Synthesis of N-succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate: a radio-iodination agent for labeling internalizing proteins and peptides

PSMA-reactive NB7 single domain antibody fragment: A potential scaffold for developing prostate cancer theranostics

INTRODUCTION

Single domain antibody fragments (sdAbs) are an appealing scaffold for radiopharmaceutical development due to their small size (~15 kDa), high solubility, high stability, and excellent tumor penetration. Previously, we developed NB7 sdAb, which has very high affinity for an epitope on PSMA that is different from those targeted by small molecule PSMA inhibitors. Herein, we evaluated NB7 after radioiodination using [*I]SGMIB (1,3,4-isomer) and iso-[*I]SGMIB (1,3,5-isomer), as well as their 211At-labeled analogues.

METHODS

[*I]SGMIB, iso-[*I]SGMIB, [211At]SAGMB, and iso-[211At]SAGMB conjugates of NB7 sdAb were synthesized and their binding affinity, cell uptake and internalization were assessed in PSMA+ PC3 PIP and PSMA- PC3 flu cells. Biodistribution studies were performed in mice bearing PSMA+ PC3 PIP xenografts. First, a single-label experiment evaluated the tissue distribution of a NB7 bearing a His6-tag (NB7H6) and labeled with iso-[125I]SGMIB. Three paired-label experiments then were performed to compare: a) NB7 labeled using [*I]SGMIB and iso-[*I]SGMIB, b) 131I- vs 211At-labeled NB7 conjugates and c) [125I]SGMIB-NB7H6 to the small molecule PSMA inhibitor [131I]YF2.

RESULTS

All NB7 radioconjugates bound specifically to PSMA with dissociation constants, Kd, in the low nM range (1.4-6.4 nM). An initial biodistribution study demonstrated good tumor uptake for iso-[125I]SGMIB-NB7H6 (7.2 ± 1.5 % ID/g at 1 h) and no deleterious effect of the His6-tag on renal activity levels, which declined to 3.1 ± 1.1 % ID/g by 4 h. Paired-label biodistribution found no distinction between the two SGMIB isomer NB7 conjugates with the [131I]SGMIB-NB7-to-iso-[125I]SGMIB-NB7 tumor uptake ratios not significantly different from unity: 1.06 ± 0.08 at 1 h, 1.04 ± 0.12 at 4 h, and 1.07 ± 0.09 at 24 h. Both isomer conjugates cleared rapidly from normal tissues and exhibited very low uptake in thyroid, lacrimal and salivary glands. Paired-label biodistribution of [131I]SGMIB-NB7H6 and [211At]SAGMB-NB7H6 demonstrated similar tumor uptake and kidney clearance for the two radioconjugates. However, levels of 211At in thyroid, stomach, salivary and lacrimal glands were significantly higher (P < 0.05) that those for 131I suggesting greater dehalogenation for [211At]SAGMB-NB7H6. Finally, co-administration of [125I]SGMIB-NB7H6 and [131I]YF2 demonstrated good tumor uptake for both with considerably more rapid renal clearance for the NB7 radioconjugate.

CONCLUSION

NB7 radioconjugates exhibited good accumulation in PSMA-positive xenografts with rapid clearance from kidney and other normal tissues. We conclude that NB7 is a potentially useful scaffold for developing PSMA-targeted theranostics with different characteristics than current small molecule and antibody-based approaches.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Synthesis of Radioiodinated Compounds. Classical Approaches and Achievements of Recent Years

This review demonstrates the progress in the synthesis of radioiodinated compounds over the past decade. The possibilities and limitations of radiopharmaceuticals with different iodine isotopes, as well as the synthesis of low and high molecular weight compounds containing radioiodine, are discussed. An analysis of synthesis strategies, substrate frameworks, isolation methods, and metabolic stability, and the possibility of industrial production of radioiodinated organic derivatives which can find applications in the synthesis of drugs and diagnostics are presented.

Evaluation of an 131I-labeled HER2-specific single domain antibody fragment for the radiopharmaceutical therapy of HER2-expressing cancers

Radiopharmaceutical therapy (RPT) is an attractive strategy for treatment of disseminated cancers including those overexpressing the HER2 receptor including breast, ovarian and gastroesophageal carcinomas. Single-domain antibody fragments (sdAbs) exemplified by the HER2-targeted VHH_1028 evaluated herein are attractive for RPT because they rapidly accumulate in tumor and clear faster from normal tissues than intact antibodies. In this study, VHH_1028 was labeled using the residualizing prosthetic agent N-succinimidyl 3-guanidinomethyl 5-[¹³¹I]iodobenzoate (iso-[¹³¹I]SGMIB) and its tissue distribution evaluated in the HER2-expressing SKOV-3 ovarian and BT474 breast carcinoma xenograft models. In head-to-head comparisons to [¹³¹I]SGMIB-2Rs15d, a HER2-targeted radiopharmaceutical currently under clinical investigation, iso-[¹³¹I]SGMIB-VHH_1028 exhibited significantly higher tumor uptake and significantly lower kidney accumulation. The results demonstrated 2.9 and 6.3 times more favorable tumor-to-kidney radiation dose ratios in the SKOV-3 and BT474 xenograft models, respectively. Iso-[¹³¹I]SGMIB-VHH_1028 was prepared using a solid-phase extraction method for purification of the prosthetic agent intermediate Boc₂-iso-[¹³¹I]SGMIB that reproducibly scaled to therapeutic-level doses and obviated the need for its HPLC purification. Single-dose (SKOV-3) and multiple-dose (BT474) treatment regimens demonstrated that iso-[¹³¹I]SGMIB-VHH_1028 was well tolerated and provided significant tumor growth delay and survival prolongation. This study suggests that iso-[¹³¹I]SGMIB-VHH_1028 is a promising candidate for RPT of HER2-expressing cancers and further development is warranted.

Radioiodine Labeling Reagents and Methods for New Chemical Entities and Biomolecules

Several radioisotopes of iodine (¹²³I, ¹²⁴I, ¹²⁵I, and ¹³¹I) are available for medical use. One of them can be used, depending on the application, for radioiodine labeling of New Chemical Entities (NCEs) and biomolecules (peptides, proteins, protein fragments, monoclonal antibodies, etc.) for the development of novel imaging and therapeutic pharmaceuticals. Direct, using inorganic and organic oxidizing agents and enzyme catalysts, and indirect, using prosthetic groups, radioiodine-labeling methods have been used routinely in the past. In this report, a comprehensive review of the physical properties of various iodine radionuclides, their medical applications, and a summary of various radioiodine labeling reagents and methods for NCEs and biomolecules are provided.

Labeling single domain antibody fragments with 18F using a novel residualizing prosthetic agent - N-succinimidyl 3-(1-(2-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate

INTRODUCTION

Labeling single domain antibody fragments (sdAbs) with ¹⁸F is an attractive strategy for immunoPET. Earlier, we developed a residualizing label, N-succinimidyl 3-((4-(4-fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([¹⁸F]RL-I), synthesized via a click reaction for labeling sdAbs with ¹⁸F, that has attractive features but suffered from modest radiochemical yields and suboptimal hydrophobicity. Herein, we have evaluated the potential utility of an analogous agent, N-succinimidyl 3-(1-(2-(2-(2-(2-[¹⁸F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate ([¹⁸F]SFETGMB; [¹⁸F]RL-III) designed to address these limitations.

