ABSTRACT Calreticulin is an emerging cancer biomarker, but current detection methods rely on expensive monoclonal antibodies that suffer from inefficient protein production, pharmacokinetic challenges and poor tissue penetration. Cal3, a calreticulin-specific nanobody, was constructed by replacing the complimentary determining region 2 (CDR2) of a soluble, clinically validated nanobody with a calreticulin-specific CDR2 isolated from a phage display library. However, the poor solubility and low yield of Cal3 limit its usefulness. In this study, we engineered CALR-Nb02 by adapting the core of Cal3 to a partial consensus framework sequence of stable nanobodies. CALR-Nb02 was purified with a 240-fold higher yield as a predominantly monomeric, soluble protein that exhibits an increased thermal stability and a higher calreticulin binding affinity (KD: 25–50 nM) compared with Cal3. These results reveal a strategy for quickly altering the specificity of a stable nanobody, and provide an improved calreticulin-binding reagent for future diagnostic, imaging, and therapeutic applications.

Rationale: Immune-targeted positron emission tomography (PET) allows for the non-invasive monitoring of immune populations with potential to provide early markers of response to novel immunotherapies in glioblastoma (GBM). Dynamic PET acquisitions can inform on tracer kinetics via dynamic modeling to enhance the GBM tumor microenvironment characterization, while its impact has yet to be understood. The objective of this study is to evaluate the quantitative description of dynamic granzyme B (GZP)-PET imaging and its ability to inform on combination immunotherapy response in orthotopic syngeneic GBM models. Methods: Orthotopic GBM murine models (GSC005-luc) were imaged with dynamic [64Cu]-NOTA-GZP PET (0-80 min) and T2 -weighted MRI one week post-treatment with saline or combination M002 virotherapy and anti-PD1 immunotherapy. One-, two-, and three-tissue compartment models were evaluated for suitability to tracer kinetics via Akaike information criterion. Biological validation consisted of ex vivo brain PET imaging, autoradiography, H&E and immunofluorescence for granzyme B. A subset of mice was longitudinally monitored via dynamic [64Cu]-NOTA-GZP PET at days 4 and 7. Changes in viable cell bioluminescence and radiological tumor volume (MRI) were used to determine response. Imaging-derived metrics including k1 , k2 , k3, k4 and SUV were evaluated across treatment and response groups via unpaired two-tailed T test. Results: Intratumoral [64Cu]-NOTA-GZP PET tracer kinetics showed improved fitting based on a two-tissue compartment model (p < 0.05). Intratumoral effector cell function (SUVmean TBR) and tracer binding rate (k3 ) were positively correlated with histological granzyme B density (p < 0.01). Increases in tumor tracer influx (k1 ) were observed in responders relative to non-responders (p < 0.01). Conclusions: Mathematical description of tracer kinetics via dynamic [64Cu]-NOTA-GZP PET offers complementary quantitative metrics for the characterization of immunotherapy response in GBM. Integration of dynamic protocols to immune-target PET approaches can provide clinically translatable metrics to differentiate immunotherapy-induced effects from tumor progression in GBM.

