A rapid and simple assay to determine the blood and urine concentrations of 1-(5-[123/125I]iodo-5-deoxyarabinofuranosyl)-2-nitroimidazole, a hypoxic cell marker.

Pharmacokinetic and dosimetric parameters of the hypoxic tissue imaging agent iodoazomycin arabinoside (123I-IAZA) have been investigated in human volunteers. In conjunction with this study it was necessary to develop an assay for low levels of the radiolabelled compound in blood and urine. A combination of high-performance liquid chromatography (HPLC) and gamma counting produced a highly selective, sensitive and rapid assay for the analysis of 123/125I-IAZA in human and animal blood and urine samples. Conventional HPLC assays for the tracer quantities of this radioactive agent in blood have not been reported previously. The addition of non-radiolabelled IAZA to the blood and urine samples containing radiolabelled IAZA allowed the pharmaceutical to serve as its own internal standard. This reverse isotope dilution approach permitted identification of the appropriate HPLC peak by UV detection, followed by highly sensitive quantification of the radiolabelled species by gamma counting. Blood samples were prepared for HPLC by a solid-phase extraction without the loss of IAZA from serum, with an extraction efficiency of 99.7 +/- 7.1% from human serum. Urine samples could be analyzed directly by HPLC, without the solid-phase extraction step. The detection limit in biological fluids depends on the specific activity of radiolabelled 123/125I-IAZA. In this study it was possible to detect serum concentrations of 123I-IAZA as low as 7.46 pg (21 fmol) per ml. The radiometric detection limit for 123I-IAZA in this assay was 10.8 Bq ml-1 of serum.