Noninvasive Imaging of PSMA in prostate tumors with (89)Zr-Labeled huJ591 engineered antibody fragments: the faster alternatives

Preclinical Evaluation of a PSMA Aptamer-Based Bifunctional PET and Fluorescent Probe

Prostate cancer is the most prevalent malignant tumor affecting male individuals worldwide. The accurate early detection of prostate cancer is crucial to preventing unnecessary diagnosis and subsequent excessive treatment. Prostate-specific membrane antigen (PSMA) has emerged as a promising biomarker for the diagnosis of prostate cancer. In this study, a dual-modality imaging probe utilizing aptamer technology was developed for positron emission tomography/near-infrared fluorescence (PET/NIRF) imaging, and the specificity and sensitivity of the probe toward PSMA were evaluated both in vitro and in vivo. The probe precursor NOTA-PSMA-Cy5 was synthesized via automated solid-phase oligonucleotide synthesis. Subsequently, the PET/NIRF dual-modality probe [68Ga]Ga-NOTA-PSMA-Cy5 was successfully prepared and exhibited favorable fluorescence properties and stability in vitro. The binding specificity of [68Ga]Ga-NOTA-PSMA-Cy5 to PSMA was assessed through flow cytometry, fluorescence imaging, and cellular uptake experiments in LNCaP cells and PC-3 cells. In vivo PET/NIRF imaging studies demonstrated the sensitive and specific binding of [68Ga]Ga-NOTA-PSMA-Cy5 to PSMA. Overall, the PET/NIRF dual-modality probe [68Ga]Ga-NOTA-PSMA-Cy5 shows promise for the diagnosis of prostate cancer and for the fluorescence-guided identification of PSMA-positive cancer lesions during surgical procedures.

Single Chelator-Minibody Theranostic Agents for 89Zr PET Imaging and 177Lu Radiopharmaceutical Therapy of PSMA-Expressing Prostate Cancer

Here we describe an anti-prostate-specific membrane antigen (PSMA) minibody (IAB2MA) conjugated to an octadentate, macrocyclic chelator based on four 1-hydroxypyridin-2-one coordinating units (Lumi804 [L804]) labeled with 89Zr (PET imaging) and 177Lu (radiopharmaceutical therapy), with the goal of developing safer and more efficacious treatment options for prostate cancer. Methods: L804 was compared with the current gold standard chelators, DOTA and deferoxamine (DFO), conjugated to IAB2MA for radiolabeling with 177Lu and 89Zr in cell binding, preclinical biodistribution, imaging, dosimetry, and efficacy studies in the PSMA-positive PC3-PIP tumor-bearing mouse model of prostate cancer. Results: Quantitative radiolabeling (>99% radiochemical yield) of L804-IAB2MA with 177Lu or 89Zr was achieved at ambient temperature in under 30 min, comparable to 89Zr labeling of DFO-IAB2MA. In contrast, DOTA-IAB2MA was radiolabeled with 177Lu for 30 min at 37°C in approximately 90% radiochemical yield, requiring further purification. Using europium(III) as a luminescent surrogate, high binding affinity of Eu-L804-IAB2MA to PSMA was demonstrated in PC3-PIP cells (dissociation constant, 4.6 ± 0.6 nM). All 4 radiolabeled constructs showed significantly higher levels of internalization after 30 min in the PC3-PIP cells than in PSMA-negative PC3-FLU cells. The accumulation of 177Lu- and 89Zr-L804-IAB2MA in PC3-PIP tumors and all organs examined (i.e., heart, liver, spleen, kidney, muscle, salivary glands, lacrimal glands, carcass, and bone) was significantly lower than that of 177Lu-DOTA-IAB2MA and 89Zr-DFO-IAB2MA at 96 and 72 h after injection, respectively. Generally, SPECT/CT and PET/CT imaging data showed no significant difference in the SUVmean of the tumors or muscle between the radiotracers. Dosimetry analysis via both organ-level and voxel-level dose calculation methods indicated significantly higher absorbed doses of 177Lu-DOTA-IAB2MA in tumors, kidney, liver, muscle, and spleen than of 177Lu-L804-IAB2MA. PC3-PIP tumor-bearing mice treated with single doses of 177Lu-L804-IAB2MA (18.4 or 22.2 MBq) exhibited significantly prolonged survival and reduced tumor volume compared with unlabeled minibody control. No significant difference in survival was observed between groups of mice treated with 177Lu-L804-IAB2MA or 177Lu-DOTA-IAB2MA (18.4 or 22.2 MBq). Treatment with 177Lu-L804-IAB2MA resulted in lower absorbed doses in tumors and less toxicity than that of 177Lu-DOTA-IAB2MA. Conclusion: 89Zr- and 177Lu-L804-IAB2MA may be a promising theranostic pair for imaging and therapy of prostate cancer.

A first-in-class dual-chelator theranostic agent designed for use with imaging-therapy radiometal pairs of different elements

A covalent adduct of DFOB and DOTA separated by a l-lysine residue (DFOB-l-Lys-N 6-DOTA) exhibited remarkable regioselective metal binding, with {1H}-13C NMR spectral shifts supporting Zr(iv) coordinating to the DFOB unit, and Lu(iii) coordinating to the DOTA unit. This first-in-class, dual-chelator theranostic design could enable the use of imaging-therapy radiometal pairs of different elements, such as 89Zr for positron emission tomography (PET) imaging and 177Lu for low-energy β--particle radiation therapy. DFOB-l-Lys-N 6-DOTA was elaborated with an amine-terminated polyethylene glycol extender unit (PEG4) to give DFOB-N 2-(PEG4)-l-Lys-N 6-DOTA (compound D2) to enable installation of a phenyl-isothiocyanate group (Ph-NCS) for subsequent monoclonal antibody (mAb) conjugation (mAb = HuJ591). D2-mAb was radiolabeled with 89Zr or 177Lu to produce [89Zr]Zr-D2-mAb or [177Lu]Lu-D2-mAb, respectively, and in vivo PET/CT imaging and in vivo/ex vivo biodistribution properties measured with the matched controls [89Zr]Zr-DFOB-mAb or [177Lu]Lu-DOTA-mAb in a murine LNCaP prostate tumour xenograft model. The 89Zr-immuno-PET imaging function of [89Zr]Zr-D2-mAb and [89Zr]Zr-DFOB-mAb showed no significant difference in tumour accumulation at 48 or 120 h post injection. [89Zr]Zr-D2-mAb and [177Lu]Lu-D2-mAb showed similar ex vivo biodistribution properties at 120 h post-injection. Tumour uptake of [177Lu]Lu-D2-mAb shown by SPECT/CT imaging at 48 h and 120 h post-injection supported the therapeutic function of D2, which was corroborated by similar therapeutic efficacy between [177Lu]Lu-D2-mAb and [177Lu]Lu-DOTA-mAb, both showing a sustained reduction in tumour volume (>80% over 65 d) compared to vehicle. The work identifies D2 as a trifunctional chelator that could expand capabilities in mixed-element radiometal theranostics to improve dosimetry and the clinical outcomes of molecularly targeted radiation.

Detection of IL12/23p40 via PET Visualizes Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), which includes both Crohn disease and ulcerative colitis, is a relapsing inflammatory disease of the gastrointestinal tract. Long-term chronic inflammatory conditions elevate the patient's risk of colorectal cancer (CRC). Currently, diagnosis requires endoscopy with biopsy. This procedure is invasive and requires a bowel-preparatory regimen, adding to patient burden. Interleukin 12 (IL12) and interleukin 23 (IL23) play key roles in inflammation, especially in the pathogenesis of IBD, and are established therapeutic targets. We propose that imaging of IL12/23 and its p40 subunit in IBD via immuno-PET potentially provides a new noninvasive diagnostic approach. Methods: Our aim was to investigate the potential of immuno-PET to image inflammation in a chemically induced mouse model of colitis using dextran sodium sulfate by targeting IL12/23p40 with a 89Zr-radiolabeled anti-IL12/23p40 antibody. Results: High uptake of the IL12/23p40 immuno-PET agent was exhibited by dextran sodium sulfate-administered mice, and this uptake correlated with increased IL12/23p40 present in the sera. Competitive binding studies confirmed the specificity of the radiotracer for IL12/23p40 in the gastrointestinal tract. Conclusion: These promising results demonstrate the utility of this radiotracer as an imaging biomarker of IBD. Moreover, IL12/23p40 immuno-PET can potentially guide treatment decisions for IBD management.

