The ultimate radiochemical nightmare: upon radio-iodination of Botulinum neurotoxin A, the introduced iodine atom itself seems to be fatal for the bioactivity of this macromolecule

EJNMMI Research

Table 3 Results of iodination of cetuximab with addition of HEPES and/or lactose and iodination of BoNT-A

Product (50 μg)	HEPES (μmol)	Lactose (mg)	IODOGEN (μg)	¹²⁵ I (MBq)	Labelling efficiency (%)	¹²⁵ I/product molar ratio
Cetuximab	0	0	2.5	19.6	80.7	0.59
Cetuximab	0	0.625	2.5	19.3	78.3	0.56
Cetuximab	1	0	2.5	15.7	43.5	0.25
Cetuximab	1	0.625	2.5	30.2	43.5	0.49
Cetuximab	1	0.625	1.25 + 1.25	15.6	50.2	0.29
Cetuximab	1	0.625	1.25 + 1.25	93.4	51.8	1.80
Cetuximab	1	0.625	1.25 + 1.25	146.2	43.0	2.34
BoNT-A	-^a	-^a	1.25 + 1.25	97.2	51.5	1.86
BoNT-A	-^a	-^a	1.25 + 1.25	98.3	51.8	1.90

Conditions: 2.5 μg IODOGEN in 35 μL IODOGEN/acetonitrile added at t = 0 or 1.25 μg IODOGEN in 17.5 μL IODOGEN/acetonitrile added at t = 0 and at t = 45. Other conditions: 250 μL reaction volume and 90-s reaction time. Radiochemical purity was >98.5% in all experiments.
^aReconstituted BoNT-A contained HEPES (20 mM) and lactose (1.25%); this was not purposely added.