Dosimetry of [212Pb]VMT01, a MC1R-Targeted Alpha Therapeutic Compound, and Effect of Free 208Tl on Tissue Absorbed Doses

[²¹²Pb]VMT01 is a melanocortin 1 receptor (MC1R) targeted theranostic ligand in clinical development for alpha particle therapy for melanoma. ²¹²Pb has an elementally matched gamma-emitting isotope ²⁰³Pb; thus, [²⁰³Pb]VMT01 can be used as an imaging surrogate for [²¹²Pb]VMT01. [²¹²Pb]VMT01 human serum stability studies have demonstrated retention of the ²¹²Bi daughter within the chelator following beta emission of parent ²¹²Pb. However, the subsequent alpha emission from the decay of ²¹²Bi into ²⁰⁸Tl results in the generation of free ²⁰⁸Tl. Due to the 10.64-hour half-life of ²¹²Pb, accumulation of free ²⁰⁸Tl in the injectate will occur. The goal of this work is to estimate the human dosimetry for [²¹²Pb]VMT01 and the impact of free ²⁰⁸Tl in the injectate on human tissue absorbed doses. Human [²¹²Pb]VMT01 tissue absorbed doses were estimated from murine [²⁰³Pb]VMT01 biodistribution data, and human biodistribution values for ²⁰¹Tl chloride (a cardiac imaging agent) from published data were used to estimate the dosimetry of free ²⁰⁸Tl. Results indicate that the dose-limiting tissues for [²¹²Pb]VMT01 are the red marrow and the kidneys, with estimated absorbed doses of 1.06 and 8.27 mGy_{RBE = 5}/MBq. The estimated percent increase in absorbed doses from free ²⁰⁸Tl in the injectate is 0.03% and 0.09% to the red marrow and the kidneys, respectively. Absorbed doses from free ²⁰⁸Tl result in a percent increase of no more than 1.2% over [²¹²Pb]VMT01 in any organ or tissue. This latter finding indicates that free ²⁰⁸Tl in the [²¹²Pb]VMT01 injectate will not substantially impact estimated tissue absorbed doses in humans.

Noninvasive Imaging of CD4+ T Cells in Humanized Mice

Antibody-based PET (immunoPET) with radiotracers that recognize specific cells of the immune system provides an opportunity to monitor immune cell trafficking at the organismal scale. We previously reported the visualization of human CD8+ T cells, including CD8+ tumor-infiltrating lymphocytes (TIL), in mice using a humanized CD8-targeted minibody. Given the important role of CD4+ T cells in adaptive immune responses of health and disease including infections, tumors, and autoimmunity, we explored immunoPET using an anti-human-CD4 minibody. We assessed the ability of [64Cu]Cu-NOTA-IAB41 to bind to various CD4+ T-cell subsets in vitro. We also determined the effect of the CD4-targeted minibody on CD4+ T-cell abundance, proliferation, and activation state in vitro. We subsequently evaluated the ability of the radiotracer to visualize CD4+ T cells in T-cell rich organs and orthotopic brain tumors in vivo. For the latter, we injected the [64Cu]Cu-NOTA-IAB41 radiotracer into humanized mice that harbored intracranial patient-derived glioblastoma (GBM) xenografts and performed in vivo PET, ex vivo autoradiography, and anti-CD4 IHC on serial brain sections. [64Cu]Cu-NOTA-IAB41 specifically detects human CD4+ T cells without impacting their abundance, proliferation, and activation. In humanized mice, [64Cu]Cu-NOTA-IAB41 can visualize various peripheral tissues in addition to orthotopically implanted GBM tumors. [64Cu]Cu-NOTA-IAB41 is able to visualize human CD4+ T cells in humanized mice and can provide noninvasive quantification of CD4+ T-cell distribution on the organismal scale.

Imaging of Cancer γ-Secretase Activity Using an Inhibitor-Based PET Probe

PURPOSE

Abnormal Notch signaling promotes cancer cell growth and tumor progression in various cancers. Targeting γ-secretase, a pivotal regulator in the Notch pathway, has yielded numerous γ-secretase inhibitors (GSIs) for clinical investigation in the last 2 decades. However, GSIs have demonstrated minimal success in clinical trials in part due to the lack of specific and precise tools to assess γ-secretase activity and its inhibition in vivo.

