RESUMEN
Vacuum ultraviolet photoionization of a gas-phase oligonucleotide anion leads to the formation of a valence hole. This hole migrates towards an energetically favorable site where it can weaken bonds and ultimately lead to bond cleavage. We have studied Vacuum UV photoionization of deprotonated oligonucleotides containing the human telomere sequence dTTAGGG and G-quadruplex structures consisting of four dTGGGGT single strands, stabilized by NH4 + counter ions. The oligonucleotide and G-quadruplex anions were confined in a radiofrequency ion trap, interfaced with a synchrotron beamline and the photofragmentation was studied using time-of-flight mass spectrometry. Oligonucleotide 12-mers containing the 5'-TTAGGG sequence were found to predominantly break in the GGG region, whereas no selective bond cleavage region was observed for the reversed 5'-GGGATT sequence. For G-quadruplex structures, fragmentation was quenched and mostly non-dissociative single and double electron removal was observed.
RESUMEN
UNLABELLED: Dosimetry in peptide receptor radionuclide therapy using (177)Lu-DOTATATE is based on patient imaging during the first week after administration and determination of the activity retention as a function of time for different tissues. For calculation of the absorbed dose, it is generally assumed that the long-term activity retention follows the pattern determined from the first week. This work aimed to investigate the validity of this assumption by performing additional patient measurements between 5 and 10 wk after administration. A further aim was to investigate to what extent absorbed dose values were affected when including these measurements, also taking into account the radionuclide impurity of (177m)Lu and build-up of secondary (177)Lu from the (177m)Lu decay. METHODS: A combination of methods was used: planar γ-camera imaging as part of the clinical dosimetry protocol, determination of the whole-body activity between 5 and 9 wk after injection using spectrometric NaI(Tl) and HPGe detectors, and imaging between 5 and 10 wk after injection for assessment of the activity distribution. From these measurements the long-term retention of activity was determined and the relative influence on absorbed doses calculated. RESULTS: The most important finding was a clearly visualized tumor uptake in images from between 5 and 7 wk after injection and in 1 patient also kidney and spleen uptake in images acquired on day 33. As a consequence, the total-body time-activity curve had a tail, which was not completely captured by imaging during the first week. The absorbed doses to total body and tumors obtained when including these late time points were on average 5%-6% higher than those obtained when using data acquired during the first week. The contributions to the absorbed dose from (177m)Lu and secondary (177)Lu were negligible. CONCLUSION: At approximately 5-7 wk after injection, there was a measureable amount of (177)Lu-DOTATATE in patients, which is mainly governed by retention in tumors. For tumor dosimetry, imaging at a later time than the routinely used 7 d may be warranted. The contribution to the absorbed dose from the radionuclide impurity of (177m)Lu was negligible.