Myeloperoxidase PET Imaging Tracks Intracellular and Extracellular Treatment Changes in Experimental Myocardial Infarction.

Myeloperoxidase (MPO) is a highly oxidative, pro-inflammatory enzyme involved in post-myocardial infarction (MI) injury and is a potential therapeutic target. While multiple MPO inhibitors have been developed, the lack of an imaging reporter to select appropriate patients and assess therapeutic efficacy has hampered clinical development. Thus, a translational imaging method to detect MPO activity non-invasively would help to better understand the role MPO plays in MI and facilitate novel therapy development and clinical validation. Interestingly, many MPO inhibitors affect both intracellular and extracellular MPO, but previous MPO imaging methods can only report extracellular MPO activity. In this study, we found that an MPO-specific PET imaging agent (18F-MAPP) can cross cell membranes to report intracellular MPO activity. We showed that 18F-MAPP can track the treatment effect of an MPO inhibitor (PF-2999) at different doses in experimental MI. The imaging results were corroborated by ex vivo autoradiography and gamma counting data. Furthermore, extracellular and intracellular MPO activity assays revealed that 18F-MAPP imaging can report the changes induced by PF-2999 on both intracellular and extracellular MPO activities. These findings support 18F-MAPP as a translational candidate to noninvasively report MPO activity and accelerate drug development against MPO and other related inflammatory targets.

Characterization of a Novel M4 PAM PET Radioligand [11C]PF06885190 in Nonhuman Primates (NHP).

Muscarinic acetylcholine receptors (mAChR), including M4, draw attention as therapeutic targets for several neurodegenerative diseases including Alzheimer's disease (AD). PET imaging of M4 positive allosteric modulator (PAM) allows qualification of the distribution as well as the expression of this receptor under physiological conditions and thereby helps to assess the receptor occupancy (RO) of a drug candidate. In this study, our aims were (a) to synthesize a novel M4 PAM PET radioligand [11C]PF06885190 (b) to evaluate the brain distribution of [11C]PF06885190 in nonhuman primates (NHP) and (c) to analyze its radiometabolites in the blood plasma of NHP. Radiolabeling of [11C]PF06885190 was accomplished via N-methylation of the precursor. Six PET measurements were performed using two male cynomolgus monkeys, where three PET measurements were at baseline, two after pretreatment with a selective M4 PAM compound CVL-231 and one after pretreatment with donepezil. The total volume of distribution (VT) of [11C]PF06885190 was examined using Logan graphical analysis with arterial input function. Radiometabolites were analyzed in monkey blood plasma using gradient HPLC system. Radiolabeling of [11C]PF06885190 was successfully accomplished and the radioligand was found to be stable in the formulation, with radiochemical purity exceeding 99% 1 h after the end of the synthesis. [11C]PF06885190 was characterized in the cynomolgus monkey brain where a moderate brain uptake was found at the baseline condition. However, it showed fast wash-out as it dropped to half of the peak at around 10 min. Change of VT from baseline was around -10% after pretreatment with a M4 PAM, CVL-231. Radiometabolite studies showed relatively fast metabolism. Although sufficient brain uptake of [11C]PF06885190 was observed, these data suggest that [11C]PF06885190 might have too low specific binding in the NHP brain to be further applied in PET imaging.

An activatable PET imaging radioprobe is a dynamic reporter of myeloperoxidase activity in vivo.

Myeloperoxidase (MPO) is a critical proinflammatory enzyme implicated in cardiovascular, neurological, and rheumatological diseases. Emerging therapies targeting inflammation have raised interest in tracking MPO activity in patients. We describe 18F-MAPP, an activatable MPO activity radioprobe for positron emission tomography (PET) imaging. The activated radioprobe binds to proteins and accumulates at sites of MPO activity. The radioprobe 18F-MAPP has a short blood half-life, remains stable in plasma, does not demonstrate cytotoxicity, and crosses the intact blood-brain barrier. The 18F-MAPP imaging detected sites of elevated MPO activity in living mice embedded with human MPO and in mice induced with chemical inflammation or myocardial infarction. The 18F-MAPP PET imaging noninvasively differentiated varying amounts of MPO activity, competitive inhibition, and MPO deficiency in living animals, confirming specificity and showing that the radioprobe can quantify changes in in vivo MPO activity. The radiosynthesis has been optimized and automated, an important step in translation. These data indicate that 18F-MAPP is a promising translational candidate to noninvasively monitor MPO activity and inflammation in patients.

Brain Penetration of the ROS1/ALK Inhibitor Lorlatinib Confirmed by PET.

