Long-term retention of sup(99m)Tc diphosphonate, 113Sn-diphosphonate and 32P-orthophosphate in rats

The kinetic behaviour of 32P orthophosphate, 113Sn-hydroxy ethylene diphosphonate and sup(99m)Tc hydroxy ethylene phosphonate in the organism of the rat was investigated over a long period of time. The effective halflives and hence radiation exposure of the various organs when applying β-radiation emitting radioisotopes 32 p and 121 Sn, as well as the exposure due to the 'β components' of the γ emitter sup(99m)Tc were calculated from the above determined biological halflives. The exposure from the β-emitter 188 rhenium was also determined from the biological data of the latter isotope due to its expected similar pharmacological behaviour. The procentual enrichment of 32P orthophosphate is considerably higher in all soft parts, lower however in the bones than that of the diphosphonate compounds. The activity is reduced more slowly in the soft parts, which additionally gives rise to a smaller output of the activity via the kidneys. The radiation exposure to the organs is considerably less with smaller activity enrichment of the diphosphonate compound than when applying 32P orthophosphonate. Only the kidney is exposed to a higher radiation dose when applying 121Sn, whereas sup(99m)Tc and 188Re exposes this organ to the same extent as 32P orthophosphonate. The ratio of the distal to the proximal femur is larger for sup(99m)Tc, 188Re and 121Sn than for 32P orthophosphate. The highest dose power in the distal femur end is achieved with sup(99m)Tc and 188Re, the lowest by far with 32P orthophosphate which is explained by the relatively long physical halflife of phosphorous and not by the average energy of the β-particles. (orig./MG)