Effects of Radiopharmaceuticals on Radioenzymatic Assays of Aminoglycoside Antibiotics: Interference by Gallium-67 and Its Elimination

Abstract

Radionuclides currently used in clinical medicine were evaluated for their possible interference with radioenzymatic assays of aminoglycoside antibiotics. Of the radiopharmaceuticals tested, only 67 Ga citrate interfered with the radioenzymatic assay of gentamicin. Radioenzymatic assay of serum samples obtained from patients receiving 67 Ga yielded gentamicin concentrations falsely elevated by more than 1 μg/ml for approximately 1 week after 67 Ga administration. A procedure was developed to eliminate 67 Ga interference with radioenzymatic assays of aminoglycoside antibiotics. After 67 Ga citrate was spotted onto phosphocellulose filter disks, the filters were immersed in 7.2 N HCl, and radioactivity was removed by successive extractions of the acid phase with diisopropyl ether. After three extractions, less than 0.1% of the original radioactivity remained. Similar extraction of disks containing 14 C-adenylylated gentamicin or tobramycin or 14 C-acetylated amikacin had no effect on 14 C radioactivity. The concentrations of aminoglycosides in serum standards supplemented with 67 Ga citrate were determined accurately by radioenzymatic assays followed by extraction with diisopropyl ether. Concentrations of gentamicin in six serum samples from patients injected with 67 Ga during gentamicin therapy, as determined by radioenzymatic assay and extraction, were within 9% of the results obtained by reassay of the same samples after the decay of 67 Ga.