(90)Y and (177)Lu labeling of a DOTA-conjugated vitronectin receptor antagonist useful for tumor therapy

Rapid Radiolabeling for Peptide Radiotracers

Peptide-based radiopharmaceuticals (PRPs) have been developed and introduced into research and clinic diagnostic imaging and targeted radionuclide therapy for more than two decades. In order to efficiently prepare PRPs, some rapid radiolabeling methods have been demonstrated. This chapter presents six common approaches for PRPs radiolabeling with metallic radioisotopes and Fluorine-18.

Homomultimer strategy for improvement of radiolabeled peptides and antibody fragments in tumor targeting

A homomultimeric radioligand is composed of multiple identical ligands connected to the linker and radionuclide to detect a variety of overexpressed receptors on cancer cells. Multimer strategy holds great potential for introducing new radiotracers based on peptide and monoclonal antibody (mAb) derivatives in molecular imaging and therapy. It offers a reliable procedure for the preparation of biological-based targeting with diverse affinities and pharmacokinetics. In this context, we provide a useful summary and interpretation of the main results by a comprehensive look at multimeric radiopharmaceuticals in nuclear oncology. Therefore, there will be explanations for the strategy mechanisms and the main variables affecting the biodistribution results. The discussion is followed by highlights of recent work in the targeting of various types of receptors. The consequences are expressed based on comparing some parameters between monomer and multimer counterparts in each relevant section.

Imaging angiogenesis in patients with head and neck squamous cell carcinomas by [68Ga]Ga-DOTA-E-[c(RGDfK)]2 PET/CT

Purpose

Angiogenesis plays an important role in the growth and metastatic spread of solid tumours and is characterised by the expression of integrins on the cell surface of endothelial cells. Radiolabelled RGD peptides specifically target angiogenesis-related α_vβ₃ integrins, expressed on the activated endothelial cells of sprouting blood vessels. Here, we validated the feasibility of ⁶⁸Ga[Ga]-DOTA-E-[c(RGDfK)]₂ (⁶⁸Ga-RGD) PET/CT to visualise angiogenesis in patients with oral squamous cell carcinoma (OSCC).

Methods

Ten patients with OSCC and scheduled for surgical resection including elective neck dissection received an intravenously administration of ⁶⁸Ga-RGD (42 ± 8 μg; 214 ± 9 MBq). All patients subsequently underwent dynamic (n = 5) or static PET/CT imaging (n = 5) for 60 min or for 4 min/bed position at 30, 60 and 90 min after injection, respectively. Quantitative tracer uptake in tumour lesions was expressed as standardised uptake values (SUV). Additionally, tumour tissue was immunohistochemically stained for α_vβ₃ integrin to assess the expression pattern.

Results

⁶⁸Ga-RGD tumour accumulation was observed in all patients. At 60 min post injection, tumour SUV_max ranged between 4.0 and 12.7. Tracer accumulation in tumour tissue plateaued at 10 min after injection. Uptake in background tissue did not change over time, resulting in tumour-to-muscle tissue of 6.4 ± 0.7 at 60 min post injection.

Conclusions

⁶⁸Ga-RGD PET/CT of α_vβ₃ integrin expression in OSCC patients is feasible with adequate tumour-to-background ratios. It will provide more insight in angiogenesis as a hallmark of the head and neck squamous cell carcinomas’ tumour microenvironment.

Trial registration

https://eudract.ema.europa.eu no. 2015-000917-31

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04766-2) contains supplementary material, which is available to authorized users.

Copper-64 Radiopharmaceuticals for Oncologic Imaging

The positron emitting radionuclide (64)Cu has a radioactive half-life of 12.7 hours. The decay characteristics of (64)Cu allow for PET images that are comparable in quality to those obtained using (18)F. Given the longer radioactive half-life of (64)Cu compared with (18)F and the versatility of copper chemistry, copper is an attractive alternative to the shorter-lived nuclides for PET imaging of peptides, antibodies, and small molecules that may require longer circulation times. This article discusses a number of copper radiopharmaceuticals, such as Cu-ATSM, that have been translated to the clinic and new developments in copper-based radiopharmaceuticals.

Radiolabeled cyclic RGD peptides as radiotracers for tumor imaging

The integrin family comprises 24 transmembrane receptors, each a heterodimeric combination of one of 18α and one of 8β subunits. Their main function is to integrate the cell adhesion and interaction with the extracellular microenvironment with the intracellular signaling and cytoskeletal rearrangement through transmitting signals across the cell membrane upon ligand binding. Integrin α_vβ₃ is a receptor for the extracellular matrix proteins containing arginine–glycine–aspartic (RGD) tripeptide sequence. The α_vβ₃ is generally expressed in low levels on the epithelial cells and mature endothelial cells, but it is highly expressed in many solid tumors. The α_vβ₃ levels correlate well with the potential for tumor metastasis and aggressiveness, which make it an important biological target for development of antiangiogenic drugs, and molecular imaging probes for early tumor diagnosis. Over the last decade, many radiolabeled cyclic RGD peptides have been evaluated as radiotracers for imaging tumors by SPECT or PET. Even though they are called “α_vβ₃-targeted” radiotracers, the radiolabeled cyclic RGD peptides are also able to bind α_vβ₅, α₅β₁, α₆β₄, α₄β₁, and α_vβ₆ integrins, which may help enhance their tumor uptake due to the “increased receptor population.” This article will use the multimeric cyclic RGD peptides as examples to illustrate basic principles for development of integrin-targeted radiotracers and focus on different approaches to maximize their tumor uptake and T/B ratios. It will also discuss important assays for pre-clinical evaluations of the integrin-targeted radiotracers, and their potential applications as molecular imaging tools for noninvasive monitoring of tumor metastasis and early detection of the tumor response to antiangiogenic therapy.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

Synthesis and evaluation of constrained phosphoramidate inhibitors of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a cell-surface enzyme-biomarker that is actively pursued for targeted delivery of imaging and therapeutic agents for prostate cancer. Our lab has developed PSMA inhibitors based on a phosphoramidate scaffold, which has shown both high selectivity for PSMA-positive tumors and rapid clearance in vivo when radiolabeled with (18)F. However, this scaffold exhibits hydrolytic instability under low pH and high temperature conditions, barring the use of other imaging or therapeutic radionuclides such as (68)Ga or (177)Lu. Previous studies in our lab have shown a trend in increasing acid stability as the distance between the phosphoramidate core and the α-carboxylate of the P1 residue is increased. Therefore, a new generation of phosphoramidate inhibitors was developed based on trans-4-hydroxyproline as the P1 residue to restrict the interaction of the α-carboxylate to the phosphoramidate core. These hydroxyproline inhibitors demonstrated comparable IC50 values to earlier generations as well as enhanced thermal and acid stability.

Targeted radionuclide therapy with RAFT-RGD radiolabelled with (90)Y or (177)Lu in a mouse model of αvβ3-expressing tumours

PURPOSE

The αvβ3 integrin plays an important role in tumour-induced angiogenesis, tumour proliferation, survival and metastasis. The tetrameric RGD-based peptide, regioselectively addressable functionalized template-(cyclo-[RGDfK])4 (RAFT-RGD), specifically targets the αvβ3 integrin in vitro and in vivo. The aim of this study was to evaluate the therapeutic potential of RAFT-RGD radiolabelled with β(-) emitters in a nude mouse model of αvβ3 integrin-expressing tumours.

