Construction and Preclinical Evaluation of a 124/131I-Labeled Radiotracer for the Detection of Mesothelin-Overexpressing Cancer

Development of a novel molecular probe for visualizing mesothelin on the tumor via positron emission tomography

OBJECTIVES

Mesothelin (MSLN) is an antigen that is overexpressed in various cancers, and its interaction with tumor-associated cancer antigen 125 plays a multifaceted role in tumor metastasis. The serum MSLN expression level can be detected using enzyme-linked immunosorbent assay; however, non-invasive visualization of its expression at the tumor site is currently lacking. Therefore, the aim of this study was to develop a molecular probe for imaging MSLN expression through positron emission tomography (PET).

METHODS

VHH 269-H4 was obtained via immunization of llama using a fragment of MSLN from residue 360 to residue 597. S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) was conjugated to VHH 269-H4 to yield precursor NOTA 269-H4 for radiolabeling. The chelator-to-VHH ratio was determined by mass spectrometry. The binding kinetics of VHH 269-H4 and NOTA 269-H4 were measured by surface plasmon resonance. Flow cytometry was carried out using the anti-mesothelin monoclonal antibody Anetumab to select MSLN-positive and MSLN-negative cell lines. After radiolabeling, the radiochemical purity and in vitro stability were tested by radio-thin-layer chromatography and size exclusion chromatography, respectively. A saturation binding assay was conducted to measure the dissociation constant (Kd) of [68Ga]Ga-NOTA-269-H4. By mircoPET/CT imaging and biodistribution studies, the in vivo performances of the novel tracer were investigated in NCG mice bearing OVCAR-8, SKOV-3, or patient-derived xenografts.

RESULTS

VHH 269-H4 targeting MSLN was obtained with a Kd value of 0.3 nM. After conjugation, approximately 27% and 3.2% of VHH were coupled to one and two NOTA chelators, respectively. This yielded precursor NOTA 269-H4 with a Kd value of 1.1 nM. The radiochemistry was accomplished with moderate radiochemical yields (34 ± 14%, n = 9, decay-corrected). [68Ga]Ga-NOTA-269-H4 was obtained with high radiochemical purity (> 99%), and was stable after 90 min incubation at room temperature. The binding affinity of the radioligand towards MSLN was kept in the nanomolar range. Flow cytometry revealed that OVCAR-8 cells possess a high level of MSLN expression, while MSLN expression on SKOV-3 cells was negligible. Consistently, in microPET/CT imaging, [68Ga]Ga-NOTA-269-H4 demonstrated clear tumor visualization using NCG mice bearing OVCAR-8 xenografts, but no radioactivity accumulation was observed in SKOV-3 xenografts, suggesting a high specificity of the tracer in vivo. In biodistribution studies, [68Ga]Ga-NOTA-269-H4 displayed radioactivity accumulation of 2.93 ± 0.39%ID/g in OVCAR-8 xenografts at 30 min post-injection, and the highest tumor-to-blood ratio (~ 3) was achieved at 90 min post-injection. In NCG mice bearing patient-derived xenografts, [68Ga]Ga-NOTA-269-H4 was able to noninvasively detect MSLN expression via microPET/CT imaging.

CONCLUSIONS

To our knowledge, our studies achieved the first-time to non-invasively detect MSLN expression clearly using a single domain antibody fragment. To sum up, [68Ga]Ga-NOTA-269-H4 is a highly promising PET probe to visualize MSLN expression in vivo and holds great potential to monitor MSLN expression during tumor development.

Construction of a 124I-Labeled Specific Antibody for the Noninvasive Detection of Mesothelin-Overexpressing Tumors

Mesothelin (MSLN) is a molecular biomarker of many types of solid tumors, such as mesothelioma, pancreatic cancer, and colon cancer. Owing to the significant difference in expression between cancer cells and normal cells, mesothelin has been widely used as a key target in cancer immunotherapy. In this study, we used iodine isotope (^nat/124/125I)-labeled mesothelin antibodies to noninvasively detect MSLN expression in mice with LS174T colon cancer. The ¹²⁴I-labeled MSLN antibody showed a high radiochemical purity (RCP, >99%) and specific activity (20.8-67.8 GBq/μmol) after purification and was stable in 5% HSA and PBS (>95% RCP at 8 days). Western blot analysis indicated that the LS174T cells showed a higher MSLN protein level than the HepG2 cells. The half maximal effective concentration (EC₅₀) values of the MSLN antibody and ^natI-anti-MSLN were 34.77 ± 3.72 ng/mL and 32.60 ± 2.52 ng/mL (P = 0.63), respectively. The dissociation constant of ¹²⁴I-anti-MSLN binding to MSLN protein was 16.0 nM. The radiotracer showed a significantly higher uptake in LS174T cells than in HepG2 tumor cells (1.56 ± 0.09 vs 0.81 ± 0.03, P = 0.0016) 2 days postinjection. The LS174T mouse models showed extremely low organ uptake and high tumor uptake 96 h after the injection of ¹²⁴I-anti-MSLN, and the T/M values were much higher than those of the other imaging groups (10.56 ± 1.20 for ¹²⁴I-anti-MSLN in LS174T mice vs 3.27 ± 0.20 for ¹²⁴I-anti-MSLN in HepG2 mice vs 3.53 ± 0.2 for ¹²⁴I-IgG in LS174T mice). The immunochemical histology results showed that LS174T tumors were strongly positive (+++) for MSLN, while those in the HepG2 group showed slight expression (+). The dosimetry estimation study showed that the effective dose of ¹²⁴I-anti-MSLN was 0.185 mSv/MBq, which is within the range of acceptable doses for further nuclear medicine translational research. Taken together, these results suggest that this radiotracer has the potential for detecting mesothelin-overexpressing tumors.

China’s radiopharmaceuticals on expressway: 2014–2021

This review provides an essential overview on the progress of rapidly-developing China’s radiopharmaceuticals in recent years (2014–2021). Our discussion reflects on efforts to develop potential, preclinical, and in-clinical radiopharmaceuticals including the following areas: (1) brain imaging agents, (2) cardiovascular imaging agents, (3) infection and inflammation imaging agents, (4) tumor radiopharmaceuticals, and (5) boron delivery agents (a class of radiopharmaceutical prodrug) for neutron capture therapy. Especially, the progress in basic research, including new radiolabeling methodology, is highlighted from a standpoint of radiopharmaceutical chemistry. Meanwhile, we briefly reflect on the recent major events related to radiopharmaceuticals along with the distribution of major R&D forces (universities, institutions, facilities, and companies), clinical study status, and national regulatory supports. We conclude with a brief commentary on remaining limitations and emerging opportunities for China’s radiopharmaceuticals.