Preclinical Efficacy of a PSMA-Targeted Thorium-227 Conjugate (PSMA-TTC), a Targeted Alpha Therapy for Prostate Cancer

PURPOSE

Prostate-specific membrane antigen (PSMA) is an attractive target for radionuclide therapy of metastatic castration-resistant prostate cancer (mCRPC). PSMA-targeted alpha therapy (TAT) has shown early signs of activity in patients with prostate cancer refractory to beta radiation. We describe a novel, antibody-based TAT, the PSMA-targeted thorium-227 conjugate PSMA-TTC (BAY 2315497) consisting of the alpha-particle emitter thorium-227 complexed by a 3,2-HOPO chelator covalently linked to a fully human PSMA-targeting antibody.

EXPERIMENTAL DESIGN

PSMA-TTC was characterized for affinity, mode of action, and cytotoxic activity in vitro. Biodistribution, pharmacokinetics, and antitumor efficacy were investigated in vivo using cell line and patient-derived xenograft (PDX) models of prostate cancer.

RESULTS

PSMA-TTC was selectively internalized into PSMA-positive cells and potently induced DNA damage, cell-cycle arrest, and apoptosis in vitro. Decrease in cell viability was observed dependent on the cellular PSMA expression levels. In vivo, PSMA-TTC showed strong antitumor efficacy with T/C values of 0.01 to 0.31 after a single injection at 300 to 500 kBq/kg in subcutaneous cell line and PDX models, including models resistant to standard-of-care drugs such as enzalutamide. Furthermore, inhibition of both cancer and cancer-induced abnormal bone growth was observed in a model mimicking prostate cancer metastasized to bone. Specific tumor uptake and efficacy were demonstrated using various PSMA-TTC doses and dosing schedules. Induction of DNA double-strand breaks was identified as a key mode of action for PSMA-TTC both in vitro and in vivo.

CONCLUSIONS

The strong preclinical antitumor activity of PSMA-TTC supports its clinical evaluation, and a phase I trial is ongoing in mCRPC patients (NCT03724747).

Synergistic Effect of a Mesothelin-Targeted 227Th Conjugate in Combination with DNA Damage Response Inhibitors in Ovarian Cancer Xenograft Models

Targeted ²²⁷Th conjugates (TTCs) represent a new class of therapeutic radiopharmaceuticals for targeted α-therapy. They comprise the α-emitter ²²⁷Th complexed to a 3,2-hydroxypyridinone chelator conjugated to a tumor-targeting monoclonal antibody. The high energy and short range of the α-particles induce antitumor activity, driven by the induction of complex DNA double-strand breaks. We hypothesized that blocking the DNA damage response (DDR) pathway should further sensitize cancer cells by inhibiting DNA repair, thereby increasing the response to TTCs. Methods: This article reports the evaluation of the mesothelin (MSLN)-TTC conjugate (BAY 2287411) in combination with several DDR inhibitors, each of them blocking different DDR pathway enzymes. MSLN is a validated cancer target known to be overexpressed in mesothelioma, ovarian, lung, breast, and pancreatic cancer, with low expression in normal tissue. In vitro cytotoxicity experiments were performed on cancer cell lines by combining the MSLN-TTC with inhibitors of ataxia telangiectasia mutated, ataxia telangiectasia and Rad3-related (ATR), DNA-dependent protein kinase, and poly[adenosine diphosphate ribose] polymerase (PARP) 1/2. Further, we evaluated the antitumor efficacy of the MSLN-TTC in combination with DDR inhibitors in human ovarian cancer xenograft models. Results: Synergistic activity was observed in vitro for all tested inhibitors (inhibitors are denoted herein by the suffix “i”) when combined with MSLN-TTC. ATRi and PARPi appeared to induce the strongest increase in potency. Further, in vivo antitumor efficacy of the MSLN-TTC in combination with ATRi or PARPi was investigated in the OVCAR-3 and OVCAR-8 xenograft models in nude mice, demonstrating synergistic antitumor activity for the ATRi combination at doses demonstrated to be nonefficacious when administered as monotherapy. Conclusion: The presented data support the mechanism-based rationale for combining the MSLN-TTC with DDR inhibitors as new treatment strategies in MSLN-positive ovarian cancer.

Synergistic Effect of a HER2 Targeted Thorium-227 Conjugate in Combination with Olaparib in a BRCA2 Deficient Xenograft Model

Targeted thorium-227 conjugates (TTCs) represent a novel class of therapeutic radiopharmaceuticals for the treatment of cancer. TTCs consist of the alpha particle emitter thorium-227 complexed to a 3,2-hydroxypyridinone chelator conjugated to a tumor-targeting monoclonal antibody. The high energy and short range of the alpha particles induce potent and selective anti-tumor activity driven by the induction of DNA damage in the target cell. Methods: The efficacy of human epidermal growth factor receptor 2 (HER2)-TTC was tested in combination in vitro and in vivo with the poly ADP ribose polymerase (PARP) inhibitor (PARPi), olaparib, in the human colorectal adenocarcinoma isogenic cell line pair DLD-1 and the knockout variant DLD-1 BRCA2 -/- Results: The in vitro combination effects were determined to be synergistic in DLD-1 BRCA2 -/- and additive in DLD-1 parental cell lines. Similarly, the in vivo efficacy of the combination was determined to be synergistic only in the DLD-1 BRCA2 -/- xenograft model, with statistically significant tumor growth inhibition at a single TTC dose of 120 kBq/kg body weight (bw) and 50 mg/kg bw olaparib (daily, i.p. for 4 weeks), demonstrating comparable tumor growth inhibition to a single TTC dose of 600 kBq/kg bw. Conclusions: This study supports the further investigation of DNA damage response inhibitors in combination with TTCs as a new strategy for the effective treatment of mutation-associated cancers.

Influx rate of 18F-fluoroaminosuberic acid reflects cystine/glutamate antiporter expression in tumour xenografts

PURPOSE

¹⁸F-fluoroaminosuberic acid (¹⁸F-FASu) is a recently developed amino acid tracer for positron emission tomography (PET) of oxidative stress that may offer improved tumour assessment over the conventional tracer ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Our aim was to evaluate and relate dynamic ¹⁸F-FASu and ¹⁸F-FDG uptake with pharmacokinetic modelling to transporter protein expression levels in a panel of diverse tumour xenograft lines.