METHODS

Biodistribution and SPECT/CT imaging studies were performed after injection of (90)Y-RAFT-RGD or (177)Lu-RAFT-RGD in nude mice subcutaneously xenografted with αvβ3 integrin-expressing U-87 MG cells. Experimental targeted radionuclide therapy with (90)Y-RAFT-RGD or (177)Lu-RAFT-RGD and (90)Y-RAFT-RAD or (177)Lu-RAFT-RAD (nonspecific controls) was evaluated by intravenous injection of the radionuclides into mice bearing αvβ3 integrin-expressing U-87 MG tumours of different sizes (small or large) or bearing TS/A-pc tumours that do not express αvβ3. Tumour volume doubling time was used to evaluate the efficacy of each treatment.

RESULTS

Injection of 37 MBq of (90)Y-RAFT-RGD into mice with large αvβ3-positive tumours or 37 MBq of (177)Lu-RAFT-RGD into mice with small αvβ3-positive tumours caused significant growth delays compared to mice treated with 37 MBq of (90)Y-RAFT-RAD or 37 MBq of (177)Lu-RAFT-RAD or untreated mice. In contrast, injection of 30 MBq of (90)Y-RAFT-RGD had no effect on the growth of αvβ3-negative tumours.

CONCLUSION

(90)Y-RAFT-RGD and (177)Lu-RAFT-RGD are potent agents targeting αvβ3-expressing tumours for internal targeted radiotherapy.

Biological evaluation of an ornithine-modified (99m)Tc-labeled RGD peptide as an angiogenesis imaging agent

INTRODUCTION

Radiolabeled RGD peptides that specifically target integrin α(ν)β(3) have great potential in early tumor detection through noninvasive monitoring of tumor angiogenesis. Based on previous findings of our group on radiopeptides containing positively charged aminoacids, we developed a new cyclic cRGDfK derivative, c(RGDfK)-(Orn)(3)-CGG. This new peptide availing the polar linker (Orn)(3) and the (99m)Tc-chelating moiety CGG (Cys-Gly-Gly) is appropriately designed for (99m)Tc-labeling, as well as consequent conjugation onto nanoparticles.

METHODS

A tumor imaging agent, c(RGDfK)-(Orn)(3)-[CGG-(99m)Tc], is evaluated with regard to its radiochemical, radiobiological and imaging characteristics.

RESULTS

The complex c(RGDfK)-(Orn)(3)-[CGG-(99m)Tc] was obtained in high radiochemical yield (>98%) and was stable in vitro and ex vivo. It presented identical to the respective, fully analytically characterized (185/187)Re complex retention time in RP-HPLC. In contrary to other RGD derivatives, we showed that the new radiopeptide exhibits kidney uptake and urine excretion due to the ornithine linker. High tumor uptake (3.87±0.48% ID/g at 60 min p.i.) was observed and was maintained relatively high even at 24 h p.i. (1.83±0.05 % ID/g), thus providing well-defined scintigraphic imaging. Accumulation in other organs was negligible. Blocking experiments indicated target specificity for integrin receptors in U87MG glioblastoma cells.

CONCLUSION

Due to its relatively high tumor uptake, renal elimination and negligible abdominal localization, the new (99m)Tc-RGD peptide is considered promising in the field of imaging α(ν)β(3)-positive tumors. However, the preparation of multifunctional SPECT/MRI contrast agents (RGD-conjugated nanoparticles) for dual modality imaging of integrin expressing tumors should be further investigated.

Stroma targeting nuclear imaging and radiopharmaceuticals

Malignant transformation of tumor accompanies profound changes in the normal neighboring tissue, called tumor stroma. The tumor stroma provides an environment favoring local tumor growth, invasion, and metastatic spreading. Nuclear imaging (PET/SPECT) measures biochemical and physiologic functions in the human body. In oncology, PET/SPECT is particularly useful for differentiating tumors from postsurgical changes or radiation necrosis, distinguishing benign from malignant lesions, identifying the optimal site for biopsy, staging cancers, and monitoring the response to therapy. Indeed, PET/SPECT is a powerful, proven diagnostic imaging modality that displays information unobtainable through other anatomical imaging, such as CT or MRI. When combined with coregistered CT data, [(18)F]fluorodeoxyglucose ([(18)F]FDG)-PET is particularly useful. However, [(18)F]FDG is not a target-specific PET tracer. This paper will review the tumor microenvironment targeting oncologic imaging such as angiogenesis, invasion, hypoxia, growth, and homing, and also therapeutic radiopharmaceuticals to provide a roadmap for additional applications of tumor imaging and therapy.

A novel PET tracer for the imaging of αvβ3 and αvβ5 integrins in experimental breast cancer bone metastases

The aim of this study was the evaluation of (68)Ga-DOTA-E-[c(RGDfK)](2) as a novel PET tracer to image αvβ3 and αvβ5 integrins. For this purpose, DOTA-E-[c(RGDfK)](2) was labeled with (68)Ga, which was obtained from a (68)Ge/(68)Ga generator, purified by solid-phase extraction and the radiochemical purity analyzed by radio-RP-HPLC. (68) Ga-DOTA-E-[c(RGDfK)](2) was obtained reproducibly in radiochemical yields of 60 ± 6% and with an excellent radiochemical purity of >99%. In nude rats bearing bone metastases after injection of MDA-MB-231 human breast cancer cells, biodistribution studies were performed to evaluate the accumulation of the radiotracer in selected organs, blood and bone metastases 0.5, 1, 2 and 3 h post injection. A rapid uptake into the bone metastases and rapid blood clearance was observed, resulting in tumor-blood ratios of up to 26.6 (3 h post injection) and tumor-muscle ratios of up to 7.9 (3 h post injection). A blocking experiment with coinjected αvβ3/αvβ5 antagonist showed the tumor uptake to be receptor-specific. In an initial in vivo micro PET evaluation of the tracer using the same animal model, the bone metastasis was clearly visualized. These results suggest that (68)Ga-DOTA-E-[c(RGDfK)](2) is a promising PET tracer suitable for the imaging of αvβ3 and αvβ5 integrins in bone metastases. This novel PET tracer should be further evaluated concerning its usefulness for early detection of bone metastases and monitoring treatment response of these lesions.

Radiolabeled Cyclic RGD Peptides as Radiotracers for Imaging Tumors and Thrombosis by SPECT

The integrin family is a group of transmembrane glycoprotein comprised of 19 α- and 8 β-subunits that are expressed in 25 different α/β heterodimeric combinations on the cell surface. Integrins play critical roles in many physiological processes, including cell attachment, proliferation, bone remodeling, and wound healing. Integrins also contribute to pathological events such as thrombosis, atherosclerosis, tumor invasion, angiogenesis and metastasis, infection by pathogenic microorganisms, and immune dysfunction. Among 25 members of the integrin family, the α(v)β(3) is studied most extensively for its role of tumor growth, progression and angiogenesis. In contrast, the α(IIb)β(3 )is expressed exclusively on platelets, facilitates the intercellular bidirectional signaling ("inside-out" and "outside-in") and allows the aggregation of platelets during vascular injury. The α(IIb)β(3) plays an important role in thrombosis by its activation and binding to fibrinogen especially in arterial thrombosis due to the high blood flow rate. In the resting state, the α(IIb)β(3) on platelets does not bind to fibrinogen; on activation, the conformation of platelet is altered and the binding sites of α(IIb)β(3 )are exposed for fibrinogen to crosslink platelets. Over the last two decades, integrins have been proposed as the molecular targets for diagnosis and therapy of cancer, thrombosis and other diseases. Several excellent review articles have appeared recently to cover a broad range of topics related to the integrin-targeted radiotracers and their nuclear medicine applications in tumor imaging by single photon emission computed tomography (SPECT) or a positron-emitting radionuclide for positron emission tomography (PET). This review will focus on recent developments of α(v)β(3)-targeted radiotracers for imaging tumors and the use of α(IIb)β(3)-targeted radiotracers for thrombosis imaging, and discuss different approaches to maximize the targeting capability of cyclic RGD peptides and improve the radiotracer excretion kinetics from non-cancerous organs. Improvement of target uptake and target-to-background ratios is critically important for target-specific radiotracers.