METHODS

[¹⁸F]RL-III was synthesized by the click reaction between 3-((2,3-bis(tert-butoxycarbonyl)guanidino)methyl)-5-ethynylbenzoate and 1-azido-2-(2-(2-(2-[¹⁸F]fluoroethoxy)ethoxy)ethoxy)ethane and subsequent deprotection. The anti-HER2 sdAbs 5F7 and 2Rs15d were labeled by conjugation with [¹⁸F]RL-III and compared in a paired-label fashion to the sdAbs labeled using N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB) or N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate (iso-[¹²⁵I]SGMIB). The ¹⁸F-labeled sdAbs were evaluated in vitro using HER2-expressing breast and ovarian carcinoma cells (BT474/BT474M1 and SKOV-3) and in vivo in athymic mice bearing subcutaneous SKOV-3 or BT474 xenografts. PET imaging of athymic mice bearing either subcutaneous BT474 or intracranial BT474M1Br-Fluc xenografts after administration of [¹⁸F]RL-III-5F7 also was performed.

RESULTS

Radiochemical yields for the synthesis of Boc₂-[¹⁸F]RL-III (21.5 ± 3.4%) were significantly higher than reported for Boc₂-[¹⁸F]RL-I. The overall radiochemical yields for the synthesis of [¹⁸F]RL-III-2Rs15d and [¹⁸F]RL-III-5F7 from aqueous [¹⁸F]fluoride were 1.7 ± 0.7% and 3.8 ± 2.3%, respectively. Both sdAbs, labeled using [¹⁸F]RL-III, retained affinity and immunoreactivity to HER2. Uptake and internalization of [¹⁸F]RL-III-5F7 in HER2-expressing cells was higher than that seen for [¹⁸F]RL-III-2Rs15d. Although different xenograft models were used, [¹⁸F]RL-III-2Rs15d showed relatively high uptake in a number of normal tissues, while uptake of [¹⁸F]RL-III-5F7 was mainly in tumor and kidneys with minimal background activity. Concordant with the necropsy experiments, microPET imaging with [¹⁸F]RL-III-5F7 in the BT474 subcutaneous model demonstrated clear delineation of the tumor (12.2 ± 5.1% ID/g) with minimal background activity except in kidneys. A tumor uptake (max) of 0.98%ID/g and a tumor-to-normal brain ratio of 9.8:1 were observed for [¹⁸F]RL-III-5F7 in the intracranial model.

CONCLUSIONS

Although higher radiochemical yields than that reported for [¹⁸F]RL-I were obtained, considerable improvements are needed for this method to be of practical utility. Despite clear tumor delineation with [¹⁸F]RL-III-5F7 as early as 1 h, high activity levels in the kidneys remain a concern.

Synthesis and preliminary evaluation of 211At-labeled inhibitors of prostate-specific membrane antigen for targeted alpha particle therapy of prostate cancer

INTRODUCTION

The high potency and short tissue range of α-particles are attractive features for targeted radionuclide therapy, particularly for cancers with micro-metastases. In the current study, we describe the synthesis of a series of ²¹¹At-labeled prostate-specific membrane antigen (PSMA) inhibitors and their preliminary evaluation as potential agents for metastatic prostate cancer treatment.

METHODS

Four novel Glu-urea based PSMA ligands containing a trialkyl stannyl group were synthesized and labeled with ²¹¹At, and for comparative purposes, ¹³¹I, via halodestannylation reactions with N-chlorosuccinimide as the oxidant. A PSMA inhibitory assay was performed to evaluate PSMA binding of the unlabeled, iodinated compounds. A series of paired-label biodistribution experiments were performed to compare each ²¹¹At-labeled PSMA ligand to its ¹³¹I-labeled counterpart in mice bearing subcutaneous PC3 PSMA+ PIP xenografts.

RESULTS

Radiochemical yields ranged from 32% to 65% for the ²¹¹At-labeled PSMA inhibitors and were consistently lower than those obtained with the corresponding ¹³¹I-labeled analogue. Good localization in PC3 PSMA+ PIP but not control xenografts was observed for all labeled molecules studied, which exhibited a variable degree of in vivo dehalogenation as reflected by thyroid and stomach activity levels. Normal tissue uptake and in vivo stability for several of the compounds was markedly improved compared with the previously evaluated compounds, [²¹¹At]DCABzL and [*I]DCIBzL.

CONCLUSIONS AND IMPLICATIONS FOR PATIENT CARE

Compared with the first generation compound [²¹¹At]DCABzL, several of the novel ²¹¹At-labeled PSMA ligands exhibited markedly improved stability in vivo and higher tumor-to-normal tissue ratios. [²¹¹At]GV-620 has the most promising characteristics and warrants further evaluation as a targeted radiotherapeutic for prostate cancer.

Labeling a TCO-functionalized single domain antibody fragment with 18F via inverse electron demand Diels Alder cycloaddition using a fluoronicotinyl moiety-bearing tetrazine derivative

Single domain antibody fragments (sdAbs) exhibit a rapid tumor uptake and fast blood clearance amenable for labeling with ¹⁸F (t_½ = 110 min) but suffer from high kidney accumulation. Previously, we developed a method for ¹⁸F-labeling of sdAbs via trans-cyclooctene (TCO)-tetrazine (Tz) inverse electron demand Diel's Alder cycloaddition reaction (IEDDAR) that incorporated a renal brush border enzyme (RBBE)-cleavable linker. Although >15 fold reduction in kidney activity levels was achieved, tumor uptake was compromised. Here we investigate whether replacing the [¹⁸F]AlF-NOTA moiety with [¹⁸F]fluoronicotinyl would rectify this problem. Anti-HER2 sdAb 5F7 was first derivatized with a TCO-containing agent that included the RBBE-cleavable linker GlyLys (GK) and a PEG chain, and then subjected to IEDDAR with 6-[¹⁸F]fluoronicotinyl-PEG₄-methyltetrazine to provide [¹⁸F]FN-PEG₄-Tz-TCO-GK-PEG₄-5F7 ([¹⁸F]FN-GK-5F7). For comparisons, a control lacking GK linker and 5F7 labeled using residualizing N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate (iso-[¹²⁵I]SGMIB) also were synthesized. Radiochemical purity, affinity (K_D) and immunoreactive fraction of [¹⁸F]FN-GK-5F7 were 99%, 5.4 ± 0.7 nM and 72.5 ± 4.3%, respectively. Tumor uptake of [¹⁸F]FN-GK-5F7 in athymic mice bearing subcutaneous SKOV3 xenografts (3.7 ± 1.2% ID/g and 3.4 ± 1.0% ID/g at 1 h and 3 h, respectively) was 2- to 3-fold lower than for co-injected iso-[¹²⁵I]SGMIB-5F7 (6.9 ± 1.9 %ID/g and 8.7 ± 3.0 %ID/g). However, due to its 6-fold lower kidney activity levels, tumor-to-kidney ratios for [¹⁸F]FN-GK-5F7 were 3-4 times higher than those for co-injected iso-[¹²⁵I]SGMIB-5F7 as well as those observed for the ¹⁸F conjugate lacking the RBBE-cleavable linker. Micro-PET/CT imaging of [¹⁸F]FN-GK-5F7 in mice with SKOV-3 subcutaneous xenografts clearly delineated tumor as early as 1 h with minimal activity in the kidneys; however, there was considerable activity in gallbladder and intestines. Although the tumor uptake of [¹⁸F]FN-GK-5F7 was unexpectedly disappointing, incorporating an alternative RBBE-cleavable linker into this labeling strategy may ameliorate this problem.

