Peritoneal carcinomatosis (PC) is a common yet deadly manifestation of gastrointestinal cancers, with few effective treatments. To identify targetable determinants of peritoneal metastasis, we focused on appendiceal adenocarcinoma (AC), which metastasizes almost exclusively to the peritoneum. No stable preclinical models of AC exist, limiting drug discovery and representing an unmet clinical need. We establish a stable biobank of 16 long-term cultured AC patient-derived tumor organoids (PDTOs). We establish an organoid orthotopic intraperitoneal xenograft model that recapitulates diffuse PC and show that PC organoids retain increased metastatic capacity, decreased growth-factor dependency, and decreased sensitivity to standard-of-care chemotherapy relative to matched primary AC organoids. Single-cell profiling reveals dedifferentiation from differentiated states in primary AC into intestinal stem cell and fetal progenitor states in AC-PC, with upregulation of oncogenic signaling pathways. We identify KRASMULTI-ON inhibitor RMC-7977 and the Wnt-targeting tyrosine kinase inhibitor WNTinib as clinically actionable strategies to target AC-PC more effectively.

264 Background: In mCRPC, standard risk models use baseline clinical and blood-based biomarkers, excluding imaging. This study developed a new predictive model incorporating baseline and early on-treatment clinical, blood, and imaging biomarkers to inform overall survival (OS). The training set was derived from COU-AA-302 and validated using Alliance 031201 (A031201). Eligibility required patients to have at least two CT and bone scans within 6 months of randomization (COU-AA-302: n = 785; A031201: n = 582). Methods: Conventional biomarker data included PSA, albumin, alkaline phosphatase, hemoglobin, and ECOG status. Imaging data included > 1000 radiomic features and the automated Bone Scan Index (aBSI). Six-month biomarker trajectories were summarized using the intercept and slope from a random effects model, yielding over 2000 risk biomarkers. Due to the high dimensionality, an elastic net proportional hazards model was applied. Predictive concordance probability, which evaluates the ability to distinguish between long-term and short-term survivors, and R-square, a calibration measure that indicates how accurately the model predicts actual survival time were determined. Results: 46 biomarkers were selected from over 2000, including 9 blood-based, 4 from aBSI, 1 tumor volume measure, and the remaining from radiomic features. In contrast, the non-imaging risk model contained 10 risk factors. Risk scores from these models were categorized into low, moderate, and high-risk groups. The median survival per group revealed significant separation, with a difference of over 30 months between low and high-risk groups. This separation was observed regardless of whether imaging was included in the model. Conclusions: This is the first model for mCRPC to integrate pre-treatment and early on-treatment serum, imaging, and clinical data from two large phase 3 trials, with external validation. While both imaging and non-imaging models showed good predictive accuracy, imaging did not significantly improve the discrimination of long-term from short-term survivors or enhance the precision of the model-based survival predictions. Clinical + Serum biomarkers with imaging Clinical + Serum biomarkers without imaging Total # of risk factors 46 10 # of clinical/serum factors 9 10 # of radiomics factors 32 0 # of aBSI factors 4 0 # of tumor volume factors 1 0 Median Survival (mos) Low risk group 55 54 Moderate risk group 38 39 High risk group 20 21 Predictive Concordance Probability 0.74 0.73 R-square Measure of Calibration 0.37 0.39