Claudin 18.2 (CLDN18.2) is a tight-junction protein overexpressed and differently exposed in solid tumors such as gastric cancer (GCa), gastroesophageal junction adenocarcinoma, and pancreatic ductal adenocarcinoma (PDAC). We sought to explore the potential of CLDN18.2 as a biomarker for molecular imaging and targeted radiopharmaceutical therapy in GCa and PDAC models. Methods: Bulk and single-cell RNA sequencing for CLDN18.2 from 31 patients with PDAC were performed. Subcutaneous xenografts of GCa with the GSU cell line and PDAC with HUPT-4 and PATU8988S cell lines were developed in nude mice. Serial PET imaging with 89Zr-labeled zolbetuximab, an anti-CLDN18.2 monoclonal antibody ([89Zr]Zr-DFO-zolbetuximab), and human IgG ([89Zr]Zr-DFO-IgG) as control (5.5-7.4 MBq) was performed 1, 3, and 6 d after injection, followed by ex vivo analysis of biodistribution. Tumor-bearing mice received a single intravenous injection of [177Lu]Lu-DOTA-zolbetuximab (7.4 or 14.8 MBq), nonradiolabeled zolbetuximab, [177Lu]Lu-DOTA-IgG, [177Lu]Lu-DOTA, or saline as the control. Toxicity of [177Lu]Lu-DOTA-zolbetuximab was evaluated by laboratory and histologic analyses over 90 d. Results: RNA sequencing confirmed significantly higher expression of CLDN18.2 in human PDAC compared with noncancerous pancreas tissue, with no significant difference between treated and untreated tumors. Serial PET imaging demonstrated a high tumor uptake of [89Zr]Zr-DFO-zolbetuximab at all 3 time points (mean uptake on day 6: 24.4 ± 7.8 %ID/g in GSU; 36.6 ± 10.1 %ID/g in PATU8988S; 16.48 ± 4.7 %ID/g in HUPT-4), and significantly higher than the uptake seen in mice imaged with [89Zr]Zr-DFO-IgG (day 6: 4.8 ± 1.9 %ID/g in GSU; P = 0.0029). The highest tumor-to-background uptake ratio was achieved on day 6 (GSU tumor-to-muscle ratio, 14.85 ± 7.8). Biodistribution analyses were consistent with the PET imaging results. High-dose [177Lu]Lu-DOTA-zolbetuximab (14.8 MBq) resulted in reduced tumor growth in GSU and PATU8988S over 4 and 8 wk, respectively, with complete regression of most HUPT-4 tumors and improved 90-d survival compared with the mice treated with control conditions. The 90-day treatment toxicity assay indicated a favorable safety profile. Conclusion: CLDN18.2 could serve as a promising biomarker for precise quantitative imaging and effective treatment of GCa and PDAC. This proof-of-concept study encourages the clinical translation of CLDN18.2 as a radiotheranostic in patients with CLDN18.2-expressing tumors.

Single domain antibodies, often known as nanobodies, are versatile molecules with therapeutic and diagnostic applications, but they are primarily developed through immunization of camelids. This approach is not scalable by automation, not effective for non-immunogenic or toxic antigens, and prevents the use of modified scaffolds for altered pharmacokinetic properties. Synthetic libraries allow for pre-selection of a single domain framework tailored to its intended downstream use. One area of interest for these biologic vectors is radiopharmaceuticals. Ideal radiopharmaceutical pharmacokinetic properties differ from most traditional therapeutics, as short plasma circulation and rapid kidney clearance are necessary to avoid dose-limiting organ radiation. Although there are a growing number of nanobody radiopharmaceuticals in clinical trials, their frameworks and corresponding pharmacokinetic properties vary. One potential method for improving the development of novel single domain antibody radiopharmaceuticals is through synthetic libraries based on nanobodies with proven clinically acceptable pharmacokinetics. We developed a modular synthetic nanobody phage display vector based on the scaffold of the 2Rs15d nanobody that allows for manipulation of the binding and framework regions. Using this vector, we created a library of nanobodies with a randomized CDR2 containing over 1.7×10 6 unique sequences/µL. As a proof-of-concept, we panned the library for nanobodies binding calreticulin (CALR), a protein critical in immunogenic cell death. One isolated clone, Cal3, has a measured affinity of 140 nM for CALR and is cross-reactive with mouse and human CALR. Using positron emission tomography (PET) imaging, the radiolabeled 64 Cu-NOTA-Cal3 demonstrated CALR binding in vivo , representing the first reported synthetic nanobody characterized by PET imaging. This study demonstrates the feasibility of building and panning synthetic libraries for high-affinity radiopharmaceutical nanobodies as an alternative to immunized camelid libraries.

