A New, Second Generation Trithiol Bifunctional Chelate for 72,77As: Trithiol(b)-(Ser)2-RM2.

Trithiol chelates are suitable for labeling radioarsenic (72As: 2.49 MeV ß+, 26 h; 77As: 0.683 MeV ß-, 38.8 h) to form potential theranostic radiopharmaceuticals for positron emission tomography (PET) imaging and therapy. A trithiol(b)-(Ser)2-RM2 bioconjugate and its arsenic complex were synthesized and characterized. The trithiol(b)-(Ser)2-RM2 bioconjugate was radiolabeled with no-carrier-added 77As in over 95% radiochemical yield and was stable for over 48 h, and in vitro IC50 cell binding studies of [77As]As-trithiol(b)-(Ser)2-RM2 in PC-3 cells demonstrated high affinity for the gastrin-releasing peptide (GRP) receptor (low nanomolar range). Limited biodistribution studies in normal mice were performed with HPLC purified 77As-trithiol(b)-(Ser)2-RM2 demonstrating both pancreatic uptake and hepatobiliary clearance.

In vitro and in vivo analysis of [(64)Cu-NO2A-8-Aoc-BBN(7-14)NH(2)]: a site-directed radiopharmaceutical for positron-emission tomography imaging of T-47D human breast cancer tumors.

INTRODUCTION: Human breast cancer, from which the T-47D cell line was derived, is known to overexpress the gastrin-releasing peptide receptor (GRPR) in some cases. Bombesin (BBN), an agonist for the GRPR, has been appended with a radionuclide capable of positron-emission tomography (PET) imaging and therapy. (64)Cu-NO2A-8-Aoc-BBN(7-14)NH(2) (NO2A=1,4,7-triazacyclononane-1,4-diacetate) has produced high-quality microPET images of GRPR-positive breast cancer xenografted tumors in mice. METHODS: The imaging probe was synthesized by solid-phase peptide synthesis followed by manual conjugation of the 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) bifunctional chelator and radiolabeling in aqueous solution. The radiolabeled conjugate was subjected to in vitro and in vivo studies to determine its specificity for the GRPR and its pharmacokinetic profile. A T-47D tumor-bearing mouse was imaged with microPET/CT and microMRI imaging. RESULTS: The (64)Cu-NO2A-8-Aoc-BBN(7-14)NH(2) targeting vector was determined to specifically localize in GRPR-positive tissue. Accumulation was observed in the tumor in sufficient quantities to allow for identification of tumors in microPET imaging procedures. For example, uptake and retention in T-47D xenografts at 1, 4 and 24 h were determined to be 2.27+/-0.08, 1.35+/-0.14 and 0.28+/-0.07 % ID/g, respectively. CONCLUSIONS: The (64)Cu-NO2A-8-Aoc-BBN(7-14)NH(2) produced high-quality microPET images. The pharmacokinetic profile justifies investigation of this bioconjugate as a potentially useful diagnostic/therapeutic agent. Additionally, the bioconjugate would serve as a good starting point for modification and optimization of similar agents to maximize tumor uptake and minimize nontarget accumulation.

Design, synthesis, and biological evaluation of an antagonist-bombesin analogue as targeting vector.

The gastrin releasing peptide receptor (GRP-R) is overexpressed on a number of tumors and cancer cell lines including pancreas, prostate, breast, gastrointestinal, and small cell lung cancer (SCLC). Radiolabeled bombesin (BBN) analogues have exhibited high binding affinity and specificity to the GRP-R. A bombesin analogue with an antagonist targeting vector at the C-terminus, DOTA-aminohexanoyl-[D-Phe(6), Leu-NHCH 2CH 2CH3(13), des Met(14)] BBN[6-14] (1, "Bomproamide"), has been synthesized and displays high binding affinity (IC50 = 1.36 +/- 0.09 nM) against (125)I-Tyr (4)-BBN in in vitro competitive assays using PC-3 cells. Maximum internalization of (111)In-1 reached 14% in PC-3 cells after 45 min of incubation. Rapid (0.25 h PI) and high (12.21 +/- 3.2%ID/g) pancreatic uptake of (111)In-1 was observed in healthy CF-1 mice, and 90% of the activity was blocked by coinjection of 100 mug of BBN. Rapid (0.25 h PI) and high uptake (6.90 +/- 1.06%ID/g) was observed in PC-3 prostate cancer xenografts in SCID mice, as well as visualized clearly in a SPECT/CT study. These results support the use of a bombesin construct with an antagonist C-terminal vector as a candidate of choice for specific in vivo imaging of tumors overexpressing GRP-receptors.

Evaluation of the pharmacokinetic effects of various linking group using the 111In-DOTA-X-BBN(7-14)NH2 structural paradigm in a prostate cancer model.