METHODS

Four different tumour xenograft lines were implanted in female athymic nude mice: MAS98.12 and HBCx3 (breast), TPMX (osteosarcoma) and A549 (lung). Dynamic PET over 60 min was performed on a small animal unit. The time-activity curves (TACs) for ¹⁸F-FASu and ¹⁸F-FDG in individual tumours were used to extract early (SUV_E; 2 min p.i.) and late (SUV_L; 55 min p.i.) standardised uptake values. Pharmacokinetic two-tissue compartment models were applied to the TACs to estimate rate constants K₁-k₄ and blood volume fraction v_B. Relative levels of cystine/glutamate antiporter subunit xCT were assessed by western blotting, and expression of GLUT1 and CD31 by immunohistochemistry.

RESULTS

¹⁸F-FASu showed higher SUV_E, whilst ¹⁸F-FDG exhibited higher SUV_L. Influx rate K₁ for ¹⁸F-FASu was significantly correlated with xCT levels (p = 0.001) and was significantly higher than K₁ for ¹⁸F-FDG (p < 0.001). K₁ for ¹⁸F-FDG was significantly correlated with GLUT1 levels (p = 0.002). v_B estimated from ¹⁸F-FASu and ¹⁸F-FDG TACs was highly consistent and significantly correlated (r = 0.85, p < 0.001). Two qualitatively different ¹⁸F-FASu uptake profiles were identified: type α with low xCT expression and low K₁ (A549 and HBCx3), and type β with high xCT expression and high K₁ (MAS98.12 and TPMX).

CONCLUSION

The influx rate of ¹⁸F-FASu reflects xCT activity in tumour xenografts. Dynamic PET with pharmacokinetic modelling is needed to fully appraise ¹⁸F-FASu distribution routes.

Abstract 5200: Preclinical pharmacology of the PSMA-targeted thorium-227 conjugate PSMA-TTC: a novel targeted alpha therapeutic for the treatment of prostate cancer

Prostate-specific membrane antigen (PSMA, FOLH1) is a type II transmembrane glycoprotein of the M28 peptidase family that acts as a glutamate carboxypeptidase on various substrates. PSMA is well established as a target antigen in prostate cancer due to its high and specific overexpression on the surface of prostate cancer cells at all tumor stages, including metastatic and hormone-refractory disease. Several PSMA targeting antibodies and ligands are currently in clinical development or compassionate use therapeutically or as imaging agents. Targeted alpha therapy (TAT) has an established clinical profile with the successful transition of Ra223, an alpha-particle emitter, from bench to bedside in prostate cancer. Thorium-227 is the immediate precursor for Ra223 via alpha-particle emission. We herein describe the generation of a novel TAT, a high energy, alpha-particle emitting PSMA-targeted thorium-227 conjugate (PSMA-TTC). PSMA-TTC consists of a fully human PSMA targeting IgG1 antibody covalently linked via an amide bond to a chelator moiety (3,2 HOPO), enabling radiolabeling with thorium-227 (227Th).

PSMA-TTC was prepared in high radiochemical yield and purity and tested for binding affinity to PSMA target (ELISA) as well as PSMA expressing cell lines (FACS). In vitro cytotoxicity experiments were carried out on prostate CA cell lines with different PSMA levels (from 3.000 to 150.000 mAbs bound/ cell). In vivo biodistribution and anti-tumor efficacy were analyzed after i.v. injection of 100-500 kBq/kg at protein doses of 0.14 mg/kg to mice bearing s.c. prostate cancer xenograft models. Additionally, anti-tumor efficacy was evaluated in a PSMA expressing orthotopic bone xenograft model (LNCaP-Luc) monitored by bioluminescence imaging, micro CT and x-ray.

PSMA-TTC retains binding affinities to PSMA target and PSMA positive cancer cells similar to the PSMA antibody. Strong in vitro potency and selectivity of PSMA-TTC was shown on different PSMA positive cells. Biodistribution studies in C4-2 xenografts demonstrated specific tumor uptake of PSMA-TTC with a maximum of 50 % of ID/g at t = 72h post dose administration. Selective significant antitumor efficacy was shown for PSMA-TTC in s.c. prostate CA xenograft models with high (C4-2) and medium/low (22Rv1) PSMA protein levels at doses of 250 and 500 kBq/kg. Furthermore, statistically significant prevention of tumor growth was observed after treatment with PSMA-TTC at a dose of 100 kBq/kg in an orthotopic bone xenograft model (LNCaP-Luc).

The promising preclinical antitumor activity of PSMA-TTC supports its development for the treatment of patients with metastatic prostate cancer.

Citation Format: Stefanie Hammer, Aasmund Larssen, Christine Ellingsen, Solene Geraudie, Derek Grant, Baard Indrevoll, Oliver von Ahsen, Alexander Kristian, Urs B Hagemann, Jenny Karlsson, Roger M Bjerke, Olav B Ryan, Dominik Mumberg, Bertolt Kreft, Alan Cuthbertson. Preclinical pharmacology of the PSMA-targeted thorium-227 conjugate PSMA-TTC: a novel targeted alpha therapeutic for the treatment of prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5200. doi:10.1158/1538-7445.AM2017-5200

Abstract 5859: HER2-targeted thorium-227 conjugate (HER2-TTC): Efficacy in preclinical models of trastuzumab and T-DM1 resistance

