adrenal vein sampling. However, MTO has low selectivity for CYP11B2 over CYP11B1. OBJECTIVE: This study aimed to determine the selectivity of (18)F-CDP2230, a new imaging agent, for CYP11B2 over CYP11B1 and determine whether the biodistribution profile of (18)F-CDP2230 is favorable for imaging CYP11B2. METHODS: The IC50 of CDP2230 for the enzymatic activities of CYP11B2 and CYP11B1 was determined using cells with stable expression of either enzyme. In vitro autoradiography of human adrenal sections with aldosterone-producing adenomas was performed to confirm the specific binding ability of (18)F-CDP2230 to CYP11B2-expressing regions. Furthermore, positron emission tomography and magnetic resonance imaging were performed to evaluate the biodistribution of (18)F-CDP2230 in rats. RESULTS: Although CDP2230 showed a significantly lower affinity for CYP11B2 and CYP11B1 than did MTO analogues, its selectivity for CYP11B2 over CYP11B1 was higher than that of MTO analogues. In vitro autoradiography revealed that the binding of (18)F-CDP2230 to CYP11B2-expressing regions in the adrenal gland was more specific than that of (123)I-IMTO. Moreover, the biodistribution study showed that (18)F-CDP2230 accumulated in adrenal glands with low background uptake. CONCLUSIONS: Our study showed a high selectivity of (18)F-CDP2230 for CYP11B2 over CYP11B1 with a favorable biodistribution for imaging CYP11B2. (18)F-CDP2230 is a promising imaging agent for detecting unilateral subtypes of primary aldosteronism.

Synthesis and biological evaluation of radioiodinated quinacrine-based derivatives for SPECT imaging of Aß plaques.

The aim of the present study was to characterize the binding property of quinacrine-based acridine derivatives for Aß plaques and to evaluate this series of compounds as Aß imaging probes. Quinacrine clearly stained Aß plaques in the brain sections of Aß deposition model transgenic mice (Tg2576 mice). Similarly, the quinacrine analog, 2-methoxy-9-(4-(dimethyl-1-methyl) -N-butyl) amino-6-iodo acridine (5), labeled Aß plaques in the brain slices of Tg2576 mice. In addition, [(125)I]5 showed modest affinity for Aß(1-42) aggregates with a K(d) value of 48 nM. Biodistribution studies using normal mice demonstrated that [(125)I]5 displayed poor initial brain uptake. Next, (125)I-labeled acridines without aliphatic amino groups were synthesized and characterized. Similar to quinacrine and 5, these compounds could detect Aß plaques in the brain sections of Tg2576 mice. It should be noted that the acridines showed much higher binding affinity for Aß aggregates and greater in vivo blood brain barrier permeability than [(125)I]5. Among them, 13 (6-Iodo-2-methoxy-9-methylaminoacridine) and 25 (2,9-Dimethoxy-6-iodo acridine) exhibited high affinity for the Aß aggregates with K(i) values of 14 and 29 nM, respectively. In the in vivo studies, [(125)I]13 and [(125)I]25 showed excellent initial brain uptake (3.0 and 4.4% dose/g, respectively, at 2 min) with fast washout from the brain (0.33 and 0.37% dose/g, respectively, at 60 min). These acridine derivatives are demonstrated to be promising SPECT imaging probes for amyloid in the living brain.

Novel Benzofurans with (99m)Tc Complexes as Probes for Imaging Cerebral ß-Amyloid Plaques.

Two novel benzofuran derivatives coupled with (99m)Tc complexes were tested as probes for imaging cerebral ß-amyloid plaques using single photon emission tomography. Although both derivatives bound to Aß(1-42) aggregates, (99m)Tc-BAT-BF showed higher affinity than (99m)Tc-MAMA-BF. In sections of brain tissue from an animal model of AD, (99m)Tc-BAT-BF clearly labeled ß-amyloid plaques. In biodistribution experiments using normal mice, (99m)Tc-BAT-BF displayed high uptake soon after its injection and washed out from the brain rapidly, a highly desirable feature for an imaging agent. (99m)Tc-BAT-BF may be a potential probe for imaging ß-amyloid plaques in Alzheimer's brains.