The high incidence of BB2 receptor (BB2r) expression in various cancers has prompted investigators to pursue the development of BB2r-targeted agents for diagnostic imaging, chemotherapy, and radiotherapy. Development of BB2r-targeted agents, based on the bombesin (BBN) peptide, has largely involved the use of the bifunctional chelate approach in which the linking group serves several key roles including pharmacokinetic modification. Understanding the in vivo properties of the various pharmacokinetic modifying linking groups is crucial for developing BB2r-targeted agents with improved targeting and clearance characteristics. The goal of this study was to systematically evaluate the pharmacokinetic profile of aliphatic hydrocarbon, aromatic, and poly(ethylene glycol) (ether) functional groups in order to obtain a better understanding of the in vivo properties of these pharmacokinetic modifiers. Specifically, we synthesized six radioconjugates with the structure 111In-DOTA- X-BBN(7-14)NH2, where X = 8-aminooctanoic acid (8-AOC), 5-amino-3-oxapentyl-succinamic acid (5-ADS), 8-amino-3,6-dioxaoctyl-succinamic acid (8-AOS), p-aminobenzoic acid (AMBA), Gly-AMBA, and Gly- p-aminomethylbenzoic acid (Gly-AM2BA). All of the (nat)In-conjugates demonstrated nanomolar binding affinities to the BB2r. In CF-1 mice, the BB2r uptake in the pancreas of radioconjugates containing aromatic linking groups was found to be significantly higher at 1 h postinjection than the radioconjugates with ether linker moieties. For PC-3 tumor-bearing SCID mice, the tumor uptake was found to be 6.66 +/- 2.00, 6.21 +/- 1.57, 6.36 +/- 1.60, 4.46 +/- 0.81, and 7.76 +/- 1.19 %ID/g for the 8-AOC, 8-ADS, AMBA, Gly-AMBA, and Gly-AM2BA radioconjugates, respectively, at 15 min postinjection. By 24 h postinjection, the radioconjugates containing aromatic groups exhibited the highest percentage tumor retention with 11.4%, 19.8%, 26.6%, 25.8%, and 25.5% relative to the 15 min values remaining in the tumor tissue for the 8-AOC, 8-ADS, AMBA, Gly-AMBA, and Gly-AM2BA radioconjugates, respectively. Fused Micro-SPECT/CT imaging studies performed at 24 h postinjection revealed substantial accumulation of radioactivity in the tumor tissue for all radioconjugates. In both biodistribution and Micro-SPECT/CT imaging studies, the radioconjugates containing aromatic linking groups typically exhibited significantly higher G.I. tract retention than the hydrocarbon or ether linking moieties. In conclusion, our studies indicate that radioconjugates incorporating aromatic linking groups, of the type investigated, generally demonstrated enhanced retention in BB2r expressing tissues in comparison to either the hydrocarbon or ether linking moieties. Furthermore, this investigation clearly demonstrates the significance of the linking group upon not only the in vivo clearance of the radiopharmaceutical, but also on the in vivo uptake and retention of the BB2r-targeted agent in tumor tissue. Future designs of BB2r-targeted agents should include a careful consideration of the effect linking group functionality has upon tumor targeting and retention.

99mTc(CO)3-DTMA bombesin conjugates having high affinity for the GRP receptor.

INTRODUCTION: Targeted diagnosis of specific human cancer types continues to be of significant interest in nuclear medicine. 99mTc is ideally suited as a diagnostic radiometal for in vivo tumor targeting due to its ideal physical characteristics and diverse labeling chemistries in numerous oxidation states. METHODS: In this study, we report a synthetic approach toward design of a new tridentate amine ligand for the organometallic aqua-ion [99mTc(H2O)3(CO)3]+. The new chelating ligand framework, 2-(N,N'-Bis(tert-butoxycarbonyl)diethylenetriamine) acetic acid (DTMA), was synthesized from a diethylenetriamine precursor and fully characterized by mass spectrometry and nuclear magnetic resonance spectroscopy (1H and 13C). DTMA was conjugated to H2N-(X)-BBN(7-14)NH2, where X=an amino acid or aliphatic pharmacokinetic modifier and BBN=bombesin peptide, by means of solid phase peptide synthesis. DTMA-(X)-BBN(7-14)NH2 conjugates were purified by reversed-phase high-performance chromatography and characterized by electrospray-ionization mass spectrometry. RESULTS: The new conjugates were radiolabeled with [99mTc(H2O)3(CO)3]+ produced via Isolink radiolabeling kits to produce [99mTc(CO)3-DTMA-(X)-BBN(7-14)NH2]. Radiolabeled conjugates were purified by reversed-phase high-performance chromatography. Effective receptor binding behavior was evaluated in vitro and in vivo. CONCLUSIONS: [99mTc(CO)3-DTMA-(X)-BBN(7-14)NH2] conjugates displayed very high affinity for the gastrin releasing peptide receptor in vitro and in vivo. Therefore, these conjugates hold some propensity to be investigated as molecular imaging agents that specifically target human cancers uniquely expressing the gastrin releasing peptide receptor subtypes.

A trithiol bifunctional chelate for 72,77As: A matched pair theranostic complex with high in vivo stability.