The Massachusetts General Hospital Radiochemistry Program, in collaboration with Pfizer, has developed unique 11C and 18F-labeling strategies to synthesize isotopologs of lorlatinib (PF-06463922) which is undergoing phase III clinical trial investigations for treatment of non-small-cell lung cancers with specific molecular alterations. A major goal in cancer therapeutics is to measure the concentrations of this drug in the brain metastases of patients with lung cancer, and penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Our recent publication in Nature Communications employed radiolabeled lorlatinib and positron emission tomography (PET) studies in preclinical models including nonhuman primates (NHPs) that demonstrated high brain permeability of this compound. Our future work with radiolabeled lorlatinib will include advanced PET evaluations in rodent tumor models and normal NHPs with the goal of clinical translation.

Liver-Targeted Small-Molecule Inhibitors of Proprotein Convertase Subtilisin/Kexin Type 9 Synthesis.

Targeting of the human ribosome is an unprecedented therapeutic modality with a genome-wide selectivity challenge. A liver-targeted drug candidate is described that inhibits ribosomal synthesis of PCSK9, a lipid regulator considered undruggable by small molecules. Key to the concept was the identification of pharmacologically active zwitterions designed to be retained in the liver. Oral delivery of the poorly permeable zwitterions was achieved by prodrugs susceptible to cleavage by carboxylesterase 1. The synthesis of select tetrazole prodrugs was crucial. A cell-free in vitro translation assay containing human cell lysate and purified target mRNA fused to a reporter was used to identify active zwitterions. In vivo PCSK9 lowering by oral dosing of the candidate prodrug and quantification of the drug fraction delivered to the liver utilizing an oral positron emission tomography 18 F-isotopologue validated our liver-targeting approach.

Synthesis and SAR of 99mTc/Re-labeled small molecule prostate specific membrane antigen inhibitors with novel polar chelates.

Prostate specific membrane antigen (PSMA) is recognized as an attractive molecular target for the development of radiopharmaceuticals to image and potentially treat metastatic prostate cancer. A series of novel (99m)Tc/Re-tricarbonyl radiolabeled PSMA inhibitors were therefore synthesized by the attachment of glutamate-urea-lysine (Glu-urea-Lys) and glutamate-urea-glutamate (Glu-urea-Glu) pharmacophore to single amino acid chelate (SAAC) where the SAAC ligand was either bis(pyridin-2-ylmethyl)amino (DPA), bis((1-methyl-1H-imidazol-2-yl)methyl)amino (NMI), bis((1-(carboxymethyl)-1H-imidazol-2-yl)methyl)amino (CIM) or bis((1-(2-(bis(carboxymethyl)amino)-2-oxoethyl)-1H-imidazol-2-yl)methyl)amino (TIM). The in vitro binding affinity of the rhenium complexes was evaluated using PSMA-expressing human prostate cancer LNCaP cells. IC(50) values ranged from 3.8 ± 2 to >2000 nM. A linker between the SAAC chelate and pharmacophore was required for high affinity binding. However, extending the length of the linker did not substantially improve binding. PSMA binding was also influenced by the nature of the SAAC chelate. One of the most potent compounds, 23b (IC(50)=4.8 ± 2.7 nM), was radiolabeled with technetium tricarbonyl ({(99m)Tc(CO)(3)}(+)) to afford the {(99m)Tc(CO)(3)}(+) complex in excellent yield and high purity. This effort has led to the identification of a diverse series of promising high affinity {(99m)Tc(CO)(3)}(+) radiolabeled PSMA inhibitors.

Target occupancy study and whole-body dosimetry with a MAGL PET ligand [11C]PF-06809247 in non-human primates.

BACKGROUND: Monoacylglycerol lipase (MAGL) is a key serine hydrolase which terminates endocannabinoid signaling and regulates arachidonic acid driven inflammatory responses within the central nervous system. To develop [11C]PF-06809247 into a clinically usable MAGL positron emission tomography (PET) radioligand, we assessed the occupancy of MAGL by an inhibitor in the non-human primate (NHP) brain. Additionally, we measured the whole-body distribution of [11C]PF-06809247 in NHP and estimated human effective radiation doses. METHODS: Seven cynomolgus monkeys were enrolled for brain PET measurements. Two PET measurements along with arterial blood sampling were performed in each NHP: one baseline and one pretreatment condition with intravenous administration of PF-06818883, a pro-drug of a selective MAGL inhibitor (total of seven doses between 0.01 and 1.27 mg/kg). Kinetic parameters K1, k2 and k3 were estimated by a two tissue compartment (2TC) model using metabolite corrected plasma radioactivity as the input function. k4 was set as 0 according to the irreversible binding of [11C]PF-06809247. Ki by 2TC and Patlak analysis were calculated as the influx constant. The target occupancy was calculated using Ki at baseline and pretreatment conditions. Two cynomolgus monkeys were enrolled for whole-body PET measurements. Estimates of the absorbed radiation dose in humans were calculated with OLINDA/EXM 1.1 using the adult male reference model. RESULTS: Radioactivity retention was decreased in all brain regions following pretreatment with PF-06818883. Occupancy was measured as 25.4-100.5% in a dose dependent manner. Whole-body PET showed high radioactivity uptake values in the liver, small intestine, kidney, and brain. The effective dose of [11C]PF-06809247 was calculated as 4.3 µSv/MBq. CONCLUSIONS: [11C]PF-06809247 is a promising PET ligand for further studies of MAGL in the human brain.

