Bidirectional Regulation of Motor Circuits Using Magnetogenetic Gene Therapy

Here we report a novel suite of magnetogenetic tools, based on a single anti-ferritin nanobody-TRPV1 receptor fusion protein, which regulated neuronal activity when exposed to magnetic fields. AAV-mediated delivery of a floxed nanobody-TRPV1 into the striatum of adenosine 2a receptor-cre driver mice resulted in motor freezing when placed in an MRI or adjacent to a transcranial magnetic stimulation (TMS) device. Functional imaging and fiber photometry both confirmed activation of the target region in response to the magnetic fields. Expression of the same construct in the striatum of wild-type mice along with a second injection of an AAVretro expressing cre into the globus pallidus led to similar circuit specificity and motor responses. Finally, a mutation was generated to gate chloride and inhibit neuronal activity. Expression of this variant in subthalamic nucleus in PitX2-cre parkinsonian mice resulted in reduced local c-fos expression and motor rotational behavior. These data demonstrate that magnetogenetic constructs can bidirectionally regulate activity of specific neuronal circuits non-invasively in-vivo using clinically available devices.

Teaser

A novel magnetogenetics toolbox to regulate neural circuits in-vivo .

Brain Fluid Clearance After Traumatic Brain Injury Measured Using Dynamic Positron Emission Tomography

Brain fluid clearance by pathways including the recently described paravascular glymphatic system is a critical homeostatic mechanism by which metabolic products, toxins, and other wastes are removed from the brain. Brain fluid clearance may be especially important after traumatic brain injury (TBI), when blood, neuronal debris, inflammatory cells, and other substances can be released and/or deposited. Using a non-invasive dynamic positron emission tomography (PET) method that models the rate at which an intravenously injected radiolabeled molecule (in this case 11C-flumazenil) is cleared from ventricular cerebrospinal fluid (CSF), we estimated the overall efficiency of brain fluid clearance in humans who had experienced complicated-mild or moderate TBI 3-6 months before neuroimaging (n = 7) as compared to healthy controls (n = 9). While there was no significant difference in ventricular clearance between TBI subjects and controls, there was a significant group difference in dependence of ventricular clearance upon tracer delivery/blood flow to the ventricles. Specifically, in controls, ventricular clearance was highly, linearly dependent upon blood flow to the ventricle, but this relation was disrupted in TBI subjects. When accounting for blood flow and group-specific alterations in blood flow, ventricular clearance was slightly (non-significantly) increased in TBI subjects as compared to controls. Current results contrast with past studies showing reduced glymphatic function after TBI and are consistent with possible differential effects of TBI on glymphatic versus non-glymphatic clearance mechanisms. Further study using multi-modal methods capable of assessing and disentangling blood flow and different aspects of fluid clearance is needed to clarify clearance alterations after TBI.

Positron Emission Tomography Quantitative Assessment of Off-Target Whole-Body Biodistribution of I-124-Labeled Adeno-Associated Virus Capsids Administered to Cerebral Spinal Fluid

Based on studies in experimental animals demonstrating that administration of adeno-associated virus (AAV) vectors to the cerebrospinal fluid (CSF) is an effective route to transfer genes to the nervous system, there are increasing number of clinical trials using the CSF route to treat nervous system disorders. With the knowledge that the CSF turns over four to five times daily, and evidence in experimental animals that at least some of CSF administered AAV vectors are distributed to systemic organs, we asked: with AAV administration to the CSF, what fraction of the total dose remains in the nervous system and what fraction goes off target and is delivered systemically? To quantify the biodistribution of AAV capsids immediately after administration, we covalently labeled AAV capsids with iodine 124 (I-124), a cyclotron generated positron emitter, enabling quantitative positron emission tomography scanning of capsid distribution for up to 96 h after AAV vector administration. We assessed the biodistribution to nonhuman primates of I-124-labeled capsids from different AAV clades, including 9 (clade F), rh.10 (E), PHP.eB (F), hu68 (F), and rh91(A). The analysis demonstrated that 60-90% of AAV vectors administered to the CSF through either the intracisternal or intrathecal (lumbar) routes distributed systemically to major organs. These observations have potentially significant clinical implications regarding accuracy of AAV vector dosing to the nervous system, evoking systemic immunity at levels similar to that with systemic administration, and potential toxicity of genes designed to treat nervous system disorders being expressed in non-nervous system organs. Based on these data, individuals in clinical trials using AAV vectors administered to the CSF should be monitored for systemic as well as nervous system adverse events and CNS dosing considerations should account for a significant AAV systemic distribution.

Efficacy of HER2-Targeted Intraperitoneal 225Ac α-Pretargeted Radioimmunotherapy for Small-Volume Ovarian Peritoneal Carcinomatosis

