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HER2-targeted radionuclide therapy might be helpful for the treatment of breast, gastric, and ovarian cancers which have developed resistance to antibody and antibody-drug conjugate-based therapies despite preserved high HER2-expression. Affibody molecules are small targeting proteins based on a non-immunoglobulin scaffold. The goal of this study was to test in an animal model a hypothesis that the second-generation HER2-targeting Affibody molecule ¹⁸⁸Re-ZHER2:41071 might be useful for treatment of HER2-expressing malignant tumors. ZHER2:41071 was efficiently labeled with a beta-emitting radionuclide rhenium-188 (¹⁸⁸Re). ¹⁸⁸Re-ZHER2:41071 demonstrated preserved specificity and high affinity (K_D = 5 ± 3 pM) of binding to HER2-expressing cells. In vivo studies demonstrated rapid washout of ¹⁸⁸Re from kidneys. The uptake in HER2-expressing SKOV-3 xenografts was HER2-specific and significantly exceeded the renal uptake 4 h after injection and later. The median survival of mice, which were treated by three injections of 16 MBq ¹⁸⁸Re-ZHER2:41071 was 68 days, which was significantly longer (<0.0001 in the log-rank Mantel-Cox test) than survival of mice in the control groups treated with vehicle (29 days) or unlabeled ZHER2:41071 (27.5 days). In conclusion, the experimental radionuclide therapy using ¹⁸⁸Re-ZHER2:41071 enabled enhancement of survival of mice with human tumors without toxicity to the kidneys, which is the critical organ.

Radiolabeled Affibody-based targeting agent ¹⁷⁷Lu-ABY-027, a fusion of an anti-HER2 Affibody molecule with albumin binding domain (ABD) site-specifically labeled at the C-terminus, has demonstrated a promising biodistribution profile in mice; binding of the construct to albumin prevents glomerular filtration and significantly reduces renal uptake. In this study, we tested the hypothesis that site-specific positioning of the chelator at helix 1 of ABD, at a maximum distance from the albumin binding site, would further increase the strength of binding to albumin and decrease the renal uptake. The new construct, ABY-271 with DOTA conjugated at the back of ABD, has been labelled with ¹⁷⁷Lu. Targeting properties of ¹⁷⁷Lu-ABY-271 and ¹⁷⁷Lu-ABY-027 were compared directly. ¹⁷⁷Lu-ABY-271 specifically accumulated in SKOV-3 xenografts in mice. The tumor uptake of ¹⁷⁷Lu-ABY-271 exceeded uptake in any other organ 24 h and later after injection. However, the renal uptake of ¹⁷⁷Lu-ABY-271 was two-fold higher than the uptake of ¹⁷⁷Lu-ABY-027. Thus, the placement of chelator on helix 1 of ABD does not provide desirable reduction of renal uptake. To conclude, minimal modification of the design of Affibody molecules has a strong effect on biodistribution, which cannot be predicted a priori. This necessitates extensive structure-properties relationship studies to find an optimal design of Affibody-based targeting agents for therapy.

Targeted molecular radiation therapy is a promising emerging treatment modality in oncology, and peptide synthesis may shorten the time to reach the clinical stage. In this study, we have explored Chemo-Enzymatic Peptide Synthesis, or CEPS, as a new means of producing a therapeutic HER2 targeted Affibody((R)) molecule, comprising a C-terminal albumin binding domain (ABD) for half-life extension and a total length of 108 amino acids. In addition, a DOTA moiety could be incorporated at N-terminus directly during the synthesis step and subsequently utilized for site-specific radiolabeling with the therapeutic radionuclide Lu-177. Retained thermodynamic stability as well as retained binding to both HER2 and albumin was verified. Furthermore, HER2 binding specificity of the radiolabeled Affibody molecule was confirmed by an in vitro saturation assay showing a significantly higher cell-bound activity of SKOV-3 (high HER2 expression) compared with BxPC3 (low HER2 expression), both in the presence and absence of HSA. In vivo evaluation in mice bearing HER2 expressing xenografts also showed specific tumor targeting as well as extended time in circulation and reduced kidney uptake compared with a HER2 targeted Affibody molecule without the ABD moiety. To conclude, we have demonstrated that CEPS can be used for production of Affibody-fusion molecules with retained in vitro and in vivo functionality.

