Towards authentically labelled bi-modal PET (SPECT)/MR-probes

Institute of Neuroscience and Medicine, INM-5: Nuclear Chemistry, Research Centre Jülich, Germany Application of radiolabelled, existing MRI probes using a suitable reporter group for multimodal PET(SPECT)/MRI imaging is limited due to the required alteration of the molecular structure and thus changing their in vivo properties. Radiolabelling of existing MRI contrast agents with PET(SPECT) isotopes of paramagnetic elements offers a simple way to address this issue. Therefore, new routes to the production of SPECT/ PET-radionuclides Gd and Mn were examined which can be applied for n.c.a. labelling of Gd(III) and Mn(II) MRI contrast agents. Additionally, Mn(II)-based complexes stable for in vivo application are to be synthesized. Reaction cross sections and experimental thick target yields were measured by irradiation of Cr or Eu2O3. Integral yields were calculated from measured excitation functions. A radiochemical separation of Mn from Cr was developed based on cationexchange chromatography [1]. Cross section data of the Eu(d,x) and Eu(p,x) reactions were measured up to 70.9 MeV and 44.8 MeV, respectively. Integral yields of up to 177.3 MBq/μAh and 81.6 MBq/μAh for Eu(d,x)Gd reactions and up to 43.3 MBq/μAh and 61.8 MBq/μAh for Eu(p,x)Gd reactions, respectively, were calculated. Those were several times higher than for aor He induced reactions on highly enriched Sm [2,3]. With n.c.a. Mn, also cross sections of co-produced V, Cr, Mn were determined in the energy range of 7.6 to 45 MeV. The production rates of Mn were measured from 8.2 to 16.9 MeV with up to 13.1 MBq/μAh which was separated from Cr by column chromatography. Production data of the SPECT nuclides Gd and the PET nuclide Mn were established. Different to Mn a practical isolation procedure for Gd is still required. Current work focuses on the radiolabelling of stable complexes of manganese (II) with the goal to develop PET/MRI tracers addressing molecular targets.