Radiolabeled nanobodies for tumor targeting: From bioengineering to imaging and therapy

A humanized trivalent Nectin-4-targeting nanobody drug conjugate displays potent antitumor activity in gastric cancer

BACKGROUND

Gastric cancer represents a highly lethal malignancy with an elevated mortality rate among cancer patients, coupled with a suboptimal postoperative survival prognosis. Nectin-4, an overexpressed oncological target for various cancers, has been exploited to create antibody-drug conjugates (ADCs) to treat solid tumors. However, there is limited research on Nectin-4 ADCs specifically for gastric cancer, and conventional immunoglobulin G (IgG)-based ADCs frequently encounter binding site barriers. Based on the excellent tumor penetration capabilities inherent in nanobodies (Nbs), we developed Nectin-4-targeting Nb drug conjugates (NDCs) for the treatment of gastric cancer.

RESULTS

An immunized phage display library was established and employed for the selection of Nectin-4-specific Nbs using phage display technology. Subsequently, these Nbs were engineered into homodimers to enhance Nb affinity. To prolong in vivo half-life and reduce immunogenicity, we fused an Nb targeting human serum albumin (HSA), resulting in the development of trivalent humanized Nbs. Further, we site-specifically conjugated a monomethyl auristatin E (MMAE) at the C-terminus of the trivalent Nbs, creating Nectin-4 NDC (huNb26/Nb26-Nbh-MMAE) with a drug-to-antibody ratio (DAR) of 1. Nectin-4 NDC demonstrated excellent in vitro cell-binding activities and cytotoxic efficacy against cells with high Nectin-4 expression. Subsequent administration of Nectin-4 NDC to mice bearing NCI-N87 human gastric cancer xenografts demonstrated rapid tissue penetration and high tumor uptake through in vivo imaging. Moreover, Nectin-4 NDC exhibited noteworthy dose-dependent anti-tumor efficacy in in vivo studies.

CONCLUSION

We have engineered a Nectin-4 NDC with elevated affinity and effective tumor uptake, further establishing its potential as a therapeutic agent for gastric cancer.

Targeted Alpha Therapy (TAT) with Single-Domain Antibodies (Nanobodies)

The persistent threat of cancer necessitates the development of improved and more efficient therapeutic strategies that limit damage to healthy tissues. Targeted alpha therapy (TαT), a novel form of radioimmuno-therapy (RIT), utilizes a targeting vehicle, commonly antibodies, to deliver high-energy, but short-range, alpha-emitting particles specifically to cancer cells, thereby reducing toxicity to surrounding normal tissues. Although full-length antibodies are often employed as targeting vehicles for TαT, their high molecular weight and the presence of an Fc-region lead to a long blood half-life, increased bone marrow toxicity, and accumulation in other tissues such as the kidney, liver, and spleen. The discovery of single-domain antibodies (sdAbs), or nanobodies, naturally occurring in camelids and sharks, has introduced a novel antigen-specific vehicle for molecular imaging and TαT. Given that nanobodies are the smallest naturally occurring antigen-binding fragments, they exhibit shorter relative blood half-lives, enhanced tumor uptake, and equivalent or superior binding affinity and specificity. Nanobody technology could provide a viable solution for the off-target toxicity observed with full-length antibody-based TαT. Notably, the pharmacokinetic properties of nanobodies align better with the decay characteristics of many short-lived α-emitting radionuclides. This review aims to encapsulate recent advancements in the use of nanobodies as a vehicle for TαT.

Large-Scale Production of Anti-RNase A VHH Expressed in pyrG Auxotrophic Aspergillus oryzae

Nanobodies, also referred to as VHH antibodies, are the smallest fragments of naturally produced camelid antibodies and are ideal affinity reagents due to their remarkable properties. They are considered an alternative to monoclonal antibodies (mAbs) with potential utility in imaging, diagnostic, and other biotechnological applications given the difficulties associated with mAb expression. Aspergillus oryzae (A. oryzae) is a potential system for the large-scale expression and production of functional VHH antibodies that can be used to meet the demand for affinity reagents. In this study, anti-RNase A VHH was expressed under the control of the glucoamylase promoter in pyrG auxotrophic A. oryzae grown in a fermenter. The feature of pyrG auxotrophy, selected for the construction of a stable and efficient platform, was established using homologous recombination. Pull-down assay, size exclusion chromatography, and surface plasmon resonance were used to confirm the binding specificity of anti-RNase A VHH to RNase A. The affinity of anti-RNase A VHH was nearly 18.3-fold higher (1.9 nM) when expressed in pyrG auxotrophic A. oryzae rather than in Escherichia coli. This demonstrates that pyrG auxotrophic A. oryzae is a practical, industrially scalable, and promising biotechnological platform for the large-scale production of functional VHH antibodies with high binding activity.