PET imaging of αvβ₃ integrin expression in tumours with ⁶⁸Ga-labelled mono-, di- and tetrameric RGD peptides

Purpose

Due to the restricted expression of α_vβ₃ in tumours, α_vβ₃ is considered a suitable receptor for tumour targeting. In this study the α_vβ₃-binding characteristics of ⁶⁸Ga-labelled monomeric, dimeric and tetrameric RGD peptides were determined and compared with their ¹¹¹In-labelled counterparts.

Methods

A monomeric (E-c(RGDfK)), a dimeric (E-[c(RGDfK)]₂) and a tetrameric (E{E[c(RGDfK)]₂}₂) RGD peptide were synthesised, conjugated with DOTA and radiolabelled with ⁶⁸Ga. In vitro α_vβ₃-binding characteristics were determined in a competitive binding assay. In vivo α_vβ₃-targeting characteristics of the compounds were assessed in mice with subcutaneously growing SK-RC-52 xenografts. In addition, microPET images were acquired using a microPET/CT scanner.

Results

The IC₅₀ values for the Ga(III)-labelled DOTA-E-c(RGDfK), DOTA-E-[c(RGDfK)]₂ and DOTA-E{E[c(RGDfK)]₂}₂ were 23.9 ± 1.22, 8.99 ± 1.20 and 1.74 ± 1.18 nM, respectively, and were similar to those of the In(III)-labelled mono-, di- and tetrameric RGD peptides (26.6 ± 1.15, 3.34 ± 1.16 and 1.80 ± 1.37 nM, respectively). At 2 h post-injection, tumour uptake of the ⁶⁸Ga-labelled mono-, di- and tetrameric RGD peptides (3.30 ± 0.30, 5.24 ± 0.27 and 7.11 ± 0.67%ID/g, respectively) was comparable to that of their ¹¹¹In-labelled counterparts (2.70 ± 0.29, 5.61 ± 0.85 and 7.32 ± 2.45%ID/g, respectively). PET scans were in line with the biodistribution data. On all PET scans, the tumour could be clearly visualised.

Conclusion

The integrin affinity and the tumour uptake followed the order of DOTA-tetramer > DOTA-dimer > DOTA-monomer. The ⁶⁸Ga-labelled tetrameric RGD peptide has excellent characteristics for imaging of α_vβ₃ expression with PET.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-010-1615-x) contains supplementary material, which is available to authorized users.

Enhancement of reaction conditions for the radiolabelling of DOTA-peptides with high activities of yttrium-90

Peptide receptor radionuclide therapy (PRRT) has recently expanded due to radiolabelling of DOTA-peptides, such as the somatostatin analogues [DOTA(0), Tyr(3)]octreotate (DOTATATE). The achievement of high specific activities during procedures has been indicated as the critical factor to consent effective therapy. Several radiochemical factors may negatively impact reaction procedures such as pH, temperature and time of reaction. Our study was undertaken to explore the influence of radiochemical parameters, such as time of incubation, on reaction kinetics during the radiolabelling of DOTATATE with (90)Y.

METHODS

Forty-five radiolabelling procedures were carried out using small volumes of yttrium-90, typically 60-78 μL. At nearly constant pH and temperature two different settings of radiolabelling procedures were implemented, removing the products from the heating water bath approximately after 30 min (group E, early; n=20) and after 39 min (group L, later; n=25). Quality controls were performed by means of both high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC).

RESULTS

Reaction kinetics for (90)Y were found to a provide suitable percentage of incorporation at pH 4.5 for both groups. Reaction temperature was not different between groups E and L. A significant difference was found between the two groups in radiochemical yield, which was 95.6% ± 0.8 for group E and 98.2% ± 1.1 for group L (p<0.0001). The specific activity of the final product was 46.9 MBq/nmol.

CONCLUSION

In order to achieve optimal specific activities, pH, temperature and time of reaction necessitate careful evaluation and setting. A statistically significant difference in labelling yield was found between a set of procedures completed at 39 min as compared to that executed at 30 min, keep the reaction pH and temperature constant.

Evaluation of 111In-labeled cyclic RGD peptides: tetrameric not tetravalent

This report presents the synthesis and evaluation of (111)In(DOTA-6G-RGD(4)) (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracetic acid; 6G-RGD(4) = E{G(3)-E[G(3)-c(RGDfK)](2)}(2) and G(3) = Gly-Gly-Gly), (111)In(DOTA-RGD(4)) (RGD(4) = E{E[c(RGDfK)](2)}(2)) and (111)In(DOTA-3G-RGD(2)) (3G-RGD(2) = G(3)-E[G(3)-c(RGDfK)](2)) as new radiotracers for imaging integrin alpha(v)beta(3)-positive tumors. The IC(50) values of DOTA-6G-RGD(4), DOTA-RGD(4), and DOTA-3G-RGD(2) were determined to be 0.4 +/- 0.1, 1.4 +/- 0.1 and 1.1 +/- 0.1 nM against (125)I-c(RGDyK) bound to integrin alpha(v)beta(3)-positive U87MG human glioma cells. (111)In(DOTA-6G-RGD(4)), (111)In(DOTA-RGD(4)), and (111)In(DOTA-3G-RGD(2)) were prepared by reacting (111)InCl(3) with the respective DOTA conjugate in NH(4)OAc buffer (100 mM, pH = 5.5). Radiolabeling could be completed by heating the reaction mixture at 100 degrees C for 15-20 min. The specific activity was approximately 1850 MBq/micromol for (111)In(DOTA-3G-RGD(2)) and approximately 1480 MBq/micromol for (111)In(DOTA-6G-RGD(4)). The athymic nude mice bearing U87MG human glioma xenografts were used to evaluate tumor uptake and excretion kinetics of (111)In(DOTA-6G-RGD(4)), (111)In(DOTA-RGD(4)), and (111)In(DOTA-3G-RGD(2)). The results from both the integrin alpha(v)beta(3) binding assay and biodistribution studies suggest that the tetrameric cyclic RGD peptides, such as RGD(4) and 6G-RGD(4), are most likely bivalent in binding to the integrin alpha(v)beta(3). Both (111)In(DOTA-6G-RGD(4)) and (111)In(DOTA-RGD(4)) had significantly higher tumor uptake than (111)In(DOTA-3G-RGD(2)) at 24-72 h postinjection due to the extra RGD motifs in RGD(4) and 6G-RGD(4). (111)In(DOTA-3G-RGD(2)) had very little metabolism, while (111)In(DOTA-6G-RGD(4)) had significant metabolism during its excretion via both renal and hepatobiliary routes over the 2 h period, probably due to its much larger size. The combination of high tumor uptake with long tumor retention suggests that their corresponding (90)Y and (177)Lu analogues M(DOTA-6G-RGD(4)) (M = (90)Y and (177)Lu) might be useful as therapeutic radiotracers for treatment of integrin alpha(v)beta(3)-positive solid tumors.

