Novel (64)Cu- and (68)Ga-labeled RGD conjugates show improved PET imaging of α(ν)β(3) integrin expression and facile radiosynthesis

Fast clearing RGD-based near-infrared fluorescent probes for in vivo tumor diagnosis

A fast clearing hydrophilic near-infrared (NIR) dye ICG-Der-02 was used to constitute tumor targeting contrast agents. Cell adhesion molecule integrin α(v)β(3) served as the target receptor because of its unique expression on almost all sprouting tumor vasculatures. The purpose of this study was to synthesize and compare the properties of integrin α(v)β(3)-targeted, fast clearing NIR probes both in vitro and in vivo for tumor diagnosis. ICG-Der-02 was covalently conjugated to three kinds of RGD peptide including linear, monoeric cyclic and dimeric RGD to form three RGD-based NIR probes. The integrin receptor specificities of these probes were evaluated in vitro by confocal microscopy. The dynamic bio-distribution and elimination ratse were in vivo real-time monitored by a near-infrared imaging system in normal mice. Further, the in vivo tumor targeting abilities of the RGD-based NIR probes were compared in α(v)β(3) -positive MDA-MB-231, U87MG and α(v)β(3)-negtive MCF-7 xenograft mice models. Three RGD-based NIR probes were successfully synthesized with good optical properties. In vitro cellular experiments indicated that the probes have a clear binding affinity to α(υ)β(3) -positive tumor cells, with a cyclic dimeric RGD probe owing the highest integrin affinity. Dynamic bio-distributions of these probes showed a rapid clearing rate through the renal pathway. In vivo tumor targeting ability of the RGD-based porbes was demonstrated on MDA-MB-231 and U87MG tumor models. As expected, the c(RGDyK)(2)-ICG-Der-02 probe displayed the highest tumor-to-normal tissue contrast. The in vitro and in vivo block experiments confirmed the receptor binding specificity of the probes. The hydrophilic dye-labeled NIR probes exhibited a fast clearing rate and deep tissue penetration capability. Further, the α(υ)β(3) receptor affinity of the three RGD-based NIR probes followed the order of dimer cyclic > monomer cyclic > linear. The results demonstrate potent fast clearing probes for in vivo early tumor diagnosis.

Quantitative analysis and parametric imaging of 18F-labeled monomeric and dimeric RGD peptides using compartment model

PURPOSE

Non-invasive PET imaging with radiolabeled RGD peptides for α(v)β(3) integrin targeting has become an important tool for tumor diagnosis and treatment monitoring in both pre-clinical and clinical studies. To better understand the molecular process and tracer pharmacokinetics, we introduced kinetic modeling in the investigation of (18)F-labeled RGD peptide monomer (18)F-FP-c(RGDyK) (denoted as (18)F-FPRGD) and dimer (18)F-FP-PEG3-E[c(RGDyK)](2) (denoted as (18)F-FPPRGD2).

PROCEDURES

MDA-MB-435 tumor-bearing mice underwent 60 min dynamic PET scans following the injection of either (18)F-FPRGD or (18)F-FPPRGD2. Blocking studies with pre-injection of a blocking mass dose were performed for both monomeric and dimeric RGD groups. (18)F-FPRAD (RAD) was used as a negative control. Kinetic parameters (K(1), k(2), k(3), k(4)) of a three-compartment model were fitted to the dynamic data to allow quantitative comparisons between the monomeric and dimeric RGD peptides.

