PET imaging facilitates antibody screening for synergistic radioimmunotherapy with a 177Lu-labeled αPD-L1 antibody

Efficacy of combined targeted radionuclide therapy and immune checkpoint Inhibition in animal tumour models: a systematic review and meta-analysis of the literature

PURPOSE

Given radiation's immunomodulatory effects and the complementary anti-cancer mechanisms of targeted radionuclide therapy (TRT) and immune checkpoint inhibition (ICI), their combination holds promise as a cancer treatment. This systematic review and meta-analysis summarize the literature on the therapeutic efficacy of combined TRT/ICI in animal tumour models.

METHODS

A systematic search in MEDLINE-PubMed and Embase-OVID was performed. Study characteristics and risk of bias were assessed. Outcome parameters included normalized area under the tumour growth curve and restricted mean survival time, of which ratios between combined treatment and untreated and monotherapy groups were analysed in a random-effects meta-analyses. Predefined subgroup analyses explored potential moderators of treatment efficacy.

RESULTS

In total, 31 studies were included. Study characteristics such as animal sex and age, cancer type, TRT target, and radionuclides, varied considerably across studies. The quality of the included studies could not always be assessed due to poor reporting. All meta-analyses indicated significantly improved survival and tumour growth of combination treatment over untreated, TRT and ICI monotherapy controls (RMST ratio 1.96 [1.72-2.23], 1.44 [ 1.34-1.55], 1.54 [1.38-1.72], and nAUC ratio 0.32 [0.25-0.42], 0.49 [0.41-0.59], 0.41 [0.31-0.55], respectively), with high between-study heterogeneity (I2 = 76.7-98.2%). The specific mode of action of ICI emerged as a potential moderator of treatment efficacy in subgroup analyses.

CONCLUSION

This systematic review highlights the therapeutic potential of combined TRT/ICI treatment, demonstrating preclinical proof-of-concept and supporting its further evaluation in clinical trials. However, the current literature remains insufficient to determine optimal treatment parameters like TRT tumour-absorbed dose and ICI type for clinical translation. Further research with improved reporting standards should systematically evaluate the impact of such parameters to enable robust comparisons.

Preclinical evaluation of 64Cu/177Lu-labelled anti-CD30 monoclonal antibody for theranostics in CD30-positive lymphoma

PURPOSE

CD30 serves as an ideal therapeutic target for lymphoma, but its variable expression and high relapse rate pose challenges in targeted therapy. This study aims to label the anti-CD30 monoclonal antibody with 64Cu/177Lu for immuno-positron emission tomography (immuno-PET) and radioimmunotherapy (RIT).

METHODS

CD30 binding kinetics of anti-CD30-IgG (IMB16) were measured by Biolayer interferometry (BLI). Western blotting screened lymphoma cell lines for CD30 expression. Flow cytometry and immunofluorescence validated the specific binding of IMB16. IMB16 was conjugated to p-SCN-Bn-NOTA(NOTA) and p-SCN-Bn-DOTA(DOTA) for radiolabeling with 64Cu and 177Lu. [64Cu]Cu-NOTA-IMB16 and [177Lu]Lu-DOTA-IMB16 were used for immuno-PET and RIT in subcutaneous lymphoma NSG mouse models.

RESULTS

IMB16 had a strong binding affinity to CD30 according to the BLI. Western blotting revealed high CD30 expression in Karpas299 cells and negative expression in Raji cells. Flow cytometry and immunofluorescence confirmed specific binding of IMB16 to CD30 on cell surface. Radiochemical purity of [64Cu]Cu-NOTA-IMB16 and [177Lu]Lu-DOTA-IMB16 exceeded 95%. In Immuno-PET imaging, CD30-positive Karpas299 tumours had a mean uptake value of 19.2 ± 0.9%ID/g (n = 3) at 24 h post-injection, significantly higher than Karpas299-blocked and Raji-negative groups (P < 0.001). A high radiation dose (300µCi) of [177Lu]Lu-DOTA-IMB16 significantly inhibited tumour growth (80.2 ± 17.6% standardized tumour volume, n = 5) at 10 days post-injection, compared to controls. Ex vivo biodistribution and histological staining supported in vivo PET imaging and RIT results.

CONCLUSIONS

Labelling IMB16 with 64Cu enabled non-invasive assessment of CD30 expression, while 177Lu labelling effectively suppressed tumour growth in CD30-positive lymphoma. CD30-targeted theranostic show promise for patient stratification and treatment enhancement, warranting further clinical evaluation.