In vitro and in vivo study on the treatment of non-small cell lung cancer with radionuclide labeled PD-L1 nanobody

PURPOSE

Nanobodies have become promising carriers due to excellent in vivo properties. Radiopharmaceutical therapy targeting programmed cell death ligand 1 (PD-L1) is an effective therapeutic strategy. Our study aimed to explore therapeutic efficacy of ¹³¹I labeled PD-L1 nanobody (Nb109) in non-small cell lung cancers (NSCLCs) in vitro and in vivo.

METHODS

¹³¹I-Nb109 was synthesized by chloramine-T method. We implemented stability analysis, SDS-PAGE and lipid-water partition coefficient test to assess its quality. Cell uptake assay and SPECT/CT scan were applied to evaluate its ability to target NSCLCs (H460 and A549). CCK8 assay and in vivo efficacy assay were conducted to estimate its therapeutic effect in H460 tumors. Damage-associated molecular patterns (DAMPs) release in H460 cells incubated with ¹³¹I-Nb109 was investigated by western blot and ATP test kit.

RESULTS

¹³¹I-Nb109 was hydrophilic with high labeling rate (69.51-98.06%), radiochemical purity (99.17% ± 0.76%) and stability. Cell uptake experiments showed that H460 cells (PD-L1 positive) compared with A549 cells (PD-L1 negative) had higher ¹³¹I-Nb109 uptake. SPECT/CT imaging revealed the accumulation of ¹³¹I-Nb109 in H460 tumor within 48 h. ¹³¹I-Nb109 inhibited H460 tumor growth without toxic side effects in contrast with control group. It also induced H460 cells to release DAMPs (adenosine triphosphate, high mobility group box 1, and heat shock protein 70).

CONCLUSION

¹³¹I-Nb109 had high stability, excellent ability to target and treatment PD-L1 positive tumors, and can improve tumor immunogenicity. The results of our study were expected to inspire the development of more novel radiopharmaceuticals to treat NSCLCs.

Structural interrogation of a trimeric prefusion RSV fusion protein vaccine candidate by a camelid nanobody

In the past decade, camelid nanobodies have been developed for multiple applications, including immuno-imaging, cancer immunotherapy, and antiviral therapeutics. Despite the prevalence of these approaches, nanobodies have rarely been used to assess the potency of vaccine antigen candidates, which are primarily based on mAb binding approaches. In this work, we demonstrate that a nanobody-based ELISA method is suitable for characterization of a leading respiratory syncytial virus (RSV) vaccine candidate, RSVPreF3. This nanobody, F-VHH-L66, compares similarly with AM14, an antibody well-known to be specific for the prefusion form of the RSV surface fusion glycoprotein, RSV F. ELISA assays based on F-VHH-L66 were specific for the trimeric, prefusion form of RSV F, the antigen conformation that best generates neutralizing antibodies. Additionally, the F-VHH-L66-based ELISA proved accurate, linear, and stability-indicating. Statistical analysis of 65 independent F-VHH-L66-based ELISA experiments indicated assay performance similar to that of ELISA assays based on AM14. Moreover, the binding kinetics of F-VHH-L66 to RSVPreF3 are comparable to those of AM14 when measured by surface plasmon resonance (SPR). Finally, F-VHH-L66 neutralized RSV(A) with similar efficacy as AM14; this bioactivity data further supports its use as an alternative to AM14 for pre-fusion specific structural characterization of RSVPreF3.

Effective Treatment of Human Breast Carcinoma Xenografts with Single-Dose 211At-Labeled Anti-HER2 Single-Domain Antibody Fragment

