Improved Efficacy of Synthesizing *MIII-Labeled DOTA Complexes in Binary Mixtures of Water and Organic Solvents. A Combined Radio- and Physicochemical Study.

Typically, the synthesis of radiometal-based radiopharmaceuticals is performed in buffered aqueous solutions. We found that the presence of organic solvents like ethanol increased the radiolabeling yields of [68Ga]Ga-DOTA (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacatic acid). In the present study, the effect of organic cosolvents [ethanol (EtOH), isopropyl alcohol, and acetonitrile] on the radiolabeling yields of the macrocyclic chelator DOTA with several trivalent radiometals (gallium-68, scandium-44, and lutetium-177) was systematically investigated. Various binary water (H2O)/organic solvent mixtures allowed the radiolabeling of DOTA at a significantly lower temperature than 95 °C, which is relevant for the labeling of sensitive biological molecules. Simultaneously, much lower amounts of the chelators were required. This strategy may have a fundamental impact on the formulation of trivalent radiometal-based radiopharmaceuticals. The equilibrium properties and formation kinetics of [M(DOTA)]- (MIII= GaIII, CeIII, EuIII, YIII, and LuIII) complexes were investigated in H2O/EtOH mixtures (up to 70 vol % EtOH). The protonation constants of DOTA were determined by pH potentiometry in H2O/EtOH mixtures (0-70 vol % EtOH, 0.15 M NaCl, 25 °C). The log K1H and log K2H values associated with protonation of the ring N atoms decreased with an increase of the EtOH content. The formation rates of [M(DOTA)]- complexes increase with an increase of the pH and [EtOH]. Complexation occurs through rapid formation of the diprotonated [M(H2DOTA)]+ intermediates, which are in equilibrium with the kinetically active monoprotonated [M(HDOTA)] intermediates. The rate-controlling step is deprotonation (and rearrangement) of the monoprotonated intermediate, which occurs through H2O (*M(HL) kH2O) and OH- (*M(HL) kOH) assisted reaction pathways. The rate constants are essentially independent of the EtOH concentration, but the M(HL) kH2O values increase from CeIII to LuIII. However, the log KM(HL)H protonation constants, analogous to the log KH2 value, decrease with increasing [EtOH], which increases the concentration of the monoprotonated M(HDOTA) intermediate and accelerates formation of the final complexes. The overall rates of complex formation calculated by the obtained rate constants at different EtOH concentrations show a trend similar to that of the complexation rates determined with the use of radioactive isotopes.

Improved radiolabeling of DOTATOC with trivalent radiometals for clinical application by addition of ethanol.

BACKGROUND: Typically, metal-based radiopharmaceuticals are synthesized in aqueous solutions with no or low ethanol content. Labeling yields are defined by temperature, period of labeling, amount of precursor, pH etc. As recently observed, radiolabeling yields (RCY) seem to increase in the presence of non-aqueous solvents. Consequently, this effect was investigated systematically using ethanol as non-aqueous solvent (n-as), which is widely utilized in medicine, and DOTATOC as model compound. METHODS: To determine the impact of ethanol on the radiolabeling efficacy, "standard" labeling conditions of 68Ga-DOTATOC (95-100 °C, 10-15 min, 20-50 µg DOTATOC, aqueous solution), i.e. 10 nmol (2.9 µM, 14.2 µg), were modified in terms of lower temperature (70 °C) to achieve lower RCY (<75 %). From those lower RCY, positive effects of increasing amounts of ethanol (0-40 vol%) could directly be observed. Labeling parameters were finally evaluated in terms of shorter reaction time and lower amount of precursor. To investigate whether the effects observed are also true for other trivalent radiometals, labeling was also performed with 44Sc. RESULTS: For increasing amounts of ethanol, 68Ga-DOTATOC RCY at 70 °C improved significantly. RCY of ~95 % can be achieved within 10 min using 30 vol% ethanol compared to 46 % in the pure aqueous system. If "standard" temperatures of 95 °C are applied, high RCY of 89 % can be achieved within 5 min with much lower amounts of precursor, i.e. even at 0.93 nmol (0.3 µM, 1.3 µg). This also represents significantly increased specific activities. Similar behavior was observed for 44Sc where RCY increase successively with increasing amounts of ethanol. CONCLUSION: There is clear experimental evidence, that adding more than 20 vol% ethanol to the reaction mixtures significantly improve labeling efficacies. This could be demonstrated for 68Ga-DOTATOC and 44Sc-DOTATOC in terms of temperature, time and concentration of required precursor. Whether this is a principal phenomenon with practical impact on the radiopharmaceutical chemistry of trivalent metals and whether this applies to other non-aqueous solvents as well - and what the physico-chemical reasons are, remains to be studied in more detail. Nevertheless, the effect observed here will improve 68Ga-DOTATOC labeling and may save at least half of the usually applied amount of precursor.

Production of large quantities of 90Y by ion-exchange chromatography using an organic resin and a chelating agent.

The performance of a system composed of an organic cation exchanger (Dowex 50Wx8) and a chelating agent (EDTA) previously described for the successful production of (90)Y via a (90)Sr/(90)Y generator is assessed under dynamic conditions. In an attempt to overcome the established limitation of ion-exchange resins for the separation of subcurie quantities of activity, (90)Y is repeatedly isolated from an 11.8-GBq (320 mCi) (90)Sr cow using a three-column tandem arrangement. The high recovery and radionuclidic purity obtained for (90)Y and the parameters of the separation (time, eluant concentration, pH and flow rate range) strongly suggest that Ci quantities of (90)Y can be handled satisfactorily by the ion-exchange method. No replacement or treatment of the cow, low waste generation and (90)Sr losses less than 0.1% after each run were observed during the present study which, in combination with the low cost of this resin, may result in an attractive alternate method for the production of large quantities of (90)Y.