Current and Future Perspectives of PDL1 PET and SPECT Imaging

Programmed Death 1 (PD1) and Programmed Death Ligand (PDL1) play a crucial role in tumor microenvironment by helping cancer cells evade innate immunity. Numerous inhibitor anticancer drugs targeting this interplay have been used in clinical practice and many more are in preclinical stage. These drugs have shown promising results in achieving good response and long-term clinical benefit, is routinely performed to identify patients who may benefit. However, there are major challenges associated with these immunohistochemistry tests which have opened the space for noninvasive imaging modalities using PD1 and PDL1 inhibitors labeled with either PET or SPECT radionuclides. These radiopharmaceuticals, although primarily developed for the field of immunotherapy, have great potential in expanding and optimizing the combination of radiopharmaceutical therapies with PD1-PDL1 targeting anticancer drugs. This review elaborates currently available PET and SPECT radiopharmaceuticals targeting PD1-PDL1 axis. It also explores the potential future role of newer targets which are being developed and tested in various preclinical studies.

Towards effective CAIX-targeted radionuclide and checkpoint inhibition combination therapy for advanced clear cell renal cell carcinoma

Background: Immune checkpoint inhibitors (ICI) are routinely used in advanced clear cell renal cell carcinoma (ccRCC). However, a substantial group of patients does not respond to ICI therapy. Radiation is a promising approach to increase ICI response rates since it can generate anti-tumor immunity. Targeted radionuclide therapy (TRT) is a systemic radiation treatment, ideally suited for precision irradiation of metastasized cancer. Therefore, the aim of this study is to explore the potential of combined TRT, targeting carbonic anhydrase IX (CAIX) which is overexpressed in ccRCC, using [177Lu]Lu-DOTA-hG250, and ICI for the treatment of ccRCC. Methods: In this study, we evaluated the therapeutic and immunological action of [177Lu]Lu-DOTA-hG250 combined with aPD-1/a-CTLA-4 ICI. First, the biodistribution of [177Lu]Lu-DOTA-hG250 was investigated in BALB/cAnNRj mice bearing Renca-CAIX or CT26-CAIX tumors. Renca-CAIX and CT26-CAIX tumors are characterized by poor versus extensive T-cell infiltration and homogeneous versus heterogeneous PD-L1 expression, respectively. Tumor-absorbed radiation doses were estimated through dosimetry. Subsequently, [177Lu]Lu-DOTA-hG250 TRT efficacy with and without ICI was evaluated by monitoring tumor growth and survival. Therapy-induced changes in the tumor microenvironment were studied by collection of tumor tissue before and 5 or 8 days after treatment and analyzed by immunohistochemistry, flow cytometry, and RNA profiling. Results: Biodistribution studies showed high tumor uptake of [177Lu]Lu-DOTA-hG250 in both tumor models. Dose escalation therapy studies in Renca-CAIX tumor-bearing mice demonstrated dose-dependent anti-tumor efficacy of [177Lu]Lu-DOTA-hG250 and remarkable therapeutic synergy including complete remissions when a presumed subtherapeutic TRT dose (4 MBq, which had no significant efficacy as monotherapy) was combined with aPD-1+aCTLA-4. Similar results were obtained in the CT26-CAIX model for 4 MBq [177Lu]Lu-DOTA-hG250 + a-PD1. Ex vivo analyses of treated tumors revealed DNA damage, T-cell infiltration, and modulated immune signaling pathways in the TME after combination treatment. Conclusions: Subtherapeutic [177Lu]Lu-DOTA-hG250 combined with ICI showed superior therapeutic outcome and significantly altered the TME. Our results underline the importance of investigating this combination treatment for patients with advanced ccRCC in a clinical setting. Further investigations should focus on how the combination therapy should be optimally applied in the future.

Therapeutic challenge for immunotherapy targeting cold colorectal cancer: A narrative review

Cold colorectal tumors are not likely to trigger a robust immune response and tend to suppress the immune response. There may be three reasons. First, the complex tumor microenvironment of cold colorectal cancer (CRC) leads to tolerance and clearance of immunotherapy. Second, the modification and concealment of tumor-specific targets in cold CRC cause immune escape and immune response interruption. Finally, the difference in number and function of immune cell subsets in patients with cold CRC makes them respond poorly to immunotherapy. Therefore, we can only overcome the challenges in immunotherapy of cold CRC through in-depth research and understanding the changes and mechanisms in the above three aspects of cold CRC.

