Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 53

Development of radioimmunotherapy with astatine-211-counjugatedantibodies
H. Takashima,∗1,∗2 Y. Koga,∗1,∗2 K. Onuki,∗3 S. Manabe,∗4,∗5 R. Tsumura,∗1 T. Anzai,∗1 N. Iwata,∗1M. Yasunaga,∗1 Y. Wang,∗2 T. Yokokita,∗2 Y. Komori,∗2 D. Mori,∗2 H. Haba,∗2 H. Fujii,∗3 and Y. Matsumura∗1
Macromolecules and macroproteins hardly extravasatefrom blood vessels at a normal organ site due to theirlarge molecular weight. By contrast, since the permeability of blood vessels at tumor sites is higher thanthat at normal organs, macromolecules and macroproteins can extravasate from tumor vessels. Moreover,due to the poor lymphatic drainage system at tumor
sites, macromolecules and macroproteins are retained
for a prolonged time after extravasation. As a result,macromolecules and macroproteines selectively and efﬁciently accumulate in tumors. This phenomenon isthe enhanced permeability and retention (EPR) eﬀect,1)
which is the rationale of drug delivery systems for cancer treatment. Antibodies such as immunoglobulin G(IgG), the molecular weight of which is approximately
150 kDa, accumulate in tumors via the EPR eﬀect. In
addition to the EPR eﬀect, antigen-antibody interaction
with target molecules expressed in tumors enhances thetumor accumulation of antibodies.2)
We successfully produced several types of monoclonal
antibodies (mAbs) that recognize target molecules
highly or speciﬁcally expressed in tumors, and appliedmAbs to drug carriers. The antibody-drug conjugate(ADC) is composed of an mAb that recognizes target molecules in tumors and low-molecular weight anticancer agents, and it is an example of armed antibodies. Previously, using our mAbs, we prepared ADCs anddemonstrated that they showed potent antitumor eﬀects
in cancer xenograft models.3–5)
Alpha particles are characterized by higher liner energy transfer (LET) and shorter range in tissue thanother types of radiation, resulting in eﬃcient deoxyribonucleic acid (DNA) double-strand breaks in accumulated cells with minor eﬀects on adjacent cells. Accordingly, since the successful delivery of alpha emitters totumor sites is expected to yield eﬃcient antitumor effects with mild toxicity, we focus on the developmentof radioimmunotherapy (RIT) with an alpha-emitterconjugated antibody.
In this study, we labeled antibodies with astatine211, one of the most promising alpha emitters for cancer
treatment, and evaluated the binding activities and invitro cytocidal eﬀects of the radioactive antibodies.
We synthesized astatine-211 in the 209 Bi(α, 2n)211 At
reaction using the RIKEN Azimuthally Varying Field∗1∗2∗3∗4∗5

Developmental Therapeutics, Exploratory Oncology Research& Clinical Trial Center (EPOC), National Cancer Center
RIKEN Nishina Center
Division of Functional Imaging, EPOC, National Cancer Center
Synthetic Cellular Chemistry Laboratory, RIKEN
Research Center for Pharmaceutical Development, Tohoku
UniversityFig. 1. Flowchart to prepare astatine-211-conjugated antibodies.

(AVF) cyclotron.
Figure 1 shows a ﬂowchart to prepare astatine211-conjugated antibodies. First, we reduced the interchain disulﬁde bonds of mAbs4,5) and attached atrimethylstannyl benzoate linker to an mAb with reactive sulfhydryl groups via a thioether bond betweenthe sulfhydryl and maleimide groups. Then, we labeled liker-antibody complexes with astatine-211 via a
halogen-exchange reaction. Subsequently, the astatine211-conjugated antibodies were puriﬁed away from unconjugated astatine-211 using a PD-10 desalting column(GE Healthcare Life Sciences, Chicago, IL, USA). To
increase the labeling eﬃciency, we optimized the procedure to purify astatine-211 from bismuth and calculated
the labeling rate by dividing the radioactivities of immunoconjugates by those of astatine-211 that was initially applied to the reaction solution (100 MBq); thelabeling rate was approximately 60%. We successfully
prepared astatine-211-conjugated antibodies using bothmAbs newly developed by us and trastuzumab, a clinically available mAb for patients with breast or gastriccancer.
We evaluated the aﬃnity of astatine-211-conjugatedantibodies by ﬂow cytometry. The radioactive antibodies bound to cancer cells depending on the expression
level of target molecules on the cell membrane, and thespeciﬁc aﬃnity of immunoconjugates was comparable
to the corresponding mAbs. As a result, astatine-211conjugated antibodies exerted in vitro cytocidal eﬀects
on cancer cells depending on the expression of target
molecules on the cell membrane.
In conclusion, we synthesized astatine-211 using theRIKEN AVF cyclotron and succeeded in constructing
astatine-211-conjugated antibodies. The present data
warrant further basic studies for future clinical development.
References

1) Y. Matsumura et al., Cancer Res. 46, 6387 (1986).
2) H. Takashima et al., Sci. Rep. 7, 12341 (2017).
3) Y. Koga et al., Int. J. Cancer. 137, 1457 (2015).
4) R. Tsumura et al., J. Control. Release. 284, 49 (2018).
5) H. Fuchigami et al., Sci. Rep. 8, 14211 (2018).

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