Development of Novel Trifunctional Chelating Agents That Enhance Tumor Retention of Radioimmunoconjugates

Development of Novel Radiotheranostic Ligand with Positively Charged Unit Targeting Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) is an ideal target of prostate cancer (PCa) for theranostics, combining diagnosis and therapy in the field of nuclear medicine. [177Lu]Lu-PSMA-617 is a gold standard in PSMA-targeting radioligands, whereas its rapid clearance from the tumor and high uptake in the kidney may compromise the efficacy of theranostics. In this study, we developed novel PSMA-targeting radioligands, [111In]In/[225Ac]Ac-PDI2 and [111In]In/[225Ac]Ac-PDI4, by introducing a positively charged diethylenetriamine (PEI2) or tetraethylenepentamine (PEI4) structure, respectively, to PSMA-617. In the biodistribution study, higher tumor retention and lower renal uptake of [111In]In-PDI2 and [111In]In-PDI4 were observed than those of [111In]In-PSMA-617, and [111In]In-PDI2 exhibited higher tumor-residualizing properties than [111In]In-PDI4. [111In]In-PDI2 and [111In]In-PDI4 clearly visualized PSMA-expressing tumors by single photon emission computed tomography/computed tomography (SPECT/CT). The administration of [225Ac]Ac-PDI2 led to a higher antitumor effect than [225Ac]Ac-PDI4 and [225Ac]Ac-PSMA-617. These findings suggest the utility of [111In]In/[225Ac]Ac-PDI2 as theranostic ligands for PCa.

Comparison of carbonic anhydrase-IX-targeted trifunctional radioligands between linear- and branched-chain arrangements

Background

Carbonic anhydrase-IX (CA-IX) is overexpressed in tumors due to hypoxic conditions and considered an attractive biomarker for tumor-targeting radioligands. The introduction of an albumin binder (ALB) to radioligands can delay their renal clearance, resulting in increased radioactivity delivered to tumors and decreased renal uptake of radioligands. In this study, we designed novel CA-IX-targeted trifunctional radioligands consisting of imidazothiadiazole sulfonamide (IS) as a CA-IX-targeted ligand, DOTA as a chelator with four free carboxylic groups, and lysine-conjugated 4-(p-iodophenyl)butyric acid (Lys-IPBA) as ALB, with IS-[111In]In-DOTADG-ALB in a linear-chain arrangement and [111In]In-DOTAGA-ALB-IS in a branched-chain arrangement. Fundamental properties of IS-[111In]In-DOTADG-ALB and [111In]In-DOTAGA-ALB-IS were evaluated by in vitro and in vivo assays.

Methods

IS-DOTADG-ALB and DOTAGA-ALB-IS were synthesized and radiolabeled with [111In]InCl3. The stability of IS-[111In]In-DOTADG-ALB and [111In]In-DOTAGA-ALB-IS was evaluated by HPLC analysis after incubation in murine plasma. A cell saturation binding assay using CA-IX-positive HT-29 cells and albumin-binding assay were performed for IS-[111In]In-DOTADG-ALB and [111In]In-DOTAGA-ALB-IS to evaluate their capacity to bind CA-IX and albumin. Biodistribution assays of IS-[111In]In-DOTADG-ALB and [111In]In-DOTAGA-ALB-IS were performed using HT-29 tumor-bearing mice to evaluate their pharmacokinetics.

Results

IS-[111In]In-DOTADG-ALB and [111In]In-DOTAGA-ALB-IS were successfully synthesized by ligand substitution reaction from their corresponding precursors. IS-[111In]In-DOTADG-ALB and [111In]In-DOTAGA-ALB-IS exhibited similar stabilities in murine plasma and affinities to CA-IX, although the affinities to albumin were higher for [111In]In-DOTAGA-ALB-IS compared with IS-[111In]In-DOTADG-ALB. In the biodistribution assays, [111In]In-DOTAGA-ALB-IS showed higher blood retention and tumor accumulation and lower renal uptake than IS-[111In]In-DOTADG-ALB, reflecting their albumin-binding affinities.

