Development of Radiohalogenated Osimertinib Derivatives as Imaging Probes for Companion Diagnostics of Osimertinib.

Osimertinib is an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor approved for treating non-small-cell lung cancer (NSCLC) with EGFR mutations. Genetic testing is required to detect the mutation for selecting patients who can use osimertinib. Here, we report an attempt to develop nuclear imaging probes that detect the EGFR mutations. We designed and synthesized I-osimertinib and Br-osimertinib with a radioactive or nonradioactive halogen atom at an indole ring in osimertinib and evaluated them. In vitro assays suggested that both I-osimertinib and Br-osimertinib exhibit a specifically high activity toward NSCLC with EGFR L858R/T790M mutations. In biodistribution experiments, the accumulation of both [125I]I-osimertinib and [77Br]Br-osimertinib in tumors with mutations was significantly higher than that in blood and muscle. However, these osimertinib derivatives showed a significantly higher accumulation in lungs than in tumors. Therefore, for detecting the mutations in lung cancer, further structural modifications of the probes are required.

Improved production of 76Br, 77Br and 80mBr via CoSe cyclotron targets and vertical dry distillation.

INTRODUCTION: The radioisotopes of bromine are uniquely suitable radiolabels for small molecule theranostic radiopharmaceuticals but are of limited availability due to production challenges. Significantly improved methods were developed for the production and radiochemical isolation of clinical quality 76Br, 77Br, and 80mBr. The radiochemical quality of the radiobromine produced using these methods was tested through the synthesis of a novel 77Br-labeled inhibitor of poly (ADP-ribose) polymerase-1 (PARP-1), a DNA damage response protein. METHODS: 76Br, 77Br, and 80mBr were produced in high radionuclidic purity via the proton irradiation of novel isotopically-enriched Co76Se, Co77Se, and Co80Se intermetallic targets, respectively. Radiobromine was isolated through thermal chromatographic distillation in a vertical furnace assembly. The 77Br-labeled PARP inhibitor was synthesized via copper-mediated aryl boronic ester radiobromination. RESULTS: Cyclotron production yields were 103 ±â€¯10 MBq∙µA-1∙h-1 for 76Br, 88 ±â€¯10 MBq∙µA-1∙h-1 for 80mBr at 16 MeV and 17 ±â€¯1 MBq∙µA-1∙h-1 for 77Br at 13 MeV. Radiobromide isolation yields were 76 ±â€¯11% in a small volume of aqueous solution. The synthesized 77Br-labeled PARP-1 inhibitor had a measured apparent molar activity up to 700 GBq/µmol at end of synthesis. CONCLUSIONS: A novel selenium alloy target enabled clinical-scale production of 76Br, 77Br, and 80mBr with high apparent molar activities, which was used to for the production of a new 77Br-labeled inhibitor of PARP-1. ADVANCES IN KNOWLEDGE: New methods for the cyclotron production and isolation of radiobromine improved the production capacity of 77Br by a factor of three and 76Br by a factor of six compared with previous methods. IMPLICATIONS FOR PATIENT CARE: Preclinical translational research of 77Br-based Auger electron radiotherapeutics, such as those targeting PARP-1, will require the production of GBq-scale 77Br, which necessitates next-generation, high-yielding, isotopically-enriched cyclotron targets, such as the novel intermetallic Co77Se.

Radiobrominated benzimidazole-quinoline derivatives as Platelet-derived growth factor receptor beta (PDGFRß) imaging probes.

Platelet-derived growth factor receptor beta (PDGFRß) affects in numerous human cancers and has been recognized as a promising molecular target for cancer therapies. The overexpression of PDGFRß could be a biomarker for cancer diagnosis. Radiolabeled ligands having high affinity for the molecular target could be useful tools for the imaging of overexpressed receptors in tumors. In this study, we aimed to develop radiobrominated PDGFRß ligands and evaluate their effectiveness as PDGFRß imaging probes. The radiolabeled ligands were designed by modification of 1-{2-[5-(2-methoxyethoxy)-1H- benzo[d]imidazol-1-yl]quinolin-8-yl}piperidin-4-amine (1), which shows selective inhibition profile toward PDGFRß. The bromine atom was introduced directly into C-5 of the quinoline group of 1, or indirectly by the conjugation of 1 with the 3-bromo benzoyl group. [77Br]1-{5-Bromo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinoline-8-yl}piperidin-4-amine ([77Br]2) and [77Br]-N-3-bromobenzoyl-1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}-piperidin-4-amine ([77Br]3) were prepared using a bromodestannylation reaction. In a cellular uptake study, [77Br]2 and [77Br]3 more highly accumulatd in BxPC3-luc cells (PDGFRß-positive) than in MCF7 cells (PDGFRß-negative), and their accumulation was significantly reduced by pretreatment with inhibitors. In biodistribution experiments, [77Br]2 accumulation was higher than [77Br]3 accumulation at 1 h postinjection. These findings suggest that [76Br]2 is more promising for positron emission tomography (PET) imaging of PDGFRß than [76Br]3.