Rationale: Pretargeted radioimmunotherapy (PRIT), which combines systemic antibody-based targeting with ionizing radiation, is promising for treating liver metastases in patients with colorectal cancer (CRC). Previously, we established a three-step DOTA-PRIT regimen to deliver DOTA radiometal payloads to CRC using an anti-tumor/anti-DOTA bispecific antibody (BsAb) targeting cell surface glycoprotein A33 (GPA33), a tumor antigen target expressed on over 95% of primary and metastatic CRC; a clearing agent; and a monovalent 177Lu radiohapten called [177Lu]Lu-ABD. More recently, we developed a bivalent 177Lu radiohapten called [177Lu]Lu-Gemini to enhance tumor uptake and radiohapten retention. Here, we aimed to compare the efficacy and safety of bivalent vs. monovalent three-step DOTA-PRIT regimens in orthotopic CRC liver metastasis models, to mimic a clinical path forward. Methods: We established two orthotopic CRC liver metastasis models by inoculating either SW1222-luc (GPA33high) or LoVo (GPA33low) human CRC cells in athymic nude mice under ultrasonographic guidance. Tumor targeting efficacy and dosimetry of the radiohaptens were compared using ex vivo biodistribution studies, SPECT/CT, and quantitative autoradiography. We also performed a DOTA-PRIT experiment to compare the efficacy and safety profiles of bivalent (single-cycle [177Lu]Lu-Gemini, 48 h pretargeting interval) vs. monovalent (multicycle [177Lu]Lu-ABD, 24 h pretargeting interval) three-step DOTA-PRIT regimens, each designed to deliver comparable total radiation doses to tumors (around 50 Gy). Results: Both radiohaptens demonstrated efficient SW1222-luc tumor targeting, with [177Lu]Lu-Gemini showing superior targeting and tumor activity retention compared with [177Lu]Lu-ABD. In LoVo tumors, [177Lu]Lu-Gemini showed superior targeting, while [177Lu]Lu-ABD showed negligible targeting. Dosimetry estimates revealed higher SW1222-luc tumor mean absorbed doses for [177Lu]Lu-Gemini (119.88 cGy/MBq, 48 h pretargeting interval) compared with [177Lu]Lu-ABD (32.88 cGy/MBq, 24 h pretargeting interval), with more favorable blood and kidney therapeutic indices (50 and 9 for [177Lu]Lu-Gemini, and 15 and 5 for [177Lu]Lu-ABD, respectively). In the DOTA-PRIT experiment, both monovalent (injected activity: 3 × 44.4 MBq, 133.2 MBq total) and bivalent (injected activity: 44.4 MBq total) radiohapten regimens increased the median survival of treated mice compared with controls: 71 days for [177Lu]Lu-ABD-treated mice, 81 days for [177Lu]Lu-Gemini-treated mice, and 18 days for controls, without a statistical difference between treatment groups. Treatments were well tolerated, without significant weight loss or hematologic changes. Radiation-induced injuries were not identified histologically in the kidneys or bone marrow of mice submitted for necropsy. Conclusions: Our study demonstrates the exceptional benefit of a multivalent radiohapten strategy when treating an advanced model of CRC liver metastasis. Three-step GPA33 DOTA-PRIT with 177Lu-Gemini demonstrated that multivalency 1) improves PRIT therapeutic indices for blood and kidney and 2) has the potential to greatly reduce the administered activity without compromising the efficiency of the PRIT platform in clinically relevant models of target-rich and target-poor metastatic CRC.

Malignant appendiceal epithelial tumors (AC) present a rare and diverse group of neoplasms.1 Treatment options are limited especially with peritoneal recurrence. Alpha and beta particle therapy using radioisotopes, especially when delivered by a pre-targeted radioimmunotherapy (PRIT) approach, can be effective with a wide therapeutic window. SADA-PRIT exploits an initial long plasma half-life of SADA tetramers for increased tumor uptake and when unbound, their disassembly to monomer with rapid renal clearance, thereby maximizing therapeutic indices across all critical normal tissues.2, 3 6-week old female BRG mice with subcutaneous (sq) or intraperitoneal (IP) patient derived AC xenograft (PDX) were treated with two doses of α-PRIT using [225Ac]-Proteus ([225Ac]Pr, or three doses of β- PRIT using [177Lu]DOTA, 4 all delivered by anti-GPA33-SADA. Isotope doses were chosen based on previous dosimetry and given one week apart.2 Toxicity, tumor response and survival were followed up to 180 days. Log rank Mantel Cox statistics was used for median survival (MS) comparison. Positive immunohistochemistry was confirmed in 26/26 AC surgical samples and 4/4 Patient derived xenografts (PDX). Similar IHC patterns were seen for B7H3 and HER2. Using GPA33-SADA in sq models α-PRIT (2 x 74 kBq 225Ac) produced an MS of 70 days, compared to 21 days in the antibody-only and [225Ac]Pr-only control group; β-PRIT (3 x 111 MBq) provided an MS of 68 days, compared to 27 days in the antibody-only and [177Lu]DOTA-only control groups. In the IP AC PDX model, all 10/10 mice in the α-PRIT group are healthy and tumor-free at 100+ days, compared to an MS of 42d in both antibody-only and [225Ac]Pr-only control groups (p <0.0001). Neither [225Ac]Pr or [177Lu]DOTA caused dose-limiting acute (<30 days) or long term (to 6 months) toxicities. Transient weight loss and leukopenia were reversible. Mild tubular degeneration was documented by histology at necropsy. Use of GPA33-SADA to deliver α-PRIT and β-PRIT induced major tumor responses and significantly prolonged survival in sq AC and ip AC tumor models. Minimal myeloid, renal, hepatic, or neural toxicities were seen. IHC results support future testing of cocktailing of SADAs (specific for GPA33, B7H3, and HER2) to overcome tumor heterogeneity while reducing on-target off-tumor side effects. Nicole Aguirre, Michelle Kim, Hong-Fen Guo, Karina Leung, Sebastian E. Carrasco, Irene Cheung, Michael B. Foote, Andrea Cercek, Sarah M. Cheal, Darren R. Veach, Steven M. Larson, Georgios Karagkounis, Garrett Nash, Nai-Kong Cheung. Pre-targeted radioimmunotherapy using self-assembling dis-assembling (SADA) bispecific antibodies for the treatment of appendiceal carcinoma [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 2150.