Conclusion

These data suggest that the branched-chain arrangement of DOTAGA-ALB-IS may be useful for the design of CA-IX-targeted radioligands consisting of an IS ligand, DOTA, and Lys-IPBA.

Improvement of tumor-to-blood ratio of radioimmunoconjugates by poly(ethyleneimine)-containing chelating agent

OBJECTIVE

Monoclonal antibody (mAb)-based radioimmunoconjugates (RICs) exhibit marked tumor uptake in cancer imaging and therapy, although their high blood retention has limited the development of RICs. In our previous study, a trifunctional chelating agent with a cationic poly(ethyleneimine) (PEI) structure of tetraethylenepentamine (PEI4), maleimide-DOTA-PEI4 (MDI4), improved the tumor-to-blood ratio of RICs by increasing tumor retention compared with a conventional bifunctional chelating agent. In this study, we developed a novel chelating agent composed of a maleimide moiety, DOTA derivative, and two PEI4 structures as a PEI4-2 unit, maleimide-DOTA-PEI4-2 (MDI4-2), a design for a highly cationized chelating agent to synthesize RICs. The properties of MDI4-2 were compared with MDI4 to evaluate the effect of the PEI4-2 unit on the pharmacokinetics of RICs.

METHODS

Trastuzumab and 111In were selected as a model mAb and radiometal, respectively. Trastuzumab-based RICs were synthesized using MDI4-2 by two-step radiolabeling, wherein conjugation with mAbs is followed by radiolabeling of chelating agents, to obtain trastuzumab-[111In]In-MDI4-2 ([111In]In-TMDI4-2). The immunoreactivity and residualizing properties of [111In]In-TMDI4-2 were evaluated using human epidermal growth factor receptor 2 (HER2)/neu-expressing SK-OV-3 cells. A biodistribution assay using SK-OV-3 tumor-bearing mice was also performed for [111In]In-TMDI4-2 and the results were compared with trastuzumab-[111In]In-MDI4 ([111In]In-TMDI4).

RESULTS

[111In]In-TMDI4-2 was successfully synthesized by two-step radiolabeling at a radiochemical yield of 37.7%. The immunoreactivity of [111In]In-TMDI4-2 was determined as 81.7%, suggesting the maintained binding ability through radiolabeling steps. The internalization assay revealed equivalent internalizing properties of [111In]In-TMDI4-2 to [111In]In-TMDI4. In the biodistribution assay, [111In]In-TMDI4-2 exhibited lower blood retention of radioactivity to and comparable tumor uptake with [111In]In-TMDI4, resulting in an improved tumor-to-blood ratio. These in vitro and in vivo results indicate that the PEI4-2 unit largely contributed to the decrease in the blood radioactivity of RICs without compromising the tumor uptake.

CONCLUSION

MDI4-2 with the PEI4-2 unit exhibited favorable properties for designing RICs with an improved tumor-to-blood ratio.

Novel Radiotheranostic Ligands Targeting Prostate-Specific Membrane Antigen Based on Dual Linker Approach

Radiotheranostics using prostate-specific membrane antigen (PSMA)-targeting radioligands offers precision medicine by performing radionuclide therapy based on results of diagnosis. Albumin binder (ALB) binds to albumin reversibly and contributes to effective radiotheranostics by enhancing tumor accumulation of PSMA-targeting radioligands. We newly developed two ALB-containing PSMA-targeting radioligands including dual functional linkers, a hydrophilic linker, d-glutamic acid, and a hydrophobic linker, 4-(aminomethyl)benzoic acid, with the opposite arrangement (PNT-DA6 and PNT-DA7). A biodistribution study of [111In]In-PNT-DA6 indicated that the introduction and arrangement of dual functional linkers contributed to improved pharmacokinetics. A single photon emission computed tomography study of [111In]In-PNT-DA6 produced a clear PSMA-expressing tumor image. Moreover, [225Ac]Ac-PNT-DA6 showed the inhibition of tumor growth in targeted radionuclide therapy in PSMA-expressing tumor-bearing mice. These results indicated that [111In]In-PNT-DA6 and [225Ac]Ac-PNT-DA6 exhibited useful characteristics as PSMA-targeting radiotheranostic ligands.

