99mTc-CD3813: A Nanobody-Based Single Photon Emission Computed Tomography Radiotracer with Clinical Potential for Myeloma Imaging and Evaluation of CD38 Expression

CD38 as theranostic target in oncology

CD38 is a multifunctional transmembrane glycoprotein found in multiple tissues and overexpressed in many cancer cells, notably in hematological malignancies such as leukemia and multiple myeloma (MM). Therefore, targeting CD38 remains an attractive strategy for cancer treatment in hematological malignancies as well as in solid tumors. It plays a critical role in the progression of these diseases through its ADP-ribosyl cyclase and cADPR-hydrolase activities. Its importance has led to the development of various anti-CD38 monoclonal antibodies (mAbs), including daratumumab and isatuximab, approved for MM treatment. These mAbs exert their anti-tumor effects through Fc-dependent immune mechanisms and immunomodulation, enhancing T-cell and NK-cell-mediated responses. However, resistance mechanisms arise during the treatment with daratumumab, creating the necessity for new therapies. This review explains current knowledge about the role of CD38 as a target in oncology and aims to delineate the use of single domain antibodies (sdAbs) as innovative theranostic tools in nuclear medicine. For diagnostic purposes, PET radionuclides like 68 Ga, 64Cu, and SPECT radionuclides like 99mTc and 111In, are commonly used. Significant progress has been made in anti-CD38 radioligand therapy (RLT), with anti-CD38 antibodies providing insights into tumor biology and treatment efficacy. In terms of therapy, RLT is a promising approach that offers precise targeting of malignant cells while minimizing exposure to healthy tissue. This involves the use of radionuclides emitting α particles, like 225Ac, 212Pb or 211At, and β--particles like 90Y, 131I, or 177Lu, to exert cytotoxic effects. Derived from Camelidae heavy chain antibodies, sdAbs offer advantages over conventional mAbs such as small size, high stability, specificity, and ability to recognize hidden epitopes. CD38-specific sdAbs, such as sdAb 2F8, characterized by our laboratory, showing excellent tumor targeting and their engineered constructs, such as biparatopic antibodies and chimeric antibodies, represent a new generation of theranostic agents for diagnosis and treatment CD38-expressing malignancies.

Development of New CD38 Targeted Peptides for Cancer Imaging

PURPOSE

Multiple myeloma (MM) affects over 35,000 patients each year in the US. There remains a need for versatile Positron Emission Tomography (PET) tracers for the detection, accurate staging, and monitoring of treatment response of MM that have optimal specificity and translational attributes. CD38 is uniformly overexpressed in MM and thus represents an ideal target to develop CD38-targeted small molecule PET radiopharmaceuticals to address these challenges.

PROCEDURES

Using phage display peptide libraries and pioneering algorithms, we identified novel CD38 specific peptides. Imaging bioconjugates were synthesized using solid phase peptide chemistry, and systematically analyzed in vitro and in vivo in relevant MM systems.

RESULTS

The CD38-targeted bioconjugates were radiolabeled with copper-64 (64Cu) with100% radiochemical purity and an average specific activity of 3.3 - 6.6 MBq/nmol. The analog NODAGA-PEG4-SL022-GGS (SL022: Thr-His-Tyr-Pro-Ile-Val-Ile) had a Kd of 7.55 ± 0.291 nM and was chosen as the lead candidate. 64Cu-NODAGA-PEG4-SL022-GGS demonstrated high binding affinity to CD38 expressing human myeloma MM.1S-CBR-GFP-WT cells, which was blocked by the non-radiolabeled version of the peptide analog and anti-CD38 clinical antibodies, daratumumab and isatuximab, by 58%, 73%, and 78%, respectively. The CD38 positive MM.1S-CBR-GFP-WT cells had > 68% enhanced cellular binding when compared to MM.1S-CBR-GFP-KO cells devoid of CD38. Furthermore, our new CD38-targeted radiopharmaceutical allowed visualization of tumors located in marrow rich bones, remaining there for up to 4 h. Clearance from non-target organs occurred within 60 min. Quantitative PET data from a murine disseminated tumor model showed significantly higher accumulation in the bones of tumor-bearing animals compared to tumor-naïve animals (SUVmax 2.06 ± 0.4 versus 1.24 ± 0.4, P = 0.02). Independently, tumor uptake of the target compound was significantly higher (P = 0.003) compared to the scrambled peptide, 64Cu-NODAGA-PEG4-SL041-GGS (SL041: Thr-Tyr-His-Ile-Pro-Ile-Val). The subcutaneous MM model demonstrated significantly higher accumulation in tumors compared to muscle at 1 and 4 h after tracer administration (SUVmax 0.8 ± 0.2 and 0.14 ± 0.04, P = 0.04 at 1 h; SUVmax 0.89 ± 0.01 and 0.09 ± 0.01, P = 0.0002 at 4 h).

CONCLUSIONS

The novel CD38-targeted, radiolabeled bioconjugates were specific and allowed visualization of MM, providing a starting point for the clinical translation of such tracers for the detection of MM.