Purpose: Curative therapies remain a significant unmet need for advanced human colorectal cancer (CRC). The aim of this study was to establish a 3-step 225Ac-DOTA pretargeted radioimmunotherapy (PRIT) system for human CRC, targeting two individual antigens: glycoprotein A33 (GPA33) and human epidermal growth factor receptor 2 (HER2). Methods: In vitro cellular uptake and internalization assays, as well as survival assays (colony forming) were performed in GPA33- and HER2-positive SW1222 human CRC cells. In vivo biodistribution and therapy studies were performed with two human CRC xenograft mouse models. Results: For both antigens, treatment with up to 74 kBq 225Ac-DOTA-PRIT in SW1222-tumored mice significantly enhanced overall survival in comparison to controls, including histologic cures. The uptake of 225Ac at the tumor correlated with antigen expression (antigen expression for GPA33:HER2 is 5:1). Cellular assays showed no significant differences in internalized fraction or nucleus-associated radioactivity between the two targets. GPA33 had a higher absorbed dose to the nucleus (1.3 Gy vs. 0.65 Gy for HER2), resulting in reduced clonogenic survival. A single cycle of either GPA33 or HER2 DOTA-PRIT (37 kBq; 193.31 Gy and 113.91 Gy (relative biological effectiveness [RBE] = 5), respectively) was equally effective. No chronic nephrotoxicity was seen at ≤ 20 Gy (RBE = 5). The efficacy of GPA33-directed 225Ac-DOTA-PRIT was also confirmed in a patient-derived xenograft model. Conclusion: In summary, 225Ac-DOTA-PRIT to GPA33 or HER2 was highly effective and safe in preclinical models of human CRC. Tumor response to treatment could not be predicted by nuclear absorbed dose alone, highlighting the importance of comprehensive micro- and macro-dosimetry.

BACKGROUND:18F-FDG PET-CT-based host metabolic (PETMet) profiling of non-tumor tissue is a novel approach to incorporate the patient-specific response to cancer into clinical algorithms. MATERIALS AND METHODS:A prospectively maintained institutional database of gastroesophageal cancer patients was queried for pretreatment PET-CTs, demographics, and clinicopathologic variables. 18F-FDG PET avidity was measured in 9 non-tumor tissue types (liver, spleen, 4 muscles, 3 fat locations). Logistic and Cox regression were used to model pathologic response (PR) and overall survival (OS) respectively. Classification and regression tree (CART) and random forest modeling were employed to create decision trees and identify PETMet features associated with outcome. RESULTS:Two-hundred and one patients with distal gastroesophageal (48 %) or gastric (52 %) adenocarcinoma were included. PET-CT-derived scores were independently associated with PR after adjusting for clinical variables. CART and Random Forest methods identified critical split points of non-tumor tissue 18F-FDG avidity that can classify patients and predict PR. PET-CT risk groups created from decision trees predicted PR significantly better than the clinical model (p < 0.001). Specifically, an elevated erector spinae-to-gluteal fat 18F-FDG avidity ratio (≥2.7) combined with low 18F-FDG avidity in the spleen (<2.9) and rectus femoris (<0.52) predict PR. No advantage of PET-CT risk groups was seen for predicting OS (p = 0.155). CONCLUSIONS:Pretreatment host PETMet features may be useful for predicting PR after neoadjuvant therapy in gastroesophageal cancer. Unsupervised decision trees indicate that low 18F-FDG avidity in visceral fat, subcutaneous fat, and muscle result in the most favorable PR, suggesting that systemic hypermetabolism adversely impacts prognosis.

