Distribution and pharmacokinetics of a potent peptide antagonist of parathyroid hormone and parathyroid hormone-related protein in the rat

Humoral hypercalcemia of malignancy results from the production by cancer cells of parathyroid hormone related protein that activates receptors in bone. Peptide antagonists that block parathyroid hormone receptors in vivo would be instrumental in the clinical treatment of humoral hypercalcemia of malignancy. We report the in vive whole body distribution and blood plasma pharmacokinetics of the parathyroid hormone receptor antagonist [Nle^8,18,D-Trp¹²,monoiodinated Tyr³⁴]bPTH(7-34)amide to determine parameters that are likely to affect its administration regimen. A single intravenously injected dose of [Nle^8,18,D-Trp¹²,monoiodinated Tyr³⁴]bPTH(7-34)amide was rapidly cleared from blood plasma. The plasma concentration reaches a maximum at 10 min (C(max) = 1.93 ± 0.27% of total injected CPM/ml), and the intact PTH derivative was detectable in plasma by HPLC analysis at this time. In vivo binding to plasma proteins was noncovalent. The peptide was rapidly cleared from blood by the liver, and more slowly by the kidney. Radiolabel was detected in excreted feces at 8 hr, but the preferred route of excretion was renal as judged by significant counts in excreted urine. Absorption of labeled peptide by skin and bone was sustained. Strong and sustained absorption also occurred in the vas deferens, seminal vesicle and hypothalamus. Given the rapid clearance of antagonist, multiple or sustained dosing schemes might be necessary to achieve the desired pharmacological effect. The high counts in liver at early time points after i.v. injection suggest that other routes of administration that do not bypass the hepatic first-pass effect would result in very low blood levels of drug.