ACTH Receptor Affinity

PURPOSE AND RATIONALE For all new compounds which have some structural similarity or relation to the corticoid scaffold. There are now many such test systems which measure the initial membrane binding to the ACTH receptor, and to define the initial steps of hormone-related activation. However the binding affinity does not necessarily imply biological activation e.g. of adrenal cortical cells. Therefore, such tests always need to be followed up by a in-vitro testing on adrenal glands or dispersed adrenal cells. 

Corticotropin (ACTH) receptors have been used as the test preparations for comparison of the binding affinities of adrenocorticotropin peptides using a cloned mouse adrenocorticotropin receptor expressed in a stably transfected HeLa cell line (Kapas et al. 1996). Such assays measure the initial membrane binding but not strictly the biological activation e.g. of adrenal cells or melanocytes. 

Binding parameters are calculated from computer programs, one example being the ligand program of 

Munson and Rodbard. Estimates of binding affinity are then compared with the binding affinity of natural corticotropin, corticotropin analogues, and more importantly with the drug concentrations of the compound to be evaluated reached in the toxicology studies, or similar pharmacokinetic studies (level of exposure). 

There are many different modifications of the method, early technology used membrane preparations, modem methods use solubilized receptors and receptor of constructs which enable fast signal detection. 

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The primary effect of ACTH seems to be mediated via cAMP production by interaction of the hormone with differing populations of receptors. The difference between the extent of steroidogenesis and cAMP formation indicates different receptor affinities in different organs. 

Corticotropin activity can be measured by the increase of corticosterone in venous blood of hypophysec-tomized or dexamethasone-blocked rats, as a singledose test when receptor affinity for ACTH receptor is found with a test compound. The test can be used to measure time-response curves of corticotropin preparations, and compounds with a adrenal stimulating activity (Vogel 1969a,b). The use of hypophysec-tomized rats for evaluation of the pituitary response... 

Haksar et al. (1973) treated rat adrenal cells with sialidase and observed a decrease in response to adrenocorticotropic hormone (ACTH) stimulation as measured by corticosterone production. No effect was observed on the cell s response to cyclic AMP or dibutyryl cyclic AMP, indicating that the observed effect of sialidase treatment must be due to impairment of some step prior to cyclic AMP formation. They proposed two possibilities (1) the sialic acid residues on the cell membrane provide affinity for the ACTH, thereby aiding in activation of the receptor, or (2) the sialic acid facilitates the transmission of the signal from the ACTH-receptor interaction to adenylcyclase. Similar studies with other hormones and isolated cell systems might indicate involvement of cell surface sialyl residues in transduction of other hormonal messages. Sialyltransferase and sialidase present in the cell membrane may serve to provide the environment and controls for transduction of the hormonal message. 

It is worth noting here that Hoffman et al. (184) recently found that a phenylalanine analog of ACTH was devoid of enzymatic activity, but exhibited affinity for the ACTH receptor. For these reasons. Coy et al. (90) suggested that the essential feature of tryptophan in maintaining hormonal action was due to its electron-donating properties. 

Endorphins, also known as P-endorphin, are released into the blood (from the pituitary gland) and into the spina] cord and the brain through the hypothalamic neurons. P-endorphin is a cleavage product of proopiomelanocortin which is also the precursor hormone for adrenocorticotrophic hormone (ACTH). In situations where the level of ACTH is increased, the level of endorphins also increases slightly. P-endorphin has the highest affinity for the pi-opioid receptor, slightly lower affinity for the p2- and 8-opioid receptors and low affinity for the k 1-opioid receptors. P-Endorphins are more hormone-like in nature. 

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