Recent Advances in Radiotracers Targeting Novel Cancer-Specific Biomarkers in China: A Brief Overview

Radiopharmaceuticals play a critical role in nuclear medicine, providing novel tools for specifically delivering radioisotopes for the diagnosis and treatment of cancers. As the starting point for developing radiopharmaceuticals, cancer-specific biomarkers are important and receive worldwide attention. This field in China is currently experiencing a rapid expansion, with multiple radiotracers targeting novel targets being developed and translated into clinical studies. This review provides a brief overview of the exploration of novel imaging targets, preclinical evaluation of their targeting ligands, and translational research in China from 2020 to 2023, for detecting cancer, guiding targeted therapy, and visualizing the immune microenvironment. We believe that China will play an even more important role in the development of nuclear medicine in the world in the future.

Noninvasive Evaluation of Tumoral PD-L1 Using a Novel 99mTc-Labeled Nanobody Tracer with Rapid Renal Clearance

The expression level of PD-L1 in tumor tissue is considered one of the effective biomarkers to guide PD-1/PD-L1 therapy. Quantifying whole-body PD-L1 expression by SPECT imaging may help in selecting patients that potentially respond to PD-1/PD-L1 therapy. Nanobody is the smallest antibody fragment with antigen-binding ability that is well suited for radionuclide imaging. Nevertheless, high retention of radioactivity in the kidney may limit its clinical translation. The present study aimed to screen, design, and prepare a nanobody-based SPECT probe with rapid renal clearance to evaluate the PD-L1 expression level in vivo noninvasively. A phage library was constructed by immunizing alpaca with recombinant human PD-L1 protein, and 17 anti-PD-L1 nanobodies were screened by the phage display technique. After sequence alignment and flow cytometry analysis, APN09 was selected as the candidate nanobody, and a GGGC chelator was attached to its C-terminus for 99mTc labeling to prepare a SPECT imaging probe. The affinity and specificity of 99mTc-APN09 were evaluated by protein and cell-binding experiments, and SPECT imaging and biodistribution were performed in a mouse model with bilateral transplantation of A549 and A549PD-L1 tumors. The ability of 99mTc-APN09 to quantify the PD-L1 expression level in vivo was validated in tumor models with different PD-L1 expression levels. 99mTc-APN09 had a radiochemical purity higher than 99% and a binding equilibrium dissociation constant of 21.44 ± 1.65 nM with hPD-L1, showing high affinity. SPECT imaging results showed that 99mTc-APN09 could efficiently detect PD-L1-positive tumors within 0.5 h, and the quantitative results of SPECT were well correlated with the expression level of PD-L1 in cell lines. SPECT imaging and biodistribution results also showed that 99mTc-APN09 was rapidly cleared from the kidney in 2 h postinjection. 99mTc-APN09 was a simple and stable tool for visualizing PD-L1 expression in the whole body. In addition, due to its significant reduction in renal retention, it has better prospects for clinical translation.

CD46-Targeted Theranostics for PET and 225Ac-Radiopharmaceutical Therapy of Multiple Myeloma

PURPOSE

Multiple myeloma is a plasma cell malignancy with an unmet clinical need for improved imaging methods and therapeutics. Recently, we identified CD46 as an overexpressed therapeutic target in multiple myeloma and developed the antibody YS5, which targets a cancer-specific epitope on this protein. We further developed the CD46-targeting PET probe [89Zr]Zr-DFO-YS5 for imaging and [225Ac]Ac-DOTA-YS5 for radiopharmaceutical therapy of prostate cancer. These prior studies suggested the feasibility of the CD46 antigen as a theranostic target in multiple myeloma. Herein, we validate [89Zr]Zr-DFO-YS5 for immunoPET imaging and [225Ac]Ac-DOTA-YS5 for radiopharmaceutical therapy of multiple myeloma in murine models.

EXPERIMENTAL DESIGN

In vitro saturation binding was performed using the CD46 expressing MM.1S multiple myeloma cell line. ImmunoPET imaging using [89Zr]Zr-DFO-YS5 was performed in immunodeficient (NSG) mice bearing subcutaneous and systemic multiple myeloma xenografts. For radioligand therapy, [225Ac]Ac-DOTA-YS5 was prepared, and both dose escalation and fractionated dose treatment studies were performed in mice bearing MM1.S-Luc systemic xenografts. Tumor burden was analyzed using BLI, and body weight and overall survival were recorded to assess antitumor effect and toxicity.

RESULTS

[89Zr]Zr-DFO-YS5 demonstrated high affinity for CD46 expressing MM.1S multiple myeloma cells (Kd = 16.3 nmol/L). In vitro assays in multiple myeloma cell lines demonstrated high binding, and bioinformatics analysis of human multiple myeloma samples revealed high CD46 expression. [89Zr]Zr-DFO-YS5 PET/CT specifically detected multiple myeloma lesions in a variety of models, with low uptake in controls, including CD46 knockout (KO) mice or multiple myeloma mice using a nontargeted antibody. In the MM.1S systemic model, localization of uptake on PET imaging correlated well with the luciferase expression from tumor cells. A treatment study using [225Ac]Ac-DOTA-YS5 in the MM.1S systemic model demonstrated a clear tumor volume and survival benefit in the treated groups.