EXPERIMENTAL DESIGN

We designed an imaging probe based on GSI Semagacestat structure and synthesized the radioiodine-labeled analogues [¹³¹I]- or [¹²⁴I]-PN67 from corresponding trimethyl-tin precursors. Both membrane- and cell-based ligand-binding assays were performed using [¹³¹I]-PN67 to determine the binding affinity and specificity for γ-secretase in vitro. Moreover, we evaluated [¹²⁴I]-PN67 by PET imaging in mammary tumor and glioblastoma mouse models.

RESULTS

The probe was synthesized through iodo-destannylation using chloramine-T as an oxidant with a high labeling yield and efficiency. In vitro binding results demonstrate the high specificity of this probe and its ability for target replacement study by clinical GSIs. PET imaging studies demonstrated a significant (P < 0.05) increased in the uptake of [¹²⁴I]-PN67 in tumors versus blocking or sham control groups across multiple mouse models, including 4T1 allograft, MMTV-PyMT breast cancer, and U87 glioblastoma allograft. Ex vivo biodistribution and autoradiography corroborate these results, indicating γ-secretase specific tumor accumulation of [¹²⁴I]-PN67.

CONCLUSIONS

[¹²⁴I]-PN67 is a novel PET imaging agent that enables assessment of γ-secretase activity and target engagement of clinical GSIs.

Exploiting the MUC5AC Antigen for Noninvasive Identification of Pancreatic Cancer

Pancreatic cancer (PC) remains the fourth leading cause of cancer death; therefore, there is a clinically unmet need for novel therapeutics and diagnostic markers to treat this devastating disease. Physicians often rely on biopsy or CT for diagnosis, but more specific protein biomarkers are highly desired to assess the stage and severity of PC in a noninvasive manner. Serum biomarkers such as carbohydrate antigen 19-9 are of particular interest as they are commonly elevated in PC but have exhibited suboptimal performance in the clinic. MUC5AC has emerged as a useful serum biomarker that is specific for PC versus inflammation. We developed RA96, an anti-MUC5AC antibody, to gauge its utility in PC diagnosis through immunohistochemical analysis and whole-body PET in PC. Methods: In this study, extensive biochemical characterization determined MUC5AC as the antigen for RA96. We then determined the utility of RA96 for MUC5AC immunohistochemistry on clinical PC and preclinical PC. Finally, we radiolabeled RA96 with 89Zr to assess its application as a whole-body PET radiotracer for MUC5AC quantification in PC. Results: Immunohistochemical staining with RA96 distinguished chronic pancreatitis, pancreatic intraepithelial neoplasia, and varying grades of pancreatic ductal adenocarcinoma in clinical samples. 89Zr-desferrioxamine-RA96 was able to detect MUC5AC with high specificity in mice bearing capan-2 xenografts. Conclusion: Our study demonstrated that RA96 can differentiate between inflammation and PC, improving the fidelity of PC diagnosis. Our immuno-PET tracer 89Zr-desferrioxamine-RA96 shows specific detection of MUC5AC-positive tumors in vivo, highlighting the utility of MUC5AC targeting for diagnosis of PC.

ERK Inhibition Improves Anti-PD-L1 Immune Checkpoint Blockade in Preclinical Pancreatic Ductal Adenocarcinoma

Patients with pancreatic ductal adenocarcinoma (PDAC) do not benefit from immune checkpoint blockade (ICB) along the PD-1/PD-L1 axis. Variable PD-L1 expression in PDAC indicates a potential access issue of PD-L1-targeted therapy. To monitor target engagement of PD-L1-targeted therapy, we generated a PD-L1-targeted PET tracer labeled with zirconium-89 (89Zr). As the MAPK signaling pathway (MEK and ERK) is known to modulate PD-L1 expression in other tumor types, we used [89Zr]Zr-DFO-anti-PD-L1 as a tool to noninvasively assess whether manipulation of the MAPK signaling cascade could be leveraged to modulate PD-L1 expression and thereby immunotherapeutic outcomes in PDAC. In this study, we observed that the inhibition of MEK or ERK is sufficient to increase PD-L1 expression, which we hypothesized could be leveraged for anti-PD-L1 immune checkpoint therapy. We found that the combination of ERK inhibition and anti-PD-L1 therapy corresponded with a significant improvement of overall survival in a syngeneic mouse model of PDAC. Furthermore, IHC analysis indicates that the survival benefit may be CD8+ T-cell mediated. The therapeutic and molecular imaging tool kit developed could be exploited to better structure clinical trials and address the therapeutic gaps in challenging malignancies such as PDAC.