First-in-Human Evaluation of 18F-PF-06445974, a PET Radioligand That Preferentially Labels Phosphodiesterase-4B.

Phosphodiesterase-4 (PDE4), which metabolizes the second messenger cyclic adenosine monophosphate (cAMP), has 4 isozymes: PDE4A, PDE4B, PDE4C, and PDE4D. PDE4B and PDE4D have the highest expression in the brain and may play a role in the pathophysiology and treatment of depression and dementia. This study evaluated the properties of the newly developed PDE4B-selective radioligand 18F-PF-06445974 in the brains of rodents, monkeys, and humans. Methods: Three monkeys and 5 healthy human volunteers underwent PET scans after intravenous injection of 18F-PF-06445974. Brain uptake was quantified as total distribution volume (V T) using the standard 2-tissue-compartment model and serial concentrations of parent radioligand in arterial plasma. Results: 18F-PF-06445974 readily distributed throughout monkey and human brain and had the highest binding in the thalamus. The value of V T was well identified by a 2-tissue-compartment model but increased by 10% during the terminal portions (40 and 60 min) of the monkey and human scans, respectively, consistent with radiometabolite accumulation in the brain. The average human V T values for the whole brain were 9.5 ± 2.4 mL ⋅ cm-3 Radiochromatographic analyses in knockout mice showed that 2 efflux transporters-permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP)-completely cleared the problematic radiometabolite but also partially cleared the parent radioligand from the brain. In vitro studies with the human transporters suggest that the parent radioligand was a partial substrate for BCRP and, to a lesser extent, for P-gp. Conclusion: 18F-PF-06445974 quantified PDE4B in the human brain with reasonable, but not complete, success. The gold standard compartmental method of analyzing brain and plasma data successfully identified the regional densities of PDE4B, which were widespread and highest in the thalamus, as expected. Because the radiometabolite-induced error was only about 10%, the radioligand is, in the opinion of the authors, suitable to extend to clinical studies.

Preclinical Evaluation of 89Zr-Df-IAB22M2C PET as an Imaging Biomarker for the Development of the GUCY2C-CD3 Bispecific PF-07062119 as a T Cell Engaging Therapy.

PURPOSE: A sensitive and specific imaging biomarker to monitor immune activation and quantify pharmacodynamic responses would be useful for development of immunomodulating anti-cancer agents. PF-07062119 is a T cell engaging bispecific antibody that binds to CD3 and guanylyl cyclase C, a protein that is over-expressed by colorectal cancers. Here, we used 89Zr-Df-IAB22M2C (89Zr-Df-Crefmirlimab), a human CD8-specific minibody to monitor CD8+ T cell infiltration into tumors by positron emission tomography. We investigated the ability of 89Zr-Df-IAB22M2C to track anti-tumor activity induced by PF-07062119 in a human CRC adoptive transfer mouse model (with injected activated/expanded human T cells), as well as the correlation of tumor radiotracer uptake with CD8+ immunohistochemical staining. PROCEDURES: NOD SCID gamma mice bearing human CRC LS1034 tumors were treated with four different doses of PF-07062119, or a non-targeted CD3 BsAb control, and imaged with 89Zr-Df-IAB22M2C PET at days 4 and 9. Following PET/CT imaging, mice were euthanized and dissected for ex vivo distribution analysis of 89Zr-Df-IAB22M2C in tissues on days 4 and 9, with additional data collected on day 6 (supplementary). Data were analyzed and reported as standard uptake value and %ID/g for in vivo imaging and ex vivo tissue distribution. In addition, tumor tissues were evaluated by immunohistochemistry for CD8+ T cells. RESULTS: The results demonstrated substantial mean uptake of 89Zr-Df-IAB22M2C (%ID/g) in PF-07062119-treated tumors, with significant increases in comparison to non-targeted BsAb-treated controls, as well as PF-07062119 dose-dependent responses over time of treatment. A moderate correlation was observed between tumor tissue radioactivity uptake and CD8+ cell density, demonstrating the value of the imaging agent for non-invasive assessment of intra-tumoral CD8+ T cells and the mechanism of action for PF-07062119. CONCLUSION: Immune-imaging technologies for quantitative cellular measures would be a valuable biomarker in immunotherapeutic clinical development. We demonstrated a qualification of 89Zr-IAB22M2C PET to evaluate PD responses (mice) to a novel immunotherapeutic.