RESULTS

Dimeric RGD peptide tracer showed significantly higher binding potential (Bp(ND) = k(3)/k(4), 5.87 ± 0.31) than that of the monomeric analog (2.75 ± 0.48, p = 0.0022, n = 4/group). The Bp(ND) values showed a significantly greater ratio (dimer/monomer ~2.1) than the difference in %ID/g uptake measured from static images (dimer/monomer ~1.5, p = 0.0045). Significant decrease in Bp(ND) was found in the blocked groups compared with the unblocked ones (dimer p = 0.00024, monomer p = 0.005, n = 4/group). Similarly, the RAD control group showed the lowest Bp(ND) value among all the test groups, as the RAD peptide does not bind to integrin α(v)β(3). Volume of distribution (V(T) = K(1)/k (2)(1 + k (3)/k (4))) could be separated into non-specific (V (ND) = K (1)/k (2)) and specific (V (S) = K (1) k (3)/(k (2) k (4))) components. Specific distribution volume (V(S)) was the dominant component of V(T) in the unblocked groups and decreased in the blocked groups. Unblocked RGD dimer also showed higher V(S) than that of the monomer (dimer V(S) = 2.38 ± 0.15, monomer V(S) = 0.90 ± 0.17, p = 0.0013, n = 4/group), well correlated with Bp(ND) calculations. Little difference in V(ND) was found among all groups. Moreover, parametric maps allowed quantitative analysis at voxel level and provided higher tumor-to-background contrast for Bp(ND) maps than the static images. Tumor heterogeneity in kinetic parameters was found in parametric images, which could not be clearly identified in static intensity images.

CONCLUSIONS

The pharmacokinetics of both monomeric and dimeric RGD peptide tracers was compared, and the RGD dimers showed significantly higher binding affinity than the monomeric analogs. Kinetic parameters were demonstrated to be valuable for separating specific and non-specific binding and may allow more sensitive and detailed quantification than simple standardized uptake value analysis.

The untapped potential of Gallium 68-PET: the next wave of ⁶⁸Ga-agents

(68)Gallium-PET ((68)Ga-PET) agents have significant clinical promise. The radionuclide can be produced from a (68)Ge/(68)Ga generator on site and is a convenient alternative to cyclotron-based PET isotopes. The short half-life of (68)Ga permits imaging applications with sufficient radioactivity while maintaining patient dose to an acceptable level. Furthermore, due to superior resolution, (68)Ga-PET agents have the ability to replace current SPECT agents in many applications. This article outlines the upcoming agents and challenges faced during the translational development of (68)Ga agents.

(68)Ga small peptide imaging: comparison of NOTA and PCTA

In this study, a bifunctional version of the chelate PCTA was compared to the analogous NOTA derivative for peptide conjugation, (68)Ga radiolabeling, and small peptide imaging. Both p-SCN-Bn-PCTA and p-SCN-Bn-NOTA were conjugated to cyclo-RGDyK. The resulting conjugates, PCTA-RGD and NOTA-RGD, retained their affinity for the peptide target, the α(v)β(3) receptor. Both PCTA-RGD and NOTA-RGD could be radiolabeled with (68)Ga in >95% radiochemical yield (RCY) at room temperature within 5 min. For PCTA-RGD, higher effective specific activities, up to 55 MBq/nmol, could be achieved in 95% RCY with gentle heating at 40 °C. The (68)Ga-radiolabeled conjugates were >90% stable in serum and in the presence of excess apo-transferrin over 4 h; (68)Ga-PCTA-RGD did have slightly lower stability than (68)Ga-NOTA-RGD, 93 ± 2% compared to 98 ± 1%, at the 4 h time point. Finally, the tumor and nontarget organ uptake and clearance of (68)Ga-radiolabeled PCTA-RGD and NOTA-RGD was compared in mice bearing HT-29 colorectal tumor xenografts. Activity cleared quickly from the blood and muscle tissue with >90% and >70% of the initial activity cleared within the first 40 min, respectively. The majority of activity was observed in the kidney, liver, and tumor tissue. The observed tumor uptake was specific with up to 75% of the tumor uptake blocked when the mice were preinjected with 160 nmol (100 μg) of unlabeled peptide. Uptake observed in the blocked tumors was not significantly different than the background activity observed in muscle tissue. The only significant difference between the two (68)Ga-radiolabeled bioconjugates in vivo was the kidney uptake. (68)Ga-radiolabeled PCTA-RGD had significantly lower (p < 0.05) kidney uptake (1.1 ± 0.5%) at 2 h postinjection compared to (68)Ga-radiolabeled NOTA-RGD (2.7 ± 1.3%). Overall, (68)Ga-radiolabeled PCTA-RGD and NOTA-RGD performed similarly, but the lower kidney uptake for (68)Ga-radiolabeled PCTA-RGD may be advantageous in some imaging applications.