Preclinical Imaging of Prostate Cancer

Prostate cancer remains a major cause of mortality and morbidity, affecting millions of men, with a large percentage expected to develop the disease as they reach advanced ages. Treatment and management advances have been dramatic over the past 50 years or so, and one aspect of these improvements is reflected in the multiple advances in diagnostic imaging techniques. Much attention has been focused on molecular imaging techniques that offer high sensitivity and specificity and can now more accurately assess disease status and detect recurrence earlier. During development of molecular imaging probes, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) must be evaluated in preclinical models of the disease. If such agents are to be translated to the clinic, where patients undergoing these imaging modalities are injected with a molecular imaging probe, these agents must first be approved by the FDA and other regulatory agencies prior to their adoption in clinical practice. Scientists have worked assiduously to develop preclinical models of prostate cancer that are relevant to the human disease to enable testing of these probes and related targeted drugs. Challenges in developing reproducible and robust models of human disease in animals are beset with practical issues such as the lack of natural occurrence of prostate cancer in mature male animals, the difficulty of initiating disease in immune-competent animals and the sheer size differences between humans and conveniently smaller animals such as rodents. Thus, compromises in what is ideal and what can be achieved have had to be made. The workhorse of preclinical animal models has been, and remains, the investigation of human xenograft tumor models in athymic immunocompromised mice. Later models have used other immunocompromised models as they have been found and developed, including the use of directly derived patient tumor tissues, completely immunocompromised mice, orthotopic methods for inducing prostate cancer within the mouse prostate itself and metastatic models of advanced disease. These models have been developed in close parallel with advances in imaging agent chemistries, radionuclide developments, computer electronics advances, radiometric dosimetry, biotechnologies, organoid technologies, advances in in vitro diagnostics, and overall deeper understandings of disease initiation, development, immunology, and genetics. The combination of molecular models of prostatic disease with radiometric-based studies in small animals will always remain spatially limited due to the inherent resolution sensitivity limits of PET and SPECT decay processes, fundamentally set at around a 0.5 cm resolution limit. Nevertheless, it is central to researcher's efforts and to successful clinical translation that the best animal models are adopted, accepted, and scientifically verified as part of this truly interdisciplinary approach to addressing this important disease.

Applications of radiolabeled antibodies in neuroscience and neuro-oncology

Positron emission tomography (PET) is a powerful tool in medicine and drug development, allowing for non-invasive imaging and quantitation of biological processes in live organisms. Targets are often probed with small molecules, but antibody-based PET is expanding because of many benefits, including ease of design of new antibodies toward targets, as well as the very strong affinities that can be expected. Application of antibodies to PET imaging of targets in the central nervous system (CNS) is a particularly nascent field, but one with tremendous potential. In this review, we discuss the growth of PET in imaging of CNS targets, present the promises and progress in antibody-based CNS PET, explore challenges faced by the field, and discuss questions that this promising approach will need to answer moving forward for imaging and perhaps even radiotherapy.

Evaluation and selection of a lead diabody for interferon-γ PET imaging

INTRODUCTION

Interferon-γ (IFN-γ) is an appealing target to evaluate immune response in cancer immunotherapy as it is a hallmark of an active immune system. Imaging and detection via immunopositron emission tomography (immunoPET) of this soluble cytokine has been made feasible using a ⁸⁹Zr-labeled (t _1/2 ~ 3.27 d) monoclonal antibody (mAb). Because of its size, using a full-length mAb as an imaging vector is not ideal for repeat serial imaging because of its prolonged blood pool residency and tumor accumulation resulting in lengthier wait times between administration and imaging. This consequently impacts the potential to image a dynamic immune response in real time. This work compares ⁸⁹Zr-labeled diabodies (Db) designed with variable linker lengths between the V_H and V_L regions with the goal of selecting a lead Db for future studies.

METHODS AND RESULTS

Four Db fragments with various linker lengths (HL-n, n = 7-13 amino acids) were each conjugated to desferrioxamine (DFO). The number of attached chelates was analyzed via mass spectrometry with all immunoconjugates exhibiting one unit of DFO attached. Db-DFO conjugates were subsequently radiolabeled with zirconium-89. All constructs radiolabeled with high yields. Each radioimmunoconjugate was tested for reactivity to IFN-γ. All tracers except for [⁸⁹Zr]Zr-DFO-NCS-anti-IFN-γ HL-9 exhibited comparable immunoreactivities (>90 %) to the radiolabeled parent mAb (95.8 %). At 24 h post-labeling, the IRF values were retained except for the HL-13 construct. Imaging scans and tissue distribution studies acquired in mice bearing CT26 syngeneic colorectal tumors between 1 and 24 h post-tracer administration demonstrated variable clearance kinetics and tumor localization of each radiotracer. HL-7 had higher binding in non-tumor tissues compared to HL-11 and HL-13 at 3 h p.i. Competitive binding studies versus unmodified parent mAb (AN-18) demonstrated blocking of radiolabeled HL-11 and HL-13. [⁸⁹Zr]Zr-DFO-NCS-anti-IFN-γ HL-7 was inadequately blocked.

CONCLUSION

Despite nuanced differences in linker lengths, our data demonstrates that [⁸⁹Zr]Zr-DFO-NCS-anti-IFN-γ HL-11 exhibited the best radiotracer properties for the assessment of IFN-γ production in vivo. Work is currently underway to test the potential of using shorter-lived isotopes, like copper-64 (t_1/2 ~ 12.7 h) to match pharmacokinetics and half-lives.

ImmunoPET: Antibody-Based PET Imaging in Solid Tumors

Immuno-positron emission tomography (immunoPET) is a molecular imaging modality combining the high sensitivity of PET with the specific targeting ability of monoclonal antibodies. Various radioimmunotracers have been successfully developed to target a broad spectrum of molecules expressed by malignant cells or tumor microenvironments. Only a few are translated into clinical studies and barely into clinical practices. Some drawbacks include slow radioimmunotracer kinetics, high physiologic uptake in lymphoid organs, and heterogeneous activity in tumoral lesions. Measures are taken to overcome the disadvantages, and new tracers are being developed. In this review, we aim to mention the fundamental components of immunoPET imaging, explore the groundbreaking success achieved using this new technique, and review different radioimmunotracers employed in various solid tumors to elaborate on this relatively new imaging modality.

Evaluation of an ImmunoPET Tracer for IL-12 in a Preclinical Model of Inflammatory Immune Responses

The immune cytokine interleukin-12 (IL-12) is involved in cancer initiation and progression, autoimmunity, as well as graft versus host disease. The ability to monitor IL-12 via imaging may provide insight into various immune processes, including levels of antitumor immunity, inflammation, and infection due to its functions in immune signaling. Here, we report the development and preclinical evaluation of an antibody-based IL-12-specific positron emission tomography (PET) tracer. To mimic localized infection and stimulate IL-12 production, BALB/c mice were administered lipopolysaccharide (LPS) intramuscularly. [⁸⁹Zr]Zr-DFO-αIL12 tracer was given one hour post LPS administration and PET images were taken after 5, 24, 48, and 72 hours. We observed significantly higher uptake in LPS-treated mice as compared to controls. Biodistribution of the tracer was evaluated in a separate cohort of mice, where tracer uptake was elevated in muscle, spleen, lymph nodes, and intestines after LPS administration. To evaluate the utility of [⁸⁹Zr]Zr-DFO-αIL12 as an indicator of antigen presenting cell activation after cancer immunotherapy, we compared PET imaging with and without intratumoral delivery of oncolytic adenovirus expressing granulocyte-macrophage colony-stimulating factor (Adv/GM-CSF), which we have shown promotes anti-tumor immunity. BALB/c mice were inoculated orthotopically with the mouse mammary carcinoma line TUBO. Once TUBO tumors reached a volume of ~50 mm³, mice were treated with either three intratumoral injections of 10⁸ PFU Adv/GM-CSF or vehicle control, given every other day. Upon the last dose, [⁸⁹Zr]Zr-DFO-αIL12 was injected intravenously and 72 hours later all mice were imaged via PET. Tumor-specific uptake of [⁸⁹Zr]Zr-DFO-αIL12 was higher in Adv/GM-CSF treated mice versus controls. Tissues were harvested after imaging, and elevated levels of macrophages and CD8⁺ T_c cells were detected in Adv/GM-CSF treated tumors by immunohistochemistry. We validated that IL-12 expression was induced after Adv/GM-CSF by qRT-PCR. Importantly, expression of genes activated by IL-12 (IFNγ, TNFα, and IL-18) were unaffected after IL-12 imaging relative to mice receiving an IgG control tracer, suggesting the tracer antibody does not significantly disrupt signaling. Our results indicate that targeting soluble cytokines such as IL-12 by PET imaging with antibody tracers may serve as a noninvasive method to evaluate the function of the immune milieu in situ.

