Abstract P099: Radionuclide imaging of low-density-lipoprotein receptor (LDLR)-overexpressing glioblastoma: A preclinical study of Gallium-68 RMX-VH

Introduction: One of the factors that limit the efficacy of the drugs, especially in primary brain tumors, is the permeability of the blood-brain barrier (BBB). The low-density lipoprotein receptor (LDL-R) expressed at the BBB mediates the transport of endogenous ligands through the BBB. We developed a new radiolabeled peptide targeting both the human and murine LDLR and able i) to cross the BBB and ii) to target tumors such as glioblastoma that express high levels of the LDLR. The objective of this study was to determine the LDLR targeting properties, the pharmacokinetics of 68Ga radiolabeled RMX-VH in a glioblastoma model that expresses the human LDLR (hLDLR). Methods: The nonclinical studies of 68Ga-RMX-VH were completed in U87MG, A172, U373 glioblastoma cancer cell lines and xenografts mice models. The studies determined in vivo time-dependent accumulation of this agent, in vitro dose-depended cellular uptake, and cellular competition studies. We compared the tumor-specific accumulation of 68Ga-RMX-VH and normal organ distribution in female and male athymic nude mice. Radiotracer, RMX-VH (10-30ug) was labeled with isotope-Ga68 (10-25mCi, ITM GmBH). U87MG and A172- derived xenografts were generated in athymic nude mice (10 weeks) and PET/CT images were acquired using G4 PET/Xray camera (Sofie Biosciences) at 1h, 2h, 3h, and 4h post-injection. The followed up biodistribution studies were done at 30 min, 1h, 2h, and 3 hrs. post-injection. The organs and tumor were collected, weighed, and the tissue radioactivity was measured with Wizard2 Gamma Counter (Perkin-Elmer, Waltham, MA). The percentage of injected dose per gram of tissue (%ID/g) was calculated and decay-corrected. Results: Our studies confirmed the in vitro and in vivo selectivity and specificity of 68Ga-RMX-VH toward LDLR-positive tumors. 68Ga-RMX-VH is a small peptide (MW: 1432.7 g/mol) and renal excretion was expected as a route of agent elimination. The tumor-specific uptake of radiotracer in U87MG xenografts was 1.8%ID/g at 1h and remained unchanged at 3h post-injection. The kidney retention of the agent reached 12.2%ID/g and decreased to 10.2%ID/g. The accumulation of radiotracer in the liver, ovaries, and intestine correlated well with the known normal expression of LDLR. The tumor-to-muscle ratio was 5.97 at 1h; increased to 26.1 and 22.39 at 2h and 3h. The elimination t1/2 of radiotracer was only 18.6 min, with a clearance CL of 502 ml/min. Adsorption from the site of administration is rapid as the Cmax was 5 min. The tumor-specific uptake and normal organ distribution of 68Ga-RMX-VH were lower in male mice than in females. This correlated with differences in lipid and lipoproteins metabolism in males and females. Conclusions: RMX-VH showed favorable hLDLR targeting properties in vitro and in vivo in glioblastoma mice models. Our results suggest that hLDLR may serve as a target for imaging for glioblastoma. The first-in-human exploratory IND study of Ga68-RMX-VH will be initiated in Q2 of 2021.

Citation Format: Izabela Tworowska, Leo Garcia Flores II, Xuewei Qu, Cédric Malicet, Nilesh Wagh, David Ranganathan, Jonathan Nowak, Pascaline Lecorche, Jamal Temsamani, Ebrahim S. Delpassand. Radionuclide imaging of low-density-lipoprotein receptor (LDLR)-overexpressing glioblastoma: A preclinical study of Gallium-68 RMX-VH [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P099.

64Cu-DOTATATE PET/CT for Imaging Patients with Known or Suspected Somatostatin Receptor-Positive Neuroendocrine Tumors: Results of the First U.S. Prospective, Reader-Masked Clinical Trial

Studies demonstrate that the investigational ⁶⁴Cu-DOTATATE radiopharmaceutical may provide diagnostic and logistical benefits over available imaging agents for patients with somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs). Accordingly, we aimed to prospectively determine the lowest dose of ⁶⁴Cu-DOTATATE that facilitates diagnostic-quality scans and evaluated the diagnostic performance and safety in a phase III study of patients with SSTR-expressing NETs. Methods: A dose-ranging study was conducted on 12 patients divided into 3 dose groups (111 MBq [3.0 mCi], 148 MBq [4.0 mCi], and 185 MBq [5.0 mCi] ± 10%) to determine the lowest dose of ⁶⁴Cu-DOTATATE that produced diagnostic-quality PET/CT images. Using the ⁶⁴Cu-DOTATATE dose identified in the dose-ranging study, 3 independent nuclear medicine physicians who were masked to all clinical information read PET/CT scans from 21 healthy volunteers and 42 NET-positive patients to determine those with disease or no disease, as well as those with localized versus metastatic status. Masked-reader evaluations were compared with a patient-specific standard of truth, which was established by an independent oncologist who used all previously available pathology, clinical, and conventional imaging data. Diagnostic performance calculated for ⁶⁴Cu-DOTATATE included sensitivity, specificity, negative predictive value, positive predictive value, and accuracy. Inter- and intrareader reliability, as well as ability to differentiate between localized and metastatic disease, was also determined. Adverse events were recorded from ⁶⁴Cu-DOTATATE injection through 48 h after injection. Results: The dose-ranging study identified 148 MBq (4.0 mCi) as the optimal dose to obtain diagnostic-quality PET/CT images. After database lock, diagnostic performance from an initial majority read of the 3 independent readers showed a significant 90.9% sensitivity (P = 0.0042) and 96.6% specificity (P < 0.0001) for detecting NETs, which translated to a 100.0% sensitivity and 96.8% specificity after correcting for an initial standard-of-truth misread. Excellent inter- and intrareader reliability, as well as ability to distinguish between localized and metastatic disease, was also noted. No adverse events were related to ⁶⁴Cu-DOTATATE, and no serious adverse events were observed. Conclusion: ⁶⁴Cu-DOTATATE PET/CT is a safe imaging technique that provides high-quality and accurate images at a dose of 148 MBq (4.0 mCi) for the detection of somatostatin-expressing NETs.

