L. Gazi et al., THE AGONIST ACTIVITIES OF THE PUTATIVE ANTIPSYCHOTIC AGENTS, L-745,870 AND U-101958 IN HEK293 CELLS EXPRESSING THE HUMAN DOPAMINE D-4.4 RECEPTOR, British Journal of Pharmacology, 124(5), 1998, pp. 889-896
1 Dopamine D-4 receptor antagonists are being developed by several pha
rmaceutical companies as putative novel antipsychotics, possibly with
low propensity to side-effects. Two such compounds, L-745,870 and U-10
1958 have been recently introduced. 2 The radioligand binding and func
tional activities of L-745,870 and U-101958 were investigated in human
embryonic kidney (HEK)293 cells expressing the human recombinant dopa
mine D-4.4 receptor (HEK293/D-4 cells). [H-3]-spiperone binding experi
ments were performed and inhibition of forskolin-stimulated cyclic AMP
accumulation was used as the functional response. 3 [H-3]-spiperone w
as found to label a homogeneous and saturable population of specific b
inding sites in HEK293/D-4 cell homogenates (B-max 505 +/- 90 fmol mg(
-1) protein, pK(D) 9.5 +/- 0.1, n=3). Inhibition of specific [H-3]-spi
perone binding was observed with spiperone (pK(i) 9.6 +/- 0.1, n=3), c
lozapine (pK(i) 7.4 +/- 0.1, n=4), L-745,870 (pK(i) 8.5 +/- 0.1, n=3)
and U-101958 (pK(i) 8.9 +/- 0.1, n=3). By contrast, raclopride was ver
y weak (pK(i) < 5, n = 3). 4 Dopamine inhibited forskolin-stimulated c
yclic AMP accumulation in HEK293/D-4 cells in a concentration-dependen
t fashion (E-max 71 +/- 2% inhibition of forskolin-stimulated levels,
pEC(50) 8.7 +/- 0.1, n=10). This effect was mimicked by the dopamine D
-2-like receptor agonists, quinpirole and 7-hydroxy-2-dipropylaminotet
ralin (7-OH-DPAT). 5 L-745,870 and U-101958 also inhibited forskolin-s
timulated cyclic AMP accumulation in HEK293/D-4 cells in a concentrati
on-dependent way. L-745,870 was less efficacious than dopamine (71% th
e efficacy of dopamine), whereas U-101958 behaved as a full agonist co
mpared to dopamine. Potencies (pEC(50)) values of L-745,870 and U-1019
58 were 9.0 +/- 0.2 (n = 4) and 8.7 +/- 0.3 (n = 3), consistent with p
K(i) values determined in radioligand binding studies. 6 Dopamine, L-7
45,870 and U-101958 (up to 1 mu M) were devoid of effect on forskolin-
stimulated cyclic AMP accumulation in control, non-transfected HEK293
cells. 7 The agonist effects of dopamine, L-745,870 and U-101958 in HE
K293/D-4 cells could be antagonized by spiperone (pK(B) 8.2-8.8) and c
lozapine (pKB 7.1), but not by raclopride (pK(B)<5). None of these ant
agonists had any significant agonist activity at concentrations up to
10 mu M. 8 These results show that the putative dopamine D-4 receptor
antagonists, L-745,870 and U-101958 are not devoid of intrinsic activi
ty at human recombinant dopamine D-4.4 receptors. Therefore, they may
not represent the most appropriate drugs for testing the benefit of D-
4 receptor antagonism in schizophrenic patients, if agonism should tra
nslate in vivo.