INTRODUCTION: Trithiol chelates are suitable for labeling radioarsenic (72As: 2.49 MeV ß+, 26 h; 77As: 0.683 MeV ß-, 38.8 h) to form potential theranostic radiopharmaceuticals for PET imaging and therapy. To investigate the in vivo stability of trithiol chelates complexed with no carrier added (nca) radioarsenic, a bifunctional trithiol chelate was developed, and conjugated to bombesin(7-14)NH2 as a model peptide. METHODS: A trithiol-BBN(7-14)NH2 bioconjugate and its arsenic complex were synthesized and characterized. The trithiol-BBN(7-14)NH2 conjugate was radiolabeled with 77As, its in vitro stability assessed, and biodistribution studies were performed in CF-1 normal mice of free [77As]arsenate and 77As-trithiol- BBN(7-14)NH2. RESULTS: The trithiol-BBN(7-14)NH2 conjugate, its precursors and its As-trithiol-BBN(7-14)NH2 complex were fully characterized. Radiolabeling studies with nca 77As resulted in over 90% radiochemical yield of 77As-trithiol-BBN, which was stable for over 48 h. Biodistribution studies were performed with both free [77As]arsenate and Sep-Pak® purified 77As-trithiol-BBN(7-14)NH2. Compared to the fast renal clearance of free [77As]arsenate, 77As-trithiol-BBN(7-14)NH2 demonstrated increased retention with clearance mainly through the hepatobiliary system, consistent with the lipophilicity of the 77As-trithiol-BBN(714)NH2 complex. CONCLUSION: The combined in vitro stability of 77As-trithiol-BBN(7-14)NH2 and the biodistribution results demonstrate its high in vivo stability, making the trithiol a promising platform for developing radioarsenic-based theranostic radiopharmaceuticals.

In vivo evaluation and small-animal PET/CT of a prostate cancer mouse model using 64Cu bombesin analogs: side-by-side comparison of the CB-TE2A and DOTA chelation systems.

UNLABELLED: The BB2 receptor subtype, of the bombesin family of receptors, has been shown to be highly overexpressed in a variety of human tumors, including prostate cancer. Bombesin (BBN), a 14-amino acid peptide, has been shown to target the BB2 receptor with high affinity. 64Cu (half-life = 12.7 h, beta+: 18%, E(beta+ max) = 653 keV; beta-: 37%, E(beta- max) = 578 keV) is a radioisotope that has clinical potential for application in both diagnostic imaging and radionuclide therapy. Recently, new chelation systems such as 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-4,11-diacetic acid (CB-TE2A) have been reported to significantly stabilize the 64Cu radiometal in vivo. The increased stability of the 64Cu-CB-TE2A chelate complex has been shown to significantly reduce nontarget retention compared with tetraazamacrocycles such as 1,4,7,10-tetraazacyclodoadecane-N,N',N'',N'''-tetraacetic acid (DOTA). The aim of this study was to determine whether the CB-TE2A chelation system could significantly improve the in vivo stability of 64Cu bombesin analogs. The study directly compares 64Cu bombesin analogs using the CB-TE2A and DOTA chelation systems in a prostate cancer xenograft SCID (severely compromised immunodeficient) mouse model. METHODS: The CB-TE2A-8-AOC-BBN(7-14)NH2 and DOTA-8-AOC-BBN(7-14)NH2 conjugates were synthesized and radiolabeled with 64Cu. The receptor-binding affinity and internalization profile of each metallated conjugate was evaluated using PC-3 cells. Pharmacokinetic and small-animal PET/CT studies were performed using female SCID mice bearing PC-3 xenografts. RESULTS: In vivo BB2 receptor targeting was confirmed by tumor uptake values of 6.95 +/- 2.27 and 4.95 +/- 0.91 %ID/g (percentage injected dose per gram) at the 15-min time point for the 64Cu-CB-TE2A and 64Cu-DOTA radioconjugates, respectively. At the 24-h time point, liver uptake was substantially reduced for the 64Cu-CB-TE2A radioconjugate (0.21 +/- 0.06 %ID/g) compared with the 64Cu-DOTA radioconjugate (7.80 +/- 1.51 %ID/g). The 64Cu-CB-TE2A-8-AOC-BBN(7-14)NH2 radioconjugate demonstrated significant clearance, 98.60 +/- 0.28 %ID, from the mouse at 24 h after injection. In contrast, only 67.84 +/- 5.43 %ID of the 64Cu activity was excreted using the 64Cu-DOTA-8-AOC-BBN(7-14)NH2 radioconjugate because of nontarget retention. CONCLUSION: The pharmacokinetic and small-animal PET/CT studies demonstrate significantly improved nontarget tissue clearance for the 64Cu-CB-TE2A8-AOC-BBN(7-14)NH2. This is attributed to the improved in vivo stability of the 64Cu-CB-TE2A chelate complex as compared with the 64Cu-DOTA chelate complex.

The effects of linking substituents on the in vivo behavior of site-directed, peptide-based, diagnostic radiopharmaceuticals.

A number of human cancers are known to over-express the gastrin-releasing peptide receptor (GRPr) on cell surfaces. The high specificity and affinity of bombesin (BBN), an amphibian analogue of mammalian gastrin-releasing peptide, for the GRPr makes it an ideal candidate for delivery of diagnostic probes, such as 99mTc radiometal, to tumor sites. An optimized targeting agent possesses high tumor uptake with minimal uptake in normal tissues. In this study, 99mTc-targeting vectors of bombesin using various amino acid/aliphatic pharmacokinetic modifiers or linking groups were evaluated to determine the effect of the spacer on receptor binding affinity, internalization/externalization and biodistribution. Conjugates of the general type [DPR-X-BBN] (X = amino acid/aliphatic pharmacokinetic modifier) were synthesized by solid phase peptide synthesis (SPPS) and metallated with either low-valent, radioactive Tc-99m(I) or non-radioactive Re(I)-tricarbonyl precursors. All of the new non-metallated and metallated conjugates were characterized by electrospray ionization mass spectrometry (ESI-MS). Receptor binding affinity, internalization/externalization and biodistribution studies in normal (CF-1) and tumor (human prostate PC-3-bearing mice) are reported. The effectiveness of targeting xenografted PC-3 tumors in rodents for two of the new 99mTc-BBN conjugates is demonstrated herein using small animal single photon emission computed tomography (SPECT).

