Monoclonal antibodies labeled with rhenium-186 using the MAG3 chelate: Relationship between the number of chelated groups and biodistribution characteristics

Our previous studies on the preparation of ¹⁸⁶Re-MAb conjugates for clinical radioimmunotherapy (RIT) were extended with the aim to derive conjugates which have a high Re:MAb molar ratio, are stable in vitro and in vivo, have favorable biodistribution characteristics and can be used together with ^99mTc-MAb conjugates as a matched pair in combined radioimmunoscintigraphy/RIT studies. Methods: Rhenium and ^99mTc- conjugates of intact MAb E48 were prepared according to our previously described multistep procedure using the MAG3 chelate and analyzed by protein mass spectrometry for the number of chelate molecules coupled to the MAb. For biodistribution analysis, tumor-free nude mice were simultaneously injected with ¹⁸⁶Re-, ^99mTc/⁹⁹Tc- and/or ¹²⁵I-labeled E48 IgG and dissected 1-48 hr postinjection. Results: Rhenium-186-MAb conjugates with up to 20 Re- MAG3 groups par MAb molecule ware prepared 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 ¹⁸⁶Re-MAG3:MAb molar ratio higher than 12 demonstrated slightly impaired immunoreactivity to a maximum of 15% decrease at the 20:1 molar ratio. Biodistribution experiments revealed that a proportion 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 intestines. The ^99mTc/⁹⁹Tc-MAb conjugates showed a similar enhanced blood clearance when containing more than eight Tc-MAG3 groups, while dual labeling of MAbs revealed that the in vive stability of the conjugated Re-MAG3 complex itself does not differ from the corresponding Tc-MAG3 complex. Conclusion: With the method described in this study, it is possible to prepare ¹⁸⁶Re-MAG3-MAb conjugates that fulfil all the aforementioned criteria for use in clinical RIT. Coupling of too many metal-MAG3 groups to MAbs results in rapid blood clearance. At the same metal-MAG3:MAb molar ratio, ^99mTc/⁹⁹Tc-MAb conjugates show a similar pharmacokinetic behavior as ¹⁸⁶Re-MAb conjugates and can thus be used to predict the localization of ¹⁸⁶Re-labeled MAbs and make dosimetric predictions in individual patients.