BACKGROUND:Median survival for patients with Diffuse Intrinsic Pontine Glioma (DIPG) is 8-12 months. METHODS:A phase 1, open label, 3 + 3 dose-escalation trial delivered radiolabeled 124I-Omburtamab, targeting B7-H3, using MR-guided stereotactic convection-enhanced delivery (CED) into the brainstem of pediatric DIPG patients. CED was performed after completion of standard-of-care external-beam radiation therapy (EBRT). Fifty children were treated and evaluable. 124I-Omburtamab activity was escalated from 0.25 to 10.0 mCi (9.25-370 MBq) and volume escalated from 0.25 mL to 10.0 mL with serial PET/MRI post administration. Safety was the primary outcome. National Cancer Institute Common Terminology Criteria for Adverse Events were assessed for 30 days following CED of 124I-Omburtamab. Secondary outcomes included overall survival and lesion-to-whole-body absorbed dose ratio. RESULTS:The maximum tolerated activity per study protocol was determined to be 6mCi (222 MBq). The overall mean (±SD) total absorbed dose in the lesion per unit injected activity was 35.2 ± 18 cGy/MBq with a high lesion-to-whole-body absorbed dose ratio averaging 816, across all activity levels. Eleven patients had treatment-related grade 3 CNS toxicities with no grade-4 or -5 CNS toxicities. Five dose-limiting toxicity events occurred. Median survival was 15.29 months from diagnosis (95% CI: 12.20-16.83 months). Survival rate estimates at 1, 2, and 3 years were 65.4% (CI 53.3-80.1%), 18.4% (CI: 10.2-33.2%), and 11.7% (CI: 5.3-25.7%), respectively. CONCLUSIONS:Administration of 124I-Omburtamab via CED is a safe treatment option for DIPG, with a maximum tolerated activity level identified. This study represents the first-in-human theranostic use of a 124I radiopharmaceutical, simultaneously, as an imaging and therapeutic agent. TRIAL REGISTRATION:NCT01502917; https://clinicaltrials.gov/study/NCT01502917.

Radioimmunotherapy using 225Ac, a highly cytotoxic α-particle emitter, has potential for treating advanced breast cancer, especially human epidermal growth factor receptor 2 (HER2)-positive cases. We use a pretargeted radioimmunotherapy (PRIT) approach consisting of a 3-step intravenous regimen (step 1: bispecific anti-HER2/anti-DOTA antibody; step 2: clearing agent; step 3: 225Ac-radiolabeled Proteus DOTA, or [225Ac]Ac-Pr). Our goal was to establish curative 225Ac-PRIT with high therapeutic indices. Methods: The impact of [225Ac]Ac-Pr specific activity was evaluated in the BT-474 breast xenograft model. We tested the effects of [225Ac]Ac-Pr dosing during PRIT on tumor-targeting efficiency and tissue biodistribution. Using a 225Ac-PRIT regimen consisting of a ratio of 1.19 nmol of bispecific antibody to 0.60-0.66 nmol of [225Ac]Ac-Pr, we evaluated therapy in the BT-474 model and a patient-derived xenograft model. BT-474-tumor-bearing mice were treated with 1 or 2 cycles of 225Ac-PRIT (37 kBq/cycle) separated by 1 wk. A dose escalation study was performed on the BT-474 model to establish an absorbed radiation dose of approximately 40 Gy (relative biological effectiveness [RBE], 5) as a nephrotoxic dose, as no such histologic findings were observed in prior studies at the 20.7-Gy (RBE, 5) renal dose level. Results: In the BT-474 model, 100% (20/20) achieved complete responses and histologic cure in 17 of 20 (85%) of the treated animals. One-cycle and 2-cycle treatments were equally effective. Treatments were well tolerated, with no chronic radiation toxicity documented during necropsy at 175 d. Dosimetry estimates (RBE, 5) per 37 kBq administered for tumors and kidneys were 210 and 3.5 Gy, respectively. In the patient-derived xenograft model, a single 225Ac-PRIT treatment led to 60% (3/5) complete response and prolonged survival (>93 d) versus no treatment (30 d; P = 0.0185). Lastly, a 225Ac-PRIT regimen was identified that induces severe chronic nephrotoxicity (41.4 Gy/592 kBq; RBE, 5). Conclusion: Safe and effective 225Ac-PRIT regimens were developed in 2 preclinical models of advanced HER2-positive human breast cancer with tumor cure without dose-limiting nephrotoxicity. This study establishes crucial preclinical dosimetry benchmarks for 225Ac-PRIT and provides a compelling rationale for its advancement into the clinic.

