Radioimmunoimaging with longer-lived positron-emitting radionuclides: potentials and challenges

Preclinical Evaluation of 99mTc-MAG3-5-Fab Targeting TREM2 in Lung Cancer Mouse Models: A Comparison with 99mTc-MAG3-5-F(ab')2

Triggering receptor expressed on myeloid cells-2 (TREM2), which is expressed on the surface of tumor-associated macrophages (TAMs), has been found to play a major role in the diagnosis and treatment of tumors. TREM2 expression is significantly upregulated in tumor tissues, and therefore, targeting TREM2 for tumor imaging may be of value. Previously, we performed TREM2 targeting imaging by using 68Ga-NOTA-COG1410 or a 124I-labeled monoclonal antibody (mAb) and F(ab')2 in mouse models of colon and gastric tumors. However, some of the shortcomings of these probes (i.e., the high uptake of 68Ga-NOTA-COG1410 in the liver, the difficulty of obtaining iodine-124, and the long half-life of iodine-124) have hindered their clinical use. Herein, we sought to synthesize novel molecular probes targeting TREM2 that are more conducive to clinical translation, eliminating the interference of isotope availability and in vivo probe biodistribution issues. Therefore, we established A549 cell lines with negative human TREM2 (hTREM2) expression (GFP tag; hTREM2- A549) or upregulated hTREM2 expression (GFP tag; hTREM2+ A549) using lentiviral transfection and confirmed these with Western blotting and immunocytochemistry. We then prepared a mouse anti-human TREM2 (5-mAb) by immunizing with the hTREM2 antigen. The antibody fragments 5-F(ab')2 and 5-Fab were prepared from 5-mAb, and 99mTc-MAG3-5-F(ab')2 and 99mTc-MAG3-5-Fab were then synthesized with excellent stability and specificity. 99mTc-MAG3-5-F(ab')2 had a slightly higher in vitro affinity than 99mTc-MAG3-5-Fab (Kd = 3.32 ± 0.05 nmol versus 4.62 ± 0.85 nmol). 99mTc-MAG3-5-F(ab')2 and 99mTc-MAG3-5-Fab both showed excellent specificity: after adding a 100-fold precursor, the two probes binding to the cells were almost blocked. In vivo pharmacokinetics showed that the distribution and elimination half-lives of 99mTc-MAG3-5-Fab (T1/2α = 1.25 ± 0.30 min and T1/2β = 21.98 ± 2.80 min, respectively) were significantly reduced compared to those of 99mTc-MAG3-5-F(ab')2 (T1/2α = 2.64 ± 0.37 min and T1/2β = 86.55 ± 26.86 min, respectively). In micro single-photon emission computed tomography/computed tomography (micro-SPECT/CT) imaging, the tumor was clearly displayed at 1 h after 99mTc-MAG3-5-Fab injection, while the blood background was extremely low at 3 h, and the probe was mainly excreted through the kidneys and biliary tract. 99mTc-MAG3-5-F(ab')2 uptake was also detected at the tumor site, although the blood background was consistently high. The biodistribution results were consistent with the micro-SPECT/CT imaging results. 99mTc-MAG3-5-Fab could clearly display hTREM2+ A549 tumors in a short time (1 h) with low uptake in nontumor organs and tissues and thus has clinical application prospects.

Immuno-PET Imaging of CD69 Visualizes T-Cell Activation and Predicts Survival Following Immunotherapy in Murine Glioblastoma

Glioblastoma (GBM) is the most common and malignant primary brain tumor in adults. Immunotherapy may be promising for the treatment of some patients with GBM; however, there is a need for noninvasive neuroimaging techniques to predict immunotherapeutic responses. The effectiveness of most immunotherapeutic strategies requires T-cell activation. Therefore, we aimed to evaluate an early marker of T-cell activation, CD69, for its use as an imaging biomarker of response to immunotherapy for GBM. Herein, we performed CD69 immunostaining on human and mouse T cells following in vitro activation and post immune checkpoint inhibitors (ICI) in an orthotopic syngeneic mouse glioma model. CD69 expression on tumor-infiltrating leukocytes was assessed using single-cell RNA sequencing (scRNA-seq) data from patients with recurrent GBM receiving ICI. Radiolabeled CD69 Ab PET/CT imaging (CD69 immuno-PET) was performed on GBM-bearing mice longitudinally to quantify CD69 and its association with survival following immunotherapy. We show CD69 expression is upregulated upon T-cell activation and on tumor-infiltrating lymphocytes (TIL) in response to immunotherapy. Similarly, scRNA-seq data demonstrated elevated CD69 on TILs from patients with ICI-treated recurrent GBM as compared with TILs from control cohorts. CD69 immuno-PET studies showed a significantly higher tracer uptake in the tumors of ICI-treated mice compared with controls. Importantly, we observed a positive correlation between survival and CD69 immuno-PET signals in immunotherapy-treated animals and established a trajectory of T-cell activation by virtue of CD69-immuno-PET measurements. Our study supports the potential use of CD69 immuno-PET as an immunotherapy response assessment imaging tool for patients with GBM.

Significance

Immunotherapy may hold promise for the treatment of some patients with GBM. There is a need to assess therapy responsiveness to allow the continuation of effective treatment in responders and to avoid ineffective treatment with potential adverse effects in the nonresponders. We demonstrate that noninvasive PET/CT imaging of CD69 may allow early detection of immunotherapy responsiveness in patients with GBM.

Recent Advances in 64Cu/67Cu-Based Radiopharmaceuticals

Copper-64 (T_1/2 = 12.7 h) is a positron and beta-emitting isotope, with decay characteristics suitable for both positron emission tomography (PET) imaging and radiotherapy of cancer. Copper-67 (T_1/2 = 61.8 h) is a beta and gamma emitter, appropriate for radiotherapy β-energy and with a half-life suitable for single-photon emission computed tomography (SPECT) imaging. The chemical identities of ⁶⁴Cu and ⁶⁷Cu isotopes allow for convenient use of the same chelating molecules for sequential PET imaging and radiotherapy. A recent breakthrough in ⁶⁷Cu production opened previously unavailable opportunities for a reliable source of ⁶⁷Cu with high specific activity and purity. These new opportunities have reignited interest in the use of copper-containing radiopharmaceuticals for the therapy, diagnosis, and theranostics of various diseases. Herein, we summarize recent (2018-2023) advances in the use of copper-based radiopharmaceuticals for PET, SPECT imaging, radiotherapy, and radioimmunotherapy.

