Yttrium-90 and Rhenium-188 Radiopharmaceuticals for Radionuclide Therapy

eagent (Sigma). The mean recovery of the reduced antibody was 98.5% ± 2.6%
(mean ± standard deviation (SD), n = 4) and the number of –SH groups generated
per molecule of antibody was 5.0 ± 1.0 (mean ± SD, n = 4).
3.2.1.2. Radiochemical quality control of 188 Re rituximab
The labelling efficiency was evaluated using instant thin layer
chromatography (ITLC). Both 0.9% NaCl and acetone were used as mobile
phases to separate free perrhenate ( 188 ReO 4 – ). Rhenium-188 tartrate and 188 ReO 4
–
moved with the solvent front (R f = 1) when 0.9% NaCl was used as the mobile
phase, while radiocolloid ( 188 ReO 2 ) and 188 Re rituximab remained at the
origin (R f = 0). When acetone was used as the mobile phase, 188 Re rituximab,
188 Re tartrate and 188 ReO 2 stayed at the origin, while 188 ReO 4 – moved to the
solvent front. ITLC silica gel (SG) strips impregnated with HSA (5%) were used
as the stationary phase and ethanol:ammonia:water (2:1:5 vol.%) was used as the
mobile phase. In this chromatographic system, 188 ReO 2 remained at the origin,
while 188 Re rituximab, 188 Re tartrate and 188 ReO 4 – moved towards the solvent
front.
3.2.1.3. Labelling of anti-CD20 and optimization of radiolabelling
The labelling studies were first performed using a liquid formulation of
rituximab, based on the literature [3.4], that contained 1 mg of rituximab, 82.8 mg
of sodium tartrate, 1.67 mg of SnCl 2 and 0.25 mg of gentisic acid. Perrhenate
eluted from the 188 W/ 188 Re generator in 0.9% NaCl was added (~642.5 MBq). The
solution was incubated for 1 h at room temperature at pH5.5. The labelling yield
was 67.0% ± 0.2%. Because of this low labelling yield, a series of parameters
was varied to identify a best formulation that could attain a high labelling
yield, including antibody mass (0.25, 0.5, 1.0 and 2.5 mg), reducing agent mass
(0.25, 0.5, 1.0, 1.67, 3.0, 5.0 and 7.0 mg), tartrate mass (20.7, 41.4, 82.8, 165.6
and 331.2 mg), reaction time (15, 30, 60 and 120 min), 188 Re volume (1 and
2 mL) and activity (3463.2 MBq). The stability for the optimized formulation
was studied at different temperatures (room temperature, cold temperature of 5°C
and dry ice) and at different times (4, 6 and 24 h). Figures 3.16–3.19 show the
results of the effects seen on the labelling efficiency of 188 Re rituximab due to
the variations made in the reducing agent mass, antibody mass, tartrate mass and
reaction time. A volume of 1 mL of 188 Re seemed to be superior to the volume
of 2 mL, with higher labelling yields and lower impurities. When rituximab was
labelled with a high activity of 188 Re, the impurity levels of 188 ReO 2 and 188 ReO 4
–
were >25%, indicating that further studies are necessary to improve the labelling
efficiency.
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