Advances in immuno-positron emission tomography: antibodies for molecular imaging in oncology

[89Zr]A2cDb Immuno-PET of Prostate Cancer in a Human Prostate Stem Cell Antigen Knock-in (hPSCA KI) Syngeneic Model

PURPOSE

A great challenge in the diagnosis and treatment of prostate cancer is distinguishing between indolent or local disease and aggressive or metastatic disease. Antibody-based positron emission tomography (immuno-PET) as a cancer-specific imaging modality could improve diagnosis of primary disease, aid the detection of metastases to regional lymph nodes as well as to distant sites (e.g., bone), and monitor response to therapy.

PROCEDURE

In search for a more physiologically relevant disease model, a human prostate stem cell antigen knock-in (hPSCA KI) mouse model was generated. The use of a syngeneic prostate cancer cell line transduced to express human PSCA (RM-9-hPSCA) enabled the evaluation of anti-PSCA immuno-PET in immunocompetent mice and in the context of normal tissue expression of PSCA. Two PSCA-specific humanized antibody fragments, A11 minibody and A2 cys-diabody, were radiolabeled with positron emitters iodine-124 and zirconium-89, respectively ([¹²⁴I]A11 Mb and [⁸⁹Zr]A2cDb), and used for immuno-PET in wild-type, hPSCA KI and tumor-bearing mice.

RESULTS

The hPSCA KI mice express PSCA at low levels in the normal prostate, bladder and stomach, reproducing the expression pattern seen in humans. [¹²⁴I]A11 Mb immuno-PET detected increased levels of PSCA expression in the stomach, and because I-124 is non-residualizing, very little activity was seen in organs of clearance (liver, kidney, spleen). However, due to the longer half-life of the 80 kDa protein, blood activity (and thus urine activity) at 20 h postinjection remains high. The smaller 50 kDa [⁸⁹Zr]A2cDb cleared faster, resulting in lower blood and background activity, despite the use of a residualizing radiometal. Importantly, [⁸⁹Zr]A2cDb immuno-PET showed antigen-specific targeting of PSCA-expressing tumors and minimal nonspecific uptake in PSCA-negative controls.

CONCLUSION

Tracer biodistribution was not significantly impacted by normal tissue expression of PSCA. [⁸⁹Zr]A2cDb immuno-PET yielded high tumor-to-blood ratio at early time points. Rapid renal clearance of the 50 kDa tracer resulted in an unobstructed view of the pelvic region at 20 h postinjection that would allow the detection of cancer in the prostate.

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.

PD-L1 checkpoint blockade delivered by retroviral replicating vector confers anti-tumor efficacy in murine tumor models

Immune checkpoint inhibitors (CPIs) are associated with a number of immune-related adverse events and low response rates. We provide preclinical evidence for use of a retroviral replicating vector (RRV) selective to cancer cells, to deliver CPI agents that may circumvent such issues and increase efficacy. An RRV, RRV-scFv-PDL1, encoding a secreted single chain variable fragment targeting PD-L1 can effectively compete with PD-1 for PD-L1 occupancy. Cell binding assays showed trans-binding activity on 100% of cells in culture when infection was limited to 5% RRV-scFv-PDL1 infected tumor cells. Further, the ability of scFv PD-L1 to rescue PD-1/PD-L1 mediated immune suppression was demonstrated in a co-culture system consisting of human-derived immune cells and further demonstrated in several syngeneic mouse models including an intracranial tumor model. These tumor models showed that tumors infected with RRV-scFv-PD-L1 conferred robust and durable immune-mediated anti-tumor activity comparable or superior to systemically administered anti-PD-1 or anti PD-L1 monoclonal antibodies. Importantly, the nominal level of scFv-PD-L1 detected in serum is ∼50–150 fold less than reported for systemically administered therapeutic antibodies targeting immune checkpoints. These results support the concept that RRV-scFv-PDL1 CPI strategy may provide an improved safety and efficacy profile compared to systemic monoclonal antibodies of currently approved therapies.

Engineering an EGFR-binding Gp2 domain for increased hydrophilicity

The Gp2 domain is a 45 amino-acid scaffold that has been evolved for specific, high-affinity binding towards multiple targets and was proven useful in molecular imaging and biological antagonism. It was hypothesized that Gp2 may benefit from increased hydrophilicity for improved physiological distribution as well as for physicochemical robustness. We identified seven exposed hydrophobic sites for hydrophilic mutations and experimentally evaluated single mutants, which yielded six mutations that do not substantially hinder expression, binding affinity or specificity (to epidermal growth factor receptor), and thermal stability. Eight combinations of these mutations improved hydrophilicity relative to the parental Gp2 clone as assessed by reverse-phase high-performance liquid chromatography (p < 0.05). Secondary structures and refolding abilities of the selected single mutants and all multimutants were unchanged relative to the parental ligand. A variant with five hydrophobic-to-hydrophilic mutations was identified with enhanced solubility as well as reasonable binding affinity ( K d  = 53-63 nM), recombinant yield (1.3 ± 0.8 mg/L), and thermal stability ( T m  = 53 ± 3°C). An alternative variant with a cluster of three leucine-to-hydrophilic mutations was identified with increased solubility, nominally increased binding affinity ( K d  = 13-28 nM) and reasonable thermal stability ( T m  = 54.0 ± 0.6°C) but reduced yield (0.4 ± 0.3 mg/L). In addition, a ≥7°C increase in the midpoint of thermal denaturation was observed in one of the single mutants (T21N). These mutants highlight the physicochemical tradeoffs associated with hydrophobic-to-hydrophilic mutation within a small protein, improve the solubility and hydrophilicity of an existent molecular imaging probe, and provide a more hydrophilic starting point for discovery of new Gp2 ligands towards additional targets.

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.

Integrating Small Animal Irradiators withFunctional Imaging for Advanced Preclinical Radiotherapy Research

Translational research aims to provide direct support for advancing novel treatment approaches in oncology towards improving patient outcomes. Preclinical studies have a central role in this process and the ability to accurately model biological and physical aspects of the clinical scenario in radiation oncology is critical to translational success. The use of small animal irradiators with disease relevant mouse models and advanced in vivo imaging approaches offers unique possibilities to interrogate the radiotherapy response of tumors and normal tissues with high potential to translate to improvements in clinical outcomes. The present review highlights the current technology and applications of small animal irradiators, and explores how these can be combined with molecular and functional imaging in advanced preclinical radiotherapy research.

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.

Dual-Modality ImmunoPET/Fluorescence Imaging of Prostate Cancer with an Anti-PSCA Cys-Minibody

Inadequate diagnostic methods for prostate cancer lead to over- and undertreatment, and the inability to intraoperatively visualize positive margins may limit the success of surgical resection. Prostate cancer visualization could be improved by combining the complementary modalities of immuno-positron emission tomography (immunoPET) for preoperative disease detection, and fluorescence imaging-guided surgery (FIGS) for real-time intraoperative tumor margin identification. Here, we report on the evaluation of dual-labeled humanized anti-prostate stem cell antigen (PSCA) cys-minibody (A11 cMb) for immunoPET/fluorescence imaging in subcutaneous and orthotopic prostate cancer models.