The human epidermal growth factor receptor 2 (HER2) is encoded by the proto-oncogene c-erbB-2 and initiates downstream signaling pathways leading to cell proliferation and tumorigenesis. HER2 is overexpressed in several cancer (Ca) types and is one of the most strongly validated targets for the treatment of breast and gastric cancer serving as both a prognostic and predictive biomarker. Several HER2-targeting antibodies as well as antibody-drug conjugates are either approved or are in clinical development. Prolonged treatment with monoclonal antibodies and antibody drug conjugates have resulted in development of resistance and so there is still an unmet medical need for drugs of new mechanism of action targeting this important receptor system. We describe herein the generation of a high energy, alpha-particle emitting HER2 targeted thorium-227 antibody-chelator conjugate. HER2-TTC consists of the humanized HER2 targeting IgG1 antibody (trastuzumab) covalently linked via an amide bond to a 3,2-hydroxypyridino-based chelator moiety, enabling efficient radiolabeling with the alpha particle emitting radionuclide thorium-227 (Th-227). HER2-TTC was prepared at high radiochemical yield and purity. When tested for binding to recombinant HER2, HER2-TTC was shown to retain comparable binding affinity to trastuzumab. In vitro cytotoxicity experiments were performed on 8 cell lines with different HER2 expression levels (from 7 000 - 500 000 mAbs bound/ cell as determined by FACS) of breast, ovarian, gastric and lung cancer origin. HER2-TTC demonstrated target mediated in vitro cytotoxicity in the pM-range. In vivo biodistribution and anti-tumor efficacy of HER2-TTC was evaluated in the dose range 100-500 kBq/kg at a protein dose of 0.14 mg/kg and i.v. injection in the s.c. KPL-4 breast and Calu-3 lung model previously described to be resistant to trastuzumab. The biodistribution study demonstrated specific tumor accumulation of HER2-TTC in both models with a maximum of 77 and 50 %ID/g 227Th at t = 168 h post dose (decay corrected to T0), respectively. Significant antitumor efficacy was shown for HER2-TTC in the JIMT-1 s.c. breast Ca xenograft model resistant to trastuzumab and T-DM1. The promising preclinical anti-tumor activity supports the development of the targeted alpha therapeutic HER2-TTC for the treatment of trastuzumab and T-DM1 resistant patients.

Citation Format: Jenny Karlsson, Urs B. Hagemann, Christoph Schatz, Derek Grant, Alexander Kristian, Christine Ellingsen, Dessislava Mihaylova, Solene Geraudie, Bård Indrevoll, Uta Wirnitzer, Roger M. Bjerke, Olav B. Ryan, Carl F. Nising, Dominik Mumberg, Alan Cuthbertson. HER2-targeted thorium-227 conjugate (HER2-TTC): Efficacy in preclinical models of trastuzumab and T-DM1 resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5859. doi:10.1158/1538-7445.AM2017-5859

Abstract 5857: HER2-targeted thorium-227 conjugate (HER2-TTC): Efficacy in a HER2 positive orthotopic bone model

In 2015, the estimated incidence of new breast cancer (Ca) cases in the US was 234.190 and number of deaths 40.730. Human epidermal growth factor receptor 2 (HER2) is encoded by the proto-oncogene c-erbB-2 and initiates downstream signaling pathways leading to cell proliferation and tumorigenesis. HER2 is overexpressed in several cancer types and has emerged as one of the most strongly validated targets for the treatment of breast and gastric cancer serving as both a prognostic and predictive biomarker. Given that 20% of breast Ca patients are HER2 positive and 70% of patients with metastatic disease will develop bone metastases and associated morbidities, there is still an unmet medical need for improved therapies targeting HER2. Radium-223 (Ra-223) is a novel targeted alpha therapeutic for treatment of patients with castration-resistant prostate cancer and bone metastases. Localized high energy alpha particle emission induces double-stranded DNA breaks and cellular apoptosis. Thorium-227 (Th-227) is the immediate precursor for Ra-223 which, in contrast to Ra-223, can be complexed by chelating agents at high affinity, allowing targeted delivery to tumor cells via antibodies. We describe herein the generation of a novel HER2-targeted Th-227 conjugate (HER2-TTC). HER2-TTC consists of the humanized HER2 targeting IgG1 antibody trastuzumab covalently linked via an amide bond to a 3,2-hydroxypyridino-based chelator moiety, enabling efficient radiolabeling with the alpha particle emitting radionuclide Th-227. Anonymized samples of consenting breast cancer patients were analyzed by Immunohistochemistry (IHC). The IHC data demonstrated HER2 positive expression in breast tumor and matched bone metastases, supporting the preclinical evaluation of the anti-tumor efficacy of HER2-TTC in the BT-474 orthotopic bone mouse model. HER2-TTC was prepared at high radiochemical yield and purity. When tested for binding to recombinant HER2, HER2-TTC was shown to retain comparable binding affinity to trastuzumab. In vitro cytotoxicity experiment of HER2-TTC demonstrated target mediated in vitro cytotoxicity in the pM-range on breast cancer cell line BT-474 (430 000 mAbs bound/ cell as determined by FACS). Anti-tumor efficacy of HER2-TTC was evaluated at 250 and 500 kBq/kg at a protein dose of 0.14 mg/kg. X-ray imaging, serum bone formation marker PINP, micro CT 3D reconstruction imaging and histological analysis demonstrated significantly reduced bone lesions and tumor induced bone remodeling. The promising preclinical anti-tumor activity supports the development of the HER2-TTC as a novel targeted alpha therapeutic for the treatment of patients with HER2 positive bone metastatic disease.

Citation Format: Jenny Karlsson, Urs B. Hagemann, Christoph Schatz, Derek Grant, Christine Ellingsen, Alexander Kristian, Dessislava Mihaylova, Steinar R. Uran, Mari Suominen, Roger M. Bjerke, Olav B. Ryan, Carl F. Nising, Dominik Mumberg, Alan Cuthbertson. HER2-targeted thorium-227 conjugate (HER2-TTC): Efficacy in a HER2 positive orthotopic bone model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5857. doi:10.1158/1538-7445.AM2017-5857

Abstract 5199: Preclinical activity of the FGFR2-targeted thorium-227 conjugate in preclinical models of colorectal, gastric and triple-negative breast cancer

