Agonist and antagonist interactions with D1 dopamine receptors: agonist-induced masking of D1 receptors depends on intrinsic activity

Pan RAS-binding compounds selected from a chemical library by inhibiting interaction between RAS and a reduced affinity intracellular antibody

Intracellular antibodies are valuable tools for target validation studies for clinical situations such as cancer. Recently we have shown that antibodies can be used for drug discovery in screening for chemical compounds surrogates by showing that compounds could be developed to the so-called undruggable RAS protein family. This method, called Antibody-derived compound (Abd) technology, employed intracellular antibodies binding to RAS in a competitive surface plasmon resonance chemical library screen. Success with this method requires a high affinity interaction between the antibody and the target. We now show that reduction in the affinity (dematuration) of the anti-active RAS antibody facilitates the screening of a chemical library using an in vitro AlphaScreen method. This identified active RAS specific-binding Abd compounds that inhibit the RAS-antibody interaction. One compound is shown to be a pan-RAS binder to KRAS, HRAS and NRAS-GTP proteins with a Kd of average 37 mM, offering the possibility of a new chemical series that interacts with RAS in the switch region where the intracellular antibody binds. This simple approach shows the druggability of RAS and is generally applicable to antibody-derived chemical library screening by affording flexibility through simple antibody affinity variation. This approach can be applied to find Abd compounds as surrogates of antibody-combining sites for novel drug development in a range of human diseases.

Long-lasting effects of chronic mu-opioid intake on the signal transmission via dopamine D1 receptors in the limbic forebrain of drug deprived rats

Rats orally self-administered the potent and selective mu-opioid receptor agonist etonitazene for 8 weeks (free choice between three opioid solutions and water resulting in low drug intake, or forced intake of a single drug solution resulting in high opioid consumption). The signal transmission in membranes of the limbic forebrain (nucleus accumbens and olfactory tubercle) was studied during acute withdrawal (2 days of abstinence) and after 6 weeks of drug deprivation. Binding experiments with the dopamine (DA) D1 receptor antagonist [3H]SCH23390 revealed in the high consuming rats an increased binding density (Bmax) by 19% during withdrawal and a decreased Bmax by 17% after long-term abstinence compared with drug-naive controls (each P < 0.05). The addition of 500 nM DA reduced the [3H]SCH23390 binding affinity (Kd increased by 60-105%) and density (by 15-23%) in each of the five groups (P < 0.001). During acute withdrawal, the portion of Bmax inhibited by DA increased by 83% in the high consuming rats vs. the controls (P < 0.05). Full concentration-response curves of adenylyl cyclase (AC) stimulation by the DA D1 receptor agonist dihydrexidine and of inhibition of forskolin stimulated AC activity by the GTP analogue guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) were performed: the former revealed a reduced maximum efficacy (Emax decreased by 23-37%), P < 0.001), the latter a reduced effective concentration (EC50 decreased by 60-103%, P < 0.05), in each etonitazene-experienced group vs. the controls.

(+/-)-3-[4'-(N,N-dimethylamino)cinnamyl]benzazepine analogs: novel dopamine D1 receptor antagonists

Neurochemical studies and structure-activity relationships of dopamine D1 receptor ligands suggest that their intrinsic activity may depend on the conformational state or binding site at which they interact on the receptor protein. Important differences in the modes of binding of these ligands may confer their agonist, partial agonist, or antagonist properties. In an effort to develop novel dopamine D1 antagonists and investigate the D1 antagonist pharmacophore, a series of (+/-)-(N-alkylamino)benzazepines were prepared in which (+/-)-7-chloro-8-hydroxy-3-[6-(N,N-dimethylamino)hexyl]-1-phenyl-2,3,4,5- tetrahydro-1H-3-benzazepine (1) demonstrated the highest binding affinity (Ki = 49.3 nM) and selectivity to dopamine D1 receptors. This compound inhibited dopamine-stimulated adenylyl cyclase, in rat caudate, confirming a D1 receptor antagonist profile. From this initial series of N-alkylamino-substituted benzazepines, structure-activity relationships suggested that the terminal amino function was necessary for optimal binding affinity and selectivity at D1 vs D2 sites. Further, addition of this side chain to the D1 agonist pharmacophore (e.g., 7,8-dihydroxy-3-[4-(N,N-dimethylamino)butyl]-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine) greatly decreased binding affinity at D1 receptors. These data suggested that a binding domain that may be unique to the D1 antagonists may have been identified. In an attempt to exploit an apparent amine-accepting binding domain on the D1 receptor, a series of (+/-)-3-[4'-(N,N-dimethylamino)cinnamyl]benzazepine analogs was designed and prepared, as D1 antagonists. In this series, (+/-)-7-chloro-8-hydroxy-3-[4'-(N,N-dimethylamino)cinnamyl]-1-phenyl-2,3,4,5 -tetrahydro-1H-3-benzazepine (6a) showed the highest binding affinity (Ki = 60.3 nM) for dopamine D1 receptors. Compound 6a was a potent dopamine D1 antagonist as evidenced by its ability to block dopamine-stimulated adenylyl cyclase activity in rat caudate (predicted Ki value = 18.4 nM). Molecular modeling studies demonstrated that the most potent and selective dopamine D1 antagonists, in both series, contained terminal amino groups 8-9 A away from the 3-position benzazepine nitrogen. Compounds that lacked a terminal amine function or where this moiety was less than 7 A away from the benzazepine nitrogen demonstrated significantly lower binding affinities. Therefore, this series of (+/-)-3-[4'-(N,N-dimethylamino)cinnamyl]benzazepines also appears to be identifying an amine-accepting binding domain on the dopamine D1 receptor protein that may be further explored for the development of novel dopamine D1 antagonists.