Radiopharmaceuticals heat anti-tumor immunity

Radiopharmaceutical therapy (RPT) has proven to be an effective cancer treatment with minimal toxicity. With several RPT agents approved by FDA, the remarkable potential of this therapy is now being recognized, and the anti-tumor immunity induced by RPT is beginning to be noticed. This review evaluates the potential of RPT for immune activation, including promoting the release of danger associated-molecular pattern molecules that recruit inflammatory cells into the tumor microenvironment, and activating antigen-presenting cells and cytotoxic T cells. We also discuss the progress of combining RPT with immunotherapy to increase efficacy.

Radioimmunotherapy study of 131I-labeled Atezolizumab in preclinical models of colorectal cancer

Background

Programmed cell death 1 ligand 1(PD-L1) is overexpressed in many tumors. The radionuclide-labeled anti-PD-L1 monoclonal antibody can be used for imaging and therapy of PD-L1 overexpressing cancer. Here, we described 131I-labeled Atezolizumab (131I-Atezolizumab, targeting PD-L1) as a therapeutic agent for colorectal cancer with PD-L1 overexpression.

Methods

131I-Atezolizumab was prepared by the Iodogen method. The expression levels of PD-L1 in different human colorectal cells were determined by flow cytometry, western blot and cell binding assay. The immunoreactivity of 131I-Atezolizumab to PD-L1 high-expressing cells was determined by immunoreactive fraction. The killing abilities of different concentrations of 131I-Atezolizumab on cells with high and low expression of PD-L1 were detected by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. Cerenkov luminescence imaging (CLI) and radioimmunotherapy (RIT) of 131I-Atezolizumab were performed on two human colorectal cancer models. The distribution and tumor targeting of 131I-Atezolizumab were evaluated by imaging. Tumor volume and survival time were used as indicators to evaluate the anti-tumor effect of 131I-Atezolizumab.

Results

The expression level of PD-L1 in vitro determined by the cell binding assay was related to the data of flow cytometry and western blot. 131I-Atezolizumab can specifically bind to PD-L1 high-expressing cells in vitro to reflect the expression level of PD-L1. Immunoreactive fraction of PD-L1 high-expressing RKO cells with 131I-Atezolizumab was 52.2%. The killing ability of 131I-Atezolizumab on PD-L1 high-expressing cells was higher than that of low-expressing cells. CLI proved that the specific uptake level of tumors depends on the expression level of PD-L1. Effect of 131I-Atezolizumab RIT showed an activity-dependent tumor suppressor effect on RKO tumor-bearing mice with high PD-L1 expression. 131I-Atezolizumab (37 MBq) can improve the median survival time of mice (34 days), compared to untreated mice (27 days) (P = 0.027). Although a single activity(37 MBq) of 131I-Atezolizumab also inhibited the tumors of HCT8 tumor-bearing mice with low PD-L1 expression (P < 0.05), it could not prolong the survival of mice(P = 0.29).

Conclusion

131I-Atezolizumab can be used as a CLI agent for screening PD-L1 expression levels. It may be used as a radioimmunotherapy drug target for PD- L1 overexpressing tumors.

Alpha radionuclide-chelated radioimmunotherapy promoters enable local radiotherapy/chemodynamic therapy to discourage cancer progression

BACKGROUND

Astatine-211 is an α-emitter with high-energy α-ray and high cytotoxicity for cancer cells. However, the targeted alpha therapy (TAT) also suffers from insufficient systematic immune activation, resulting in tumor metastasis and relapse. Combined immune checkpoint blockade (ICB) with chemodynamic therapy (CDT) could boost antitumor immunity, which may magnify the immune responses of TAT. This study aims to discourage tumor metastasis and relapse by tri-model TAT-CDT-ICB strategy.

METHODS

We successfully designed Mn-based radioimmunotherapy promoters (²¹¹At-ATE-MnO₂-BSA), which are consisting of ²¹¹At, MnO₂ and bovine serum albumin (BSA). The efficacy of ²¹¹At-ATE-MnO₂-BSA was studied as monotherapy or in combination with anti-PD-L1 in both metastatic and relapse models. The immune effects of radioimmunotherapy promoters on cytotoxic T lymphocytes and dendritic cells (DCs) were analyzed by flow cytometry. Enzyme-linked immunosorbent assay and immunofluorescence were used to explore the underlying mechanism.