Imaging Tumor-Infiltrating Lymphocytes in Brain Tumors with [64Cu]Cu-NOTA-anti-CD8 PET

PURPOSE

Glioblastoma (GBM) is the most common malignant brain tumor in adults. Various immunotherapeutic approaches to improve patient survival are being developed, but the molecular mechanisms of immunotherapy resistance are currently unknown. Here, we explored the ability of a humanized radiolabeled CD8-targeted minibody to noninvasively quantify tumor-infiltrating CD8-positive (CD8+) T cells using PET.

EXPERIMENTAL DESIGN

We generated a peripheral blood mononuclear cell (PBMC) humanized immune system (HIS) mouse model and quantified the absolute number of CD8+ T cells by flow cytometry relative to the [64Cu]Cu-NOTA-anti-CD8 PET signal. To evaluate a patient-derived orthotopic GBM HIS model, we intracranially injected cells into NOG mice, humanized cohorts with multiple HLA-matched PBMC donors, and quantified CD8+ tumor-infiltrating lymphocytes by IHC. To determine whether [64Cu]Cu-NOTA-anti-CD8 images brain parenchymal T-cell infiltrate in GBM tumors, we performed PET and autoradiography and subsequently stained serial sections of brain tumor tissue by IHC for CD8+ T cells.

RESULTS

Nontumor-bearing NOG mice injected with human PBMCs showed prominent [64Cu]Cu-NOTA-anti-CD8 uptake in the spleen and minimal radiotracer localization to the normal brain. NOG mice harboring intracranial human GBMs yielded high-resolution PET images of tumor-infiltrating CD8+ T cells. Radiotracer retention correlated with CD8+ T-cell numbers in spleen and tumor tissue. Our study demonstrates the ability of [64Cu]Cu-NOTA-anti-CD8 PET to quantify peripheral and tumor-infiltrating CD8+ T cells in brain tumors.

CONCLUSIONS

Human CD8+ T cells infiltrate an orthotopic GBM in a donor-dependent manner. Furthermore, [64Cu]Cu-NOTA-anti-CD8 quantitatively images both peripheral and brain parenchymal human CD8+ T cells.

Polyazamacrocycle Ligands Facilitate 89Zr Radiochemistry and Yield 89Zr Complexes with Remarkable Stability

Over the last three decades, the chemistry of zirconium has facilitated antibody development and the clinical management of disease in the precision medicine era. Scientists have harnessed its reactivity, coordination chemistry, and nuclear chemistry to develop antibody-based radiopharmaceuticals incorporating zirconium-89 (⁸⁹Zr: t_1/2 = 78.4 h, β⁺: 22.8%, E_β+max = 901 keV; EC: 77%, E_γ = 909 keV) to improve disease detection, identify patients for individualized therapeutic interventions. and monitor their response to those interventions. However, release of the ⁸⁹Zr⁴⁺ ion from the radiopharmaceutical remains a concern, since it may confound the interpretation of clinical imaging data, negatively affect dosimetric calculations, and hinder treatment planning. In this report, we relate our novel observations involving the use of polyazamacrocycles as zirconium-89 chelators. We describe the synthesis and complete characterization of zirconium 2,2',2″,2‴-(1,4,7,10-tetraazacyclotridecane-1,4,7,10-tetrayl)tetraacetic acid (Zr-TRITA), zirconium 3,6,9,15-Tetraazabicyclo[9.3.1] pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (Zr-PCTA), and zirconium 2,2',2″-(1,4,7-triazacyclononane-1,4,7-triyl)triacetic acid (Zr-NOTA). In addition, we elucidate the solid-state structure of each complex using single-crystal X-ray diffraction analysis. Finally, we found that [⁸⁹Zr]Zr-PCTA and [⁸⁹Zr]Zr-NOTA demonstrate excellent stability in vitro and in vivo and provide a rationale for these observations. These innovative findings have the potential to guide the development of safer and more robust immuno-PET agents to improve precision medicine applications.