Targeted agents, such as nanobodies, can be covalently linked to radionuclide chelators for theranostics. Nanobodies are single-domain immunoglobulin variable regions with a short blood half-lives and high specificity. Calreticulin (CALR) is endoplasmic reticular (ER) protein that translocates to the cell surface upon induction with certain agents, namely doxorubicin. We hypothesized surface CALR on tumors could be targeted using a novel nanobody after induction with doxorubicin. Studies were conducted using in vitro and in vivo models of pancreatic ductal adenocarcinoma (PDAC). Surface CALR expression was determined by western blot on fractionated lysates, flow cytometry, or immunofluorescence (IF). Doxorubicin was administered by intraperitoneal injection to mouse models of PDAC. The anti-CALR nanobody was developed via the panning of a synthetic nanobody phage display library. The top CALR-binding nanobody was confirmed by ELISA, immunohistochemistry (IHC), and flow cytometry. For in vivo studies, the nanobody was conjugated with the chelator NOTA and radiolabeled with 64Cu. PET images were acquired 1-hour after injection. Significance was determined using an independent t-test and p values < 0.05 were considered statistically significant. PDAC cells treated with doxorubicin showed peak surface CALR expression between 24 and 48 hours. Flow cytometry revealed that 61.7% of live PDAC cultured cells treated for 24 hours expressed surface CALR, compared to 2.2% in controls (p < 0.001). In a subcutaneous syngeneic PDAC mouse model, two doses of doxorubicin 48 hours apart significantly increased surface CALR, with 30.4% of live-cells expressing CALR in treated tumors versus 1.5% in controls (p < 0.001). Similarly, doxorubicin-treated tumors from a PDAC patient derived xenograft (PDX) model had a 6.7-fold increase in live-cells expressing surface CALR compared to controls (p < 0.01). IF confirmed a 1.4-fold increase in CALR expression in doxorubicin-treated tumors from the subcutaneous PDAC mouse model (p < 0.01) and a 1.7-fold increase in the PDAC PDX model (p < 0.01). Our anti-CALR nanobody has an apparent 37 nM affinity for recombinant CALR and successfully detected surface CALR by IHC and flow cytometry. In vivo PET imaging showed significant increased uptake of the 64Cu-NOTA-nanobody in tumors from doxorubicin-treated mice over controls, with no significant changes in heart, kidney, or liver accumulation by PET quantification and flow cytometry (p < 0.05). Systemic doxorubicin treatment induces CALR surface translocation in tumors that can be successfully targeted with a radiolabeled nanobody for PET imaging. Targeting chemotherapy-induced surface CALR using our radiolabeled nanobody could provide a theragnostic strategy for the detection and treatment of PDAC. Rachael Guenter, Yuvasri Golivi, Lucinda Hall, Amro Abdelrahman, Chloe La Prairie, Caleb Miller, Tejeshwar C. Rao, Danielle M. Carlson, Mark J. Truty, Benjamin Larimer, J. Bart Rose. Detection of tumor cell surface calreticulin after doxorubicin treatment using a novel radiolabeled nanobody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 574.

Obesity and type 2 diabetes (T2D) influence the tumor microenvironment by altering glucose metabolism, which has been shown to decrease immune cell infiltration and activation. Positron emission tomography (PET) imaging provides a non-invasive method to detect molecular markers of immune populations in the tumor microenvironment and systemic organs. The goal of this study is to utilize advanced molecular imaging to quantify differences in innate and adaptive immune responses in diabetic obese mice systemically and within the tumor microenvironment. 5–6-week-old female C57BL6/J mice were placed on a high-fat diet (HFD) composed of 60

Background/Objectives: The identification of inflammatory mediators and the involvement of CD206 macrophages in anti-inflammatory responses, along with the synthesis of fibrotic mediators, are crucial for the diagnosis and treatment of Idiopathic Pulmonary Fibrosis (IPF). Methods: In this study, the assessment of 68Ga-labeled linear and cyclic forms of the RP832c peptide, which demonstrate a specific affinity for CD206 macrophages, was performed to evaluate efficacy for CD206 imaging through PET/CT, biodistribution studies, and CD206 staining in a bleomycin-induced lung injury mouse model (BLM). This model serves as a representative framework for inflammation and fibrosis. Results: The findings reveal significant peak PET/CT signals (SUV means), ID/gram values, and CD206 staining scores in lung tissues at one week post bleomycin instillation, likely due to the heightened expression of CD206 in the bleomycin-induced lung injury model. In contrast, the healthy mice exhibited no detectable CD206 staining, lower PET signals, and reduced radiopharmaceutical accumulation in lung tissues at the same timepoint. Conclusions: These findings suggest that both linear and cyclic [68Ga]Ga-RP832c may function as promising PET imaging agents for CD206 macrophages, and thereby a strategy to non-invasively explore the role of macrophages during fibrogenesis.