FGFR2 is a transmembrane tyrosine kinase receptor, consisting of three extracellular N-terminal immunoglobulin-like domains which are involved in ligand-binding as well as in receptor dimerization. Ligand-independent activation of FGFR2 signaling either via genomic amplification, gene fusion events, mRNA overexpression, or mutations has been observed e.g. in gastric cancer, colorectal cancer (CRC), and triple-negative breast cancer (TNBC). As such, FGFR2 has been described to be involved in cancer progression, promotion of oncogenesis, neoangiogenesis, as well as resistance to targeted therapies. Overexpression of FGFR2 and relatively low levels of cell surface expression of FGFR2 in normal human tissues renders FGFR2 an attractive candidate to explore targeted alpha therapy (TAT). We describe the generation of a high energy, alpha-particle emitting FGFR2 targeted thorium-227 conjugate (FGFR2-TTC). The FGFR2-TTC consists of a fully human FGFR2 binding IgG1 antibody (BAY 1179470) cross-reactive with mouse FGFR2, covalently linked via an amide bond to a chelator moiety (3,2 HOPO), enabling radiolabeling with the alpha particle emitting thorium-227 (227Th). In vitro cytotoxicity experiments with FGFR2-TTC demonstrated potency in the sub-picomolar range compared to a non-targeting control-TTC and a correlation between decrease in cell viability and increasing number of anti-FGFR2 antibodies bound per cell (ABC counts) in a panel of FGFR2-positive cancer cell lines. Upon treatment of cells with FGFR2-TTC, the DNA damage response marker protein γH2AX was up-regulated indicating that the mode-of-action involves induction of DNA double strand breaks. Furthermore, induction of the immunogenic cell death marker calreticulin was observed Biodistribution studies of the FGFR2-TTC in mouse models, evaluated by whole body autoradiography and acquisition of gamma-spectra specific for thorium-227, demonstrated specific accumulation of thorium-227 in FGFR2-positive tumors and very limited signal in murine organs and tissues. FGFR2-TTC exhibited in vivo tumor growth inhibition after a single dose in mouse xenograft models of CRC (NCI-H716) and gastric cancer (SNU-16). In addition, FGFR2-TTC showed anti-tumor activity in the aggressive murine syngeneic orthotopic 4T1 TNBC model. In summary, FGFR2-TTC has been established as a promising targeted alpha therapy (TAT) for efficacious and selective delivery of alpha emitter-based radiotherapy in several FGFR2-positive cancer indications. Further exploration for cancer therapy may thus be of interest.

Citation Format: Urs B. Hagemann, Anette Sommer, Alexander Kristian, Ellen Wang, Åsmund Larsen, Uta Wirnitzer, Heidrun Ellinger-Ziegelbauer, Steffen Sandmann, Thorsten Poethko, Jenny Karlsson, Roger M. Bjerke, Lars Linden, Bertolt Kreft, Hanno Wild, Alan S. Cuthbertson. Preclinical activity of the FGFR2-targeted thorium-227 conjugate in preclinical models of colorectal, gastric and triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5199. doi:10.1158/1538-7445.AM2017-5199

Abstract 2467: SPECT-imaging guided development and evaluation of targeted alpha therapy (TAT) for colorectal liver metastasis

BACKGROUND: Only a small fraction of patients with colorectal liver metastases (CLM) is eligible for curative resection, and novel treatment options are highly needed. The high linear-energy transfer of short-ranged alpha particles can induce complex double-stranded DNA breaks, leading to cell death. With no known resistance mechanism, alpha-particle emitting isotopes represent a promising tool in cancer management. In targeted alpha therapy (TAT), an alpha-particle emitting isotope is attached to a tumor-targeting antibody (hereafter TAT antibody), ensuring specific delivery to the tumor. By substituting the alpha-particle emitting isotope (Actinium-225, 225Ac) with a diagnostic isotope (Zirconium-89, 89Zr), the in-vivo distribution of the TAT antibody can be monitored using PET/SPECT/CT. The aim of this work was to set up robust labeling and PET/SPECT protocols to evaluate TAT in CLM using novel experimental models.

METHODS: For SPECT-imaging of 225Ac daughter isotopes, the combined gamma emission spectra were gated at 218 keV ± 20% and 440 keV ± 20% for Francium-221 and Bismuth-213, respectively. 89Zr was labeled to a desferrioxamine* (DFO*) conjugated TAT antibody, which recognizes both the murine and human antigen. Indium-111 (111In) labeling was performed through diethylenetriaminepentaacetic acid (DTPA) conjugated to an isotype antibody. Radiochemical purity and protein integrity were assessed by size exclusion HPLC, iTLC and SDS-PAGE. For orthotopic CLM model development, 106 cells of human colorectal cancer cell lines HT-55 or LS1034 were injected in the spleen of female Rj:NMRI-Foxn1nu mice, followed by splenectomy. Tumor growth was monitored by T2-weighted MR-imaging. Non-tumor bearing mice (n = 12, 3 per group) were injected with 4 MBq of 10, 20, 50 or 100 µg 89Zr-TAT antibody to evaluate biodistribution and the mice scanned on day 2, 4 and 6 after iv injection.

RESULTS: Francium-221 and Bismuth-213 could be separately visualized after free Actinium-225 administration (i.v.). 89Zr and 111In were routinely labeled to their respective antibody with up to 500 MBq/mg with &gt;95% radiochemical purity. 89Zr-DFO*-TAT antibody showed &gt;95% radiochemical purity and &gt;95% protein integrity after 7 days stability test in serum, shown by iTLC and SDS-PAGE. SPECT/CT of 89Zr-DFO*-TAT antibody in non-tumor bearing mice showed little uptake in healthy tissues apart from the liver due to hepatic antibody clearance. Orthotopic models of CLM show reproducible development of metastases for HT-55 and LS1034 within 3 weeks.

CONCLUSION: Robust labeling methods, PET/SPECT imaging protocols and suitable animal models of CLM were established. Results of our dual-isotope SPECT/CT imaging of our 89Zr-DFO*-TAT antibody in comparison with an 111In-labeled isotype antibody, together with efficacy data of the TAT, will be presented at the meeting.

Citation Format: Frans V. Suurs, Alexander Kristian, Gebregziabher Petros, Yuan Zeng Feng, Karianne G. Fleten, Roger M. Bjerke, Alan Cuthbertson, Kjersti Flatmark. SPECT-imaging guided development and evaluation of targeted alpha therapy (TAT) for colorectal liver metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2467.