Measuring oncogenic signaling pathways in cancer with PET: an emerging paradigm from studies in castration-resistant prostate cancer

As parallel advances in cancer biology and drug development continue to elevate the role of targeted therapies in oncology, the need for imaging biomarkers that systematically measure the biology associated with therapeutic intervention has become more urgent. Although the molecular imaging community has a commitment to develop technologies to this end, few investigational radiotracers directly measure the biology of common oncogenic signaling pathways often addressed by targeted therapies. Visible progress has been achieved with a handful of radiotracers rationally designed to intercalate the pathobiology of prostate cancer, a molecularly heterogeneous disease nevertheless broadly defined by a fairly small repertoire of recurrent oncogenic lesions.

SIGNIFICANCE

That variable treatment responses or emergent resistance phenotypes are often documented in humans argues strongly for diagnostic technologies that can be realistically applied posttherapy to capture the dynamic patterns of disease response. The purpose of this review is to describe a collection of radiotracers developed to measure the pathobiology of prostate cancer for improved treatment monitoring, placing particular emphasis on the biologic rationale for their preparation. A chronologic description of radiotracer development programs is outlined, primarily to stress how an ongoing dialectic between earlier and more contemporary imaging technologies has accelerated discovery.

(64)Cu-NODAGA-c(RGDyK) Is a Promising New Angiogenesis PET Tracer: Correlation between Tumor Uptake and Integrin α(V)β(3) Expression in Human Neuroendocrine Tumor Xenografts

Purpose. The purpose of this paper is to evaluate a new PET tracer ⁶⁴Cu-NODAGA-c(RGDyK) for imaging of tumor angiogenesis using gene expression of angiogenesis markers as reference and to estimate radiation dosimetry for humans. Procedures. Nude mice with human neuroendocrine tumor xenografts (H727) were administered ⁶⁴Cu-NODAGA-c(RGDyK) i.v. for study of biodistribution as well as for dynamic PET. Gene expression of angiogenesis markers integrin α_V, integrin β₃, and VEGF-A were analyzed using QPCR and correlated to the tracer uptake in the tumors (%ID/g). From biodistribution data human radiation-absorbed doses were estimated using OLINDA/EXM. Results. Tumor uptake was 1.2%ID/g with strong correlations between gene expression and tracer uptake, for integrin α_V  R = 0.76, integrin β₃  R = 0.75 and VEGF-A R = 0.81 (all P < 0.05). The whole body effective dose for humans was estimated to be 0.038 and 0.029 mSv/MBq for females and males, respectively, with highest absorbed dose in bladder wall. Conclusion. ⁶⁴Cu-NODAGA-c(RGDyK) is a promising new angiogenesis PET tracer with potential for human use.

Be spoilt for choice with radiolabelled RGD peptides: preclinical evaluation of ⁶⁸Ga-TRAP(RGD)₃

