86Y based PET radiopharmaceuticals: radiochemistry and biological applications

Med Chem. Author manuscript; available in PMC 2012 September 1.

Published in final edited form as:

Med Chem. 2011 September 1; 7(5): 380–388.

PMCID: PMC3184346

NIHMSID: NIHMS281760

⁸⁶Y based PET radiopharmaceuticals: radiochemistry and biological applications

Tapan K. Nayak¹ and Martin W. Brechbiel²^*

¹ Imaging Sciences, Translational Research Sciences, Pharma Research and Early Development, F. Hoffmann-La Roche Ltd., Basel, BS-4072, Switzerland

² Radioimmune & Inorganic Chemistry Section, Radiation Oncology Branch, National Cancer Institute, National Institute of Health, Bethesda, MD-20892, USA

^* Corresponding author: Martin W. Brechbiel, 10 Center Drive MSC 1002, Room B3B69, NCI-Bethesda, Bethesda, MD 20892., Fax: 301-402-1923, Phone: 301-496-0591, Email: martinwb/at/mail.nih.gov

The publisher's final edited version of this article is available at Med Chem

Abstract

Development of targeted radionuclide therapy with ⁹⁰Y labeled antibodies and peptides has gained momentum in the past decade due to the successes of ⁹⁰Y-ibritumomab tiuxetan and ⁹⁰Y-DOTA-Phe¹-Tyr³-octreotide in treatment of cancer. ⁹⁰Y is a pure β⁻-emitter and cannot be imaged for patient-specific dosimetry which is essential for pre-therapeutic treatment planning and accurate absorbed dose estimation in individual patients to mitigate radiation related risks. This review article describes the utility of ⁸⁶Y, a positron emitter (33%) with a 14.7-h half-life that can be imaged by positron emission tomography and used as an isotopically matched surrogate radionuclide for ⁹⁰Y radiation doses estimations. This review discusses various aspects involved in the development of ⁸⁶Y labeled radiopharmaceuticals with the specific emphasis on the radiochemistry and biological applications with antibodies and peptides.

Keywords: PET imaging, radiochemistry, ⁹⁰Y, ⁸⁶Y, bioconjugate chemistry, peptides, and antibodies

Introduction

Recent advances in radiochemistry and biomedical sciences offer diverse receptor-avid and immune-derived molecular vectors as well as a plethora of therapeutic radionuclides [1,2]. Several systems are currently being investigated as potential means for targeting of radionuclide to desired site. Of these, largely monoclonal antibodies (mAb) and peptides have successfully been used in clinic [1,2]. Targeted radionuclide therapy with antibodies and peptides are at the forefront of molecular cancer treatment modalities. ⁹⁰Y is one of the promising radionuclides for targeted radionuclide therapy of hematologic malignancies and solid tumors [3–6]. However, ⁹⁰Y labeled antibodies and peptides may pose challenges in managing radiation toxicity if the radiation doses exceed the critical limit to organs such as the bone marrow and the kidneys [7,8]. Since ⁹⁰Y is a pure β⁻-emitter, its biodistribution cannot be readily imaged for patient-specific dosimetry which is essential for pre-therapeutic treatment planning and accurate absorbed dose estimation in individual patients to mitigate radiation risks. Attempts have been made to image the Bremsstrahlung radiation generated by the slowing down of high-energy electrons in tissue. However, the low photon yield and the polychromatic nature of the Bremsstrahlung spectrum result in limited quantitative accuracy with ⁹⁰Y [9,10].