Synthesis and bioevaluation of radioiodinated nitroimidazole hypoxia imaging agents by one-pot click reaction

Eight radioiodinated 2-nitroimidazole derivatives for use as hypoxia imaging agents were synthesized by one-pot click reaction using four azides, two alkynes, and [¹³¹I]iodide ions and evaluated by hypoxic cellular uptake and biodistribution experiments. The results suggested that radiotracers with suitable partition coefficients (log P: -0.2-1.2) were more likely to have higher hypoxic cellular uptake. Among these eight molecules, [¹³¹I]15 ([¹³¹I]-(5-iodo-1-(2-(2-(2-nitro-1H-imidazol-1-yl)ethoxy)ethyl)-4-((2-nitro-1H-imidazol-1-yl)methyl)-1H-1,2,3-triazole)) had a suitable log P (0.05 ± 0.03) and contained two 2-nitroimidazole groups. The hypoxic/aerobic cellular uptake ratio of [¹³¹I]15 was 4.4 ± 0.5, and the tumor/blood (T/B) and tumor/muscle (T/M) ratios were 2.03 ± 0.45 and 6.82 ± 1.70, respectively. These results suggested that [¹³¹I]15 was a potential hypoxia imaging agent.

Site-specific radioiodination of an anti-HER2 single domain antibody fragment with a residualizing prosthetic agent

INTRODUCTION

As a consequence of their small size, high stability and high affinity, single domain antibody fragments (sdAbs) are appealing targeting vectors for radiopharmaceutical development. With sdAbs binding to internalizing receptors like HER2, residualizing prosthetic agents can enhance tumor retention of radioiodine, which until now has been done with random labeling approaches. Herein we evaluate a site-specific strategy utilizing a radioiodinated, residualizing maleimido moiety and the anti-HER2 sdAb 5F7 bearing a GGC tail for conjugation.

METHODS

Maleimidoethyl 3-(guanidinomethyl)-5-iodobenzoate ([¹³¹I]MEGMB) and its N-succinimidyl ester analogue, iso-[¹²⁵I]SGMIB, were labeled by halodestannylation and conjugated with 5F7GGC and 5F7, respectively. Radiochemical purity, immunoreactivity and binding affinity were determined. Paired-label experiments directly compared iso-[¹²⁵I]SGMIB-5F7 and [¹³¹I]MEGMIB-5F7GGC with regard to internalization/residualization and affinity on HER2-expressing SKOV-3 ovarian carcinoma cells as well as biodistribution and metabolite distribution in athymic mice with subcutaneous SKOV-3 xenografts.

RESULTS

[¹³¹I]MEGMIB-5F7GGC had an immunoreactivity of 81.3% and K_d = 0.94 ± 0.27 nM. Internalization assays demonstrated high intracellular trapping for both conjugates, For example, at 1 h, intracellular retention was 50.30 ± 3.36% for [¹³¹I]MEGMIB-5F7GGC and 55.95 ± 3.27% for iso-[¹²⁵I]SGMIB-5F7, while higher retention was seen for iso-[¹²⁵I]SGMIB-5F7 at later time points. Peak tumor uptake was similar for both conjugates (8.35 ± 2.66%ID/g and 8.43 ± 2.84%ID/g for iso-[¹²⁵I]SGMIB-5F7 and [¹³¹I]MEGMIB-5F7GGC at 1 h, respectively); however, more rapid normal tissue clearance was seen for [¹³¹I]MEGMIB-5F7GGC, with a 2-fold higher tumor-to-kidney ratio and a 3-fold higher tumor-to-liver ratio compared with co-injected iso-[¹²⁵I]SGMIB-5F7. Consisted with this, generation of labeled catabolites in the kidneys was higher for [¹³¹I]MEGMIB-5F7GGC.

CONCLUSION

[¹³¹I]MEGMIB-5F7GGC offers similar tumor targeting as iso-[¹²⁵I]SGMIB-5F7 but with generally lower normal tissue uptake.

ADVANCES IN KNOWLEDGE AND IMPLICATION FOR PATIENT CARE

The site specific nature of the [¹³¹I]MEGMIB reagent may facilitate clinical translation, particularly for sdAb with compromised affinity after random labeling.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as ⁸⁹Zr-Df-pertuzumab and ⁸⁹Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer. Thus, the advantages of AE emitters for cancer therapy are most likely to be realized by their selective delivery into the nucleus of the malignant cells. To achieve this goal, delivery systems must combine a challenging complex of properties that not only provide cancer cell preferential recognition but also cell entry followed by transport into the cell nucleus. A promising strategy for achieving this is the recruitment of natural cell transport processes of macromolecules, involved in each of the aforementioned steps. To date, a number of constructs exploiting intracellular transport systems have been proposed for AE emitter delivery to the nucleus of a targeted cell. An example of such a multifunctional vehicle that provides smart step-by-step delivery is the so-called modular nanotransporter, which accomplishes selective recognition, binding, internalization, and endosomal escape followed by nuclear import of the delivered radionuclide. The current review will focus on delivery systems utilizing various intracellular transport pathways and their combinations in order to provide efficient targeting of AE to the cancer cell nucleus.

Observations on the Effects of Residualization and Dehalogenation on the Utility of N-Succinimidyl Ester Acylation Agents for Radioiodination of the Internalizing Antibody Trastuzumab

Trastuzumab is an antibody used for the treatment of human epidermal growth factor receptor 2 (HER2)-overexpressing breast cancers. Since trastuzumab is an internalizing antibody, two factors could play an important role in achieving high uptake and prolonged retention of radioactivity in HER2-positive tumors after radioiodination-residualizing capacity after receptor-mediated internalization and susceptibility to dehalogenation. To evaluate the contribution of these two factors, trastuzumab was radiolabeled using the residualizing reagent N-succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate ([*I]SGMIB) and the nonresidualizing reagent N-succinimidyl-3-[*I]iodobenzoate ([*I]SIB), both of which are highly dehalogenation-resistant. Paired-label uptake and intracellular retention of [¹²⁵I]SGMIB-trastuzumab and [¹³¹I]SIB-trastuzumab was compared on HER2-expressing BT474 human breast carcinoma cells. Tumor uptake and normal tissue distribution characteristics for the two labeled conjugates were assessed in mice bearing BT474M1 xenografts. The internalization and intracellular retention of initially-bound radioactivity in BT474 cells was similar for the two labeled conjugates up to 4 h, but were significantly higher for [¹²⁵I]SGMIB-trastuzumab at 6 and 24 h. Similarly, [*I]SGMIB labeling resulted in significantly higher uptake and retention of radioactivity in BT474M1 xenografts at all studied time points. Moreover, tumor-to-tissue ratios for [¹²⁵I]SGMIB-trastuzumab were consistently higher than those for [¹³¹I]SIB-trastuzumab starting at 12 h postinjection. Thus, optimal targeting of HER2-positive breast cancers with a radioiodinated trastuzumab conjugate requires an acylation agent that imparts residualizing capacity in addition to high stability towards dehalogenation in vivo.