Abstract 3937: MSLN-TTC (BAY 2287411) demonstrates increased activity in comparison to standard of care chemotherapy in models of acquired drug resistance

Mesothelin (MSLN)-targeted thorium conjugate (MSLN-TTC; BAY 2287411) is a targeted alpha therapy consisting of the MSLN-targeting antibody anetumab, covalently attached to a 3,2-HOPO chelator complexing the alpha-emitter thorium-227. We previously demonstrated the potent in vivo activity of MSLN-TTC in cell line and patient-derived xenograft models, and further, demonstrated its combination potential with DNA damage repair inhibitors. MSLN-TTC is currently in clinical development for the treatment of patients with MSLN-positive mesothelioma and ovarian cancer (NCT03507452). Development of cellular resistance mechanisms are often co-evolutionary to the treatment regimen given to the respective individual. As such, it is well described that ovarian cancer patients have a decreased overall survival when tested positive for markers of acquired drug resistance (ADR), e.g. phospho-glycoprotein (P-gp) (Herzog TJ et al; Oncotarget 2016). However, MSLN-TTC should also be effective independent of P-gp expression, as the major mode of action of MSLN-TTC is the induction of irreparable DNA double-strand breaks upon alpha decay by thorium-227, resulting in apoptotic cell death. To confirm this hypothesis, we first determined the involvement of P-gp in cellular resistance mechanisms in ovarian cancer. To this end, immunohistochemical staining of ovarian cancer patient samples for P-gp was performed. Expression of P-gp was observed in 15% (7/48) primary and relapsing specimens. We then evaluated the potency of MSLN-TTC in MSLN-expressing cell line models in vitro using the isogenic cell line pair OVCAR-8 (P-gp negative) and NCI-RES-ADR (P-gp positive) in comparison to the standard of care (SOC) therapies cisplatin, paclitaxel, doxorubicin and vinorelbine. Whereas paclitaxel and doxorubicin showed a 100- and 10-fold decrease in in vitro potency on the P-gp positive cell line NCI-RES-ADR vs the P-gp negative cell line OVCAR-8, the potency of MSLN-TTC on the two isogenic cell lines remained unaffected.

To further confirm and explore these observations, in vivo xenograft models were conducted. Similar to the in vitro results, the in vivo efficacy of MSLN-TTC was not affected by the P-gp status in the OVCAR-8 vs NCI-RES-ADR xenograft model. In contrast, limited efficacy of paclitaxel and doxorubicin was observed in the P-gp-positive MSLN-expressing NCI-RES-ADR and Hela-MATU-ADR models. Similar, vinorelbine showed no efficacy in the Hela-MATU-ADR xenograft. In summary, these data demonstrate that the efficacy of MSLN-TTC is independent of the P-gp status. Further, these data suggest that MSLN-TTC may be active in patients relapsing after standard therapy.

Citation Format: Urs B. Hagemann, Sabine Zitzmann-Kolbe, Alexander Kristian, Carolyn Sperl, Christoph A. Schatz, Roger M. Bjerke, Alan S. Cuthbertson, Hartwig Hennekes, Karl Ziegelbauer, Dominik Mumberg. MSLN-TTC (BAY 2287411) demonstrates increased activity in comparison to standard of care chemotherapy in models of acquired drug resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3937.

Abstract 5226: In vivo imaging and molecular characterization of site-specific growth of malignant melanoma: a study of melanoma metastasis in experimental animal models

Metastatic melanoma is a very aggressive and fatal disease in need of novel therapeutic targets. Melanomas are notorious for the ability to disseminate to almost any organ. The ability to develop metastases at distant sites is not only a function of intrinsic properties of the malignant cells themselves, but is also influenced by a wide range of factors in the host microenvironment. Understanding of the biological role of the microenvironment in the establishment of metastases and identifying implicated factors, might uncover novel potential targets for anti-metastatic therapy. The aim of our study was to reveal characteristics of metastatic growth of melanoma in different anatomical sites in animal models through in vivo imaging and molecular approaches.

We have developed experimental metastasis models in nude mice based on intracardial (left ventricle) injection of the melanoma cell lines Melmet 1 and Melmet 5, established from two metastatic melanoma patients with different clinical presentations. A follow-up of melanoma cell migration and micro/macrometastases formation assessed by molecular and functional imaging in vivo (IVIS and PET), indicated that Melmet 5 could establish metastases in multiple organs (including brain, lung, lymph nodes, kidney, bone), while Melmet 1 displayed a more consistent preference for the brain. Flow cytometric analysis of metastatic organs, specifically brain and lungs, revealed the recruitment of bone marrow derived cells, identified as CD11b+/Gr-1+ myeloid cells and CD11b+/F4/80+ macrophages. These cell subpopulations have been linked to the formation of a (pre)metastatic niche, and we are presently investigating their contribution to the metastatic growth of melanoma. Further, we performed a comparative microarray study to investigate differential gene expression with respect to organ-specific environments. Specifically, we identified a set of genes altered in immunoselected melanoma cells purified from brain tissue. Among these genes were receptors involved in glutamate signaling, indicating metabolic changes associated with metastatic colonization of the brain. Furthermore, after separation of tumor cells, the remaining host brain tissue was also analyzed by gene expression profiling in order to reveal molecular changes induced in the stroma compartment by metastases. Compared to non-diseased brain, we found altered expression of genes implicated in biological activities such as immune defense, evasion of apoptosis and cell adhesion and migration. Based on the results obtained, we are currently performing functional studies on candidate genes to elucidate their role in metastatic growth of melanoma. Targeting focal points of communication between metastatic tumor cells and the host in distinct microenvironments may represent an anti-metastatic therapeutic strategy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5226. doi:10.1158/1538-7445.AM2011-5226

Tumor growth inhibition and immune system activation following treatment with thorium-227 conjugates and PD-1 check-point inhibition in the MC-38 murine model