Affibody molecules are artificial proteins that can recognize cancer-related molecular abnormalities in the living body. Clinical studies demonstrated that Affibody molecules can be successfully used for radionuclide diagnostics. Targeted radionuclide therapy selectively delivers cytotoxic radionuclides to malignant tumors, thus sparing normal tissues. For radionuclide therapy, Affibody molecules were re-engineered to decrease accumulation in the kidneys. This study has demonstrated that radionuclide therapy using re-engineered Affibody molecules increases the survival of immunodeficient mice bearing human tumors. The therapy was more efficient than the treatment with a monoclonal antibody, which is currently used in clinical practice. The best results were obtained when both the antibody and radiolabeled Affibody molecules were used simultaneously. This work provides a preclinical rationale for a potentially more efficient treatment in HER2-positive cancers.ABY-027 is a scaffold-protein-based cancer-targeting agent. ABY-027 includes the second-generation Affibody molecule Z(HER2:2891), which binds to human epidermal growth factor receptor type 2 (HER2). An engineered albumin-binding domain is fused to Z(HER2:2891) to reduce renal uptake and increase bioavailability. The agent can be site-specifically labeled with a beta-emitting radionuclide Lu-177 using a DOTA chelator. The goals of this study were to test the hypotheses that a targeted radionuclide therapy using [Lu-177]Lu-ABY-027 could extend the survival of mice with HER2-expressing human xenografts and that co-treatment with [Lu-177]Lu-ABY-027 and the HER2-targeting antibody trastuzumab could enhance this effect. Balb/C nu/nu mice bearing HER2-expressing SKOV-3 xenografts were used as in vivo models. A pre-injection of trastuzumab did not reduce the uptake of [Lu-177]Lu-ABY-027 in tumors. Mice were treated with [Lu-177]Lu-ABY-027 or trastuzumab as monotherapies and a combination of these therapies. Mice treated with vehicle or unlabeled ABY-027 were used as controls. Targeted monotherapy using [Lu-177]Lu-ABY-027 improved the survival of mice and was more efficient than trastuzumab monotherapy. A combination of therapies utilizing [Lu-177]Lu-ABY-027 and trastuzumab improved the treatment outcome in comparison with monotherapies using these agents. In conclusion, [Lu-177]Lu-ABY-027 alone or in combination with trastuzumab could be a new potential agent for the treatment of HER2-expressing tumors.

Purpose

Fusion of Affibody molecules with an albumin-binding domain (ABD) provides targeting agents, which are suitable for radionuclide therapy. To facilitate clinical translation, the low immunogenic potential of such constructs with targeting properties conserved is required.

Methods

The HER2-targeting Affibody molecule ZHER2:2891 was fused with a deimmunized ABD variant and DOTA was conjugated to a unique C-terminal cysteine. The novel construct, PEP49989, was labelled with ¹⁷⁷Lu. Affinity, specificity, and in vivo targeting properties of [¹⁷⁷Lu]Lu-PEP49989 were characterised. Experimental therapy in mice with human HER2-expressing xenografts was evaluated.

Results

The maximum molar activity of 52 GBq/µmol [¹⁷⁷Lu]Lu-PEP49989 was obtained. [¹⁷⁷Lu]Lu-PEP49989 bound specifically to HER2-expressing cells in vitro and in vivo. The HER2 binding affinity of [¹⁷⁷Lu]Lu-PEP49989 was similar to the affinity of [¹⁷⁷Lu]Lu-ABY-027 containing the parental ABD035 variant. The renal uptake of [¹⁷⁷Lu]Lu-PEP49989 was 1.4-fold higher, but hepatic and splenic uptake was 1.7-2-fold lower than the uptake of [¹⁷⁷Lu]Lu-ABY-027. The median survival of xenograft-bearing mice treated with 21 MBq [¹⁷⁷Lu]Lu-PEP49989 (> 90 days) was significantly longer than the survival of mice treated with vehicle (38 days) or trastuzumab (45 days). Treatment using a combination of [¹⁷⁷Lu]Lu-PEP49989 and trastuzumab increased the number of complete tumour remissions. The renal and hepatic toxicity was minimal to mild.

Conclusion

In preclinical studies, [¹⁷⁷Lu]Lu-PEP49989 demonstrated favourable biodistribution and a strong antitumour effect, which was further enhanced by co-treatment with trastuzumab.