Background: Differentiated thyroid cancer (DTC) is the most prevalent cancer of thyroid and is among the most frequently diagnosed cancers in the United States. The practice guidelines of the American Thyroid Association (ATA) for DTC management in adult patients (previously combined with thyroid nodules) were published initially in 1996, with subsequent revisions based on advances in the field. The goal of this update is to provide clinicians, patients, researchers, and those involved in health policy with rigorous, comprehensive, and contemporary guidelines to assist in the management of adult patients with DTC, emphasizing the patient journey beginning with a thyroid cancer diagnosis. Methods: The questions addressed were based, in part, on prior versions of the guidelines, with input from a larger, more diverse complement of stakeholders. The panel included members from multiple specialties involved in thyroid cancer care, including a patient advocate and an expert in systematic reviews/meta-analyses/guidelines who educated and supported task force members. The panel conducted systematic literature reviews to inform the recommendations and commissioned two additional systematic reviews. Published English-language articles were eligible for inclusion, with a final search date of July 1, 2024. A modified Grading of Recommendations Assessment, Development and Evaluation system was used for critical appraisal of evidence and determining the quality of data. The guidelines panel had editorial independence from the ATA. Competing interests of task force members were pre-vetted, regularly updated, communicated with task force members, and assessed and managed by ATA leadership and the Clinical Practice Guidelines and Statements Committee. Results: These revised guidelines begin with the initial cancer diagnosis and continue with recommendations for staging and risk assessment, initial treatment decisions, assessment of treatment responses, monitoring approaches, diagnostic testing, and subsequent therapies based on the strength of evidence for response and consideration of side effects and outcomes. Patient-reported outcomes and identified areas of need for additional high-quality research are highlighted. Conclusions: These revised evidence-based recommendations inform clinical decision-making in the management of DTC that reflect the changing science and optimize the evidence-based clinical care of patients throughout their journey with DTC. Critical areas of need for additional research are highlighted.

TPS752 Background: CAIX is a cell surface glycoprotein expressed in >90% of ccRCC but rarely in normal tissues, providing a target for imaging and therapeutic application. Radiolabeling the anti-CAIX monoclonal antibody girentuximab with 89Zr has shown promise as a novel PET tracer and labeling with 177Lu promise as a therapeutic agent in ccRCC (Muselaers, Eur Urol, 2016). Targeted delivery of radiation to ccRCC cells may prime the immune response by enhancing tumor antigen presentation, providing rationale for combining 177Lu-girentuximab with the anti-PD-1 antibody, nivolumab. This phase 2, open-label, single arm study (NCT05239533) is being conducted to evaluate 177Lu-girentuximab in combination with nivolumab in pts with previously treated ccRCC. Methods: Pts with biopsy-proven ccRCC, progressive disease after prior systemic therapy including ≥1 immunotherapy (IO) agent, adequate organ/marrow function, and ≥1 evaluable lesion by RECIST 1.1 that is also avid on 89Zr-girentuximab PET will be enrolled. There is no limit on number of prior lines of systemic therapy, but pts who stopped IO for immune toxicity are excluded. Treatment consists of 177Lu-girentuximab every 12-14 weeks for a maximum of 3 doses plus nivolumab 240mg every 2 weeks until progressive disease (PD) or unacceptable toxicity. Due to expected cumulative myelosuppression, each subsequent 177Lu-girentuximab dose to the same patient is reduced by 25% (dose 2 = 75% of dose 1; dose 3 = 75% of dose 2). Tumor imaging is performed every 12 weeks. Pts will be evaluated in a safety lead-in phase followed by an expansion phase. In the safety lead-in phase, the MTD of 177Lu-girentuximab in combination with nivolumab will be determined with a 3+3 design using a starting dose of 1804 MBq/m2 (75% of single agent MTD). For cohort 2, dose escalation to 2405 MBq/m2 (single agent MTD) or de-escalation to 1353 MBq/m2 will be based on dose-limiting toxicities. In the expansion phase, a Simon 2-stage optimal design is used to evaluate the primary endpoint of response rate by RECIST 1.1 within 24 weeks. With ≥1 response in the first Simon stage (n=10; includes pts treated at MTD in the safety lead-in phase), a second stage will open (n=19) for a total of 29 pts with ≥3 responses indicating the regimen worthy of further study. Secondary endpoints include PFS, OS, and toxicity including a continuous safety monitoring rule during expansion. Exploratory imaging with 89Zr-girentuximab PET is performed at baseline and before each 177Lu-girentuximab dose with results correlated with RECIST response on conventional imaging. In addition, whole body planar and SPECT imaging are performed after each 177Lu-girentuximab dose to evaluate distribution, lesion uptake and dosimetry of 177Lu-girentuximab. The trial is currently accruing to the safety lead-in phase. Clinical trial information: NCT05239533 .