Labeling Single Domain Antibody Fragments with Fluorine-18 Using 2,3,5,6-Tetrafluorophenyl 6-[18F]Fluoronicotinate Resulting in High Tumor-to-Kidney Ratios

ImmunoPET agents are being investigated to assess the status of epidermal growth factor receptor 2 (HER2) in breast cancer patients with the goal of selecting those likely to benefit from HER2-targeted therapies and monitoring their progress after these treatments. We have been exploring the use of single domain antibody fragments (sdAbs) labeled with ¹⁸F using residualizing prosthetic agents for this purpose. In this study, we have labeled two sdAbs that bind to different domains on the HER2 receptor, 2Rs15d and 5F7, using 2,3,5,6-tetrafluorophenyl 6-[¹⁸F]fluoronicotinate ([¹⁸F]TFPFN) and evaluated their HER2 targeting properties in vitro and in vivo. The overall decay-corrected radiochemical yield for the synthesis of [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 was 5.7 ± 3.6 and 4.0 ± 2.0%, respectively. The radiochemical purity of labeled sdAbs was >95%, immunoreactive fractions were about 60%, and affinity was in the low nanomolar range. Intracellularly trapped activity from [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 in HER2-expressing SKOV-3 ovarian and BT474M1 breast carcinoma cells were similar to the sdAbs labeled using the previously validated radioiodination residualizing prosthetic agents N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB) and N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate ( iso-[¹²⁵I]SGMIB). Intracellular activity was about 2-fold higher for radiolabeled 5F7 compared with 2Rs15d for both ¹⁸F and ¹²⁵I. While tumor uptake of both [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 was comparable to those for the coadministered ¹²⁵I-labeled sdAb, renal uptake of the ¹⁸F-labeled sdAbs was substantially lower. In microPET images, the tumor was clearly delineated in SKOV-3 and BT474 xenograft-bearing athymic mice with low levels of background activity in normal tissues, except the bladder. These results indicate that the [¹⁸F]TFPFN prosthetic group could be a valuable reagent for developing sdAb-based immunoPET imaging agents.

Fluorine-18 Labeling of the HER2-Targeting Single-Domain Antibody 2Rs15d Using a Residualizing Label and Preclinical Evaluation

PURPOSE

Our previous studies with F-18-labeled anti-HER2 single-domain antibodies (sdAbs) utilized 5F7, which binds to the same epitope on HER2 as trastuzumab, complicating its use for positron emission tomography (PET) imaging of patients undergoing trastuzumab therapy. On the other hand, sdAb 2Rs15d binds to a different epitope on HER2 and thus might be a preferable vector for imaging in these patients. The aim of this study was to evaluate the tumor targeting of F-18 -labeled 2Rs15d in HER2-expressing breast carcinoma cells and xenografts.

PROCEDURES

sdAb 2Rs15d was labeled with the residualizing labels N-succinimidyl 3-((4-(4-[¹⁸F]fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([¹⁸F]RL-I) and N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB), and the purity and HER2-specific binding affinity and immunoreactivity were assessed after labeling. The biodistribution of I-125- and F-18-labeled 2Rs15d was determined in SCID mice bearing subcutaneous BT474M1 xenografts. MicroPET/x-ray computed tomograph (CT) imaging of [¹⁸F]RL-I-2Rs15d was performed in this model and compared to that of nonspecific sdAb [¹⁸F]RL-I-R3B23. MicroPET/CT imaging was also done in an intracranial HER2-positive breast cancer brain metastasis model after administration of 2Rs15d-, 5F7-, and R3B23-[¹⁸F]RL-I conjugates.

RESULTS

[¹⁸F]RL-I was conjugated to 2Rs15d in 40.8 ± 9.1 % yield and with a radiochemical purity of 97-100 %. Its immunoreactive fraction (IRF) and affinity for HER2-specific binding were 79.2 ± 5.4 % and 7.1 ± 0.4 nM, respectively. [¹²⁵I]SGMIB was conjugated to 2Rs15d in 58.4 ± 8.2 % yield and with a radiochemical purity of 95-99 %; its IRF and affinity for HER2-specific binding were 79.0 ± 12.9 % and 4.5 ± 0.8 nM, respectively. Internalized radioactivity in BT474M1 cells in vitro for [¹⁸F]RL-I-2Rs15d was 43.7 ± 3.6, 36.5 ± 2.6, and 21.7 ± 1.2 % of initially bound radioactivity at 1, 2, and 4 h, respectively, and was similar to that seen for [¹²⁵I]SGMIB-2Rs15d. Uptake of [¹⁸F]RL-I-2Rs15d in subcutaneous xenografts was 16-20 %ID/g over 1-3 h. Subcutaneous tumor could be clearly delineated by microPET/CT imaging with [¹⁸F]RL-I-2Rs15d but not with [¹⁸F]RL-I-R3B23. Intracranial breast cancer brain metastases could be visualized after intravenous administration of both [¹⁸F]RL-I-2Rs15d and [¹⁸F]RL-I-5F7.

CONCLUSIONS

Although radiolabeled 2Rs15d conjugates exhibited lower tumor cell retention both in vitro and in vivo than that observed previously for 5F7, given that it binds to a different epitope on HER2 from those targeted by the clinically utilized HER2-targeted therapeutic antibodies trastuzumab and pertuzumab, F-18-labeled 2Rs15d has potential for assessing HER2 status by PET imaging after trastuzumab and/or pertuzumab therapy.

Development of a new thiol-reactive prosthetic group for site-specific labeling of biomolecules with radioactive iodine

In this report, we describe the radiosynthesis of a new thiol-targeting prosthetic group for efficient radioactive iodine labeling of biomolecules. Radioiodination using the precursor 3 was performed to obtain ¹²⁵I-labeled tetrazole 4b with high radiochemical yield (73%) and radiochemical purity. Using the radiolabeled 4b, a single free cysteine containing peptide and human serum albumin were labeled with ¹²⁵I in modest-to-good radiochemical yields (65-99%) under mildly reactive conditions. A biodistribution study of [¹²⁵I]7 in normal ICR mice exhibited lower thyroid uptake values than those of ¹²⁵I-labeled human serum albumin prepared via a traditional radiolabeling method. Thus, [¹²⁵I]7 could be employed as an effective radiotracer for molecular imaging and biodistribution studies. The results clearly demonstrate that 4b has the potential to be effectively implemented as a prosthetic group in the preparation of radiolabeled biomolecules.

Fluorine-18 labeling of an anti-HER2 VHH using a residualizing prosthetic group via a strain-promoted click reaction: Chemistry and preliminary evaluation

In a previous study, we evaluated a HER2-specific single domain antibody fragment (sdAb) 2Rs15d labeled with ¹⁸F via conjugation of a residualizing prosthetic agent that was synthesized by copper-catalyzed azide-alkyne cycloaddition (CuAAC). In order to potentially increase overall efficiency and decrease the time required for labeling, we now investigate the use of a strain-promoted azide-alkyne cycloaddition (SPAAC) between the 2Rs15d sdAb, which had been pre-derivatized with an azide-containing residualizing moiety, and an ¹⁸F-labeled aza-dibenzocyclooctyne derivative. The HER2-targeted sdAb 2Rs15d and a nonspecific sdAb R3B23 were pre-conjugated with a moiety containing both azide- and guanidine functionalities. The thus derivatized sdAbs were radiolabeled with ¹⁸F using an ¹⁸F-labeled aza-dibenzocyclooctyne derivative ([¹⁸F]F-ADIBO) via SPAAC, generating the desired conjugate ([¹⁸F]RL-II-sdAb). For comparison, unmodified 2Rs15d was labeled with N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB), the prototypical residualizing agent for radioiodination. Radiochemical purity (RCP), immunoreactive fraction (IRF), HER2-binding affinity and cellular uptake of [¹⁸F]RL-II-2Rs15d were assessed in vitro. Paired label biodistribution of [¹⁸F]RL-II-2Rs15d and [¹²⁵I]SGMIB-2Rs15d, and microPET/CT imaging of [¹⁸F]RL-II-2Rs15d and the [¹⁸F]RL-II-R3B23 control sdAb were performed in nude mice bearing HER2-expressing SKOV-3 xenografts. A radiochemical yield of 23.9 ± 6.9% (n = 8) was achieved for the SPAAC reaction between [¹⁸F]F-ADIBO and azide-modified 2Rs15d and the RCP of the labeled sdAb was >95%. The affinity (K_d) and IRF for the binding of [¹⁸F]RL-II-2Rs15d to HER2 were 5.6 ± 1.3 nM and 73.1 ± 22.5% (n = 3), respectively. The specific uptake of [¹⁸F]RL-II-2Rs15d by HER2-expressing BT474M1 breast carcinoma cells in vitro was 14-17% of the input dose at 1, 2, and 4 h, slightly higher than seen for co-incubated [¹²⁵I]SGMIB-2Rs15d. The uptake of [¹⁸F]RL-II-2Rs15d in SKOV-3 xenografts at 1 h and 2 h p.i. were 5.54 ± 0.77% ID/g and 6.42 ± 1.70% ID/g, respectively, slightly higher than those for co-administered [¹²⁵I]SGMIB-2Rs15d (4.80 ± 0.78% ID/g and 4.78 ± 1.39% ID/g). MicroPET/CT imaging with [¹⁸F]RL-II-2Rs15d at 1-3 h p.i. clearly delineated SKOV-3 tumors while no significant accumulation of activity in tumor was seen for [¹⁸F]RL-II-R3B23. With the exception of kidneys, normal tissue levels for [¹⁸F]RL-II-2Rs15d were low and cleared rapidly. To our knowledge, this is the first time SPAAC method has been used to label an sdAb with ¹⁸F, especially with residualizing functionality.