Epithelial ovarian cancer (EOC) is often asymptomatic and presents clinically in an advanced stage as widespread peritoneal microscopic disease that is generally considered to be surgically incurable. Targeted α-therapy with the α-particle-emitting radionuclide 225Ac (half-life, 9.92 d) is a high-linear-energy-transfer treatment approach effective for small-volume disease and even single cells. Here, we report the use of human epidermal growth factor receptor 2 (HER2) 225Ac-pretargeted radioimmunotherapy (PRIT) to treat a mouse model of human EOC SKOV3 xenografts growing as peritoneal carcinomatosis (PC). Methods: On day 0, 105 SKOV3 cells transduced with a luciferase reporter gene were implanted intraperitoneally in nude mice, and tumor engraftment was verified by bioluminescent imaging (BLI). On day 15, treatment was started using 1 or 2 cycles of 3-step anti-HER2 225Ac-PRIT (37 kBq/cycle as 225Ac-Proteus DOTA), separated by a 1-wk interval. Efficacy and toxicity were monitored for up to 154 d. Results: Untreated PC-tumor-bearing nude mice showed a median survival of 112 d. We used 2 independent measures of response to evaluate the efficacy of 225Ac-PRIT. First, a greater proportion of the treated mice (9/10 1-cycle and 8/10 2-cycle; total, 17/20; 85%) survived long-term compared with controls (9/27, 33%), and significantly prolonged survival was documented (log-rank [Mantel-Cox] P = 0.0042). Second, using BLI, a significant difference in the integrated BLI signal area to 98 d was noted between controls and treated groups (P = 0.0354). Of a total of 8 mice from the 2-cycle treatment group (74 kBq total) that were evaluated by necropsy, kidney radiotoxicity was mild and did not manifest itself clinically (normal serum blood urea nitrogen and creatinine). Dosimetry estimates (relative biological effectiveness-weighted dose, where relative biological effectiveness = 5) per 37 kBq administered for tumors and kidneys were 56.9 and 16.1 Gy, respectively. One-cycle and 2-cycle treatments were equally effective. With immunohistology, mild tubular changes attributable to α-toxicity were observed in both therapeutic groups. Conclusion: Treatment of EOC PC-tumor-bearing mice with anti-HER2 225Ac-PRIT resulted in histologic cures and prolonged survival with minimal toxicity. Targeted α-therapy using the anti-HER2 225Ac-PRIT system is a potential treatment for otherwise incurable EOC.

Choroid Plexus Calcification Correlates with Cortical Microglial Activation in Humans: A Multimodal PET, CT, MRI Study

BACKGROUND AND PURPOSE

The choroid plexus (CP) within the brain ventricles is well-known to produce cerebrospinal fluid (CSF). Recently, the CP has been recognized as critical in modulating inflammation. MRI-measured CP enlargement has been reported in neuroinflammatory disorders like MS as well as with aging and neurodegeneration. The basis of MRI-measured CP enlargement is unknown. On the basis of tissue studies demonstrating CP calcification as a common pathology associated with aging and disease, we hypothesized that previously unmeasured CP calcification contributes to MRI-measured CP volume and may be more specifically associated with neuroinflammation.

MATERIALS AND METHODS

We analyzed 60 subjects (43 healthy controls and 17 subjects with Parkinson's disease) who underwent PET/CT using 11C-PK11195, a radiotracer sensitive to the translocator protein expressed by activated microglia. Cortical inflammation was quantified as nondisplaceable binding potential. Choroid plexus calcium was measured via manual tracing on low-dose CT acquired with PET and automatically using a new CT/MRI method. Linear regression assessed the contribution of choroid plexus calcium, age, diagnosis, sex, overall volume of the choroid plexus, and ventricle volume to cortical inflammation.

RESULTS

Fully automated choroid plexus calcium quantification was accurate (intraclass correlation coefficient with manual tracing = .98). Subject age and choroid plexus calcium were the only significant predictors of neuroinflammation.

CONCLUSIONS

Choroid plexus calcification can be accurately and automatically quantified using low-dose CT and MRI. Choroid plexus calcification-but not choroid plexus volume-predicted cortical inflammation. Previously unmeasured choroid plexus calcium may explain recent reports of choroid plexus enlargement in human inflammatory and other diseases. Choroid plexus calcification may be a specific and relatively easily acquired biomarker for neuroinflammation and choroid plexus pathology in humans.

Monitoring the biodistribution of radiolabeled therapeutics in mice

Radiopharmaceutical therapy is a rapidly growing field for the treatment of cancer due to its high specificity and ability to target individual affected cells. A key component of the pre-clinical development of a new therapeutic radiopharmaceutical is the determination of its time-dependent distribution in tumors, normal tissues, and the whole body in mouse tumor models. Here, we provide an overview of the available instrumentation for the novice in radiation measurement. We also detail the methodology for assessing distribution and kinetics of a radiopharmaceutical and calculating radiation absorbed dose in mice using a gamma counter or a PET or SPECT camera.

Abstract 5084: Targeted delivery of low-dose radiation alleviates tumor resistance to CAR-T cell therapy

Purpose: Current understanding of resistance to CAR-T cell therapy in solid tumors implicates inadequate CAR-T cell potency in the immunosuppressive tumor microenvironment (TME). We have previously developed a platform using somatostatin receptor 2 (SSTR2) as a positron emission tomography (PET) reporter to detect CAR-T cell expansion and trafficking. The current study aimed to leverage SSTR2 for low-dose targeted radionuclide therapy (177Lu-DOTATATE, Lutathera), which under dosimetry guidance might enhance antitumor immunity by reprograming the TME and promoting T-cell reinvigoration.

Methods: Using intercellular adhesion molecule 1 (ICAM-1) as a model antigen, we evaluated the immunomodulatory effects of low-dose radiation in a gastric cancer animal model. NSG mice were inoculated subcutaneously with firefly luciferase-expressing Hs 746T cells (0.1 × 106 per mouse) and treated 5 days later with 10 × 106 SSTR2-expressing ICAM-1 CAR-T cells. CAR-T cell expansion was monitored weekly by PET scan using 18F-NOTA-Octreotide, a radiotracer targeting SSTR2. Three weeks post-T cell infusion, a cohort of mice were injected with 7.4 MBq of 177Lu-DOTATATE via the tail vein. SPECT imaging was performed for dosimetry analysis. Tumor growth was monitored by bioluminescence imaging and tumor size measurement. Serum cytokines were analyzed.

