Current prospects for 125I in tumour therapy

To evaluate 125I as a tool in tumour therapy, microdosimetric data, results from animal experiments and information obtained from clinical applications are presented. Numbers and energies of electrons and photons emitted were calculated to determine the energy deposition in critical biological targets (spheres of 20 nm diameter) after incorporation of 125IUdR. The total energy deposited per decay includes the energy imparted to Auger electrons (0.96 keV) and the ionization potential (1.07 keV) which is built up in consequence of the electron cascade. The latter will be locally absorbed. Thus, 125I acts like a high-LET radiator. In animal experiments tumour bearing mice were continuously injected intravenously with cold IUdR and 125IUdR. Tumour regression improved from 10% for the controls to 25% after infusion of 2x10-4mol/L IUdR and to 31% after infusion of 200 μCi of 125IUdR per mouse, respectively. In clinical applications two patients with advanced squamous carcinomas of the tongue and buccal cavity were locally intra-arterially perfused with 125IUdR. To the first patient 50 μCi 125IUdR were administered via the arteria lingualis every 12 h followed at day 12 by 500 μCi 125IUdR per 12 h period. In the second patient 'interport' catheters were implanted in the arteria carotis externa. Twenty-five μCi were infused 10-18 h daily for 44 days, to a total of 1080 μCi. Retained activities remained below the expected levels in both patients and the treatments were discontinued. To render more promising the application of 125I in tumour therapy it appears necessary to specifically augment 125IUdR uptake in tumour DNA. (author)