RESULTS

Such radioimmunotherapy promoters could not only enhance the therapeutic outcomes of TAT and CDT, but also induce robust anti-cancer immune activity by activating dendritic cells. More intriguingly, ²¹¹At-ATE-MnO₂-BSA could effectively suppress the growths of primary tumors and distant tumors when combined with immune checkpoint inhibitors.

CONCLUSIONS

The tri-model TAT-CDT-ICB strategy provides a long-term immunological memory, which can protect against tumor rechallenge after eliminating original tumors. Therefore, this work presents a novel approach for TAT-CDT-ICB tri-modal cancer therapy with repressed metastasis and relapse in clinics.

Combining Targeted Radionuclide Therapy and Immune Checkpoint Inhibition for Cancer Treatment

The development of immunotherapy, in particular immune checkpoint inhibitors (ICI), has revolutionized cancer treatment in the past decades. However, its efficacy is still limited to subgroups of patients with cancer. Therefore, effective treatment combination strategies are needed. Here, radiotherapy is highly promising, as it can induce immunogenic cell death, triggering the release of pro-inflammatory cytokines, thereby creating an immunogenic phenotype and sensitizing tumors to ICI. Recently, targeted radionuclide therapy (TRT) has attained significant interest for cancer treatment. In this approach, a tumor-targeting radiopharmaceutical is used to specifically deliver a therapeutic radiation dose to all tumor cells, including distant metastatic lesions, while limiting radiation exposure to healthy tissue. However, fundamental differences between TRT and conventional radiotherapy make it impossible to directly extrapolate the biological effects from conventional radiotherapy to TRT. In this review, we present a comprehensive overview of studies investigating the immunomodulatory effects of TRT and the efficacy of combined TRT-ICI treatment. Preclinical studies have evaluated a variety of murine cancer models in which α- or β-emitting radionuclides were directed to a diverse set of targets. In addition, clinical trials are ongoing to assess safety and efficacy of combined TRT-ICI in patients with cancer. Taken together, research indicates that combining TRT and ICI might improve therapeutic response in patients with cancer. Future research has to disclose what the optimal conditions are in terms of dose and treatment schedule to maximize the efficacy of this combined approach.

Can Radiotherapy Empower the Host Immune System to Counterattack Neoplastic Cells? A Systematic Review on Tumor Microenvironment Radiomodulation

Despite the rising evidence in favor of immunotherapy (IT), the treatment of oncological patients affected by so-called "cold tumors" still represents an open issue. Cold tumors are characterized by an immunosuppressive (so-called cold) tumor microenvironment (TME), which favors host immune system suppression, cancer immune-escape, and a worse response to IT. However, the TME is not a static element, but dynamically mutates and can be changed. Radiotherapy (RT) can modulate a cold microenvironment, rendering it better at tumor killing by priming the quiescent host immune system, with a consequent increase in immunotherapy response. The combination of TME radiomodulation and IT could therefore be a strategy for those patients affected by cold tumors, with limited or no response to IT. Thus, this review aims to provide an easy, rapid, and practical overview of how RT could convert the cold TME and why cold tumor radiomodulation could represent an interesting strategy in combination with IT.

PD-L1-Targeted Radionuclide Therapy Combined with αPD-L1 Antibody Immunotherapy Synergistically Improves the Antitumor Effect

Immune checkpoint blockers (ICBs) targeting programmed death receptor 1 (PD-1) ligand 1 (PD-L1) for immunotherapy have radically reformed oncology. It is of great significance to enhance the response rate of ICB in cancer patients. Here, a radioiodinated anti-PD-L1 antibody (¹³¹I-αPD-L1) was developed for PD-L1-targeted single-photon emission computed tomography (SPECT) imaging and αPD-L1 immunotherapy. Flow cytometry and immunofluorescence staining were performed to identify PD-L1 upregulation in a time- and dose-dependent manner after being induced by ¹³¹I-αPD-L1. ImmunoSPECT imaging and biodistributions of ¹³¹I-αPD-L1 in CT26, MC38, 4T1, and B16F10 tumor models were conducted to visualize the high tumor uptake and low background signal. Compared to monotherapy alone, concurrent administration of αPD-L1 mAb and ¹³¹I-αPD-L1 revealed improved tumor control in murine tumor models. The combination of 11.1 MBq of ¹³¹I-αPD-L1 and 200 μg of αPD-L1 mAb resulted in significant tumor growth delay and prolonged survival. This radioligand synergized immunotherapy strategy holds great potential for cancer management.

