Functionalised macrocycles for tumour targeting
Monoclonal antibodies which recognise tumour-associated antigens provide a means of targeting radionuclides selectively to tumour cells. (^99m)Tc and (^64)Cu are potentially useful isotopes for radioimmunoimaging;(^ 90)Y and (^67)Cu may be suitable for radioimmunotherapy. The synthesis of functionalised macrocycles for binding these four radioisotopes to antibodies is described. In each case, a macrocycle has been selected to provide a complex which is kinetically inert, thereby preventing dissociation of the radiolabel in vivo. A novel strategy for conjugating a C-alkylated cyclam derivative (for binding Tc and Cu) to an antibody is described. This method facilitates the selective acylation of an exocyclic primary amino group in the presence of the secondary ring nitrogens. Unfortunately, the labelling of antibody-bound cyclam with (^99m)Tc required conditions (pH 11) which produced extensive binding of the radiolabel to the protein backbone. "Non-specific" (^99m) Tc was subsequently found to dissociate in vivo. Pre-labelling the macrocycle with (^99m)Tc solved the "non-specifics" problem but required a pH which meant that the conjugation step was too slow for sufficient specific activity to be bound. A phenol-pendent derivative of cyclam was found to incorporate (^99m)Tc at a lower pH than cyclam itself. The "non-specific" binding of copper to the protein was minimised using a low pH labelling strategy in conjunction with a chelate wash. Macrocycle antibody conjugates labelled manner provide very promising biodistribution profiles in normal mice. A labelling buffer was selected to enhance the rate of uptake of copper by the macrocycle at low pH. Macrocycle-antibody conjugates containing 13N(_4), which was found to provide faster association kinetics than cyclam, have been prepared and await radiolabelling studies. A derivative of I3N(_4), containing 4 carboxylic acid donor sites, has been functionalised for conjugation to an antibody to act as a (^90)Y binder.