Synthesis and radiolabeling of a polar [125 I]I-1,2,4,5-tetrazine

Pretargeting imaging has gained a lot of prominence, due to its excellent bioorthogonality and improved imaging contrast compared to conventional imaging. A new iodo tetrazine (Tz) derivative has been synthesized and further developed into the corresponding iodine-125 (¹²⁵ I) analog (12), via the trimethylstannane precursor. Radiolabeling with either N-chlorosuccinimide or chloramine-T, in either MeCN or MeOH proceeded with a radiochemical conversion (RCC) of >80%. Subsequent deprotection only proved successful, among the tested conditions, when the radiolabeled Tz was stirred in 6-M HCl_(aq.) at 60°C for 2.5 h. To the best of our knowledge, this is the first H-tetrazine labeled with iodine. In vivo investigations on the pretargeting ability of 12 are currently under way.

Novel Chelating Agents for Zirconium-89-Positron Emission Tomography (PET) Imaging: Synthesis, DFT Calculation, Radiolabeling, and In Vitro and In Vivo Complex Stability

We report the synthesis and evaluation of novel chelating agents for zirconium-89 (⁸⁹Zr) with positron emission tomography (PET) imaging applications. New chelating agents NODHA, NOTHA, and NODHA-PY were constructed on 1,4,7-triazacyclononane (TACN) and possess hydroxamic acid or a pyridine ring as an acyclic binding moiety. The new chelating agents were theoretically studied for complexation with Zr(IV). Structures of Zr(IV)-NODHA, Zr(IV)-NOTHA, and Zr(IV)-NODHA-PY were predicted using density functional methods. NODHA was found to form stronger bonds with Zr(IV) when compared to NOTHA and NODHA-PY. The new chelating agents were evaluated for radiolabeling efficiency in binding ⁸⁹Zr. The corresponding [⁸⁹Zr]Zr-labeled chelators were evaluated for complex stability in human serum. All new chelating agents rapidly bound to ⁸⁹Zr in excellent radiolabeling efficiency at room temperature. Among the new [⁸⁹Zr]Zr-labeled chelators evaluated, [⁸⁹Zr]Zr-NODHA showed the highest stability in human serum without losing ⁸⁹Zr, and [⁸⁹Zr]Zr-NODHA-PY released a considerable amount of ⁸⁹Zr in human serum. [⁸⁹Zr]Zr-NODHA, [⁸⁹Zr]Zr-NODHA-PY, and [⁸⁹Zr]Zr-DFO were comparatively evaluated for in vivo complex stability by performing biodistribution studies using normal mice. [⁸⁹Zr]Zr-DFO had the lowest bone uptake at all time points, while [⁸⁹Zr]Zr-NODHA-PY showed poor stability in mice as evidenced by high bone accumulation at the 24 h time point. [⁸⁹Zr]Zr-NODHA exhibited better renal clearance but higher bone uptake than [⁸⁹Zr]Zr-DFO.

On the Versatility of Nanozeolite Linde Type L for Biomedical Applications: Zirconium-89 Radiolabeling and In Vivo Positron Emission Tomography Study

Porous materials, such as zeolites, have great potential for biomedical applications, thanks to their ability to accommodate positively charged metal-ions and their facile surface functionalization. Although the latter aspect is important to endow the nanoparticles with chemical/colloidal stability and desired biological properties, the possibility for simple ion-exchange enables easy switching between imaging modalities and/or combination with therapy, depending on the envisioned application. In this study, the nanozeolite Linde type L (LTL) with already confirmed magnetic resonance imaging properties, generated by the paramagnetic gadolinium (GdIII) in the inner cavities, was successfully radiolabeled with a positron emission tomography (PET)-tracer zirconium-89 (89Zr). Thereby, exploiting 89Zr-chloride resulted in a slightly higher radiolabeling in the inner cavities compared to the commonly used 89Zr-oxalate, which apparently remained on the surface of LTL. Intravenous injection of PEGylated 89Zr/GdIII-LTL in healthy mice allowed for PET-computed tomography evaluation, revealing initial lung uptake followed by gradual migration of LTL to the liver and spleen. Ex vivo biodistribution confirmed the in vivo stability and integrity of the proposed multimodal probe by demonstrating the original metal/Si ratio being preserved in the organs. These findings reveal beneficial biological behavior of the nanozeolite LTL and hence open the door for follow-up theranostic studies by exploiting the immense variety of metal-based radioisotopes.

Synthesis and evaluation of Diaza-Crown Ether-Backboned chelator containing hydroxamate groups for Zr-89 chelation chemistry

Zirconium-89 (⁸⁹Zr) has been explored for molecularly targeted positron emission tomography (PET) imaging of various diseases. We synthesized and evaluated a novel chelator (DA-18C6-BHA) for ⁸⁹Zr. The new chelator is structured on a macrocyclic backbone (1,10-diaza-18-crown-6) and contains hydroxamates as acyclic donor groups. The new chelator ((DA-18C6-BHA) was rapidly labeled with ⁸⁹Zr under mild conditions. The ⁸⁹Zr-labeled DA-18C6-BHA complex remained stable in human serum and apotransferrin for 7 days. When challenged with excess EDTA solution, ⁸⁹Zr-labeled DA-18C6-BHA was shown to hold ⁸⁹Zr without losing considerable radioactivity to EDTA. The ⁸⁹Zr-labeled DA-18C6-BHA complex displayed high complex stability in normal mice as evidenced by low bone uptake.

Pretargeted PET Imaging with a TCO-Conjugated Anti-CD44v6 Chimeric mAb U36 and [89Zr]Zr-DFO-PEG5-Tz

The recent advances in the production of engineered antibodies have facilitated the development and application of tailored, target-specific antibodies. Positron emission tomography (PET) of these antibody-based drug candidates can help to better understand their in vivo behavior. In this study, we report an in vivo proof-of-concept pretargeted immuno-PET study where we compare a pretargeting vs targeted approach using a new ⁸⁹Zr-labeled tetrazine as a bio-orthogonal ligand in an inverse electron demand Diels–Alder (IEDDA) in vivo click reaction. A CD44v6-selective chimeric monoclonal U36 was selected as the targeting antibody because it has potential in immuno-PET imaging of head-and-neck squamous cell carcinoma (HNSCC). Zirconium-89 (t_1/2 = 78.41 h) was selected as the radionuclide of choice to be able to make a head-to-head comparison of the pretargeted and targeted approaches. [⁸⁹Zr]Zr-DFO-PEG₅-Tz ([⁸⁹Zr]Zr-3) was synthesized and used in pretargeted PET imaging of HNSCC xenografts (VU-SCC-OE) at 24 and 48 h after administration of a trans-cyclooctene (TCO)-functionalized U36. The pretargeted approach resulted in lower absolute tumor uptake than the targeted approach (1.5 ± 0.2 vs 17.1 ± 3.0% ID/g at 72 h p.i. U36) but with comparable tumor-to-non-target tissue ratios and significantly lower absorbed doses. In conclusion, anti-CD44v6 monoclonal antibody U36 was successfully used for ⁸⁹Zr-immuno-PET imaging of HNSCC xenograft tumors using both a targeted and pretargeted approach. The results not only support the utility of the pretargeted approach in immuno-PET imaging but also demonstrate the challenges in achieving optimal in vivo IEDDA reaction efficiencies in relation to antibody pharmacokinetics.