¹¹¹In and ⁸⁹Zr were used as surrogate single photon emission computerized tomography (SPECT) and positron emission tomography (PET) radionuclides for ⁹⁰Y, respectively; however, disparities were observed in the biodistribution of these radionuclides and the ⁹⁰Y labeled antibodies and peptides [11–15]. The somatostatin receptor-targeted ⁹⁰Y-DOTA-Phe¹-Tyr³-octreotide is the most extensively studied radiolabeled peptide in patients for treatment of cancer. Clinical trials performed in several countries showed complete and partial remissions in 10% to 30% of patients [1]. In spite of encouraging efficacy results, loss of renal function or renal failure was observed in some patients treated with ⁹⁰Y-DOTA-Phe¹-Tyr³-octreotide and therefore pre-treatment radiation dose assessments are critical for ⁹⁰Y-DOTA-Phe¹-Tyr³-octreotide [1,8]. In a clinical study, a ⁸⁶Y labeled peptide and ¹¹¹In labeled peptide were compared for pre-treatment dosimetry assessments of ⁹⁰Y radionuclide therapy [11] Compared to ⁸⁶Y-DOTA-Phe¹-Tyr³-octreotide, dosimetry with ¹¹¹In-pentetreotide overestimated doses to kidneys and spleen, whereas the radiation dose to the tumor-free liver was then underestimated for ⁹⁰Y-DOTA-Phe1-Tyr3-octreotide radiotherapy [11].

In recent years, ⁸⁶Y has gained popularity as an attractive surrogate for studying ⁹⁰Y due its half-life (14. 7 h) and positron emission (33 % β⁺) which allows quantitative imaging over 2–3 days [11,16]. Since the chemical form is identical to ⁹⁰Y, ⁸⁶Y labeled antibodies and peptides have identical biodistributions with ⁹⁰Y labeled antibodies and peptides, and therefore should inherently enable more accurate absorbed dose estimates for ⁹⁰Y [11,17,18]. This review discusses various aspects involved in the development of ⁸⁶Y labeled radiopharmaceuticals with the specific emphasis on the radiochemistry and biological applications with antibodies and peptides.

Radiochemistry

Yttrium belongs to group IIIB of the periodic table and has an atomic radius of 2.3 Å and electronegativity of 1.22. ⁸⁶Y can be produced via the (p,n) or (d,2n) nuclear reaction on enriched ⁸⁶Sr (³He,2n) on natural rubidium, (p,3n) on enriched ⁸⁸Sr, or (d,x) reaction on natural zirconium target [19,20]. However, the ⁸⁶Sr(p,n)⁸⁶Y reaction is the more commonly used method as it can be applied on medical cyclotrons by irradiating SrCO₃ or SrO with 2–6 μA of beam current for <4 h [21,22]. As compared to SrCO₃, SrO is a superior target because it can withstand at least a 6 μA of beam current, which is a significant improvement over a maximum of 2 μA on the SrCO₃ target, resulting in yields that are almost double with minimum contaminants [21]. After irradiation, ⁸⁶Y is separated from the target material using a commercially available Sr(II) selective resin and purified in order to accomplish efficient radiolabeling [22]. Electrochemical method of separation is also commonly used to produce no-carrier-added ⁸⁶Y of high purity [23]. Other separation techniques include the combination of co-precipitation and cation-exchange chromatography and ion specific resin chromatography, or paper filtration from alkaline solution [18,19,24].

Once high purity ⁸⁶Y is produced, it is then attached to the targeting vector such as an antibody and a peptide via a chelating agent. A chelating agent is a molecule containing more than one ligand or an atom such as N, O and S that can donate a lone pair of electrons. The oxidation state and electronegativity play a major role in the formation of metal-ligand complexes. Yttrium is a transition metal ion that exists in a tricationic state, Y(III), that can readily reach coordination numbers of 8 and 9 in its complexes and prefers octadentate coordinating ligands. Acyclic polyaminocarboxylates such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA) and macrocyclic polyaminocarboxylates such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) have been used as chelating agents for yttrium (Figure 1).

Figure 1

Chemical structures of acyclic and macrocylic polyaminocarboxylates chelating agents used for radiolabeling with yttrium

