Pharmacokinetics of synthetic human parathyroid hormone 1-34 in man measured by cytochemical bioassay and radioimmunoassay.

Synthetic human parathyroid hormone (hPTH) 1-34 was given by intravenous injection to two healthy men. The time course of its appearance in and disappearance from the plasma was monitored both by cytochemical bioassay and by a specific radioimmunoassay (RIA) system. Immunoreactive N-region parathyroid hormone (iPTH) reached peak concentrations in plasma at 2 min after injection, whereas peak concentrations of biologically active parathyroid hormone (bioPTH) were delayed until 4-6 min. Bioassayable PTH-like activity then disappeared from the plasma (mean transit times 5.8 and 8.6 min), approximately twice as fast as immuno-reactivity. After separate subcutaneous administrations, a calculated 22-37% of administered hPTH 1-34 was subsequently detected in the plasma, by both assay systems. It was not possible to explain fully the non-parallel appearances of bio- and immuno-reactivities in the plasma after intravenous injection nor the non-parallel disappearances after both intravenous and subcutaneous injections on the basis of the present data. It seems likely, however, that in the process of biological degradation the immuno-reactive locus is inactivated by a different reaction from that which destroys bioactivity. To investigate these activity dissociations further will require the application of micro-fractionation procedures in conjunction with both types of assay system.