Applications of trimerbodies in cancer immunotherapy

Trimerbodies, with their unique structural and functional properties, are the basis of a new generation of therapeutic antibodies, which due to their small size and plasticity are ideal for the generation of novel biological protein drugs with multiple competitive advantages over conventional full-length monoclonal antibodies. Since their emergence, trimerbodies have been used in preclinical cancer diagnosis and therapy. Trimerbodies are highly adaptable molecules, as they allow target-specific modulation of T cell-mediated anti-tumor immunity to enhance preexisting responses or to generate de novo immune responses. In fact, a tumor-specific humanized 4-1BB-agonistic trimerbody has shown a rather impressive safety and efficacy profile in preclinical studies making it a realistic option for clinical development. Moreover, thanks to the avidity effect they are endowed with considerable therapeutic potential as carriers to deliver cytotoxic payloads to tumors. In addition, molecular imaging studies could benefit from some intermediate-sized trivalent trimerbodies as promising candidates for targeted therapy and tumor imaging.

Pilot study of the diagnostic utility of 89 Zr-df-IAB2M and 68 Ga-PSMA-11 PET imaging and multiparametric MRI in localized prostate cancer

BACKGROUND

Accurate diagnosis of localized prostate cancer (PCa) is limited by inadequacy of multiparametric (mp) MRI to fully identify and differentiate localized malignant tissue from benign pathologies. Prostate-specific membrane antigen (PSMA) represents an excellent target for molecular imaging. IAB2M, an 85-kD minibody derived from a de-immunized monoclonal antibody directed at the extracellular domain of human PSMA (huJ591), and PSMA-11, a small molecule ligand have been previously tested as probes for visualization of recurrent/metastatic PCa with PET/CT. This pilot, non-randomized trial studied their diagnostic utility in patients (pts) with localized PCa.

METHODS

Pts planned for radical prostatectomy (RP) were enrolled and underwent mpMRI and PET/CT imaging with ⁸⁹ Zr-df-IAB2M and/or ⁶⁸ Ga-PSMA-PET/CT. Image results were read by a radiologist blinded to clinical information and pathology results, mapped and compared to corresponding histopathology findings from all lesions, both clinically significant and nonsignificant. The detection rates of all three imaging modalities were measured and correlated.

RESULTS

20 pts with median age of 64.5 (46-79) years and PSA level of 7.5 (1.6-36.56) ng/ml were enrolled. 19 pts underwent RP and were imaged pre-operatively with ⁸⁹ Zr-Df-IAB2M PET/CT and mpMRI. Nine of those were imaged using ⁶⁸ Ga-PSMA-11 as well. Out of 48 intraprostatic lesions verified on surgical pathology, IAB2M PET/CT was able to detect 36 (75%). A similar proportion of pathologically confirmed, clinically significant lesions (22/29, 76%) was detected. IAB2M PET/CT was also able to identify 14/19 (74%) extraprostatic lesions. The performance of mpMRI was inferior, with 24/48 detectable lesions (50%) and 18/29 clinically significant intraprostatic lesions (62%). Compared to the current standard (mpMRI), IAB2M PET/CT had a sensitivity of 88%, specificity 38%, positive predictive value 58%, and accuracy 63%. In 9 pts who underwent Ga-PSMA-11 as well, the latter yielded a detection rate of 70% (14/20), which was also seen in clinically significant lesions (10/14, 71%). Ga-PSMA-11 PET/CT also detected 4/6 (67%) extraprostatic lesions.

CONCLUSIONS

In this pilot study, the performance of ⁸⁹ Zr-df-IAB2M was superior to mpMRI and similar to ⁶⁸ Ga-PSMA-11 PET/CT. The higher detection rate of PSMA-PET supports its use as a diagnostic tool with consequent management change implications in men with localized PCa.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives

Simple Summary

One of the most frequently diagnosed cancer in men is adenocarcinoma of the prostate. Once the disease is metastatic, only very limited treatment options are available, resulting in a very short median survival time of 13 months; however, this reality is gradually changing due to the discovery of prostate-specific membrane antigen (PSMA), a protein that is present in cancerous prostate tissue. Researchers have developed pharmaceuticals specific for PSMA, ranging from antibodies (mAb) to low-molecular weight molecules coupled to beta minus and alpha-emitting radionuclides for their use in targeted radionuclide therapy (TRT). TRT offers the possibility of selectively removing cancer tissue via the emission of radiation or radioactive particles within the tumour. In this article, the major milestones in PSMA ligand research and the therapeutic developments are summarised, together with a future perspective on the enhancement of current therapeutic approaches.

Abstract

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

Review of commonly used prostate specific PET tracers used in prostate cancer imaging in current clinical practice

¹⁸F-Fluorodeoxyglucose positron emission tomography (FDG-PET) underperforms in detecting prostate cancer (PCa) due to inherent characteristics of primary and metastatic tumors, including relatively low rate of glucose utilization. Consequently, alternate PCa PET imaging agents targeting other aspects of PCa cell biology have been developed for clinical practice. The most common dedicated PET imaging tracers include ⁶⁸Ga/¹⁸F prostate-specific membrane antigen (PSMA), ¹¹C-Choline, and ¹⁸F-fluciclovine (Axumin™). This review will describe how these agents target specific inherent characteristics of PCa and explore the current literature for these agents for both primary and recurrent PCa, comparing the advantages and limitations of each tracer. Both ¹¹C-Choline and ¹⁸F-Fluciclovine PET have been shown to detect nodal and osseous disease at higher rates compared to FDG-PET but offer no additional benefit in detecting prostate disease, especially in primary staging. As a result, PSMA PET, specifically ⁶⁸Ga-PSMA-11, has emerged as a key imaging option for both primary and recurrent cancer. PSMA PET may be more sensitive than MRI at the local level and more sensitive than ¹¹C-Choline and ¹⁸F-Fluciclovine PET for distant disease. Furthermore, compared to ¹¹C-Choline and ¹⁸F-Fluciclovine PET, ⁶⁸Ga-PSMA-11 PET has higher detection rates at low PSA levels (<2 ng/dL). With improved delineation of disease, PSMA imaging has influenced treatment planning; radiation fields can be narrowed, and patients with isolated or oligo-metastatic disease can be spared systemic therapy. The retrospective nature of many of the studies describing these PCa imaging modalities complicates their assessment and comparison.

Evaluation of [131I]I- and [177Lu]Lu-DTPA-A11 Minibody for Radioimmunotherapy in a Preclinical Model of PSCA-Expressing Prostate Cancer

PURPOSE

Radioimmunotherapy uses tumor-specific antibodies to deliver therapeutic radionuclides, but hematological toxicity due to the long serum half-life of intact antibodies remains a challenge. We evaluated a smaller antibody fragment, the minibody, with faster kinetics and a potentially improved therapeutic index.

PROCEDURES

The anti-prostate stem cell antigen (PSCA) minibody (A11 Mb) was radiolabeled with iodine-124 ([124I]I-A11 Mb) or conjugated with deferoxamine (DFO) and labeled with zirconium-89 ([89Zr]Zr-DFO-A11 Mb) for surrogate immunoPET to profile pharmacokinetics in a human prostate cancer xenograft model. Subsequently, minibodies labeled with two therapeutic beta emitters, directly iodinated [131I]I-A11 Mb (non-residualizing) and 177Lu chelated using DTPA ([177Lu]Lu-DTPA-A11 Mb) (residualizing), were compared for in vitro antigen-specific cytotoxicity. Full biodistribution studies (in 22Rv1-PSCA tumor bearing and hPSCA knock-in mice) were conducted for dosimetry calculations. Finally, the lead candidate [131I]I-A11 Mb was evaluated in a radioimmunotherapy experiment. Escalating single doses (3.7, 11, or 37 MBq) and saline control were administered to 22Rv1-PSCA tumor bearing mice and anti-tumor effects (tumor volume) and toxicity (body weight) were monitored.