The chelating agents themselves are modified to provide a method to link them to antibodies and peptides (e.g., cyclic and mixed anhydrides of DTPA), alternatively linking groups such as isothiocyanatobenzyl group (SCN-Bz) are incorporated to the chelating agent by covalent linkage to the carbon backbone of the chelating agents [25]. In a pre-clinical study, the biodistribution of yttrium- and indium-labeled mAb B72.3 was compared in athymic mice bearing human colorectal cancer LS-174T tumors using three different chelate conjugates (SCN-Bz-EDTA, Cyclic anhydride-DTPA (ca-DTPA), and SCN-Bz-DTPA) [26]. The yttrium uptake in the bone with the SCN-Bz-EDTA and ca-DTPA conjugated mAb was greater then 14 and 11% ID/g, respectively, while ⁸⁸Y-SCN-Bz-DTPA-B72.3 showed only 3% ID/g. In contrast, the ¹¹¹In-labeled B72.3 uptake in the bone with all three chelate conjugates was 2–3% ID/g. The differences in yttrium- versus indium-labeled mAb biodistributions demonstrates the differences in the in vivo stability of different forms of acyclic polyaminocarboxylates for In(III) and Y(III); all three chelating agents were stable for ¹¹¹In, but only the full DTPA with 3 amines and 5 carboxylate donors bound to the Y(III) was stable for ⁸⁸Y, thereby elucidating the need for both better and more stable chelating agents for Y(III) and the need for a radioistopically matched imageable yttrium radionuclide to predict more accurate biodistribution and dosimetry of ⁹⁰Y-labeled agent [26].

To further understand and improve the in vivo stability, additional studies were performed with backbone-substituted forms of the SCN-Bz-DTPA and DOTA [27]. In this study, the biodistribution of yttrium-and indium-labeled mAb, B3, using two different chelate conjugates, i.e. 2-(p-SCN-Bz)-6-methyl-DTPA (1B4M-DTPA) and 2-(p-SCN-Bz)-1,4,7,10 tetraazacyclododecane tetraacetic acid (C-DOTA) (Figure 1) were compared in tumor bearing athymic mice [27]. Mice injected with indium and yttrium labeled 1B4M-DTPA-B3 showed a similar biodistribution in all tissues except the bones, where significantly higher accretion of yttrium than indium was observed, with 2.8% +/− 0.2% vs. 1.3% +/− 0.16% ID/g in the femur at 168 h, respectively (P < 0.0001) [27]. In contrast, mice receiving indium and yttrium labeled C-DOTA-B3 showed significantly higher accumulation of indium than yttrium in most tissues, including the bones and therefore further illustrating the differences between two radionuclides and the chelating agents. From these studies, it was observed that the SCN-Bz substituted onto the carbon backbone of DTPA for use in linkage to the antibody and the methyl groups incorporated onto the backbone sterically hindered the release of yttrium from the chelate and therefore demonstrated the superiority of backbone-substituted DTPA chelating agents over cyclic anhydride-DTPA chelating agents.

With a better understanding of the coordination chemistry and geometry of DTPA derivatives, a cyclohexyl derivative of DTPA (CHX-DTPA) was developed, which demonstrated excellent stability in vitro and in vivo [25,28,29]. Interestingly, the stereochemistry of CHX-DTPA played a major role in the in vivo stability of yttrium labeled agents and subsequent bone uptake. The single enantiomeric [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A″-DTPA) radioimmunoconjugate was found to be more stable as compared to the corresponding diastereomeric pair of CHX-B-DTPA radioimmunoconjugates (Figure 1). After 7 days post-injection, bone uptake of the yttrium labeled CHX-B′-DTPA or CHX-B″-DTPA radioimmunoconjugates was 5 times higher than that of the yttrium labeled CHX-A″-DTPA radioimmunoconjugate [28,30]. For labeling of peptides at the N-terminus, an N-hydroxysuccinimidyl penta-tert-butyl ester derivative of CHX-A″–DTPA featuring a glutaric acid spacer was developed (Figure 2) [31] In order to further improve the stability, DOTA derivatives such as SCN-Bz-DOTA were explored for radiolabeling antibodies and peptides with yttrium [32–34]. In a comparative study, it was found that DOTA conjugated antibody was more stable than the DTPA conjugated [33]. Since then, numerous pre-clinical and clinical studies have been published reporting DTPA and DOTA based chelating agents for ⁹⁰Y radionuclide therapy and thus extrapolating to ⁸⁶Y for PET imaging [15,16,31,35].

Figure 2

Succinimidyl ester coupling chemistry for N-terminal conjugation of octreotide with N-hydroxysuccinimidyl penta-tert-butyl ester derivative of CHX-A″-DTPA with a glutaric acid spacer.