Methods: A11 cMb was site-specifically conjugated with the near-infrared fluorophore Cy5.5 and radiolabeled with ¹²⁴I or ⁸⁹Zr. ¹²⁴I-A11 cMb-Cy5.5 was used for successive immunoPET/fluorescence imaging of prostate cancer xenografts expressing high or moderate levels of PSCA (22Rv1-PSCA and PC3-PSCA). ⁸⁹Zr-A11 cMb-Cy5.5 dual-modality imaging was evaluated in an orthotopic model. Ex vivo biodistribution at 24 h was used to confirm the uptake values, and tumors were visualized by post-mortem fluorescence imaging.

Results: A11 cMb-Cy5.5 retained low nanomolar affinity for PSCA-positive cells. Conjugation conditions were established (dye-to-protein ratio of 0.7:1) that did not affect the biodistribution, pharmacokinetics, or clearance of A11 cMb. ImmunoPET using dual-labeled ¹²⁴I-A11 cMb-Cy5.5 showed specific targeting to both 22Rv1-PSCA and PC3-PSCA s.c. xenografts in nude mice. Ex vivo biodistribution confirmed specific uptake to PSCA-expressing tumors with 22Rv1-PSCA:22Rv1 and PC3-PSCA:PC3 ratios of 13:1 and 5.6:1, respectively. Consistent with the immunoPET, fluorescence imaging showed a strong signal from both 22Rv1-PSCA and PC3-PSCA tumors compared with non-PSCA expressing tumors. In an orthotopic model, ⁸⁹Zr-A11 cMb-Cy5.5 immunoPET was able to detect intraprostatically implanted 22Rv1-PSCA cells. Importantly, fluorescence imaging clearly distinguished the prostate tumor from surrounding seminal vesicles.

Conclusion: Dual-labeled A11 cMb specifically visualized PSCA-positive tumor by successive immunoPET/fluorescence, which can potentially be translated for preoperative whole-body prostate cancer detection and intraoperative surgical guidance in patients.

Immuno-PET of Innate Immune Markers CD11b and IL-1β Detects Inflammation in Murine Colitis

Inflammatory bowel disease (IBD) is a chronic relapsing and remitting inflammatory disease of the gastrointestinal tract. The diagnosis and monitoring of IBD are reliant on endoscopy, which is invasive and does not provide information on specific mediators. Symptom flare in IBD is associated with increased activation of innate immune pathways. Immuno-PET approaches have previously demonstrated the ability to detect colitis; however, a direct comparison of antibodies targeted to innate immune mediators and cells has not been done. We aimed to compare immuno-PET of antibodies to IL-1β and CD11b against standard ¹⁸F-FDG and MRI approaches to detect colonic inflammation. Methods: Colonic concentrations of IL-1β and myeloperoxidase were determined by ELISA, and colonic infiltration by CD11b-positive CD3-negative innate immune cells was determined by flow cytometry and compared between healthy and dextran sodium sulphate-treated colitic mice. PET of ⁸⁹Zr-lα-IL-1β, ⁸⁹Zr-α-CD11b, and ¹⁸F-FDG was compared by volume-of-interest analysis and with MRI by region-of-interest analysis. Imaging results were confirmed by ex vivo biodistribution analysis. Results: Colonic inflammation was associated with impaired colonic epithelial barrier permeability, increased colonic IL-1β and myeloperoxidase concentrations, and increased CD11b-positive CD3-negative innate immune cell infiltration into the colon. ⁸⁹Zr-α-IL-1β and ⁸⁹Zr-α-CD11b immuno-PET detected colonic inflammation, as did ¹⁸F-FDG, and all PET tracers were more sensitive than MRI. Although ¹⁸F-FDG volumes of interest correlated with colitis severity and a strong trend was observed with ⁸⁹Zr-α-IL-1β, no correlation was observed for ⁸⁹Zr-α-CD11b or MRI. ⁸⁹Zr-α-IL-1β was distributed mainly to the gastrointestinal tract, whereas ⁸⁹Zr-α-CD11b was distributed to more tissue types. Conclusion: Immuno-PET using antibodies directed to innate immune markers detected colonic inflammation, with ⁸⁹Zr-α-IL-1β providing a more tissue-specific signal than ⁸⁹Zr-α-CD11b. Development of these technologies for human subjects will potentially provide a less invasive approach than endoscopy for diagnosing and monitoring IBD.

Role of noninvasive molecular imaging in determining response

The intersection of immunotherapy and radiation oncology is a rapidly evolving area of preclinical and clinical investigation. The strategy of combining radiation and immunotherapy to enhance local and systemic antitumor immune responses is intriguing yet largely unproven in the clinical setting because the mechanisms of synergy and the determinants of therapeutic response remain undefined. In recent years, several noninvasive molecular imaging approaches have emerged as a platform to interrogate the tumor immune microenvironment. These tools have the potential to serve as robust biomarkers for cancer immunotherapy and may hold several advantages over conventional anatomic imaging modalities and contemporary invasive tissue acquisition techniques. Given the key and expanding role of precision imaging in radiation oncology for patient selection, target delineation, image guided treatment delivery, and response assessment, noninvasive molecular-specific imaging may be uniquely suited to evaluate radiation/immunotherapy combinations. Herein, we describe several experimental imaging-based strategies that are currently being explored to characterize in vivo immune responses, and we review a growing body of preclinical data and nascent clinical experience with immuno-positron emission tomography molecular imaging as a putative biomarker for cancer immunotherapy. Finally, we discuss practical considerations for clinical translation to implement noninvasive molecular imaging of immune checkpoint molecules, immune cells, or associated elements of the antitumor immune response with a specific emphasis on its potential application at the interface of radiation oncology and immuno-oncology.

Pretargeted Imaging with Gallium-68-Improving the Binding Capability by Increasing the Number of Tetrazine Motifs

The inverse electron-demand Diels-Alder reaction between 1,2,4,5-tetrazine (Tz) and trans-cyclooct-2-ene (TCO) has gained increasing attraction among extensive studies on click chemistry due to its exceptionally fast reaction kinetics and high selectivity for in vivo pretargeting applications including PET imaging. The facile two-step approach utilizing TCO-modified antibodies as targeting structures has not made it into clinics yet. An increase in the blood volume of humans in comparison to mice seems to be the major limitation. This study aims to show if the design of multimeric Tz-ligands by chelator scaffolding can improve the binding capacity and may lead to enhanced PET imaging with gallium-68. We utilized for this purpose the macrocyclic siderophore Fusarinine C (FSC) which allows conjugation of up to three Tz-residues due to three primary amines available for site specific modification. The resulting mono- di- and trimeric conjugates were radiolabelled with gallium-68 and characterized in vitro (logD, protein binding, stability, binding towards TCO modified rituximab (RTX)) and in vivo (biodistribution- and imaging studies in normal BALB/c mice using a simplified RTX-TCO tumour surrogate). The ⁶⁸Ga-labelled FSC-based Tz-ligands showed suitable hydrophilicity, high stability and high targeting specificity. The binding capacity to RTX-TCO was increased according to the grade of multimerization. Corresponding in vivo studies showed a multimerization typical profile but generally suitable pharmacokinetics with low accumulation in non-targeted tissue. Imaging studies in RTX-TCO tumour surrogate bearing BALB/c mice confirmed this trend and revealed improved targeting by multimerization as increased accumulation in RTX-TCO positive tissue was observed.