Pyrazolyl conjugates of bombesin: a new tridentate ligand framework for the stabilization of fac-[M(CO)3]+ moiety.

We have described the synthesis of tridentate pyrazolyl ligand frameworks for coordination to the fac-[*M(CO)(3)](+) metal fragment (*M=(186/188)Re or (99m)Tc). These ligands impart a degree of kinetic inertness on the metal center, warranting their study in biological systems. We herein report in vitro/in vivo radiolabeling investigations of a new series of pyrazolyl bombesin (BBN) conjugates radiolabeled via the Isolink kit. These new conjugates are based on the general structure [(99m)Tc-pyrazolyl-X-BBN[7-14]NH(2)], where X=beta-alanine, serylserylserine or glycylglycylglycine. The pyrazolyl ligand is a tridentate ligand framework that coordinates the metal center through nitrogen donor atoms. The results of these investigations demonstrate the ability of these new conjugates to specifically target the gastrin-releasing peptide receptor subtype 2, which is overexpressed on human prostate PC-3 cancerous tissues. Therefore, these studies suggest the tridentate pyrazolyl ligand framework to be an ideal candidate for the design and development of low-valent (99m)Tc-based diagnostic radiopharmaceuticals based on BBN or other targeting vectors.

Monooxorhenium(V) complexes with 222-N2S2 MAMA ligands for bifunctional chelator agents: Syntheses and preliminary in vivo evaluation.

INTRODUCTION: Targeted radiotherapy using the bifunctional chelate approach with 186/188Re(V) is challenging because of the susceptibility of monooxorhenium(V)-based complexes to oxidize in vivo at high dilution. A monoamine-monoamide dithiol (MAMA)-based bifunctional chelating agent was evaluated with both rhenium and technetium to determine its utility for in vivo applications. METHODS: A 222-MAMA chelator, 222-MAMA(N-6-Ahx-OEt) bifunctional chelator, and 222-MAMA(N-6-Ahx-BBN(7-14)NH2) were synthesized, complexed with rhenium, radiolabeled with 99mTc and 186Re (carrier added and no carrier added), and evaluated in initial biological distribution studies. RESULTS: An IC50 value of 2.0±0.7nM for natReO-222-MAMA(N-6-Ahx-BBN(7-14)NH2) compared to [125I]-Tyr4-BBN(NH2) was determined through competitive cell binding assays with PC-3 tumor cells. In vivo evaluation of the no-carrier added 99mTc-222-N2S2(N-6-Ahx-BBN(7-14)NH2) complex showed little gastric uptake and blockable pancreatic uptake in normal mice. CONCLUSIONS: The 186ReO-222-N2S2(N-6-Ahx-BBN(7-14)NH2) complex showed stability in biological media, which indicates that the 222-N2S2 chelator is appropriate for chelating 186/188Re in radiopharmaceuticals involving peptides. Additionally, the in vitro cell studies showed that the ReO-222-N2S2(N-6-Ahx-BBN(7-14)NH2) complex (macroscopically) bound to PC3-tumor cell surface receptors with high affinity. The 99mTc analog was stable in vivo and exhibited pancreatic uptake in mice that was blockable, indicating BB2r targeting.

Characterization and evaluation of DOTA-conjugated Bombesin/RGD-antagonists for prostate cancer tumor imaging and therapy.

INTRODUCTION: Here we present the metallation, characterization, in vivo and in vitro evaluations of dual-targeting, peptide-based radiopharmaceuticals with utility for imaging and potentially treating prostate tumors by virtue of their ability to target the αVß3 integrin or the gastrin releasing peptide receptor (GRPr). METHODS: [RGD-Glu-6Ahx-RM2] (RGD: Arg-Gly-Asp; Glu: glutamic acid; 6-Ahx: 6-amino hexanoic acid; RM2: (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2)) was conjugated to a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) bifunctional chelator (BFCA) purified via reversed-phase high-performance liquid chromatography (RP-HPLC), characterized by electrospray ionization-mass spectrometry (ESI-MS), and radiolabeled with (111)In or (177)Lu. Natural-metallated compounds were assessed for binding affinity for the αVß3 integrin or GRPr in human glioblastoma U87-MG and prostate PC-3 cell lines and stability prior to in vivo evaluation in normal CF-1 mice and SCID mice xenografted with PC-3 cells. RESULTS: Competitive displacement binding assays with PC-3 and U87-MG cells revealed high to moderate binding affinity for the GRPr or the αVß3 integrin (IC50 range of 5.39±1.37 nM to 9.26±0.00 nM in PC-3 cells, and a range of 255±47 nM to 321±85 nM in U87-MG cells). Biodistribution studies indicated high tumor uptake in PC-3 tumor-bearing mice (average of 7.40±0.53% ID/g at 1h post-intravenous injection) and prolonged retention of tracer (mean of 4.41±0.91% ID/g at 24h post-intravenous injection). Blocking assays corroborated the specificity of radioconjugates for each target. Micro-single photon emission computed tomography (microSPECT) confirmed favorable radiouptake profiles in xenografted mice at 20h post-injection. CONCLUSIONS: [RGD-Glu-[(111)In-DO3A]-6-Ahx-RM2] and [RGD-Glu-[(177)Lu- DO3A]-6-Ahx-RM2] show favorable pharmacokinetic and radiouptake profiles, meriting continued evaluation for molecular imaging in murine U87-MG/PC-3 xenograft models and radiotherapy studies with (177)Lu and (90)Y conjugates. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: These heterovalent, peptide-targeting ligands perform comparably with many mono- and multivalent conjugates with the potential benefit of increased sensitivity for detecting cancer cells exhibiting differential expression of target receptors.