Results: Single dose 177Lu-DOTATATE treatment (delivering 1-6 Gy to tumor) improved response of established gastric cancer tumor (>1,000 mm3) that was not responsive to CAR-T cell treatment alone and significantly prolonged survival. All mice receiving CAR-T cells plus 177Lu-DOTATATE displayed rapid tumor shrinkage, with 83% of mice achieving complete remission within 3 weeks of 177Lu-DOTATATE treatment. SPECT imaging confirmed specific delivery of 177Lu-DOTATATE by tumor-infiltrating CAR-T cells, with tumor uptake of 0.43 ± 0.24 and 0.19 ± 0.08 MBq/g at 24 and 144 hours post 177Lu-DOTATATE injection, respectively. Most radioactivity reduction over time (73%) is explained by physical decay of 177Lu, indicating persistent tumor retention of 177Lu-DOTATATE. In contrast, rapid clearance of 177Lu-DOTATATE was observed in liver and kidneys. Importantly, longitudinal CAR-T cell imaging using 18F-NOTA-Octreotide revealed increased CAR-T cell expansion induced by low-dose radiation. Furthermore, we detected high levels of IFN-γ and perforin in serum after 177Lu-DOTATATE treatment, which were >10 folds higher than that in control mice receiving CAR-T cell only, indicating activation of T cells by low-dose radiation.

Conclusions: We developed a translatable radioimmunotherapy platform that incorporates a FDA-approved theranostic endoradiotherapy (Lutathera) to improve tumor response to CAR-T cell therapy. The next steps would be to explore the immunomodulatory effects of low-dose radiation systematically and elucidate the mechanism of action.

Citation Format: Yanping Yang, Yago Alcaina, Yogindra Vedvyas, Maria Cristina Riascos, Edward K. Fung, Brett Vaughn, Sarah M. Cheal, Irene M. Min, Claire Vanpouille-Box, Moonsoo M. Jin. Targeted delivery of low-dose radiation alleviates tumor resistance to CAR-T cell therapy. [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 5084.

Intraperitoneal Pretargeted Radioimmunotherapy for Colorectal Peritoneal Carcinomatosis

Peritoneal carcinomatosis (PC) is considered incurable, and more effective therapies are needed. Herein we test the hypothesis that GPA33-directed intracompartmental pretargeted radioimmunotherapy (PRIT) can cure colorectal peritoneal carcinomatosis. Nude mice were implanted intraperitoneally with luciferase-transduced GPA33-expressing SW1222 cells for aggressive peritoneal carcinomatosis (e.g., resected tumor mass 0.369 ± 0.246 g; n = 17 on day 29). For GPA33-PRIT, we administered intraperitoneally a high-affinity anti-GPA33/anti-DOTA bispecific antibody (BsAb), followed by clearing agent (intravenous), and lutetium-177 (Lu-177) or yttrium-86 (Y-86) radiolabeled DOTA-radiohapten (intraperitoneal) for beta/gamma-emitter therapy and PET imaging, respectively. The DOTA-radiohaptens were prepared from S-2-(4-aminobenzyl)-1,4,7, 10-tetraazacyclododecane tetraacetic acid chelate (DOTA-Bn). Efficacy and toxicity of single- versus three-cycle therapy were evaluated in mice 26-27 days post-tumor implantation. Single-cycle treatment ([177Lu]LuDOTA-Bn 111 MBq; tumor dose: 4,992 cGy) significantly prolonged median survival (MS) approximately 2-fold to 84.5 days in comparison with controls (P = 0.007). With three-cycle therapy (once weekly, total 333 MBq; tumor dose: 14,975 cGy), 6/8 (75%) survived long-term (MS > 183 days). Furthermore, for these treated long-term survivors, 1 mouse was completely disease free (microscopic "cure") at necropsy; the others showed stabilized disease, which was detectable during PET-CT using [86Y]DOTA-Bn. Treatment controls had MS ranging from 42-52.5 days (P < 0.001) and 19/20 mice succumbed to progressive intraperitoneal disease by 69 days. Multi-cycle GPA33 DOTA-PRIT significantly prolongs survival with reversible myelosuppression and no chronic marrow (929 cGy to blood) or kidney (982 cGy) radiotoxicity, with therapeutic indices of 12 for blood and 12 for kidneys. MTD was not reached.

Tau PET following acute TBI: Off-target binding to blood products, tauopathy, or both?

Repeated mild Traumatic Brain Injury (TBI) is a risk factor for Chronic Traumatic Encephalopathy (CTE), characterized pathologically by neurofibrillary tau deposition in the depths of brain sulci and surrounding blood vessels. The mechanism by which TBI leads to CTE remains unknown but has been posited to relate to axonal shear injury leading to release and possibly deposition of tau at the time of injury. As part of an IRB-approved study designed to learn how processes occurring acutely after TBI may predict later proteinopathy and neurodegeneration, we performed tau PET using 18F-MK6240 and MRI within 14 days of complicated mild TBI in three subjects. PET radiotracer accumulation was apparent in regions of traumatic hemorrhage in all subjects, with prominent intraparenchymal PET signal in one young subject with a history of repeated sports-related concussions. These results are consistent with off-target tracer binding to blood products as well as possible on-target binding to chronically and/or acutely-deposited neurofibrillary tau. Both explanations are highly relevant to applying tau PET to understanding TBI and CTE. Additional study is needed to assess the potential utility of tau PET in understanding how processes occurring acutely after TBI, such as release and deposition of tau and blood from damaged axons and blood vessels, may relate to development CTE years later.