S1. Hierarchical clustering based on Euclidean distance of RNA-seq data demonstrates separation of EBRT-treated (B1-3) and untreated (C1-3) samples.S2. Multi-dimensional scaling plot of RNA-seq data demonstrates separation of EBRT-treated (B1-3) and untreated (C1-3) samples.

Peritoneal carcinomatosis (PC), characterized by the dissemination of metastatic tumor cells throughout the peritoneal cavity from several gastrointestinal and gynecological malignancies, has significantly compromised patient survival. The standard of care is cytoreductive surgery with or without intraperitoneal chemotherapy. However, surgical resection often leaves behind microscopic or clinically occult disease due to the complex anatomy of the peritoneum, where intraperitoneal chemotherapy and systemic chemotherapy have shown limited success. To improve the therapeutic outcome, targeted therapy using radionuclides such as alpha, beta, and Auger emitters delivered by antibodies is actively being investigated. While preclinical murine models of PC have shown the potential of radioimmunotherapy (RIT) using various radioisotopes across a wide spectrum of antigen targets and tumor diagnoses with acceptable toxicities, successful clinical trials are lacking. Here, we retrospectively summarize preclinical and clinical PC studies, consider their translational potential, and examine paths to development that maximize the clinical benefit of RIT in this context.

Peritoneal carcinomatosis is a common yet deadly manifestation of gastrointestinal cancers, with few effective treatments. To identify targetable determinants of peritoneal metastasis, we focused on appendiceal adenocarcinoma (AC), a gastrointestinal cancer that metastasizes almost exclusively to the peritoneum. Current treatments are extrapolated from colorectal cancer (CRC), yet AC has distinct genomic alterations, mucinous morphology and peritoneum restricted metastatic pattern. Further, no stable preclinical models of AC exist, limiting drug discovery and representing an unmet clinical need. We establish a first-in-class stable biobank of 16 long-term cultured AC patient-derived organoids (PDOs), including 3 matched, simultaneously resected primary AC-peritoneal carcinomatosis (AC-PC) pairs. By enriching for cancer cells, AC PDOs enable accurate genomic characterization relative to paucicellular AC tissue. We establish an organoid orthotopic intraperitoneal xenograft model that recapitulates diffuse peritoneal carcinomatosis and show that PC-organoids retain increased metastatic capacity, decreased growth factor dependency and sensitivity to standard of care chemotherapy relative to matched primary AC organoids. Single cell profiling of AC-PC pairs reveals dedifferentiation from mucinous differentiated states in primary AC into intestinal stem cell and fetal progenitor states in AC-PC, with upregulation of oncogenic signaling pathways. Through hypothesis-driven drug testing, we identify KRASMULTI-ON inhibitor RMC-7977 and Wnt-targeting tyrosine kinase inhibitor WNTinib as novel, clinically actionable strategies to target AC-PC more effectively.

