title: DNA double strand breaks as predictor of efficacy of the alpha-particle emitter Ac-225 and the electron emitter Lu-177 for somatostatin receptor targeted radiotherapy.
abstract: BACKGROUND
  Key biologic effects of the alpha-particle emitter Actinium-225 in comparison to the beta-particle emitter Lutetium-177 labeled somatostatin-analogue DOTATOC in vitro and in vivo were studied to evaluate the significance of γH2AX-foci formation.
  
  
  METHODS
  To determine the relative biological effectiveness (RBE) between the two isotopes (as - biological consequence of different ionisation-densities along a particle-track), somatostatin expressing AR42J cells were incubated with Ac-225-DOTATOC and Lu-177-DOTATOC up to 48 h and viability was analyzed using the MTT assay. DNA double strand breaks (DSB) were quantified by immunofluorescence staining of γH2AX-foci. Cell cycle was analyzed by flow cytometry. In vivo uptake of both radiolabeled somatostatin-analogues into subcutaneously growing AR42J tumors and the number of cells displaying γH2AX-foci were measured. Therapeutic efficacy was assayed by monitoring tumor growth after treatment with activities estimated from in vitro cytotoxicity.
  
  
  RESULTS
  Ac-225-DOTATOC resulted in ED50 values of 14 kBq/ml after 48 h, whereas Lu-177-DOTATOC displayed ED50 values of 10 MBq/ml. The number of DSB grew with increasing concentration of Ac-225-DOTATOC and similarly with Lu-177-DOTATOC when applying a factor of 700-fold higher activity compared to Ac-225. Already 24 h after incubation with 2.5-10 kBq/ml, Ac-225-DOTATOC cell-cycle studies showed up to a 60% increase in the percentage of tumor cells in G2/M phase. After 72 h an apoptotic subG1 peak was also detectable. Tumor uptake for both radio peptides at 48 h was identical (7.5%ID/g), though the overall number of cells with γH2AX-foci was higher in tumors treated with 48 kBq Ac-225-DOTATOC compared to tumors treated with 30 MBq Lu-177-DOTATOC (35% vs. 21%). Tumors with a volume of 0.34 ml reached delayed exponential tumor growth after 25 days (44 kBq Ac-225-DOTATOC) and after 21 days (34 MBq Lu-177-DOTATOC).
  
  
  CONCLUSIONS
  γH2AX-foci formation, triggered by beta- and alpha-irradiation, is an early key parameter in predicting response to internal radiotherapy.
authors: University Medical Centre, Department of Nuclear Medicine, Mainz, Germany.
authors: Franziska
authors: F
authors: Graf
authors: University Medical Centre, Institute of Toxicology, Mainz, Germany.
authors: Jörg
authors: J
authors: Fahrer
authors: University Medical Centre, Department of Nuclear Medicine, Mainz, Germany.
authors: Stephan
authors: S
authors: Maus
authors: European Commission, Joint Research Centre - Institute for Transuranium Elements, Karlsruhe, Germany.
authors: Alfred
authors: A
authors: Morgenstern
authors: European Commission, Joint Research Centre - Institute for Transuranium Elements, Karlsruhe, Germany.
authors: Frank
authors: F
authors: Bruchertseifer
authors: University Medical Centre, Department of Endocrinology, Mainz, Germany.
authors: Senthil
authors: S
authors: Venkatachalam
authors: University Medical Centre, Department of Endocrinology, Mainz, Germany.
authors: Christian
authors: C
authors: Fottner
authors: University Medical Centre, Department of Endocrinology, Mainz, Germany.
authors: Matthias M
authors: MM
authors: Weber
authors: University Medical Centre, Institute of Toxicology, Mainz, Germany.
authors: Johannes
authors: J
authors: Huelsenbeck
authors: University Medical Centre, Department of Nuclear Medicine, Mainz, Germany.
authors: Mathias
authors: M
authors: Schreckenberger
authors: University Medical Centre, Institute of Toxicology, Mainz, Germany.
authors: Bernd
authors: B
authors: Kaina
authors: University Medical Centre, Department of Nuclear Medicine, Mainz, Germany.
authors: Matthias
authors: M
authors: Miederer
mesh_terms: D000186:Actinium / Q000627:therapeutic use*; D000512:Alpha Particles / Q000627:therapeutic use*; D000818:Animals; D002453:Cell Cycle / Q000528:radiation effects; D016923:Cell Death / Q000528:radiation effects; D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded; D008187:Lutetium / Q000627:therapeutic use*; D018358:Neuroendocrine Tumors / Q000473:pathology / Q000532:radiotherapy*; D011868:Radioisotopes / Q000627:therapeutic use; D051381:Rats; D017481:Receptors, Somatostatin / Q000235:genetics / Q000378:metabolism*
chemical_list: D011868:Radioisotopes; D017481:Receptors, Somatostatin; D008187:Lutetium; D000186:Actinium