Radiolabeled cyclic RGD peptides as integrin alpha(v)beta(3)-targeted radiotracers: maximizing binding affinity via bivalency

Integrin alpha(v)beta(3) plays a significant role in tumor angiogenesis and is a receptor for the extracellular matrix proteins with the exposed arginine-glycine-aspartic (RGD) tripeptide sequence. These include vitronectin, fibronectin, fibrinogen, lamin, collagen, Von Willibrand's factor, osteoponin, and adenovirus particles. Integrin alpha(v)beta(3) is expressed at low levels on epithelial cells and mature endothelial cells, but it is overexpressed on the activated endothelial cells of tumor neovasculature and some tumor cells. The restricted expression of integrin alpha(v)beta(3) during tumor growth, invasion, and metastasis presents an interesting molecular target for both early detection and treatment of rapidly growing solid tumors. Over the past decade, many radiolabeled linear and cyclic RGD peptide antagonists have been evaluated as integrin alpha(v)beta(3)-targeted radiotracers. Significant progress has been made on their use for imaging integrin alpha(v)beta(3)-positive tumors by SPECT or PET. Among the radiotracers evaluated in preclinical tumor-bearing models, [18F]Galacto-RGD (2-[18F]fluoropropanamide c(RGDfK(SAA); SAA = 7-amino-L-glyero-L-galacto-2,6-anhydro-7-deoxyheptanamide) and [18F]-AH111585 are currently under clinical investigation for visualization of integrin alpha(v)beta(3) expression in cancer patients. However, their low tumor uptake, high cost, and lack of preparative modules for routine radiosynthesis will limit their continued clinical application. Thus, there is a continuing need for more efficient integrin alpha(v)beta(3)-targeted radiotracers that are readily prepared from a kit formulation without further postlabeling purification. This article will focus on different approaches to maximize the targeting capability of cyclic RGD peptides and to improve the radiotracer excretion kinetics from noncancerous organs. Improvement of tumor uptake and tumor-to-background ratios is important for early detection of integrin alpha(v)beta(3)-positive tumors and/or noninvasive monitoring of therapeutic efficacy of antiangiogenic therapy.

Radionuclide imaging of tumor angiogenesis

Angiogenesis is a multistep process regulated by pro- and antiangiogenic factors. In order to grow and metastasize, tumors need a constant supply of oxygen and nutrients. For growth beyond 1-2 mm in size, tumors are dependent on angiogenesis. Inhibition of angiogenesis is a new cancer treatment strategy that is now widely investigated clinically. Researchers have begun to search for objective measures that indicate pharmacologic responses to antiangiogenic drugs. Therefore, there is a great interest in techniques to visualize angiogenesis in growing tumors noninvasively. Several markers have been described that are preferentially expressed on newly formed blood vessels in tumors (alpha(v)beta(3) integrin, vascular endothelial growth factor, and its receptor, prostate-specific membrane antigen) and in the extracellular matrix surrounding newly formed blood vessels (extra domain B of fibronectin, Tenascin-C, matrix metalloproteinases, and Robo-4). Several ligands targeting these markers have been tested as a radiotracer for imaging angiogenesis in tumors. The potential of some of these tracers, such as radiolabeled cyclic RGD peptides and radiolabeled anti-PSMA antibodies, has already been tested in cancer patients, while for markers such as Robo-4, the ligand has not yet been identified. In this review, an overview on the currently used nuclear imaging probes for noninvasive visualization of tumor angiogenesis is given.

2-Mercaptoacetylglycylglycyl (MAG2) as a bifunctional chelator for 99mTc-labeling of cyclic RGD dimers: effect of technetium chelate on tumor uptake and pharmacokinetics

This report describes the synthesis of MAG(2)-PEG(4)-E[c(RGDfK)](2) (MAG(2)-P-RGD(2): MAG(2) = S-benzoylmercaptoacetylglycylglycyl; PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) and MAG(2)-PEG(4)-E[PEG(4)-c(RGDfK)](2) (MAG(2)-3P-RGD(2)), and the evaluation of (99m)TcO(MAG(2)-P-RGD(2)) and (99m)TcO(MAG(2)-3P-RGD(2)) as new radiotracers for tumor imaging in the athymic nude mice bearing U87MG human glioma xenografts. We found that MAG(2) is such an efficient bifunctional chelating agent that (99m)TcO(MAG(2)-P-RGD(2)) and (99m)TcO(MAG(2)-3P-RGD(2)) coul d be prepared in high yield (>90%) with high specific activity (∼5 Ci/μmol) using a kit formulation. (99m)TcO(MAG(2)-P-RGD(2)) and (99m)TcO(MAG(2)-3P-RGD(2)) have very high solution stability in the kit matrix. Biodistribution data in athymic nude mice bearing U87MG human glioma xenografts indicated that replacing the highly charged [(99m)Tc(HYNIC = 6-hydrazinonicotinyl and TPPTS = trisodium triphenylphosphine-3,3',3''-trisulfonate) with smaller (99m)TcO(MAG(2)) resulted in a significant increase in the radiotracer uptake in the tumor and normal organs most likely due to the higher lipophilicity of (99m)TcO(MAG(2)-3P-RGD(2)) (log P = -3.15 ± 0.10) than that for [(99m)Tc(HYNIC-3P-RGD(2))(tricine)(TPPTS)] ((99m)Tc-3P-RGD(2): log P = -3.96 ± 0.05). Even though (99m)TcO(MAG(2)-3P-RGD(2)) has better tumor uptake (15.36 ± 2.17 %ID/g at 60 min postinjection (p.i.)) than (99m)Tc-3P-RGD(2) (9.15 ± 2.13 %ID/g at 60 min p.i.), its tumor-to-background (T/B) ratios (tumor/blood = 13.52 ± 4.57; tumor/liver = 4.25 ± 0.88; tumor/lung = 3.17 ± 0.60; and tumor/muscle = 8.34 ± 2.34) are not as good as those of (99m)Tc-3P-RGD(2) (tumor/blood = 36.0 ± 11.5; tumor/liver = 5.14 ± 1.46; tumor/lung = 4.36 ± 0.54; and tumor/muscle = 13.70 ± 2.21) at 60 min p.i. On the basis of these results, we believe that (99m)Tc-3P-RGD(2) remains a better radiotracer because of its higher T/B ratios.

