Serum stability of ⁶⁷Cu chelates: Comparison with ¹¹¹In and ⁵⁷Co

Simple chelates and chelate conjugated monoclonal antibodies labeled with ¹¹¹In, ⁵⁷Co and ⁶⁷Cu demonstrate marked differences in stability when exposed to a serum environment. Among these radiometals, on DTPA, the order of stability is ¹¹¹In > ⁵⁷Co ≥ ⁶⁷Cu. On benzyl-EDTA, the order of stability is ¹¹¹In ~ ⁵⁷Co ≥ ⁶⁷Cu. Among those investigated, the only serum stable ⁶⁷Cu chelate found was ⁶⁷Cu-TETA. The order of stability observed for ⁵⁷Co vs ⁶⁷Cu is contrary to published equilibrium constants. These in vitro studies suggest that the in vivo behavior of metal chelates exposed to a complex molecular environment may not be predicted by classically determined equilibrium constants.