Targeted thorium-227 conjugates comprise the combination of a monoclonal antibody with specificity for a tumor cell antigen and a 3,2-HOPO chelator enabling complexation of thorium-227 (Th-227). The radiolabeled conjugate functions as an effective delivery system of alpha-particle radiation to the surface of the tumor cell inducing difficult to repair complex DNA damage and cell death. In addition, the mechanism of action of targeted alpha therapy (TAT) appears to involve a significant component linked to stimulation of the immune system. We report herein evidence of immune activation and long-lasting immune protection of a TAT in a syngeneic model using the MC-38 murine cell line. Firstly, MC-38 cells were irradiated ex vivo with the thorium labeled antibody before subcutaneous implantation into mice. These mice were then rechallenged with MC-38 cells contra-laterally. In the group receiving irradiated cells, 9 out of 10 animals had no measurable tumor growth compared to aggressive tumor growth in the control group. Secondly, in an efficacy study, 500 kBq/kg of thorium labeled antibody alone or in combination with PD-1 checkpoint inhibitor gave statistically significant tumor growth inhibition compared to vehicle control. Animals with no measurable tumors were once again rechallenged contra-laterally with MC-38 cells. The re-growth of tumors was significantly delayed (approx. 60 days) in the treatment group compared to age-matched controls (approx. 30 days) in the monotherapy group. Interestingly, in the TAT/ PD-1 combination group no re-growth was observed demonstrating the potential of combining a TAT with checkpoint inhibition therapy. Finally, tumors were excised from treated mice and analyzed by flow cytometry and immunohistochemistry (IHC). Analysis revealed significant infiltration of CD8+ T-cells and mature dendritic cells compared to vehicle controls. Together these results indicated that an ongoing immune response from treatment with alpha radiation could be enhanced by check-point inhibition.

Abstract 5440: Development of a novel PSMA-targeting small molecule-Thorium-227 conjugate for the treatment of prostate cancer

Aim/Introduction: PSMA (prostate-specific membrane antigen) is a membrane-bound N-carboxypeptidase that is highly expressed on prostate cancer cells and has been used extensively as a homing target for the treatment of prostate cancer. Peptidomimetic inhibitors of the enzymatic functionality of PSMA have been shown to be able to deliver conjugated cargo into prostate cancer cells. Most prominently PSMA-617 carrying a DOTA chelator has been labelled with therapeutic radionuclides like beta-emitting lutetium-177 or alpha-emitting actinium-225 and has shown clinical activity in late stage prostate cancer patients. We sought to develop a new PSMA-targeting small molecule conjugate that shows high uptake in PSMA-expressing tumors, is quickly cleared from the circulation, and can easily be labeled with the alpha emitter thorium-227 (half-life 18.7d).

Materials and Methods: Using PSMA-617 as a starting point for our venture we were able to identify a novel linker motif that showed improved competition against 3H-PSMA-617 on PSMA-expressing LNCaP cells. Furthermore, we developed a new 2,3-hydroxypyridinone chelator (carboxy-HOPO) modified with carboxyl groups to increase hydrophilicity. The carboxy-HOPO conjugates were labelled with thorium-227 and evaluated in cellular binding, internalization, and antiproliferation assays. Furthermore, the thorium-227 labelled conjugates were tested in vivo for pharmacokinetics and efficacy in mouse xenograft models.

Results: The conjugates were readily labeled with thorium-227 at room temperature and possessed high stability and improved binding, internalization and antiproliferative activity in vitro. In vivo therapeutic studies with our new thorium-227-labelled carboxy-HOPO-PSMA small molecule conjugate in mice with PSMA-expressing tumor LNCaP xenografts showed high uptake and retention in the tumors and very fast clearance from the blood. Besides uptake in the kidneys no major uptake or retention into other organs was observed. Low uptake of free thorium-227 into the bones confirms the high complex stability in vivo with no leakage of thorium-227 from the carboxy-HOPO chelator. Single dosing with 1.5MBq/kg led to good treatment efficacy. No signs of toxicity were observed in the treated mice during the course of the study encouraging further development of the compound.

Conclusion: In summary we developed a novel PSMA-targeting carboxy-HOPO small molecule conjugate that readily complexed thorium-227, showed high uptake into PSMA-expressing cells and gave statistically significant inhibition of tumor growth in tumor xenograft models compared to control groups.

Citation Format: Niels Boehnke, Bård Indrevoll, Stefanie Hammer, Alex Papple, Alexander Kristian, Alan Cuthbertson, Sabine Zitzmann-Kolbe. Development of a novel PSMA-targeting small molecule-Thorium-227 conjugate for the treatment of prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5440.

Abstract 4909: The oncolytic peptide LTX-315 enhances tumor-specific immune responses and tumor regression in murine 4T1 breast cancer when combined with doxorubicin

The oncolytic effect of LTX-315 involves perturbation of the plasma membrane and distortion of the mitochondrial membrane with subsequent release of DAMPs (Damage-Associated Molecular Pattern molecules) such as ATP, cytochrome C and HMGB1 (1,2). Also multidomain proteins from the BCL-2 family seem to be partially involved in LTX-315 mediated killing (3). In addition, LTX-315 induces the hallmarks of immunogenic cell death (4). Preclinical studies have demonstrated that LTX-315 is able to induce complete tumour regression after intra-tumoral treatment (5,6). In this study the effect of combining intratumoral administration of LTX-315 and systemic treatment with Doxorubicin was investigated in the murine 4T1 breast cancer model. The combination treatment was investigated both in s.c. tumors and in tumors established in the mammary fat pad. The 4T1 cell were labeled with mcherry reporter gene in order to assess tumor growth by whole body imaging. LTX-315 (1 mg in 50 ìl) was injected intratumorally for three consecutive days. Doxorubicin (8mg/kg) was injected into the tail vein as a single dose together with the first injection of LTX-315. Tumor growth was assessed by caliper measurements and whole body luminescence. The combination of local administration of LTX-315 and systemic Doxorubicin, demonstrated significant synergy in both s.c. tumors and in tumors growing in the mammary fat pad. Furthermore, the combination induced a systemic tumour specific immune response since tumors did not develop in cured animals when re-challenged with tumor cells. The tolerable safety profile and novel mode of action of LTX-315 indicate a rationale for combining LTX-315 with other immunotherapies, targeted agents, and chemotherapy.