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.

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.

Targeting prostate cancer: Prostate-specific membrane antigen based diagnosis and therapy

The high incidence rates of prostate cancer (PCa) raise demand for improved therapeutic strategies. Prostate tumors specifically express the prostate-specific membrane antigen (PSMA), a membrane-bound protease. As PSMA is highly overexpressed on malignant prostate tumor cells and as its expression rate correlates with the aggressiveness of the disease, this tumor-associated biomarker provides the possibility to develop new strategies for diagnostics and therapy of PCa. Major advances have been made in PSMA targeting, ranging from immunotherapeutic approaches to therapeutic small molecules. This review elaborates the diversity of PSMA targeting agents while focusing on the radioactively labeled tracers for diagnosis and endoradiotherapy. A variety of radionuclides have been shown to either enable precise diagnosis or efficiently treat the tumor with minimal effects to nontargeted organs. Most small molecules with affinity for PSMA are based on either a phosphonate or a urea-based binding motif. Based on these pharmacophores, major effort has been made to identify modifications to achieve ideal pharmacokinetics while retaining the specific targeting of the PSMA binding pocket. Several tracers have now shown excellent clinical usability in particular for molecular imaging and therapy as proven by the efficiency of theranostic approaches in current studies. The archetypal expression profile of PSMA may be exploited for the treatment with alpha emitters to break radioresistance and thus to bring the power of systemic therapy to higher levels.

Cancer Drug Development of Carbonic Anhydrase Inhibitors beyond the Active Site

Carbonic anhydrases (CAs) catalyze the reversible hydration of carbon dioxide to produce bicarbonate and a proton. Multiple CA isoforms are implicated in a range of diseases, including cancer. In solid tumors, continuously dividing cells create hypoxic conditions that eventually lead to an acidic microenvironment. Hypoxic tumor cells have different mechanisms in place to regulate and adjust the surrounding microenvironment for survival. These mechanisms include expression of CA isoform IX (CA IX) and XII (CA XII). These enzymes help maintain a physiological intracellular pH while simultaneously contributing to an acidic extracellular pH, leading to tumor cell survival. Expression of CA IX and CA XII has also been shown to promote tumor cell invasion and metastasis. This review discusses the characteristics of CA IX and CA XII, their mechanism of action, and validates their prospective use as anticancer targets. We discuss the current status of small inhibitors that target these isoforms, both classical and non-classical, and their future design in order to obtain isoform-specificity for CA IX and CA XII. Biologics, such as monoclonal antibodies, monoclonal-radionuclide conjugated chimeric antibodies, and antibody-small molecule conjugates are also discussed.

Dual-Modality Immuno-PET and Near-Infrared Fluorescence Imaging of Pancreatic Cancer Using an Anti-Prostate Stem Cell Antigen Cys-Diabody

Pancreatic cancer has a high mortality rate due to late diagnosis and the tendency to invade surrounding tissues and metastasize at an early stage. A molecular imaging agent that enables both presurgery antigen-specific PET (immuno-PET) and intraoperative near-infrared fluorescence (NIRF) guidance might benefit diagnosis of pancreatic cancer, staging, and surgical resection, which remains the only curative treatment. Methods: We developed a dual-labeled probe based on A2 cys-diabody (A2cDb) targeting the cell-surface prostate stem cell antigen (PSCA), which is expressed in most pancreatic cancers. Maleimide-IRDye800CW was site-specifically conjugated to the C-terminal cys-tag (A2cDb-800) without impairing integrity or affinity (half-maximal binding, 4.3 nM). Direct radioiodination with ¹²⁴I (¹²⁴I-A2cDb-800) yielded a specific activity of 159 ± 48 MBq/mg with a radiochemical purity exceeding 99% and 65% ± 4.5% immunoreactivity (n = 3). In vivo specificity for PSCA-expressing tumor cells and biodistribution of the dual-modality tracer were evaluated in a prostate cancer xenograft model and compared with single-labeled ¹²⁴I-A2cDb. Patient-derived pancreatic ductal adenocarcinoma xenografts (PDX-PDACs) were grown subcutaneously in NSG mice and screened for PSCA expression by immuno-PET. Small-animal PET/CT scans of PDX-PDAC-bearing mice were obtained using the dual-modality ¹²⁴I-A2cDb-800 followed by postmortem NIRF imaging with the skin removed. Tumors and organs were analyzed ex vivo to compare the relative fluorescent signals without obstruction by other organs. Results: Specific uptake in PSCA-positive tumors and low nonspecific background activity resulted in high-contrast immuno-PET images. Concurrent with the PET studies, fluorescent signal was observed in the PSCA-positive tumors of mice injected with the dual-tracer ¹²⁴I-A2cDb-800, with low background uptake or autofluorescence in the surrounding tissue. Ex vivo biodistribution confirmed comparable tumor uptake of both ¹²⁴I-A2cDb-800 and ¹²⁴I-A2cDb. Conclusion: Dual-modality imaging using the anti-PSCA cys-diabody resulted in high-contrast immuno-PET/NIRF images of PDX-PDACs, suggesting that this imaging agent might offer both noninvasive whole-body imaging to localize PSCA-positive pancreatic cancer and fluorescence image-guided identification of tumor margins during surgery.

RAGE-specific single chain Fv for PET imaging of pancreatic cancer

Noninvasive detection of both early pancreatic neoplasia and metastases could enhance strategies to improve patient survival in this disease that is notorious for an extremely poor prognosis. There are almost no identifiable targets for non-invasive diagnosis by positron emission tomography (PET) for patients with pancreatic ductal adenocarcinoma (PDAC). Over-expression of the receptor for advanced glycation end products (RAGE) is found on the cell surface of both pre-neoplastic lesions and invasive PDAC. Here, a RAGE-specific single chain (scFv) was developed, specific for PET imaging in syngeneic mouse models of PDAC. An anti-RAGE scFv conjugated with a sulfo-Cy5 fluorescence molecule showed high affinity and selectivity for RAGE expressing pancreatic tumor cells and genetically engineered KRAS^G12D mouse models of PDAC. An in vivo biodistribution study was performed with the ⁶⁴Cu-radiolabled scFv in a syngeneic murine pancreatic cancer model, demonstrating both the feasibility and potential of an anti-RAGE scFv for detection of PDAC. These studies hold great promise for translation into the clinic.

