Derivatives of cyclosporin compatible with antibody-based assays: I. The generation of [125I]-labeled cyclosporin.

The immunosuppressive drug cyclosporin A, has been successfully iodinated to a specific activity of 300 Ci per gram. 125I-labeled cyclosporin and [3H]cyclosporin are nearly equivalent as tracers in a radioimmunoassay in producing standard lines (suppression by unlabeled cyclosporin) and in assigning values to clinical samples. In addition, the [125I]-labeled cyclosporin has greater than twice the sensitivity, and it is stable to long-term storage. Use of a [125I]-labeled cyclosporin tracer is more convenient, more reproducible, more precise, and easier than the tritiated-cyclosporin alternative in radioimmunoassay of this compound.