Astatine-211 labeled anti-HER2 5F7 single domain antibody fragment conjugates: radiolabeling and preliminary evaluation

INTRODUCTION

Derived from heavy chain only camelid antibodies, ~15-kDa single-domain antibody fragments (sdAbs) are an attractive platform for developing molecularly specific imaging probes and targeted radiotherapeutics. The rapid tumor accumulation and normal tissue clearance of sdAbs might be ideal for use with 211At, a 7.2-h half-life α-emitter, if appropriate labeling chemistry can be devised to trap 211At in cancer cells after sdAb binding. This study evaluated two reagents, [211At]SAGMB and iso-[211At]SAGMB, for this purpose.

METHODS

[211At]SAGMB and iso-[211At]SAGMB, and their radioiodinated analogues [131I]SGMIB and iso-[131I]SGMIB, were synthesized by halodestannylation and reacted with the anti-HER2 sdAb 5F7. Radiochemical purity, immunoreactivity and binding affinity were determined. Paired-label internalization assays on HER2-expressing BT474M1 breast carcinoma cells directly compared [131I]SGMIB-5F7/[211At]SAGMB-5F7 and iso-[131I]SGMIB-5F7/iso-[211At]SAGMB-5F7 tandems. The biodistribution of the two tandems was evaluated in SCID mice with subcutaneous BT474M1 xenografts.

RESULTS

Radiochemical yields for Boc2-iso-[211At]SAGMB and Boc2-[211At]SAGMB synthesis, and efficiencies for coupling of iso-[211At]SAGMB and [211At]SAGMB to 5F7 were similar, with radiochemical purities of [211At]SAGMB-5F7 and iso-[211At]SAGMB-5F7 >98%. iso-[211At]SAGMB-5F7 and [211At]SAGMB-5F7 had immunoreactive fractions >80% and HER2 binding affinities of less than 5 nM. Internalization assays demonstrated high intracellular trapping of radioactivity, with little difference observed between corresponding 211At- and 131I-labeled 5F7 conjugates. Higher BT474M1 intracellular retention was observed from 1-6 h for the iso-conjugates (iso-[211At]SAGMB-5F7, 74.3 ± 2.8%, vs. [211At]SAGMB-5F7, 63.7 ± 0.4% at 2 h) with the opposite behavior observed at 24 h. Peak tumor uptake for iso-[211At]SAGMB-5F7 was 23.4 ± 2.2% ID/g at 4 h, slightly lower than its radioiodinated counterpart, but significantly higher than observed with [211At]SAGMB-5F7. Except in kidneys and lungs, tumor-to-normal organ ratios for iso-[211At]SAGMB-5F7 were greater than 10:1 by 2 h, and significantly higher than those for [211At]SAGMB-5F7.

CONCLUSION

These 211At-labeled sdAb conjugates, particularly iso-[211At]SAGMB-5F7, warrant further evaluation for targeted α-particle radiotherapy of HER2-expressing cancers.

131I-labeled Anti-HER2 Camelid sdAb as a Theranostic Tool in Cancer Treatment

Purpose: Camelid single-domain antibody-fragments (sdAb) have beneficial pharmacokinetic properties, and those targeted to HER2 can be used for imaging of HER2-overexpressing cancer. Labeled with a therapeutic radionuclide, they may be used for HER2-targeted therapy. Here, we describe the generation of a ¹³¹I-labeled sdAb as a theranostic drug to treat HER2-overexpressing cancer.Experimental Design: Anti-HER2 sdAb 2Rs15d was labeled with ¹³¹I using [¹³¹I]SGMIB and evaluated in vitro Biodistribution was evaluated in two HER2⁺ murine xenograft models by micro-SPECT/CT imaging and at necropsy, and under challenge with trastuzumab and pertuzumab. The therapeutic potential of [¹³¹I]SGMIB-2Rs15d was investigated in two HER2⁺ tumor mouse models. A single-dose toxicity study was performed in mice using unlabeled [¹²⁷I]SGMIB-sdAb at 1.4 mg/kg. The structure of the 2Rs15d-HER2 complex was determined by X-ray crystallography.Results: [¹³¹I]SGMIB-2Rs15d bound specifically to HER2⁺ cells (K_d = 4.74 ± 0.39 nmol/L). High and specific tumor uptake was observed in both BT474/M1 and SKOV-3 tumor xenografted mice and surpassed kidney levels by 3 hours. Extremely low uptake values were observed in other normal tissues at all time points. The crystal structure revealed that 2Rs15d recognizes HER2 Domain 1, consistent with the lack of competition with trastuzumab and pertuzumab observed in vivo [¹³¹I]SGMIB-2Rs15d alone, or in combination with trastuzumab, extended median survival significantly. No toxicity was observed after injecting [¹²⁷I]SGMIB-2Rs15d.Conclusions: These findings demonstrate the theranostic potential of [¹³¹I]SGMIB-2Rs15d. An initial scan using low radioactive [*I]SGMIB-2Rs15d allows patient selection and dosimetry calculations for subsequent therapeutic [¹³¹I]SGMIB-2Rs15d and could thereby impact therapy outcome on HER2⁺ breast cancer patients. Clin Cancer Res; 23(21); 6616-28. ©2017 AACR.

Systems Level Analysis of Histone H3 Post-translational Modifications (PTMs) Reveals Features of PTM Crosstalk in Chromatin Regulation

Histones are abundant chromatin constituents carrying numerous post-translational modifications (PTMs). Such PTMs mediate a variety of biological functions, including recruitment of enzymatic readers, writers and erasers that modulate DNA replication, transcription and repair. Individual histone molecules contain multiple coexisting PTMs, some of which exhibit crosstalk, i.e. coordinated or mutually exclusive activities. Here, we present an integrated experimental and computational systems level molecular characterization of histone PTMs and PTM crosstalk. Using wild type and engineered mouse embryonic stem cells (mESCs) knocked out in components of the Polycomb Repressive Complex 2 (PRC2, Suz12(-/-)), PRC1 (Ring1A/B(-/-)) and (Dnmt1/3a/3b(-/-)) we performed comprehensive PTM analysis of histone H3 tails (50 aa) by utilizing quantitative middle-down proteome analysis by tandem mass spectrometry. We characterized combinatorial PTM features across the four mESC lines and then applied statistical data analysis to predict crosstalk between histone H3 PTMs. We detected an overrepresentation of positive crosstalk (codependent marks) between adjacent mono-methylated and acetylated marks, and negative crosstalk (mutually exclusive marks) among most of the seven characterized di- and tri-methylated lysine residues in the H3 tails. We report novel features of PTM interplay involving hitherto poorly characterized arginine methylation and lysine methylation sites, including H3R2me, H3R8me and H3K37me. Integration of the H3 data with RNAseq data by coabundance clustering analysis of histone PTMs and histone modifying enzymes revealed correlations between PTM and enzyme levels. We conclude that middle-down proteomics is a powerful tool to determine conserved or dynamic interdependencies between histone marks, which paves the way for detailed investigations of the histone code. Histone H3 PTM data is publicly available in the CrossTalkDB repository at http://crosstalkdb.bmb.sdu.dk.

