Organ Distribution and Biological Activity of 125-I-Labelled LH-RH Analogues

Abstract

The high biological activity of certain LH-RH analogues may be due to increased receptor binding and/or reduced enzyme stability. The organ distribution of 125-I-labelled analogues was studied to estimate the in vivo degree of receptor binding and the inactivating tissues. The following peptides were labelled with 125-I by the chloramine-T method: LH-RH(1–9) nonapeptide-ethylamide, [D-Glu⁶] LH-RH(1–9) nonapeptide-ethylamide, [D-Ser(Bu^t)⁶] LH-RH(1–9) nonapeptide-ethylamide and the corresponding (3–9) heptapeptide fragment. Biological activity was tested in ovariectomized, steroid-blocked rats by LH-release. Organ distribution was determined 15, 30 and 60 minutes after iv-injection of labelled peptides. After iodination, biological activity was retained to 30–50%. The order of potency of the labelled analogues was LH-RH: LH-RH(1–9) EA:[D-Glu⁶]LH-RH(1–9)EA:[D-Ser (Bu^t)⁶]EA = 1:2:20:40. Pituitary accumulation of the weak analogue LH-RH(1–9) EA was only found after 15 minutes (tissue/plasma ratio 1.23), [D-Glu⁶]LH-RH(1–9) EA reached a T/P ratio of 1.33 after 30 minutes and the highly active [D-Ser(Bu^t)⁶] LH-RH(1–9) EA reached high pituitary accumulation even after 60 minutes (T/P ratio 2.33). The peptides were also concentrated in liver and kidney, the main inactivating organs fop LH-RH. The (3–9) heptapeptide fragment of [D-Ser(Bu^t)⁶] LH-RH(1–9)EA did not show pituitary accumulation, even though it had some residual LH releasing activity.