Molecular Imaging of Mesothelin-Expressing Ovarian Cancer with a Human and Mouse Cross-Reactive Nanobody

Mesothelin is an epithelial marker highly expressed at the cell surface of cancer cells from diverse origins, including ovarian and pancreatic adenocarcinomas and mesotheliomas. Previously, we identified and characterized an antimesothelin nanobody (NbG3a) for in vitro diagnostic applications. The main goal of this research was to establish the potential of NbG3a as a molecular imaging agent. Site-specific biotinylated NbG3a (bNbG3a) was bound to streptavidin-conjugated reagents for in vitro and in vivo assays. Initially, we performed microscale thermophoresis to determine the binding affinity between bNbG3a and human ( K_d = 46 ± 8 nM) or mouse ( K_d = 4.8 ± 0.4 nM) mesothelin protein. The human and mouse cross-reactivity was confirmed by in vivo optical imaging using bNbG3a bound to fluorescent streptavidin. We also localized the binding site of nNbG3a on human mesothelin using overlapping peptide scan. NbG3a recognized an epitope within residues 21-65 of the mature membrane bound form of human mesothelin, which is part of the N-terminal region of mesothelin that is important for interactions between mesothelin on peritoneal cells and CA125 on tumor cells. Next, the bNbG3a in vivo half-life after intravenous injection in healthy mice was estimated by ELISA assay to be 5.3 ± 1.3 min. In tumor-bearing animals, fluorescent bNbG3a accumulated in a subcutaneous ovarian xenograft (A1847) and in two syngeneic, orthotopic ovarian tumors (intraovary and intraperitoneal ID8) within an hour of intravenous injection that peaked by 4 h and persisted up to 48 h. MRI analysis of bNbG3a-targeted streptavidin-labeled iron oxides showed that the MRI signal intensity decreased 1 h after injection for a subcutaneous xenograft model of ovarian cancer for bNbG3a-labeled iron oxides compared to unlabeled iron oxides. The signal intensity differences continued up to the final time point at 24 h post injection. Finally, in vivo immunofluorescence 24 or 48 h after bNbG3a intravenous injection showed bNbG3a diffuse distribution of both xenograft and syngeneic ovarian tumors, with local areas of high concentration throughout A1847 human tumor. The data support the use of NbG3a for continued preclinical development and translation to human applications for cancers that overexpress mesothelin.

Evaluation of a Pretargeting Strategy for Molecular Imaging of the Prostate Stem Cell Antigen with a Single Chain Antibody

In pretargeted radio-immunotherapy, the gradual administration of a non-radioactive tumor antigen-addressing antibody-construct and the subsequent application of a radioactive labeled, low molecular weight substance enable a highly effective and selective targeting of tumor tissue. We evaluated this concept in prostate stem cell antigen (PSCA)-positive cancers using the antigen-specific, biotinylated single chain antibody scFv(AM1)-P-BAP conjugated with tetrameric neutravidin. To visualize the systemic biodistribution, a radiolabeled biotin was injected to interact with scFv(AM1)-P-BAP/neutravidin conjugate. Biotin derivatives conjugated with different chelators for complexation of radioactive metal ions and a polyethylene glycol linker (n = 45) were successfully synthesized and evaluated in vitro and in a mouse xenograft model. In vivo, the scFv(AM1)-P-BAP showed highly PSCA-specific tumor retention with a PSCA⁺ tumor/PSCA^- tumor accumulation ratio of ten. PEGylation of radiolabeled biotin resulted in lower liver uptake improving the tumor to background ratio.

Incorporating Radiation Oncology into Immunotherapy: proceedings from the ASTRO-SITC-NCI immunotherapy workshop

Radiotherapy (RT) has been a fundamental component of the anti-cancer armamentarium for over a century. Approximately half of all cancer patients are treated with radiotherapy during their disease course. Over the two past decades, there has been a growing body of preclinical evidence supporting the immunomodulatory effects of radiotherapy, particularly when combined with immunotherapy, but only anecdotal clinical examples existed until recently. The renaissance of immunotherapy and the recent U.S. Food and Drug Administration (FDA) approval of several immune checkpoint inhibitors (ICIs) and other immuno-oncology (IO) agents in multiple cancers provides the opportunity to investigate how localized radiotherapy can induce systemic immune responses. Early clinical experiences have demonstrated feasibility of this approach but additional preclinical and clinical investigation is needed to understand how RT and immunotherapy can be optimally combined.

To address questions that are critical to successful incorporation of radiation oncology into immunotherapy, the American Society for Radiation Oncology (ASTRO), the Society for Immunotherapy of Cancer (SITC) and the National Cancer Institute (NCI) organized a collaborative scientific workshop, Incorporating Radiation Oncology into Immunotherapy, that convened on June 15 and 16 of 2017 at the Natcher Building, NIH Campus in Bethesda, Maryland. This report summarizes key data and highlights from each session.

Pretargeted PET Imaging Using a Bioorthogonal 18F-Labeled trans-Cyclooctene in an Ovarian Carcinoma Model

In cancer research, pretargeted positron emission tomography (PET) imaging has emerged as an effective two-step approach that combines the excellent target affinity and selectivity of antibodies with the advantages of using short-lived radionuclides such as fluorine-18. One possible approach is based on the bioorthogonal inverse-electron-demand Diels-Alder (IEDDA) reaction between tetrazines and trans-cyclooctene (TCO) derivatives. Here, we report the first successful use of an ¹⁸F-labeled small TCO compound, [¹⁸F]1 recently developed in our laboratory, to perform pretargeted immuno-PET imaging. The study was performed in an ovarian carcinoma mouse model, using a trastuzumab-tetrazine conjugate.

Image quality of Zr-89 PET imaging in the Siemens microPET Focus 220 preclinical scanner

PURPOSE

Zr-89 positron emission tomography (PET) is a valuable tool for understanding the biodistribution and pharmacokinetics of antibody-based therapeutics. We compared the image quality of Zr-89 PET and F-18 PET in the Siemens microPET Focus 220 preclinical scanner using different reconstruction methods.

PROCEDURES

Image quality metrics were measured in various Zr-89 and F-18 PET phantoms, including the NEMA NU 4-2008 image quality phantom. Images were reconstructed using various algorithms.

RESULTS

Zr-89 PET had greater image noise, inferior spatial resolution, and greater spillover than F-18 PET, but comparable recovery coefficients for cylinders of various diameters. Of the reconstruction methods, OSEM3D resulted in the lowest noise, highest recovery coefficients, best spatial resolution, but also the greatest spillover. Scatter correction results were found to be sensitive to varying object sizes.

CONCLUSIONS

Zr-89 PET image quality was inferior to that of F-18, and no single reconstruction method was superior in all aspects of image quality.