A heterodimeric [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2] αvß3/GRPr-targeting antagonist radiotracer for PET imaging of prostate tumors.

INTRODUCTION: In the present study, we describe a (64)Cu-radiolabeled heterodimeric peptide conjugate for dual αvß3/GRPr (αvß3 integrin/gastrin releasing peptide receptor) targeting of the form [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2] (RGD: the amino acid sequence [Arg-Gly-Asp], a nonregulatory peptide used for αvß3 integrin receptor targeting; Glu: glutamic acid; NO2A: 1,4,7-triazacyclononane-1,4-diacetic acid; 6-Ahx: 6-amino hexanoic acid; and RM2: (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2), an antagonist analogue of bombesin (BBN) peptide used for GRPr targeting). METHODS: RGD-Glu-6Ahx-RM2] was conjugated to a NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) complexing agent to produce [RGD-Glu-[NO2A]-6-Ahx-RM2], which was purified by reversed-phase high-performance liquid chromatography (RP-HPLC) and characterized by electrospray ionization-mass spectrometry (ESI-MS). Radiolabeling of the conjugate with (64)Cu produced [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2 in high radiochemical yield (≥95%). In vivo behavior of the radiolabeled peptide conjugate was investigated in normal CF-1 mice and in the PC-3 human prostate cancer experimental model. RESULTS: A competitive displacement receptor binding assay in human prostate PC-3 cells using (125)I-[Tyr(4)]BBN as the radioligand showed high binding affinity of [RGD-Glu-[(nat)Cu-NO2A]-6-Ahx-RM2] conjugate for the GRPr (3.09±0.34 nM). A similar assay in human, glioblastoma U87-MG cells using (125)I-Echistatin as the radioligand indicated a moderate receptor-binding affinity for the αvß3 integrin (518±37.5 nM). In vivo studies of [RGD-Glu-[(64)Cu-NO2A]-6-Ahx-RM2] showed high accumulation (4.86±1.01 %ID/g, 1h post-intravenous injection (p.i.)) and prolonged retention (4.26±1.23 %ID/g, 24h p.i.) of tracer in PC-3 tumor-bearing mice. Micro-positron emission tomography (microPET) molecular imaging studies produced high-quality, high contrast images in PC-3 tumor-bearing mice at 4h p.i. CONCLUSIONS: The favorable pharmacokinetics and enhanced tumor uptake of (64)Cu-NOTA-RGD-Glu-6Ahx-RM2 warrant further investigations for dual integrin and GRPr-positive tumor imaging and possible radiotherapy.

Synthesis and biological evaluation of copper-64 radiolabeled [DUPA-6-Ahx-(NODAGA)-5-Ava-BBN(7-14)NH2], a novel bivalent targeting vector having affinity for two distinct biomarkers (GRPr/PSMA) of prostate cancer.