Comparison of Radioiodine- or Radiobromine-Labeled RGD Peptides between Direct and Indirect Labeling Methods.

Radiolabeled cyclic peptides containing the (Arg-Gly-Asp) RGD sequence for use in positron emission tomography (PET) imaging, single-photon emission computed tomography (SPECT) imaging, and targeted radionuclide therapy of cancer have been reported. In this study, RGD was used as a model carrier peptide for diagnosis and therapy of cancer. To evaluate the characteristics of radiohalogen-labeled peptides, several kinds of labeled RGD peptides [125I-c(RGDyK), 77Br-c(RGDyK), [125I]SIB-c(RGDfK), [77Br]SBrB-c(RGDfK), [125I]SIB-EG2-c(RGDfK), and [77Br]SBrB-EG2-c(RGDfK)] were designed, prepared, and evaluated. In these initial studies, 77Br (t1/2=57.0 h) and 125I (t1/2=59.4 d) were used because of their longer half-lives. Precursor peptides were synthesized using a standard 9-fluorenylmethyloxycarbonyl (Fmoc)-based solid-phase methodology. Radiolabeled peptides were prepared by chloramine-T method or conjugation of RGD peptides with [125I]N-succinimidyl 3-iodobenzoate ([125I]SIB) or [77Br]N-succinimidyl 3-bromobenzoate ([77Br]SBrB). Measurement of the partition coefficients, integrin binding assay, and biodistribution experiments in tumor-bearing mice were performed. 125I and 77Br labeling were successfully performed using similar methods, and in vitro characteristics and biodistributions were similar between the 125I-labeled and corresponding 77Br-labeled peptides. [125I]SIB- and [77Br]SBrB-conjugated RGD peptides showed higher partition coefficients, lower tumor uptakes, and higher intestinal uptake than 125I-c(RGDyK) and 77Br-c(RGDyK). [125I]SIB-EG2-c(RGDfK) and [77Br]SBrB-EG2-c(RGDfK), which possess an ethylene glycol linker, decreased lipophilicity and uptake in intestine compared with [125I]SIB-c(RGDfK) and [77Br]SBrB-c(RGDfK), which possess no linker. However, the improvement in biodistribution of [125I]SIB-EG2-c(RGDfK) and [77Br]SBrB-EG2-c(RGDfK)] was insufficient. In conclusion, directly radiohalogenated c(RGDyK) peptides are potentially more useful for tumor imaging and therapy than indirectly radiohalogenated ones.

Evaluation of aromatic radiobromination by nucleophilic substitution using diaryliodonium salt precursors.

Radiobromine-labeled compounds can be used for positron emission tomography (PET) imaging (ie, 76 Br) and for radiation therapy (ie, 77 Br). However, the commonly used electrophilic substitution reaction using no-carrier-added radiobromide does not always afford the desired product due to the high reactivity of the brominating intermediate. A nucleophilic substitution by bromide, such as radiobromination of iodonium precursors, provides an alternative route for the synthesis of bromo-radiopharmaceuticals. The applicability of aromatic radiobromination by nucleophilic substitution using diaryliodonium salt precursors was evaluated using iodonium model compounds and [76 Br]/[77 Br]bromide. Radiobromination was observed under all conditions tested, in up to quantitative yields. A QMA cartridge treatment method and a base-free method have been developed, and no extra base is needed for either methods. The base-free conditions are mild and afford much cleaner reactions. Up to 20% water is tolerated in the reactions without reducing the radiochemical yields. No-carrier-added and carrier-added reactions afforded similar results. 4-Bromobenzaldehyde and 4-bromobenzoate have been radiosynthesized reliably and in good yields. These results indicate that this method is robust and efficient and thus will provide a route for radiobromination of electron-deficient arenes and an alternative route for the synthesis of bromo-radiopharmaceuticals for biological evaluations.