For in vivo applications, kinetic inertness of yttrium–chelator complexes or conjugates is more relevant than thermodynamic stability [36]. For yttrium, acyclic chelating agents are less kinetically inert than macrocyclic complexes of comparable stability, but at the same time acyclic chelating agents have faster metal-binding kinetics at room temperature compared with their macrocyclic analogues, which can be a significant advantage for radiolabeling of antibodies which are thermosensitive.

Biological applications

⁸⁶Y labeled peptides

⁸⁶Y labeled somatostatin receptor-targeted peptides were evaluated in rodents, non-human primates and humans [11,18,31]. For peptides, CHX-A″-DTPA, a widely applicable chelating agent for yttrium was modified via standard succinimidyl ester coupling chemistry for N-terminal conjugation to somatostatin peptide [31]. Briefly, an N-hydroxysuccinimidyl penta-tert-butyl ester derivative of CHX-A″-DTPA with a glutaric acid spacer moiety was developed to lessen steric effects and interactions of the chelator–metal complex on peptide–receptor binding affinity. CHX-A″-DTPA-octreotide was radiolabeled with ⁸⁶Y and evaluated in AR42J tumor bearing male rats. ⁸⁶Y-CHX-A″-DTPA-octreotide was prepared with high radiochemical yields (> 97 %) and specific activity of 6.49 × 10⁶ MBq/mmole. Somatostatin-receptor mediated uptake was observed in the AR42J tumor (0.94 ± 0.22 % ID/g) in addition to somatostatin receptor expressing organs such as pituitary gland (1.70 ± 0.08 % ID/g), adrenal glands (0.95 ± 0.30 % ID/g), and pancreas (0.65 ± 0.24 % ID/g) [31]. Tumor was clearly visualized at 4 h after injection by PET imaging [31].

In order to quantify the in vivo parameters of ⁹⁰Y-DOTA-DPhe¹-Tyr³-octreotide and the radiation doses delivered to healthy organs, the analogue ⁸⁶Y-DOTA-DPhe¹-Tyr³-octreotide was synthesized and its uptake measured in baboons using PET [18]. ⁸⁶Y-DOTA-DPhe¹-Tyr³-octreotide was further evaluated in a Phase I clinical study involving 24 patients with somatostatin-receptor positive neuroendocrine tumors [15]. Most patients were injected with 370 MBq of ⁸⁶Y-DOTA-DPhe¹-Tyr³-octreotide along with amino acids to reduce renal uptake. PET scans were performed at 4, 24 and 48 h after injection. Primary and metastatic lesions were clearly visualized at 4, 24 and 48 h after injection [15]. In another study comprising 8 patients with progressive metastatic neuroendocrine tumors, ⁸⁶Y-DOTA-DPhe1-Tyr3-octreotide was compared with ¹¹¹In-pentetreotide for predicting the radiation doses after 3 cycles of ⁹⁰Y-DOTA-DPhe1-Tyr3-octreotide [11]. The range of doses (mGy/MBq ⁹⁰Y-DOTA-Phe¹-Tyr³-octreotide) for kidneys, spleen, liver and tumor masses was 0.6–2.8, 1.5–4.2, 0.3–1.3 and 2.1–29.5 (⁸⁶Y-DOTA-Phe¹-Tyr³-octreotide), respectively, versus 1.3–3.0, 1.8–4.4, 0.2–0.8 and 1.4–19.7 (¹¹¹In-pentetreotide), with wide inter-subject variability. Compared with ⁸⁶Y-DOTA-Phe¹-Tyr³-octreotide, dosimetry with ¹¹¹In-pentetreotide overestimated doses to kidneys and spleen, whereas the radiation dose to the tumor-free liver was underestimated [11].

