Evaluation of an antibody-PNA conjugate as a clearing agent for antibody-based PNA-mediated radionuclide pretargeting

Optimizing peptide nucleic acid-based pretargeting for enhanced targeted radionuclide therapy

Radiolabeled targeting agents have emerged as valuable tools for the treatment of disseminated cancer. Monoclonal antibodies (mAbs) are widely employed as carriers for diagnostic and therapeutic radionuclides due to their exceptional specificity and affinity. However, their prolonged circulatory half-life can diminish diagnostic efficacy and increase radiation exposure to non-target tissues in therapeutic applications, resulting in dose-limiting toxicities. To overcome this limitation, pretargeting technologies emerge as promising strategies to enhance tumor-to-background ratio and reduce radiation exposure of healthy tissues. Our previous work introduced a pretargeting concept leveraging the specific interaction between two peptide nucleic acid (PNA) probes, HP1 and HP2, as the recognition mechanism. This early iteration of the PNA-based concept showed limited efficacy when used with mAb-based vectors. To improve its performance, we re-engineered the primary and secondary targeting agents by incorporating newly designed PNA-probes. As the primary targeting agent, we functionalized trastuzumab (T), a well-characterized human epidermal growth factor receptor 2 (HER2)-targeting IgG1 mAb, with a 9-mer PNA probe (HP9). Both FcIII-based covalent UV-light crosslinking and enzyme-mediated glyco-engineering click-chemistry methods were applied to generate trastuzumab-PNA conjugates T-FcIII-HP9 and T-gly-HP9, respectively. As a radionuclide-carrying secondary agent, we utilized a 9-mer complementary PNA probe, HP16, which forms a stable duplex with HP9 as well as displaying favorable in vivo kinetics. Biacore and flow cytometry assessment of the HP9-conjugated trastuzumab agents demonstrated retained HER2-binding properties. The secondary HP16 probe, labeled with either a dye or a radionuclide, showed cell surface accumulation contingent on the presence of HP9 on the primary HER2-targeting agents. In vivo, T-gly-HP9 exhibited significantly longer blood circulation half-life and superior tumor uptake compared to T-FcIII-HP9. Further, therapeutic dosing with [177Lu]-HP16 of trastuzumab-HP9 pretargeted HER2+ tumor models resulted in significantly delayed disease progression and extended survival compared to untreated subjects. Furthermore, pretargeted [177Lu]-HP16 exhibited comparable efficacy to [177Lu]-trastuzumab in both delaying disease progression and prolonging survival. In conclusion, the optimization of our PNA-based pretargeting system has resulted in exceptional in vivo targeting characteristics and therapeutic efficacy, validating the potential of this novel approach.

Modified poly-L-lysine for use as a clearing agent in pretargeted radioimmunotherapy

BACKGROUND

Pretargeted radioimmunotherapy of cancer has the potential to increase tumor specific uptake of activity when compared with conventional radioimmunotherapy. This is especially true in radioimmunotherapy with nuclides that exhibit a relatively short half-life. When administering antibody-based pretargeting molecules systemically, the antibodies often show a relatively slow clearance from the blood. Therefore, the use of a clearing agent is advantageous to remove unbound pretargeting molecules from the circulation, facilitating a reduction in the nonspecific radiation exposure to normal tissue while maximizing the dose delivered to the tumors.

RESULTS

In the current study, two types of poly-L-lysine based clearing agents were produced for two different pretargeting systems: (strept)avidin/biotin and Tetrazine/Transcyclooctene. Poly-L-lysine was used as scaffold for production of clearing agents. The polymer is available in multiple sizes and can readily be modified with several functional groups, allowing different pretargeting strategies to be used. In vivo evaluation of the biotin-functionalized poly-L-lysine clearing agent, 110 repeating units, resulted in a decrease in blood concentration of the Iodine-125 labeled pretargeting agent of 50%, circa 23 h after injection, compared to controls. Two sizes, 68 and 143 repeating units, of the tetrazine-functionalized poly-L-lysine clearing agent were also evaluated, which at 23 h after injection decreased the blood concentration of the Iodine-125 labeled pretargeting agent to 58 and 38% respectively.

CONCLUSION

The straightforward synthesis of poly-L-lysine based clearing agents makes kit preparation possible and these agents show good potential for further evaluation, especially within the Tetrazine/Transcyclooctene pretargeting system where no liver or kidney accumulation was observed.