Highly efficient method for 125I-radiolabeling of biomolecules using inverse-electron-demand Diels-Alder reaction

In this report, we present a rapid and highly efficient method for radioactive iodine labeling of trans-cyclooctene group conjugated biomolecules using inverse-electron-demand Diels-Alder reaction. Radioiodination reaction of the tetrazine structure was carried out using the stannylated precursor 2 to give 125I-labeled product ([125I]1) with high radiochemical yield (65±8%) and radiochemical purity (>99%). For radiolabeling application of [125I]1, trans-cyclooctene derived cRGD peptide and human serum albumin were prepared. These substrates were reacted with [125I]1 under mild condition to provide the radiolabeled products [125I]6 and [125I]8, respectively, with excellent radiochemical yields. The biodistribution study of [125I]8 in normal ICR mice showed significantly lower thyroid uptake values than that of 125I-labeled human serum albumin prepared by a traditional radiolabeling method. Therefore [125I]8 will be a useful radiolabeled tracer in various molecular imaging and biological studies. Those results clearly demonstrate that [125I]1 will be used as a valuable prosthetic group for radiolabeling of biomolecules.

N-Succinimidyl 3-((4-(4-[(18)F]fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([(18)F]SFBTMGMB): a residualizing label for (18)F-labeling of internalizing biomolecules

Residualizing labeling methods for internalizing peptides and proteins are designed to trap the radionuclide inside the cell after intracellular degradation of the biomolecule. The goal of this work was to develop a residualizing label for the (18)F-labeling of internalizing biomolecules based on a template used successfully for radioiodination. N-Succinimidyl 3-((4-(4-[(18)F]fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(bis-Boc-guanidinomethyl)benzoate ([(18)F]SFBTMGMB-Boc2) was synthesized by a click reaction of an azide precursor and [(18)F]fluorohexyne in 8.5 ± 2.8% average decay-corrected radiochemical yield (n = 15). An anti-HER2 nanobody 5F7 was labeled with (18)F using [(18)F]SFBTMGMB ([(18)F]RL-I), obtained by the deprotection of [(18)F]SFBTMGMB-Boc2, in 31.2 ± 6.7% (n = 5) conjugation efficiency. The labeled nanobody had a radiochemical purity of >95%, bound to HER2-expressing BT474M1 breast cancer cells with an affinity of 4.7 ± 0.9 nM, and had an immunoreactive fraction of 62-80%. In summary, a novel residualizing prosthetic agent for labeling biomolecules with (18)F has been developed. An anti-HER2 nanobody was labeled using this prosthetic group with retention of affinity and immunoreactivity to HER2.

N-Succinimidyl guanidinomethyl iodobenzoate protein radiohalogenation agents: influence of isomeric substitution on radiolabeling and target cell residualization

INTRODUCTION

N-succinimidyl 4-guanidinomethyl-3-[(*)I]iodobenzoate ([(*)I]SGMIB) has shown promise for the radioiodination of monoclonal antibodies (mAbs) and other proteins that undergo extensive internalization after receptor binding, enhancing tumor targeting compared to direct electrophilic radioiodination. However, radiochemical yields for [(131)I]SGMIB synthesis are low, which we hypothesize is due to steric hindrance from the Boc-protected guanidinomethyl group ortho to the tin moiety. To overcome this, we developed the isomeric compound, N-succinimidyl 3-guanidinomethyl-5-[(131)I]iodobenzoate (iso-[(131)I]SGMIB) wherein this bulky group was moved from ortho to meta position.

METHODS

Boc2-iso-SGMIB standard and its tin precursor, N-succinimidyl 3-((1,2-bis(tert-butoxycarbonyl)guanidino)methyl)-5-(trimethylstannyl)benzoate (Boc2-iso-SGMTB), were synthesized using two disparate routes, and iso-[*I]SGMIB synthesized from the tin precursor. Two HER2-targeted vectors - trastuzumab (Tras) and a nanobody 5F7 (Nb) - were labeled using iso-[(*)I]SGMIB and [(*)I]SGMIB. Paired-label internalization assays in vitro with both proteins, and biodistribution in vivo with trastuzumab, labeled using the two isomeric prosthetic agents were performed.

RESULTS

When the reactions were performed under identical conditions, radioiodination yields for the synthesis of Boc2-iso-[(131)I]SGMIB were significantly higher than those for Boc2-[(131)I]SGMIB (70.7±2.0% vs 56.5±5.5%). With both Nb and trastuzumab, conjugation efficiency also was higher with iso-[(131)I]SGMIB than with [(131)I]SGMIB (Nb, 33.1±7.1% vs 28.9±13.0%; Tras, 45.1±4.5% vs 34.8±10.3%); however, the differences were not statistically significant. Internalization assays performed on BT474 cells with 5F7 Nb indicated similar residualizing capacity over 6h; however, at 24h, radioactivity retained intracellularly for iso-[(131)I]SGMIB-Nb was lower than for [(125)I]SGMIB-Nb (46.4±1.3% vs 56.5±2.5%); similar results were obtained using Tras. Likewise, a paired-label biodistribution of Tras labeled using iso-[(125)I]SGMIB and [(131)I]SGMIB indicated an up to 22% tumor uptake advantage at later time points for [(131)I]SGMIB-Tras.

CONCLUSION

Given the higher labeling efficiency obtained with iso-SGMIB, this residualizing agent might be of value for use with shorter half-life radiohalogens.

Radiolabeling and in vitro evaluation of (67)Ga-NOTA-modular nanotransporter--a potential Auger electron emitting EGFR-targeted radiotherapeutic

INTRODUCTION

Modular nanotransporters (MNTs) are vehicles designed to transport drugs from the cell surface via receptor-mediated endocytosis and endosomal escape to nucleus. Hence their conjugation to Auger electron emitters, can cause severe cell killing, by nuclear localization. Herein we evaluate the use of MNT as a platform for targeted radiotherapy with (67)Ga.

METHODS

EGF was the targeting ligand on the MNT, and NOTA was selected for its radiolabeling with (67)Ga. In the radiolabeling study we dealt with the precipitation of MNT (pI 5.7) at the labeling pH (4.5-5.5) of (67)Ga. Cellular and nuclei uptake of (67)Ga-NOTA-MNT by the A431 cell line was determined. Its specific cytotoxicity was compared to that of (67)Ga-EDTA, (67)Ga-NOTA-BSA and (67)Ga-NOTA-hEGF, in A431 and U87MGWTT, cell lines, by clonogenic assay. Dosimetry studies were also performed.

RESULTS

(67)Ga-NOTA-MNT was produced with 90% yield and specific activity of 25.6mCi/mg. The in vitro kinetics revealed an increased uptake over 24h. 55% of the internalized radioactivity was detected in the nuclei at 1h. The cytotoxicity of (67)Ga-NOTA-MNT on A431 cell line was 17 and 385-fold higher when compared to non-specific (67)Ga-NOTA-BSA and (67)Ga-EDTA. While its cytotoxic potency was 13 and 72-fold higher when compared to (67)Ga-NOTA-hEGF in the A431 and the U87MGWTT cell lines, respectively, validating its nuclear localization. The absorbed dose, for 63% cell killing, was 8Gy, confirming the high specific index of (67)Ga.