Immune checkpoint inhibitor (ICI) therapy is effective and in routine clinical use for various cancers, but accurately identifying which patients will respond remains a significant challenge. The PET agent 18F-FDG has uptake by cancer cells as well as inflammation induced by ICI therapy, complicating and often limiting the utility of 18F-FDG for early response assessment during ICI therapy. An imaging agent that accurately distinguishes responders from nonresponders early in the course of ICI therapy could enable intensification or change of therapy for nonresponders. In this study, the 18F-labeled amino acid 18F-MeFAMP, a fluorinated analog selectively targeting system A amino acid transport, was compared with 18F-FDG in the MC38 syngeneic mouse model of ICI therapy. 18F-MeFAMP was chosen because of the relatively low uptake of system A substrates in inflammatory tissues combined with growing evidence suggesting system A transporters are involved in immunotherapy. Methods: PET/CT imaging was used to compare tumor uptake of 18F-MeFAMP with tumor uptake of 18F-FDG before and 6 d after starting dual ICIs in MC38 tumor-bearing female C57BL/6 mice. SUVs, biologic tumor volumes, and total lesion activity were measured along with selected tumor-to-organ ratios. Histogram analysis of tracer uptake was performed to assess differences in tumor activity distribution between responders and nonresponders. Results: 18F-FDG showed no significant differences at baseline or after ICI regardless of response. In contrast, 18F-MeFAMP SUVs defined using a 40% of SUVmax threshold (SUV40%) decreased significantly in responders (-60.0% ± 15.6%, P < 0.0001), whereas nonresponders showed no significant change (+45.5% ± 51.2%, P = 0.09). Similar patterns were observed with SUVmax, biologic tumor volume, and total lesion activity measures with 18F-MeFAMP. Histogram analysis revealed significant 18F-MeFAMP uptake differences between groups before and after imaging (P < 0.05). 18F-MeFAMP demonstrated low uptake in common metastatic sites, including liver, lungs, and brain. Conclusion: 18F-MeFAMP better detected early ICI response than 18F-FDG with favorable whole-body imaging properties. These findings support further investigation of 18F-MeFAMP for early evaluation of response to ICI and the role of system A substrates in cancer and immune cells before and during ICI.

The effects of obesity on cancer treatment efficacy remain unclear, as both beneficial and detrimental modulations of the tumor immune microenvironment have been reported. We compared 68Ga-NOTA-GZP (βAla-Gly-Gly-Ile-Glu-Phe-Asp-CHO) PET images with those obtained with the gold standard, 18F-FDG PET, to quantify biologic variations in a diet-induced obesity model of triple-negative breast cancer to understand how obesity influences the tumor immune landscape and response to immunotherapy. Methods: C57BL6/J mice were fed a high-fat diet (n = 24) or low-fat diet (n = 18) for 14 wk. EO771 tumor-bearing mice were treated with a fixed or weight-based dose of saline or checkpoint-blockade immunotherapy, and tumor volume was evaluated for long-term response. Mice were imaged via 68Ga-NOTA-GZP PET on day 7 to quantify immune activation, and those images were compared with 18F-FDG PET images to characterize changes in glucose metabolism on days 0 and 6. SUV was quantified from imaging data, and a cohort of mice was euthanized to validate biologic changes via flow cytometry. Results: Mice fed a high-fat diet demonstrated increased tumor glucose metabolism at baseline, as measured by 18F-FDG PET. No significant differences were observed in 18F-FDG SUV for responder tumors on day 6. The 68Ga-NOTA-GZP PET signal was increased in tumors responsive to immunotherapy on day 7 and was highly sensitive in predicting response via analysis of receiver-operating-characteristic curves. Conclusion: Obesity decreases response to immunotherapy by altering metabolism and the tumor immune microenvironment. 68Ga-NOTA-GZP PET imaging is a sensitive and predictive imaging biomarker of immunotherapy response, but weight-based dosing is needed to achieve effective changes in tumor volume.

Abstract Background Immune-positron emission tomography (PET) imaging with tracers that target CD8 and granzyme B has shown promise in predicting the therapeutic response following immune checkpoint blockade (ICB) in immunologically “hot” tumors. However, immune dynamics in the low T-cell infiltrating “cold” tumor immune microenvironment during ICB remain poorly understood. This study uses molecular imaging to evaluate changes in CD4 + T cells and CD8 + T cells during ICB in breast cancer models and examines biomarkers of response. Methods [89Zr]Zr-DFO-CD4 and [89Zr]Zr-DFO-CD8 radiotracers were used to quantify changes in intratumoral and splenic CD4 T cells and CD8 T cells in response to ICB treatment in 4T1 and MMTV-HER2 mouse models, which represent immunologically “cold” tumors. A correlation between PET quantification metrics and long-term anti-tumor response was observed. Further biological validation was obtained by autoradiography and immunofluorescence. Results Following ICB treatment, an increase in the CD8-specific PET signal was observed within 6 days, and an increase in the CD4-specific PET signal was observed within 2 days in tumors that eventually responded to immunotherapy, while no significant differences in CD4 or CD8 were found at the baseline of treatment that differentiated responders from nonresponders. Furthermore, mice whose tumors responded to ICB had a lower CD8 PET signal in the spleen and a higher CD4 PET signal in the spleen compared to non-responders. Intratumoral spatial heterogeneity of the CD8 and CD4-specific PET signals was lower in responders compared to non-responders. Finally, PET imaging, autoradiography, and immunofluorescence signals were correlated when comparing in vivo imaging to ex vivo validations. Conclusions CD4- and CD8-specific immuno-PET imaging can be used to characterize the in vivo distribution of CD4 + and CD8 + T cells in response to immune checkpoint blockade. Imaging metrics that describe the overall levels and distribution of CD8 + T cells and CD4 + T cells can provide insight into immunological alterations, predict biomarkers of response to immunotherapy, and guide clinical decision-making in those tumors where the kinetics of the response differ.