⁸⁶Y labeled peptides targeting the alpha-melanocyte stimulating hormone (alpha-MSH) and gastrin-releasing peptide receptors have also been reported [37,38]. The α-MSH targeted DOTA–ReCCMSH(Arg¹¹) was labeled with ⁶⁴Cu and ⁸⁶Y and evaluated in tumor bearing mice [37]. Tumor concentration for both the ⁸⁶Y- and ⁶⁴Cu-complexes reached a maximum at 30 min, while co-administering 20 μg of unlabeled complex reduced tumor uptake significantly. Non-target organ concentration was considerably lower with ⁸⁶Y–DOTA–ReCCMSH(Arg¹¹) than its ⁶⁴Cu analogue, except in the kidneys, where the ⁶⁴Cu complex had lower accumulation at all time points [37]. In order to improve specific activity, CHX-A″-DTPA as chelating agent for ReCCMSH(Arg¹¹) was examined due to its rapid and efficient metal chelation properties [39]. CHX-A″-DTPA has been reported to chelate yttrium and other trivalent metal cations nearly instantaneously at a chelator/metal ratio of 1:1 at 22°C, whereas a minimum molar ratio of DOTA-ReCCMSH(Arg¹¹)/radiometal of 20:1 and incubation at ≥75°C were necessary for efficient peptide labeling and therefore post-labeling purification is required [39]. Gastrin-releasing peptide receptors targeted peptide, MP2346 was conjugated with DOTA and radiolabeled with ⁶⁴Cu and ⁸⁶Y [38]. ⁶⁴Cu and ⁸⁶Y labeled MP2346 were evaluated in mice bearing PC-3 tumor xenografts. Biodistribution in PC3 tumor-bearing mice demonstrated higher tumor uptake, but lower liver retention, in animals injected with ⁸⁶Y-MP2346 compared to ⁶⁴Cu-MP2346. It was possible to delineate PC-3 tumors in vivo with ⁶⁴Cu-MP2346, but superior ⁸⁶Y-MP2346-PET images were obtained due to lower uptake in clearance organs and lower background activity and therefore the ⁸⁶Y analogue demonstrated superior targeting characteristics than ⁶⁴Cu analogue [38]. The studies with α-MSH and gastrin-releasing peptide receptors targeted peptides demonstrated the superiority of ⁸⁶Y labeled over ⁶⁴Cu labeled peptides.

⁸⁶Y labeled antibodies

Numerous ⁹⁰Y labeled intact monoclonal antibodies were evaluated in pre-clinical and clinical settings [7,40–43]. Of the numerous ⁹⁰Y labeled mAbs evaluated, ⁹⁰Y-ibritumomab tiuxetan (Zevalin^®) was approved by the US FDA in 2002 for treatment of patients with relapsed or refractory, low-grade or follicular B-cell non-Hodgkin’s lymphoma (NHL), including patients with rituximab refractory follicular NHL[43]. Since then many antibodies and proteins have been labeled with ⁸⁶Y for PET imaging and dosimetry of counterpart ⁹⁰Y labeled antibodies and proteins [16,22,44]. Human epidermal growth factor 2 (HER2) targeted intact IgG₁ mAb, Trastuzumab (Herceptin^®) was approved by US FDA in 1999 for treatment of cancer [43].

1B4M-DTPA-trastuzumab was radiolabeled with ⁸⁶Y and ¹¹¹In, and evaluated in LS-174T tumor bearing athymic mice [22]. The biodistribution study revealed that ¹¹¹In-1B4M-DTPA-trastuzumab, while a suitable surrogate for ⁹⁰Y in the major organs, did not parallel the uptake of ⁸⁶Y-1B4M-DTPA-trastuzumab in the bone, and thus may not accurately predict the level of ⁹⁰Y accumulation in the bone for clinical radioimmunotherapy applications [22]. Examination of the results for the uptake of the two radiolabeled antibodies at the bone over time revealed that ¹¹¹In-1B4M-DTPA-trastuzumab reached a maximum of 2.15 ± 0.49 %ID/g at 12 h and remained constant until 72 h. The amount of radioactivity in the bone declined thereafter at 120 h to 1.07 ± 0.41 %ID/g. On the contrary, accumulation of ⁸⁶Y-1B4M-DTPA-trastuzumab in the bone was observed over a period of 120 h. The level of ⁸⁶Y-1B4M-DTPA-trastuzumab at 12 h was higher than ¹¹¹In-1B4M-DTPA-trastuzumab (2.69 ± 0.71 %ID/g, p = 0.345), however the differences were not found to be significant. This trend continued until the 72 h time point when a marked increase in the ⁸⁶Y-1B4M-DTPA-trastuzumab level was noted (3.07 ± 0.63% ID/g versus 1.83 ± 0.19 %ID/g; ⁸⁶Y versus ¹¹¹In, p = 0.019), which was a significant difference. Finally, the level of ⁸⁶Y in the bone at the 120 h time point continued to increase from being merely 25% greater than the comparable ¹¹¹In value at 72 h to 331% greater than the comparable ¹¹¹In value at 120 h. This study once again exemplified the requirement of employing appropriate matched pair isotopes for imaging and therapy to insure that dosimetry considerations may be addressed accurately [22].