Quantitative Whole-Body Imaging of I-124-Labeled Adeno-Associated Viral Vector Biodistribution in Nonhuman Primates

A method is presented for quantitative analysis of the biodistribution of adeno-associated virus (AAV) gene transfer vectors following in vivo administration. We used iodine-124 (I-124) radiolabeling of the AAV capsid and positron emission tomography combined with compartmental modeling to quantify whole-body and organ-specific biodistribution of AAV capsids from 1 to 72 h following administration. Using intravenous (IV) and intracisternal (IC) routes of administration of AAVrh.10 and AAV9 vectors to nonhuman primates in the absence or presence of anticapsid immunity, we have identified novel insights into initial capsid biodistribution and organ-specific capsid half-life. Neither I-124-labeled AAVrh.10 nor AAV9 administered intravenously was detected at significant levels in the brain relative to the administered vector dose. Approximately 50% of the intravenously administered labeled capsids were dispersed throughout the body, independent of the liver, heart, and spleen. When administered by the IC route, the labeled capsid had a half-life of ∼10 h in the cerebral spinal fluid (CSF), suggesting that by this route, the CSF serves as a source with slow diffusion into the brain. For both IV and IC administration, there was significant influence of pre-existing anticapsid immunity on I-124-capsid biodistribution. The methodology facilitates quantitative in vivo viral vector dosimetry, which can serve as a technique for evaluation of both on- and off-target organ biodistribution, and potentially accelerate gene therapy development through rapid prototyping of novel vector designs.

Radiation segmentectomy of hepatic metastases with Y-90 glass microspheres

PURPOSE

To evaluate safety and efficacy of radiation segmentectomy (RS) with ⁹⁰Y glass microspheres in patients with limited metastatic liver disease not amenable to resection or percutaneous ablation.

METHODS

Patients with ≤ 3 tumors treated with RS from 6/2015 to 12/2017 were included. Target tumor radiation dose was > 190 Gy based on medical internal radiation dose (MIRD) dosimetry. Tumor response, local tumor progression (LTP), LTP-free survival (LTPFS) and disease progression rate in the treated segment were defined using Choi and RECIST 1.1 criteria. Toxicities were evaluated using modified SIR criteria.

RESULTS

Ten patients with 14 tumors underwent 12 RS. Median tumor size was 3 cm (range 1.4-5.6). Median follow-up was 17.8 months (range 1.6-37.3). Response rates per Choi and RECIST 1.1 criteria were 8/8 (100%) and 4/9 (44%), respectively. Overall LTP rate was 3/14 (21%) during the study period. One-, two- and three-year LTPFS was 83%, 83% and 69%, respectively. Median LTPFS was not reached. Disease progression rate in the treated segment was 6/18 (33%). Median overall survival was 41.5 months (IQR 16.7-41.5). Median delivered tumor radiation dose was 293 Gy (range 163-1303). One major complication was recorded in a patient post-Whipple procedure who suffered anaphylactic reaction to prophylactic cefotetan and liver abscess in RS region 6.5 months post-RS. All patients were alive on last follow-up.

CONCLUSION

RS of ≤ 3 hepatic segments can safely provide a 2-year local tumor control rate of 83% in selected patients with limited metastatic liver disease and limited treatment options. Optimal dosimetry methodology requires further investigation.

A pilot study treatment of malignant tumors using low-dose 18F-fluorodeoxyglucose (18F-FDG)

Photons, electrons and protons have therapeutic use however positrons have only been used for diagnostic imaging purposes. The energies of positrons (β⁺) from F-18 (0.633 MeV) and electrons (β^-) from I-131 (0.606 MeV) are very close and have similar equilibrium dose constants. Since [¹⁸F]-fluorodeoxyglucose (¹⁸F-FDG) clears rapidly from circulation, administration of 37-74 GBq (1-2 Ci) of ¹⁸F-FDG is relatively safe from an internal radiation dosimetry point of view. We initiated a phase I dose escalation study to assess the safety, toxicity, and potential therapeutic utility of administering 100-200 mCi/m^{2 18}F-FDG delivered over a 1 to 5 day period in patients with advanced lymphomas and solid tumors refractory to standard of care treatment (SCT). Here we report the results of the first four patients treated. Four patients with advanced cancers received a single dose of 3.7-7.4 GBq/m² (100-200 mCi/m²) ¹⁸F-FDG. We monitored the patients for adverse effects and for response. No treatment-related toxicities were observed. There was no increased radiation exposure to personnel. Two patients showed decrease in the index lesions' SUVs by 17-33% (Day 1) and 25-31% (Day 30) post treatment. The two other patients showed stable disease on ¹⁸F-PET-CT. Interestingly, responses were seen at low radiotherapy doses (below 1 Gy). This exploratory study demonstrated the safety of therapeutic administration of up to 14.2 GBq (385 mCi) ¹⁸F-FDG. In patients with ¹⁸F-FDG-avid cancers, targeted radionuclide ¹⁸F-FDG therapy appears safe and may offer clinical benefit.

A Self-Assembling and Disassembling (SADA) Bispecific Antibody (BsAb) Platform for Curative Two-step Pretargeted Radioimmunotherapy

PURPOSE

Many cancer treatments suffer from dose-limiting toxicities to vital organs due to poor therapeutic indices. To overcome these challenges we developed a novel multimerization platform that rapidly removes tumor-targeting proteins from the blood to substantially improve therapeutic index.

EXPERIMENTAL DESIGN

The platform was designed as a fusion of a self-assembling and disassembling (SADA) domain to a tandem single-chain bispecific antibody (BsAb, anti-ganglioside GD2 × anti-DOTA). SADA-BsAbs were assessed with multiple in vivo tumor models using two-step pretargeted radioimmunotherapy (PRIT) to evaluate tumor uptake, dosimetry, and antitumor responses.

RESULTS

SADA-BsAbs self-assembled into stable tetramers (220 kDa), but could also disassemble into dimers or monomers (55 kDa) that rapidly cleared via renal filtration and substantially reduced immunogenicity in mice. When used with rapidly clearing DOTA-caged PET isotopes, SADA-BsAbs demonstrated accurate tumor localization, dosimetry, and improved imaging contrast by PET/CT. When combined with therapeutic isotopes, two-step SADA-PRIT safely delivered massive doses of alpha-emitting (²²⁵Ac, 1.48 MBq/kg) or beta-emitting (¹⁷⁷Lu, 6,660 MBq/kg) S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA) payloads to tumors, ablating them without any short-term or long-term toxicities to the bone marrow, kidneys, or liver.