Interleukin-13 receptor α-2 (IL13Rα2) is a cell surface receptor frequently expressed in solid malignancies, such as glioblastoma and melanoma, with limited expression in healthy tissue, rendering it an ideal target for noninvasive and specific tumor delineation. In this study, we report the development of 5 novel IL13Rα2-targeted human monoclonal antibodies (mAbs) KLG-1-5; in subsequent in vitro and in vivo studies after radiolabeling with 89Zr, we evaluate their performance to identify a lead candidate. Methods: Five novel human anti-IL13Rα2 mAbs KLG-1-5 were developed and in vitro binding properties and target specificity assessed. In vivo 89Zr-immuno-PET using KLG-1-5 was conducted in a subcutaneous U-87 MG glioblastoma mouse model, and a mass dose titration study was conducted with lead candidate KLG-3. Ex vivo biodistribution results were used to derive prospective dosimetry of 177Lu-labeled KLG-3. Targeting with KLG-3 was also verified in an A-375 melanoma model using the optimized conditions determined in the U-87 MG xenograft model. Results: In vitro studies confirmed target specificity and pico- to low nanomolar binding affinity. Immuno-PET studies with KLG-1-5 in U-87 MG xenografts demonstrated continuously increasing tumoral uptake with maximal uptake at 144 h after tracer injection, clearance of the unbound tracer from the blood pool, and little uptake in any other normal tissues, leading to high-contrast images. KLG-3 provided the highest tumoral uptake and tumor-to-normal tissue ratios and was chosen as the lead candidate, and further dose optimization with this antibody led to tumoral uptake of 97 ± 6 maximum percent of injected dose per gram at 144 h after tracer injection. Ex vivo biodistribution-derived prospective dosimetry for 177Lu-labeled KLG-3 predicted a favorable therapeutic index, encouraging the development of IL13Rα2-targeted radioimmunotherapy. Of note, KLG-3 performed similarly well in a melanoma model, emphasizing the versatility of this antibody. Conclusion: Lead candidate anti-IL13Rα2 mAb KLG-3 validated highly specific target binding in human glioblastoma and melanoma models, resulting in high-contrast PET images with minimal accumulation in off-target healthy tissues. Prospective dosimetry of its 177Lu-labeled counterpart suggested therapeutic efficacy at relatively low injected activities, supporting further pursuit of KLG-3 in future translational radioimmunotherapy applications.

Radiolabeled small-molecule DOTA-haptens can be combined with antitumor/anti-DOTA bispecific antibodies (BsAbs) for pretargeted radioimmunotherapy (PRIT). For optimized delivery of the theranostic γ- and β-emitting isotope

Abstract Background Fluorine 18-labelled tetrafluoroborate ([18F]TFB) is a substrate for the sodium/iodide symporter. In thyroid cancer, [18F]TFB-PET/CT may be an alternative to iodine imaging to evaluate the extent of disease, eligibility for radioiodine treatment, and success of redifferentiation therapies. We report the results of a pilot study to determine tumor uptake of [18F]TFB and compare its properties to [124I]IodinePET/CT in patients with metastatic thyroid cancer. Methods Five patients were included in a prospective study. All patients received PET/CT 1 h after injection of 356 ± 12 MBq [18F]TFB and were given 230 ± 9 MBq [124I]Iodine orally on the same day, followed by PET/CT after 48 h. Before redifferentiation therapy, patients underwent an additional baseline [124I]Iodine PET/CT. Cases were analyzed by two board-certified specialists. Detection rates and Spearman correlation for [18F]TFB and [124I]Iodine were calculated. Results Three patients had poorly differentiated thyroid cancer and received trametinib in a redifferentiation trial. Two patients had papillary thyroid cancer and did not receive redifferentiation therapy. Of the 33 lesions seen before/without redifferentiation therapy, 19 (58%) were visible on [18F]TFB and 30 (91%) on [124I]Iodine imaging. In the patients who underwent redifferentiation therapy, 48 lesions were newly seen on [124I]Iodine PET/CT with a median SUVmax of 3.3 (range, 0.4–285.0). All of these lesions were [18F]TFB-negative. Conclusion [18F]TFB failed to predict radioactive iodine uptake in patients with poorly differentiated thyroid cancer who underwent redifferentiation therapy with trametinib. It is unclear whether such discrepancies may also occur in other redifferentiation therapies or may even be encountered in redifferentiation-naïve thyroid cancer. Trial registration number NCT03196518, registered on June 22, 2017.