RESULTS

Minibodies radiolabeled with therapeutic beta emitters [131I]I-A11 Mb and [177Lu]Lu-DTPA-A11 Mb exhibited comparable tumor cell growth inhibition in vitro. In vivo surrogate immunoPET imaging using [89Zr]Zr-DFO-A11 Mb showed activity retention in liver and kidney up to 72 h, while [124I]I-A11 Mb cleared from liver, kidney, and blood by 48 h. Based on full biodistribution and dosimetry calculations, administering 37 MBq [131I]I-A11 Mb was predicted to deliver a favorable dose to the tumor (35 Gy), with a therapeutic index of 22 (tumor:bone marrow). For [177Lu]Lu-DTPA-A11 Mb, the kidneys would be dose-limiting, and the maximum tolerated activity (7.4 MBq) was not predicted to deliver an effective radiation dose to tumor. Radioimmunotherapy with a single dose of [131I]I-A11 Mb showed dose-dependent tumor inhibition with minimal off-target toxicity and improved median survival (19 and 24 days, P < 0.001) compared with untreated mice (12 days).

CONCLUSIONS

These findings show the potential of the anti-PSCA minibody for targeted radioimmunotherapy with minimal toxicity, and the application of immunoPET and dosimetry for personalized treatment.

Acute Statin Treatment Improves Antibody Accumulation in EGFR- and PSMA-Expressing Tumors

PURPOSE

Statins are cholesterol-depleting drugs used to treat patients with hypercholesterolemia. Preclinically, statins disrupt trafficking of receptors present at the cell membrane. Membrane receptors, defined as tumor biomarkers and therapeutic targets, are often internalized by an endocytic pathway. Indeed, receptor endocytosis and recycling are dynamic mechanisms that often affect receptor density at the cell surface. In therapies using monoclonal antibodies (mAb), a downregulation in receptor density at the cell surface decreases antibody binding to the extracellular domain of the membrane receptor. Here, we determined the potential of lovastatin, simvastatin, and rosuvastatin in preclinically modulating epidermal growth factor receptor (EGFR) and prostate-specific membrane antigen (PSMA) receptor density at the tumor cell surface.

EXPERIMENTAL DESIGN

Small-animal PET was used to study the binding of ⁸⁹Zr-labeled antibodies in ectopic xenografts. Ex vivo analyses were performed to determine changes in endocytic proteins, EGFR, and PSMA surface levels.

RESULTS

Acute statin treatment using lovastatin, simvastatin, or rosuvastatin enhanced tumors' avidity for the mAbs panitumumab, cetuximab, and huJ591. Statins temporarily modulated caveolin-1, cavin-1, endophilin, clathrin, and dynamin proteins in EGFR- and PSMA-overexpressing xenografts.

CONCLUSIONS

These data show the potential of statins as pharmacologic modulators of endocytic proteins for improved tumors' accumulation of mAbs. The translational significance of these findings lies in the potential of statins to temporarily modulate the heterogeneous presence of receptors at the cell membrane, a characteristic often associated with poor response in tumors to therapeutic antibodies.

Engineered Fragments of the PSMA-Specific 5D3 Antibody and Their Functional Characterization

Prostate-Specific Membrane Antigen (PSMA) is an established biomarker for the imaging and experimental therapy of prostate cancer (PCa), as it is strongly upregulated in high-grade primary, androgen-independent, and metastatic lesions. Here, we report on the development and functional characterization of recombinant single-chain Fv (scFv) and Fab fragments derived from the 5D3 PSMA-specific monoclonal antibody (mAb). These fragments were engineered, heterologously expressed in insect S2 cells, and purified to homogeneity with yields up to 20 mg/L. In vitro assays including ELISA, immunofluorescence and flow cytometry, revealed that the fragments retain the nanomolar affinity and single target specificity of the parent 5D3 antibody. Importantly, using a murine xenograft model of PCa, we verified the suitability of fluorescently labeled fragments for in vivo imaging of PSMA-positive tumors and compared their pharmacokinetics and tissue distribution to the parent mAb. Collectively, our data provide an experimental basis for the further development of 5D3 recombinant fragments for future clinical use.

Imaging using radiolabelled targeted proteins: radioimmunodetection and beyond

The use of radiolabelled antibodies was proposed in 1970s for staging of malignant tumours. Intensive research established chemistry for radiolabelling of proteins and understanding of factors determining biodistribution and targeting properties. The use of radioimmunodetection for staging of cancer was not established as common practice due to approval and widespread use of [¹⁸F]-FDG, which provided a more general diagnostic use than antibodies or their fragments. Expanded application of antibody-based therapeutics renewed the interest in radiolabelled antibodies. RadioimmunoPET emerged as a powerful tool for evaluation of pharmacokinetics of and target engagement by biotherapeutics. In addition to monoclonal antibodies, new radiolabelled engineered proteins have recently appeared, offering high-contrast imaging of expression of therapeutic molecular targets in tumours shortly after injection. This creates preconditions for noninvasive determination of a target expression level and stratification of patients for targeted therapies. Radiolabelled proteins hold great promise to play an important role in development and implementation of personalised targeted treatment of malignant tumours. This article provides an overview of biodistribution and tumour-seeking features of major classes of targeting proteins currently utilized for molecular imaging. Such information might be useful for researchers entering the field of the protein-based radionuclide molecular imaging.

Current Perspectives on 89Zr-PET Imaging

⁸⁹Zr is an emerging radionuclide that plays an essential role in immuno-positron emission tomography (PET) imaging. The long half-life of ⁸⁹Zr (t_1/2 = 3.3 days) is favorable for evaluating the in vivo distribution of monoclonal antibodies. Thus, the use of ⁸⁹Zr is promising for monitoring antibody-based cancer therapies. Immuno-PET combines the sensitivity of PET with the specificity of antibodies. A number of studies have been conducted to investigate the feasibility of ⁸⁹Zr immuno-PET imaging for predicting the efficacy of radioimmunotherapy and antibody therapies, imaging target expression, detecting target-expressing tumors, and the monitoring of anti-cancer chemotherapies. In this review, we summarize the current status of PET imaging using ⁸⁹Zr in both preclinical and clinical studies by highlighting the use of immuno-PET for the targets of high clinical relevance. We also present ⁸⁹Zr-PET applications other than immuno-PET, such as nanoparticle imaging and cell tracking. Finally, we discuss the limitations and the ongoing research being performed to overcome the remaining hurdles.

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.

Cetuximab PET delineated changes in cellular distribution of EGFR upon dasatinib treatment in triple negative breast cancer

Background

At least 50% of triple negative breast cancer (TNBC) overexpress the epidermal growth factor receptor, EGFR, which paved the way for clinical trials investigating its blockade. Outcomes remained dismal stemming from mechanisms of resistance particularly the nuclear cycling of EGFR, which is enhanced by Src activation. Attenuation of Src reversed nuclear translocation, restoring EGFR to the cell surface. Herein, we hypothesize that changes in cellular distribution of EGFR upon Src inhibition with dasatinib can be annotated through the EGFR immunopositron emission tomography (immunoPET) radiotracer, [⁸⁹Zr]Zr-cetuximab.

Methods

Nuclear and non-nuclear EGFR levels of dasatinib-treated vs. untreated MDA-MB-231 and MDA-MB-468 cells were analyzed via immunoblots. Both treated and untreated cells were exposed to [⁸⁹Zr]Zr-cetuximab to assess binding at 4 °C and 37 °C. EGFR-positive MDA-MB-231, MDA-MB-468, and a patient-derived xenograft were treated with dasatinib or vehicle followed by cetuximab PET imaging to compare EGFR levels. After imaging, the treated mice were separated into two groups: one cohort continued with dasatinib with the addition of cetuximab while the other cohort received dasatinib alone. Correlations between the radiotracer uptake vs. changes in tumor growth and EGFR expression from immunoblots were analyzed.

Results

Treated cells displayed higher binding of [⁸⁹Zr]Zr-cetuximab to the cell membrane at 4 °C and with greater internalized activity at 37 °C vs. untreated cells. In all tumor models, higher accumulation of the radiotracer in dasatinib-treated groups was observed compared to untreated tumors. Treated tumors displayed significantly decreased pSrc (Y416) with retained total Src levels compared to control. In MDA-MB-468 and PDX tumors, the analysis of cetuximab PET vs. changes in tumor volume showed an inverse relationship where high tracer uptake in the tumor demonstrated minimal tumor volume progression. Furthermore, combined cetuximab and dasatinib treatment showed better tumor regression compared to control and dasatinib-only-treated groups. No benefit was achieved in MDA-MB-231 xenografts with the addition of cetuximab, likely due to its KRAS-mutated status.