Synthesis and Biological Evaluation of (S)-Amino-2-methyl-4-[(76)Br]bromo-3-(E)-butenoic Acid (BrVAIB) for Brain Tumor Imaging.

The novel compound, (S)-amino-2-methyl-4-[(76)Br]bromo-3-(E)-butenoic acid (BrVAIB, [(76)Br]5), was characterized against the known system A tracer, IVAIB ([(123)I]8). [(76)Br]5 was prepared in a 51% ± 19% radiochemical yield with high radiochemical purity (≥98%). The biological properties of [(76)Br]5 were compared with those of [(123)I]8. Results showed that [(76)Br]5 undergoes mixed amino acid transport by system A and system L transport, while [(123)I]8 had less uptake by system L. [(76)Br]5 demonstrated higher uptake than [(123)I]8 in DBT tumors 1 h after injection (3.7 ± 0.4% ID/g vs 1.5 ± 0.3% ID/g) and also showed higher uptake vs [(123)I]8 in normal brain. Small animal PET studies with [(76)Br]5 demonstrated good tumor visualization of intracranial DBTs up to 24 h with clearance from normal tissues. These results indicate that [(76)Br]5 is a promising PET tracer for brain tumor imaging and lead compound for a mixed system A and system L transport substrate.

Evaluation of excitation functions of proton, ³He- and α-particle induced reactions for production of the medically interesting positron-emitter bromine-76.

Cross section data for production of the medically interesting radionuclide 76Br (T(½)=16.2 h) via the proton induced reactions on 76Se, 77Se, 78Se and 79Br, and ³He- and α-particle induced reactions on 75As were evaluated. The nuclear model codes STAPRE, EMPIRE and TALYS were used to check the consistency in the experimental data and a statistical procedure was applied to derive the recommended excitation functions. A comparison of various production routes of 76Br (and of 75Br) is presented.

Bromination from the macroscopic level to the tracer radiochemical level: (76)Br radiolabeling of aromatic compounds via electrophilic substitution.

No-carrier-added (NCA) (76)Br labeling of 4-(5-acetoxy-7-bromobenzoxazol-2-yl)phenyl acetate, a diacetate-protected estrogen-receptor beta (ERbeta) selective ligand, was carried out successfully using [(76)Br]bromide ion. The labeling was achieved via oxidative electrophilic destannylation of an organotin precursor molecule by modification of the leaving group (from Bu(3)Sn to Me(3)Sn) and the addition of methanol to the reaction mixture. The differences between the oxidative bromination reaction under small-scale macroscopic vs tracer level radiochemical conditions were explored in terms of effective brominating agents, which depend greatly on the nature of the solvent during the radiochemical bromination, and the potential interference by trace levels of highly reactive impurities in the reaction that compete for the desired bromination at the NCA level. Our observations, and our development of experimental protocols for successful radiobromination at the tracer NCA-scale, should be applicable to the synthesis of other radiobromine-labeled organic compounds of potential interest as PET radiopharmaceuticals and radiotherapy agents.

Synthesis and characterization of [76Br]-labeled high-affinity A3 adenosine receptor ligands for positron emission tomography.

INTRODUCTION: Bromine-76-radiolabeled analogues of previously reported high-affinity A(3) adenosine receptor (A(3)AR) nucleoside ligands have been prepared as potential radiotracers for positron emission tomography. METHODS: The radiosyntheses were accomplished by oxidative radiobromination on the N(6)-benzyl moiety of trimethyltin precursors. Biodistribution studies of the kinetics of uptake were conducted in awake rats. RESULTS: We prepared an agonist ligand {[(76)Br](1'S,2'R,3'S,4'R,5'S)-4'-{2-chloro-6-[(3-bromophenylmethyl)amino]purin-9-yl}-1'-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2',3'-diol (MRS3581)} in 59% radiochemical yield with a specific activity of 19.5 GBq/micromol and an antagonist ligand {[(76)Br](1'R,2'R,3'S,4'R,5'S)-4'-(6-(3-bromobenzylamino)-2-chloro-9H-purin-9-yl)bicyclo[3.1.0]hexane-2',3'-diol (MRS5147)} in 65% radiochemical yield with a specific activity of 22 GBq/micromol. The resultant products exhibited the expected high affinity (K(i) approximately 0.6 nM) and specific binding at the human A(3)AR in vitro. Biodistribution studies in the rat showed uptake in the organs of excretion and metabolism. The antagonist MRS5147 exhibited increasing uptake in testes, an organ that contains significant quantities of A(3)AR, over a 2-h time course, which suggests the presence of a specific A(3)AR retention mechanism. CONCLUSION: We were able to compare uptake of the [(76)Br]-labeled antagonist MRS5147 to [(76)Br]agonist MRS3581. The antagonist MRS5147 shows increasing uptake in the testes, an A(3)AR-rich tissue, suggesting that this ligand may have promise as a molecular imaging agent.