Gallium-68 is rapidly gaining importance, as this generator-produced PET isotope is available independent of on-site cyclotrons, enabling radiopharmaceutical production with comparably simple techniques at low cost. The recently introduced TRAP chelator combines the advantage of straightforward design of multimeric ⁶⁸Ga-radiopharmaceuticals with very fast and efficient ⁶⁸Ga-labeling. We synthesized a series of five cyclo(RGDfK) peptide trimers and determined their α(v)β₃ integrin affinities in competition assays on α(v)β₃-expressing M21 human melanoma cells against ¹²⁵I-echistatin. The compound with highest IC₅₀, Ga-TRAP(RGD)₃, showed more than 7-fold higher affinity compared to the monomers F-Galacto-RGD and Ga-NODAGA-c(RGDyK). TRAP(RGD)₃ was radiolabeled with ⁶⁸Ga in a fully automated GMP compliant manner. CD-1 athymic nude mice bearing M21/M21L human melanoma xenografts were used for biodistribution studies, blockade experiments, metabolite studies and PET imaging. ⁶⁸Ga-TRAP(RGD)₃ exhibited high M21 tumor uptake (6.08±0.63% ID/g, 60 min p.i.), was found to be fully stable in vivo, and showed a fast renal clearance. Blockade studies showed that uptake in the tumor, as well as in all other tissues, is highly integrin specific. A comparison of biodistribution and PET data of ⁶⁸Ga-TRAP(RGD)₃ with those of ⁶⁸Ga-NODAGA-c(RGDyK) and ¹⁸F-Galacto-RGD showed that the higher affinity of the trimer effects a larger dynamic response of tracer uptake to integrin expression, i.e., enhanced integrin-specific uptake in all tissues. We conclude that ⁶⁸Ga-TRAP(RGD)₃ could allow for imaging of low-level integrin expression in tissues which are not visible with the two competitors. Overall, the study constitutes proof of concept for the favourable in vivo properties of TRAP-based ⁶⁸Ga radiopharmaceuticals.

Novel imaging chelates for drug discovery

The introduction of organic frameworks designed to coordinate metal ions or chelates to medical imaging has allowed for significant breakthroughs in the identification of many disease states. These identified disease states such as cancerous, cardiovascular and neurological, which provide excellent targets for novel drugs, and without the imaging chelates would certainly not have been discovered. In this article we will present a series of novel imaging chelates that have influenced drug discovery.

Radiopharmaceutical development of radiolabelled peptides

Receptor targeting with radiolabelled peptides has become very important in nuclear medicine and oncology in the past few years. The overexpression of many peptide receptors in numerous cancers, compared to their relatively low density in physiological organs, represents the molecular basis for in vivo imaging and targeted radionuclide therapy with radiolabelled peptide-based probes. The prototypes are analogs of somatostatin which are routinely used in the clinic. More recent developments include somatostatin analogs with a broader receptor subtype profile or with antagonistic properties. Many other peptide families such as bombesin, cholecystokinin/gastrin, glucagon-like peptide-1 (GLP-1)/exendin, arginine-glycine-aspartic acid (RGD) etc. have been explored during the last few years and quite a number of potential radiolabelled probes have been derived from them. On the other hand, a variety of strategies and optimized protocols for efficient labelling of peptides with clinically relevant radionuclides such as (99m)Tc, M(3+) radiometals ((111)In, (86/90)Y, (177)Lu, (67/68)Ga), (64/67)Cu, (18)F or radioisotopes of iodine have been developed. The labelling approaches include direct labelling, the use of bifunctional chelators or prosthetic groups. The choice of the labelling approach is driven by the nature and the chemical properties of the radionuclide. Additionally, chemical strategies, including modification of the amino acid sequence and introduction of linkers/spacers with different characteristics, have been explored for the improvement of the overall performance of the radiopeptides, e.g. metabolic stability and pharmacokinetics. Herein, we discuss the development of peptides as radiopharmaceuticals starting from the choice of the labelling method and the conditions to the design and optimization of the peptide probe, as well as some recent developments, focusing on a selected list of peptide families, including somatostatin, bombesin, cholecystokinin/gastrin, GLP-1/exendin and RGD.