⁸⁶Y-trastuzumab was further evaluated in mice bearing intraperitoneal ovarian (SKOV-3) and colorectal (LS-174T) tumors [35,45]. Both these studies demonstrated the utility of ⁸⁶Y-trastuzumab as a PET surrogate for ⁹⁰Y-trastuzumab distribution. ⁸⁶Y-trastuzumab localized to sites of disease with minimal normal organ uptake as revealed in microPET and MRI data (Figure 3) [35,45]. Dosimetry calculations with ⁸⁶Y-trastuzumab in intraperitoneal ovarian carcinoma model revealed the absorbed dose to the kidneys was 0.31 Gy/MBq for ⁹⁰Y-trastuzumab [35]. The liver received 0.48 Gy/MBq, and the spleen received 0.56 Gy/MBq and the absorbed dose to tumors varied from 0.10 Gy/MBq for radius = 0.1 mm to 3.7 Gy/MBq for radius = 5 mm [35]. Cetuximab (Erbitux^®), panitumumab (Vectibix^®) and bevacizumab (Avastin^®) are other mAbs that are approved by the US FDA for treatment of cancer. Cetuximab and panitumumab target HER1, whereas bevacizumab targets vascular endothelial growth factor (VEGF). All these mAbs have been radiolabeled with ⁸⁶Y and evaluated in pre-clinical models [16,17,46].

Figure 3

PET maximum intensity projections (3 d p.i.) of female athymic mouse bearing (A) subcutaneous LS-174T and (B) intraperitoneal LS-174T tumors injected with ⁸⁶Y-CHX-A″-DTPA-trastuzumab. (C) MRI of the female athymic mouse bearing intraperitoneal (more ...)

⁸⁶Y-CHX-A″-DTPA-panitumumab was developed as a noninvasive molecular imaging tool for selecting patients for HER1-targeted panitumumab therapy and for dosimetry assessment for possible targeted ⁹⁰Y therapy [16]. Panitumumab was modified with the CHX-A″-DTPA at a 10:1 molar excess of chelate to protein, yielding a final chelate-to-protein ratio of 1.6 chelate molecules per protein molecule. The ⁸⁶Y-CHX-A″-DTPA-panitumumab conjugate was successfully prepared, with the radiochemical yields ranging from 60% to 75% and specific activity exceeding 2 GBq/mg [16]. Biodistribution, non-compartmental pharmacokinetics, and imaging data revealed HER1-mediated uptake of ⁸⁶Y-CHX-A″-DTPA-panitumumab and accumulation in HER1-expressing tumor xenografts [16]. Similarly, ⁸⁶Y-CHX-A″-DTPA-bevacizumab was developed as a potential PET imaging agent of VEGF-A mediated tumor angiogenesis and as a surrogate marker for ⁹⁰Y-based radioimmunotherapy [17]. In vivo biodistribution and PET imaging studies were performed on mice bearing VEGF-A–secreting human colorectal (LS-174T), human ovarian (SKOV-3) and VEGF-A–negative human mesothelioma (MSTO-211H) xenografts. Biodistribution and PET imaging studies demonstrated highly specific tumor uptake of the radioimmunoconjugate. In mice bearing VEGF-A–secreting LS-174T, SKOV-3 and VEGF-A–negative MSTO-211H tumors, the tumor uptake at 3 days post-injection was 13.6 ± 1.5, 17.4 ± 1.7 and 6.8 ± 0.7 % ID/g, respectively [17]. The corresponding tumor uptake in mice co-injected with 0.05 mg cold bevacizumab were 5.8 ± 1.3, 8.9 ± 1.9 and 7.4 ± 1.0 % ID/g, respectively at the same time point, demonstrating specific blockage of the target in VEGF-A–secreting tumors. The LS-174T and SKOV3 tumors were clearly visualized by PET imaging after injecting 1.8–2.0 MBq ⁸⁶Y-CHX-A″-DTPA-bevacizumab [17].