References:

1) Forveille et al., Cell Cycle (2015)

2) Eike et al., Oncotarget (2015)

3) Zhou et al., Oncotarget (2015)

4) Zhou et al. Cell Death & Differentiation (In rev)

5) Camilio et al., Cancer Immunol Immunother (2014)

6) Camilio et al., Oncoimmunology (2014)

Citation Format: Meng Yu Wang, Alexander Kristian, Ketil Camilio, Baldur Sveinbjornsson, Gunhild Mari Mælandsmo, Gunnar Kvalheim, Øystein Rekdal. The oncolytic peptide LTX-315 enhances tumor-specific immune responses and tumor regression in murine 4T1 breast cancer when combined with doxorubicin. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4909.

The SRC-family serves as a therapeutic target in triple negative breast cancer with acquired resistance to chemotherapy

BACKGROUND

Resistance to chemotherapy, combined with heterogeneity among resistant tumors, represents a significant challenge in the clinical management of triple negative breast cancer (TNBC). By dissecting molecular pathways associated with treatment resistance, we sought to define patient sub-groups and actionable targets for next-line treatment.

METHODS

Bulk RNA sequencing and reverse phase protein array profiling were performed on isogenic patient-derived xenografts (PDX) representing paclitaxel-sensitive and -resistant tumors. Pathways identified as upregulated in the resistant model were further explored as targets in PDX explants. Their clinical relevance was assessed in two distinct patient cohorts (NeoAva and MET500).

RESULTS

Increased activity in signaling pathways involving SRC-family kinases (SFKs)- and MAPK/ERK was found in treatment resistant PDX, with targeted inhibitors being significantly more potent in resistant tumors. Up-regulation of SFKs- and MAPK/ERK-pathways was also detected in a sub-group of chemoresistant patients after neoadjuvant treatment. Furthermore, High SFK expression (of either SRC, FYN and/or YES1) was detected in metastatic lesions of TNBC patients with fast progressing disease (median disease-free interval 27 vs 105 months).

CONCLUSIONS

Upregulation of SFK-signaling is found in a subset of chemoresistant tumors and is persistent in metastatic lesions. Based on pre-clinical results, these patients may respond favorably to treatment targeting SFKs.

Abstract 850: Mesothelin targeted thorium-227 conjugate (MSLN-TTC): Preclinical evaluation of a new targeted alpha therapeutic in mesothelin-positive cancers

Targeted Thorium-227 Conjugates (TTCs) represent a new class of targeted alpha therapy. In this compound family a 3,2-HOPO chelator, which binds thorium-227 with high affinity, is covalently attached to an antibody. This enables the specific delivery of the alpha particle emitter thorium-227 to tumor cells. Thorium-227 has a half-life of 18.7 days and decays via emission of an alpha particle to radium-223 (half-life of 11.4 days), a calcium-mimetic used in the treatment of CRPC [Henriksen et al. J Nucl Med, 2003]. The high linear energy transfer from the alpha-emitter thorium-227 induces clustered DNA double-strand breaks. Its short penetration range of 2-10 cell diameters limits the damage to the normal tissue surrounding the tumor. We present the preclinical evaluation of a mesothelin targeted thorium-227 conjugate (MSLN-TTC), the first TTC that will enter clinical development in MSLN-positive solid tumor indications, based on the fully human anti-MSLN monoclonal antibody anetumab. MSLN is a 40 kDa membrane-anchored glycoprotein with prominent overexpression in mesothelioma, ovarian, pancreatic, lung and breast cancer. In normal tissue, MSLN is confined mainly to the mesothelial cells of pleura, peritoneum and pericardium. In vitro, the mode of action of MSLN-TTC in cellular assays was demonstrated to induce DNA double strand breaks, leading to cell cycle arrest and subsequent reduced cell viability. In vivo, MSLN-TTC demonstrated potent tumor growth inhibition administered as a single-dose in cell- and patient-derived xenograft tumor models. Similar anti-tumor activity to single dose application was observed when the MSLN-TTC was applied at fractionated doses. A trend for dependence of anti-tumor activity on MSLN expression levels in preclinical tumor models was observed. Biodistribution studies evaluated the tumor accumulation of MSLN-TTC in xenograft models. These studies served to develop a mechanistic PK/PD model, which was used to predict the efficacious dose in humans. The initiation of clinical investigation of the MSLN-TTC in mesothelin positive cancers' is planned for 2018.

Citation Format: Urs B. Hagemann, Alexander Kristian, Christine Ellingsen, Katrine Wickstroem, Anne Mobergslien, Jenny Karlsson, Roger M. Bjerke, Christoph Schatz, Christoph Kneip, Joachim Schuhmacher, Liv-Ingrid Oedegaardstuen, Hartwig Hennekes, Anna Tafuri, Dominik Mumberg, Hanno Wild, Karl Ziegelbauer, Alan S. Cuthbertson. Mesothelin targeted thorium-227 conjugate (MSLN-TTC): Preclinical evaluation of a new targeted alpha therapeutic in mesothelin-positive cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 850.

Efficacy of a HER2-Targeted Thorium-227 Conjugate in a HER2-Positive Breast Cancer Bone Metastasis Model

Human epidermal growth factor receptor 2 (HER2) is overexpressed in 15-30% of breast cancers but has low expression in normal tissue, making it attractive for targeted alpha therapy (TAT). HER2-positive breast cancer typically metastasizes to bone, resulting in incurable disease and significant morbidity and mortality. Therefore, new strategies for HER2-targeting therapy are needed. Here, we present the preclinical in vitro and in vivo characterization of the HER2-targeted thorium-227 conjugate (HER2-TTC) TAT in various HER2-positive cancer models. In vitro, HER2-TTC showed potent cytotoxicity in various HER2-expressing cancer cell lines and increased DNA double strand break formation and the induction of cell cycle arrest in BT-474 cells. In vivo, HER2-TTC demonstrated dose-dependent antitumor efficacy in subcutaneous xenograft models. Notably, HER2-TTC also inhibited intratibial tumor growth and tumor-induced abnormal bone formation in an intratibial BT-474 mouse model that mimics breast cancer metastasized to bone. Furthermore, a match in HER2 expression levels between primary breast tumor and matched bone metastases samples from breast cancer patients was observed. These results demonstrate proof-of-concept for TAT in the treatment of patients with HER2-positive breast cancer, including cases where the tumor has metastasized to bone.

