Correlation between binding characteristics and functional antagonism in human glioma cells by tachykinin NK1 receptor antagonists

Binding characteristics and functional antagonism exerted by two structural ly related tachykinin NK1 receptor antagonists, MEN 11467 ((1 R,2S)-2N[1(H) indol-3-yl-carbonyl]-1-N-{N-alpha(p-tolylacetyl)-N-alpha(methyl)-D-3-(2-nap hthyl)alanyl}diaminocyclohexane and FK888 (N-2-[(4 R)-3-hydroxy-1-(1-methyl -1H-indol-3-yl)carbonyl-L-prolyl]-N-methyl-N-phenylmethyl-L-3-(2-naphthyl)a laninamide), were investigated in the human astrocytoma cell line U373 MG. In radioligand binding studies, conducted with [H-3]substance P and intact cells at 37 degrees C, MEN 11467 bound to tachykinin NKi receptors in an ir reversible manner with a K-i value of 1.2 +/- 0.5 nM while FK888 bound in c ompetitive manner with a K-i value of 4.6 +/- 7.2 nM. Receptor blockade by both antagonists resulted in a powerful and complete inhibition of function al responses induced by substance P, such as accumulation of the second mes senger inositol monophosphate or interleukin-6 release. However, MEN 11467 showed a greater potency for blocking functional responses than FK888 despi te their similar affinity for human tachykinin NKi receptors. Moreover, MEN 11467 was more potent in inhibiting late rather than early phases of subst ance P-induced inositol monophosphate accumulation and its antagonism was e nhanced by drug preincubation and barely affected by removal of unbound dru g from the external medium, suggesting that MEN 11467 bound in a tight mann er to the receptor. Such behaviour was not observed with the competitive an d rapidly reversible antagonist FK888. These data indicate that the small d ifferences in the chemical structure of MEN 11467 and FK888 determine the d ifferent binding characteristics at the tachykinin NK, receptor and which a re responsible for the greater potency of MEN 11467 for blocking functional responses. The K-i value obtained in binding studies may be inadequate to reveal the real potency of tachykinin NKi receptor antagonists. (C) 1999 El sevier Science B.V. All rights reserved.