1 research outputs found
Comparison of <sup>64</sup>Cu-Complexing Bifunctional Chelators for Radioimmunoconjugation: Labeling Efficiency, Specific Activity, and <i>in Vitro</i>/<i>in Vivo</i> Stability
High radiolabeling efficiency, preferably to high specific
activity,
and good stability of the radioimmunoconjugate are essential features
for a successful immunoconjugate for imaging or therapy. In this study,
the radiolabeling efficiency, <i>in vitro</i> stability,
and biodistribution of immunoconjugates with eight different bifunctional
chelators labeled with <sup>64</sup>Cu were compared. The anti-CD20
antibody, rituximab, was conjugated to four macrocyclic bifunctional
chelators (<i>p</i>-SCN-Bn-DOTA, <i>p</i>-SCN-Bn-Oxo-DO3A, <i>p</i>-SCN-NOTA, and <i>p</i>-SCN-PCTA), three DTPA
derivatives (<i>p</i>-SCN-Bn-DTPA, <i>p</i>-SCN-CHX-A″-DTPA,
and ITC-2B3M-DTPA), and a macrobicyclic hexamine (sarcophagine) chelator
(sar-CO<sub>2</sub>H) = (1-NH<sub>2</sub>-8-NHCOÂ(CH<sub>2</sub>)<sub>3</sub>CO<sub>2</sub>H)Âsar where sar = sarcophagine = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]Âicosane).
Radiolabeling efficiency under various conditions, <i>in vitro</i> stability in serum at 37 °C, and <i>in vivo</i> biodistribution
and imaging in normal mice over 48 h were studied. All chelators except
sar-CO<sub>2</sub>H were conjugated to rituximab by thiourea bond
formation with an average of 4.9 ± 0.9 chelators per antibody
molecule. Sar-CO<sub>2</sub>H was conjugated to rituximab by amide
bond formation with 0.5 chelators per antibody molecule. Efficiencies
of <sup>64</sup>Cu radiolabeling were dependent on the concentration
of immunoconjugate. Notably, the <sup>64</sup>Cu-NOTA-rituximab conjugate
demonstrated the highest radiochemical yield (95%) under very dilute
conditions (31 nM NOTA-rituximab conjugate). Similarly, sar-CO-rituximab,
containing 1/10th the number of chelators per antibody compared to
that of other conjugates, retained high labeling efficiency (98%)
at an antibody concentration of 250 nM. In contrast to the radioimmunoconjugates
containing DTPA derivatives, which demonstrated poor serum stability,
all macrocyclic radioimmunoconjugates were very stable in serum with <6%
dissociation of <sup>64</sup>Cu over 48 h. <i>In vivo</i> biodistribution profiles in normal female Balb/C mice were similar
for all the macrocyclic radioimmunoconjugates with most of the activity
remaining in the blood pool up to 48 h. While all the macrocyclic
bifunctional chelators are suitable for molecular imaging using <sup>64</sup>Cu-labeled antibody conjugates, NOTA and sar-CO<sub>2</sub>H show significant advantages over the others in that they can be
radiolabeled rapidly at room temperature, under dilute conditions,
resulting in high specific activity