CONCLUSIONS

The SADA-BsAb platform safely delivered large doses of radioisotopes to tumors and demonstrated no toxicities to the bone marrow, kidneys, or liver. Because of its modularity, SADA-BsAbs can be easily adapted to most tumor antigens, tumor types, or drug delivery approaches to improve therapeutic index and maximize the delivered dose.See related commentary by Capala and Kunos, p. 377.

Alpha radioimmunotherapy using 225Ac-proteus-DOTA for solid tumors - safety at curative doses

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using ²²⁵Ac and its theranostic pair, ¹¹¹In. We call our novel tumor-targeting DOTA-hapten PRIT system "proteus-DOTA" or "Pr." Herein we report the first results of radiochemistry development, radiopharmacology, and stoichiometry of tumor antigen binding, including the role of specific activity, anti-tumor efficacy, and normal tissue toxicity with the Pr-PRIT approach (as α-DOTA-PRIT). A series of α-DOTA-PRIT therapy studies were performed in three solid human cancer xenograft models of colorectal cancer (GPA33), breast cancer (HER2), and neuroblastoma (GD2), including evaluation of chronic toxicity at ~20 weeks of select survivors. Methods: Preliminary biodistribution experiments in SW1222 tumor-bearing mice revealed that ²²⁵Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for ¹⁷⁷Lu or ⁹⁰Y, leading to poor tumor uptake in vivo. Therefore, we synthesized Pr consisting of an empty DOTA-chelate for ²²⁵Ac, tethered via a short polyethylene glycol linker to a lutetium-complexed DOTA for picomolar anti-DOTA chelate single-chain variable fragment (scFv) binding. Pr was radiolabeled with ²²⁵Ac and its imaging surrogate, ¹¹¹In. In vitro studies verified anti-DOTA scFv recognition of [²²⁵Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered ²²⁵Ac- or ¹¹¹In-radiolabeled Pr in mice showed rapid renal clearance and minimal normal tissue retention. In vivo pretargeting studies show high tumor accumulation of Pr (16.71 ± 5.11 %IA/g or 13.19 ± 3.88 %IA/g at 24 h p.i. for [²²⁵Ac]Pr and [¹¹¹In]Pr, respectively) and relatively low uptake in normal tissues (all average ≤ 1.4 %IA/g at 24 h p.i.). Maximum tolerated dose (MTD) was not reached for either [²²⁵Ac]Pr alone or pretargeted [²²⁵Ac]Pr at administered activities up to 296 kBq/mouse. Single-cycle treatment consisting of α-DOTA-PRIT with either huA33-C825 bispecific anti-tumor/anti-DOTA-hapten antibody (BsAb), anti-HER2-C825 BsAb, or hu3F8-C825 BsAb for targeting GPA33, HER2, or GD2, respectively, was highly effective. In the GPA33 model, no complete responses (CRs) were observed but prolonged overall survival of treated animals was 42 d for α-DOTA-PRIT vs. 25 d for [²²⁵Ac]Pr only (P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, 37%) and histologic cures (10/19, 56%) with no acute or chronic toxicity. Conclusions: [²²⁵Ac]Pr and its imaging biomarker [¹¹¹In]Pr demonstrate optimal radiopharmacologic behavior for theranostic applications of α-DOTA-PRIT. For this initial evaluation of efficacy and toxicity, single-cycle treatment regimens were performed in all three systems. Histologic toxicity was not observed, so MTD was not observed. Prolonged overall survival, CRs, and histologic cures were observed in treated animals. In comparison to RIT with anti-tumor IgG antibodies, [²²⁵Ac]Pr has a much improved safety profile. Ultimately, these data will be used to guide clinical development of toxicity and efficacy studies of [²²⁵Ac]Pr, with the goal of delivering massive lethal doses of radiation to achieve a high probability of cure without toxicity.

Biodistribution and Dosimetry of Intraventricularly Administered 124I-Omburtamab in Patients with Metastatic Leptomeningeal Tumors

Radiation dose estimations are key for optimizing therapies. We studied the role of ¹²⁴I-omburtamab (8H9) given intraventricularly in assessing the distribution and radiation doses before ¹³¹I-omburtamab therapy in patients with metastatic leptomeningeal disease and compared it with the estimates from cerebrospinal fluid (CSF) sampling. Methods: Patients with histologically proven malignancy and metastatic disease to the central nervous system or leptomeninges who met eligibility criteria for ¹³¹I-omburtamab therapy underwent immuno-PET imaging with ¹²⁴I-8H9 followed by ¹³¹I-8H9 antibody therapy. Patients were imaged with approximately 74 MBq of intraventricular ¹²⁴I-omburtamab via an Ommaya reservoir. Whole-body PET images were acquired at approximately 4, 24, and 48 h after administration and analyzed for dosimetry calculations. Peripheral blood and CSF samples were obtained at multiple time points for dosimetry estimation. Results: Forty-two patients with complete dosimetry and therapy data were analyzed. ¹²⁴I-omburtamab PET-based radiation dosimetry estimations revealed mean (±SD) absorbed dose to the CSF for ¹³¹I-8H9 of 0.62 ± 0.40 cGy/MBq, compared with 2.22 ± 2.19 cGy/MBq based on ¹²⁴I-omburtamab CSF samples and 1.53 ± 1.37 cGy/MBq based on ¹³¹I-omburtamab CSF samples. The mean absorbed dose to the blood was 0.051 ± 0.11 cGy/MBq for ¹²⁴I-omburtamab samples and 0.07 ± 0.04 cGy/MBq for ¹³¹I-omburtamab samples. The effective whole-body radiation dose for ¹²⁴I-omburtamab was 0.49 ± 0.27 mSv/MBq. The mean whole-body clearance half-time was 44.98 ± 16.29 h. Conclusion: PET imaging with ¹²⁴I-omburtamab antibody administered intraventricularly allows for noninvasive estimation of dose to CSF and normal organs. High CSF-to-blood absorbed-dose ratios are noted, allowing for an improved therapeutic index to leptomeningeal disease and reduced systemic doses. PET imaging-based estimates were less variable and more reliable than CSF sample-based dosimetry.