Development of 18F-Labeled Bispyridyl Tetrazines for In Vivo Pretargeted PET Imaging

Pretargeted PET imaging is an emerging and fast-developing method to monitor immuno-oncology strategies. Currently, tetrazine ligation is considered the most promising bioorthogonal reaction for pretargeting in vivo. Recently, we have developed a method to ¹⁸F-label ultrareactive tetrazines by copper-mediated fluorinations. However, bispyridyl tetrazines—one of the most promising structures for in vivo pretargeted applications—were inaccessible using this strategy. We believed that our successful efforts to ¹⁸F-label H-tetrazines using low basic labeling conditions could also be used to label bispyridyl tetrazines via aliphatic nucleophilic substitution. Here, we report the first direct ¹⁸F-labeling of bispyridyl tetrazines, their optimization for in vivo use, as well as their successful application in pretargeted PET imaging. This strategy resulted in the design of [¹⁸F]45, which could be labeled in a satisfactorily radiochemical yield (RCY = 16%), molar activity (A_m = 57 GBq/µmol), and high radiochemical purity (RCP > 98%). The [¹⁸F]45 displayed a target-to-background ratio comparable to previously successfully applied tracers for pretargeted imaging. This study showed that bispyridyl tetrazines can be developed into pretargeted imaging agents. These structures allow an easy chemical modification of ¹⁸F-labeled tetrazines, paving the road toward highly functionalized pretargeting tools. Moreover, bispyridyl tetrazines led to near-instant drug release of iTCO-tetrazine-based ‘click-to-release’ reactions. Consequently, ¹⁸F-labeled bispyridyl tetrazines bear the possibility to quantify such release in vivo in the future.

Tuberculosis: Role of Nuclear Medicine and Molecular Imaging With Potential Impact of Neutrophil-Specific Tracers

With Tuberculosis (TB) affecting millions of people worldwide, novel imaging modalities and tools, particularly nuclear medicine and molecular imaging, have grown with greater interest to assess the biology of the tuberculous granuloma and evolution thereof. Much early work has been performed at the pre-clinical level using gamma single photon emission computed tomography (SPECT) agents exploiting certain characteristics of Mycobacterium tuberculosis (MTb). Both antituberculous SPECT and positron emission tomography (PET) agents have been utilised to characterise MTb. Other PET tracers have been utilised to help to characterise the biology of MTb (including Gallium-68-labelled radiopharmaceuticals). Of all the tracers, 2-[¹⁸F]FDG has been studied extensively over the last two decades in many aspects of the treatment paradigm of TB: at diagnosis, staging, response assessment, restaging, and in potentially predicting the outcome of patients with latent TB infection. Its lower specificity in being able to distinguish different inflammatory cell types in the granuloma has garnered interest in reviewing more specific agents that can portend prognostic implications in the management of MTb. With the neutrophil being a cell type that portends this poorer prognosis, imaging this cell type may be able to answer more accurately questions relating to the tuberculous granuloma transmissivity and may help in characterising patients who may be at risk of developing active TB. The formyl peptide receptor 1(FPR1) expressed by neutrophils is a key marker in this process and is a potential target to characterise these areas. The pre-clinical work regarding the role of radiolabelled N-cinnamoyl –F-(D) L – F – (D) –L F (cFLFLF) (which is an antagonist for FPR1) using Technetium 99m-labelled conjugates and more recently radiolabelled with Gallium-68 and Copper 64 is discussed. It is the hope that further work with this tracer may accelerate its potential to be utilised in responding to many of the current diagnostic dilemmas and challenges in TB management, thereby making the tracer a translatable option in routine clinical care.

[nat/89Zr][Zr(pypa)]: Thermodynamically Stable and Kinetically Inert Binary Nonadentate Complex for Radiopharmaceutical Applications

H₄pypa is a nonadentate nonmacrocyclic chelator, which previously demonstrated high affinity for scandium-44, lutetium-177, and indium-111. Herein, we report the highly stable binary [Zr(pypa)] complex; the nonradioactive complex was synthesized and characterized in detail using high-resolution electrospray-ionization mass spectroscopy (HR-ESI-MS) and various nuclear magnetic resonance spectroscopies (NMR), which revealed C_2v symmetry of the complex. The geometry of [Zr(pypa)] was further detailed via X-ray crystallography and compared with the structure of [Fe(Hpypa)]. Despite a slow complexation rate with an association half-life of 31.4 h at pH 2 and room temperature, the [Zr(pypa)] complex is thermodynamically stable (log K_ML = 38.92, pZr = 39.4). Radiochemical studies demonstrated quantitative radiolabeling achieved at 10 μM chelator concentration within 2 h at 40 °C and pH = 7, antibody-compatible conditions. Of the utmost importance, [⁸⁹Zr][Zr(pypa)] is highly kinetically inert upon challenge with excess EDTA and DFO ligands, superior to [⁸⁹Zr][Zr(DFO)]⁺, and maintains inertness toward human serum.

Preclinical Molecular PET-CT Imaging Targeting CDCP1 in Colorectal Cancer

Colorectal cancer (CRC) is the third most common malignancy in the world, with 22% of patients presenting with metastatic disease and a further 50% destined to develop metastasis. Molecular imaging uses antigen-specific ligands conjugated to radionuclides to detect and characterise primary cancer and metastases. Expression of the cell surface protein CDCP1 is increased in CRC, and here we sought to assess whether it is a suitable molecular imaging target for the detection of this cancer. CDCP1 expression was assessed in CRC cell lines and a patient-derived xenograft to identify models suitable for evaluation of radio-labelled 10D7, a CDCP1-targeted, high-affinity monoclonal antibody, for preclinical molecular imaging. Positron emission tomography-computed tomography was used to compare zirconium-89 (⁸⁹Zr)-10D7 avidity to a nonspecific, isotype control ⁸⁹Zr-labelled IgGκ1 antibody. The specificity of CDCP1-avidity was further confirmed using CDCP1 silencing and blocking models. Our data indicate high avidity and specificity for of ⁸⁹Zr-10D7 in CDCP1 expressing tumors at. Significantly higher levels than normal organs and blood, with greatest tumor avidity observed at late imaging time points. Furthermore, relatively high avidity is detected in high CDCP1 expressing tumors, with reduced avidity where CDCP1 expression was knocked down or blocked. The study supports CDCP1 as a molecular imaging target for CRC in preclinical PET-CT models using the radioligand ⁸⁹Zr-10D7.