Carrier systems of radiopharmaceuticals and the application in cancer therapy

Radiopharmaceuticals play a vital role in cancer therapy. The carrier of radiopharmaceuticals can precisely locate and guide radionuclides to the target, where radionuclides kill surrounding tumor cells. Effective application of radiopharmaceuticals depends on the selection of an appropriate carrier. Herein, different types of carriers of radiopharmaceuticals and the characteristics are briefly described. Subsequently, we review radiolabeled monoclonal antibodies (mAbs) and their derivatives, and novel strategies of radiolabeled mAbs and their derivatives in the treatment of lymphoma and colorectal cancer. Furthermore, this review outlines radiolabeled peptides, and novel strategies of radiolabeled peptides in the treatment of neuroendocrine neoplasms, prostate cancer, and gliomas. The emphasis is given to heterodimers, bicyclic peptides, and peptide-modified nanoparticles. Last, the latest developments and applications of radiolabeled nucleic acids and small molecules in cancer therapy are discussed. Thus, this review will contribute to a better understanding of the carrier of radiopharmaceuticals and the application in cancer therapy.

Site-Specific Photoaffinity Bioconjugation for the Creation of 89Zr-Labeled Radioimmunoconjugates

PURPOSE

Site-specific approaches to bioconjugation produce well-defined and homogeneous immunoconjugates with potential for superior in vivo behavior compared to analogs synthesized using traditional, stochastic methods. The possibility of incorporating photoaffinity chemistry into a site-specific bioconjugation strategy is particularly enticing, as it could simplify and accelerate the preparation of homogeneous immunoconjugates for the clinic. In this investigation, we report the synthesis, in vitro characterization, and in vivo evaluation of a site-specifically modified, 89Zr-labeled radioimmunoconjugate created via the reaction between an mAb and an Fc-binding protein bearing a photoactivatable 4-benzoylphenylalanine residue.

PROCEDURES

A variant of the Fc-binding Z domain of protein A containing a photoactivatable, 4-benzoylphenylalanine residue - Z(35BPA) - was modified with desferrioxamine (DFO), combined with the A33 antigen-targeting mAb huA33, and irradiated with UV light. The resulting immunoconjugate - DFOZ(35BPA)-huA33 - was purified and characterized via SDS-PAGE, MALDI-ToF mass spectrometry, surface plasmon resonance, and flow cytometry. The radiolabeling of DFOZ(35BPA)-huA33 was optimized to produce [89Zr]Zr-DFOZ(35BPA)-huA33, and the immunoreactivity of the radioimmunoconjugate was determined with SW1222 human colorectal cancer cells. Finally, the in vivo performance of [89Zr]Zr-DFOZ(35BPA)-huA33 in mice bearing subcutaneous SW1222 xenografts was interrogated via PET imaging and biodistribution experiments and compared to that of a stochastically labeled control radioimmunoconjugate, [89Zr]Zr-DFO-huA33.

RESULTS

HuA33 was site-specifically modified with Z(35BPA)-DFO, producing an immunoconjugate with on average 1 DFO/mAb, high in vitro stability, and high affinity for its target. [89Zr]Zr-DFOZ(35BPA)-huA33 was synthesized in 95% radiochemical yield and exhibited a specific activity of 2 mCi/mg and an immunoreactive fraction of ~ 0.85. PET imaging and biodistribution experiments revealed that high concentrations of the radioimmunoconjugate accumulated in tumor tissue (i.e., ~ 40%ID/g at 120 h p.i.) but also that the Z(35BPA)-bearing immunoPET probe produced higher uptake in the liver, spleen, and kidneys than its stochastically modified cousin, [89Zr]Zr-DFO-huA33.

CONCLUSIONS

Photoaffinity chemistry and an Fc-binding variant of the Z domain were successfully leveraged to create a novel site-specific strategy for the synthesis of radioimmunoconjugates. The probe synthesized using this method - DFOZ(35BPA)-huA33 - was well-defined and homogeneous, and the resulting radioimmunoconjugate ([89Zr]Zr-DFOZ(35BPA)-huA33) boasted high specific activity, stability, and immunoreactivity. While the site-specifically modified radioimmunoconjugate produced high activity concentrations in tumor tissue, it also yielded higher uptake in healthy organs than a stochastically modified analog, suggesting that optimization of this system is necessary prior to clinical translation.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

Antibody-Based In Vivo Imaging of Central Nervous System Targets-Evaluation of a Pretargeting Approach Utilizing a TCO-Conjugated Brain Shuttle Antibody and Radiolabeled Tetrazines