ImmunoPET of Malignant and Normal B Cells with 89Zr- and 124I-Labeled Obinutuzumab Antibody Fragments Reveals Differential CD20 Internalization In Vivo

Purpose: The B-cell antigen CD20 provides a target for antibody-based positron emission tomography (immunoPET). We engineered antibody fragments targeting human CD20 and studied their potential as immunoPET tracers in transgenic mice (huCD20TM) and in a murine lymphoma model expressing human CD20.Experimental Design: Anti-CD20 cys-diabody (cDb) and cys-minibody (cMb) based on rituximab and obinutuzumab (GA101) were radioiodinated and used for immunoPET imaging of a murine lymphoma model. Pairwise comparison of obinutuzumab-based antibody fragments labeled with residualizing (⁸⁹Zr) versus non-residualizing (¹²⁴I) radionuclides by region of interest analysis of serial PET images was conducted both in the murine lymphoma model and in huCD20TM to assess antigen modulation in vivoResults:¹²⁴I-GAcDb and ¹²⁴I-GAcMb produced high-contrast immunoPET images of B-cell lymphoma and outperformed the respective rituximab-based tracers. ImmunoPET imaging of huCD20TM showed specific uptake in lymphoid tissues. The use of the radiometal ⁸⁹Zr as alternative label for GAcDb and GAcMb yielded greater target-specific uptake and retention compared with ¹²⁴I-labeled tracers. Pairwise comparison of ⁸⁹Zr- and ¹²⁴I-labeled GAcDb and GAcMb allowed assessment of in vivo internalization of CD20/antibody complexes and revealed that CD20 internalization differs between malignant and endogenous B cells.Conclusions: These obinutuzumab-based PET tracers have the ability to noninvasively and quantitatively monitor CD20-expression and have revealed insights into CD20 internalization upon antibody binding in vivo Because they are based on a humanized mAb they have the potential for direct clinical translation and could improve patient selection for targeted therapy, dosimetry prior to radioimmunotherapy, and prediction of response to therapy. Clin Cancer Res; 23(23); 7242-52. ©2017 AACR.

Immuno-PET imaging based radioimmunotherapy in head and neck squamous cell carcinoma model

The epidermal growth factor receptor (EGFR) is one of the most comprehensively studied molecular targets in head and neck squamous cell carcinoma (HNSCC). However, inherent and acquired resistance are serious problems and are responsible for limited clinical efficacy and tumor recurrence. In this study, we evaluated the feasibility of immuno-positron emission tomography (PET) imaging and radioimmunotherapy (RIT) with ⁶⁴Cu-/¹⁷⁷Lu-PCTA-cetuximab in cetuximab-resistant SNU-1066 HNSCC xenografted model. The cellular uptake of ⁶⁴Cu/¹⁷⁷Lu-3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-1(15),11,13-triene-3,6,9,-triacetic acid (PCTA)-cetuximab showed good correlation with western blot and flow cytometry analysis in EGFR expression level of various HNSCC cells. ¹⁷⁷Lu-PCTA-cetuximab selectively killed cetuximab-resistant SNU-1066 cells in vitro. ⁶⁴Cu-/¹⁷⁷Lu-PCTA-cetuximab specifically accumulated in SNU-1066 tumor and those uptakes were peaked at 48 h and 7 day, respectively in biodistribution, PET and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging. RIT with single dose of ¹⁷⁷Lu-PCTA-cetuximab exhibited significant tumor regression and markedly reduced 2-[¹⁸F]fluoro-2-deoxy-D-glucose (¹⁸F-FDG) uptake, compared to other groups. Proliferation index were dramatically decreased and apoptotic index increased in RIT group. These results suggest that a diagnostic and therapeutic convergence radiopharmaceutical, ⁶⁴Cu-/¹⁷⁷Lu-PCTA-cetuximab has the potential of target selection using immuno-PET imaging and targeted therapy by RIT in EGFR expressing cetuximab-resistant HNSCC tumors.

Clinical characteristics of patient selection and imaging predictors of outcome in solid tumors treated with checkpoint-inhibitors

The rapidly evolving knowledge on tumor immunology and the continuous implementation of immunotherapy in cancer have recently led to the FDA and EMA approval of several checkpoint inhibitors as immunotherapic agents in clinical practice. Anti-CTLA-4, anti-PD-1, and anti-PDL-1 antibodies are becoming standard of care in advanced melanoma, as well as in relapsed or metastatic lung and kidney cancer, demonstrating higher and longer response compared to standard chemotherapy. These encouraging results have fostered the evaluation of these antibodies either alone or in combination with other therapies in several dozen clinical trials for the treatment of multiple tumor types. However, not all patients respond to immune checkpoint inhibitors, hence, specific biomarkers are necessary to guide and monitor therapy. The utility of PD-L1 expression as a biomarker has varied in different clinical trials, but, to date, no consensus has been reached on whether PD-L1 expression is an ideal marker for response and patient selection; approximately 20-25% of patients will respond to immunotherapy with checkpoint inhibitors despite a negative PD-L1 staining. On the other hand, major issues concern the evaluation of objective response in patients treated with immunotherapy. Pure morphological criteria as commonly used in solid tumors (i.e. RECIST) are not sufficient because change in size is not an early and reliable marker of tumor response to biological therapies. Thus, the scientific community has required a continuous adaptation of immune-related response criteria (irRC) to overcome the problem. In this context, metabolic information and antibody-based imaging with positron emission tomography (PET) have been investigated, providing a powerful approach for an optimal stratification of patients at staging and during the evaluation of the response to therapy. In the present review we provide an overview on the clinical characteristics of patient selection when using imaging predictors of outcome in solid tumors treated with checkpoint-inhibitors.

Noninvasive Interrogation of DLL3 Expression in Metastatic Small Cell Lung Cancer

The Notch ligand DLL3 has emerged as a novel therapeutic target expressed in small cell lung cancer (SCLC) and high-grade neuroendocrine carcinomas. Rovalpituzumab teserine (Rova-T; SC16LD6.5) is a first-in-class DLL3-targeted antibody-drug conjugate with encouraging initial safety and efficacy profiles in SCLC in the clinic. Here we demonstrate that tumor expression of DLL3, although orders of magnitude lower in surface protein expression than typical oncology targets of immunoPET, can serve as an imaging biomarker for SCLC. We developed ⁸⁹Zr-labeled SC16 antibody as a companion diagnostic agent to facilitate selection of patients for treatment with Rova-T based on a noninvasive interrogation of the in vivo status of DLL3 expression using PET imaging. Despite low cell-surface abundance of DLL3, immunoPET imaging with ⁸⁹Zr-labeled SC16 antibody enabled delineation of subcutaneous and orthotopic SCLC tumor xenografts as well as distant organ metastases with high sensitivity. Uptake of the radiotracer in tumors was concordant with levels of DLL3 expression and, most notably, DLL3 immunoPET yielded rank-order correlation for response to SC16LD6.5 therapy in SCLC patient-derived xenograft models. Cancer Res; 77(14); 3931-41. ©2017 AACR.

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).

Harnessing Preclinical Molecular Imaging to Inform Advances in Personalized Cancer Medicine

Comprehensive molecular analysis of individual tumors provides great potential for personalized cancer therapy. However, the presence of a particular genetic alteration is often insufficient to predict therapeutic efficacy. Drugs with distinct mechanisms of action can affect the biology of tumors in specific and unique ways. Therefore, assays that can measure drug-induced perturbations of defined functional tumor properties can be highly complementary to genomic analysis. PET provides the capacity to noninvasively measure the dynamics of various tumor biologic processes in vivo. Here, we review the underlying biochemical and biologic basis for a variety of PET tracers and how they may be used to better optimize cancer therapy.