Effect of Linker Entities on Pharmacokinetics of 111In-Labeled Prostate-Specific Membrane Antigen-Targeting Ligands with an Albumin Binder

In the field of radiopharmaceutical development targeting cancer, an albumin binder (ALB) is commonly used to improve accumulation of radioligands in tumors because it has high binding affinity for albumin and extends the circulation time of radioligands. The further development of ALB-containing radioligands is also expected to regulate their pharmacokinetics. In this study, we newly designed and synthesized [111In]In-PNT-DA1 derivatives, prostate-specific membrane antigen (PSMA)-targeting radioligands including a functional linker (d-glutamic acid or 4-(aminomethyl)benzoic acid), and evaluated the relationships among the structure, albumin-binding affinity, and pharmacokinetics. These derivatives showed a different binding affinity for albumin by the introduction of a linker. Biodistribution studies revealed that the introduction of a linker affects the pharmacokinetics of each derivative. The biodistribution studies also suggested that moderate albumin-binding affinity enhances the tumor/kidney ratio of the derivative. SPECT imaging using [111In]In-PNT-DA3 with the highest tumor/kidney ratio among [111In]In-PNT-DA1 derivatives led to clear visualization of a PSMA-positive LNCaP tumor. The results suggest that the appropriate introduction of linker entities may be necessary to improve the pharmacokinetics of PSMA-targeting radioligands.

Synthesis and Evaluation of a Cathepsin B-Recognizing Trifunctional Chelating Agent to Improve Tumor Retention of Radioimmunoconjugates

Cathepsin B (CTSB) is a lysosomal protease that is overexpressed in tumor cells. Radioimmunoconjugates (RICs) composed of CTSB-recognizing chelating agents are expected to increase the molecular weights of their radiometabolites by forming conjugates with CTSB in cells, resulting in their improved retention in tumor cells. We designed a novel CTSB-recognizing trifunctional chelating agent, azide-[111In]In-DOTA-CTSB-substrate ([111In]In-ADCS), to synthesize a RIC, trastuzumab-[111In]In-ADCS ([111In]In-TADCS), and evaluated its utility to improve tumor retention of the RIC. [111In]In-ADCS and [111In]In-TADCS were synthesized with satisfactory yield and purity. [111In]In-ADCS was markedly stable in murine plasma until 96 h postincubation. [111In]In-ADCS showed binding to CTSB in vitro, and the conjugation was blocked by the addition of CTSB inhibitor. In the internalization assay, [111In]In-TADCS exhibited high-level retention in SK-OV-3 cells, indicating the in vitro utility of the CTSB-recognizing unit. In the biodistribution assay, [111In]In-TADCS showed high-level tumor accumulation, but the retention was hardly improved. In the first attempt to combine a CTSB-recognizing unit and RIC, these findings show the fundamental properties of the CTSB-recognizing trifunctional chelating agent to improve tumor retention of RICs.

Synthesis and evaluation of novel trifunctional chelating agents for pretargeting approach using albumin binder to improve tumor accumulation

INTRODUCTION

The pretargeting approach consists of in vivo ligation between pre-injected antibodies and low-molecular-weight radiolabeled effectors. The advantage of the pretargeting approach is to improve a tumor-to-background ratio, but the disadvantage is to compromise tumor accumulation. In this study, we applied albumin binder (ALB) to the pretargeting approach to overcome low tumor accumulation.

