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

Ho-166, with its favourable radiation characteristics of t(1/2) 26.8 h and E-beta 1.85 and 1.75 MeV, is proposed as a suitable choice for the endovasc ular radionuclide therapy (EVRT) technique of liquid filled, low pressure b alloon angioplasty. Ho-166 was produced by the (n,gamma) reaction on a natu ral Ho2O3 target. The specific activity obtained was similar to 100 mCi.mg( -1) when irradiated at a flux of 2 x 10(13) n.cm(-2)s(-1) for similar to7 d ays, and the possible contaminant Ho-166(m) was not detected. Ho-166 was ea sily complexed with diethylenetriaminepentaacetic acid (DTPA) at a ligand t o metal molar ratio ([L]:[M]) of 1:1 at room temperature (22-23 degreesC) a nd 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 degreesC 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 Ho -166-DTPA complex could be an alternative to the more expensive and difficu lt to access Re-188 based products for EVRT, and provide adequate uniform r adiation dose for the arterial vessel wall under treatment. ((C) 2001 Lippi ncott Williams & Wilkins).