Preparation and biological evaluation of (64)Cu labeled Tyr(3)-octreotate using a phosphonic acid-based cross-bridged macrocyclic chelator

Somatostatin receptors (SSTr) are overexpressed in a wide range of neuroendocrine tumors, making them excellent targets for nuclear imaging and therapy, and radiolabeled somatostatin analogues have been investigated for positron emission tomography imaging and radionuclide therapy of SSTr-positive tumors, especially of the subtype-2 (SSTr2). The aim of this study was to develop a somatostatin analogue, Tyr(3)-octreotate (Y3-TATE), conjugated to a novel cross-bridged macrocyclic chelator, 11-carboxymethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-4-methanephosphonic acid (CB-TE1A1P). Unlike traditional cross-bridged macrocycles, such as 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A), CB-TE1A1P-Y3-TATE was radiolabeled with (64)Cu in high purity and high specific activity using mild conditions. Saturation binding assays revealed that (64)Cu-CB-TE1A1P-Y3-TATE had comparable binding affinity but bound to more binding sites in AR42J rat pancreatic tumor cell membranes than (64)Cu-CB-TE2A-Y3-TATE. Both radiopharmaceuticals showed comparable uptake in SSTr2 positive tissues in AR42J tumor-bearing rats. (64)Cu-CB-TE1A1P-Y3-TATE demonstrated improved blood clearance compared to (64)Cu-CB-TE2A-Y3-TATE, as the tumor/blood ratios of (64)Cu-CB-TE1A1P-Y3-TATE were shown to be significantly higher than those of (64)Cu-CB-TE2A-Y3-TATE at 4 and 24 h postinjection. (64)Cu-CB-TE1A1P-Y3-TATE, in spite of a relatively high kidney uptake, accumulated less in nontarget organs such as liver, lung, and bone. Small animal PET/CT imaging of (64)Cu-CB-TE1A1P-Y3-TATE in AR42J tumor bearing rats validated significant uptake and good contrast in the tumor. This study suggests that CB-TE1A1P is a promising bifunctional chelator for (64)Cu-labeled for Y3-TATE, owing to high binding affinity and target tissue uptake, the ability to radiolabel the agent at lower temperatures, and improved tumor/nontarget organ ratios over (64)Cu-CB-TE2A-Y3-TATE.

Radiolabeled peptides: valuable tools for the detection and treatment of cancer

Human cancer cells overexpress many peptide receptors as molecular targets. Radiolabeled peptides that bind with high affinity and specificity to the receptors on tumor cells hold great potential for both diagnostic imaging and targeted radionuclide therapy. The advantage of solid-phase peptide synthesis, the availability of different chelating agents and prosthetic groups and bioconjugation techniques permit the facile preparation of a wide variety of peptide-based targeting molecules with diverse biological and tumor targeting properties. Some of these peptides, including somatostatin, bombesin, vasoactive intestinal peptide, gastrin, neurotensin, exendin and RGD are currently under investigation. It is anticipated that in the near future many of these peptides may find applications in nuclear oncology. This article presents recent developments in the field of small peptides, and their applications in the diagnosis and treatment of cancer.

Radiolabeling of RGD peptide and preliminary biological evaluation in mice bearing U87MG tumors

2-[(18)F]Fluoroethyl azide ([(18)F]FEA) and terminal alkynyl modified propioloyl RGDfK were selected in this study. [(18)F]FEA was prepared by nucleophilic radiofluorination of 2-azidoethyl 4-toluenesulfonate with radiochemical yield of 71 ± 4% (n = 5, decay-corrected). We assessed the various conditions of the CuAAC reaction between [(18)F]FEA and propioloyl RGDfK, which included peptide concentration, reaction time, temperature and catalyst dosage. The (18)F-labeled-RGD peptide ([(18)F]F-RGDfK) could be obtained in 60 min by a two-step radiochemical synthesis route, with total radiochemical yield of 60 ± 2% (n = 3, decay-corrected) through click chemistry. [(18)F]F-RGDfK showed high stability in phosphate buffered saline and new-born calf serum. Micro-PET imaging at 1 h post injection of [(18)F]F-RGDfK showed medium concentration of radioactivity in tumors while much decreased concentration in tumors in the blocking group. These results showed that [(18)F]F-RGDfK obtained by click chemistry maintained the affinity and specificity of the RGDfK peptide to integrin α(v)β(3). This study provided useful information for peptide radiofluorination by using click chemistry.