Novel polar single amino acid chelates for technetium-99m tricarbonyl-based radiopharmaceuticals with enhanced renal clearance: application to octreotide.

Single amino acid chelate (SAAC) systems for the incorporation of the M(CO)(3) moiety (M = Tc/Re) have been successfully incorporated into novel synthetic strategies for radiopharmaceuticals and evaluated in a variety of biological applications. However, the lipophilicity of the first generation Tc(CO)(3)-dipyridyl complexes has resulted in substantial hepatobiliary uptake when either examined as lysine derivatives or integrated into biologically active small molecules and peptides. Here we designed, synthesized, and evaluated novel SAAC systems that have been chemically modified to promote overall Tc(CO)(3)L(3) complex hydrophilicity with the intent of enhancing renal clearance. A series of lysine derived SAAC systems containing functionalized polar imidazole rings and/or carboxylic acids were synthesized via reductive alkylation of the epsilon amino group of lysine. The SAAC systems were radiolabeled with (99m)Tc, purified, and evaluated for radiochemical stability, lipophilicity, and tissue distribution in rats. The log P values of the (99m)Tc complexes were determined experimentally and ranged from -0.91 to -2.33. The resulting complexes were stable (>90%) for at least 24 h. Tissue distribution in normal rats of the lead (99m)Tc complexes demonstrated decreased liver (<1 %ID/g) and gastrointestinal clearance (<1.5%ID/g) and increased kidney clearance (>15 %ID/g) at 2 h after injection compared to the dipyridyl lysine complex (DpK). One of the new SAAC ligands, [(99m)Tc]bis-carboxymethylimidazole lysine, was conjugated to the N-terminus of Tyr-3 octreotide and evaluated for localization in nude mice bearing AR42J xenografts to examine tissue distribution, tumor uptake and retention, clearance, and route of excretion for comparison to (111)In-DOTA-Tyr-3-octreotide and (99m)Tc-DpK-Tyr-3-octreotide. (99m)Tc-bis-(carboxymethylimidazole)-lysine-Tyr-3-octreotide exhibited significantly less liver uptake and gastrointestinal clearance compared to (99m)Tc-DpK-Tyr-3-octreotide while maintaining tumor uptake in the same mouse model. These novel chelators demonstrate that lipophilicity can be controlled and organ distribution significantly altered, opening up broad application of these novel SAAC systems for radiopharmaceutical design.

Comprehensive radiolabeling, stability, and tissue distribution studies of technetium-99m single amino acid chelates (SAAC).

Technetium tricarbonyl chemistry has been a subject of interest in radiopharmaceutical development over the past decade. Despite the extensive work done on developing chelates for Tc(I), a rigorous investigation of the impact of changing donor groups and labeling conditions on radiochemical yields and/or distribution has been lacking. This information is crucially important if these platforms are going to be used to develop molecular imaging probes. Previous studies on the coordination chemistry of the {M(CO)(3)}(+) core have established alkylamine, aromatic nitrogen heterocycles, and carboxylate donors as effective chelating ligands. These observations led to the design of tridentate ligands derived from the amino acid lysine. Such amino acid analogues provide a tridentate donor set for chelation to the metal and an amino acid functionality for conjugation to biomolecules. We recently developed a family of single amino acid chelates (SAAC) that serve this function and can be readily incorporated into peptides via solid-phase synthesis techniques. As part of these continuing studies, we report here on the radiolabeling with technetium-99m ((99m)Tc) and stability of a series of SAAC analogues of lysine. The complexes studied include cationic, neutral, and anionic complexes. The results of tissue distribution studies with these novel complexes in normal rats demonstrate a range of distribution in kidney, liver, and intestines.

Single amino acid chelates (SAAC): a strategy for the design of technetium and rhenium radiopharmaceuticals.