Improving tumor-targeting capability and pharmacokinetics of (99m)Tc-labeled cyclic RGD dimers with PEG(4) linkers

This report describes the synthesis of two cyclic RGD (Arg-Gly-Asp) conjugates, HYNIC-2PEG(4)-dimer (HYNIC = 6-hydrazinonicotinyl; 2PEG(4)-dimer = E[PEG(4)-c(RGDfK)](2); and PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) and HYNIC-3PEG(4)-dimer (3PEG(4)-dimer = PEG(4)-E[PEG(4)-c(RGDfK)](2)), and evaluation of their (99m)Tc complexes [(99m)Tc(HYNIC-2PEG(4)-dimer)(tricine)(TPPTS)] ((99m)Tc-2PEG(4)-dimer: TPPTS = trisodium triphenylphosphine-3,3',3''-trisulfonate) and [(99m)Tc(HYNIC-3PEG(4)-dimer)(tricine)(TPPTS)] ((99m)Tc-3PEG(4)-dimer) as novel radiotracers for imaging integrin alpha(v)beta(3) expression in athymic nude mice bearing U87MG glioma and MDA-MB-435 breast cancer xenografts. The integrin alpha(v)beta(3) binding affinities of RGD peptides were determined by competitive displacement of (125)I-c(RGDyK) on U87MG glioma cells. It was found that the two PEG(4) linkers between RGD motifs in HYNIC-2PEG(4)-dimer (IC(50) = 2.8 +/- 0.5 nM) and HYNIC-3PEG(4)-dimer (IC(50) = 2.4 +/- 0.7 nM) are responsible for their higher integrin alpha(v)beta(3) binding affinity than that of HYNIC-PEG(4)-dimer (PEG(4)-dimer = PEG(4)-E[c(RGDfK)](2); IC(50) = 7.5 +/- 2.3 nM). Addition of extra PEG(4) linker in HYNIC-3PEG(4)-dimer has little impact on integrin alpha(v)beta(3) binding affinity. (99m)Tc-2PEG(4)-dimer and (99m)Tc-3PEG(4)-dimer were prepared in high yield with >95% radiochemical purity and the specific activity of >10 Ci/mumol. Biodistribution studies clearly demonstrated that PEG(4) linkers are particularly useful for improving the tumor uptake and clearance kinetics of (99m)Tc-2PEG(4)-dimer and (99m)Tc-3PEG(4)-dimer from noncancerous organs. It was also found that there was a linear relationship between the tumor size and radiotracer tumor uptake expressed as %ID (percentage of the injected dose) in U87MG glioma and MDA-MB-435 breast tumor models. The blocking experiment showed that the tumor uptake of (99m)Tc-2PEG(4)-dimer is integrin alpha(v)beta(3)-mediated. In the metabolism study, (99m)Tc-2PEG(4)-dimer had high metabolic stability during its excretion from renal and hepatobiliary routes. (99m)Tc-3PEG(4)-dimer also remained intact during thee excretion from the renal route, but, had approximately 30% metabolism during the excretion from the hepatobiliary route. Planar imaging studies in U87MG glioma and MDA-MB-435 breast tumor models showed that the tumors of approximately 5 mm in diameter could be readily visualized with excellent contrast. Thus, (99m)Tc-3PEG(4)-dimer is a very promising radiotracer for the early detection of integrin alpha(v)beta(3)-positive tumors, and may have the potential for noninvasive monitoring of tumor growth or treatment efficacy.

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.

Linker effects on biological properties of 111In-labeled DTPA conjugates of a cyclic RGDfK dimer

In this report, we present in vitro and in vivo evaluation of three 111 In-labeled DTPA conjugates of a cyclic RGDfK dimer: DTPA-Bn-SU016 (SU016 = E[c(RGDfK)] 2; DTPA-Bn = 2-( p-isothioureidobenzyl)diethylenetriaminepentaacetic acid), DTPA-Bn-E-SU016 ( E = glutamic acid) and DTPA-Bn-Cys-SU016 (Cys = cysteic acid). The integrin alpha vbeta 3 binding affinities of SU016, DTPA-Bn-SU016, DTPA-Bn-E-SU016, and DTPA-Bn-Cys-SU016 were determined to be 5.0 +/- 0.7 nM, 7.9 +/- 0.6 nM, 5.8 +/- 0.6 nM, and 6.9 +/- 0.9 nM, respectively, against 125 I-c(RGDyK) in binding to integrin alpha vbeta3, suggesting that E or Cys residue has little effect on the integrin alpha vbeta3 affinity of E[c(RGDfK)] 2. It was also found that the 111 In-labeling efficiency of DTPA-Bn-SU016 and DTPA-Bn-E-SU016 is 3-5 times better than that of DOTA analogues due to fast chelation kinetics and high-yield 111 In-labeling under mild conditions (e.g., room temperature). Biodistribution studies were performed using BALB/c nude mice bearing U87MG human glioma xenografts. 111 In-DTPA-Bn-SU016, 111 In-DTPA-Bn-E-SU016, and 111 In-DTPA-Bn-Cys-SU016 all displayed rapid blood clearance. Their tumor uptake was comparable between 0.5 and 4 h postinjection (p.i.) within experimental error. 111 In-DTPA-Bn-E-SU016 had a significantly lower ( p < 0.01) kidney uptake than 111 In-DTPA-Bn-SU016 and 111 In-DTPA-Bn-Cys-SU016. The liver uptake of 111 In-DTPA-Bn-SU016 was 1.69 +/- 0.18% ID/g at 24 h p.i., while the liver uptake values of 111 In-DTPA-Bn-E-SU016 and 111 In-DTPA-Bn-Cys-SU016 were 0.55 +/- 0.11% ID/g and 0.79 +/- 0.15% ID/g at 24 h p.i., respectively. Among the three 111 In radiotracers evaluated in this study, 111 In-DTPA-Bn-E-SU016 has the lowest liver and kidney uptake and the best tumor/liver and tumor/kidney ratios. Results from metabolism studies indicated that there is little metabolism (<10%) for three 111 In radiotracers at 1 h p.i. Imaging data showed that tumors can be clearly visualized at 4 h p.i. with good contrast in the tumor-bearing mice administered with 111 In-DTPA-Bn-E-SU016. It is concluded that using a glutamic acid linker can significantly improve excretion kinetics of the 111 In-labeled E[c(RGDfK)] 2 from liver and kidneys.

Effects of linker variation on the in vitro and in vivo characteristics of an 111In-labeled RGD peptide

INTRODUCTION

Due to the selective expression of the alpha(v)beta3 integrin in tumors, radiolabeled arginine-glycine-aspartic acid (RGD) peptides are attractive candidates for tumor targeting. Minor modifications of these peptides could have a major impact on in vivo characteristics. In this study, we systematically investigated the effects of linker modification between two cyclic RGD sequences and DOTA (1,4,7,10-tetraazadodecane-N,N',N",N'''-tetraacetic acid) on the in vitro and in vivo characteristics of the tracer.

METHODS

A dimeric RGD peptide was synthesized and conjugated either directly with DOTA or via different linkers: PEG4 (polyethylene glycol), glutamic acid or lysine. The RGD peptides were radiolabeled with 111In, and their in vitro and in vivo alpha(v)beta3-binding characteristics were determined.

RESULTS

LogP values varied between -2.82+/-0.06 and -3.95+/-0.33. The IC50 values for DOTA-E-[c(RGDfK)]2, DOTA-PEG4-E-[c(RGDfK)]2, DOTA-E-E-[c(RGDfK)]2 and DOTA-K-E-[c(RGDfK)]2 were comparable. Two hours after injection, the tumor uptakes of the 111In-labeled compounds were not significantly different. The kidney accumulation of [111In]-DOTA-K-E-[c(RGDfK)]2 [4.05+/-0.20% of the injected dose per gram (ID/g)] was significantly higher as compared with that of [111In]-DOTA-E-[c(RGDfK)]2 (2.63+/-0.19% ID/g; P<.05) as well as that of [111In]-DOTA-E-E-[c(RGDfK)]2 (2.16+/-0.21% ID/g; P<.01). The liver uptake of [111In]-DOTA-E-E-[c(RGDfK)]2 (2.12+/-0.09% ID/g) was significantly higher as compared with that of [111In]-DOTA-E-[c(RGDfK)]2 (1.64+/-0.1% ID/g; P<.05) as well as that of [111In]-DOTA-K-E-[c(RGDfK)]2 (1.52+/-0.04% ID/g; P<.01).