Synthesis, Cu(II) complexation, 64Cu-labeling and biological evaluation of cross-bridged cyclam chelators with phosphonate pendant arms

A new class of cross-bridged cyclam-based macrocycles featuring phosphonate pendant groups has been developed. 1,4,8,11-tetraazacyclotetradecane-1,8-di(methanephosphonic acid) (CB-TE2P, 1) and 1,4,8,11-tetraazacyclotetradecane-1-(methanephosphonic acid)-8-(methanecarboxylic acid) (CB-TE1A1P, 2) have been synthesized and have been shown to readily form neutral copper(II) complexes at room temperature as the corresponding dianions. Both complexes showed high kinetic inertness to demetallation and crystal structures confirmed complete encapsulation of copper(II) ion within each macrocycle's cleft-like structure. Unprecedented for cross-bridged cyclam derivatives, both CB-TE2P (1) and CB-TE1A1P (2) can be radiolabeled with (64)Cu at room temperature in less than 1 h with specific activities >1 mCi μg(-1). The in vivo behavior of both (64)Cu-CB-TE2P and (64)Cu-CB-TE1A1P were investigated through biodistribution studies using healthy male Lewis rats. Both new compounds showed rapid clearance with similar or lower accumulation in non-target organs/tissues when compared to other copper chelators including CB-TE2A, NOTA and Diamsar.

68Ga-NODAGA-RGDyK for αvβ3 integrin PET imaging. Preclinical investigation and dosimetry

AIM

To visualize neovasculature and/or tumour integrin αvβ3 we selected the binding moiety Arg-Gly-Asp-D-Tyr-Lys (RGDyK) coupled to NODAGA for labeling with 68Ga.

METHODS

NODAGA-RGDyK (ABX) was labeled with the 68Ga eluate from the 68Ge generator IGG100 using the processor unit PharmTracer. Biodistribution was measured in female Hsd mice sacrificed 10, 30, 60 and 90 min after i.v. injection of 68Ga-NODAGA-RGDyK for OLINDA dosimetry extrapolated to humans. Tumour targeting was studied in SCID mice bearing A431 and other tumour transplants using microPET and biodistribution measurements.

RESULTS

Effective half-life of 68Ga-NODAGA-RGDyK was ~25 min for total body and most organs except liver and spleen that showed stable activity retention. With a bladder voiding interval of 0.5 h the calculated effective dose (ED) was 0.012 and 0.016 mSv/MBq for males and females, respectively. Rapid uptake within 10 min was observed in A431 tumours with dynamic PET followed by a slow release. Biodistribution measurements showed a 68Ga-NODAGA-RGDyK uptake in A431 tumours of 3.4±0.4 and 2.7±0.3%ID/g at 1 and 2 h, respectively. Similar uptakes were observed in a mouse and human breast and ovarian cancer xenografts. Co-injection of excess (5 mg/kg) unlabeled NODAGA-RGDyK with the radiotracer reduced tumour uptake at one hour to 0.23±0.01%ID/g, but similarly decreased uptake in normal organs as well. When unlabeled peptide was injected 15 min after 68Ga-NODAGA-RGDyK, uptake diminished particularly in tumour and adrenals, suggestive of a different binding mode compared with other normal tissues.

CONCLUSION

NODAGA-RGDyK was reliably labeled with 68Ga and revealed a predicted ED of 0.014 mSv/MBq. Tumour uptake was rapid and significant and was chased with unlabeled RGDyK in a similar manner as adrenal uptake.