Chemokine receptors are important components of cellular signaling and play a critical role in directing leukocytes during inflammatory reactions. Their importance extends to numerous pathological processes, including tumor differentiation, angiogenesis, metastasis, and associations with multiple inflammatory disorders. The necessity to monitor the in vivo interactions of cellular chemokine receptors has been driven the recent development of novel positron emission tomography (PET) imaging agents. This imaging modality provides non-invasive localization and quantitation of these receptors that cannot be provided through blood or tissue-based assays. Herein, we provide a review of PET imaging of the chemokine receptors that have been imaged to date, namely CXCR3, CXCR4, CCR2, CCR5, and CMKLR1. The quantification of these receptors can aid in understanding various diseases, including cancer, atherosclerosis, idiopathic pulmonary fibrosis, and acute respiratory distress syndrome. The development of specific radiotracers targeting these receptors will be discussed, including promising results for disease diagnosis and management. However, challenges persist in fully translating these imaging advancements into practical therapeutic applications. Given the success of CXCR4 PET imaging to date, future research should focus on clinical translation of these approaches to understand their role in the management of a wide variety of diseases.

ABSTRACT:Cancer immunotherapy, including checkpoint blockade and cellular therapy, has become a cornerstone in cancer treatment. However, understanding the factors driving patient response or resistance to these therapies remains challenging. The dynamic interplay between the immune system and tumors requires new approaches for characterization. Biopsies and blood tests provide valuable information, but their limitations have led to increased interest in positron emission tomography (PET)/computed tomography imaging to complement these strategies. The noninvasive nature of PET imaging makes it ideal for monitoring the dynamic tumor immune microenvironment. This review discusses various PET imaging approaches, including immune cell lineage markers, immune functional markers, immune cell metabolism, direct cell labeling, and reporter genes, highlighting their potential in targeted immunotherapies and cell-based approaches. Although PET imaging has limitations, its integration into diagnostic strategies holds promise for improving patient outcomes and accelerating drug development in cancer immunotherapy.

Context: Hypoxia within the tumor microenvironment is a critical factor influencing the efficacy of immunotherapy, including immune checkpoint inhibition. Insufficient oxygen supply, characteristic of hypoxia, has been recognized as a central determinant in the progression of various cancers. The reemergence of evofosfamide, a hypoxia-activated prodrug, as a potential treatment strategy has sparked interest in addressing the role of hypoxia in immunotherapy response. This investigation sought to understand the kinetics and heterogeneity of tumor hypoxia and their implications in affecting responses to immunotherapeutic interventions with and without evofosfamide. Purpose: This study aimed to investigate the influence of hypoxia on immune checkpoint inhibition, evofosfamide monotherapy, and their combination on colorectal cancer (CRC). Employing positron emission tomography (PET) imaging, we developed novel analytical methods to quantify and characterize tumor hypoxia severity and distribution. Procedures: Murine CRC models were longitudinally imaged with [18F]-fluoromisonidazole (FMISO)-PET to quantify tumor hypoxia during checkpoint blockade (anti-CTLA-4 + and anti-PD1 +/- evofosfamide). Metrics including maximum tumor [18F]FMISO uptake (FMISOmax) and mean tumor [18F]FMISO uptake (FMISOmean) were quantified and compared with normal muscle tissue (average muscle FMISO uptake (mAvg) and muscle standard deviation (mSD)). Histogram distributions were used to evaluate heterogeneity of tumor hypoxia. Findings: Severe hypoxia significantly impeded immunotherapy effectiveness consistent with an immunosuppressive microenvironment. Hypoxia-specific PET imaging revealed a striking degree of spatial heterogeneity in tumor hypoxia, with some regions exhibiting significantly more severe hypoxia than others. The study identified FMISOmax as a robust predictor of immunotherapy response, emphasizing the impact of localized severe hypoxia on tumor volume control during therapy. Interestingly, evofosfamide did not directly reduce hypoxia but markedly improved the response to immunotherapy, uncovering an alternative mechanism for its efficacy. Conclusions: These results enhance our comprehension of the interplay between hypoxia and immune checkpoint inhibition within the tumor microenvironment, offering crucial insights for the development of personalized cancer treatment strategies. Non-invasive hypoxia quantification through molecular imaging evaluating hypoxia severity may be an effective tool in guiding treatment planning, predicting therapy response, and ultimately improving patient outcomes across diverse cancer types and tumor microenvironments. It sets the stage for the translation of these findings into clinical practice, facilitating the optimization of immunotherapy regimens by addressing tumor hypoxia and thereby enhancing the efficacy of cancer treatments.