UNLABELLED: Gastrin-releasing peptide receptors (GRPr) and prostate-specific membrane antigen (PSMA) are two identifying biomarkers expressed in very high numbers on prostate cancer cells and could serve as a useful tool for molecular targeting and diagnosis of disease via positron-emission tomography (PET). The aim of this study was to produce the multipurpose, bivalent [DUPA-6-Ahx-((64)Cu-NODAGA)-5-Ava-BBN(7-14)NH2] radioligand for prostate cancer imaging, where DUPA = (2-[3-(1,3-dicarboxypropyl)-ureido]pentanedioic acid), a small-molecule, PSMA-targeting probe, 6Ahx = 6-aminohexanoic acid, 5-Ava = 5-aminovaleric acid, NODAGA = [2-(4,7-biscarboxymethyl)-1,4,7-(triazonan-1-yl)pentanedioic acid] (a derivative of NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid)), and BBN(7-14)NH2 = bombesin, a GRPr-specific peptide targeting probe. METHODS: The PSMA/GRPr dual targeting ligand precursor [DUPA-6-Ahx-K-5-Ava-BBN(7-14)NH2], was synthesized by solid-phase and manual peptide synthesis, after which NODAGA was added via manual conjugation to the ε-amine of lysine (K). The new bivalent GRPr/PSMA targeting vector was purified by reversed-phase high performance liquid chromatography (RP-HPLC), characterized by electrospray-ionization mass spectrometry (ESI-MS), and metallated with (64)CuCl2 and (nat)CuCl2. The receptor binding affinity was evaluated in human, prostate, PC-3 (GRPr-positive) and LNCaP (PSMA-positive) cells and the tumor-targeting efficacy determined in severe combined immunodeficient (SCID) and athymic nude mice bearing PC-3 and LNCaP tumors. Whole-body maximum intensity microPET/CT images of PC-3/LNCaP tumor-bearing mice were obtained 18 h post-injection (p.i.). RESULTS: Competitive binding assays in PC-3 and LNCaP cells indicated high receptor binding affinity for the [DUPA-6-Ahx-((nat)Cu-NODAGA)-5-Ava-BBN(7-14)NH2] conjugate. MicroPET scintigraphy in PC-3/LNCaP tumor-bearing mice indicated that xenografted tumors were visible at 18h p.i. with collateral, background radiation also being observed in non-target tissue. CONCLUSIONS: DUPA-6-Ahx-((64)Cu-NODAGA)-5-Ava-BBN(7-14)NH2] targeting vector, as described herein, is the first example of a dual GRPr-/PSMA-targeting radioligand for molecular of imaging prostate tumors. Detailed in vitro studies and microPET molecular imaging investigations of [DUPA-6-Ahx-((64)Cu-NODAGA)-5-Ava-BBN(7-14)NH2 in tumor-bearing mice indicate that further studies are necessary to optimize uptake and retention of tracer in GRPr- and PSMA-positive tissues.

Non-invasive microCT imaging characterization and in vivo targeting of BB2 receptor expression of a PC-3 bone metastasis model.

PURPOSE: A devastating progression of human prostate cancer is the development of bone metastasis. Animal models of bone metastasis induced by inoculating human prostate cell lines into mice are well established. Here, we report the characterization of a mouse model of prostatic bone metastasis using non-invasive microCT and targeted microSPECT imaging of bone tumors using the bombesin receptor (BB2r)-avid radiolabeled peptide, (111)In-DOTA-8-Aoc-BBN[7­14]NH(2). PROCEDURES: Immunocompromised mice were inoculated with human prostate cancer cells by intracardiac injection. Metastatic lesion development was monitored by serially imaging mice weekly with microCT. Mice with CT imaging-confirmed bone lesions were administered (111)In-DOTA-8-Aoc-BBN[7­14]NH(2) for microSPECT imaging of BB2r expressing lesions. RESULTS: Metastatic bone lesions as small as 0.3 mm in diameter were detected by microCT image analysis as early as 21 days after tumor cell inoculation and had wide anatomical distribution. MicroSPECT imaging using (111)In-DOTA-8-Aoc-BBN[7­14]NH(2) successfully targeted BB2r expressing metastatic bone lesions of the tibia at day 29. CONCLUSIONS: MicroCT imaging can accurately and non-invasively follow the onset and progression of metastatic bone lesions in mouse models of prostate cancer. Micro-CT coupled with BB2r Micro-SPECT imaging affords the opportunity to obtain a combined receptor/anatomic map of metastatic bone lesion status in this mouse model.

Bombesin analogues for gastrin-releasing peptide receptor imaging.

OBJECTIVES: The present study describes the design and development of a series of new bombesin (BBN) antagonist peptide ligands of the form [(64)Cu-(NO2A-X-D-Phe(6)-BBN(6-13)NHEt)], where Cu-64=a positron emitting radiometal; NO2A=1,4,7-triazacyclononane-1,4-diacetic acid; X=6-amino hexanoic acid, 8-amino octanoic acid or 9-Aminononanoic acid; and BBN(6-13)NHEt=Gln-Trp-Ala-Val-Gly-His-Leu-NHEt, an antagonist analogue of bombesin peptide for specific targeting of the gastrin-releasing peptide receptor (GRPR). METHODS: [NO2A-X-D-Phe(6)-BBN(6-13)NHEt] conjugates were manually conjugated with NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid), and the resulting conjugates were labeled with (64)Cu to yield [(64)Cu-(NO2A-X-D-Phe(6)-BBN(6-13)NHEt)]. The metallated and nonmetallated conjugates were purified via reversed-phase high-performance liquid chromatography and characterized by electrospray ionization-mass spectrometry. RESULTS: Competitive displacement binding assays displayed nanomolar binding affinities toward human GRPR for all of the newly formed peptide analogues. Biodistribution studies showed very high uptake and retention of tumor-associated radioactivity in PC-3 (a prostate tumor model known to express the GRPR) tumor-bearing rodent models. The radiolabeled conjugates also exhibited rapid urinary excretion and very high tumor to background ratios. Micro-positron emission tomography (PET) molecular imaging investigations showed clear visualization of tumors in female PC-3 tumor-bearing mice 15 h postinjection. CONCLUSION: The biodistribution and molecular imaging study suggests that these conjugates can be considered as potential PET tracer candidates for the diagnosis of GRPR-positive tumors in human patients.

64Cu-NO2A-RGD-Glu-6-Ahx-BBN(7-14)NH2: a heterodimeric targeting vector for positron emission tomography imaging of prostate cancer.