CONCLUSION

These results demonstrate the feasibility of using MNT as a platform for single cell kill targeted radiotherapy by Auger electron emitters.

Improved tumor targeting of anti-HER2 nanobody through N-succinimidyl 4-guanidinomethyl-3-iodobenzoate radiolabeling

Nanobodies are approximately 15-kDa proteins based on the smallest functional fragments of naturally occurring heavy chain-only antibodies and represent an attractive platform for the development of molecularly targeted agents for cancer diagnosis and therapy. Because the human epidermal growth factor receptor type 2 (HER2) is overexpressed in breast and ovarian carcinoma, as well as in other malignancies, HER2-specific Nanobodies may be valuable radiodiagnostics and therapeutics for these diseases. The aim of the present study was to evaluate the tumor-targeting potential of anti-HER2 5F7GGC Nanobody after radioiodination with the residualizing agent N-succinimidyl 4-guanidinomethyl 3-(125/131)I-iodobenzoate (*I-SGMIB).

METHODS

The 5F7GGC Nanobody was radiolabeled using *I-SGMIB and, for comparison, with N(ε)-(3-*I-iodobenzoyl)-Lys(5)-N(α)-maleimido-Gly(1)-GEEEK (*I-IB-Mal-d-GEEEK), another residualizing agent, and by direct radioiodination using IODO-GEN ((125)I-Nanobody). The 3 labeled Nanobodies were evaluated in affinity measurements, and paired-label internalization assays were performed on HER2-expressing BT474M1 breast carcinoma cells and in paired-label tissue distribution measurements in mice bearing subcutaneous BT474M1 xenografts.

RESULTS

*I-SGMIB-Nanobody was produced in 50.4% ± 3.6% radiochemical yield and exhibited a dissociation constant of 1.5 ± 0.5 nM. Internalization assays demonstrated that intracellular retention of radioactivity was up to 1.5-fold higher for *I-SGMIB-Nanobody than for coincubated (125)I-Nanobody or *I-IB-Mal-d-GEEEK-Nanobody. Peak tumor uptake for *I-SGMIB-Nanobody was 24.50% ± 9.89% injected dose/g at 2 h, 2- to 4-fold higher than observed with other labeling methods, and was reduced by 90% with trastuzumab blocking, confirming the HER2 specificity of localization. Moreover, normal-organ clearance was fastest for *I-SGMIB-Nanobody, such that tumor-to-normal-organ ratios greater than 50:1 were reached by 24 h in all tissues except lungs and kidneys, for which the values were 10.4 ± 4.5 and 5.2 ± 1.5, respectively.

CONCLUSION

Labeling anti-HER2 Nanobody 5F7GGC with *I-SGMIB yields a promising new conjugate for targeting HER2-expressing malignancies. Further research is needed to determine the potential utility of *I-SGMIB-5F7GGC labeled with (124)I, (123)I, and (131)I for PET and SPECT imaging and for targeted radiotherapy, respectively.

A one-pot three-component radiochemical reaction for rapid assembly of 125I-labeled molecular probes

Nuclear imaging in conjunction with radioactive tracers enables noninvasive measurements of biochemical events in vivo. However, access to tracers remains limited due to the lack of methods for rapid assembly of radiolabeled molecules with the prerequisite biological activity. Herein, we report a one-pot, three-component, copper(II)-mediated reaction of azides, alkynes, and [(125)I]iodide to yield 5-[(125)I]iodo-1,2,3-triazoles. Using a selection of azides and alkynes in a combinatorial approach, we have synthesized a library of structurally diverse (125)I-labeled triazoles functionalized with bioconjugation groups, fluorescent dyes, and biomolecules. Our preliminary biological evaluation suggests that 5-[(125)I]iodo-1,2,3-triazoles are resistant to deiodination in vivo, both as small molecular probes and as antibody conjugates. The ability to incorporate radioactive iodide into triazoles directly from the parent azides and alkynes makes the method broadly applicable and offers the potential to rapidly assemble molecular probes from an array of structurally diverse, and readily available, building blocks.

Modular nanotransporters: a versatile approach for enhancing nuclear delivery and cytotoxicity of Auger electron-emitting 125I

Background

This study evaluates the potential utility of a modular nanotransporter (MNT) for enhancing the nuclear delivery and cytotoxicity of the Auger electron emitter ¹²⁵I in cancer cells that overexpress the epidermal growth factor receptor (EGFR).

Methods

MNTs are recombinant multifunctional polypeptides that we have developed for achieving selective delivery of short-range therapeutics into cancer cells. MNTs contain functional modules for receptor binding, internalization, endosomal escape and nuclear translocation, thereby facilitating the transport of drugs from the cell surface to the nucleus. The MNT described herein utilized EGF as the targeting ligand and was labeled with ¹²⁵I using N-succinimidyl-4-guanidinomethyl-3-[¹²⁵I]iodobenzoate (SGMIB). Membrane binding, intracellular and nuclear accumulation kinetics, and clonogenic survival assays were performed using the EGFR-expressing A431 epidermoid carcinoma and D247 MG glioma cell lines.

Results

[¹²⁵I]SGMIB-MNT bound to A431 and D247 MG cells with an affinity comparable to that of native EGF. More than 60% of internalized [¹²⁵I]SGMIB-MNT radioactivity accumulated in the cell nuclei after a 1-h incubation. The cytotoxic effectiveness of [¹²⁵I]SGMIB-MNT compared with ¹²⁵I-labeled bovine serum albumin control was enhanced by a factor of 60 for D247 MG cells and more than 1,000-fold for A431 cells, which express higher levels of EGFR.

Conclusions

MNT can be utilized to deliver ¹²⁵I into the nuclei of cancer cells overexpressing EGFR, significantly enhancing cytotoxicity. Further evaluation of [¹²⁵I]SGMIB-MNT as a targeted radiotherapeutic for EGFR-expressing cancer cells appears warranted.

SIB-DOTA: a trifunctional prosthetic group potentially amenable for multi-modal labeling that enhances tumor uptake of internalizing monoclonal antibodies

A major drawback of internalizing monoclonal antibodies (mAbs) radioiodinated with direct electrophilic approaches is that tumor retention of radioactivity is compromised by the rapid washout of iodo-tyrosine, the primary labeled catabolite for mAbs labeled via this strategy. In our continuing efforts to develop more versatile residualizing labels that could overcome this problem, we have designed SIB-DOTA, a prosthetic labeling template that combines the features of the prototypical, dehalogenation-resistant N-succinimidyl 3-iodobenzoate (SIB) with DOTA, a useful macrocyclic chelator for labeling with radiometals. Herein we describe the synthesis of the unlabeled standard of this prosthetic moiety, its protected tin precursor, and radioiodinated SIB-DOTA. An anti-EGFRvIII-reactive mAb, L8A4 was radiolabeled with [(131)I]SIB-DOTA in 27.1±6.2% (n=2) conjugation yields and its targeting properties to the same mAb labeled with [(125)I]SGMIB both in vitro and in vivo using U87MG·ΔEGFR cells and xenografts were compared. In vitro paired-label internalization assays showed that the intracellular radioactivity from [(131)I]SIB-DOTA-L8A4 was 21.4±0.5% and 26.2±1.1% of initially bound radioactivity at 16 and 24h, respectively. In comparison, these values for [(125)I]SGMIB-L8A4 were 16.7±0.5% and 14.9±1.1%. Similarly, the SIB-DOTA prosthetic group provided better tumor targeting in vivo than SGMIB over 8 d period. These results suggest that SIB-DOTA warrants further evaluation as a residualizing agent for labeling internalizing mAbs including those targeted to EGFRvIII.