Synthesis, in vitro and in vivo evaluation of radiolabeled nanoparticles.

Recent advances in the field of nanomedicine offer the promise of better diagnostic and therapeutic options. Synthetic chemists are making strides in developing nanoparticle constructs that can be used as platforms for attaching different functionalities for the purposes of molecular imaging and targeted drug delivery. As new nanoparticles are developed, it is imperative to evaluate their biological effectiveness by in vitro and in vivo screening techniques. While the in vitro results give insight into the cellular structure and function at sub-cellular level, the in vivo and ex vivo data give vital information about the pharmacokinetics of these novel particles, along with their ability to reach the desired target. This three-way information is pertinent to developing effective drugs and imaging probes for targeting key cancer/inflammation biomarkers such as the alphavbeta3 integrin. Labeling nanoparticles with positron emitting radionuclides can speed up this evaluation. In fact, small animal Positron Emission Tomography (PET) scanners allow researchers to quantitatively image the uptake of candidate nanocarriers at the target site with high sensitivity. In addition to conventional ex vivo biodistribution techniques, the pharmacokinetic profile of new nanomaterials with potential medical application can be obtained with dynamic and/or longitudinal PET studies on a relatively small number of laboratory animals. This article will focus on some of the approaches to label nanoparticles with positron emitting radionuclides along with in vitro and in vivo protocols that have been optimized and are being used for evaluating nanoparticles.

Synthesis and evaluation of a bromine-76-labeled PPARgamma antagonist 2-bromo-5-nitro-N-phenylbenzamide.

Peroxisome proliferator activated-receptor gamma (PPARgamma) binds to peroxisome receptor response elements with its heterodimeric partner, retinoid X receptor, and regulates downstream gene expression. PPARgamma transcriptionally modulates fat metabolism, and receptor agonists have been developed to treat type II diabetes. PPARgamma is also overexpressed in some tumor cell lines and primary tumors, including breast and prostate tumors. Two PPARgamma antagonists, 2-chloro-5-nitro-N-phenylbenzamide (GW9662) and 2-chloro-5-nitro-N-pyridin-4-yl-benzamide (T0070907), represent good lead compounds for radiotracer development. In the current study, four additional halogen substituted analogs were synthesized and evaluated in a whole cell screening assay for PPARgamma binding activity. Two bromine-containing analogs having EC50 values <5 nM were chosen for bromine-76 radiolabeling. Bromine-76-labeled 2-bromo-5-nitro-N-phenyl-benzamide was selected for subsequent in vitro and in vivo studies due to its superior radiolabeling yield (approximately 70%) and the well-characterized pharmacological properties of its analog GW9662. An in vitro stability study showed that 40% of the compound remained intact in plasma and about 25% in whole blood after 30 min. Biodistribution studies in MDA-MB-435 human breast tumor-bearing nude mice were carried out at 5 min, 30 min, 2 h and 24 h post injection of the radiotracer. Although in vivo metabolite studies demonstrated rapid compound degradation, at least 10% of the parent compound was delivered to the tumor. We are currently exploring second generation analogs of these lead compounds for the development of radiolabeled antagonists of the PPARgamma receptor.

Synthesis and in vivo evaluation of 2 high-affinity 76Br-labeled sigma2-receptor ligands.

