J Med Assoc Thai 2001; 84 (4):545

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Factors Affecting Chemistry of Reduction-Mediated 99m. Tc-Labelling of Monoclonal Antibodies and Polyclonal Human Immunoglobulins
Boonkitticharoen V Mail, Puchinda D , Ngonrath A , Kraiphibul P

DUANGPEN PUCHINDA, M.Sc. *,
PUANGTONG KRAIPHIBUL, M.D.*
In developing a new method for preparing a radiopharmaceutical for clinical investiga-
tion, a thorough understanding of reaction stoichiometry is crucial in optimizing the labelling
chemistry. Factors determining labelling efficiency of the 2 -mercaptoethanol (2-ME) -mediated
99mTc-labelling of antibody molecules were elucidated using anti-tumor monoclonal antibodies
of different lgG subclasses (i.e. IOR-CEA(IgG
1
),
Ml70(1gG
1
),
3F8(IgG
3
)
and EMD (lgG
2
.))
and
polyclonal human immunoglobulins (Sandoglobulin). Antibodies which were sensitive to 2-ME
reduction (i.e. required 500-1000 molar excess of 2-ME) could tag 99mTc with high efficiency since
they possessed abundant reactive sites (i.e. sulfydryl groups) for 99mTc binding. Reduction sensi-
tivity of antibodies was unlikely to be affected by IgG subclass and could be rated as follows :
Sandoglobulin > IOR-CEA > 3F8 > Ml70 > EMD. Concentrations of the reduced antibodies for
effective labelling appeared to be related to the reduction sensitivity, i.e. 0.2, 0.4 and 0.6 mg/ml
were required for labelling of IOR-CEA, 3F8 and Ml70 respectively. In addition, susceptibility to
2-ME reduction seemed to reflect the rate of antibody labelling. For 2-ME resistant molecules, i.e.
M 170 and EMD, successful labelling could be achieved by using a slow 99mTc reducing agent such
as SnC1
2
instead of SnF
2
which reacted more rapidly. Since 2-ME generates reactive sulfhydryl
groups that are distal to antigen binding sites, the immunoreactivity of the modified antibody was
not affected by the effect of reduction.
Key word : 99mTc-Labelling, Labelling Chemistry, Anti-Tumor Monoclonal Antibody, Human
Immunoglobulin

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