89Zr-PET imaging in humans: a systematic review

Purpose

The remarkable amount of preclinical data achieved on 89Zr-PET imaging led to a significant clinical translation, concerning mainly immuno-PET applications. The aim of this systematic review is to provide a complete overview on clinical applications of 89Zr-PET imaging, using a systematic approach to identify and collect published studies performed in humans, sorted by field of application and specific disease subsections.

Methods

A systematic literature search of articles suiting the inclusion criteria was conducted on Pubmed, Scopus, Central, and Web Of Science databases, including papers published from January 1967 to November 2020. Eligible studies had to be performed on humans through PET imaging with 89Zr-labeled compounds. The methodological quality was assessed through the Quality Assessment of Diagnostic accuracy Studies-2 tool.

Results

A total of 821 articles were screened. 74 studies performed on humans were assessed for eligibility with the exclusion of further 18, thus 56 articles were ultimately selected for the qualitative analysis.

Conclusions

89Zr has shown to be a powerful PET-imaging tool, in particular for radiolabeling antibodies in order to study antigen expression, biodistribution, anticancer treatment planning and follow-up. Other than oncologic applications, 89Zr-radiolabeled antibodies have been proposed for use in inflammatory and autoimmune disorders with interesting results. 89Zr-labeled nanoparticles represent groundbreaking radiopharmaceuticals with potential huge fields of application. To evaluate the clinical usefulness of 89Zr PET-imaging in different conditions and in real-world settings, and to widen its use in clinical practice, further translation of preclinical to clinical data is needed.

Novel theranostic agent for PET imaging and targeted radiopharmaceutical therapy of tumour-infiltrating immune cells in glioma

BACKGROUND

Malignant gliomas are deadly tumours with few therapeutic options. Although immunotherapy may be a promising therapeutic strategy for treating gliomas, a significant barrier is the CD11b+ tumour-associated myeloid cells (TAMCs), a heterogeneous glioma infiltrate comprising up to 40% of a glioma's cellular mass that inhibits anti-tumour T-cell function and promotes tumour progression. A theranostic approach uses a single molecule for targeted radiopharmaceutical therapy (TRT) and diagnostic imaging; however, there are few reports of theranostics targeting the tumour microenvironment.

METHODS

Utilizing a newly developed bifunctional chelator, Lumi804, an anti-CD11b antibody (αCD11b) was readily labelled with either Zr-89 or Lu-177, yielding functional radiolabelled conjugates for PET, SPECT, and TRT.

FINDINGS

89Zr/177Lu-labeled Lumi804-αCD11b enabled non-invasive imaging of TAMCs in murine gliomas. Additionally, 177Lu-Lumi804-αCD11b treatment reduced TAMC populations in the spleen and tumour and improved the efficacy of checkpoint immunotherapy.

INTERPRETATION

89Zr- and 177Lu-labeled Lumi804-αCD11b may be a promising theranostic pair for monitoring and reducing TAMCs in gliomas to improve immunotherapy responses.

FUNDING

A full list of funding bodies that contributed to this study can be found in the Acknowledgements section.

Re-Evaluations of Zr-DFO Complex Coordination Chemistry for the Estimation of Radiochemical Yields and Chelator-to-Antibody Ratios of 89Zr Immune-PET Tracers

(1) Background: Deferoxamine B (DFO) is the most widely used chelator for labeling of zirconium-89 (⁸⁹Zr) to monoclonal antibody (mAb). Despite the remarkable developments of the clinical ⁸⁹Zr-immuno-PET, chemical species and stability constants of the Zr-DFO complexes remain controversial. The aim of this study was to re-evaluate their stability constants by identifying species of Zr-DFO complexes and demonstrate that the stability constants can estimate radiochemical yield (RCY) and chelator-to-antibody ratio (CAR). (2) Methods: Zr-DFO species were determined by UV and ESI-MS spectroscopy. Stability constants and speciation of the Zr-DFO complex were redetermined by potentiometric titration. Complexation inhibition of Zr-DFO by residual impurities was investigated by competition titration. (3) Results: Unknown species, ZrH_qDFO₂, were successfully detected by nano-ESI-Q-MS analysis. We revealed that a dominant specie under radiolabeling condition (pH 7) was ZrHDFO, and its stability constant (logβ₁₁₁) was 49.1 ± 0.3. Competition titration revealed that residual oxalate inhibits Zr-DFO complex formation. RCYs in different oxalate concentration (0.1 and 0.04 mol/L) were estimated to be 86% and >99%, which was in good agreement with reported results (87%, 97%). (4) Conclusion: This study succeeded in obtaining accurate stability constants of Zr-DFO complexes and estimating RCY and CAR from accurate stability constants established in this study.

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.

Combination Therapy, a Promising Approach to Enhance the Efficacy of Radionuclide and Targeted Radionuclide Therapy of Prostate and Breast Cancer

In recent years, radionuclide therapy (RT) and targeted radionuclide therapy (TRT) have gained great interest in cancer treatment. This is due to promising results obtained in both preclinical and clinical studies. However, a complete response is achieved in only a small percentage of patients that receive RT or TRT. As a consequence, there have been several strategies to improve RT and TRT outcomes including the combination of these treatments with other well-established anti-cancer therapies, for example, chemotherapy. Combinations of RT and TRT with other therapies with distinct mechanisms of action represent a promising strategy. As for prostate cancer and breast cancer, the two most prevalent cancer types worldwide, several combination-based therapies have been evaluated. In this review, we will provide an overview of the RT and TRT agents currently used or being investigated in combination with hormone therapy, chemotherapy, immunotherapy, and external beam radiation therapy for the treatment of prostate cancer and breast cancer.