UNLABELLED: The sigma(2)-receptor has been shown to be upregulated in proliferating tumors cells. The purpose of this study was to compare 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) and 2 new (76)Br-radiolabeled compounds that have a high affinity and selectivity for the sigma(2)-receptor. These are 5-bromo-N-(4-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)butyl)-2,3-dimethoxybenzamide (compound (1)) and 5-bromo-N-(2-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)ethyl)-2-methoxybenzamide (compound (2)). METHODS: Two sigma(2)-receptor-binding ligands were prepared, from the corresponding tributylstannyl precursors using standard electrophilic chemistry, (76)Br-compound (1) ((76)Br-1) and (76)Br-compound (2) ((76)Br-2). (18)F-FLT, (76)Br-1, and (76)Br-2 were compared using allograft tumors of the EMT-6 cell line (mouse mammary adenocarcinoma) in biodistribution studies at 5 min, 0.5, 1, and 2 h. Imaging of (76)Br-1 and (18)F-FLT was also performed at 2 and 1 h, respectively. RESULTS: (76)Br-1 and (76)Br-2 were synthesized with yields between 50% and 70% with high specific activity. Both compounds showed uptake into the tumor with tumor-to-normal tissue ratios of (76)Br-1 being greater than both (76)Br-2 and (18)F-FLT. Except for the liver and kidney, all ratios were greater than 1 and uptake into the tumor was shown with microPET imaging for (76)Br-1. CONCLUSION: We were able to synthesize two (76)Br-radiolabeled compounds with a high yield and specific activity that target the sigma(2) receptor with high affinity and selectivity. The studies presented show that both of the flexible benzamide compounds can identify EMT-6 breast tumors in vivo. (76)Br-1 also has higher tumor-to-normal tissue ratios when compared with (76)Br-2 and (18)F-FLT. The high affinity and low nonspecific binding of (76)Br-1 indicates that it can be a potential PET radiotracer for imaging solid tumors.

Cellular processing in the SW1222 cell line of mAb A33 directly and indirectly radiohalogenated.

Investigations into the cellular processing of radiolabeled monoclonal antibodies (mAbs) for their further use in radioimmunodiagnosis and cancer therapy are needed in order to understand the fate of internalized and catabolized mAbs. The anti-colorectal cancer mAb, A33, was labelled with 76Br and 125I using the direct Chloramine-T method, or by labelling N-succinimidyl para-(tri-methylstannyl) benzoate and its further conjugation to the mAb. The cellular processing of the four conjugates was investigated in SW1222 cells in vitro. Uptake of mAb was rapid, peaking after 14-16 h. Intracellular degradation was slow and the early loss of radioactivity was due to dissociation of cell-surface bound mAb. The indirect labelling resulted in stronger binding of the mAb as well as prolonged intracellular retention of the radiolabel. Direct and indirect halogen radiolabelling results in different cell-processing patterns of radiolabels, and radioactive catabolic products follow different routes of cellular excretion. The results of this cellular study indicate that indirect labelling is preferable to the direct Chloramine-T method.

Evaluation of ((4-hydroxyphenyl)ethyl)maleimide for site-specific radiobromination of anti-HER2 affibody.

Affibody molecules are a new class of small phage-display selected proteins using a scaffold domain of the bacterial receptor protein A. They can be selected for specific binding to a large variety of protein targets. An affibody molecule binding with high affinity to a tumor antigen HER2 was recently developed for radionuclide diagnostics and therapy in vivo. The use of the positron-emitting nuclide (76)Br (T(1/2) = 16.2 h) could improve the sensitivity of detection of HER2-expressing tumors. A site-specific radiobromination of a cysteine-containing variant of the anti-HER2 affibody, (Z(HER2:4))(2)-Cys, using ((4-hydroxyphenyl)ethyl)maleimide (HPEM), was evaluated in this study. It was found that HPEM can be radiobrominated with an efficiency of 83 +/- 0.4% and thereafter coupled to freshly reduced affibody with a yield of 65.3 +/- 3.9%. A "one-pot" labeling enabled the radiochemical purity of the conjugate to exceed 97%. The label was stable against challenge with large excess of nonlabeled bromide and in a high molar strength solution. In vitro cell tests demonstrated that radiobrominated affibody binds specifically to the HER2-expressing cell-line, SK-OV-3. Biodistribution studies in nude mice bearing SK-OV-3 xenografts have shown tumor accumulation of 4.8 +/- 2.2% IA/g and good tumor-to-normal tissue ratios.

Radiobromination of humanized anti-HER2 monoclonal antibody trastuzumab using N-succinimidyl 5-bromo-3-pyridinecarboxylate, a potential label for immunoPET.