Theranostic pretargeted radioimmunotherapy of internalizing solid tumor antigens in human tumor xenografts in mice: Curative treatment of HER2-positive breast carcinoma

In recent reports, we have shown that optimized pretargeted radioimmunotherapy (PRIT) based on molecularly engineered antibody conjugates and ¹⁷⁷Lu-DOTA chelate (DOTA-PRIT) can be used to cure mice bearing human solid tumor xenografts using antitumor antibodies to minimally internalizing membrane antigens, GPA33 (colon) and GD2 (neuroblastoma). However, many solid tumor membrane antigens are internalized after antibody binding and it is generally believed that internalizing tumor membrane antigens are not suitable targets for PRIT. In this study, we tested the hypothesis that DOTA-PRIT can be performed successfully to target HER2, an internalizing membrane antigen widely expressed in breast, ovarian, and gastroesophageal junction cancers.

Methods: DOTA-PRIT was carried out in athymic nude mice bearing BT-474 xenografts, a HER2-expressing human breast cancer, using a three-step dosing regimen consisting of sequential intravenous administrations of: 1) a bispecific IgG-scFv (210 kD) format (BsAb) carrying the IgG sequence of the anti-HER2 antibody trastuzumab and the scFv “C825” with high-affinity, hapten-binding antibody for Bn-DOTA (metal) (BsAb: anti-HER2-C825), 2) a 500 kD dextran-based clearing agent, followed by 3) ¹⁷⁷Lu-DOTA-Bn. At the time of treatment, athymic nude mice bearing established subcutaneous BT-474 tumors (medium- and smaller-sized tumors with tumor volumes of 209 ± 101 mm³ and ranging from palpable to 30 mm³, respectively), were studied along with controls. We studied single- and multi-dose regimens. For groups receiving fractionated treatment, we verified quantitative tumor targeting during each treatment cycle using non-invasive imaging with single-photon emission computed tomography/computed tomography (SPECT/CT).

Results: We achieved high therapeutic indices (TI, the ratio of radiation-absorbed dose in tumor to radiation-absorbed dose to critical organs, such as bone marrow) for targeting in blood (TI = 28) and kidney (TI = 7), while delivering average radiation-absorbed doses of 39.9 cGy/MBq to tumor. Based on dosimetry estimates, we implemented a curative fractionated therapeutic regimen for medium-sized tumors that would deliver approximately 70 Gy to tumors, which required treatment with a total of 167 MBq ¹⁷⁷Lu-DOTA-Bn/mouse (estimated absorbed tumor dose: 66 Gy). This regimen was well tolerated and achieved 100% complete responses (CRs; defined herein as tumor volume equal to or smaller than 4.2 mm³), including 62.5% histologic cure (5/8) and 37.5% microscopic residual disease (3/8) at 85 days (d). Treatment controls showed tumor progression to 207 ± 201% of pre-treatment volume at 85 d and no CRs. Finally, we show that treatment with this curative ¹⁷⁷Lu regimen leads to a very low incidence of histopathologic abnormalities in critical organs such as bone marrow and kidney among survivors compared with non-treated controls.

Conclusion: Contrary to popular belief, we demonstrate that DOTA-PRIT can be successfully adapted to an internalizing antigen-antibody system such as HER2, with sufficient TIs and absorbed tumor doses to achieve a high probability of cures of established human breast cancer xenografts while sparing critical organs of significant radiotoxicity.

ImmunoPET Imaging of Endogenous and Transfected Prolactin Receptor Tumor Xenografts

Antibodies labeled with positron-emitting isotopes have been used for tumor detection, predicting which patients may respond to tumor antigen-directed therapy, and assessing pharmacodynamic effects of drug interventions. Prolactin receptor (PRLR) is overexpressed in breast and prostate cancers and is a new target for cancer therapy. We evaluated REGN2878, an anti-PRLR monoclonal antibody, as an immunoPET reagent. REGN2878 was labeled with Zr-89 after conjugation with desferrioxamine B or labeled with I-131/I-124. In vitro determination of the half-maximal inhibitory concentration (IC50) of parental REGN2878, DFO-REGN2878, and iodinated REGN2878 was performed by examining the effect of the increasing amounts of these on uptake of trace-labeled I-131 REGN2878. REGN1932, a non-PRLR binding antibody, was used as a control. Imaging and biodistribution studies were performed in mice bearing tumor xenografts with various expression levels of PRLR, including MCF-7, transfected MCF-7/PRLR, PC3, and transfected PC3/PRLR and T4D7v11 cell lines. The specificity of uptake in tumors was evaluated by comparing Zr-89 REGN2878 and REGN1932, and in vivo competition compared Zr-89 REGN2878 uptake in tumor xenografts with and without prior injection of 2 mg of nonradioactive REGN2878. The competition binding assay of DFO-REGN2878 at ratios of 3.53-5.77 DFO per antibody showed IC50 values of 0.4917 and 0.7136 nM, respectively, compared to 0.3455 nM for parental REGN2878 and 0.3343 nM for I-124 REGN2878. Imaging and biodistribution studies showed excellent targeting of Zr-89 REGN2878 in PRLR-positive xenografts at delayed times of 189 h (presented as mean ± 1 SD, percent injected activity per mL (%IA/mL) 74.6 ± 33.8%IA/mL). In contrast, MCF-7/PRLR tumor xenografts showed a low uptake (7.0 ± 2.3%IA/mL) of control Zr-89 REGN1932 and a very low uptake and rapid clearance of I-124 REGN2878 (1.4 ± 0.6%IA/mL). Zr-89 REGN2878 has excellent antigen-specific targeting in various PRLR tumor xenograft models. We estimated, using image-based kinetic modeling, that PRLR antigen has a very rapid in vivo turnover half-life of ∼14 min from the cell membrane. Despite relatively modest estimated tumor PRLR expression numbers, PRLR-expressing cells have shown final retention of the Zr-89 REGN2878 antibody, with an uptake that appeared to be related to PRLR expression. This reagent has the potential to be used in clinical trials targeting PRLR.

