Liposomes as carriers of radionuclides: From imaging to therapy

Liposomes have been studied quite extensively during the last twenty years or so as carriers for a variety of diagnostic molecules, in order to enhanc e the obtained in vivo target-to-background contrast, thus improve imaging resolution. In such applications, unlike chemotherapy and even more radioth erapy, low radiation doses are generally administered hence, apart from ris k assessment purposes, the need for radiation dosimetry is not critical for their success. In the present article we will try to present and analyze c ritical factors that may affect any attempt to construct and investigate li posomes as carriers of particle-emitting radionuclides for radiotherapeutic purposes. Since dosimetric considerations for this kind of applications ar e considered essential for establishing radiotoxicity levels and dose-respo nse relations for both healthy and tumor tissues, a description of current dosimetry methodologies will be included. It is aimed that the conceptual d ifferences between imaging and therapy using radiolabeled liposome conjugat es, along with those between chemotherapy and radiotherapy will be elucidat ed. The complex interplay of different pharmacokinetic (liposome biodistrib ution), physical (radionuclide half-life, particle emission characteristics ), physicochemical (liposome particle size, surface charge), and physiologi cal (tumor dimensions, vasculature, pH) parameters will be qualitatively in vestigated towards optimal selection and design of liposome-radionuclide co mplexes.