Conclusions

Cetuximab PET can monitor effects of dasatinib on EGFR cellular distribution and potentially inform treatment response in wild-type KRAS TNBC.

Targeted Radionuclide Therapy of Prostate Cancer-From Basic Research to Clinical Perspectives

Prostate cancer is the most commonly diagnosed malignancy in men and the second leading cause of cancer-related deaths in Western civilization. Although localized prostate cancer can be treated effectively in different ways, almost all patients progress to the incurable metastatic castration-resistant prostate cancer. Due to the significant mortality and morbidity rate associated with the progression of this disease, there is an urgent need for new and targeted treatments. In this review, we summarize the recent advances in research on identification of prostate tissue-specific antigens for targeted therapy, generation of highly specific and selective molecules targeting these antigens, availability of therapeutic radionuclides for widespread medical applications, and recent achievements in the development of new-generation small-molecule inhibitors and antibody-based strategies for targeted prostate cancer therapy with alpha-, beta-, and Auger electron-emitting radionuclides.

Alpha-Particle Therapy for Multifocal Osteosarcoma: A Hypothesis

Osteosarcoma (OST) is the most common bone tumor in children and adolescents with a second peak of incidence in elderly adults usually diagnosed as secondary tumors in Paget's disease or irradiated bone. Subjects with metastatic disease or whose disease relapses after the initial therapy have a poor prognosis. Moreover, multifocal OST contains tumor-initiating cells that are resistant to chemotherapy. The use of aggressive therapies in an attempt to eradicate these cells can have long-term negative consequences in these vulnerable patient populations. ²²⁷Th-labeled molecular probes based on ligands to OST-associated receptors such as IGF-1R (insulin-like growth factor receptor 1), HER2 (human epidermal growth factor receptor 2), and PSMA (prostate-specific membrane antigen) are expected to detect and treat osseous and nonosseous sites of multifocal OST. Published reports indicate that ²²⁷Th has limited myelotoxicity, can be stably chelated to its carriers and, as it decays at targeted sites, ²²⁷Th produces ²²³Ra that is subsequently incorporated into the areas of increased osteoblastic activity, that is, osseous metastatic lesions. Linear energy transfer of α particles emitted by ²²⁷Th and its daughter ²²³Ra is within the range of the optimum relative biological effectiveness. The radiotoxicity of α particles is virtually independent of the phase in the cell cycle, oxygenation, and the dose rate. For these reasons, even resistant OST cells remain susceptible to killing by high-energy α particles, which can also kill adjacent quiescent OST cells or cells with low expression of targeted receptors. Systemic side effects are minimized by the limited range of these intense radiations. Quantitative single-photon emission computed tomography of ²²⁷Th and ²²³Ra is feasible. Additionally, the availability of radionuclide pairs, for example, ⁸⁹Zr for positron emission tomography and ²²⁷Th for therapy, establish a strong basis for the theranostic use of ²²⁷Th in the individualized treatment of multifocal OST.

Current outlook on radionuclide delivery systems: from design consideration to translation into clinics

Radiopharmaceuticals have proven to be effective agents, since they can be successfully applied for both diagnostics and therapy. Effective application of relevant radionuclides in pre-clinical and clinical studies depends on the choice of a sufficient delivery platform. Herein, we provide a comprehensive review on the most relevant aspects in radionuclide delivery using the most employed carrier systems, including, (i) monoclonal antibodies and their fragments, (ii) organic and (iii) inorganic nanoparticles, and (iv) microspheres. This review offers an extensive analysis of radionuclide delivery systems, the approaches of their modification and radiolabeling strategies with the further prospects of their implementation in multimodal imaging and disease curing. Finally, the comparative outlook on the carriers and radionuclide choice, as well as on the targeting efficiency of the developed systems is discussed.

Ion Channel Targeting with Antibodies and Antibody Fragments for Cancer Diagnosis

The antibody era has greatly impacted cancer management in recent decades. Indeed, antibodies are currently applied for both cancer diagnosis and therapy. For example, monoclonal antibodies are the main constituents of several in vitro diagnostics, which are applied at many levels of cancer diagnosis. Moreover, the great improvement provided by in vivo imaging, especially for early-stage cancer diagnosis, has traced the path for the development of a complete new class of antibodies, i.e., engineered antibody fragments. The latter embody the optimal characteristics (e.g., low renal retention, rapid clearance, and small size) which make them ideal for in vivo applications. Furthermore, the present review focuses on reviewing the main applications of antibodies and antibody fragments for solid cancer diagnosis, both in vitro and in vivo. Furthermore, we review the scientific evidence showing that ion channels represent an almost unexplored class of ideal targets for both in vitro and in vivo diagnostic purposes. In particular, we review the applications, in solid cancers, of monoclonal antibodies and engineered antibody fragments targeting the voltage-dependent ion channel Kv 11.1, also known as hERG1.

Recent Advances in Prostate-Specific Membrane Antigen-Based Radiopharmaceuticals

BACKGROUND

Prostate cancer (PCa) is the most common sex-related malignancy with high mortality in men worldwide. Prostate-specific membrane antigen (PSMA) is overexpressed on the surface of most prostate tumor cells and considered a valuable target for both diagnosis and therapy of prostate cancer. A series of radiolabeled agents have been developed based on the featured PSMA ligands in the previous decade and have demonstrated promising outcomes in clinical research of primary and recurrent PCa. Furthermore, the inspiring response and safety of lutetium-177-PSMA-617 (177Lu-PSMA-617) radiotherapy represent the potential for expanded therapeutic options for metastatic castration-resistant PCa. Retrospective cohort studies have revealed that radiolabeled PSMA agents are the mainstays of the current success, especially in detecting prostate cancer with metastasis and biochemical recurrence.

OBJECTIVE

This review is intended to present a comprehensive overview of the current literature on PSMA ligand-based agents for both radionuclide imaging and therapeutic approaches, with a focus on those that have been clinically adopted.

CONCLUSION

PSMA-based diagnosis and therapy hold great promise for improving the clinical management of prostate cancer.

David vs. Goliath: The Structure, Function, and Clinical Prospects of Antibody Fragments

Since the licensing of the first monoclonal antibody therapy in 1986, monoclonal antibodies have become the largest class of biopharmaceuticals with over 80 antibodies currently approved for a variety of disease indications. The development of smaller, antigen binding antibody fragments, derived from conventional antibodies or produced recombinantly, has been growing at a fast pace. Antibody fragments can be used on their own or linked to other molecules to generate numerous possibilities for bispecific, multi-specific, multimeric, or multifunctional molecules, and to achieve a variety of biological effects. They offer several advantages over full-length monoclonal antibodies, particularly a lower cost of goods, and because of their small size they can penetrate tissues, access challenging epitopes, and have potentially reduced immunogenicity. In this review, we will discuss the structure, production, and mechanism of action of EMA/FDA-approved fragments and of those in clinical and pre-clinical development. We will also discuss current topics of interest surrounding the potential use of antibody fragments for intracellular targeting and blood-brain barrier (BBB) penetration.

Comparison of prostate-specific membrane antigen ligands in clinical translation research for diagnosis of prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA), overexpressed on prostate cancer (PCa), is a well-characterized cell surface protein to selectively diagnose PCa. PSMA's unique characteristics and its 1000-fold higher expression in PCa compared with other tissues renders it as a suitable biomarker for detection of PCa in its early stage. In this report, we critically analyze and recommend the requirements needed for the development of variety of PSMA-targeted molecular imaging agents based on antibodies, small molecule ligands, peptides, and aptamers. The targeting moieties are either conjugated to radionuclear isotopes or near-infrared agents for efficient diagnosis of PCa.

RECENT FINDINGS

From the analysis, it was found that several small molecule-derived PCa imaging agents are approved for clinical trials in Europe and the United States, and few are already in the clinical use for diagnosis of PCa. Even though ¹¹¹In-labeled capromab pendetide was approved by the Food and Drug Administration (FDA) and other engineered antibodies are available for detection of PCa, but high production cost, low shelf life (less than 1 month at 4°C), possibility of human immuno reactions, and low blood clearance rate necessitated a need for developing new imaging agents, which are serum stable, cost-effective, and possesses longer shelf life (6 months), have fast clearance rate from nontargeted tissues during the diagnosis process. It is found that small molecule ligand-derived imaging agents possesses most of the desired properties expected for an ideal diagnostic agent when compared with other targeting moieties.