66Ga-PET-imaging of GRPR-expression in prostate cancer: production and characterization of [66Ga]Ga-NOTA-PEG2-RM26

Molecular imaging of the gastrin-releasing peptide receptor (GRPR) could improve patient management in prostate cancer. This study aimed to produce gallium-66 (T½ = 9.5 h) suitable for radiolabeling, and investigate the imaging properties of gallium-66 labeled GRPR-antagonist NOTA-PEG2-RM26 for later-time point PET-imaging of GRPR expression. Gallium-66 was cyclotron-produced using a liquid target, and enriched [66Zn]Zn(NO3)2. In vitro, [66Ga]Ga-NOTA-PEG2-RM26 was characterized in GRPR-expressing PC-3 prostate cancer cells. In vivo, specificity test and biodistribution studies were performed 3 h and 22 h pi in PC-3 xenografted mice. microPET/MR was performed 3 h and 22 h pi. Biodistribution of [66Ga]Ga-NOTA-PEG2-RM26 was compared with [68Ga]Ga-NOTA-PEG2-RM26 3 h pi. [66Ga]Ga-NOTA-PEG2-RM26 was successfully prepared with preserved binding specificity and high affinity towards GRPR. [66Ga]Ga-NOTA-PEG2-RM26 cleared rapidly from blood via kidneys. Tumor uptake was GRPR-specific and exceeded normal organ uptake. Normal tissue clearance was limited, resulting in no improvement of tumor-to-organ ratios with time. Tumors could be clearly visualized using microPET/MR. Gallium-66 was successfully produced and [66Ga]Ga-NOTA-PEG2-RM26 was able to clearly visualize GRPR-expression both shortly after injection and on the next day using PET. However, delayed imaging did not improve contrast for Ga-labeled NOTA-PEG2-RM26.

Evaluation of the nuclear reaction cross sections via proton induced reactions on 72Ge and 76Se to produce 72As: A potential entrant for the theranostic pairs

Theranostic applications of radiopharmaceuticals have revolutionized present era specially, dealing with cancer diseases. Increase in the uses of radionuclides in nuclear medicine has resulted in the demands of optimized new radionuclides to be produced focussing on the economy, simplicity and maximum yield. Two radionuclides of arsenic offer a well agreed theranostic systems namely ⁷⁷As and ⁷²As. Some arsenic radionuclides are capable of positron-emission, with range of hour to weeks half-lives and have potential to be used for nuclear medicine. Present work will elucidate the production of ⁷²As on Germanium and Selenium via proton induced nuclear reactions. The experimental results obtained by several nuclear reactions were analyzed. The results of nuclear model codes namely ALICE-IPPE, EMPIRE-3.2.3 and TALYS-1.9 are compared with the experimental cross sections to generate recommended cross section data. Recommended excitation functions are used to compute the thick target yields (TTY) of ⁷²As. Assessment of radionuclidic impurities are also studied and comparison of several radionuclidic impurities is done. To produce ⁷²As,⁷²Ge (p, n)⁷²As, ⁷³Ge (p, 2n)⁷²As, ⁷⁴Ge (p, 3n)⁷²As and ⁷⁶Se (p, x)⁷²As reactions in different energy ranges are discussed. We have identified ⁷²Ge (p, n)⁷²As reaction; gives pre-eminent yield with least impurities mark it as feasible entrant to be applied in Positron Emission Tomography (PET) and theranostic applications.

New Bifunctional Chelator 3p-C-NEPA for Potential Applications in Lu(III) and Y(III) Radionuclide Therapy and Imaging

We have developed structurally unique bifunctional chelators in the NETA, NE3TA, and DEPA series for potential radiopharmaceutical applications. As part of our continued research efforts to generate efficient bifunctional chelators for targeted radionuclide therapy and imaging of various diseases, we designed a scorpion-like chelator that is proposed to completely saturate the coordination spheres of Y(III) and Lu(III). We herein report the synthesis and evaluation of a new chelator (3p-C-NEPA) with 10 donor groups for complexation with β-emitting radionuclides ⁹⁰Y(III), ⁸⁶Y(III), and ¹⁷⁷Lu(III). The chelator was synthesized and evaluated for radiolabeling kinetics with the readily available radioisotopes ⁹⁰Y and ¹⁷⁷Lu, and the corresponding ⁹⁰Y or ¹⁷⁷Lu-radiolabeled complexes were evaluated for in vitro stability in human serum and in vivo complex stability in mice. The new chelator rapidly bound ⁹⁰Y or ¹⁷⁷Lu and formed a stable complex with the radionuclides. The new chelator 3p-C-NEPA radiolabeled with either ⁹⁰Y or ¹⁷⁷Lu remains stable in human serum without dissociation for 10 days. ¹⁷⁷Lu-labeled 3p-C-NEPA produced a favorable in vivo biodistribution profile in normal mice.

Strategies for improving stability and pharmacokinetic characteristics of radiolabeled peptides for imaging and therapy

Tumor cells overexpress a variety of receptors that are emerging targets in cancer chemotherapy. Radiolabeled peptides with high affinity and selectivity for these overexpressed receptors have been designed for both imaging and therapy purposes. Such peptides display advantages such as high selectivity for tumor cells, rapid tumor tissue penetration, and rapid clearance from non-target tissues and the circulation. However, the very short in vivo half-life of radiolabeled peptides, arising from enzymatic degradation and/or efficient clearance by the kidney, limits their accumulation in tumors. This review presents various strategies that have been applied to extend the half-life extension and improve the pharmacokinetic characteristics of radiolabeled peptides. These include amino acid substitution, modification of the peptide termini, dimerization and multimerization of the peptide, cyclization, conjugation with polymers, sugars and albumin and use of peptidase inhibitors.

Construction and Preclinical Evaluation of a 124/131I-Labeled Radiotracer for the Detection of Mesothelin-Overexpressing Cancer

Mesothelin is a molecular biomarker of many types of solid cancers, which may represent a highly promising new target in the development of cancer-targeted diagnostic agents. A human anti-mesothelin antibody with a low molecular weight, ET210sc, was applied; this antibody has potent affinity and can penetrate tissue quickly and stably without causing immunoreactions. We developed a new ^124/131I-labeled radiotracer of ET210sc. The ^124/131I-labeled ET210sc radiotracer showed excellent radiochemical quality (with over 99% radiolabeling yield, 0.07 GBq/μmol specific activity) and remarkable stability in phosphate-buffered saline (>95% at 3 days). The radiotracer retained its potent affinity (dissociation constant, K_d = 0.101 nM). The radiotracer specifically bound to mesothelin-positive cells in vitro. Interestingly, the radiotracer exhibited significant positive-to-negative tumor uptake ratios (1.5:1) 3 days postinjection. The estimated absorbed doses of each organ (e.g., 0.704 mGy/MBq for the rectum; 0.341 mGy/MBq for the spleen) met the medical safety standards for further clinical applications. Our findings provide an initial proof of concept for the potential use of ^124/131I-labeled ET210sc radiotracers to detect mesothelin-overexpressing cancer. ¹²⁴I-ET210sc is proposed to be an ideal imaging agent for further clinical applications.