Albumin Binder-Conjugated Fibroblast Activation Protein Inhibitor Radiopharmaceuticals for Cancer Therapy

Fibroblast activation protein (FAP) has become an attractive target for diagnosis and therapy, and a series of FAP inhibitor (FAPI)-based radiotracers has been developed and had excellent performance for diagnosis outcomes in clinical applications. Yet, their fast clearance and insufficient tumor retention have hampered their further clinical application in cancer treatment. In this study, we developed 2 albumin binder-conjugated FAPI radiotracers, TEFAPI-06 and TEFAPI-07. They were derived from FAPI-04 and were optimized by conjugating 2 types of well-studied albumin binders, 4-(p-iodophenyl) butyric acid moiety (TEFAPI-06) and truncated Evans blue moiety (TEFAPI-07), to try to overcome the above limitations at the expense of prolonging the blood circulation. Methods: TEFAPI-06 and TEFAPI-07 were synthesized and labeled with 68Ga, 86Y, and 177Lu successfully. A series of cell assays was performed to identify the binding affinity and FAP specificity in vitro. PET imaging, SPECT imaging, and biodistribution studies were performed to evaluate the pharmacokinetics in pancreatic cancer patient-derived xenograft (PDX) animal models. The cancer treatment efficacy of 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 were evaluated in pancreatic cancer PDX-bearing mice. Results: The binding affinities (dissociation constants) to FAP of 68Ga-TEFAPI-06 and 68Ga-TEFAPI-07 were 10.16 ± 2.56 nM and 7.81 ± 2.28 nM, respectively, which were comparable with that of 68Ga-FAPI-04. Comparative PET imaging of HT-1080-FAP and HT-1080 tumor-bearing mice and a blocking study showed the FAP-targeting ability in vivo of these 2 tracers. Compared with 177Lu-FAPI-04, PET imaging, SPECT imaging, and biodistribution studies of TEFAPI-06 and TEFAPI-07 demonstrated their remarkably enhanced tumor accumulation and retention, respectively. Notable tumor growth inhibition by 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 were observed, whereas the control group and the group treated by 177Lu-FAPI-04 showed a slight therapeutic effect. Conclusion: Two albumin binder-conjugated FAPI radiopharmaceuticals have been developed and evaluated in vitro and in vivo. Significantly improved tumor uptake and retention were observed, compared with the original FAPI tracer. Both 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 showed remarkable growth inhibition of PDX tumors, whereas the side effects were almost negligible, demonstrating that these radiopharmaceuticals are promising for further clinical translational studies.

Implications of physics, chemistry and biology for dosimetry calculations using theranostic pairs

Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy (using emitted beta, alpha or Auger-electron particles) for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic (e.g. chemical differences) or extrinsic (e.g. injecting different amounts of each radiopharmaceutical) to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them.

A Novel Small Cyclic Peptide-Based 68Ga-Radiotracer for Positron Emission Tomography Imaging of PD-L1 Expression in Tumors

