In the past, many
diphosphonates were introduced as bone scan
radiopharmaceuticals. In addition,
diphosphonates have been labeled with beta-emitted
isotopes and developed into useful therapeutic drugs for bone
metastases. However, it is not clear which diphosphonate is the best choice when labeling with Re-188. In this study, we labeled
methylene diphosphonate (MDP), hydroxyethylidene diphosphonate (
HEDP), and hydroxymethane diphosphonate (HDP) with Re-188. Each
radiopharmaceutical was further evaluated in two conditions (with and without carrier). Twenty-four rabbits were used (four in each group) for the analysis of the biodistributions and bone uptakes of these
radiopharmaceuticals to assess their potential for clinical applicability. Four hours after
intravenous injection of approximately 37 MBq (1 mCi) Re-188-labeled diphosphonate preparations, whole body scans were performed using a large-field
gamma camera equipped with a high resolution collimator. Bone-to-soft tissue ratios (B/S ratio) were calculated using a computer program. Our data showed that Re-188
HEDP with carrier (10(-4) M carrier) could accumulate in the skeletal system whereas very little absorption by bone was observed in the rabbits that were injected with carrier-free Re-188
HEDP. In addition, no significant bone uptake was demonstrated for Re-188 MDP or Re-188 HDP, with or without carrier. The B/S ratio was 25.06 in the Re-188
HEDP with carrier group but less than 3 in the other groups. In conclusion,
HEDP is the best choice among these three bone-seeking drugs when labeled with Re-188. But, it is necessary to add carrier when preparing Re-188
HEDP for the treatment of bone
metastases.