Abstract 855: Increased in vitro potency and in vivo efficacy of FGFR2-targeted thorium-227 conjugate (FGFR2-TTC) in combination with the ATR inhibitor BAY 1895344

Targeted Thorium-227 Conjugates (TTCs) consist of the alpha emitter Thorium-227 (227Th) coupled, by a 3, 2-HOPO chelator, to a tumor specific antibody. The alpha particles release high energy over a short range (2- 10 cell diameters), resulting in a potent local irradiation of the tumor with limited damage to surrounding tissue. Here, we describe the in vitro and in vivo evaluation of an FGFR2 targeted thorium-227 conjugate (FGFR2-TTC) in combination with the ATR inhibitor BAY 1895344. FGFR2 (fibroblast growth factor receptor 2) is a receptor tyrosine kinase and overexpression of FGFR2 has been described in different cancers, while its expression in healthy human tissues is moderate to low. This renders FGFR2 an attractive antigen to explore the concept of targeted alpha therapy (TAT). The mode-of-action of TTCs is based on the induction of clustered DNA double strand breaks and G2 cell cycle arrest. We hypothesized that combination of FGFR2-TTC with inhibitors of DNA damage response (DDRi's) may enhance potency and efficacy. The ataxia telangiectasia and Rad3-related protein (ATR) kinase is a central mediator of DDR. ATR kinase responds to a broad spectrum of DNA damage, including double-strand breaks (DSB) and lesions derived from interference with DNA replication as well as increased replication stress. Inhibition of ATR kinase activity induces cell death especially in tumors with increased DNA damage, deficiency in DNA damage repair or replication stress. Therefore, we investigated whether the combination of the FGFR2-TTC with the ATRi BAY 1895344 results in enhanced tumor sensitivity in vitro and in vivo. In in vitro cytotoxicity assays, the combination of FGFR2-TTC and BAY 1895344 resulted in increased potency of the FGFR2-TTC on three different cancer cell lines (KATO III (gastric), MFM-223 (triple negative breast cancer), SUM52-PE (triple negative breast cancer)). Mechanistic analysis demonstrated that the combination treatment resulted in reduced levels of G2 arrest and increased levels of DNA damage in comparison to single agent treatments. The combination was further evaluated in vivo using the MFM-223 breast cancer xenograft model. An increased anti-tumor efficacy of a low dose of FGFR2-TTC (100 kBq/kg) was observed in combination with BAY 1895344 compared to animals treated with vehicle. The presented data support the mechanism-based rationale for combining DNA damage induction by FGFR2-TTC with DNA damage repair inhibition using ATRi BAY 1895344. Our findings warrant further exploration of TTCs in combination with BAY 1895344 for cancer therapy.

Citation Format: Katrine Wickstroem, Urs B. Hagemann, Antje M. Wengner, Anette Sommer, Alexander Kristian, Christine Ellingsen, Roger M. Bjerke, Jenny Karlsson, Olav B. Ryan, Lars Linden, Bertolt Kreft, Dominik Mumberg, Hanno Wild, Karl Ziegelbauer, Alan Cuthbertson. Increased in vitro potency and in vivo efficacy of FGFR2-targeted thorium-227 conjugate (FGFR2-TTC) in combination with the ATR inhibitor BAY 1895344 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 855.

Mono- and multimeric PSMA-targeting small molecule-thorium-227 conjugates for optimized efficacy and biodistribution in preclinical models

PURPOSE

PSMA (prostate-specific membrane antigen) is highly expressed on prostate cancer (PrCa) cells and extensively used as a homing target for PrCa treatment. Most prominently, PSMA-targeting conjugate PSMA-617, carrying a DOTA chelator and labeled with therapeutic radionuclides like beta-emitting lutetium-177 or alpha-emitting actinium-225, has shown clinical activity in PrCa patients. We sought to develop PSMA-targeting small molecule (SMOL) conjugates that show high uptake in PSMA-expressing tumors and fast clearance, and can easily be labeled with the alpha emitter thorium-227 (half-life 18.7 days).

METHODS

A novel linker motif with improved competition against 3H-PSMA-617 on PSMA-expressing LNCaP cells was identified. A 2,3-hydroxypyridinone chelator modified with carboxyl groups (carboxy-HOPO) with increased hydrophilicity and robust labeling with thorium-227 was developed and allowed the synthesis of mono-, di-, tri-, and tetrameric conjugates. The resulting monomeric and multimeric PSMA SMOL-TTCs (targeted thorium conjugate) were evaluated for cellular binding, internalization, and antiproliferative activity. The in vivo antitumor efficacy of the PSMA SMOL-TTCs was determined in ST1273 and KUCaP-1 PrCa models in mice, and their biodistribution was assessed in cynomolgus monkeys, minipigs, and mice.

RESULTS

The monomeric and multimeric PSMA SMOL conjugates were readily labeled with thorium-227 at room temperature and possessed high stability and good binding, internalization, and antiproliferative activity in vitro. In vivo, the monomeric, dimeric, and trimeric PSMA SMOL-TTCs showed fast clearance, potent antitumor efficacy, and high uptake and retention in prostate tumors in mice. No major uptake or retention in other organs was observed beyond kidneys. Low uptake of free thorium-227 into bone confirmed high complex stability in vivo. Salivary gland uptake remained inconclusive as mini pigs were devalidated as a relevant model and imaging controls failed in cynomolgus monkeys.

CONCLUSION

Monomeric and multimeric PSMA SMOL-TTCs show high tumor uptake and fast clearance in preclinical models and warrant further therapeutic exploration.