Curative Multicycle Radioimmunotherapy Monitored by Quantitative SPECT/CT-Based Theranostics, Using Bispecific Antibody Pretargeting Strategy in Colorectal Cancer

Radioimmunotherapy of solid tumors using antibody-targeted radionuclides has been limited by low therapeutic indices (TIs). We recently reported a novel 3-step pretargeted radioimmunotherapy (PRIT) strategy based on a glycoprotein A33 (GPA33)-targeting bispecific antibody and a small-molecule radioactive hapten, a complex of ¹⁷⁷Lu and S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (¹⁷⁷Lu-DOTA-Bn), that leads to high TIs for radiosensitive tissues such as blood (TI = 73) and kidney (TI = 12). We tested our hypothesis that a fractionated anti-GPA33 DOTA-PRIT regimen calibrated to deliver a radiation absorbed dose to tumor of more than 100 Gy would lead to a high probability of tumor cure while being well tolerated by nude mice bearing subcutaneous GPA33-positive SW1222 xenografts. Methods: We treated groups of nude mice bearing 7-d-old SW1222 xenografts with a fractionated 3-cycle anti-GPA33 DOTA-PRIT regimen (total administered ¹⁷⁷Lu-DOTA-Bn activity, 167 MBq/mouse; estimated radiation absorbed dose to tumor, 110 Gy). In randomly selected mice undergoing treatment, serial SPECT/CT imaging was used to monitor treatment response and calculate radiation absorbed doses to tumor. Necropsy was done on surviving animals 100-200 d after treatment to determine frequency of cure and assess select normal tissues for treatment-related histopathologies. Results: Rapid exponential tumor progression was observed in control treatment groups (i.e., no treatment or ¹⁷⁷Lu-DOTA-Bn only), leading to euthanasia due to excessive tumor burden, whereas 10 of 10 complete responses were observed for the DOTA-PRIT-treated animals within 30 d. Treatment was well tolerated, and 100% histologic cure was achieved in 9 of 9 assessable animals without detectable radiation damage to critical organs, including bone marrow and kidney. Radiation absorbed doses to tumor derived from SPECT/CT (102 Gy) and from biodistribution (110 Gy) agreed to within 6.9%. Of the total dose of approximately 100 Gy, the first dose contributes 30%, the second dose 60%, and the third dose 10%. Conclusion: In a GPA33-positive human colorectal cancer xenograft mouse model, we validated a SPECT/CT-based theranostic PRIT regimen that led to 100% complete responses and 100% cures without any treatment-related toxicities, based on high TIs for radiosensitive tissues. These studies support the view that anti-GPA33 DOTA-PRIT will be a potent radioimmunotherapy regimen for GPA33-positive colorectal cancer tumors in humans.

Targeting of radiolabeled J591 antibody to PSMA-expressing tumors: optimization of imaging and therapy based on non-linear compartmental modeling

Background

We applied a non-linear immunokinetic model to quantitatively compare absolute antibody uptake and turnover in subcutaneous LNCaP human prostate cancer (PCa) xenografts of two radiolabeled forms of the humanized anti-prostate-specific membrane antigen (PSMA) monoclonal antibody J591 (¹²⁴I-J591 and ⁸⁹Zr-J591). Using the model, we examined the impact of dose on the tumor and plasma positron emission tomography (PET)-derived time-activity curves. We also sought to predict the optimal targeting index (ratio of integrated-tumor-to-integrated-plasma activity concentrations) for radioimmunotherapy.

Methods

The equilibrium rates of antibody internalization and turnover in the tumors were derived from PET images up to 96 h post-injection using compartmental modeling with a non-linear transfer rate. In addition, we serially imaged groups of LNCaP tumor-bearing mice injected with ⁸⁹Zr-J591 antibody doses ranging from antigen subsaturating to saturating to examine the suitability of using a non-linear approach and derived the time-integrated concentration (in μM∙hours) of administered tracer in tumor as a function of the administered dose of antibody.

Results

The comparison of ¹²⁴I-J591 and ⁸⁹Zr-J591 yielded similar model-derived values of the total antigen concentration and internalization rate. The association equilibrium constant (k_a) was twofold higher for ¹²⁴I, but there was a ~tenfold greater tumoral efflux rate of ¹²⁴I from tumor compared to that of ⁸⁹Zr. Plots of surface-bound and internalized radiotracers indicate similar behavior up to 24 h p.i. for both ¹²⁴I-J591 and ⁸⁹Zr-J591, with the effect of differential clearance rates becoming apparent after about 35 h p.i. Estimates of J591/PSMA complex turnover were 3.9–90.5 × 10¹² (for doses from 60 to 240 μg) molecules per hour per gram of tumor (20 % of receptors internalized per hour).