Molecular imaging of the tumor microenvironment

The tumor microenvironment plays a critical role in tumor initiation, progression, metastasis, and resistance to therapy. It is different from normal tissue in the extracellular matrix, vascular and lymphatic networks, as well as physiologic conditions. Molecular imaging of the tumor microenvironment provides a better understanding of its function in cancer biology, and thus allowing for the design of new diagnostics and therapeutics for early cancer diagnosis and treatment. The clinical translation of cancer molecular imaging is often hampered by the high cost of commercialization of targeted imaging agents as well as the limited clinical applications and small market size of some of the agents. Because many different cancer types share similar tumor microenvironment features, the ability to target these biomarkers has the potential to provide clinically translatable molecular imaging technologies for a spectrum of cancers and broad clinical applications. There has been significant progress in targeting the tumor microenvironment for cancer molecular imaging. In this review, we summarize the principles and strategies of recent advances made in molecular imaging of the tumor microenvironment, using various imaging modalities for early detection and diagnosis of cancer.

Antibody Engineering for Pursuing a Healthier Future

Since the development of antibody-production techniques, a number of immunoglobulins have been developed on a large scale using conventional methods. Hybridoma technology opened a new horizon in the production of antibodies against target antigens of infectious pathogens, malignant diseases including autoimmune disorders, and numerous potent toxins. However, these clinical humanized or chimeric murine antibodies have several limitations and complexities. Therefore, to overcome these difficulties, recent advances in genetic engineering techniques and phage display technique have allowed the production of highly specific recombinant antibodies. These engineered antibodies have been constructed in the hunt for novel therapeutic drugs equipped with enhanced immunoprotective abilities, such as engaging immune effector functions, effective development of fusion proteins, efficient tumor and tissue penetration, and high-affinity antibodies directed against conserved targets. Advanced antibody engineering techniques have extensive applications in the fields of immunology, biotechnology, diagnostics, and therapeutic medicines. However, there is limited knowledge regarding dynamic antibody development approaches. Therefore, this review extends beyond our understanding of conventional polyclonal and monoclonal antibodies. Furthermore, recent advances in antibody engineering techniques together with antibody fragments, display technologies, immunomodulation, and broad applications of antibodies are discussed to enhance innovative antibody production in pursuit of a healthier future for humans.

Noninvasive Imaging of Human Immune Responses in a Human Xenograft Model of Graft-Versus-Host Disease

The immune system plays a crucial role in many diseases. Activation or suppression of immunity is often related to clinical outcome. Methods to explore the dynamics of immune responses are important to elucidate their role in conditions characterized by inflammation, such as infectious disease, cancer, or autoimmunity. Immuno-PET is a noninvasive method by which disease and immune cell infiltration can be explored simultaneously. Using radiolabeled antibodies or fragments derived from them, it is possible to image disease-specific antigens and immune cell subsets. Methods: We developed a method to noninvasively image human immune responses in a relevant humanized mouse model. We generated a camelid-derived single-domain antibody specific for human class II major histocompatibility complex products and used it to noninvasively image human immune cell reconstitution in nonobese diabetic severe combined immune deficiency γ-/- mice reconstituted with human fetal thymus, liver, and liver-derived hematopoietic stem cells (BLT mice). Results: We showed imaging of infiltrating immunocytes in BLT mice that spontaneously developed a graft-versus-host-like condition, characterized by alopecia and blepharitis. In diseased animals, we showed an increased PET signal in the liver, attributable to infiltration of activated class II major histocompatibility complex⁺ T cells. Conclusion: Noninvasive imaging of immune infiltration and activation could thus be of importance for diagnosis and evaluation of treatment of graft-versus-host disease and holds promise for other diseases characterized by inflammation.

Emerging biomarkers for PD-1 pathway cancer therapy

The field of immuno-oncology has witnessed unprecedented success in recent years, with several PD=1 and PD-L1 inhibitors obtaining US FDA registration and breakthrough drug therapy designation in multiple tumor types. Despite its clear efficacy in certain cancers, treatment with these agents carries a risk of immune-related toxicities and substantial financial burden. It is, therefore, critical to identify patients likely to benefit from such immunotherapies and develop strategies to differentiate responders from nonresponders early during treatment. Here we discuss the development of predictive and treatment response biomarkers for immune checkpoint inhibitors. We first examine the role of PD-L1 expression, the most extensively studied predictive biomarker of response, and further discuss emerging putative predictive biomarkers. We also detail challenges faced in the development of response assessments for immunotherapeutics and propose other biomarkers that may be useful as surrogate intermediate end points of response.

Non-internalizing antibody-drug conjugates display potent anti-cancer activity upon proteolytic release of monomethyl auristatin E in the subendothelial extracellular matrix

Antibody-drug conjugates (ADCs) represent a promising class of biopharmaceuticals with the potential to localize at the tumor site and improve the therapeutic index of cytotoxic drugs. While it is generally believed that ADCs need to be internalized into tumor cells in order to display optimal therapeutic activity, it has recently been shown that non-internalizing antibodies can efficiently liberate disulfide-linked drugs at the extracellular tumor site, leading to potent anti-cancer activity in preclinical animal models. Here, we show that engineered variants of the F16 antibody, specific to a splice isoform of tenascin-C, selectively localize to the subendothelial tumor extracellular matrix in three mouse models of human cancer (U87, A431, MDA-MB-231). A site-specific coupling of F16 in IgG format with a monomethyl auristatin E (MMAE) derivative, featuring a valine-citrulline dipeptide linker equipped with a self-immolative spacer, yielded an ADC product, which cured tumor-bearing mice at a dose of 7 mg/Kg. The observation of an efficient extracellular proteolytic cleavage of the valine-citrulline linker was surprising, as it has generally been assumed that this peptidic structure would be selectively cleaved by cathepsin B in intracellular compartments. The products described in this article may be useful for the treatment of human malignancies, as their cognate antigen is strongly expressed in the majority of human solid tumors, lymphomas and aggressive leukemias, while being virtually undetectable in most normal adult tissues.

SPECT and fluorescence imaging of vulnerable atherosclerotic plaque with a vascular cell adhesion molecule 1 single-chain antibody fragment

BACKGROUND AND AIMS

Early detection and evaluation of vulnerable atherosclerotic plaque are important for risk stratification and timely intervention, and vascular cell adhesion molecule 1 (VCAM1) assists in adhesion and recruitment of inflammatory cells to vulnerable lesions. We labeled a single-chain variable fragment (scFv) of VCAM1 with ^99mtechnetium (^99mTc) and fluorescent markers to investigate its potential utility in detecting vulnerable plaques in animal models of atherosclerosis.

METHODS

We labeled VCAM1 scFv with ^99mTc and cyanine5 (CY5) and evaluated the probes on apolipoprotein E gene-deficient mice and New Zealand White rabbits with induced atherosclerosis. Histopathology and Western blot examinations confirmed atherosclerotic plaque and VCAM1 expression in the aortas. In vivo biodistribution of ^99mTc-scFv-VCAM1 was studied. Abdominal organs of mice were removed after CY5-scFv-VCAM1 administration for aortic fluorescence imaging. Rabbits SPECT imaging of ^99mTc-scFv-VCAM1 was performed and autoradiography (ARG) of the aortas was checked to confirm the tracer uptake.