CONCLUSION

This report discusses in detail the homing moieties used in the development of targeted diagnostic tools for detection of PCa. The merits and demerits of monoclonal antibodies, small molecule ligands, peptides, and aptamers for imaging of PCa and intraoperative guided surgery are extensively analyzed. Among all, urea-based ligands were found to be most successful in preclinical and clinical trials and show a major promise for future commercialization.

Immuno-PET Imaging and Pharmacokinetics of an Anti-CEA scFv-based Trimerbody and Its Monomeric Counterpart in Human Gastric Carcinoma-Bearing Mice

Monoclonal antibodies (mAbs) are currently used as therapeutic agents in different types of cancer. However, mAbs and antibody fragments developed so far show suboptimal properties in terms of circulation time and tumor penetration/retention. Here, we report the radiolabeling, pharmacokinetic evaluation, and determination of tumor targeting capacity of the previously validated anti-CEA MFE23-scFv-based N-terminal trimerbody (MFE23^N-trimerbody), and the results are compared to those obtained for the monomeric MFE23-scFv. Dissection and gamma-counting studies performed with the ¹³¹I-labeled protein scaffolds in normal mice showed slower blood clearance for the trimerbody, and accumulation in the kidneys, the spleen, and the liver for both species. These, together with a progressive uptake in the small intestine, confirm a combined elimination scheme with hepatobiliary and urinary excretion. Positron emission tomography studies performed in a xenograft mouse model of human gastric adenocarcinoma, generated by subcutaneous administration of CEA-positive human MKN45 cells, showed higher tumor accumulation and tumor-to-muscle (T/M) ratios for ¹²⁴I-labeled MFE23^N-trimerbody than for MFE23-scFv. Specific uptake was not detected with PET imaging in CEA negative xenografts as indicated by low T/M ratios. Our data suggest that engineered intermediate-sized trivalent antibody fragments could be promising candidates for targeted therapy and imaging of CEA-positive tumors.

Antibody Fragment and Affibody ImmunoPET Imaging Agents: Radiolabelling Strategies and Applications

Antibodies have long been recognised as potent vectors for carrying diagnostic medical radionuclides, contrast agents and optical probes to diseased tissue for imaging. The area of ImmunoPET combines the use of positron emission tomography (PET) imaging with antibodies to improve the diagnosis, staging and monitoring of diseases. Recent developments in antibody engineering and PET radiochemistry have led to a new wave of experimental ImmunoPET imaging agents that are based on a range of antibody fragments and affibodies. In contrast to full antibodies, engineered affibody proteins and antibody fragments such as minibodies, diabodies, single-chain variable region fragments (scFvs), and nanobodies are much smaller but retain the essential specificities and affinities of full antibodies in addition to more desirable pharmacokinetics for imaging. Herein, recent key developments in the PET radiolabelling strategies of antibody fragments and related affibody molecules are highlighted, along with the main PET imaging applications of overexpressed antigen-associated tumours and immune cells.

Monitoring Src status after dasatinib treatment in HER2+ breast cancer with 89Zr-trastuzumab PET imaging

Background

De novo or acquired resistance in breast cancer leads to treatment failures and disease progression. In human epidermal growth factor receptor 2 (HER2)-positive (HER2+) breast cancer, Src, a non-receptor tyrosine kinase, is identified as a major mechanism of trastuzumab resistance, with its activation stabilizing aberrant HER2 signaling, thus making it an attractive target for inhibition. Here, we explored the causal relationship between Src and HER2 by examining the potential of ⁸⁹Zr-trastuzumab as a surrogate imaging marker of Src activity upon inhibition with dasatinib in HER2+ breast cancer.

Methods

HER2+ primary breast cancer cell lines BT-474 and trastuzumab-resistant JIMT-1 were treated with dasatinib and assessed for expression and localization of HER2, Src, and phosphorylated Src (pSrc) (Y416) through western blots and binding assays. Mice bearing BT-474 or JIMT-1 tumors were treated for 7 or 14 days with dasatinib. At the end of each treatment, tumors were imaged with ⁸⁹Zr-trastuzumab. The results of ⁸⁹Zr-trastuzumab positron emission tomography (PET) was compared against tumor uptake of fluorodeoxyglucose (¹⁸F-FDG) obtained the day before in the same group of mice. Ex vivo western blots and immunohistochemical staining (IHC) were performed for validation.

Results

In BT-474 and JIMT-1 cells, treatment with dasatinib resulted in a decrease in internalized ⁸⁹Zr-trastuzumab. Confirmation with immunoblots displayed abrogation of pSrc (Y416) signaling; binding assays in both cell lines demonstrated a decrease in cell surface and internalized HER2-bound tracer. In xenograft models, dasatinib treatment for 7 days (BT-474, 11.05 ± 2.10 % injected dose per gram of tissue %(ID)/g; JIMT-1, 3.88 ± 1.47 %ID/g)) or 14 days (BT-474, 9.20 ± 1.85 %ID/g; JIMT-1, 4.45 ± 1.23 %ID/g) resulted in a significant decrease in ⁸⁹Zr-trastuzumab uptake on PET compared to untreated control (BT-474, 17.88 ± 2.18 %ID/g; JIMT-1, 8.04 ± 1.47 %ID/g). No difference in ¹⁸F-FDG uptake was observed between control and treated cohorts. A parallel decrease in membranous HER2 and pSrc (Y416) staining was observed in tumors post treatment on IHC. Immunoblots further validated the ⁸⁹Zr-trastuzumab-PET readout. Positive correlation was established between ⁸⁹Zr-trastuzumab tumor uptake versus tumor regression, pSrc and pHER2 expression.

Conclusions

⁸⁹Zr-trastuzumab can potentially assess tumor response to dasatinib in HER2+ breast cancer and could be used as a surrogate tool to monitor early changes in Src signaling downstream of HER2.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-1055-2) contains supplementary material, which is available to authorized users.

IFNγ PET Imaging as a Predictive Tool for Monitoring Response to Tumor Immunotherapy

IFNγ is an attractive target for imaging active antitumor immunity due to its function in the T-cell signaling axis. Here, we test an IFNγ immuno-PET (immunoPET) probe for its capacity to identify adaptive immunotherapy response after HER2/neu vaccination in both spontaneous salivary and orthotopic neu+ mouse mammary tumors. IFNγ immunoPET detected elevated cytokine levels in situ after vaccination, which inversely correlated with tumor growth rate, an indicator of response to therapy. In a model of induced T-cell anergy where CD8 T cells infiltrate the tumor, but upregulate PD-1, IFNγ tracer uptake was equivalent to isotype control, illustrating a lack of antitumor T-cell activity. The IFNγ immunoPET tracer detected IFNγ protein sequestered on the surface of tumor cells, likely in complex with the IFNγ receptor, which may explain imaging localization of this soluble factor in vivo Collectively, we find that the activation status of cytotoxic T cells is annotated by IFNγ immunoPET, with reduced off-target binding to secondary lymphoid tissues compared with imaging total CD3+ tumor-infiltrating lymphocytes. Targeting of soluble cytokines such as IFNγ by PET imaging may provide valuable noninvasive insight into the function of immune cells in situ Significance: This study presents a novel approach to monitor therapeutic outcomes via IFNγ-targeted positron emission tomography. Cancer Res; 78(19); 5706-17. ©2018 AACR.

Aligning physics and physiology: Engineering antibodies for radionuclide delivery

The exquisite specificity of antibodies and antibody fragments renders them excellent agents for targeted delivery of radionuclides. Radiolabeled antibodies and fragments have been successfully used for molecular imaging and radioimmunotherapy (RIT) of cell surface targets in oncology and immunology. Protein engineering has been used for antibody humanization essential for clinical applications, as well as optimization of important characteristics including pharmacokinetics, biodistribution, and clearance. Although intact antibodies have high potential as imaging and therapeutic agents, challenges include long circulation time in blood, which leads to later imaging time points post-injection and higher blood absorbed dose that may be disadvantageous for RIT. Using engineered fragments may address these challenges, as size reduction and removal of Fc function decreases serum half-life. Radiolabeled fragments and pretargeting strategies can result in high contrast images within hours to days, and a reduction of RIT toxicity in normal tissues. Additionally, fragments can be engineered to direct hepatic or renal clearance, which may be chosen based on the application and disease setting. This review discusses aligning the physical properties of radionuclides (positron, gamma, beta, alpha, and Auger emitters) with antibodies and fragments and highlights recent advances of engineered antibodies and fragments in preclinical and clinical development for imaging and therapy.