Radiolabeled biomolecules can be used to visualize a variety of diseases through interaction with specific cell receptors. A key step is the introduction of a molecular entity that allows facile labeling with the medically useful radionuclide (99m)Tc without significant alteration of the structure and function of the biomolecule. One strategy focuses on the design of single amino acid chelates (SAACs), novel bifunctional chelators constructed from derivatized amino acids or amino acid analogues. The chelating terminus of the SAAC has been designed for effective coordination to the {(99m)Tc(CO)(3)}(+) core, while the other terminus allows incorporation into any position along a peptide sequence or into a variety of biomolecules. In applications to peptidic materials, the approach affords significant flexibility in the choice of donors for (99m)Tc coordination combined with the considerable advantages of routine solid phase synthetic techniques. The methodology allows libraries of peptide-based (99m)Tc(i) and (186,188)Re(i) radiopharmaceuticals to prepared using conventional automated peptides synthesis. Other biomolecules, including nucleosides, carbohydrates, folic acid and vitamin B12 are also readily modified using analogous methods. The approach also allows the preparation of isostructural (99m)Tc and Re complexes for the correlation of in vivo and in vitro imaging studies.

Myeloperoxidase inhibition in mice alters atherosclerotic lesion composition.

Myeloperoxidase (MPO) is a highly abundant protein within the neutrophil that is associated with lipoprotein oxidation, and increased plasma MPO levels are correlated with poor prognosis after myocardial infarct. Thus, MPO inhibitors have been developed for the treatment of heart failure and acute coronary syndrome in humans. 2-(6-(5-Chloro-2-methoxyphenyl)-4-oxo-2-thioxo-3,4-dihydropyrimidin-1(2H)-yl)acetamide PF-06282999 is a recently described selective small molecule mechanism-based inactivator of MPO. Here, utilizing PF-06282999, we investigated the role of MPO to regulate atherosclerotic lesion formation and composition in the Ldlr-/- mouse model of atherosclerosis. Though MPO inhibition did not affect lesion area in Ldlr-/- mice fed a Western diet, reduced necrotic core area was observed in aortic root sections after MPO inhibitor treatment. MPO inhibition did not alter macrophage content in and leukocyte homing to atherosclerotic plaques. To assess non-invasive monitoring of plaque inflammation, [18F]-Fluoro-deoxy-glucose (FDG) was administered to Ldlr-/- mice with established atherosclerosis that had been treated with clinically relevant doses of PF-06282999, and reduced FDG signal was observed in animals treated with a dose of PF-06282999 that corresponded with reduced necrotic core area. These data suggest that MPO inhibition does not alter atherosclerotic plaque area or leukocyte homing, but rather alters the inflammatory tone of atherosclerotic lesions; thus, MPO inhibition could have utility to promote atherosclerotic lesion stabilization and prevent atherosclerotic plaque rupture.

Identification and Development of an Irreversible Monoacylglycerol Lipase (MAGL) Positron Emission Tomography (PET) Radioligand with High Specificity.

Monoacylglycerol lipase (MAGL), a serine hydrolase extensively expressed throughout the brain, serves as a key gatekeeper regulating the tone of endocannabinoid signaling. Preclinically, inhibition of MAGL is known to provide therapeutic benefits for a number of neurological disorders. The availability of a MAGL-specific positron emission tomography (PET) ligand would considerably facilitate the development and clinical characterization of MAGL inhibitors via noninvasive and quantitative PET imaging. Herein, we report the identification of the potent and selective irreversible MAGL inhibitor 7 (PF-06809247) as a suitable radioligand lead, which upon radiolabeling was found to exhibit a high level of MAGL specificity; this enabled cross-species measurement of MAGL brain expression (Bmax), assessment of in vivo binding in the rat, and nonhuman primate PET imaging.

Synthesis and evaluation of a series of 99mTc(CO)3+ lisinopril complexes for in vivo imaging of angiotensin-converting enzyme expression.