Pre-targeted Imaging of Protease Activity through In Situ Assembly of Nanoparticles

The pre-targeted imaging of enzyme activity has not been reported, likely owing to the lack of a mechanism to retain the injected substrate in the first step for subsequent labeling. Herein, we report the use of two bioorthogonal reactions-the condensation reaction of aromatic nitriles and aminothiols and the inverse-electron demand Diels-Alder reaction between tetrazine and trans-cyclooctene (TCO)-to develop a novel strategy for pre-targeted imaging of the activity of proteases. The substrate probe (TCO-C-SNAT4) can be selectively activated by an enzyme target (e.g. caspase-3/7), which triggers macrocyclization and subsequent in situ self-assembly into nanoaggregates retained at the target site. The tetrazine-imaging tag conjugate labels TCO in the nanoaggregates to generate selective signal retention for imaging in vitro, in cells, and in mice. Owing to the decoupling of enzyme activation and imaging tag immobilization, TCO-C-SNAT4 can be repeatedly injected to generate and accumulate more TCO-nanoaggregates for click labeling.

Trans-Cyclooctene-Functionalized PeptoBrushes with Improved Reaction Kinetics of the Tetrazine Ligation for Pretargeted Nuclear Imaging

Tumor targeting using agents with slow pharmacokinetics represents a major challenge in nuclear imaging and targeted radionuclide therapy as they most often result in low imaging contrast and high radiation dose to healthy tissue. To address this challenge, we developed a polymer-based targeting agent that can be used for pretargeted imaging and thus separates tumor accumulation from the imaging step in time. The developed targeting agent is based on polypeptide-graft-polypeptoid polymers (PeptoBrushes) functionalized with trans-cyclooctene (TCO). The complementary ¹¹¹In-labeled imaging agent is a 1,2,4,5-tetrazine derivative, which can react with aforementioned TCO-modified PeptoBrushes in a rapid bioorthogonal ligation. A high degree of TCO loading (up to 30%) was achieved, without altering the physicochemical properties of the polymeric nanoparticle. The highest degree of TCO loading resulted in significantly increased reaction rates (77-fold enhancement) compared to those with small molecule TCO moieties when using lipophilic tetrazines. Based on computer simulations, we hypothesize that this increase is a result of hydrophobic effects and significant rearrangements within the polymer framework, in which hydrophobic patches of TCO moieties are formed. These patches attract lipophilic tetrazines, leading to increased reaction rates in the bioorthogonal ligation. The most reactive system was evaluated as a targeting agent for pretargeted imaging in tumor-bearing mice. After the setup was optimized, sufficient tumor-to-background ratios were achieved as early as 2 h after administration of the tetrazine imaging agent, which further improved at 22 h, enabling clear visualization of CT-26 tumors. These findings show the potential of PeptoBrushes to be used as a pretargeting agent when an optimized dose of polymer is used.

Evaluation of a 68Ga-Labeled DOTA-Tetrazine as a PET Alternative to 111In-SPECT Pretargeted Imaging

The bioorthogonal reaction between a tetrazine and strained trans-cyclooctene (TCO) has garnered success in pretargeted imaging. This reaction was first validated in nuclear imaging using an ¹¹¹In-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-linked bispyridyl tetrazine (Tz) ([¹¹¹In]In-DOTA-PEG₁₁-Tz) and a TCO functionalized CC49 antibody. Given the initial success of this Tz, it has been paired with TCO functionalized small molecules, diabodies, and affibodies for in vivo pretargeted studies. Furthermore, the single photon emission tomography (SPECT) radionuclide, ¹¹¹In, has been replaced with the β-emitter, ¹⁷⁷Lu and α-emitter, ²¹²Pb, both yielding the opportunity for targeted radiotherapy. Despite use of the ‘universal chelator’, DOTA, there is yet to be an analogue suitable for positron emission tomography (PET) using a widely available radionuclide. Here, a ⁶⁸Ga-labeled variant ([⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz) was developed and evaluated using two different in vivo pretargeting systems (Aln-TCO and TCO-CC49). Small animal imaging and ex vivo biodistribution studies were performed and revealed target specific uptake of [⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz in the bone (3.7 %ID/g, knee) in mice pretreated with Aln-TCO and tumor specific uptake (5.8 %ID/g) with TCO-CC49 in mice bearing LS174 xenografts. Given the results of this study, [⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz can serve as an alternative to [¹¹¹In]In-DOTA-PEG₁₁-Tz.

Site-Specific Immuno-PET Tracer to Image PD-L1

The rapid ascension of immune checkpoint blockade treatments has placed an emphasis on the need for viable, robust, and noninvasive imaging methods for immune checkpoint proteins, which could be of diagnostic value. Immunoconjugate-based positron emission tomography (immuno-PET) allows for sensitive and quantitative imaging of target levels and has promising potential for the noninvasive evaluation of immune checkpoint proteins. However, the advancement of immuno-PET is currently limited by available imaging tools, which heavily rely on full-length IgGs with Fc-mediated effects and are heterogeneous mixtures upon random conjugation with chelators for imaging. Herein, we have developed a site-specific αPD-L1 Fab conjugate with the chelator 1,4,7-triazacyclononane- N, N', N″-triacetic acid (NOTA), enabling radiolabeling for PET imaging, using the amber suppression-mediated genetic incorporation of unnatural amino acid (UAA), p-azidophenylalanine. This Fab conjugate is homogeneous and demonstrated tight binding toward the PD-L1 antigen in vitro. The radiolabeled version, ⁶⁴Cu-NOTA-αPD-L1, has been employed in PET imaging to allow for effective visualization and mapping of the biodistribution of PD-L1 in two normal mouse models, including the capturing of different PD-L1 expression levels in the spleens of the different mouse types. Follow-up in vivo blocking studies and ex vivo fluorescent staining further validated specific tissue uptakes of the imaging agent. This approach illustrates the utility of UAA-based site-specific Fab conjugation as a general strategy for making sensitive PET imaging probes, which could facilitate the elucidation of the roles of a wide variety of immune checkpoint proteins in immunotherapy.