METHODS

We synthesized two novel trifunctional effectors containing an ALB moiety, a chelator, and a different tetrazine and two corresponding effectors without an ALB moiety. Albumin-binding assays and stability assays were performed using 111In-labeled effectors. Measurements of reaction rate constant were conducted using 111In-labeled effectors and anti-HER2 antibody trastuzumab modified by trans-cyclooctene, which drives the click reaction with tetrazine. Biodistribution studies using HER2-expressing tumor-bearing mice were performed with or without the pretargeting approach.

RESULTS

In albumin-binding assays, ALB-containing effectors exhibited a marked binding to albumin. Two ALB-containing effectors showed the difference in the reactivity and the slight difference in the stability. In biodistribution studies without the pretargeting approach, two ALB-containing effectors showed different pharmacokinetics in blood retention. With the pretargeting approach, the tumor accumulation was improved by the introduction of ALB and the highest tumor accumulation was observed in using the ALB-containing effector with higher blood retention.

CONCLUSION

These results suggest that the application of ALB to the pretargeting approach is effective to improve tumor accumulation, and the structure of tetrazine influences the utility of ALB-containing effectors.

Reduction of the hepatic radioactivity levels of [111In]In-DOTA-labeled antibodies via cleavage of a linkage metabolized in lysosomes

INTRODUCTION

Radiolabeled antibodies are promising tools for cancer diagnosis using nuclear medicine. A DOTA-chelating system is useful for preparing immuno-positron emission tomography and immuno-single-photon emission computed tomography probes with various radiometals. Radiolabeled antibodies are generally metabolized in the reticuloendothelial system, producing radiometabolites after proteolysis in hepatic lysosomes. Because of the bulkiness and extremely high hydrophilicity of DOTA, radiometabolites containing a radiometal-DOTA complex typically exhibit high and persistent localization in hepatic lysosomes. Radioactivity in the liver impairs the accurate diagnosis of cancer surrounding the liver and liver metastasis, and a high tumor/liver ratio is desirable. In this study, we reduced the hepatic radioactivity of radiometal-labeled antibodies containing a DOTA-chelating system. A cleavable linkage was inserted to liberate the radiometabolite, which exhibited a short residence time in hepatocytes.

METHODS

Using indium-111 (111In)-labeled antibodies, we prepared 111In-labeled galactosyl-neoglycoalbumins (NGAs) because they are useful for evaluating the residence time of radiometabolites in the liver. An 111In-labeled NGA with a cleavable linkage ([111In]In-DO3AiBu-Bn-FGK-NGA) was administered to normal mice, and biodistribution studies and metabolic analyses of urinary and fecal samples were performed with comparison to an 111In-labeled NGA prepared by a conventional method ([111In]In-DOTA-Bn-SCN-NGA). Then, 111In-labeled antibodies ([111In]In-DO3AiBu-Bn-FGK-IgG and [111In]In-DOTA-Bn-SCN-IgG) were prepared using a procedure similar to that for 111In-labeled NGAs. In vitro plasma stability and biodistribution were investigated for both 111In-labeled antibodies in U87MG tumor-bearing mice.

RESULTS

Through the liberation of radiometabolites including [111In]In-DO3AiBu-Bn-F, [111In]In-DO3AiBu-Bn-FGK-NGA was cleared more rapidly from the liver than [111In]In-DOTA-Bn-SCN-NGA (4.07 ± 1.54%ID VS 71.68 ± 3.03%ID at 6 h postinjection). [111In]In-DO3AiBu-Bn-FGK-IgG exhibited lower tumor accumulation (8.83 ± 1.48%ID/g) but a significantly higher tumor/liver ratio (2.21 ± 0.53) than [111In]In-DOTA-Bn-SCN-IgG (11.65 ± 2.17%ID/g in the tumor and a tumor/liver ratio of 0.85 ± 0.18) at 72 h after injection.

CONCLUSION

A molecular design that reduces the high and persistent hepatic radioactivity of radiolabeled antibodies by liberating radiometabolites with a short hepatic residence time in lysosomes would be applicable for radiometal-labeled antibodies using a DOTA-chelating system.