In animal models of cardiac disease and in human congestive heart failure, expression of angiotensin-converting enzyme (ACE) is upregulated in the failing heart and has been associated with disease progression leading to cardiac failure and fibrosis. To develop probes for imaging ACE expression, a series of di(2-pyridylmethyl)amine (D) chelates capable of binding M(CO)3+ (M = technetium, rhenium) was conjugated to lisinopril by acylation of the epsilon-amine of the lysine residue with a series of di(2-pyridylmethylamino)alkanoic acids where the distance of the chelator from the lisinopril core was investigated by varying the number of methylene spacer groups to produce di(2-pyridylmethyl)amine(Cx)lisinopril analogs: D(C4)lisinopril, D(C5)lisinopril, and D(C8)lisinopril. The inhibitory activity of each rhenium complex was evaluated in vitro against purified rabbit lung ACE and was shown to vary directly with the length of the methylene spacer: Re[D(C8)lisinopril], inhibitory concentration of 50% (IC50) = 3 nM; Re[D(C5)lisinopril], IC50 = 144 nM; and Re[D(C4)lisinopril], IC50 = 1,146 nM, as compared with lisinopril, IC50 = 4 nM. The in vivo specificity for ACE was determined by examining the biodistribution of the 99mTc-[D(C8)lisinopril] analog in rats with and without pretreatment with unlabeled lisinopril. Uptake in the lungs, a tissue that constitutively expresses ACE, was 15.2 percentage injected dose per gram at 10 min after injection and was dramatically reduced by pretreatment with lisinopril, supporting ACE-mediated binding in vivo. Planar anterior imaging analysis of 99mTc-[D(C8)lisinopril] corroborated these data. Thus, high-affinity 99mTc-labeled ACE inhibitor has been designed with potency similar to that of lisinopril and has been demonstrated to specifically localize to tissues that express ACE in vivo. This agent may be useful in monitoring ACE as a function of disease progression in relevant diseases such as heart failure.

Synthesis and preliminary PET imaging of 11C and 18F isotopologues of the ROS1/ALK inhibitor lorlatinib.

Lorlatinib (PF-06463922) is a next-generation small-molecule inhibitor of the orphan receptor tyrosine kinase c-ros oncogene 1 (ROS1), which has a kinase domain that is physiologically related to anaplastic lymphoma kinase (ALK), and is undergoing Phase I/II clinical trial investigations for non-small cell lung cancers. An early goal is to measure the concentrations of this drug in brain tumour lesions of lung cancer patients, as penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Here we prepare both 11C- and 18F-isotopologues of lorlatinib to determine the biodistribution and whole-body dosimetry assessments by positron emission tomography (PET). Non-traditional radiolabelling strategies are employed to enable an automated multistep 11C-labelling process and an iodonium ylide-based radiofluorination. Carbon-11-labelled lorlatinib is routinely prepared with good radiochemical yields and shows reasonable tumour uptake in rodents. PET imaging in non-human primates confirms that this radiotracer has high brain permeability.

New directions in the coordination chemistry of 99mTc: a reflection on technetium core structures and a strategy for new chelate design.

Bifunctional chelates offer a general approach for the linking of radioactive metal cations to macromolecules. In the specific case of 99mTc, a variety of technologies have been developed for assembling a metal-chelate-biomolecule complex. An evaluation of these methodologies requires an appreciation of the coordination characteristics and preferences of the technetium core structures and oxidation states, which serve as platforms for the development of the imaging agent. Three technologies, namely, the MAG3-based bifunctional chelates, the N-oxysuccinimidylhydrazino-nicotinamide system and the recently described single amino acid chelates for the {Tc(CO)3}1+ core, are discussed in terms of the fundamental coordination chemistry of the technetium core structures. In assessing the advantages and disadvantages of these technologies, we conclude that the single amino acid analogue chelates (SAAC), which are readily conjugated to small peptides by solid-phase synthesis methods and which form robust complexes with the {Tc(CO)3}1+ core, offer an effective alternative to the previously described methods.

Synthesis and Characterization of Indium Compounds with Phosphinothiol Ligands. The Crystal and Molecular Structures of [In{2-(Ph(2)P)C(6)H(4)S}(3)], [In{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S}(2){2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}], and [NMe(4)][In{PhP(C(6)H(4)S-2)(2)}(2)].CH(3)CN.

The electrochemical oxidation of anodic indium metal in an acetonitrile solution of phosphinothiol ligands affords [In{2-(Ph(2)P)C(6)H(4)S}(3)] (1), [In{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S)}(2){2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}] (2), [In{2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}(3)] (3), and [NMe(4)][In{PhP(C(6)H(4)S-2)(2)}(2)].CH(3)CN (4) complexes exhibiting distorted six-coordinate geometries based on {InS(3)P(3)}, {InS(3)P(2)O}, and {InS(4)P(2)} cores, respectively. In all cases, the In-P bond distances are anomalously long, presumably as a consequence of steric crowding. The anion of 4 provides an unusual example of an In(III)-thiolate coordination complex ion. Crystal data: 1, C(54)H(42)InP(3)S(3), monoclinic, P2(1)/c, a = 16.6579(1) Å, b = 12.6628(2) Å, c = 22.5520(3) Å, beta = 96.42(1) degrees, V = 4727.15(1) Å(3), Z = 4, 6176 reflections, R = 0.0579; 2, C(63)H(66)InOP(3)S(3)Si(3), monoclinic, P2(1)/c, a = 11.53090(10) Å, b = 26.2505(3) Å, c = 20.4206(2) Å, beta = 94.0870(10) degrees, V = 6165.43(11) Å(3), Z = 4, 10 699 reflections, R = 0.0621; 4, C(42)H(41)InN(2)P(2)S(4), monoclinic, P2(1), a = 13.0052(2) Å, b = 11.2240(2) Å, c = 14.3070(3) Å, beta = 93.190(1) degrees, V = 2085.16(7) Å(3), Z = 2, 6651 reflections, R = 0.0352.