Anti-EGFRvIII monoclonal antibody armed with 177Lu: in vivo comparison of macrocyclic and acyclic ligands

INTRODUCTION

Monoclonal antibody (mAb) L8A4 binds specifically to the epidermal growth factor receptor variant III (EGFRvIII) that is present on gliomas but not on normal tissues, and is internalized rapidly after receptor binding. Because of the short range of its β-emissions, labeling this mAb with (177)Lu would be an attractive approach for the treatment of residual tumor margins remaining after surgical debulking of brain tumors.

MATERIALS AND METHODS

L8A4 mAb was labeled with (177)Lu using the acyclic ligands [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A″-DTPA) and 2-(4-isothiocyanatobenzyl)-6-methyldiethylene-triaminepentaacetic acid (1B4M-DTPA), and the macrocyclic ligands S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (C-DOTA) and α-(5-isothiocyanato-2-methoxyphenyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (MeO-DOTA). Paired-label tissue distribution experiments were performed in athymic mice bearing subcutaneous EGFRvIII-expressing U87.ΔEGFR glioma xenografts over a period of 1 to 8 days to directly compare (177)Lu-labeled L8A4 to L8A4 labeled with (125)I using N-succinimidyl 4-guanidinomethyl-3-[(125)I]iodobenzoate ([(125)I]SGMIB).

RESULTS

Except with C-DOTA, tumor uptake for the (177)Lu-labeled mAb was significantly higher than the co-administered radioiodinated preparation; however, this was also the case for spleen, liver, bone and kidneys. Tumor/normal tissue ratios for (177)Lu-1B4M-DTPA-L8A4 and, to an even greater extent, (177)Lu-MeO-DOTA-L8A4 were higher than those for [(125)I]SGMIB-L8A4 in most other tissues.

CONCLUSIONS

Tumor and normal tissue distribution patterns for this anti-EGFRvIII mAb were dependent on the nature of the bifunctional chelate used for (177)Lu labeling. Optimal results were obtained with 1B4M-DTPA and MeO-DOTA, suggesting no clear advantage for acyclic vs. macrocyclic ligands for this application.

Evaluation of an anti-p185(HER2) (scFv-C(H)2-C(H)3)2 fragment following radioiodination using two different residualizing labels: SGMIB and IB-Mal-D-GEEEK

INTRODUCTION

A 105-kDa double mutant single-chain Fv-Fc fragment (scFv-Fc DM) derived from the anti-p185(HER2) hu4D5v8 antibody (trastuzumab; Herceptin) has been described recently. The goal of this study was to investigate whether improved tumor targeting could be achieved with this fragment through the use of residualizing radioiodination methods.

METHODS

The scFv-Fc DM fragment was radioiodinated using N-succinimidyl 4-guanidinomethyl 3-[(131)I]iodobenzoate ([(131)I]SGMIB) and N(epsilon)-(3-[(131)I]iodobenzoyl)-Lys(5)-N(alpha)- maleimido-Gly(1)-GEEEK ([(131)I]IB-Mal-D-GEEEK), two residualizing radioiodination agents that have been used successfully with intact antibodies. Paired-label internalization assays of the labeled fragments were performed in vitro using MCF7 human breast cancer cells transfected to express HER2 (MCF7-HER2); comparisons were made to scFv-Fc DM directly radioiodinated using Iodogen. The tissue distribution of the scFv-Fc DM labeled with [(125)I]IB-Mal-d-GEEEK and [(131)I]SGMIB was compared in athymic mice bearing MCF7-HER2 xenografts.

RESULTS

The scFv-Fc DM fragment was labeled with [(131)I]SGMIB and [(131)I]IB-Mal-d-GEEEK in conjugation yields of 53% and 25%, respectively, with preservation of immunoreactivity for HER2. Internalization assays indicated that labeling via SGMIB resulted in a 1.6- to 3.5-fold higher (P<.05) retention of radioactivity, compared to that from the directly labeled fragment, in HER2-expressing cells during a 24-h observation period. Likewise, the amount of radioactivity retained in cells from the IB-Mal-d-GEEEK-labeled fragment was 1.4- to 3.3-fold higher (P<.05). Tumor uptake of radioiodine activity in athymic mice bearing MCF7-HER2 xenografts in vivo was significantly higher for the [(125)I]IB-Mal-d-GEEEK-labeled scFv-Fc DM fragment compared with that of the [(131)I]SGMIB-labeled fragment, particularly at later time points. The uptake of (125)I was threefold (3.6+/-1.1 %ID/g vs. 1.2+/-0.4 %ID/g) and fourfold (3.1+/-1.7 %ID/g vs. 0.8+/-0.4 %ID/g) higher than that for (131)I at 24 and 48 h, respectively. However, the [(125)I]IB-Mal-d-GEEEK-labeled scFv-Fc DM fragment also exhibited considerably higher levels of radioiodine activity in liver, spleen and kidney.

CONCLUSIONS

The overall results further demonstrate the potential utility of these two prosthetic groups for the radiohalogenation of internalizing monoclonal antibodies and their fragments. Specifically, the trastuzumab-derived double mutant fragment in combination with these residualizing agents warrants further evaluation for imaging and possibly treatment of HER2 expressing malignancies.

Quantitative PET imaging of Met-expressing human cancer xenografts with 89Zr-labelled monoclonal antibody DN30

PURPOSE

Targeting the c-Met receptor with monoclonal antibodies (MAbs) is an appealing approach for cancer diagnosis and treatment because this receptor plays a prominent role in tumour invasion and metastasis. Positron emission tomography (PET) might be a powerful tool for guidance of therapy with anti-Met MAbs like the recently described MAb DN30 because it allows accurate quantitative imaging of tumour targeting (immuno-PET). We considered the potential of PET with either (89)Zr-labelled (residualising radionuclide) or (124)I-labelled (non-residualising radionuclide) DN30 for imaging of Met-expressing tumours.

MATERIALS AND METHODS

The biodistribution of co-injected (89)Zr-DN30 and iodine-labelled DN30 was compared in nude mice bearing either the human gastric cancer line GLT-16 (high Met expression) or the head-and-neck cancer line FaDu (low Met expression). PET images were acquired in both xenograft models up to 4 days post-injection (p.i.) and used for quantification of tumour uptake.

RESULTS

Biodistribution studies in GTL-16-tumour-bearing mice revealed that (89)Zr-DN30 achieved much higher tumour uptake levels than iodine-labelled DN30 (e.g. 19.6%ID/g vs 5.3%ID/g, 5 days p.i.), while blood levels were similar, indicating internalisation of DN30. Therefore, (89)Zr-DN30 was selected for PET imaging of GLT-16-bearing mice. Tumours as small as 11 mg were readily visualised with immuno-PET. A distinctive lower (89)Zr uptake was observed in FaDu compared to GTL-16 xenografts (e.g. 7.8%ID/g vs 18.1%ID/g, 3 days p.i.). Nevertheless, FaDu xenografts were also clearly visualised with (89)Zr-DN30 immuno-PET. An excellent correlation was found between PET-image-derived (89)Zr tumour uptake and ex-vivo-assessed (89)Zr tumour uptake (R(2)=0.98).

CONCLUSIONS

The long-lived positron emitter (89)Zr seems attractive for PET-guided development of therapeutic anti-c-Met MAbs.