INTRODUCTION: The present study describes the design and development of a new heterodimeric RGD-bombesin (BBN) agonist peptide ligand for dual receptor targeting of the form (64)Cu-NO2A-RGD-Glu-6-Ahx-BBN(7-14)NH(2) in which Cu-64=a positron emitting radiometal; NO2A=1,4,7-triazacyclononane-1,4-diacetic acid; Glu=glutamic acid; 6-Ahx=6-aminohexanoic acid; RGD=the amino acid sequence [Arg-Gly-Asp], a nonregulatory peptide that has been used extensively to target α(v)ß(3) receptors up-regulated on tumor cells and neovasculature; and BBN(7-14)NH(2)=Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH(2), an agonist analogue of bombesin peptide for specific targeting of the gastrin-releasing peptide receptor (GRPr). METHODS: RGD-Glu-6-Ahx-BBN(7-14)NH(2) was manually coupled with NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid), and the resulting conjugate was labeled with (64)Cu to yield (64)Cu-NO2A-RGD-Glu-6-Ahx-BBN(7-14)NH(2). Purification was achieved via reversed-phase high-performance liquid chromatography and characterization confirmed by electrospray ionization-mass spectrometry. RESULTS: Competitive displacement binding assays displayed single-digit nanomolar IC(50) values showing very high binding affinities toward the GRPr for the new heterodimeric peptide analogues. In vivo biodistribution studies showed high uptake and retention of tumor-associated radioactivity in PC-3 tumor-bearing rodent models with little accumulation and retention in nontarget tissues. The radiolabeled conjugate also exhibited rapid urinary excretion and high tumor-to-background ratios. Micro-positron emission tomography (microPET) molecular imaging investigations produced high-quality, high-contrast images in PC-3 tumor-bearing mice 15 h postinjection. CONCLUSIONS: Based on microPET imaging experiments that show high-quality, high-contrast images with virtually no residual gastrointestinal radioactivity, this new heterodimeric RGD-BBN conjugate can be considered as a promising PET tracer candidate for the diagnosis of GRPr-positive tumors in human patients.

Positron-emission tomography (PET) imaging agents for diagnosis of human prostate cancer: agonist vs. antagonist ligands.

AIM: The present study adds scientific support to the growing debate regarding the superiority of radiolabeled bombesin-based antagonist peptides over agonists for molecular imaging and therapy of human tumors overexpressing the gastrin-releasing peptide receptor (GRPR) and describes a detailed in vitro and in vivo comparison of 64Cu-NODAGA-6-Ahx-BBN(7-14)NH2 agonist and 64Cu-NODAGA-6-Ahx-DPhe6-BBN(6-13)NHEt antagonist ligands. MATERIALS AND METHODS: Conjugates were synthesized by solid-phase peptide synthesis, purified by reversed-phase high-performance liquid chromatography, and characterized by electrospray ionization-mass spectroscopy. The conjugates were radiolabeled with 64Cu. RESULTS: In vitro and in vivo data support the hypothesis for targeting of the GRPR by these tracer molecules. Maximum-intensity micro Positron Emission Tomography (microPET) imaging studies show the agonist ligand to provide high-quality, high-contrast images with very impressive tumor uptake and background clearance, with virtually no residual gastrointestinal or renal-urinary radioactivity. CONCLUSION: Based on microPET imaging experiments, we conclude the agonist peptide ligand to be a superior molecular imaging agent for targeting GRPR.

Tetradentate bis-phosphine ligands (P(2)N(2) and P(2)S(2)) and their Rh(III), Ni(II) and (105)Rh complexes: X-ray crystal structures of trans-[RhCl(2)(L2)]PF(6), [Ni(L2)](PF(6))(2) and µ-O(2)SO(2)-[Ni(L5)](2)(PF(6))(2).

INTRODUCTION: Tetradentate acyclic and macrocyclic diphosphine ligands (P(2)N(2) and P(2)S(2)) have been synthesized and characterized as potential chelates for Rh(III). METHODS: The coordination complexes [RhCl(2)(L1)]Cl, trans-[RhCl(2)(L2)]PF(6), [Ni(L2)](PF(6))(2), [Ni(L3)](PF(6))(2), [RhCl(2)(L4)]PF(6) and [RhCl(2)(L5)]PF(6) have been synthesized and characterized. In addition, radiochemistry studies of the (105)Rh complexes with the ligands N,N'-bis[2-(diphenylphosphino)phenyl]-1,3-diaminopropane (L1), 4,8-diphenyl-1,11-diaza-4,8-diphosphaundecane (L2), 5,9-diphenyl-5,9-diphospha-2,12-dithiatridecane (L3) and 1,4,8,11-tetraphenyl-4,8-diphospha-1,11-dithiaundecane (L4) are reported, including normal mouse biodistributions of (105)Rh-L2. RESULTS: trans-[RhCl(2)(L2)]PF(6) crystallized in the monoclinic space group P2(1)/c with a = 9.9353(5) Å, b = 9.0929(5) Å, c = 28.689(1) Å, ß = 93.1400(10) deg, Z = 4, R = 0.037 and R(w) = 0.053. [Ni(L2)](PF(6))(2) crystallized in the monoclinic space group P2(1)/c with a = 11.9665(6) Å, b = 14.8903(7) Å, c = 31.148(1) Å, ß = 91.587(1) deg, Z = 8, R = 0.056 and R(w) = 0.083. µ-O(2)SO(2)-[Ni(L5)](2)(PF(6))(2), an unusual sulfate-bridged Ni(II) dimer, crystallized in the triclinic space group P1 bar with a = 15.179(2) Å, b = 15.514(2) Å, c = 16.128(2) Å, α = 105.280(7) deg, ß = 113.074(6) deg, γ = 101.657(8) deg, Z = 2, R = 0.050 and R(w) = 0.072. CONCLUSIONS: Phosphine-containing ligands allowed for lower temperatures and lower ethanol concentrations in the formulations for (105)Rh(L) complexation, but at the expense of higher ligand concentrations.