CONCLUSIONS

Our study showed that the CD46-targeted probe [89Zr]Zr-DFO-YS5 can successfully image CD46-expressing multiple myeloma xenografts in murine models, and [225Ac]Ac-DOTA-YS5 can effectively inhibit the growth of multiple myeloma. These results demonstrate that CD46 is a promising theranostic target for multiple myeloma, with the potential for clinical translation.

Microarray-Based CD38 Peptide Probe Screening for Multiple Myeloma Imaging

Assessing CD38 expression in vivo has become a significant element in multiple myeloma (MM) therapy, as it can be used to detect lesions and forecast the effectiveness of treatment. Accurate diagnosis requires a multifunctional, high-throughput probe screening platform to develop molecular probes for tumor-targeted multimodal imaging and treatment. Here, we investigated a microarray chip-based strategy for high-throughput screening of peptide probes for CD38. We obtained two new target peptides, CA-1 and CA-2, from a 105 peptide library with a dissociation constant (KD) of 10-7 M. The specificity and affinity of the target peptides were confirmed at the molecular and cellular levels. Peptide probes were labeled with indocyanine green (ICG) dye and 68Ga-DOTA, which were injected into a CD38-positive Ramos tumor-bearing mouse via its tail vein, and small animal fluorescence and positron emission tomography (PET) imaging showed that the peptide probes could show specific enrichment in the tumor tissue. Our study shows that a microchip-based screening of peptide probes can be used as a promising imaging tool for MM diagnosis.

Immuno-PET of colorectal cancer with a CEA-targeted [68 Ga]Ga-nanobody: from bench to bedside

PURPOSE

An accurate diagnosis of colorectal carcinoma (CRC) can assist physicians in developing reasonable therapeutic regimens, thereby significantly improving the patient's prognosis. Carcinoembryonic antigen (CEA)-targeted PET imaging shows great potential for this purpose. Despite showing remarkable abilities to detect primary and metastatic CRC, previously reported CEA-specific antibody radiotracers or pretargeted imaging are not suitable for clinical use due to poor pharmacokinetics and complicated imaging procedures. In contrast, radiolabeled nanobodies exhibit ideal characteristics for PET imaging, for instance, rapid clearance rates and excellent distribution profiles, allowing same-day imaging with sufficient contrast. In this study, we developed a novel CEA-targeted nanobody radiotracer, [68 Ga]Ga-HNI01, and assessed its tumor imaging ability and biodistribution profile in preclinical xenografts and patients with primary and metastatic CRC.

METHODS

The novel nanobody HNI01 was acquired by immunizing the llama with CEA proteins. [68 Ga]Ga-HNI01 was synthesized by site-specifically conjugating [68 Ga]Ga with tris(hydroxypyridinone) (THP). Small-animal PET imaging and biodistribution studies were performed in CEA-overexpressed LS174T and CEA-low-expressed HT-29 tumor models. Following successful preclinical assessment, a phase I study was conducted on 9 patients with primary and metastatic CRC. Study participants received 151.21 ± 25.25 MBq of intravenous [68 Ga]Ga-HNI01 and underwent PET/CT scans at 1 h and 2 h post injection. Patients 01-03 also underwent whole-body dynamic PET imaging within 0-40 min p.i. All patients underwent [18F]F-FDG PET/CT imaging within 1 week after [68 Ga]Ga-HNI01 imaging. Tracer distribution, pharmacokinetics, and radiation dosimetry were calculated.

RESULTS

[68 Ga]Ga-HNI01 was successfully synthesized within 10 min under mild conditions, and the radiochemical purity was more than 98% without purification. Micro-PET imaging with [68 Ga]Ga-HNI01 revealed clear visualization of LS174T tumors, while signals from HT-29 tumors were significantly lower. Biodistribution studies indicated that uptake of [68 Ga]Ga-HNI01 in LS174T and HT-29 was 8.83 ± 3.02%ID/g and 1.81 ± 0.87%ID/g, respectively, at 2 h p.i. No adverse events occurred in all clinical participants after the injection of [68 Ga]Ga-HNI01. A fast blood clearance and low background uptake were observed, and CRC lesions could be visualized with high contrast as early as 30 min after injection. [68 Ga]Ga-HNI01 PET could clearly detect metastatic lesions in the liver, lung, and pancreas and showed superior ability in detecting small metastases. A significant accumulation of radioactivity was observed in the kidney, and normal tissues physiologically expressing CEA receptors showed slight uptakes of [68 Ga]Ga-HNI01. An interesting finding was that strong uptake of [68 Ga]Ga-HNI01 was found in non-malignant colorectal tissues adjacent to the primary tumor in some patients, suggesting abnormal CEA expression in these healthy tissues.

CONCLUSION

[68 Ga]Ga-HNI01 is a novel CEA-targeted PET imaging radiotracer with excellent pharmacokinetics and favorable dosimetry profiles. [68 Ga]Ga-HNI01 PET is an effective and convenient imaging tool for detecting CRC lesions, particularly for identifying small metastases. Furthermore, its high specificity for CEA in vivo makes it an ideal tool for selecting patients for anti-CEA therapy.