Conclusions

Using quantitative compartmental model methods, surface binding and internalization rates were shown to be similar for both ¹²⁴I-J591 and ⁸⁹Zr-J591 forms, as expected. The large difference in clearance rates of the radioactivity from the tumor is likely due to differential trapping of residualizing zirconium versus non-residualizing iodine. Our non-linear model was found to be superior to a conventional linear model. This finding and the calculated activity persistence time in tumor have important implications for radioimmunotherapy and other antibody-based therapies in patients.

Theranostic pretargeted radioimmunotherapy of colorectal cancer xenografts in mice using picomolar affinity ⁸⁶Y- or ¹⁷⁷Lu-DOTA-Bn binding scFv C825/GPA33 IgG bispecific immunoconjugates

PURPOSE

GPA33 is a colorectal cancer (CRC) antigen with unique retention properties after huA33-mediated tumor targeting. We tested a pretargeted radioimmunotherapy (PRIT) approach for CRC using a tetravalent bispecific antibody with dual specificity for GPA33 tumor antigen and DOTA-Bn-(radiolanthanide metal) complex.

METHODS

PRIT was optimized in vivo by titrating sequential intravenous doses of huA33-C825, the dextran-based clearing agent, and the C825 haptens (177)Lu-or (86)Y-DOTA-Bn in mice bearing the SW1222 subcutaneous (s.c.) CRC xenograft model.

RESULTS

Using optimized PRIT, therapeutic indices (TIs) for tumor radiation-absorbed dose of 73 (tumor/blood) and 12 (tumor/kidney) were achieved. Estimated absorbed doses (cGy/MBq) to tumor, blood, liver, spleen, and kidney for single-cycle PRIT were 65.8, 0.9 (TI 73), 6.3 (TI 10), 6.6 (TI 10), and 5.3 (TI 12), respectively. Two cycles of PRIT (66.6 or 111 MBq (177)Lu-DOTA-Bn) were safe and effective, with a complete response of established s.c. tumors (100 - 700 mm(3)) in nine of nine mice, with two mice alive without recurrence at >140 days. Tumor log kill in this model was estimated to be 2.1 - 3.0 based on time to 500-mm(3) tumor recurrence. In addition, PRIT dosimetry/diagnosis was performed by PET imaging of the positron-emitting DOTA hapten (86)Y-DOTA-Bn.

CONCLUSION

We have developed anti-GPA33 PRIT as a triple-step theranostic strategy for preclinical detection, dosimetry, and safe targeted radiotherapy of established human colorectal mouse xenografts.

Cerebral blood flow with [15O]water PET studies using an image-derived input function and MR-defined carotid centerlines

Full quantitative analysis of brain PET data requires knowledge of the arterial input function into the brain. Such data are normally acquired by arterial sampling with corrections for delay and dispersion to account for the distant sampling site. Several attempts have been made to extract an image-derived input function (IDIF) directly from the internal carotid arteries that supply the brain and are often visible in brain PET images. We have devised a method of delineating the internal carotids in co-registered magnetic resonance (MR) images using the level-set method and applying the segmentations to PET images using a novel centerline approach. Centerlines of the segmented carotids were modeled as cubic splines and re-registered in PET images summed over the early portion of the scan. Using information from the anatomical center of the vessel should minimize partial volume and spillover effects. Centerline time-activity curves were taken as the mean of the values for points along the centerline interpolated from neighboring voxels. A scale factor correction was derived from calculation of cerebral blood flow (CBF) using gold standard arterial blood measurements. We have applied the method to human subject data from multiple injections of [(15)O]water on the HRRT. The method was assessed by calculating the area under the curve (AUC) of the IDIF and the CBF, and comparing these to values computed using the gold standard arterial input curve. The average ratio of IDIF to arterial AUC (apparent recovery coefficient: aRC) across 9 subjects with multiple (n = 69) injections was 0.49 ± 0.09 at 0-30 s post tracer arrival, 0.45 ± 0.09 at 30-60 s, and 0.46 ± 0.09 at 60-90 s. Gray and white matter CBF values were 61.4 ± 11.0 and 15.6 ± 3.0 mL/min/100 g tissue using sampled blood data. Using IDIF centerlines scaled by the average aRC over each subjects' injections, gray and white matter CBF values were 61.3 ± 13.5 and 15.5 ± 3.4 mL/min/100 g tissue. Using global average aRC values, the means were unchanged, and intersubject variability was noticeably reduced. This MR-based centerline method with local re-registration to [(15)O]water PET yields a consistent IDIF over multiple injections in the same subject, thus permitting the absolute quantification of CBF without arterial input function measurements.

A Multimodal Approach to Image-Derived Input Functions for Brain PET

Many methods have been proposed for generating an image-derived input function (IDIF) exclusively from PET images. The purpose of this study was to assess the viability of a multimodality approach utilizing registered MR images. 3T-MR and HRRT-PET data were acquired from human subjects. Segmentation of both the left and right carotid arteries was performed in MR images using a 3D level sets method. Vessel centerlines were extracted by parameterization of the segmented voxel coordinates with either a single polynomial curve or a B-spline curve fitted to the segmented data. These centerlines were subsequently re-registered to static PET data to maximize the accurate classification of PET voxels in the ROI. The accuracy of this approach was assessed by comparison of the area under the curve (AUC) of the IDIF to that measured from conventional automated arterial blood sampling.Our method produces curves similar in shape to that of blood sampling. The mean AUC ratio of the centerline region was 0.40±0.19 before re-registration and 0.69±0.26 after re-registration. Increasing the diameter of the carotid ROI produced a smooth reduction in AUC. Thus, even with the high resolution of the HRRT, partial volume correction is still necessary. This study suggests that the combination of PET information with MR segmented regions will demonstrate an improvement over regions based solely on MR or PET alone.