Optimization, biological evaluation and microPET imaging of copper-64-labeled bombesin agonists, [64Cu-NO2A-(X)-BBN(7-14)NH2], in a prostate tumor xenografted mouse model.

UNLABELLED: Gastrin-releasing peptide receptors (GRPr) are a member of the bombesin (BBN) receptor family. GRPr are expressed in high numbers on specific human cancers, including human prostate cancer. Therefore, copper-64 ((64)Cu) radiolabeled BBN(7-14)NH(2) conjugates could have potential for diagnosis of human prostate cancer via positron-emission tomography (PET). The aim of this study was to produce [(64)Cu-NO2A-(X)-BBN(7-14)NH(2)] conjugates for prostate cancer imaging, where X=pharmacokinetic modifier (beta-alanine, 5-aminovaleric acid, 6-aminohexanoic acid, 8-aminooctanoic acid, 9-aminonanoic acid or para-aminobenzoic acid) and NO2A=1,4,7-triazacyclononane-1,4-diacetic acid [a derivative of NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid)]. METHODS: [(X)-BBN(7-14)NH(2)] Conjugates were synthesized by solid-phase peptide synthesis (SPPS), after which NOTA was added via manual conjugation. The new peptide conjugates were radiolabeled with (64)Cu radionuclide. The receptor-binding affinity was determined in human prostate PC-3 cells, and tumor-targeting efficacy was determined in PC-3 tumor-bearing severely combined immunodeficient (SCID) mice. Whole-body maximum intensity microPET/CT images of PC-3 tumor-bearing SCID mice were obtained 18 h postinjection (pi). RESULTS: Competitive binding assays in PC-3 cells indicated high receptor-binding affinity for the [NO2A-(X)-BBN(7-14)NH(2)] and [(nat)Cu-NO2A-(X)-BBN(7-14)NH(2)] conjugates. In vivo biodistribution studies of the [(64)Cu-NO2A-(X)-BBN(7-14)NH(2)] conjugates at 1, 4 and 24 h pi showed very high uptake of the tracer in GRPr-positive tissue with little accumulation and retention in nontarget tissues. High-quality, high-contrast microPET images were obtained, with xenografted tumors being clearly visible at 18 h pi. CONCLUSIONS: NO2A chelator sufficiently stabilizes copper(II) radiometal under in vivo conditions, producing conjugates with very high uptake and retention in targeted GRPr. Preclinical evaluation of these new peptide conjugates in tumor-bearing mice provides some impetus for clinical evaluation in human patients.

Gold(III) bis-thiosemicarbazonato complexes: synthesis, characterization, radiochemistry and X-ray crystal structure analysis.

INTRODUCTION: A variety of (bis)thiosemicarbazone-based ligand systems have been investigated as chelating agents for Au(III) complexes with potential radiotherapeutic applications. Ligand systems containing an ethyl, propyl or butyl backbone between the two imine N donors have been synthesized to evaluate chelate ring size effects on the resultant Au(III) complex stability at the macroscopic and radiotracer levels. METHODS: The Au(III) complexes were synthesized and characterized by NMR, electrospray ionization mass spectra, elemental analysis and X-ray crystallography. The (198)Au complexes were evaluated in vitro at the tracer level for stability in phosphate-buffered saline at pH 7.4 and 37 degrees C. One of these complexes [(198)Au(3,4-HxTSE)] showed high in vitro stability and was further evaluated in vivo in normal mice. RESULTS: [Au(ATSM)]AuCl(4).2CH(3)OH, (ATSM=diacetyl-bis(N(4)-methylthiosemicarbazone)) H(14)C(8)N(6)O(2)S(2)Cl(4)Au(2).2CH(3)OH, crystallized from methanol in the monoclinic space group P21/n with a=14.7293(13) A, b=7.7432(7) A, c=20.4363(18) A, beta=100.140(2) degrees, V=2294.4 (4) A(3), Z=4; [Au(3,4-HxTSE)]Cl.CH(3)CH(2)OH/AuCl(2), (3,4-HxTSE=3,4-hexanedione-bis(N(4)-ethylthiosemicarbazone)) H(26)C(13.6)N(6)O(0.8)S(2)Cl(1.2)Au(1.2), crystallized from ethanol in the monoclinic space group P21/c with a=10.1990(10) A, b=13.8833(14) A, c=15.1752(15) A, beta=99.353(2) degrees , V=2120.2 (4) A(3), Z=4. CONCLUSIONS: These studies revealed poor stability of the [(198)Au][Au(3,4-HxTSE)](+) complex; however, crystal structure data suggest potential alterations to the ligand backbone may increase stability.