PURPOSE:This study aimed to provide a novel noninvasive method to quantify abscopal immune activation and predict combinational treatment response using [68Ga]-NOTA-GZP positron emission tomography (PET) imaging. METHODS AND MATERIALS:4T1 breast cancer cells were implanted bilaterally in the mammary fat pad of Balb/c mice and Lewis's lung cancer cells (LLC) were implanted bilaterally on the shoulders of C57/Bl6 mice. One of the tumors received a single fraction of 12 Gy irradiation followed by combination of concurrent PD-1 and CTLA-4 inhibitors or controls. Tumor growth of the irradiated and nonirradiated tumors was measured and compared with 12 Gy irradiation only, checkpoint inhibitor only, and no treatment control group. Changes in granzyme B activity were assessed with [68Ga]-NOTA-GZP PET imaging from baseline and every 3 days until day 9. RESULTS:In the 4T1 model, concurrent treatment with dual checkpoint inhibitors and radiation resulted in reduction of the irradiated tumor volume at day 30. At this same time point, the nonirradiated tumor volume for combination treatment decreased significantly, consistent with abscopal immune activation. Similarly, in the LLC model, concurrent treatment inhibited tumor growth on the nonirradiated tumor at day 15. On day 9, granzyme B PET signal in both 4T1 and LLC models was significantly higher in the nonirradiated tumors that responded to concurrent treatment compared with subsequent nonresponding tumors. A similar lack of granzyme B signal was observed in the nonirradiated tumors from mice that received radiation or checkpoint inhibitors only and control tumors. Receiver operating characteristic analysis identified a PET threshold of 1.505 and 1.233 on day 9 that predicted treatment response in 4T1 and LLC models, respectively. CONCLUSIONS:[68Ga]-NOTA-GZP PET imaging was able to noninvasively predict abscopal immune activation before subsequent tumor volume changes after combination treatment. It provides a potential translational paradigm for investigating distal immune activation postradiation in a clinical setting.