Human mindin homologue, mindin/RG-1, is expressed in >80% of prostate cancers metastatic to bone or lymph nodes as well as in locally recurrent tumors in androgen-unresponsive patients [47,48]. A fully human antibody, 19G9, was generated against mindin/RG-1 protein and conjugated with CHX-A″-DTPA, and radiolabeled with either ¹¹¹In, ⁹⁰Y, or ⁸⁶Y for in vivo imaging and radionuclide therapy [47]. PET with ⁸⁶Y-CHX-A″-DTPA-19G9 showed very specific accumulation of the antibody in LNCaP tumor xenografts with clear tumor delineation apparent 4 h after injection [47]. Based on the imaging data with ⁸⁶Y-CHX-A″-DTPA-19G9 the therapeutic efficacy of ⁹⁰Y-CHX-A″-DTPA-19G9 was evaluated in mice bearing LNCaP xenografts. A dose-finding study identified a nontoxic therapeutic dose to be approximately 2.7 MBq. Significant antitumor effects were seen with a single administration of radiolabeled antibody to animals bearing 200 to 400 mm³ tumors [47]. To further develop mindin/RG-1-targeted radionuclide therapy, 19G9 was engineered to obtain the matched diabody, single-chain variable domain (scFv) and novel miniantibody format. The intact antibody, 19G9 and its fragments were conjugated with CHX-A″-DTPA and radiolabeled with ⁸⁶Y for PET imaging and in vivo evaluation in tumor bearing mice [48].

The size of the construct profoundly affected the pharmacokinetic characteristics of each agent (Figure 4) [48]. The smallest construct (scFv) was rapidly and efficiently extracted from the blood and removed, primarily via the renal system, before significant tumor accumulation. In contrast, the largest construct (IgG) demonstrated prolonged blood retention and significant tumor uptake. PET detected IgG tumor deposition at 24 h. The diabody and miniantibody fragments both demonstrated intermediate blood clearance and tumor localization. Diabody blood levels were rapidly reduced during the initial hours, and tumor deposition was probably not sufficient for radiotherapy. High kidney levels were found for the diabody at 3 and 6 h. PET of the diabody at 48 h did, however, show comparable tumor delineation to that of the IgG and miniantibody [48]. The above studies demonstrate the utility of ⁸⁶Y labeled antibody not only in understanding the biodistribution of ⁹⁰Y labeled antibody but also selection of the optimum construct for improved outcomes.

Figure 4

Comparison of PET imaging of ⁸⁶Y labeled IgG, scFv, diabody and miniantibody construct of 19G9 in tumor bearing mice. Reprinted by permission of the Society of Nuclear Medine from Schneider at al. [48]

Besides antibodies and peptides, there are other reported applications of ⁸⁶Y. ⁸⁶Y-citrate was used an analog of the commercially available radiotherapeutic ⁹⁰Y-citrate [20]. Additionally, a mirror-image oligonucleotide (L-RNA) was radiolabeled with ⁸⁶Y for PET imaging [49].

Discussion

Antibodies and peptides function as carriers of cytotoxic substances, such as radioisotopes for treatment of cancer [1,43]. In NHL, anti-CD20 radiolabeled antibodies have superior antitumor activity compared with their unconjugated antibody counterparts, but there is increased, albeit manageable, hematological toxicity if the careful pretreatment planning and dosimetry is performed [7,41]. Similarly, for neuroendocrine and somatostatin receptor expressing tumors, radiolabeled peptides have demonstrated superior antitumor activity than unlabeled peptides [1,6]. However, renal toxicity is a primary concern and therefore careful pretreatment dosimetry and screening is required [8].