Synthesis and Characterization of Zinc and Cadmium Compounds with Arenephosphinothiol Ligands. Crystal and Molecular Structures of [Cd(2){2-(Ph(2)PO)C(6)H(4)S}(4)], [Zn{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S}(2)], [Cd{2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}(2)(CH(3)OH)], and [Zn{PhPO(C(6)H(4)S-2)(2)}(bipy)].

The electrochemical oxidation of a metallic anode (zinc or cadmium) in an acetonitrile solution of a series of arenephosphinothiol ligands, 2-(Ph(2)P)C(6)H(4)SH, 2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)SH, 2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)SH, and PhP(C(6)H(4)SH-2)(2) [abbreviated RP-(SH)(x)(), x = 1 or 2], affords [M(RP-S)(2)] and [M(RP-S(2))], M = Zn, Cd. Adducts of several of these compounds with 1,10-phenanthroline and 2,2'-bipyridine have also been obtained by addition of these coligands to the electrolysis phase. The compounds obtained have been characterized by microanalysis, IR, UV-visible, FAB spectrometry and (1)H, (31)P NMR spectroscopic studies. The compounds, [Cd(2){2-(Ph(2)PO)C(6)H(4)S}(4)]CH(3)CN (1), [Zn{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S}(2)] (2), [Cd{2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}(2)(CH(3)OH)] (3), and [Zn{PhPO(C(6)H(4)S-2)(2)}(bipy)] (4), have been also characterized by single-crystal X-ray diffraction. Compound 1 is binuclear with a {Cd(2)S(2)} core and distorted trigonal bipyramidal {CdO(2)S(3)} geometry about the Cd sites. Compounds 2, 3, and 4 are mononuclear with distorted tetrahedral {ZnP(2)S(2)}, distorted square pyramidal {CdO(3)S(2)}, and distorted trigonal bipyramidal {ZnON(2)S(2)} geometries, respectively. Crystal data: 1, C(42)H(37)N(3)O(2)P(2)S(2)Cd, triclinic, P&onemacr;, a = 13.5780(2) Å, b = 13.8505(2) Å, c = 13.9526(2) Å, alpha = 106.622(1) degrees, beta = 109.693(1) degrees, gamma = 107.137(1) degrees, V = 2133.30(5) Å(3), Z = 2, 9560 reflections, R = 0.0483; 2, C(46)H(50)N(2)P(2)S(2)Si(2)Zn, monoclinic, C2/c, a = 21.332(4) Å, b = 9.391(2) Å, c = 25.938(5) Å, beta = 113.84(3) degrees, V = 4753(2) Å(3), Z = 4, 2564 reflections, R = 0.0377; 3, C(43)H(48)CdO(3)P(2)S(2)Si(2), triclinic, P&onemacr;, a = 12.1237(4) Å, b = 14.0568(4) Å, c = 15.0938(2) Å, alpha = 70.836(2) degrees, beta = 83.410(2) degrees, gamma = 65.397(2) degrees, V = 2208.7(1) Å(3), Z = 2, 4936 reflections, R = 0.0738; 4, C(29)H(22)Cl(3)N(2)OP(2)S(2)Zn, triclinic, P&onemacr;, a = 8.9556(3) Å, b = 12.7911(4) Å, c = 14.0598(5) Å, alpha = 82.671(1) degrees, beta = 73.140(1) degrees, gamma = 74.113(1) degrees, V = 1480.44(9)(1) Å(3), Z = 2, 3820 reflections, R = 0.0511.

Synthesis and Characterization of Organohydrazino Complexes of Technetium, Rhenium, and Molybdenum with the {M(eta(1)-H(x)()NNR)(eta(2)-H(y)()NNR)} Core and Their Relationship to Radiolabeled Organohydrazine-Derivatized Chemotactic Peptides with Diagnostic Applications.

