Positron emission tomography and radioimmunotargeting - General aspects

To optimize radioimmunotherapy, in vivo information on individual patients, such as radionuclide uptake, kinetics, metabolic patterns and optimal administration methods, is important. An overriding problem is to determine accurately the absorbed dose in the target organ as well as critical organs. Positron Emission Tomography (PET) is a superior technique to quantify regional kinetics in vivo with a spatial resolution better than 1 cm³ and a temporal resolution better than 10 s. However, target molecules often have distribution times of several hours to days. Conventional PET nuclides are not applicable and alternative positron-emitting nuclides with matching half-lives and with suitable labelling properties are thus necessary. Over many years we have systematically developed convenient production methods and labelling techniques of suitable positron nuclides, such as ¹¹⁰In(T 1/2 = 1.15 h), ⁸⁶Y(T 1/2 = 14 h), ⁷⁶Br(T 1/2 = 16 h) and ¹²⁴I(T 1/2 = 4 days). 'Dose planning' can be done, for example, with ⁸⁶Y- or ¹²⁴I-labelled ligands before therapy, and ⁹⁰Y- and ¹³¹I-labelled analogues and double-labelling, e.g. with a ⁸⁶Y/⁹⁰Y-labelled ligand, can be used to determine the true radioactivity integral from a pure beta- emitting nuclide. The usefulness of these techniques was demonstrated in animal and patient studies by halogen-labelled MAbs and EGF-dextran conjugates and peptides chelated with metal ions.