Abstract LB-259: Pb203-AR-RMX conjugates for image-guided TAT of neuroendocrine tumors (NETs)

Objective: The peptide receptor radionuclide therapy (PPRT) for somatostatin receptor positive (SSTR) neuroendocrine tumors (NETs) has emerged more than 15 years ago. The beta-emitter-PRRT has shown to induce objective response in 30-45% of metastatic NETs patients with hematologic/renal toxicity reduced by dose fractionation. The complete response to therapy is rare due to the heterogeneity of NETs; advanced stage of disease at the time of diagnosis; and patient resistance to nonradioactive octreotide and 90Y/177Lu PRRT developed during the therapy. The targeted alpha-emitter therapy of NETs can overcome these limitations. It can enhance the therapeutic response of patients and decrease side-effect and overcome patient resistance of beta-emitter PRRT without significant acute and mid-term toxicity. The RadioMedix and AREVA Med teams together have recently developed several novel 203Pb-peptide derivatives targeting SSTR(+) cancer cells, 203Pb-AR-RMX. The 203Pb is a gamma emitter (279 keV) with t1/2=51.9 h, suitable for single-photon emission computed tomography (SPECT) imaging. The 203Pb is an ideal surrogate for 212Pb α-particle therapy because both isotopes share identical chemical properties. The objective of these studies were: (1) to evaluate the SSTR targeting properties of novel conjugates 203Pb-AR-RMX; and (2) to determine their PK and biodistribution in vivo in SSTR overexpressing xenographs; and (3) to select lead candidate for further pre-IND clinical studies.

Methods: AR-RMX was manufactured under GMP by Macrocylics Inc. The 203Pb-radiolabeling of AR-RMX was carried out under mild conditions. The SSTR targeting properties of 203Pb-AR-RMX were determined in the cellular uptake/competition studies in AR-42J cancer cells and in vivo in SSTR(+) AR42J xenograph mice.

Results: 203Pb-AR-RMX-15 shows exceptionally high tumor-specific accumulation and retention in SSTR(+) AR42J xenograph mice. Tumor uptake of 203Pb-AR-RMX-15 was &gt;14.4 %ID/g at 1h post injection and it remained at this level at least for 24h. The kidneys accumulation of 203Pb-AR-RMX-15 was &gt;13 %ID/g at 1h and varies in different strains of mice but decreased progressively over the 24h. The kidney uptake of agent is similar to previously observed for octreotate labeled with other isotopes. The tumor uptake is significantly higher possible as a result of the change of charge of the 203Pb-AR-RMX-15. The acute hematotoxicity and chronic kidney toxicity is known to limit the doses of 90Y/177Lu PRRT. Preliminary studies of 203Pb-AR-RMX-15 showed that its kidney retention can be reduced by &gt;32% by co-injection of our proprietary AminoMedixTM or Gelofusine-Lysine composition. 203Pb-AR-RMX-15 has shown &gt;98% radio/chemical stability up 7 days post-formulation; no bone marrow uptake of agent was observed in bioD studies of 203Pb AR-RMX-15 done up to 24h post injection. These results allow us to hypothesize that the therapeutic dose of 203Pb/212Pb-AR-RMX-15 is expected to be significantly higher than the dose limiting activity.

Conclusions: 203Pb-AR-RMX-15 showed promising results in vitro and in vivo studies and will be further investigated as a 203Pb/212Pb-labeled theranostics agent. Our “theranostics” approach using 203Pb/212Pb-PRRT has a potential to advance image-guided radionuclide therapy that can detect and deliver therapeutic radiation dose precisely to SSTR(+) NETs.

Citation Format: Izabela Tworowska, Tania Stallons, Amal Saidi, Nilesh Wagh, Federico Rojas-Quijano, Paul Jurek, Garry Kiefer, Julien Torgue, Ebrahim Delpassand. Pb203-AR-RMX conjugates for image-guided TAT of neuroendocrine tumors (NETs) [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 LB-259. doi:10.1158/1538-7445.AM2017-LB-259