Combining the specificity of radioimmunoscintigraphy and the high sensitivity of PET in an in vivo detection technique could improve the quality of nuclear diagnostics. Positron-emitting nuclide (76)Br (T(1/2)=16.2 h) might be a possible candidate for labeling monoclonal antibodies (mAbs) and their fragments, provided that the appropriate labeling chemistry has been established. For internalizing antibodies, such as the humanized anti-HER2 monoclonal antibody, trastuzumab, radiobromine label should be residualizing, i.e., ensuring that radiocatabolites are trapped intracellularly after the proteolytic degradation of antibody. This study evaluated the chemistry of indirect radiobromination of trastuzumab using N-succinimidyl 5-(tributylstannyl)-3-pyridinecarboxylate. Literature data indicated that the use of this method provided residualizing properties for iodine and astatine labels on some antibodies. An optimized "one-pot" procedure produced an overall labeling efficiency of 45.5+/-1.2% over 15 min. The bromine label was stable under physiological and denaturing conditions. The labeled trastuzumab retained its capacity to bind specifically to HER2-expressing SKOV-3 ovarian carcinoma cells in vitro (immunoreactivity more than 75%). However, in vitro cell test did not demonstrate that the radiobromination of trastuzumab using N-succinimidyl 5-bromo-3-pyridinecarboxylate improves cellular retention of radioactivity in comparison with the use of N-succinimidyl 4-bromobenzoate.

Radiobromination of anti-HER2/neu/ErbB-2 monoclonal antibody using the p-isothiocyanatobenzene derivative of the [76Br]undecahydro-bromo-7,8-dicarba-nido-undecaborate(1-) ion.

The monoclonal humanized anti-HER2 antibody trastuzumab was radiolabeled with the positron emitter (76)Br (T(1/2) =16.2 h). Indirect labeling was performed using the p-isothiocyanatobenzene derivative of the [(76)Br]undecahydro-bromo-7,8-dicarba-nido-undecaborate(1-) ((76)Br-NBI) as a precursor molecule. (76)Br-NBI was prepared by bromination of the 7-(p-isothiocyanato-phenyl)dodecahydro-7,8-dicarba-nido-undecaborate(1-) ion (NBI) with a yield of 93-95% using Chloramine-T (CAT) as an oxidant. Coupling of radiobrominated NBI to antibody was performed without intermediate purification, in an "one pot" reaction. An overall labeling yield of 55.7 +/- 4.8% (mean +/- maximum error) was achieved when 300 microg of antibody was labeled. The label was stable in vitro in physiological and denaturing conditions. In a cell binding test, trastuzumab remained immunoreactive after labeling.

Fluoro-, bromo-, and iodopaclitaxel derivatives: synthesis and biological evaluation.

Paclitaxel (Taxol) is a clinically important chemotherapeutic agent. We describe the synthesis of fluoro-, bromo-, and iodopaclitaxel and their [(18)F]fluoro-, [(76)Br]bromo-, and [(124)I]iodo- analogues. [(18)F]Fluoropaclitaxel shows high uptake and rapid clearance from tissues in rats. Preadministration of paclitaxel in normal rats significantly increases (p < 0.005) retention of [(18)F]fluoropaclitaxel and [(76)Br]bromopaclitaxel in blood (33.0%), heart (32.0%), lung (37.6%) kidney (142.4%); and blood (33.4%), lung (42.3%), kidney (62.4%), respectively. [(18)F]Fluoropaclitaxel uptake in the brain of mdr1a/1b(-/-) mice is increased 1400% (p < 1.3e-07) relative to wild-type controls. Preadministration of paclitaxel or XR9576, a modulator, had little effect on the biodistribution in these mdr1a/1b(-/-) mice. As a result, [(18)F]fluoropaclitaxel will be a useful radiopharmaceutical for the study of multidrug resistant tumors.

Preparation and pharmacological characterization of [76Br]-meta-bromobenzylguanidine ([76Br]MBBG).

[76Br]-meta-Bromobenzylguanidine ([76Br]MBBG) was prepared from the iodinated analog (MIBG) and [76Br]NH4 using a Cu(+)-assisted halogen exchange reaction. [76Br]MBBG was produced in a 60-65% radiochemical yield with a specific activity of 20 MBq/nmol. In rats, biodistribution kinetic studies showed a high uptake of [76Br]MBBG in heart tissues with its maximum of 5% ID/S at 2 h p.i.; whereas 4 h p.i., the maximum of the heart-to-lung concentration ratio of 8 was observed. Metabolic studies in rats indicated that [76Br]MBBG was rapidly metabolized in plasma. However in heart tissue, 25 h p.i., 85% of the radioactivity still represented unchanged radiotracer. Pharmacological studies in rats showed that the myocardial uptake of [76Br]MBBG was similar to that of norepinephrine. After pretreatment of the rats, the uptake of [76Br]MBBG was reduced 4 h p.i. to the following values: after desipramine (DMI) to 37%, after dexamethasone (DXM) to 88% and after 6-hydroxydopamine (6-OHDA) to 16%. These preliminary results suggest that [76Br]MBBG can be useful for the assessment of heart catecholamine reuptake disorders with PET.