Demarcation of Sepsis-Induced Peripheral and Central Acidosis with pH (Low) Insertion Cycle Peptide

Acidosis is a key driver for many diseases, including cancer, sepsis, and stroke. The spatiotemporal dynamics of dysregulated pH across disease remain elusive, and current diagnostic strategies do not provide localization of pH alterations. We sought to explore if PET imaging using hydrophobic cyclic peptides that partition into the cellular membrane at low extracellular pH (denoted as pH [low] insertion cycles, or pHLIC) can permit accurate in vivo visualization of acidosis. Methods: Acid-sensitive cyclic peptide c[E₄W₅C] pHLIC was conjugated to bifunctional maleimide-NO2A and radiolabeled with ⁶⁴Cu (half-life, 12.7 h). C57BL/6J mice were administered lipopolysaccharide (15 mg/kg) or saline (vehicle) and serially imaged with [⁶⁴Cu]Cu-c[E₄W₅C] over 24 h. Ex vivo autoradiography was performed on resected brain slices and subsequently stained with cresyl violet to enable high-resolution spatial analysis of tracer accumulation. A non-pH-sensitive cell-penetrating control peptide (c[R₄W₅C]) was used to confirm specificity of [⁶⁴Cu]Cu-c[E₄W₅C]. CD11b (macrophage/microglia) and TMEM119 (microglia) immunostaining was performed to correlate extent of neuroinflammation with [⁶⁴Cu]Cu-c[E₄W₅C] PET signal. Results: [⁶⁴Cu]Cu-c[E₄W₅C] radiochemical yield and purity were more than 95% and more than 99%, respectively, with molar activity of more than 0.925 MBq/nmol. Significantly increased [⁶⁴Cu]Cu-c[E₄W₅C] uptake was observed in lipopolysaccharide-treated mice (vs. vehicle) within peripheral tissues, including blood, lungs, liver, and small intestines (P < 0.001-0.05). Additionally, there was significantly increased [⁶⁴Cu]Cu-c[E₄W₅C] uptake in the brains of lipopolysaccharide-treated animals. Autoradiography confirmed increased uptake in the cerebellum, cortex, hippocampus, striatum, and hypothalamus of lipopolysaccharide-treated mice (vs. vehicle). Immunohistochemical analysis revealed microglial or macrophage infiltration, suggesting activation in brain regions containing increased tracer uptake. [⁶⁴Cu]Cu-c[R₄W₅C] demonstrated significantly reduced uptake in the brain and periphery of lipopolysaccharide mice compared with the acid-mediated [⁶⁴Cu]Cu-c[E₄W₅C] tracer. Conclusion: Here, we demonstrate that a pH-sensitive PET tracer specifically detects acidosis in regions associated with sepsis-driven proinflammatory responses. This study suggests that [⁶⁴Cu]Cu-pHLIC is a valuable tool to noninvasively assess acidosis associated with both central and peripheral innate immune activation.

A PET Imaging Strategy for Interrogating Target Engagement and Oncogene Status in Pancreatic Cancer

PURPOSE

Pancreatic ductal adenocarcinoma (PDAC) is one of the most deadly cancers, with a 5-year survival rate of less than 10%. Physicians often rely on biopsy or CT to guide treatment decisions, but these techniques fail to reliably measure the actions of therapeutic agents in PDAC. KRAS mutations are present in >90% of PDAC and are connected to many signaling pathways through its oncogenic cascade, including extracellular regulated kinase (ERK) and MYC. A key downstream event of MYC is transferrin receptor (TfR), which has been identified as a biomarker for cancer therapeutics and imaging.