89Zr-DFO-Cetuximab as a Molecular Imaging Agent to Identify Cetuximab Resistance in Head and Neck Squamous Cell Carcinoma

Background: Despite the improvement in clinical outcomes for head and neck squamous cell carcinoma (HNSCC) as the result of cetuximab, patients may present with or develop resistance that increases tumor recurrence rates and limits clinical efficacy. Therefore, identifying those patients who are or become resistant is essential to tailor the best therapeutic approach. Materials and Methods: Cetuximab was conjugated to p-NCS-Bz-DFO and labeled with ⁸⁹Zr. The resistance model was developed by treating FaDu cells with cetuximab. Western blotting (WB) and specific binding assays were performed to evaluate epidermal growth factor receptor (EGFR) expression and ⁸⁹Zr-DFO-cetuximab uptake in FaDu cetuximab-resistant (FCR) and FaDu cetuximab-sensitive (FCS) cells. Positron emission tomography imaging and biodistribution were conducted in NU/NU nude mice implanted with FCR or FCS cells. Results: Cetuximab was successfully radiolabeled with ⁸⁹Zr (≥95%). Binding assays performed in FCR and FCS cells showed significantly lower ⁸⁹Zr-DFO-cetuximab uptake in FCR (p < 0.0001). WB suggests that the resistance mechanism is associated with EGFR downregulation (p = 0.038). This result is in agreement with the low uptake of ⁸⁹Zr-DFO-cetuximab in FCR cells. Tumor uptake of ⁸⁹Zr-DFO-cetuximab in FCR was significantly lower than FCS tumors (p = 0.0340). Conclusions: In this work, the authors showed that ⁸⁹Zr-DFO-cetuximab is suitable for identification of EGFR downregulation in vitro and in vivo. This radiopharmaceutical may be useful for monitoring resistance in HNSCC patients during cetuximab therapy.

A radiopharmaceutical [89Zr]Zr-DFO-nimotuzumab for immunoPET with epidermal growth factor receptor expression in vivo

INTRODUCTION

The potential of the positron-emitting zirconium-89 (⁸⁹Zr) (t_1/2 = 78.4 h) has been recently reported for immune positron emission tomography (immunoPET) radioimmunoconjugates design. In our work, we explored the optimized preparation of [⁸⁹Zr]Zr-DFO-nimotuzumab, and evaluated ⁸⁹Zr-labeled monoclonal antibody (mAb) construct for targeted imaging of epidermal growth factor receptor (EGFR) overexpressed in glioma.

METHODS

To optimize the radiolabeling efficiency of ⁸⁹Zr with DFO-nimotuzumab, multiple immunoconjugates and radiolabeling were performed. Radiolabeling yield, radiochemical purity, stability, and activity assay were investigated to characterize [⁸⁹Zr]Zr-DFO-nimotuzumab for chemical and biological integrity. The in vivo behavior of this tracer was studied in mice bearing subcutaneous U87MG (EGFR-positive) tumors received a 3.5 ± 0.2 MBq/dose using PET/CT imaging. One group mice bearing subcutaneous U87MG (EGFR-positive) tumors received [⁸⁹Zr]Zr-DFO-nimotuzumab (3.5 ± 0.2 MBq, ~3 μg) (nonblocking) for immunoPET; the other group had 30 μg predose (blocking) of cold nimotuzumab 24 h prior to [⁸⁹Zr]Zr-DFO-nimotuzumab.

RESULTS

[⁸⁹Zr]Zr-DFO-nimotuzumab was prepared with high radiochemical yield (>90%), radiochemical purity (>99%), and specific activity (115 ± 0.8 MBq/mg). In vitro validation showed that [⁸⁹Zr]Zr-DFO-nimotuzumab had an initial immunoreactive fraction of 0.99 ± 0.05 and remained active for up to 5 days. A biodistribution study revealed excellent stability of [⁸⁹Zr]Zr-DFO-nimotuzumab in vivo compared with ⁸⁹Zr as a bone seeker. High uptake in the liver and heart and modest penetration in the brain were observed, with no significant accumulation of activity in other organs. ImmunoPET studies also indicated prominent image contrast that remarkably high uptake up to ~20%ID/g for nonblocking and ~2%ID/g for blocking in tumor between 12 and 120 h after administration.

CONCLUSION

These studies developed a radiopharmaceutical [⁸⁹Zr]Zr-DFO-nimotuzumab with optimized synthesis. The potential utility of [⁸⁹Zr]Zr-DFO-nimotuzumab in assessing EGFR status in glioma was demonstrated in this study.

Cross-section measurements and production of 72Se with medium to high energy protons using arsenic containing targets

Experiments were performed to evaluate production of 72Se, parent radionuclide of the positron emitter 72As, at high energy at the Brookhaven Linac Isotope Producer (BLIP). Excitation functions for 75As(p, xn)72/75Se in the 52-105 MeV energy range were measured by irradiating thin gallium arsenide (GaAs) wafers. Maximum cross section value for the natAs(p, 4n)72Se reaction in the energy range was 103±9 mb at 52±1 MeV. Production size GaAs and arsenic metal (As°) targets were irradiated with 136 μA and 165 μA beam current possessing an initial Linac energy of 117 MeV. A total of 3.77±0.1 GBq (102±3 mCi) of 72Se was produced from a GaAs target at a calculated target entrance energy of 105.4 MeV, and 13.8±0.3 GBq (373±8 mCi) of 72Se from an As° target at a calculated incident energy of 49.5 MeV irradiated for 116.5 h and 68.9 h, respectively.

89 Zr-ImmunoPET companion diagnostics and their impact in clinical drug development

Therapeutic monoclonal antibodies have been used in cancer treatment for 30 years, with around 24 mAb and mAb:drug conjugates approved by the FDA to date. Despite their specificity, efficacy has remained limited, which, in part, derails nascent initiatives towards precision medicine. An image-guided approach to reinforce treatment decisions using immune positron emission tomography (immunoPET) companion diagnostic is warranted. This review provides a general overview of current translational research using Zr-89 immunoPET and opportunities for utilizing and harnessing this tool to its full potential. Patient case studies are cited to illustrate immunoPET probes as tools for profiling molecular signatures. Discussions on its utility in reinforcing clinical decisions as it relates to histopathological tumor assessment and standard diagnostic methods, and its potential as predictive biomarkers, are presented. We finally conclude with an overview of practical considerations to its utility in the clinic.

The chemistry of PET imaging with zirconium-89

This Tutorial Review aims to provide an overview of the use of zirconium-89 complexes in biomedical imaging. Over the past decade there have been many new papers in this field, ranging from chemistry through to preclinical and clinical applications. Here we attempt to summarise the main developments that have occurred in this period. The primary focus is on coordination chemistry but other aspects such as isotope production, isotope properties, handling and radiochemical techniques and characterisation of cold and labelled complexes are included. Selected results from animal and human clinical studies are presented in the context of the stabilities and properties of the labelled bioconjugates.