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with ²¹¹At, because of their rapid accumulation in tumor and clearance from normal tissues. Here, we evaluate the therapeutic potential of this strategy with 5F7 and VHH_1028-2 sdAbs that bind with high affinity to domain IV of human epidermal growth factor receptor type 2 (HER2). Methods: The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using N-succinimidyl-3-²¹¹At-astato-5-guanidinomethyl benzoate (iso-²¹¹At-SAGMB). The cytotoxicity of iso- ²¹¹At-SAGMB-5F7 and iso- ²¹¹At-SAGMB-VHH_2001 were compared on HER2-expressing BT474 breast carcinoma cells. Three experiments in mice with subcutaneous BT474 xenografts were performed to evaluate the therapeutic effectiveness of single doses of iso- ²¹¹At-SAGMB-5F7 (0.7-3.0 MBq), iso- ²¹¹At-SAGMB-VHH_1028 (1.0-3.0 MBq), and iso- ²¹¹At-SAGMB-VHH_1028 and iso- ²¹¹At-SAGMB-VHH_2001 (∼1.0 MBq). Results: Clonogenic survival of BT474 cells was reduced after exposure to iso- ²¹¹At-SAGMB-5F7 (D₀ = 1.313 kBq/mL) whereas iso- ²¹¹At-SAGMB-VHH_2001 was ineffective. Dose-dependent tumor growth inhibition was observed with ²¹¹At-labeled HER2-specific 5F7 and VHH_1028 but not with HER2-irrelevant VHH_2001. At the 3.0-MBq dose, complete tumor regression was seen in 3 of 4 mice treated with iso- ²¹¹At-SAGMB-5F7 and 8 of 11 mice treated with iso- ²¹¹At-SAGMB-VHH_1028; prolongation in median survival was 495% and 414%, respectively. Conclusion: Combining rapidly internalizing, high-affinity HER2-targeted sdAbs with the iso- ²¹¹At-SAGMB residualizing prosthetic agent is a promising strategy for targeted α-particle therapy of HER2-expressing cancers.

Site-Specific Radiohalogenation of a HER2-Targeted Single-Domain Antibody Fragment Using a Novel Residualizing Prosthetic Agent

Because of their rapid tumor accumulation and normal tissue clearance, single-domain antibody fragments (sdAbs) are an attractive vehicle for developing radiotherapeutics labeled with the α-emitter ²¹¹At. Herein, we have evaluated iso-[²¹¹At]AGMB-PODS, a prosthetic agent that combines a functionality for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone (PODS) moiety for site-specific sdAb conjugation. Iso-[²¹¹At]AGMB-PODS and its radioiodinated analogue were evaluated for thiol-selective conjugation to anti-HER2 5F7 sdAb bearing a C-terminus GGC tail. Both radiohalogenated PODS-5F7GGC conjugates were synthesized in good radiochemical yields and retained high binding affinity on HER2-positive BT474 breast carcinoma cells. Iso-[²¹¹At]AGMB-PODS-5F7GGC was considerably more stable in vitro than its maleimide analogue in the presence of cysteine and human serum albumin (HSA) and exhibited excellent tumor uptake and high in vivo stability. Superior tumor-to-kidney activity ratios were seen for both radiohalogenated PODS-5F7GGC conjugates compared with [¹⁷⁷Lu]Lu-DOTA-PODS-5F7GGC. These results suggest that iso-[²¹¹At]AGMB-PODS-5F7GGC warrants further evaluation for the treatment of HER2-expressing malignancies.

Evaluation of an 131I-labeled HER2-specific single domain antibody fragment for the radiopharmaceutical therapy of HER2-expressing cancers

Radiopharmaceutical therapy (RPT) is an attractive strategy for treatment of disseminated cancers including those overexpressing the HER2 receptor including breast, ovarian and gastroesophageal carcinomas. Single-domain antibody fragments (sdAbs) exemplified by the HER2-targeted VHH_1028 evaluated herein are attractive for RPT because they rapidly accumulate in tumor and clear faster from normal tissues than intact antibodies. In this study, VHH_1028 was labeled using the residualizing prosthetic agent N-succinimidyl 3-guanidinomethyl 5-[¹³¹I]iodobenzoate (iso-[¹³¹I]SGMIB) and its tissue distribution evaluated in the HER2-expressing SKOV-3 ovarian and BT474 breast carcinoma xenograft models. In head-to-head comparisons to [¹³¹I]SGMIB-2Rs15d, a HER2-targeted radiopharmaceutical currently under clinical investigation, iso-[¹³¹I]SGMIB-VHH_1028 exhibited significantly higher tumor uptake and significantly lower kidney accumulation. The results demonstrated 2.9 and 6.3 times more favorable tumor-to-kidney radiation dose ratios in the SKOV-3 and BT474 xenograft models, respectively. Iso-[¹³¹I]SGMIB-VHH_1028 was prepared using a solid-phase extraction method for purification of the prosthetic agent intermediate Boc₂-iso-[¹³¹I]SGMIB that reproducibly scaled to therapeutic-level doses and obviated the need for its HPLC purification. Single-dose (SKOV-3) and multiple-dose (BT474) treatment regimens demonstrated that iso-[¹³¹I]SGMIB-VHH_1028 was well tolerated and provided significant tumor growth delay and survival prolongation. This study suggests that iso-[¹³¹I]SGMIB-VHH_1028 is a promising candidate for RPT of HER2-expressing cancers and further development is warranted.