Radiolabelled peptides: now and the future.

Designed by nature for stimulating, inhibiting or regulating numerous life functions, for a long time peptides have been considered ideal agents for therapeutic applications. In recent years, radiolabelled peptides have emerged as a new, useful class of radiopharmaceuticals for diagnosis of a variety of endocrine tumours. Although clinical results with 111In- and 125I-labelled peptides have been excellent, extensive efforts have been made in preparing and evaluating peptides labelled with 99Tcm. This is the raidonuclide of choice by virtue of its cost-effectiveness, availability and desirable nuclear characteristics. With 99Tcm, certain peptides can be labelled 'directly' without a loss of functional specificity, but certain peptides must be labelled using bifunctional chelating agents. The bifunctional chelating agent methods are chemically complex and frequently inefficient. Instant kit preparations are, however, possible. Important tests in the evaluation of these agents are receptor binding assays as well as examinations in experimental pre-clinical models. Peptides are small and can be taken up in target tissues non-specifically by increased capillary permeability. However, something that is frequently forgotten during such evaluations are control studies with non-specific peptides of similar size and charge. Radiolabelled peptides aimed at instant in vivo interaction with neutrophils or platelets must also be examined carefully to ensure that they do not induce neutropaenia or thrombocytopaenia. The preparation of radiolabelled peptides is not only challenging but also exciting, since these agents will continue to be promising and rewarding radiopharmaceuticals for decades to come.