Noninvasive Trafficking of Brentuximab Vedotin and PET Imaging of CD30 in Lung Cancer Murine Models

CD30 has been considered a unique diagnostic and therapeutic target for CD30-positive lymphomas and some lung diseases. Additionally, CD30 has shown high expression in clinical lung cancer samples. In this study, ⁸⁹Zr-radiolabeled brentuximab vedotin (BV) was developed for in vivo tracking of BV and imaging CD30 expression in lung cancer models via conjugation with desferrioxamine (Df). CD30 expression in three lung cancer cell lines (H460, H358, and A549) was quantified by Western blot. Flow cytometry and saturation binding assays were used to evaluate the binding capabilities of the tracer in vitro. After longitudinal positron emission tomography (PET) imaging and quantitative analysis were performed, ex vivo biodistribution and histological studies were used to verify PET results. Finally, dosimetric extrapolation of murine data to humans was performed. At the cellular level, CD30 was found to be expressed on H460 and A549 cells with the highest and lowest levels of expression, respectively. Both Df-BV and ⁸⁹Zr-Df-BV displayed high binding affinity to H460 cells. PET images and their quantification verified that BV accumulated in H460 tumor models (9.93 ± 2.70% ID/g at 24 h after injection; n = 4) at the highest level, followed by H358 and A549 tumors (8.05 ± 2.43 and 5.00 ± 1.56% ID/g; n = 4). The nonspecific ⁸⁹Zr-labeled IgG showed a low tumor uptake of 5.2 ± 1.0% ID/g for H460 models. Ex vivo biodistribution and fluorescence immunohistochemistry also corroborated these findings. Dosimetric results displayed safe dose estimations. Therefore, ⁸⁹Zr-Df-BV provides a potential agent for evaluating CD30 expression noninvasively in lung cancer, and also for imaging of brentuximab vedotin for better understanding of its pharmacokinetics.

Use of PET Imaging to Evaluate Transporter-Mediated Drug-Drug Interactions

Several membrane transporters belonging to the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families can transport drugs and drug metabolites and thereby exert an effect on drug absorption, distribution, and excretion, which may potentially lead to transporter-mediated drug-drug interactions (DDIs). Some transporter-mediated DDIs may lead to changes in organ distribution of drugs (eg, brain, liver, kidneys) without affecting plasma concentrations. Positron emission tomography (PET) is a noninvasive imaging method that allows studying of the distribution of radiolabeled drugs to different organs and tissues and is therefore the method of choice to quantitatively assess transporter-mediated DDIs on a tissue level. There are 2 approaches to how PET can be used in transporter-mediated DDI studies. When the drug of interest is a potential perpetrator of DDIs, it may be administered in unlabeled form to assess its influence on tissue distribution of a generic transporter-specific PET tracer (probe substrate). When the drug of interest is a potential victim of DDIs, it may be radiolabeled with carbon-11 or fluorine-18 and used in combination with a prototypical transporter inhibitor (eg, rifampicin). PET has already been used both in preclinical species and in humans to assess the effects of transporter-mediated DDIs on drug disposition in different organ systems, such as brain, liver, and kidneys, for which examples are given in the present review article. Given the growing importance of membrane transporters with respect to drug safety and efficacy, PET is expected to play an increasingly important role in future drug development.

CD44v6-Targeted Imaging of Head and Neck Squamous Cell Carcinoma: Antibody-Based Approaches

Head and neck squamous cell carcinoma (HNSCC) is a common and severe cancer with low survival rate in advanced stages. Noninvasive imaging of prognostic and therapeutic biomarkers could provide valuable information for planning and monitoring of the different therapy options. Thus, there is a major interest in development of new tracers towards cancer-specific molecular targets to improve diagnostic imaging and treatment. CD44v6, an oncogenic variant of the cell surface molecule CD44, is a promising molecular target since it exhibits a unique expression pattern in HNSCC and is associated with drug- and radio-resistance. In this review we summarize results from preclinical and clinical investigations of radiolabeled anti-CD44v6 antibody-based tracers: full-length antibodies, Fab, F(ab′)₂ fragments, and scFvs with particular focus on the engineering of various antibody formats and choice of radiolabel for the use as molecular imaging agents in HNSCC. We conclude that the current evidence points to CD44v6 imaging being a promising approach for providing more specific and sensitive diagnostic tools, leading to customized treatment decisions and functional diagnosis. Improved imaging tools hold promise to enable more effective treatment for head and neck cancer patients.

Establishing Reliable Cu-64 Production Process: From Target Plating to Molecular Specific Tumor Micro-PET Imaging

Copper-64 is a useful radioisotope for positron emission tomography (PET). Due to the wide range of applications, the demand of ⁶⁴Cu with low metallic impurities is increasing. Here we report a simple method for the efficient production of high specific activity ⁶⁴Cu using a cyclotron for biomedical application. We designed new equipment based on the plating of enriched ⁶⁴Ni as the target, and used automated ion exchange chromatography to purify copper-64 efficiently after irradiation and dissolution of the target in good radiochemical and chemical yield and purity. The ⁶⁴Cu radionuclide produced using 99.32% enriched ⁶⁴Ni with a density of 61.4 ± 5.0 mg/cm², reaching a total radioactivity greater than 200 mCi, with specific activity up to 5.6 GBq/μmoL. It was further incorporated into modified monoclonal antibody DOTA-rituximab to synthesize ⁶⁴Cu-DOTA-rituximab, which was used successfully for micro-PET imaging.

Chemistry and radiochemistry of As, Re and Rh isotopes relevant to radiopharmaceutical applications: high specific activity radionuclides for imaging and treatment

Advances in production, separation, target recovery, and chelation chemistry of high specific activity radionuclides will promote new theranostic agent development.

Siderophores for molecular imaging applications

This review covers publications on siderophores applied for molecular imaging applications, mainly for radionuclide-based imaging. Siderophores are low molecular weight chelators produced by bacteria and fungi to scavenge essential iron. Research on these molecules has a continuing history over the past 50 years. Many biomedical applications have been developed, most prominently the use of the siderophore desferrioxamine (DFO) to tackle iron overload related diseases. Recent research described the upregulation of siderophore production and transport systems during infection. Replacing iron in siderophores by radionuclides, the most prominent Ga-68 for PET, opens approaches for targeted imaging of infection; the proof of principle has been reported for fungal infections using ⁶⁸Ga-triacetylfusarinine C (TAFC). Additionally, fluorescent siderophores and therapeutic conjugates have been described and may be translated to optical imaging and theranostic applications. Siderophores have also been applied as bifunctional chelators, initially DFO as chelator for Ga-67 and more recently for Zr-89 where it has become the standard chelator in Immuno-PET. Improved DFO constructs and bifunctional chelators based on cyclic siderophores have recently been developed for Ga-68 and Zr-89 and show promising properties for radiopharmaceutical development in PET. A huge potential from basic biomedical research on siderophores still awaits to be utilized for clinical and translational imaging.