CONCLUSIONS

Linker variation did not affect affinity for alpha(v)beta3 and tumor uptake. Insertion of lysine caused enhanced kidney retention; that of glutamic acid also resulted in enhanced retention in the kidneys. PEG4 appeared to be the most suitable linker as compared with glutamic acid and lysine because it has the highest tumor-to-blood ratio and the lowest uptake in the kidney and liver.

Synthesis of DOTA-conjugated multivalent cyclic-RGD peptide dendrimers via 1,3-dipolar cycloaddition and their biological evaluation: implications for tumor targeting and tumor imaging purposes

This report describes the design and synthesis of a series of alpha(V)beta(3) integrin-directed monomeric, dimeric and tetrameric cyclo[Arg-Gly-Asp-d-Phe-Lys] dendrimers using "click chemistry". It was found that the unprotected N-epsilon-azido derivative of cyclo[Arg-Gly-Asp-d-Phe-Lys] underwent a highly chemoselective conjugation to amino acid-based dendrimers bearing terminal alkynes using a microwave-assisted Cu(I)-catalyzed 1,3-dipolar cycloaddition. The alpha(V)beta(3) binding characteristics of the dendrimers were determined in vitro and their in vivoalpha(V)beta(3) targeting properties were assessed in nude mice with subcutaneously growing human SK-RC-52 tumors. The multivalent RGD-dendrimers were found to have enhanced affinity toward the alpha(V)beta(3) integrin receptor as compared to the monomeric derivative as determined in an in vitro binding assay. In case of the DOTA-conjugated (111)In-labeled RGD-dendrimers, it was found that the radiolabeled multimeric dendrimers showed specifically enhanced uptake in alpha(V)beta(3) integrin expressing tumors in vivo. These studies showed that the tetrameric RGD-dendrimer had better tumor targeting properties than its dimeric and monomeric congeners.

Improved targeting of the alpha(v)beta (3) integrin by multimerisation of RGD peptides

PURPOSE

The integrin alpha(v)beta(3) is expressed on sprouting endothelial cells and on various tumour cell types. Due to the restricted expression of alpha(v)beta(3) in tumours, alpha(v)beta(3) is considered a suitable receptor for tumour targeting. In this study the alpha(v)beta(3) binding characteristics of an (111)In-labelled monomeric, dimeric and tetrameric RGD analogue were compared.

METHODS

A monomeric (E-c(RGDfK)), dimeric (E-[c(RGDfK)](2)), and tetrameric (E{E[c(RGDfK)](2)}(2)) RGD peptide were synthesised, conjugated with DOTA and radiolabelled with (111)In. In vitro alpha(v)beta(3) binding characteristics were determined in a competitive binding assay. In vivo alpha(v)beta(3) targeting characteristics of the compounds were assessed in mice with SK-RC-52 xenografts.

RESULTS

The IC(50) values for DOTA-E-c(RGDfK), DOTA-E-[c(RGDfK)](2), and DOTA-E{E[c(RGDfK)](2)}(2)were 120 nM, 69.9 nM and 19.6 nM, respectively. At all time points, the tumour uptake of the dimer was significantly higher as compared to that of the monomer. At 8 h p.i., tumour uptake of the tetramer (7.40+/-1.12%ID/g) was significantly higher than that of the monomer (2.30+/-0.34%ID/g), p<0.001, and the dimer (5.17+/-1.22%ID/g), p<0.05. At 24 h p.i., the tumour uptake was significantly higher for the tetramer (6.82+/-1.41%ID/g) than for the dimer (4.22+/-0.96%ID/g), p<0.01, and the monomer (1.90+/-0.29%ID/g), p<0.001.

CONCLUSION

Multimerisation of c(RGDfK) resulted in enhanced affinity for alpha(v)beta(3) as determined in vitro. Tumour uptake of a tetrameric RGD peptide was significantly higher than that of the monomeric and dimeric analogues, presumably owing to the enhanced statistical likelihood for rebinding to alpha(v)beta(3).

Evaluation of a new biotin-DOTA conjugate for pretargeted antibody-guided radioimmunotherapy (PAGRIT)

PURPOSE

A novel biotin-DOTA conjugate (r-BHD: reduced biotinamidohexylamine-DOTA) was investigated in order to provide an efficient pretargeted antibody-guided radioimmunotherapy (PAGRIT) application. Preclinical and clinical results are described.

METHODS

(90)Y and (177)Lu were used to label r-BHD. The effect of pH and a wide range of specific activities were studied. Radiolabelled r-BHD was tested for affinity towards avidin and for stability in saline or in human serum with and without ascorbic acid. Pharmacokinetic data were collected and organ biodistribution evaluated in a tumour-bearing pretargeted animal model. A pilot study was performed in a metastatic melanoma patient and dosimetry was estimated.

RESULTS

High radiochemical purity (>99%) was routinely achieved with (90)Y or (177)Lu in sodium acetate buffer (1.0 M, pH 5.0) at a specific activity of 2.6 MBq/nmol. Both (90)Y- and (177)Lu-r-BHD were also prepared at higher specific activities. Radiolabelled r-BHD was stable up to 96 h in human serum and saline with the addition of ascorbic acid. The structural modifications proposed for the r-BHD stabilised it against enzymatic degradation while retaining high binding affinity for avidin. Renal clearance appeared to be the main route of excretion in animals, and high tumour uptake was observed in the pretargeted animals. The patient study showed a total body clearance of approximately 85% in 24 h, with a kidney absorbed dose of 1.5 mGy/MBq. Tumour uptake was rapid and the calculated dose to a 10-mm tumour lesion was approximately 12 mGy/MBq.

CONCLUSION

These results indicate that the new biotin-DOTA conjugate may be a suitable candidate for pretargeting trials.

RGD-based strategies for selective delivery of therapeutics and imaging agents to the tumour vasculature

During the past decade, RGD-peptides have become a popular tool for the targeting of drugs and imaging agents to alphavbeta3-integrin expressing tumour vasculature. RGD-peptides have been introduced by recombinant means into therapeutic proteins and viruses. Chemical means have been applied to couple RGD-peptides and RGD-mimetics to liposomes, polymers, peptides, small molecule drugs and radiotracers. Some of these products show impressive results in preclinical animal models and a RGD targeted radiotracer has already successfully been tested in humans for the visualization of alphavbeta3-integrin, which demonstrates the feasibility of this approach. This review will summarize the structural requirements for RGD-peptides and RGD-mimetics as ligands for alphavbeta3. We will show how they have been introduced in the various types of constructs by chemical and recombinant techniques. The importance of multivalent RGD-constructs for high affinity binding and internalization will be highlighted. Furthermore the in vitro and in vivo efficacy of RGD-targeted therapeutics and diagnostics reported in recent years will be reviewed.