Abstract Introduction: Obesity is known to reduce efficacy for cancer treatment patients and is thought to modulate the tumor microenvironment and glucose metabolism. In this study, we aim to quantify obesity-induced differences in the breast cancer tumor microenvironment with positron emission tomography (PET) imaging of hypoxia, glucose metabolism, and granzyme B. These experiments are testing the hypothesis that tumors in obese mice have increased glucose metabolism and hypoxia, measured by [18F]F-fluorodeoxyglucose (FDG) and [18F]F-fluoromisonidazole (FMISO) uptake. We also hypothesize that immunotherapy (IMT) increases immune activation, as measured by [68Ga]Ga-Granzyme B peptide (GZP), and that obese mice will respond less to immunotherapy than lean mice. Methods: C57/Bl6 mice were placed on a high-fat diet (HFD) with 60% kcal fat (n=18) or low-fat diet (LFD) composed of 10% kcal fat (n=18) to induce obesity. Blood serum was collected for analysis of cholesterol levels prior to tumor implantation. Body weight and food intake were measured weekly. 14 weeks after initiating each diet, 5x105 E0771 cells were implanted into the 3rd mammary fat pad and allowed to grow to 50-150mm3. Tumor-bearing mice were imaged with [18F]F-FDG and [18F]F-FMISO PET to measure glucose metabolism and hypoxia, respectively prior to treatment. Following imaging, mice began treatment with either saline (n=6/diet) or checkpoint blockade IMT with 200 ug anti-PD-1 and 100 ug anti-CTLA-4 (n=12/diet). After 3 doses of IMT, mice were imaged with [68Ga]Ga-GZP PET to measure immune activation. Tumor volume measurements and treatment administration occurred every 3 days. Metrics of standardized uptake value (SUV) including mean, max, and peak were quantified from imaging data. A one-way independent ANOVA or independent t-test was used to compare differences between groups. Results: HFD-fed mice had significantly increased body weight (p<0.0001) and serum cholesterol (p=0.005) compared to LFD-fed mice. SUVmean for tumor hypoxia (p=0.003) and glucose metabolism (p=0.02) were significantly increased at baseline in HFD-fed mice compared to LFD-fed mice. IMT increases immune activation for both LFD-fed (p=0.009) and HFD-fed mice (p=0.05). IMT treatment significantly reduces tumor burden for LFD mice (p<0.0001), but not HFD mice (p=0.096). Conclusions: Obesity plays a role in the reduction of the effectiveness of immunotherapy. At baseline, tumors of obese mice have increased tumor glucose metabolism and hypoxia. Although immunotherapy increases immune activation, this is not sufficient to reduce tumor burden or improve overall survival. Understanding obesity-induced differences in tumor biology may provide imaging-directed treatment decision making. Citation Format: Shannon E. Lynch, Corinne Crawford, Addison Hunt, Luke Sligh, Benjamin M. Larimer, Suzanne E. Lapi, Anna G. Sorace. PET imaging to characterize the tumor microenvironment in a breast cancer model of obesity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4154.

This study aimed to provide a novel noninvasive method to quantify abscopal immune activation and predict combinational treatment response using [

Background: Pancreatic ductal adenocarcinoma (PDAC) is the 3rd leading cause of cancer related death with a 5-year survival rate at 11%. New systemic therapies for patients with PDAC are desperately needed. Upregulation or exposure of a new protein on the tumor cell surface can serve as a therapeutic or diagnostic target. Here, we highlight our recently developed strategy to induce localization of the reticular protein calreticulin (CALR) to the cell surface in PDAC cells and its subsequent detection using a novel radiolabeled peptide. Methods: Surface translocation of CALR was detected by flow cytometry, western blot, and total internal reflection fluorescence (TIRF) microscopy in PDAC cells treated with either doxorubicin or gemcitabine. The radionuclide-binding chelator ‘DOTA’ was covalently linked to a CALR-specific peptide ‘KLGFFKR’ and then labeled with 68Ga. Samples were analyzed on HPLC with an average radiolabeling efficiency of 93%. Mice bearing Panc02 allografts were treated with doxorubicin for 24h, injected with ~3 MBq (5 μg) of radiopeptide, and sacrificed after 1 hour to determine biodistribution. Results: Using flow cytometry, we found that treating PDAC cells with doxorubicin or gemcitabine increased both the total number and the median fluorescence intensity of surface staining for live cells expressing CALR. When membrane proteins were isolated from PDAC cells treated with doxorubicin at various time points, a peak in surface CALR protein was detected at 30 minutes with persistent expression lasting for 24 hours. TIRF microscopy showed that Panc02 cells treated with doxorubicin had approximately a 2-fold higher surface CALR expression as detected by enhanced membrane fluorescence compared to controls. In vivo, our novel [68Ga]-CALR peptide showed rapid clearance through the kidneys with no significant uptake in vital organs (n=4). In Panc02 allograft-bearing mice treated intratumorally with either vehicle or doxorubicin, biodistribution analysis after radiopeptide injection showed a significant increase in radiopeptide uptake in the treated tumors (n=6, p < 0.05). Conclusions: CALR is translocated to the cell surface in PDAC cells, where it can subsequently be targeted by a novel radiopeptide agent. Future studies are needed to determine if induced CALR can be targeted for therapeutic effect. Citation Format: Rachael Guenter, Maxwell Ducharme, Brendon Herring, Tejeshwar C. Rao, Odalyz Montes, Tyler McCaw, Herbert Chen, Suzanne E. Lapi, Benjamin Larimer, J. Bart Rose. Using a novel [68Ga]-radiolabeled peptide to detect cell surface expression of calreticulin in pancreatic adenocarcinoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3578.