RESULTS

The radiochemical purity of ^99mTc-scFv-VCAM1 was 98.72± 1.04% (n = 5) and its specific activity was 7.8 MBq/μg. Biodistribution study indicated predominant probe clearance by kidneys. In fluorescence imaging, stronger signal from CY5-scFv-VCAM1 in the aorta was observed in atherosclerotic mice than that in controls. SPECT imaging with ^99mTc-scFv-VCAM1 showed tracer uptake in the abdominal aorta and the aortic arch of atherosclerotic animals. ARG confirmed tracer uptake in the aortas of atherosclerotic rabbits, with higher uptake ratios of aortic arch/descending aorta in experimental animals (4.45 ± 0.63, n = 5) than controls (1.12 ± 0.15, n = 5; p < 0.05).

CONCLUSIONS

SPECT and fluorescence imaging results showed the feasibility and effectiveness of detecting vulnerable plaque with scFv of VCAM1, indicating its potential for early diagnosis and evaluation of atherosclerosis.

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.

Biodistribution and Targeting of Anti-5T4 Antibody-Drug Conjugate Using Fluorescence Molecular Tomography

Understanding a drug's whole-body biodistribution and tumor targeting can provide important information regarding efficacy, safety, and dosing parameters. Current methods to evaluate biodistribution include in vivo imaging technologies like positron electron tomography and single-photon emission computed tomography or ex vivo quantitation of drug concentrations in tissues using autoradiography and standard biochemical assays. These methods use radioactive compounds or are cumbersome and do not give whole-body information. Here, for the first time, we show the utility of fluorescence molecular tomography (FMT) imaging to determine the biodistribution and targeting of an antibody-drug conjugate (ADC). An anti-5T4-antibody (5T4-Ab) and 5T4-ADC were conjugated with a near-infrared (NIR) fluorophore VivoTag 680XL (VT680). Both conjugated compounds were stable as determined by SEC-HPLC and plasma stability studies. Flow cytometry and fluorescence microscopy studies showed that VT680-conjugated 5T4-ADC specifically bound 5T4-expressing cells in vitro and also exhibited a similar cytotoxicity profile as the unconjugated 5T4-ADC. In vivo biodistribution and tumor targeting in an H1975 subcutaneous xenograft model demonstrated no significant differences between accumulation of VT680-conjugated 5T4-Ab or 5T4-ADC in either normal tissues or tumor. In addition, quantitation of heart signal from FMT imaging showed good correlation with the plasma pharmacokinetic profile suggesting that it (heart FMT imaging) may be a surrogate for plasma drug clearance. These results demonstrate that conjugation of VT680 to 5T4-Ab or 5T4-ADC does not change the behavior of native biologic, and FMT imaging can be a useful tool to understand biodistribution and tumor-targeting kinetics of antibodies, ADCs, and other biologics. Mol Cancer Ther; 15(10); 2530-40. ©2016 AACR.

L-Tyrosine Confers Residualizing Properties to a d-Amino Acid-Rich Residualizing Peptide for Radioiodination of Internalizing Antibodies

PURPOSE

The aims of the study were to develop and evaluate a novel residualizing peptide for labeling internalizing antibodies with (124)I to support clinical development using immuno-positron emission tomography (PET).

METHODS

The anti-epidermal growth factor receptor antibody ch806 was radiolabeled directly or indirectly with isotopes and various residualizing peptides. Azido-derivatized radiolabeled peptides were conjugated to dibenzylcyclooctyne-derivatized ch806 antibody via click chemistry. The radiochemical purities, antigen-expressing U87MG.de2-7 human glioblastoma cell-binding properties, and targeting of xenografts at 72 hours post injection of all radioconjugates were compared. Biodistribution of (124)I-PEG4-tptddYddtpt-ch806 and immuno-PET imaging were evaluated in tumor-bearing mice.

RESULTS

Biodistribution studies using xenografts at 72 hours post injection showed that (131)I-PEG4-tptddYddtpt-ch806 tumor uptake was similar to (111)In-CHX-A″-DTPA-ch806. (125)I-PEG4-tptddyddtpt-ch806 showed a lower tumor uptake value but higher than directly labeled (125)I-ch806. (124)I-PEG4-tptddYddtpt-ch806 was produced at 23% labeling efficiency, 98% radiochemical purity, 25.9 MBq/mg specific activity, and 64% cell binding in the presence of antigen excess. Tumor uptake for (124)I-PEG4-tptddYddtpt-ch806 was similar to (111)In-CHX-A″-DTPA-ch806. High-resolution immuno-PET/magnetic resonance imaging of tumors showed good correlation with biodistribution data.

CONCLUSIONS

The mixed d/l-enantiomeric peptide, dThr-dPro-dThr-dAsp-dAsp-Tyr-dAsp-dAsp-dThr-dPro-dThr, is suitable for radiolabeling antibodies with radiohalogens such as (124)I for high-resolution immuno-PET imaging of tumors and for evaluation in early-phase clinical trials.

Synthesis and pre-clinical evaluation of an (18)F-labeled single-chain antibody fragment for PET imaging of epithelial ovarian cancer

Anti-CA125 antibodies have been used in immunoassays to quantify levels of shed antigen in the serum of patients who are under surveillance for epithelial ovarian cancer (EOC). However, there is currently no molecular imaging probe in the clinic for the assessment of CA125 expression in vivo. The present study describes the development of an (18)F-labeled single-chain variable fragment (scFv) for PET imaging of CA125 in preclinical EOC models. Anti-CA125 scFv was derived from MAb-B43.13 by recombinant expression of the fragment in E.coli. Fragment scFv-B43.13 was purified via immobilized metal affinity chromatography and characterized for antigen binding via immuno-staining and flow cytometry. Prosthetic group N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB) was used for radiolabeling of scFv-B43.13. Preclinical ovarian cancer models were developed based on ovarian cancer cell lines OVCAR3 (CA125-positive) and SKOV3 (CA125-negative) in NIH-III mice. The radiopharmacological profile of (18)F-labeled scFv-B43.13 ([(18)F]FBz-scFv-B43.13) was studied with PET. [(18)F]FBz-scFv-B43.13 was prepared in radiochemical yields of 3.7 ± 1.8% (n = 5) at an effective specific activity of 3.88 ± 0.76 GBq/µmol (n = 5). The radiotracer demonstrated selective uptake in CA125-positive OVCAR3 cells and virtually no uptake in CA125-negative SKOV3 cells. Standardized uptake values (SUV) of radioactivity uptake in OVCAR3 tumors was 0.5 (n = 3) and 0.3 (n = 2) in SKOV3 tumors after 60 min post injection (p.i.).

ImmunoPET for assessing the differential uptake of a CD146-specific monoclonal antibody in lung cancer

PURPOSE

Overexpression of CD146 in solid tumors has been linked to disease progression, invasion, and metastasis. We describe the generation of a ⁶⁴Cu-labeled CD146-specific antibody and its use for quantitative immunoPET imaging of CD146 expression in six lung cancer models.