αv Integrin Affinity/Specificity and Antiangiogenesis Effect of a Novel Tetraaza Cyclic Peptide Derivative, SU015, in Various Species

The present study was undertaken to define the alpha v beta3 and alpha v beta5 binding potency and specificity of SU015, an integrin antagonist. SU015 inhibited alpha v beta3-mediated human umbilical vein endothelial cell or 293/beta3-transfected CHO cell adhesion to fibrinogen, with IC50 values of 0.21 +/- 0.11 muM and 0.32 +/- 0.02 microM. SU015 demonstrated comparable affinity to alpha v beta5 as compared with alpha v beta3 affinity, as well as a relatively high degree of specificity for human alpha v beta3- and alpha v beta5-mediated functions, as compared with other human integrins, including alphaIIbbeta3 (IC50 >100 microM), alpha5/beta1 (IC50 >100 microM), and alpha4/beta1 (IC50 >100 microM). SU015 demonstrated different degrees of species specificity in blocking alpha v beta3-mediated cellular adhesion, with relatively higher affinity to monkey (IC50 = 0.10 microM) and dog (IC50 = 1.30 microM) endothelial or smooth muscle cell alpha v beta3-mediated adhesion. Additionally, SU015 demonstrated a high degree of alpha v beta3 and alpha v beta5 specificity as compared with alpha4beta1-, alpha5beta1-, or alpha IIb beta3-mediated binding in the above species. In conclusion, SU015 is an alpha v beta3 and alpha v beta5 antagonist with relatively higher potency and specificity as compared with alpha IIb beta3, alpha5beta1, or alpha4beta1 integrins. Additionally, comparable alpha v beta3 and alpha v beta5 affinity for SU015 was demonstrated with human and monkey endothelial cells. These data also suggest that this bicyclic RGD analogue linked to a linker at the bottom leaves the RGD at the top available for binding and allows for conjugation with radioisotope for imaging and radiotherapy.

Improved tumor targeting of radiolabeled RGD peptides using rapid dose fractionation

Arginine-glycine-aspartic acid (RGD) peptides preferentially bind to alphavbeta3 integrin, an integrin expressed on newly formed endothelial cells and on various tumor cells. When labeled with beta-emitting radionuclides, these peptides can be used for peptide-receptor radionuclide therapy of malignant tumors. These studies aimed to investigate whether tumor targeting and tumor therapy could be optimized by dose fractionation. The RGD-peptide DOTA-E-[c(RGDfK)]2 was labeled with 111In for biodistribution experiments and with 90Y for therapy experiments. In mice with NIH:OVCAR-3 ovarian carcinoma xenografts, optimal tumor uptake was obtained at peptide doses up to 1.0 microg (4.8 %ID/g). A peptide dose of 5 microg, required to administer the maximum tolerable dose (MTD) 90Y-DOTA-E-[c(RGDfK)]2, was administered as 5 portions of 1.0 microg. Tumor uptake of the fifth portion was significantly higher than that of the single 5.0 microg portion (3.3 %ID/g versus 2.1 %ID/g). The therapeutic efficacy of 37 MBq 90Y-DOTA-E-[c(RGDfK)]2 (1 x 5.0 microg) was compared with that of 37 MBq administered in five equal portions (5 x 1.0 microg). No difference in tumor growth between the fractionated and the nonfractionated therapy was observed. In conclusion, dose fractionation resulted in higher radiation doses. However, therapeutic efficacy of the radiolabeled peptide was not significantly improved by dose fractionation.

Synthesis, Characterization, and Structures of Indium In(DTPA-BA2) and Yttrium Y(DTPA-BA2)(CH3OH) Complexes (BA = Benzylamine): Models for111In- and90Y-Labeled DTPA-Biomolecule Conjugates

To explore structural differences in In3+, Y3+, and Lu3+ chelates, we prepared M(DTPA-BA2) complexes (M = In, Y, and Lu; DTPA-BA2 = N,N' '-bis(benzylcarbamoylmethyl)diethylenetriamine-N,N',N' '-triacetic acid) by reacting the trisodium salt of DTPA-BA2 with 1 equiv of metal chloride or nitrate. All three complexes have been characterized by elemental analysis, HPLC, IR, ES-MS, and NMR (1H and 13C) methods. ES-MS spectral and elemental analysis data are consistent with the proposed formula for M(DTPA-BA2) (M = In, Y, and Lu) and have been confirmed by the X-ray crystal structures of both In(DTPA-BA2) x 2H2O and Y(DTPA-BA2)(CH3OH) complexes. By a reversed-phase HPLC method, it was found that In(DTPA-BA2) is more hydrophilic than M(DTPA-BA2) (M = Y and Lu), most likely due to the dissociation of the two carbonyl oxygen donors in solution. The X-ray crystal structure of In(DTPA-BA2) revealed a rare example of an eight-coordinated In3+ complex with DTPA-BA2 bonding to the In3+ in a distorted square antiprism coordination geometry. Both benzylamine groups are in the trans position relative to the acetate-chelating arm that is attached to the central N atom. The Y3+ in Y(DTPA-BA2)(CH3OH) is nine-coordinated with an octadentate DTPA-BA2 and a methanol oxygen. The coordination geometry is best described as a tricapped trigonal prism. One benzylamine group is trans and the other cis to the acetate-chelating arm that is attached to the central N atom. All three M(DTPA-BA2) complexes (M = In, Y, and Lu) exist as at least three isomers in solution (approximately 10 mM), as shown by the presence of 6-8 overlapped 1H NMR signals from the methylene hydrogens of the benzylamine groups. The coordinated DTPA-BA2 remains rigid even at temperatures > 85 degrees C. The exchange rate between different isomers in M(DTPA-BA2) (M = In, Y, and Lu) is relatively slow at high concentrations (> 1.0 mM), but it is fast due to the partial dissociation and rapid interconversion of different isomers at lower concentrations ( approximately 10 mircroM). It is not surprising that M(DTPA-BA2) complexes (M = In, Y, and Lu) appear as a single peak in their respective HPLC chromatogram.

Micro-PET Imaging of α v β 3 -Integrin Expression with 18 F-Labeled Dimeric RGD Peptide

The alphav integrins, which act as cell adhesion molecules, are closely involved with tumor invasion and angiogenesis. In particular, alphavbeta3 integrin, which is specifically expressed on proliferating endothelial cells and tumor cells, is a logical target for development of a radiotracer method to assess angiogenesis and anti-angiogenic therapy. In this study, a dimeric cyclic RGD peptide E[c(RGDyK)]2 was labeled with 18F (t(1/2) = 109.7 min) by using a prosthetic 4-[18F]fluorobenzoyl moiety to the amino group of the glutamate. The resulting [18F]FB-E[c(RGDyK)]2, with high specific activity (200-250 GBq/micromol at the end of synthesis), was administered to subcutaneous U87MG glioblastoma xenograft models for micro-PET and autoradiographic imaging as well as direct tissue sampling to assess tumor targeting efficacy and in vivo kinetics of this PET tracer. The dimeric RGD peptide demonstrated significantly higher tumor uptake and prolonged tumor retention in comparison with a monomeric RGD peptide analog [18F]FB-c(RGDyK). The dimeric RGD peptide had predominant renal excretion, whereas the monomeric analog was excreted primarily through the biliary route. Micro-PET imaging 1 hr after injection of the dimeric RGD peptide exhibited tumor to contralateral background ratio of 9.5 +/- 0.8. The synergistic effect of polyvalency and improved pharmacokinetics may be responsible for the superior imaging characteristics of [18F]FB-E[c(RGDyK)]2.