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.

Preclinical optimization of antibody-based radiopharmaceuticals for cancer imaging and radionuclide therapy-Model, vector, and radionuclide selection

Intact antibodies and their truncated counterparts (eg, Fab, scFv fragments) are generally exquisitely specific and selective vectors, enabling recognition of individual cancer-associated molecular phenotypes against a complex and dynamic biomolecular background. Complementary alignment of these advantages with unique properties of radionuclides is a defining paradigm in both radioimmunoimaging and radioimmunotherapy, which remain some of the most adept and promising tools for cancer diagnosis and treatment. This review discusses how translational potency can be maximized through rational selection of antibody-nuclide couples for radioimmunoimaging/therapy in preclinical models.

Imaging EGFR and HER3 through 89Zr-labeled MEHD7945A (Duligotuzumab)

Tumor resistance to treatment paved the way toward the development of single agent drugs that target multiple molecular signatures amplified within the malignancy. The discovered crosstalk between EGFR and HER3 as well as the role of HER3 in mediating EGFR resistance made these two receptor tyrosine kinases attractive targets. MEHD7945A or duligotuzumab is a single immunotherapy agent that dually targets both molecular signatures. In this study, a positron emission tomography (PET) companion diagnostic to MEHD7945A is reported and evaluated in pancreatic cancer. Tumor accretion and whole body pharmacokinetics of 89Zr-MEHD7945A were established. Specificity of the probe for EGFR and/or HER3 was further examined.

PSMA-Based [(18)F]DCFPyL PET/CT Is Superior to Conventional Imaging for Lesion Detection in Patients with Metastatic Prostate Cancer

PURPOSE

Current standard of care conventional imaging modalities (CIM) such as X-ray computed tomography (CT) and bone scan can be limited for detection of metastatic prostate cancer and therefore improved imaging methods are an unmet clinical need. We evaluated the utility of a novel second-generation low molecular weight radiofluorinated prostate-specific membrane antigen (PSMA)-targeted positron emission tomography (PET) radiotracer, [(18)F]DCFPyL, in patients with metastatic prostate cancer.

PROCEDURES

Nine patients with suspected prostate cancer recurrence, eight with CIM evidence of metastatic prostate cancer and one with biochemical recurrence, were imaged with [(18)F]DCFPyL PET/CT. Eight of the patients had contemporaneous CIM for comparison. A lesion-by-lesion comparison of the detection of suspected sites of metastatic prostate cancer was carried out between PET and CIM. Statistical analysis for estimated proportions of inter-modality agreement for detection of metastatic disease was calculated accounting for intra-patient correlation using general estimating equation (GEE) intercept-only regression models.

RESULTS

One hundred thirty-nine sites of PET positive [(18)F]DCFPyL uptake (138 definite, 1 equivocal) for metastatic disease were detected in the eight patients with available comparison CIM. By contrast, only 45 lesions were identified on CIM (30 definite, 15 equivocal). When lesions were negative or equivocal on CIM, it was estimated that a large portion of these lesions or 0.72 (95 % confidence interval (CI) 0.55-0.84) would be positive on [(18)F]DCFPyL PET. Conversely, of those lesions negative or equivocal on [(18)F]DCFPyL PET, it was estimated that only a very small proportion or 0.03 (95 % CI 0.01-0.07) would be positive on CIM. Delayed 2-h-post-injection time point PET yielded higher tumor radiotracer uptake and higher tumor-to-background ratios than an earlier 1-h-post-injection time point.

CONCLUSIONS

A novel PSMA-targeted PET radiotracer, [(18)F]DCFPyL, was able to a large number of suspected sites of prostate cancer, many of which were occult or equivocal by CIM. This study provides strong preliminary evidence for the use of this second-generation PSMA-targeted PET radiotracer for detection of metastatic prostate cancer and lends further support for the importance of PSMA-targeted PET imaging in prostate cancer.

PSMA-Targeted Theranostic Nanocarrier for Prostate Cancer

Herein, we report the use of a theranostic nanocarrier (Folate-HBPE(CT20p)) to deliver a therapeutic peptide to prostate cancer tumors that express PSMA (folate hydrolase 1). The therapeutic peptide (CT20p) targets and inhibits the chaperonin-containing TCP-1 (CCT) protein-folding complex, is selectively cytotoxic to cancer cells, and is non-toxic to normal tissue. With the delivery of CT20p to prostate cancer cells via PSMA, a dual level of cancer specificity is achieved: (1) selective targeting to PSMA-expressing prostate tumors, and (2) specific cytotoxicity to cancer cells with minimal toxicity to normal cells. The PSMA-targeting theranostic nanocarrier can image PSMA-expressing cells and tumors when a near infrared dye is used as cargo. Meanwhile, it can be used to treat PSMA-expressing tumors when a therapeutic, such as the CT20p peptide, is encapsulated within the nanocarrier. Even when these PSMA-targeting nanocarriers are taken up by macrophages, minimal cell death is observed in these cells, in contrast with doxorubicin-based therapeutics that result in significant macrophage death. Incubation of PSMA-expressing prostate cancer cells with the Folate-HBPE(CT20p) nanocarriers induces considerable changes in cell morphology, reduction in the levels of integrin β1, and lower cell adhesion, eventually resulting in cell death. These results are relevant as integrin β1 plays a key role in prostate cancer invasion and metastatic potential. In addition, the use of the developed PSMA-targeting nanocarrier facilitates the selective in vivo delivery of CT20p to PSMA-positive tumor, inducing significant reduction in tumor size.

Evaluation of Castration-Resistant Prostate Cancer with Androgen Receptor-Axis Imaging

Castration-resistant prostate cancer (CRPC) is the lethal form of prostate cancer, and more than 26,000 men will die from this disease in 2016. The pathophysiology of CRPC is clearly multifactorial, but most often, androgen receptor (AR) upregulation is associated with its earliest beginnings and the AR increase is part of the multimolecular complex including downstream effector proteins linked to AR (AR-axis) responsible for rapid proliferation and malignant features of the malignant cell. In both animal models and patients, glycolysis (Warburg effect) is also an early manifestation of CRPC transformation. At Memorial Sloan Kettering Cancer Center, we have focused our energies on imaging studies of the AR-axis in CRPC, using ¹⁸F-FDG, ¹⁸F-16β-fluoro-5α-dihydrotestosterone (¹⁸F-FDHT), and a variety of radiolabeled antibodies targeting downstream effectors, such as prostate-specific membrane antigen (PSMA). Small-molecular-weight PSMA-targeting agents are not part of this review. In this review, we will focus on molecular imaging of the AR-axis in metastatic CRPC (mCRPC) and discuss our personal experience with these tracers. Our goal is to put these radiopharmaceuticals in the context of mCRPC biology and diagnosis (e.g., ¹⁸F-FDHT).

Metal-Based PSMA Radioligands

Prostate cancer is one of the most common malignancies for which great progress has been made in identifying appropriate molecular targets that would enable efficient in vivo targeting for imaging and therapy. The type II integral membrane protein, prostate specific membrane antigen (PSMA) is overexpressed on prostate cancer cells in proportion to the stage and grade of the tumor progression, especially in androgen-independent, advanced and metastatic disease, rendering it a promising diagnostic and/or therapeutic target. From the perspective of nuclear medicine, PSMA-based radioligands may significantly impact the management of patients who suffer from prostate cancer. For that purpose, chelating-based PSMA-specific ligands have been labeled with various diagnostic and/or therapeutic radiometals for single-photon-emission tomography (SPECT), positron-emission-tomography (PET), radionuclide targeted therapy as well as intraoperative applications. This review focuses on the development and further applications of metal-based PSMA radioligands.