BACKGROUND:PET-CT-based patient metabolic profiling is a novel concept to incorporate patient-specific metabolism into gastric cancer care. METHODS:Staging PET-CTs, demographics, and clinicopathologic variables of gastric cancer patients were obtained from a prospectively maintained institutional database. PET-CT avidity was measured in tumor, liver, spleen, four paired muscles, and two paired fat areas in each patient. The liver to rectus femoris (LRF) ratio was defined as the ratio of SUVmean of liver to the average SUVmean of the bilateral rectus femoris muscles. Kaplan-Meier and Cox-proportional hazards models were used to identify the impact of LRF ratio on OS. RESULTS:Two hundred and one patients with distal gastroesophageal (48%) or gastric (52%) adenocarcinoma were included. Median age was 65 years, and 146 (73%) were male. On univariate analysis, rectus femoris PET-CT avidity and LRF ratio were significantly associated with overall survival (p < 0.05). LRF ratio was significantly higher in males, early-stage cancer, patients with an ECOG 0 or 1 performance status, patients with albumin > 3.5 mg/dL, and those with moderately differentiated tumor histology. In multivariable regression, gastric cancer stage, albumin, and LRF ratio were significant independent predictors of overall survival (LRF ratio HR = 0.73 (0.56-0.96); p = 0.024). Survival curves showed that the prognostic impact of LRF was associated with metastatic gastric cancer (p = 0.009). CONCLUSIONS:Elevated LRF ratio, a patient-specific PET-CT-based metabolic parameter, was independently associated with an improvement in OS in patients with metastatic gastric cancer. With prospective validation, LRF ratio may be a useful, host-specific metabolic parameter for prognostication in gastric cancer.

Radiolabeled small-molecule DOTA-haptens can be combined with antitumor/anti-DOTA bispecific antibodies (BsAbs) for pretargeted radioimmunotherapy (PRIT). For optimized delivery of the theranostic γ- and β-emitting isotope 177Lu with DOTA-based PRIT (DOTA-PRIT), bivalent Gemini (DOTA-Bn-thiourea-PEG4-thiourea-Bn-DOTA, aka (3,6,9,12-tetraoxatetradecane-1,14-diyl)bis(DOTA-benzyl thiourea)) was developed. Methods: Gemini was synthesized by linking 2 S-2-(4-isothiocyanatobenzyl)-DOTA molecules together via a 1,14-diamino-PEG4 linker. [177Lu]Lu-Gemini was prepared with no-carrier-added 177LuCl3 to a molar-specific activity of 123 GBq/μmol and radiochemical purity of more than 99%. The specificity of BsAb-177Lu-Gemini was verified in vitro. Subsequently, we evaluated biodistribution and whole-body clearance for [177Lu]Lu-Gemini and, for comparison, our gold-standard monovalent [177Lu]Lu-S-2-(4-aminobenzyl)-DOTA ([177Lu]Lu-DOTA-Bn) in naïve (tumor-free) athymic nude mice. For our proof-of-concept system, a 3-step pretargeting approach was performed with an established DOTA-PRIT regimen (anti-GPA33/anti-DOTA IgG-scFv BsAb, a clearing agent, and [177Lu]Lu-Gemini) in mouse models. Results: Initial in vivo studies showed that [177Lu]Lu-Gemini behaved similarly to [177Lu]Lu-DOTA-Bn, with almost identical blood and whole-body clearance kinetics, as well as biodistribution and mouse kidney dosimetry. Pretargeting [177Lu]Lu-Gemini to GPA33-expressing SW1222 human colorectal xenografts was highly effective, leading to absorbed doses of [177Lu]Lu-Gemini for blood, tumor, liver, spleen, and kidneys of 3.99, 455, 6.93, 5.36, and 14.0 cGy/MBq, respectively. Tumor-to-normal tissue absorbed-dose ratios (i.e., therapeutic indices [TIs]) for the blood and kidneys were 114 and 33, respectively. In addition, we demonstrate that the use of bivalent [177Lu]Lu-Gemini in DOTA-PRIT leads to improved TIs and augmented [177Lu]Lu-Gemini tumor uptake and retention in comparison to monovalent [177Lu]Lu-DOTA-Bn. Finally, we established efficacy in SW1222 tumor-bearing mice, demonstrating that a single injection of anti-GPA33 DOTA-PRIT with 44 MBq (1.2 mCi) of [177Lu]Lu-Gemini (estimated tumor-absorbed dose, 200 Gy) induced complete responses in 5 of 5 animals and a histologic cure in 2 of 5 (40%) animals. Moreover, a significant increase in survival compared with nontreated controls was noted (maximum tolerated dose not reached). Conclusion: We have developed a bivalent DOTA-radiohapten, [177Lu]Lu-Gemini, that showed improved radiopharmacology for DOTA-PRIT application. The use of bivalent [177Lu]Lu-Gemini in DOTA-PRIT, as opposed to monovalent [177Lu]Lu-DOTA-Bn, allows curative treatments with considerably less administered 177Lu activity while still achieving high TIs for both the blood (>100) and the kidneys (>30).