Recoil and conversion electron considerations of the 166Dy/166Ho in vivo generator

The use of radionuclides as potential therapeutic radiopharmaceuticals is increasingly investigated. An important aspect is the delivery of the radionuclide to the target, i.e. the radionuclide is not lost from the chelating agent. For in vivo generators, it is not only the log K of complexation between the metal ion and the chelator that is important, but also whether the daughter radionuclide stays inside the chelator after decay of the parent radionuclide. In our previous work, we showed that the classical recoil effect is only applicable for decays with a Q value higher than 0.6 MeV (in the atomic mass range around 100). However, Zhernosekov et al. [1] published a result for140Nd/140Pr (Q = 0.222 MeV) which indicated that >95% of the daughter (140Pr) was lost by a DOTA chelator upon decay of140Nd. The authors ascribed this to the & ldquo;post-effect”. Their experiment was repeated with the166Dy/166Ho generator to ascertain whether our calculations were correct. It was found that 72% of the daughter (166Ho) was liberated from the DOTA chelator, indicating that the “post effect” does exist in contrast to our recoil calculations. Upon further investigation, we determined that one should not only consider recoil energy levels but also the mode of decay which was able to explain the partial recoil found for166Dy/166Ho. It is concluded for the166Dy/166Ho system that the low recoil energy of the daughter nucleus166Ho is not a sufficient reason to rule out release of the nuclide from chelators. On the other hand, we found that the ratio of the166Ho that gets released corresponds to the ratio of relaxation of Ho atoms via the Auger process.