Potential 166Ho radiopharmaceuticals for endovascular radionuclide therapy. II. Preparation and evaluation of 166Ho-DTPA.

166Ho, with its favourable radiation characteristics of t(1/2) 26.8 h and Ebeta 1.85 and 1.75 MeV, is proposed as a suitable choice for the endovascular radionuclide therapy (EVRT) technique of liquid filled, low pressure balloon angioplasty. 166Ho was produced by the (n,gamma) reaction on a natural Ho2O3 target. The specific activity obtained was approximately 100 mCi x mg(-1) when irradiated at a flux of 2 x 10(13) n x cm(-2) s(-1) for approximately 7 days, and the possible contaminant 166Ho(m) was not detected. 166Ho was easily complexed with diethylenetriaminepentaacetic acid (DTPA) at a ligand to metal molar ratio ([L]:[M]) of 1:1 at room temperature (22-23 degrees C) and a reaction time of a few minutes. The radiochemical purity was >99%, as determined by paper chromatography using a mixture of pyridine, ethanol and water (1:2:4) as solvent. The complex had good stability up to 72 h at 37 degrees C in a serum environment. In a study using Swiss mice > 85% of the injected dose was cleared into the urine within 30 min post-injection, with insignificant retention in any major tissues. The studies show that the 166Ho-DTPA complex could be an alternative to the more expensive and difficult to access 188Re based products for EVRT, and provide adequate uniform radiation dose for the arterial vessel wall under treatment.