In the tumor microenvironment, programmed death protein 1 and programmed death protein ligand 1 (PD-L1) signaling pathways help tumors escape the immune system. We designed a gallium-68 (⁶⁸Ga)-labeled small-molecule peptide-targeting PD-L1 and used positron emission tomography/computed tomography (PET/CT) to detect and dynamically monitor the expression level of PD-L1 in tumors. S-Cyclo(ETSK)-SF-NH₂ (SETSKSF) is a cyclic peptide inhibitor comprising seven amino-acid residues. We connected it with the chelating agent DOTA, labeled DOTA-SETSKSF, with the short half-life nuclide Ga-68, and measured the stability of ⁶⁸Ga-2,2',2″-(10-(2-((S)-1-((3S,6S,9S,18S)-18-((S)-1-((S)-1-amino-1-oxo-3-henylpropan-2-ylamino)-3-hydroxy-1-oxopropan-2-ylcarbamoyl)-6-((R)-1-hydroxyethyl)-3-(hydroxymethyl)-2,5,8,12-tetraoxo-1,4,7,13-tetraazacyclooctadecan-9-ylamino)-3-ydroxy-1-oxopropan-2-ylamino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (⁶⁸Ga-DOTA-SETSKSF) in normal saline (NS), phosphate-buffered saline (PBS), and fetal bovine serum (FBS) in vitro. We conducted the ⁶⁸Ga-DOTA-SETSKSF affinity test, cell-specific uptake experiments, time-combined experiments, western blotting, and laser confocal experiments to confirm the expression and localization of PD-L1 at the cell level and determine the uptake. Biodistribution and imaging experiments were performed using the H1975, B16F10, and A549 tumor models. ⁶⁸Ga-DOTA-SETSKSF was successfully synthesized, and the radiochemical purity was >99% after purification. The in vitro stability of ⁶⁸Ga-DOTA-SETSKSF was >95% in NS, PBS, and FBS at 37 °C after 4 h of incubation. Cell-binding experiments confirmed that ⁶⁸Ga-DOTA-SETSKSF exhibited high uptake in H1975 tumors with high PD-L1 expression and low uptake in A549 tumors with low PD-L1 expression. The clear half-life (T_1/2) of ⁶⁸Ga-DOTA-SETSKSF from the blood was 14.48 ± 3.26 min. The percentages of the injected dose per gram of tissue (%ID/g) for H1975 and A549 tumors were 5.29 ± 0.21 and 0.89 ± 0.10 at 1 h after injection, respectively. The H1975 tumor-to-muscle and tumor-to-blood ratios were 41.79 ± 5.81 and 4.75 ± 0.19 at 4 h, respectively. Apart from the H1975 tumor, the kidney and the bladder showed high accumulation because ⁶⁸Ga-DOTA-SETSKSF was excreted through the urinary system. PET/CT images showed high accumulation of ⁶⁸Ga-DOTA-SETSKSF in H1975 tumors and low uptake in A549 tumors, which was consistent with the results of biodistribution experiments. ⁶⁸Ga-DOTA-SETSKSF is convenient to prepare, has high stability, can be used to monitor the expression of PD-L1, and has an extremely high clinical application value.

Current status and future perspective of radiopharmaceuticals in China

Radiopharmaceuticals are essential components of nuclear medicine and serve as one of the cornerstones of molecular imaging and precision medicine. They provide new means and approaches for early diagnosis and treatment of diseases. After decades of development and hard efforts, a relatively matured radiopharmaceutical production and management system has been established in China with high-quality facilities. This review provides an overview of the current status of radiopharmaceuticals on production and distribution, clinical application, and regulatory supervision and also describes some important advances in research and development and clinical translation of radiopharmaceuticals in the past 10 years. Moreover, some prospects of research and development of radiopharmaceuticals in the near future are discussed.

Fatty acid-conjugated radiopharmaceuticals for fibroblast activation protein-targeted radiotherapy

INTRODUCTION

Radiopharmaceuticals that target cancer-associated fibroblasts (CAFs) have become an increasingly attractive strategy for cancer theranostics. Recently, a series of fibroblast activation protein inhibitor (FAPI)-based radiopharmaceuticals have been successfully applied to the diagnosis of a variety of cancers and exhibited excellent tumor selectivity. Nevertheless, CAF-targeted radionuclide therapy encounters difficulties in cancer treatment, as the tumor uptake and retention of FAPIs are insufficient. To meet this challenge, we tried to conjugate albumin-binding moiety to FAPI molecule for prolonged circulation that may increase the accumulation and retention of radiopharmaceuticals in tumor.

METHODS

Two fatty acids, lauric acid (C12) and palmitic acid (C16), were conjugated to FAPI-04 to give two albumin-binding FAPI radiopharmaceuticals, denoted as FAPI-C12 and FAPI-C16, respectively. They had been radiolabeled with gallium-68, yttrium-86, and lutecium-177 for stability study, binding affinity assay, PET and SPECT imaging, biodistribution, and radionuclide therapy study to systematically evaluate their potential for CAF-targeted radionuclide therapy.

