Abstract A147: A HER2 targeted polylysine dendrimer nanoparticle radiotheranostic demonstrates excellent tumor accumulation, rapid clearance from circulation, and promising performance in PET-CT imaging
Authors: Johnstone Cameron N , Heery Graham P , Kelly Brian D , Humphries James , Akhter Dewan T , Thurecht Kristofer J , Yuen Daniel , Giannis Michael , Yong Ken , Chadwick Gareth , Barker Helena , Cargill Anne , Eglezos Anthony , Castellarnau Alex , Paull Jeremy R A , Hufton Richard 
Published: December 2023

Abstract
Background: Development of targeted radiotheranostics for human epidermal growth factor receptor type 2 positive (HER2+) tumors has lagged advances made in other receptor systems such as prostate-specific membrane antigen (PSMA) in prostate cancer. Myelosuppression is a dose limiting toxicity in clinical studies of trastuzumab radiotheranostics due to slow clearance of this large radiotherapeutic from circulation. Conjugation of chemotherapeutics such as taxanes and irinotecan/SN38 to highly optimized dendrimer nanoparticles improves pharmacokinetics (PK), tumor uptake and antitumor efficacy in vivo. Dendrimers can be used as targeted radiotheranostics with potential to achieve superior PK, enhanced tumor killing, and reduced toxicity (improved therapeutic index) compared to traditional antibody-based radiotheranostics. A radio-imaging and biodistribution study in mice implanted with HER2+ tumors directly compared the imaging performance, in positron emission tomography-computed tomography (PET-CT), and kinetics of tumor and normal tissue uptake of a HER2-targeted dendrimer radiotheranostic to radiolabeled trastuzumab.
Methods: A HER2 targeted VHH (single domain antibody [sdAb], or antigen binding fragment of heavy chain only camelid antibody) was covalently linked to a generation 4 dendrimer with 16 deferoxamine (DFO) chelation groups on its surface (DEP-HER2-DFO). Trastuzumab was covalently linked to DFO (trastuzumab-DFO). DEP-HER2-DFO and trastuzumab-DFO were successfully radiolabeled with 89Zr to high specific activity. Female BALB/c nude mice were inoculated subcutaneously with BT474 HER2+ breast cancer cells. When tumors reached 150mm3, a single dose of DEP-HER2-DFO-89Zr and trastuzumab-DFO-89Zr (3MBq) was administered intravenously. Uptake was evaluated in tumor and normal tissues by ex vivo biodistribution (n=3/group). In vivo biodistribution (n=3/group) was determined using quantified PET-CT data (4h-5d). PET-CT images were obtained on day 2 and 4.
Results: Both DEP-HER2-DFO-89Zr and trastuzumab-DFO-89Zr showed sustained tumor accumulation of &gt;20% injected dose per gram (ID/g) over several days with low levels of normal tissue uptake (&lt;5% ID/g), as determined by ex vivo biodistribution. However, DEP-HER2-DFO-89Zr was cleared from the vascular circulation more quickly than trastuzumab-DFO-89Zr resulting in superior tumor/blood, tumor/heart, and tumor/lung ratios. These findings were corroborated by quantified PET-CT data. Excellent signal to noise (tumor to normal tissue) was found for DEP-HER2-DFO-89Zr on whole body PET-CT images. DEP-HER2-DFO-89Zr was stable in serum and not susceptible to radiolysis over 7 days.
Conclusions: DEP-HER2-DFO-89Zr achieved high levels of accumulation in tumor, low uptake in normal tissues and relatively fast clearance from circulation, yielding favorable tumor/blood ratios compared with trastuzumab-DFO-89Zr. The dendrimer-based DEP-HER2 radiotheranostic offers advantages over antibody-based radiotheranostics and is under development ahead of a clinical biodistribution study.
Citation Format: Cameron N Johnstone, Graham P Heery, Brian D Kelly, James Humphries, Dewan T Akhter, Kristofer J Thurecht, Daniel Yuen, Michael Giannis, Ken Yong, Gareth Chadwick, Helena Barker, Anne Cargill, Anthony Eglezos, Alex Castellarnau, Jeremy R A Paull, Richard Hufton. A HER2 targeted polylysine dendrimer nanoparticle radiotheranostic demonstrates excellent tumor accumulation, rapid clearance from circulation, and promising performance in PET-CT imaging [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A147.