Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 53

Anti-cancer eﬀect of 211 At-labeled antibody on xenografted mice withintravenous injection
Y. Kanayama,∗1 K. Sakamoto,∗2 R. Zochi,∗1 Y. Wang,∗3 T. Yokokita,∗3 Y. Komori,∗3 D. Mori,∗3 H. Haba,∗3 andY. Watanabe∗1Uptake rates [%ID/g]

35303h

2524h

48h

26.10
21.47

201510
50UL

BLHE

LU

SPPALIKI

STIN

FM

TH A431 C6GBFig. 2. Biodistribution in xenografted mice (n = 4). A431,HER2 positive; C6, HER2 negative.
1200Tumor size [mm^3]

Astatine-211 is a promising α emitter for targeted
α therapy because of its simple decay scheme, which
can avoid undesirable radiation from daughter nuclides.
In a previous study, we investigated the radiolabeling method of 211 At on monoclonal antibodies using astannyl compound (m-MeATE). The synthesis of 211 Atlabeled IgG was successful, but we observed the detachment of 211 At 24 h after intravenous (IV) injection.
To overcome the deastatination, the use of decaborane,
a boron cluster compound, is a possible solution because its binding to astatine is more stable in vivo.1)
However, as we have previously reported,2) the 211 Atdecaborate moiety may increase hydrophobicity, which
probably causes its poor bioavailability, resulting in thelow tumor uptake by IV injection. In the present study,
we introduced a PEG linker between decaborate and IgG
to decrease the hydrophobicity. By using this conjugate,
we investigated the anti-cancer eﬀect of 211 At-labeledtrastuzumab on HER2-positive cancer xenografted micewith IV injection.
The 211 At-decaborate (B10)-PEG5-trastuzumab wasobtained through the 3-step synthesis (Fig. 1). We used
46 MBq of 211 At produced in the 209 Bi(α, 2n)211 At reaction at the RIKEN AVF cyclotron for labeling 40 µg of
the antibody-linker conjugate, and the mixture after thereaction was applied to ﬁltration with G50 DNA-grade
gel to remove free 211 At and replace it with physiologicalsaline. The radioactivity yield was 65% (30.2 MBq wasretained on the antibody), with the protein recovery rate
being 64%. The speciﬁc radioactivity was 1.08 MBq/µg.
We ﬁrst investigated the bio-distribution of the 211 Atlabeled trastuzumab in xenografted mice 3, 24, and48 h after IV injection. Figure 2 shows that 211 Attrastuzumab was in the blood at signiﬁcant levels(around 15%ID/g) for 48 h and progressively accumulated in the target A431 tumor region (up to approximately 20%ID/g) during the same period. On the otherhand, the control C6 tumor region did not show a signiﬁcant accumulation from 24 to 48 h. This result wasconsistent with the typical outcome of a PET imaging

1.8 MBq At-Tra (n=4)

1000

1.0 MBq At-Tra (n=4)
1.0 MBq Free At (n=5)

800
600
400
2000061218Day

243036Fig. 3. Average tumor size of the mice after IV injection of211
At-B10-PEG5-trastuzumab.

study using 64 Cu-labeled trastuzumab.
Subsequently, we proceeded to examine the anticancer eﬀect of 211 At-trastuzumab via IV injection. Twogroups of mice were administered 1.8 or 1.0 MBq of211
At-trastuzumab (1.7 µg), while the control group wasadministered 1.0 MBq of free 211 At. The changes in thebody weight and tumor size, as well as the survival ratewere observed up to 36 days after the IV injection (Day0) (the tumor size is shown in Fig. 3).
A larger dose (1.8 MBq) of 211 At-Trastuzumab caused
a severe decrease in the body weight within a week, implying a serious side eﬀect of the injection, and led tothe euthanization of a mouse. The 211 At-trastuzumabin either dose successfully suppressed the growth of thetumor in size with only one shot of the IV injection, andno mice died because of the tumor. No adverse eﬀects
were observed in the 1.0 MBq At-Tra group. In con2. Cu-free click chemistry 1. Amine coupling
trast, the tumor grew much more rapidly in the control
211(SPAAC)
3. Covalent aaching of At
group, with no mice alive until Day 36. Thus, we suc211At
cessfully demonstrated the favorable anti-tumor eﬀect of
NH2
211
At-B10-labeled trastuzumab conjugated with a PEG
DBCO-PEG4-decaborate NHS-PEG1-azide Trastuzumab linker, which was probably supported by the improved
biodistribution.
211
Fig. 1. Synthesis of

∗1
∗2
∗3

At-B10-PEG5-trastuzumab.

Laboratory for Pathophysiological and Health Science,RIKENLaboratory for Nonnatural Amino Acid Technology, RIKEN
RIKEN Nishina Center

References

1) F. Guerard et al., Cancer Biother. Radiopharm. 28, 1(2013).
2) K. Fujiki et al., Chem. Sci. 10, 1936 (2019).

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