RESULTS

FAPI-C12 and FAPI-C16 showed high binding affinity to FAP with the IC₅₀ of 6.80 ± 0.58 nM and 5.06 ± 0.69 nM, respectively. They were stable in both saline and plasma. The tumor uptake of [⁶⁸Ga]Ga-FAPI-04 decreased by 56.9% until 30 h after treated with FAPI-C16 before, and the uptakes of [⁸⁶Y]Y-FAPI-C12 and [⁸⁶Y]Y-FAPI-C16 in HT-1080-FAP tumor were both much higher than that of HT-1080-Vehicle tumor which identified the high FAP specific of these two radiopharmaceuticals. Both FAPI-C12 and FAPI-C16 showed notably longer circulation and significantly enhanced tumor uptake than those of FAPI-04. [¹⁷⁷Lu]Lu-FAPI-C16 had the higher tumor uptake at both 24 h (11.22 ± 1.18%IA/g) and 72 h (6.50 ± 1.19%IA/g) than that of [¹⁷⁷Lu]Lu-FAPI-C12 (24 h, 7.54 ± 0.97%IA/g; 72 h, 2.62 ± 0.65%IA/g); both of them were much higher than [¹⁷⁷Lu]Lu-FAPI-04 with the value of 1.24 ± 0.54%IA/g at 24 h after injection. Significant tumor volume inhibition of [¹⁷⁷Lu]Lu-FAPI-C16 at the high activity of 29.6 MBq was observed, and the median survival was 28 days which was much longer than that of the [¹⁷⁷Lu]Lu-FAPI-04 treated group of which the median survival was only 10 days.

CONCLUSION

This proof-of-concept study validates the hypothesis that conjugation of albumin binders may shift the pharmacokinetics and enhance the tumor uptake of FAPI-based radiopharmaceuticals. This could be a general strategy to transform the diagnostic FAP-targeted radiopharmaceuticals into their therapeutic pairs.

Novel theranostic agent for PET imaging and targeted radiopharmaceutical therapy of tumour-infiltrating immune cells in glioma

BACKGROUND

Malignant gliomas are deadly tumours with few therapeutic options. Although immunotherapy may be a promising therapeutic strategy for treating gliomas, a significant barrier is the CD11b+ tumour-associated myeloid cells (TAMCs), a heterogeneous glioma infiltrate comprising up to 40% of a glioma's cellular mass that inhibits anti-tumour T-cell function and promotes tumour progression. A theranostic approach uses a single molecule for targeted radiopharmaceutical therapy (TRT) and diagnostic imaging; however, there are few reports of theranostics targeting the tumour microenvironment.

METHODS

Utilizing a newly developed bifunctional chelator, Lumi804, an anti-CD11b antibody (αCD11b) was readily labelled with either Zr-89 or Lu-177, yielding functional radiolabelled conjugates for PET, SPECT, and TRT.

FINDINGS

89Zr/177Lu-labeled Lumi804-αCD11b enabled non-invasive imaging of TAMCs in murine gliomas. Additionally, 177Lu-Lumi804-αCD11b treatment reduced TAMC populations in the spleen and tumour and improved the efficacy of checkpoint immunotherapy.

INTERPRETATION

89Zr- and 177Lu-labeled Lumi804-αCD11b may be a promising theranostic pair for monitoring and reducing TAMCs in gliomas to improve immunotherapy responses.

FUNDING

A full list of funding bodies that contributed to this study can be found in the Acknowledgements section.

Covalent Organic Polymer as a Carborane Carrier for Imaging-Facilitated Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is an atomic targeted radiotherapy that shows fantastic suppression impact on locally intrusive threatening tumors. One key factor for effective BNCT is to aggregate an adequate concentration (>20 ppm) of ¹⁰B in the cytoplasm of the tumor. Carborane-loaded polymer nanoparticles are promising because of their outstanding biocompatibility and plasma steadiness. In this study, a new class of carborane-loaded nanoscale covalent organic polymers (BCOPs) was prepared by a Schiff base condensation reaction, and their solubility was greatly improved in common solvents via alkyl chain engineering and size tailoring. The obtained BCOP-5T was further functionalized by biocompatible 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene-glycol)-2000] (DSPE-PEG, molecular weight 2000) to form stable aqueous-phase nanoparticles with a hydrodynamic diameter of around 100 nm. After chelating with radioactive copper-64, DSPE-BCOP-5T was tracked by positron emission tomography (PET) imaging and showed significant accumulation in the tumor. DSPE-BCOP-5T + neutron radiation showed remarkable tumor suppression in 4T1 tumor-bearing mice (murine breast cancer). No obvious physical tissue damage and abnormal behavior were observed, demonstrating that the boron delivery was successful and tumor-selective. To conclude, this study presents a theranostic COP-based platform with a well-defined composition, good biocompatibility, and satisfactory tumor accumulation, which is promising for PET imaging, drug delivery, and BNCT.