EXPERIMENTAL DESIGN

In this study, we aimed to test whether zirconium-89 transferrin ([⁸⁹Zr]Zr-Tf) could measure changes in MYC depending on KRAS status of PDAC, and assess target engagement of anti-MYC and anti-ERK-targeted therapies.

RESULTS

Mice bearing iKras*p53* tumors showed significantly higher (P < 0.05) uptake of [⁸⁹Zr]Zr-Tf in mice withdrawn from inducible oncogenic KRAS. A therapy study with JQ1 showed a statistically significant decrease (P < 0.05) of [⁸⁹Zr]Zr-Tf uptake in drug versus vehicle-treated mice bearing Capan-2 and Suit-2 xenografts. IHC analysis of resected PDAC tumors reflects the data observed via PET imaging and radiotracer biodistribution.

CONCLUSIONS

Our study demonstrates that [⁸⁹Zr]Zr-Tf is a valuable tool to noninvasively assess oncogene status and target engagement of small-molecule inhibitors downstream of oncogenic KRAS, allowing a quantitative assessment of drug delivery.

Clinical Potential of Human Epidermal Growth Factor Receptor 2 and Human Epidermal Growth Factor Receptor 3 Imaging in Breast Cancer

Increased expression of the human epidermal growth factor receptor (HER) protein family are targets in breast cancer for imaging and therapy. Imaging modalities targeting HER2 and HER3 can diagnose breast cancer with a specific, biologically relevant target. Repeat biopsies do not address heterogeneity intratumorally or between primary disease and metastasis. HER2- and HER3-targeted PET is an important tool to diagnose disease in breast cancer and evaluate response to targeted therapies. PET and single photon emission computed tomography with radiolabeled biomolecules can be used to detect and quantify specific targets, conferring a better understanding of the behavior and effectiveness of treatments.

Human Epidermal Growth Factor Receptor 2-Targeted PET/Single- Photon Emission Computed Tomography Imaging of Breast Cancer: Noninvasive Measurement of a Biomarker Integral to Tumor Treatment and Prognosis

Increased human epidermal growth factor receptor 2 (HER2) expression is a hallmark of aggressive breast cancer. Imaging modalities have the potential to diagnose HER2-positive breast cancer and detect distant metastases. The heterogeneity of HER2 expression between primary and metastatic disease sites limits the value of tumor biopsies. Molecular imaging is a noninvasive tool to assess HER2-positive primary lesions and metastases. Radiolabeled antibodies, antibody fragments, and affibody molecules devise a reliable and quantitative method for detecting HER2-positive cancer using PET. HER2-targeted PET imaging is a valuable clinical tool with respect to both the care and maintenance of patients with breast cancer.

Noninvasive 89Zr-Transferrin PET Shows Improved Tumor Targeting Compared with 18F-FDG PET in MYC-Overexpressing Human Triple-Negative Breast Cancer

The current standard for breast PET imaging is 18F-FDG. The heterogeneity of 18F-FDG uptake in breast cancer limits its utility, varying greatly among receptor status, histopathologic subtypes, and proliferation markers. 18F-FDG PET often exhibits nonspecific internalization and low specificity and sensitivity, especially with tumors smaller than 1 cm3 MYC is a protein involved in oncogenesis and is overexpressed in triple-negative breast cancer (TNBC). Increased surface expression of transferrin receptor (TfR) is a downstream event of MYC upregulation and has been validated as a clinically relevant target for molecular imaging. Transferrin labeled with 89Zr has successfully identified MYC status in many cancer subtypes preclinically and been shown to predict response and changes in oncogene status via treatment with small-molecule inhibitors that target MYC and PI3K signaling pathways. We hypothesized that 89Zr-transferrin PET will noninvasively detect MYC and TfR and improve upon the current standard of 18F-FDG PET for MYC-overexpressing TNBC. Methods: In this study, 89Zr-transferrin and 18F-FDG imaging were compared in preclinical models of TNBC. TNBC cells (MDA-MB-157, MDA-MB-231, and Hs578T) were treated with bromodomain-containing protein 4 (BRD4) inhibitors JQ1 and OTX015 (0.5-1 μM). Cell proliferation, gene expression, and protein expression were assayed to explore the effects of these inhibitors on MYC and TfR. Results: Head-to-head comparison showed that 89Zr-transferrin targets TNBC tumors significantly better (P < 0.05-0.001) than 18F-FDG through PET imaging and biodistribution studies in MDA-MB-231 and MDA-MB-157 xenografts and a patient-derived xenograft model of TNBC. c-Myc and TfR gene expression was decreased upon treatment with BRD4 inhibitors and c-MYC small interfering RNA (P < 0.01-0.001 for responding cell lines), compared with vehicle treatment. MYC and TfR protein expression, along with receptor-mediated internalization of transferrin, was also significantly decreased upon drug treatment in MDA-MB-231 and MDA-MB-157 cells (P < 0.01-0.001). Conclusion:89Zr-transferrin targets human TNBC primary tumors significantly better than 18F-FDG, as shown through PET imaging and biodistribution studies. 89Zr-transferrin is a useful tool to interrogate MYC via TfR-targeted PET imaging in TNBC.

