An initial investigation of radiolabeling with higher oxidation states of astatine-211. Evaluation of chelation with DOTA and NOTA

Objectives The low in vivo stability of many organic compounds labeled with At-211 prompted investigation of whether compounds that contain higher oxidation states of At are stabilized to in vivo deastatination. The initial objective was to examine chelation of At with macrocyclic chelation reagents DOTA and NOTA.

Methods Reaction conditions were evaluated for oxidation of NaI and Na[I-125]I with H₂O₂, Na₂S₂O₈ and NaIO₄ to be used as HPLC reference standards. HPLC conditions were developed to separate NaI, NaIO₃, and NaIO₄ for identification of oxidized forms of At-211. At(-) was oxidized to form AtO₃(-) and AtO₄(-). Studies of the chelation of I-125 and At-211 with DOTA and NOTA were conducted under oxidizing conditions at various temperatures. A microwave unit was used at higher temperatures.

Results HPLC conditions were developed for separation of Na[I-125]I [t_R=4.0 min], Na[I-125]IO₃ [3.0 min], NaIO₄ [6.1 min], using a JMC J’Sphere ODS M80 column eluting with 1:1 mixture of 50 mM aqueous (Bu₄N)H₂PO₄ and CH₃CN. Using this HPLC method, At(-) [t_R = 5.5 min] was separated from AtO₃(-) [4.8 min] and AtO₄(-) [9.0 min]. None of the oxidizing conditions studied provided HPLC evidence of chelation I-125 with DOTA or NOTA. Contrary to this, reaction of At(-) with NaIO₄ / 0.1N H₂SO₄ at 120°C for 10 min (microwave) resulted in new HPLC peaks with DOTA [11.2 min] or NOTA [15.1 min]. Examination of the reaction mixtures by HPLC over 24 hours indicated that the compounds formed were stable under the reaction conditions. However, attempts to isolate the labeled compounds by evaporation and dissolution in PBS resulted in formation of At(-) and AtO₃(-).

Conclusions HPLC conditions have been developed which provide a method of determining the oxidation state of radioiodine and At-211. The initial studies have provided evidence that At-211 may be chelated with DOTA and NOTA, but indicate that the complexes are not very stable.

Research Support Financial support for these studies was provided from DOE (DE-FG02-08ER64666)