Establishment of the In Vivo Efficacy of Pretargeted Radioimmunotherapy Utilizing Inverse Electron Demand Diels-Alder Click Chemistry

The pretargeting system based on the inverse electron demand Diels-Alder reaction (IEDDA) between trans-cyclooctene (TCO) and tetrazine (Tz) combines the favorable pharmacokinetic properties of radiolabeled small molecules with the affinity and specificity of antibodies. This strategy has proven to be an efficient method for the molecularly targeted delivery of pharmaceuticals, including isotopes for radiological imaging. Despite encouraging results from in vivo PET imaging studies, this promising system has yet to be thoroughly evaluated for pretargeted radioimmunotherapy (PRIT). Toward that end, we synthesized two novel ¹⁷⁷Lu-labeled tetrazine-bearing radioligands. Next, we compared the usefulness of our ligands for PRIT when paired with TCO-modified 5B1-a human, anti-CA19.9 mAb-in preclinical murine models of pancreatic cancer. The exemplary ligand, ¹⁷⁷Lu-DOTA-PEG₇-Tz, showed rapid (4.6 ± 0.8% ID/g at 4 hours) and persistent (16.8 ± 3.9% ID/g at 120 hours) uptake in tumors while concurrently clearing from blood and nontarget tissues. Single-dose therapy studies using 5B1-TCO and varying amounts of ¹⁷⁷Lu-DOTA-PEG₇-Tz (400, 800, and 1,200 μCi) showed that our system elicits a dose-dependent therapeutic response in mice bearing human xenografts. Furthermore, dosimetry calculations suggest that our approach is amenable to clinical applications with its excellent dosimetric profile in organs of clearance (i.e., liver and kidneys) as well as in dose-limiting tissues, such as red marrow. This study established that a pretargeted methodology utilizing the IEDDA reaction can rapidly and specifically deliver a radiotherapeutic payload to tumor tissue, thus illustrating its excellent potential for clinical translation. Mol Cancer Ther; 16(1); 124-33. ©2016 AACR.

Improved decision making for prioritizing tumor targeting antibodies in human xenografts: Utility of fluorescence imaging to verify tumor target expression, antibody binding and optimization of dosage and application schedule

Preclinical efficacy studies of antibodies targeting a tumor-associated antigen are only justified when the expression of the relevant antigen has been demonstrated. Conventionally, antigen expression level is examined by immunohistochemistry of formalin-fixed paraffin-embedded tumor tissue section. This method represents the diagnostic "gold standard" for tumor target evaluation, but is affected by a number of factors, such as epitope masking and insufficient antigen retrieval. As a consequence, variances and discrepancies in histological staining results can occur, which may influence decision-making and therapeutic outcome. To overcome these problems, we have used different fluorescence-labeled therapeutic antibodies targeting human epidermal growth factor receptor (HER) family members and insulin-like growth factor-1 receptor (IGF1R) in combination with fluorescence imaging modalities to determine tumor antigen expression, drug-target interaction, and biodistribution and tumor saturation kinetics in non-small cell lung cancer xenografts. For this, whole-body fluorescence intensities of labeled antibodies, applied as a single compound or antibody mixture, were measured in Calu-1 and Calu-3 tumor-bearing mice, then ex vivo multispectral tumor tissue analysis at microscopic resolution was performed. With the aid of this simple and fast imaging method, we were able to analyze the tumor cell receptor status of HER1-3 and IGF1R, monitor the antibody-target interaction and evaluate the receptor binding sites of anti-HER2-targeting antibodies. Based on this, the most suitable tumor model, best therapeutic antibody, and optimal treatment dosage and application schedule was selected. Predictions drawn from obtained imaging data were in excellent concordance with outcome of conducted preclinical efficacy studies. Our results clearly demonstrate the great potential of combined in vivo and ex vivo fluorescence imaging for the preclinical development and characterization of monoclonal antibodies.

Cerenkov Luminescence Imaging as a Modality to Evaluate Antibody-Based PET Radiotracers

Antibodies, and engineered antibody fragments, labeled with radioisotopes are being developed as radiotracers for the detection and phenotyping of diseases such as cancer. The development of antibody-based radiotracers requires extensive characterization of their in vitro and in vivo properties, including their ability to target tumors in an antigen-selective manner. In this study, we investigated the use of Cerenkov luminescence imaging (CLI) as compared with PET as a modality for evaluating the in vivo behavior of antibody-based radiotracers.

METHODS

The anti-prostate-specific membrane antigen (PSMA) huJ591 antibody (IgG; 150 kDa) and its minibody (Mb; 80 kDa) format were functionalized with the chelator 1,4,7-triazacyclononane-1-glutaric acid-4,7-diacetic acid (NODAGA) and radiolabeled with the positron-emitting radionuclide ⁶⁴Cu (half-life, 12.7 h). Immunoreactive preparations of the radiolabeled antibodies were injected into NCr nu/nu mice harboring PSMA-positive CWR22Rv1 and PSMA-negative PC-3 tumor xenografts. Tumor targeting was evaluated by both PET and CLI.

RESULTS

⁶⁴Cu-NODAGA-PSMA-IgG and ⁶⁴Cu-NODAGA-PSMA-Mb retained the ability to bind cell surface PSMA, and both radiotracers exhibited selective uptake into PSMA-positive tumors. Under the experimental conditions used, PSMA-selective uptake of ⁶⁴Cu-NODAGA-PSMA-IgG and ⁶⁴Cu-NODAGA-PSMA-Mb was observed by CLI as early as 3 h after injection, with tumor-to-background ratios peaking at 24 (IgG) and 16 (Mb) h after injection. Targeting data generated by CLI correlated with that generated by PET and necropsy.

CONCLUSION

CLI provided a rapid and simple assessment of the targeting specificity and pharmacokinetics of the antibody-based PET radiotracers that correlated well with the behavior observed by standard PET imaging. Moreover, CLI provided clear discrimination between uptake kinetics of an intact IgG and its small-molecular-weight derivative Mb. These data support the use of CLI for the evaluation of radiotracer performance.

First-in-Human Imaging with 89Zr-Df-IAB2M Anti-PSMA Minibody in Patients with Metastatic Prostate Cancer: Pharmacokinetics, Biodistribution, Dosimetry, and Lesion Uptake

We conducted a phase I dose-escalation study with ⁸⁹Zr-desferrioxamine-IAB2M (⁸⁹Zr-IAB2M), an anti-prostate-specific membrane antigen minibody, in patients with metastatic prostate cancer.

METHODS

Patients received 185 MBq (5 mCi) of ⁸⁹Zr-IAB2M and Df-IAB2M at total mass doses of 10 (n = 6), 20 (n = 6), and 50 mg (n = 6). Whole-body and serum clearance, normal-organ and lesion uptake, and radiation absorbed dose were estimated, and the effect of mass escalation was analyzed.

RESULTS

Eighteen patients were injected and scanned without side effects. Whole-body clearance was monoexponential, with a median biologic half-life of 215 h, whereas serum clearance showed biexponential kinetics, with a median biologic half-life of 3.7 (12.3%/L) and 33.8 h (17.9%/L). The radiation absorbed dose estimates were 1.67, 1.36, and 0.32 mGy/MBq to liver, kidney, and marrow, respectively, with an effective dose of 0.41 mSv/MBq (1.5 rem/mCi). Both skeletal and nodal lesions were detected with ⁸⁹Zr-IAB2M, most visualized by 48-h imaging.

CONCLUSION

⁸⁹Zr-IAB2M is safe and demonstrates favorable biodistribution and kinetics for targeting metastatic prostate cancer. Imaging with 10 mg of minibody mass provides optimal biodistribution, and imaging at 48 h after injection provides good lesion visualization. Assessment of lesion targeting is being studied in detail in an expansion cohort.

Immuno-PET Imaging of Engineered Human T Cells in Tumors

Sensitive in vivo imaging technologies applicable to the clinical setting are still lacking for adoptive T-cell-based immunotherapies, an important gap to fill if mechanisms of tumor rejection or escape are to be understood. Here, we propose a highly sensitive imaging technology to track human TCR-transgenic T cells in vivo by directly targeting the murinized constant TCR beta domain (TCRmu) with a zirconium-89 ((89)Zr)-labeled anti-TCRmu-F(ab')2 fragment. Binding of the labeled or unlabeled F(ab')2 fragment did not impair functionality of transgenic T cells in vitro and in vivo Using a murine xenograft model of human myeloid sarcoma, we monitored by Immuno-PET imaging human central memory T cells (TCM), which were transgenic for a myeloid peroxidase (MPO)-specific TCR. Diverse T-cell distribution patterns were detected by PET/CT imaging, depending on the tumor size and rejection phase. Results were confirmed by IHC and semiquantitative evaluation of T-cell infiltration within the tumor corresponding to the PET/CT images. Overall, these findings offer a preclinical proof of concept for an imaging approach that is readily tractable for clinical translation. Cancer Res; 76(14); 4113-23. ©2016 AACR.