Bioorthogonal pretargeted imaging using the inverse-electron-demand Diels-Alder (IEDDA) reaction between a tetrazine (Tz) and a trans-cyclooctene (TCO) represents an attractive strategy for molecular imaging via antibodies. The advantages of using a pretargeted imaging approach are on the one hand the possibility to achieve a high signal-to-noise ratio and imaging contrast; on the other hand, the method allows the uncoupling of the biological half-life of antibodies from the physical half-life of short-lived radionuclides. A brain-penetrating antibody (mAb) specific for β-amyloid (Aβ) plaques was functionalized with TCO moieties for pretargeted labeling of Aβ plaques in vitro, ex vivo, and in vivo by a tritium-labeled Tz. The overall aim was to explore the applicability of mAbs for brain imaging, using a preclinical model system. In vitro clicked mAb-TCO-Tz was able to pass the blood-brain barrier of transgenic PS2APP mice and specifically visualize Aβ plaques ex vivo. Further experiments showed that click reactivity of the mAb-TCO construct in vivo persisted up to 3 days after injection by labeling Aβ plaques ex vivo after incubation of brain sections with the Tz in vitro. An attempted in vivo click reaction between injected mAb-TCO and Tz did not lead to significant labeling of Aβ plaques, most probably due to unfavorable in vivo properties of the used Tz and a long half-life of the mAb-TCO in the blood stream. This study clearly demonstrates that pretargeted imaging of CNS targets via antibody-based click chemistry is a viable approach. Further experiments are warranted to optimize the balance between stability and reactivity of all reactants, particularly the Tz.

Synthesis of Protein-Oligonucleotide Conjugates

Nucleic acids and proteins form two of the key classes of functional biomolecules. Through the ability to access specific protein-oligonucleotide conjugates, a broader range of functional molecules becomes accessible which leverages both the programmability and recognition potential of nucleic acids and the structural, chemical and functional diversity of proteins. Herein, we summarize the available conjugation strategies to access such chimeric molecules and highlight some key case study examples within the field to showcase the power and utility of such technology.

Barnase*Barstar-guided two-step targeting approach for drug delivery to tumor cells in vivo

For precise ligation of a targeting and cytotoxic moiety, the use of Barnase-Barstar pair as a molecular glue is proposed for the first time. Targeting was mediated through the use of a scaffold protein DARPin_9-29 specific for the human epidermal receptor 2 (HER2) antigen that is highly expressed on some types of cancer and Barnase*Barstar native bacterial proteins interacted with each other with K_d 10^-14 M. The approach proposed consists of prelabeling a target tumor with hybrid protein DARPin-Barnase prior to administration of cytotoxic component-loaded liposomes that have Barstar covalently attached to their surface. Based on in vivo bioimaging we have proven that DARPin-based Barnase*Barstar-mediated pretargeting possesses precise tumor-targeting capability as well as antitumor activity leading to apparent tumor-growth inhibition of primary tumors and distant metastases in experimental animals. The results obtained indicate that the new system combining DARPin and Barnase*Barstar can be useful both for the drug development and for monitoring the response to treatment in vivo in preclinical studies.

IEDDA: An Attractive Bioorthogonal Reaction for Biomedical Applications

The pretargeting strategy has recently emerged in order to overcome the limitations of direct targeting, mainly in the field of radioimmunotherapy (RIT). This strategy is directly dependent on chemical reactions, namely bioorthogonal reactions, which have been developed for their ability to occur under physiological conditions. The Staudinger ligation, the copper catalyzed azide-alkyne cycloaddition (CuAAC) and the strain-promoted [3 + 2] azide–alkyne cycloaddition (SPAAC) were the first bioorthogonal reactions introduced in the literature. However, due to their incomplete biocompatibility and slow kinetics, the inverse-electron demand Diels-Alder (IEDDA) reaction was advanced in 2008 by Blackman et al. as an optimal bioorthogonal reaction. The IEDDA is the fastest bioorthogonal reaction known so far. Its biocompatibility and ideal kinetics are very appealing for pretargeting applications. The use of a trans-cyclooctene (TCO) and a tetrazine (Tz) in the reaction encouraged researchers to study them deeply. It was found that both reagents are sensitive to acidic or basic conditions. Furthermore, TCO is photosensitive and can be isomerized to its cis-conformation via a radical catalyzed reaction. Unfortunately, the cis-conformer is significantly less reactive toward tetrazine than the trans-conformation. Therefore, extensive research has been carried out to optimize both click reagents and to employ the IEDDA bioorthogonal reaction in biomedical applications.