Chemical and Physical Characterisation of Macroaggregated Human Serum Albumin: Strength and Specificity of Bonds with 99mTc and 68Ga

Abstract

Background: Macroaggregated human serum albumin (MAA) properties are widely used in nuclear medicine, labelled with 99mTc. The aim of this study is to improve the knowledge about the morphology, size, dimension and physical–chemical characteristics of MAA and their bond with 99mTc and 68Ga. Methods: Commercial kits of MAA (Pulmocis®) were used. Characterisation through experiments based on SEM, DLS and Stokes’ Law were carried out. In vitro experiments for Langmuir isotherms and pH studies on radiolabelling were performed and the stability of the radiometal complex was verified through competition reactions. Results: The study settles the MAA dimension within the range 43–51 μm. The Langmuir isotherm reveals for [99mTc]MAA: Bmax (46.32), h (2.36); for [68Ga]MAA: Bmax (44.54), h (0.893). Dual labelling reveals that MAA does not discriminate different radioisotopes. Experiments on pH placed the optimal pH for labelling with 99mTc at 6. Conclusion: Radiolabelling of MAA is possible with high efficiency. The nondiscriminatory MAA bonds make this drug suitable for radiolabelling with different radioisotopes or for dual labelling. This finding illustrates the need to continue investigating MAA chemical and physical characteristics to allow for secure labelling with different isotopes.

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“…Some authors have assumed that 99m TcO 4 − reduced by the Sn(II)- albumin aggregates probably formed a (Tc = O) 3+ complex with the aggregates [ 15 ]. More recently, high positive cooperativity was shown between 99m Tc and MAA, although MAA particles did not seem to have binding pockets [ 18 , 19 ]. Moreover, it has been shown that the speed of radiolabelling increased from HAS to albumin nanocolloids (NC) to MAA due to the greater reaction surface [ 18 ].…”

“…More recently, high positive cooperativity was shown between 99m Tc and MAA, although MAA particles did not seem to have binding pockets [ 18 , 19 ]. Moreover, it has been shown that the speed of radiolabelling increased from HAS to albumin nanocolloids (NC) to MAA due to the greater reaction surface [ 18 ]. This result agreed with the hypothesis, which assumed that in HAS labelling kits, Sn 2+ may be enclosed in the tertiary structure of the protein and that it may take some time for the 99m TcO 4 − added to diffuse the site of Sn 2+ for reduction reaction [ 17 ].…”

“…This result agreed with the hypothesis, which assumed that in HAS labelling kits, Sn 2+ may be enclosed in the tertiary structure of the protein and that it may take some time for the 99m TcO 4 − added to diffuse the site of Sn 2+ for reduction reaction [ 17 ]. Furthermore, MAA has very complex shapes with larger surfaces than a spherical shape (as for NC) of equivalent diameter, enhancing the reactivity properties [ 18 , 19 ].…”

“…The 99m Tc is obtained from a 99 Mo/ 99m Tc generator as sodium pertechnetate ( 99m TcO 4 − , Na + ). The MAA labelling with 99m Tc, which occurred at pH 6, is a simple and fast (about 15 min) process, which allows the production of GMP [ 99m Tc]Tc-MAA without heating step [ 18 ].…”

“…As Ga is present as Ga(OH) 4 − at a basic pH, 68 Ga does not bind to MAA at a pH above 7. The MAA behavior matches with solvent-exposed glutamate and aspartate amino acids, which should be binding sites for multivalent cations with low affinity and low cation specificity [ 18 ]. Furthermore, Jain et al found that stannous chloride (SnCl 2 ) present in the MAA kits for the reduction of 99m Tc had a strong influence on [ 68 Ga]Ga-MAA formation (radiochemical yield, mean particle diameter, serum stability), suggesting that Sn could be linked to MAA or 68 Ga [ 29 ].…”