The reduction of perrhenate, molybdate and pertechnetate with 2-hydrazinopyridine dihydrochloride in methanol has led to the preparation of a class of complexes containing the {M(eta(1)-NNC(5)H(4)NH(x)())(eta(2)-HNNH(y)()C(5)H(4)N)} core, represented by [TcCl(3)(NNC(5)H(4)NH)(HNNC(5)H(4)N)] (2), [ReCl(3)(NNC(5)H(4)NH)(HNNC(5)H(4)N)] (3), and [MoCl(3)(NNC(5)H(4)NH)(HNNHC(5)H(4)N)] (6). The reaction of 3 with NEt(3) results in the formation of [HNEt(3)][[ReCl(3)(NNC(5)H(4)N)(HNNC(5)H(4)N)].H(2)O (4) by deprotonation of the pyridine nitrogen site. Similarly, the reduction of perrhenate with 2-hydrazino-2-imidazoline hydrobromide has led to the preparation of the analogous [ReCl(3)(NNC(3)H(4)N(2)H)(HNNHC(3)H(4)N(2)H)] (5). Reaction of 3 with pyridine-2-thiol and pyrimidine-2-thiol yields two structurally characterized derivatives with a modified {Re(eta(1)-NNC(5)H(4)N)(eta(2)-HNNC(5)H(4)N)} core, [Re(C(5)H(4)NS)(2)(NNC(5)H(4)N)(HNNC(5)H(4)N)] (8) and [Re(C(4)H(3)N(2)S)(2)(NNC(5)H(4)N)(HNNC(5)H(4)N)] (9), respectively. Reaction of 6 with pyrimidine-2-thiol led to the isolation of the analogous [Mo(C(4)H(3)N(2)S)(2)(NNC(5)H(4)N)(HNNHC(5)H(4)N)] (11) and the seven-coordinate monohydrazine core complex [Mo(C(4)H(3)N(2)S)(3)(NNC(5)H(4)N)].CH(2)Cl(2) (12). In similar fashion, the reaction of 2 with pyridine-2-thiol yielded a complex structurally analogous to 8, [Tc(C(5)H(4)NS)(2)(NNC(5)H(4)N)(HNNC(5)H(4)N)] (7). Crystal data for 3, C(10)H(10)Cl(3)N(6)Re: triclinic, P&onemacr;, a = 7.527(2) Å, b = 7.599(2) Å, c = 13.118(3) Å, alpha = 106.55(3) degrees, beta = 90.28(3) degrees, gamma = 93.83(3) degrees, V = 717.4(4) Å(3), Z = 2. For 4, C(16)H(27)Cl(3)N(7)ORe: orthorhombic, P2(1)2(1)2(1), a = 7.503(2) Å, b = 10.3643(2) Å, c = 30.1590(5) Å, V = 2345.20(6) Å(3), Z = 2. For 5, C(6)H(12)Cl(3)N(8)Re: monoclinic, P2(1)/n, a = 9.093(2) Å, b = 11.105(2) Å, c = 14.295(3) Å, beta = 94.71(3) degrees, V = 1438.6(7) Å(3), Z = 4. For 6, C(10)H(11)Cl(3)N(6)Mo: monoclinic, P2(1)/c, a = 15.366(3) Å, b = 7.804(2) Å, c = 12.378(3) Å, beta = 95.92(3) degrees, V = 1476.4(5) Å(3), Z = 4. For 7, C(20)H(17)N(8)S(2)Tc: monoclinic, P2(1), a = 8.827(2) Å, b = 9.278(2) Å, c = 13.304(3) Å, beta = 98.92(3) degrees, V = 1076.5(5) Å(3), Z = 2, 2564 reflections. For 8, C(20)H(17)N(8)S(2)Re: monoclinic, P2(1), a = 8.848(2) Å, b = 9.190(2) Å, c = 13.293(3) Å, beta = 98.89(3) degrees, V = 1067.9(5) Å(3), Z = 2. For 9, C(18)H(15)N(10)S(2)Re: monoclinic, P2(1), a = 8.796(2) Å, b = 9.008(2) Å, c = 13.208(3) Å, beta = 97.90(3) degrees, V = 1036.6(5) Å(3), Z = 2. For 12, C(18)H(15)N(9)S(3)Cl(2)Mo: monoclinic, P2(1)/n, a = 10.52900(10) Å, b = 15.1116(3) Å, c = 15.8193(3) Å, beta = 108.4790(10) degrees, V = 2387.23(7) Å(3), Z = 4. Complexes 2 and 3 serve as models for the binding of Tc(V)-oxo and Re(V)-oxo species to hydrazinonicotinamide (HYNIC)-conjugated chemotactic peptides. Furthermore, since the use of the pyrimidinethiol coligand in the {(99m)Tc-HYNIC-peptide} radiochemical species results in favorable pharmacokinetics, the thiolate derivatives 8 and 9 provide models for possible modes of interaction of metal-hydrazine cores with coligands in the radiopharmaceutical reagents.