For ⁹⁰Y labeled agents, ⁸⁶Y offers an attractive alternative to ¹¹¹In for pretreatment dosimetry and imaging. In pre-clinical studies, the superiority of ⁸⁶Y over ¹¹¹In in predicting ⁹⁰Y has been demonstrated [11,22]. However, ⁸⁶Y itself has some limitations such as the high-energy gamma-emission and a lack of adequate commercial availability (as compared to ¹¹¹In). More than 65% of ⁸⁶Y decays are accompanied by additional gamma-rays with energies from 200 to 3,000 keV that are mostly emitted simultaneously with positron emissions and the subsequent annihilation photons resulting into increased scatter and random events [50,51]. The resulting large number of non-true coincidences evoke a low-frequency background noise that decreases the contrast in reconstructed images and may lead to erroneous quantitation of the ⁸⁶Y radioactivity concentration in individual regions of interest, thus resulting in false ⁹⁰Y organ doses [50,51]. Successful attempts have been made to improve the image characteristics of ⁸⁶Y by developing methods to correct for scatter fractions and spurious coincidences [50–52]. Image reconstruction algorithms have been developed, which include models of the statistical nature of nuclear decays and tomography responses that improve the spatial resolution and at the same time reduce the image noise [52,53]. Further advances in instrumentation and image processing may additionally improve spatial resolution and the ability to perform more accurate quantitative analysis with ⁸⁶Y.

The available choice of radionuclides used for PET imaging of antibodies and peptides are ¹²⁴I, ⁶⁴Cu, ⁸⁹Zr, ⁸⁶Ga, ¹⁸F and ⁸⁶Y. Each of these radionuclides has their own specific advantage and drawback. For rapidly internalizing antibodies and peptides, ¹²⁴I will be dehalogenated rapidly in vivo and result in poor tumor to background ratio. ⁶⁴Cu-TETA-1A3 has previously been reported for clinical PET imaging of metastatic colorectal cancer [54]. Detection of lung and liver metastasis was seriously hindered by non-specific uptake in the liver and the blood due to dissociation of the ⁶⁴Cu from the currently used chelates for radiolabeling mAbs [54]. ⁸⁹Zr is an attractive positron emitter due its longer half-life, however preparation of ⁸⁹Zr labeled mAbs is a multi-step, tedious process and ⁸⁹Zr has been shown to dissociate from the currently used chelates and to localize in the bone thereafter [55,56]. While for peptides and antibody fragments, ⁶⁸Ga has gained immense popularity in the recent years [57–59], despite the lack of the availability of radiopharmaceutical grade ⁸⁶Ge/⁶⁸Ga generator remains an issue (at least in the United States). Clinical translation of all these unconventional radionuclides will largely depend on the commercial availability and supply of radiopharmaceutical grade radionuclide. Availability and high energy gamma emission poses the biggest challenge for clinical translation of ⁸⁶Y.

It is clear from the selected examples cited in this report that efforts are continuing to develop ⁸⁶Y labeled radiopharmaceuticals as PET imaging agents and for dosimetry of ⁹⁰Y labeled radiopharmaceuticals. ⁸⁶Y labeled radiopharmaceuticals hold significant promises, but several scientific and non-scientific challenges such as availability have to be conquered before realizing its true potential in PET imaging agents and as a surrogate radioisotope for dosimetry of ⁹⁰Y.

Acknowledgments

This report was supported by the Intramural Research Program of the NIH, NCI, Center for Cancer Research, and the United States Department of Health and Human Services. Gratitude is expressed to Prof. Hari Seldon for useful discussion.

List of abbreviations


mAbs	Monoclonal antibodies

PET	Positron emission tomography

MRI	Magnetic resonance imaging

EDTA	Ethylenediaminetetraacetic acid

DTPA	Diethylenetriamine pentaacetic acid

DOTA	1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid

SCN-Bz	Isothiocyanatobenzyl

ca	Cyclic anhydride

% ID/g	Percent injected dose per gram

alpha-MSH	Alpha-melanocyte stimulating hormone

NHL	Non-Hodgkin’s lymphoma

HER	Human epidermal growth factor

VEGF	Vascular endothelial growth factor

scFv	Single-chain variable domain antibody fragment

IgG	immunoglobulin

Footnotes

Conflict of Interest: Herceptin^® and Avastin^® discussed in the article are manufactured by F. Hoffmann-La Roche, Ltd. TN is an employee of F. Hoffmann-La Roche, Ltd.

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