METHODS

The anti-CD146 antibody (YY146) was conjugated to 1,4,7-triazacyclononane-triacetic acid (NOTA) and radiolabeled with ⁶⁴Cu. CD146 expression was evaluated in six human lung cancer cell lines (A549, NCI-H358, NCI-H522, HCC4006, H23, and NCI-H460) by flow cytometry and quantitative western blot studies. The biodistribution and tumor uptake of ⁶⁴Cu-NOTA-YY146 was assessed by sequential PET imaging in athymic nude mice bearing subcutaneous lung cancer xenografts. The correlation between CD146 expression and tumor uptake of ⁶⁴Cu-NOTA-YY146 was evaluated by graphical software while ex vivo biodistribution and immunohistochemistry studies were performed to validate the accuracy of PET data and spatial expression of CD146.

RESULTS

Flow cytometry and western blot studies showed similar findings with H460 and H23 cells showing high levels of expression of CD146. Small differences in CD146 expression levels were found among A549, H4006, H522, and H358 cells. Tumor uptake of ⁶⁴Cu-NOTA-YY146 was highest in CD146-expressing H460 and H23 tumors, peaking at 20.1 ± 2.86 and 11.6 ± 2.34 %ID/g at 48 h after injection (n = 4). Tumor uptake was lowest in the H522 model (4.1 ± 0.98 %ID/g at 48 h after injection; n = 4), while H4006, A549 and H358 exhibited similar uptake of ⁶⁴Cu-NOTA-YY146. A positive correlation was found between tumor uptake of ⁶⁴Cu-NOTA-YY146 (%ID/g) and relative CD146 expression (r ² = 0.98, p < 0.01). Ex vivo biodistribution confirmed the accuracy of the PET data.

CONCLUSION

The strong correlation between tumor uptake of ⁶⁴Cu-NOTA-YY146 and CD146 expression demonstrates the potential use of this radiotracer for imaging tumors that elicit varying levels of CD146. In the future, this tool may promote enhanced monitoring of therapeutic response and improved patient stratification.

Direct flow separation strategy, to isolate no-carrier-added 90Nb from irradiated Mo or Zr targets

90Nb has an intermediate half-life of 14.6 h, a high positron branching of 53% and optimal β + emission energy of only E mean 0.35 MeV per decay. These favorable characteristics suggest it may be a potential candidate for application in immuno-PET. Our recent aim was to conduct studies on distribution coefficients for ZrIV and NbV in mixtures of HCl/H2O2 and HCl/oxalic acid for anion exchange resin (AG 1 × 8) and UTEVA resin to develop a “direct flow” separation strategy for 90Nb. The direct flow concept refers to a separation accomplished using a single eluent on multiple columns, effectively streamlining the separation process and increasing the time efficiency. Finally, we also demonstrated that this separation strategy is applicable to the production of the positron emitter 90Nb via the irradiation of molybdenum targets and isolation of 90Nb from the irradiated molybdenum target.

Dual PET and Near-Infrared Fluorescence Imaging Probes as Tools for Imaging in Oncology

OBJECTIVE

The purpose of this article is to summarize advances in PET fluorescence resolution, agent design, and preclinical imaging that make a growing case for clinical PET fluorescence imaging.

CONCLUSION

Existing SPECT, PET, fluorescence, and MRI contrast imaging techniques are already deeply integrated into the management of cancer, from initial diagnosis to the observation and management of metastases. Combined positron-emitting fluorescent contrast agents can convey new or substantial benefits that improve on these proven clinical contrast agents.

Advances in PET Detection of the Antitumor T Cell Response

Positron emission tomography (PET) is a powerful noninvasive imaging technique able to measure distinct biological processes in vivo by administration of a radiolabeled probe. Whole-body measurements track the probe accumulation providing a means to measure biological changes such as metabolism, cell location, or tumor burden. PET can also be applied to both preclinical and clinical studies providing three-dimensional information. For immunotherapies (in particular understanding T cell responses), PET can be utilized for spatial and longitudinal tracking of T lymphocytes. Although PET has been utilized clinically for over 30 years, the recent development of additional PET radiotracers have dramatically expanded the use of PET to detect endogenous or adoptively transferred T cells in vivo. Novel probes have identified changes in T cell quantity, location, and function. This has enabled investigators to track T cells outside of the circulation and in hematopoietic organs such as spleen, lymph nodes, and bone marrow, or within tumors. In this review, we cover advances in PET detection of the antitumor T cell response and areas of focus for future studies.

Dual Targeting of Tissue Factor and CD105 for Preclinical PET Imaging of Pancreatic Cancer

PURPOSE

Pancreatic adenocarcinoma is a highly aggressive cancer, currently treated with limited success and dismal outcomes. New diagnostic and treatment strategies offer the potential to reduce cancer mortality. Developing highly specific noninvasive imaging probes for pancreatic cancer is essential to improving diagnostic accuracy and monitoring therapeutic intervention.

EXPERIMENTAL DESIGN

A bispecific heterodimer was synthesized by conjugating an anti-tissue factor (TF) Fab with an anti-CD105 Fab, via the bio-orthogonal "click" reaction between tetrazine (Tz) and trans-cyclooctene (TCO). The heterodimer was labeled with (64)Cu for PET imaging of nude mice bearing BXPC-3 xenograft and orthotopic pancreatic tumors.

RESULTS

PET imaging of BXPC-3 (TF/CD105(+/+)) xenograft tumors with (64)Cu-labeled heterodimer displayed significantly enhanced tumor uptake (28.8 ± 3.2 %ID/g; n = 4; SD) at 30 hours postinjection, as compared with each of their monospecific Fab tracers (12.5 ± 1.4 and 7.1 ± 2.6 %ID/g; n = 3; SD). In addition, the activity-concentration ratio allowed for effective tumor visualization (tumor/muscle ratio 75.2 ± 9.4 at 30 hours postinjection.; n = 4; SD). Furthermore, (64)Cu-NOTA-heterodimer enabled sensitive detection of orthotopic pancreatic tumor lesions with an uptake of 17.1 ± 4.9 %ID/g at 30 hours postinjection and tumor/muscle ratio of 72.3 ± 46.7.

CONCLUSIONS

This study demonstrates that dual targeting of TF and CD105 provided synergistic improvements in binding affinity and tumor localization of the heterodimer. Dual-targeted imaging agents of pancreatic and other cancers may assist in diagnosing pancreatic malignancies as well as reliable monitoring of therapeutic response. Clin Cancer Res; 22(15); 3821-30. ©2016 AACR.

Imaging Biomarkers in Immunotherapy

Immune-based therapies have been in use for decades but recent work with immune checkpoint inhibitors has now changed the landscape of cancer treatment as a whole. While these advances are encouraging, clinicians still do not have a consistent biomarker they can rely on that can accurately select patients or monitor response. Molecular imaging technology provides a noninvasive mechanism to evaluate tumors and may be an ideal candidate for these purposes. This review provides an overview of the mechanism of action of varied immunotherapies and the current strategies for monitoring patients with imaging. We then describe some of the key researches in the preclinical and clinical literature on the current uses of molecular imaging of the immune system and cancer.