Solution Structure and Constrained Molecular Dynamics Study of Vitamin B12 Conjugates of the Anorectic Peptide PYY(3-36)

Vitamin B12 -peptide conjugates have considerable therapeutic potential through improved pharmacokinetic and/or pharmacodynamic properties imparted on the peptide upon covalent attachment to vitamin B12 (B12 ). There remains a lack of structural studies investigating the effects of B12 conjugation on peptide secondary structure. Determining the solution structure of a B12 -peptide conjugate or conjugates and measuring functions of the conjugate(s) at the target peptide receptor may offer considerable insight concerning the future design of fully optimized conjugates. This methodology is especially useful in tandem with constrained molecular dynamics (MD) studies, such that predictions may be made about conjugates not yet synthesized. Focusing on two B12 conjugates of the anorectic peptide PYY(3-36), one of which was previously demonstrated to have improved food intake reduction compared with PYY(3-36), we performed NMR structural analyses and used the information to conduct MD simulations. The study provides rare structural insight into vitamin B12 conjugates and validates the fact that B12 can be conjugated to a peptide without markedly affecting peptide secondary structure.

Vitamin B12 conjugation of peptide-YY(3-36) decreases food intake compared to native peptide-YY(3-36) upon subcutaneous administration in male rats

Challenges to peptide-based therapies include rapid clearance, ready degradation by hydrolysis/proteolysis, and poor intestinal uptake and/or a need for blood brain barrier transport. This work evaluates the efficacy of conjugation of vitamin B12 (B12) on sc administered peptide tyrosine tyrosine (PYY)(3-36) function. In the current experiments, a B12-PYY(3-36) conjugate was tested against native PYY(3-36), and an inactive conjugate B12-PYYC36 (null control) in vitro and in vivo. In vitro experiments demonstrated similar agonism for the neuropeptide Y2 receptor by the B12-PYY(3-36) conjugate (EC50 26.5 nM) compared with native PYY(3-36) (EC50 16.0 nM), with the null control having an EC50 of 1.8 μM. In vivo experiments were performed in young adult male Sprague Dawley rats (9 wk). Daily treatments were delivered sc in five 1-hour pulses, each pulse delivering 5-10 nmol/kg, by implanted microinfusion pumps. Increases in hindbrain Fos expression were comparable 90 minutes after B12-PYY(3-36) or PYY3-36 injection relative to saline or B12-PYYC36. Food intake was reduced during a 5-day treatment for both B12-PYY(3-36)- (24%, P = .001) and PYY(3-36)-(13%, P = .008) treated groups relative to baseline. In addition, reduction of food intake after the three dark cycle treatment pulses was more consistent with B12-PYY(3-36) treatment (-26%, -29%, -27%) compared with the PYY(3-36) treatment (-3%, -21%, -16%), and B12-PYY(3-36) generated a significantly longer inhibition of food intake vs. PYY(3-36) treatment after the first two pulses (P = .041 and P = .036, respectively). These findings demonstrate a stronger, more consistent, and longer inhibition of food intake after the pulses of B12-PYY(3-36) conjugate compared with the native PYY(3-36).