Synthesis, characterization, and X-ray crystal structure of In(DOTA-AA) (AA = p-aminoanilide): a model for 111In-labeled DOTA-biomolecule conjugates

This report describes the synthesis and structural characterization of the indium complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-aminoanilide) (DOTA-AA), a model compound for (111)In-labeled DOTA-biomolecule conjugates. In(DOTA-AA) was prepared by reacting DOTA-AA with 1 equiv of InCl(3) in 0.5 M ammonium acetate buffer (pH approximately 6). It was characterized by spectroscopic methods (IR, ES-MS, and (1)H NMR), elemental analysis, and X-ray crystallography. For comparison purposes, we also prepared the complex Y(DOTA-AA). ES-MS and (1)H NMR data are consistent with the proposed structure. HPLC analysis using a reversed phase method shows that the retention time of In(DOTA-AA) is approximately 2.0 min shorter than that of Y(DOTA-AA), demonstrating that In(DOTA-monoamide) is more hydrophilic than Y(DOTA-monoamide). In the solid state, In(DOTA-AA) has a twisted square antiprismatic coordination geometry with all eight donor atoms (N(4)O(4)) bonded to the In center. The average In-N and In-O distances are almost identical to those of Y-N and Y-O bonds found in Y(DOTA-d-Phe-NH(2)) even though the ionic radius of Y(3+) is much longer than that of In(3+). It seems that In(3+) does not fit the coordination cavity of DOTA-AA perfectly. The (1)H NMR data clearly demonstrated that In(DOTA-AA) becomes fluxional at room temperature, most likely due to dissociation of the acetamide-oxygen, rotation of acetate chelating arms, and inversion of ethylenic groups of the macrocyclic ring. Results from this study and our previous studies (Liu, S.; Pietryka, J.; Ellars C. E.; Edwards D. S. Bioconjugate Chem. 2002, 13, 902-913) suggest that the In(3+) complex of DOTA-monoamide in the solid state might be different from that in solution due to dissociation of the carbonyl-oxygen donor. Although Y(3+) and In(3+) complexes of DOTA-monoamide are both eight-coordinate in the solid state, the difference in their solution structures is most likely responsible for their difference in lipophilicity.

Production logistics of 177Lu for radionuclide therapy

Owing to its favourable decay characteristics 177Lu [T(1/2)=6.71 d, Ebeta(max)=497 keV] is an attractive radionuclide for various therapeutic applications. Production of 177Lu using [176Lu (n,gamma)177Lu] reaction by thermal neutron bombardment on natural as well as enriched lutetium oxide target is described. In all, approximately 4 TBq/g (108 Ci/g) of 177Lu was obtained using natural Lu target after 7 d irradiation at 3 x 10(13) n/cm2/s thermal neutron flux while it was approximately 110 TBq/g (3000 Ci/g) of 177Lu when 60.6% enriched 176Lu target was used. In both the cases, radionuclidic purity was approximately 100%, only insignificant quantity of 177mLu [T(1/2)=160.5 d, Ebeta(max)=200 keV] could be detected as the radionuclidic impurity. Production logistics using different routes of production is compared. Possible therapeutic applications of 177Lu are discussed and its merits highlighted by comparison with other therapeutic radionuclides.

Optimising conditions for radiolabelling of DOTA-peptides with 90Y, 111In and 177Lu at high specific activities

DOTA-conjugated peptides, such as [DOTA(0),Tyr(3)]octreotide (DOTATOC) and [DOTA(0),Tyr(3)]octreotate (DOTA-tate), can be labelled with radionuclides such as (90)Y, (111)In and (177)Lu. These radiolabelled somatostatin analogues are used for peptide receptor radionuclide therapy (PRRT). Radioligands for PRRT require high specific activities. However, although these radionuclides are produced without addition of carrier, contaminants are introduced during production and as decay products. In this study, parameters influencing the kinetics of labelling of DOTA-peptides were investigated and conditions were optimised to obtain the highest achievable specific activity. The effects of contaminants were systematically investigated, concentration dependently, in a test model mimicking conditions for labelling with minimal molar excess of DOTA-peptides over radionuclide. Kinetics of labelling of DOTA-peptides were optimal at pH 4-4.5; pH <4 strongly slowed down the kinetics. Above pH 5, reaction kinetics varied owing to the formation of radionuclide hydroxides. Labelling with (90)Y and (177)Lu was completed after 20 min at 80 degrees C, while labelling with (111)In was completed after 30 min at 100 degrees C. The effects of contaminants were systematically categorised, e.g. Cd(2+) is the target and decay product of (111)In, and it was found to be a strong competitor with (111)In for incorporation in DOTA. In contrast, Zr(4+) and Hf(4+), decay products of (90)Y and (177)Lu, respectively, did not interfere with the incorporation of these radionuclides. The following conclusions are drawn: (a) DOTA-peptides can be radiolabelled at high specific activity; (b) reaction kinetics differ for each radionuclide; and (c) reactions can be hampered by contaminants, such as target material and decay products.

Antibody therapy of non-Hodgkin's B-cell lymphoma

Engineering antibodies with reduced immunogenicity and enhanced effector functions, and selecting antigen targets with the appropriate specificity, density, and/or functionality, have contributed to the recent clinical successes in using unconjugated "naked" antibody therapies of B-cell lymphoma (rituximab) and breast carcinoma (Herceptin). The non-overlapping toxicities of naked antibodies and chemotherapy, together with their potential synergy, which is based on unique and complementary mechanisms of action, have contributed to the creation of new standards of care in cancer therapy and management. Clinical trial results supporting these concepts are presented. Furthermore, the exquisite specificity of antibodies renders them ideal vehicles for selective delivery of toxic payloads such as drugs or radionuclides. Although successful in therapy of hematological cancers (Zevalin, Mylotarg), the broader application of these technologies to carcinomas still remains to be proven in clinical testing. Engineering of antibody constructs with optimal blood clearance and tumor-targeting kinetics, and selecting the radionuclide that may deliver sufficient radiation energy to kill the more radio-resistant carcinomas, are discussed. With the advent of genomics and proteomics, new membrane-associated tumor antigens are being discovered and will provide novel targets for future antibody therapy of cancer.

Optimization of the small-scale synthesis of DOTA-Tyr3 -octreotide

The clinical potential of 111In and 90Y labelled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid conjugated Tyr3-octreotide (DOTA-TOC) have been reported in a number of publications, and Phase II clinical trials of 90Y-DOTA-TOC are currently in progress. However, to date, only a summary of the large-scale preparation of these radiopharmaceuticals has been published. This publication aims to describe our experience of the small-scale synthesis of DOTA-TOC in the hope that this will assist others in the preparation of this and other similar radioconjugates. DOTA in the form of the tri-t-butyl ester was coupled to the Lys5 (BOC) protected Tyr3-octreotide in N,N-dimethylformamide or N-methyl-2-pyrolidinone, in a three-step reaction involving conjugation, using O-(7-azabenzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU) and diisopropylethyamine (DIPEA) as coupling reagents, deprotection with trifluoroacetic acid and HPLC purification of the conjugates. The product was obtained in final yields of 60+/-5%. The purified product was characterized by mass spectroscopy, showing a molecular weight of 1421.55+/-0.08. In somatostatin receptor binding assays, the unlabelled DOTA-TOC showed an effective displacement of 99mTc labelled HYNIC-TOC (where HYNIC is hydrazinonicotinamide) (IC50=0.31+/-0.07 nm), confirming the retention of receptor-binding affinity. The conjugate could be efficiently labelled with 111In by addition of 111InCl3 and ammonium acetate buffer pH5 and heating (95 degrees C, 20 min).