MONOCLONAL-ANTIBODIES LABELED WITH RE-186 USING THE MAG3 CHELATE - RELATIONSHIP BETWEEN THE NUMBER OF CHELATED GROUPS AND BIODISTRIBUTION CHARACTERISTICS

Our previous studies on the preparation of Re-186-MAb conjugates for c linical radioimmunotherapy (RIT) were extended with the aim to derive conjugates which have a high Re:MAb molar ratio, are stable in vitro a nd in vivo, have favorable biodistribution characteristics and can be used together with Tc-99m-MAb conjugates as a matched pair in combined radioimmunoscintigraphy/RIT studies. Methods: Rhenium and Tc-99m-conj ugates of intact MAb E48 were prepared according to our previously des cribed multistep procedure using the MAG3 chelate and analyzed by prot ein mass spectrometry for the number of chelate molecules coupled to t he MAb. For biodistribution analysis, tumor-free nude mice were simult aneously injected with Re-186-, (TC)-T-99m/(TC)-T-99- and/or (125)l-la beled E48 IgG and dissected 1-48 hr postinjection. Results: Rhenium-18 6-MAb conjugates with up to 20 Re-MAG3 groups per MAb molecule were pr epared with an overall radiochemical yield of 40%-60%. The conjugates did not contain empty MAG3 groups and no aggregates were formed. Only conjugates with a Re-186-MAG3:MAb molar ratio higher than 12 demonstra ted slightly impaired immunoreactivity to a maximum of 15% decrease at the 20:1 molar ratio. Biodistribution experiments revealed that a pro portion of the conjugate became rapidly eliminated from the blood for conjugates with a Re-MAG3:MAb molar ratio higher than 8. In this case, an increased uptake of activity was observed in the liver and intesti nes. The Tc-99m/Tc-99-MAb conjugates showed a similar enhanced blood c learance when containing more than eight Tc-MAG3 groups, while dual la beling of MAbs revealed that the in vivo stability of the conjugated R e-MAG3 complex itself does not differ from the corresponding Tc-MAG3 c omplex. Conclusion: With the method described in this study, it is pos sible to prepare Re-186-MAG3-MAb conjugates that fulfil all the aforem entioned criteria for use in clinical RIT. Coupling of too many metal- MAG3 groups to MAbs results in rapid blood clearance. At the same meta l-MAG3:MAb molar ratio, Tc-99m/Tc-99-MAb conjugates show a similar pha rmacokinetic behavior as Re-186-MAb conjugates and can thus be used to predict the localization of Re-186-labeled MAbs and make dosimetric p redictions in individual patients.