Preferential Formation and Stability of Copper Porphyrins amidst Other Metals in Dilute Aqueous Solution─Applications for Radiopharmaceuticals

Two worlds exist for cupration of a ligand: the preparative lab where natural-abundance copper (in pure form) is typically used in excess versus the ligand, and the radiochemistry lab where the ligand is in great excess versus the copper radionuclide (unavoidably accompanied by contaminating metal ions). Here, seven synthetic free base water-soluble porphyrins have been examined to assess molecular designs suitable for targeted radiotheranostics. Studies were carried out with equimolar quantities of a porphyrin and natural abundance (nonradioactive) CuCl₂ (10^-4-10^-7 M) and other metals in aqueous HEPES buffer (1 M) at room temperature (detection by absorption and fluorescence spectroscopy). Studies also were carried out with ⁶⁴CuCl₂ and excess porphyrin at 37 °C. Key results are as follows: (1) competition experiments with CoCl₂, NiCl₂, CuCl₂, ZnCl₂, FeCl₂, and FeCl₃ gave selective cupration. (2) The copper chelates of the porphyrins were stable in the aqueous solution for at least 24 h. (3) Copper porphyrins were obtained in apparent molar activities (AMAs) ∼6000 mCi/μmol (222 GBq/μmol) even in the presence of other metals. The results augur well for use of copper chelates of designer porphyrins in radiomedicine.