Nefiracetam (DM-9384) reverses apomorphine-induced amnesia of a passive avoidance response: delayed emergence of the memory retention effects

Nefiracetam is a novel pyrrolidone derivative which attenuates scopolamine-induced learning and post-training consolidation deficits. Given that apomorphine inhibits passive avoidance retention when given during training or in a defined 10-12h post-training period, we evaluated the ability of nefiracetam to attenuate amnesia induced by dopaminergic agonism. A step-down passive avoidance paradigm was employed and nefiracetam (3 mg/kg) and apomorphine (0.5 mg/kg) were given alone or in combination during training and at the 10-12h post-training period of consolidation. Co-administration of nefiracetam and apomorphine during training or 10h thereafter produced no significant anti-amnesic effect. However, administration of nefiracetam during training completely reversed the amnesia induced by apomorphine at the 10h post-training time and the converse was also true. These effects were not mediated by a dopaminergic mechanism as nefiracetam, at millimolar concentrations, failed to displace either [3H]SCH 23390 or [3H]spiperone binding from D1 or D2 dopamine receptor subtypes, respectively. It is suggested that nefiracetam augments molecular processes in the early stages of events which ultimately lead to consolidation of memory.

Self-administration of D1 receptor agonists by squirrel monkeys

Dopaminergic mechanisms are believed to play a prominent role in the self-administration of cocaine and other abused stimulants. The contribution of D2 receptors is now well established, but less is known about the role of D1 receptors in the reinforcing effects of these drugs. To help clarify the role of D1 mechanisms in stimulant self-administration, agonists differing in D1 receptor selectivity (SKF 81297 > SKF 82958 > SKF 77434) and efficacy (SKF 82958 > SKF 81297 > SKF 77434) were studied for their ability to maintain IV self-administration in squirrel monkeys previously trained to self-administer cocaine. Up to a 100-fold range of doses of each D1 agonist was studied under both a fixed-ratio (FR) and a second-order fixed-interval (FI) schedule of reinforcement. Parallel studies were conducted with the D2 receptor agonists, (+)-PHNO and quinpirole, under the second-order FI schedule. Of the three D1 agonists, only SKF 82958 maintained consistent self-administration under both the FR and second-order FI schedules and had dose-related effects that were qualitatively similar to those of (+)-PHNO and quinpirole under the latter condition. SKF 81297, which has high selectivity at D1 receptors and intermediate agonist efficacy, maintained self-administration in the majority of monkeys under the FR schedule, but did not maintain self-administration under the second-order FI schedule. SKF 77434, which has moderate selectivity at D1 receptors and low agonist efficacy, did not maintain self-administration under either schedule. The results suggest that the ability of D1 agonists to maintain IV self-administration in squirrel monkeys depends both on the type of schedule and on the pharmacological properties (i.e. selectivity and efficacy) of the particular drug. These results are also consistent with the view that D1, in addition to D2, receptor mechanisms play a role in the self-administration of abused stimulants.

In vivo binding of [11C]SKF 75670 and [11C]SKF 82957 in rat brain: two dopamine D-1 receptor agonist ligands

The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 labeled with C-11 were evaluated in vivo in rats as potential radioligands for imaging dopamine D1 receptors with positron emission tomography (PET). Their in vivo pharmacological profile revealed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen. The more lipophilic [11C]SKF 82957 (6-chloro-[11C]SKF 75670) showed a higher brain uptake (more than 2-fold up to 30 min), higher specific uptake in the striatum and higher signal-to-noise ratio (striatum-to-cerebellum = 3.2 +/- 0.4 for [11C]SKF 75670 and 9.7 +/- 2.5 for [11C]SKF 82957 at 60 min post-injection) as compared to [11C]SKF 75670. Both radiotracers exhibited high specificity and selectivity for D1 receptors, since only D1 competitors but not the D2 antagonist sulpiride or the 5-HT2 antagonist ritanserin reduced significantly their binding the striatum with [11C]SKF 75670 or the striatum and olfactory tubercles with [11C]SKF 82957. Previous reports have shown that only D1 agonists can recognize the functional high-affinity state from the low-affinity state of D1 receptors. [11C]SKF 75670 and especially [11C]SKF 82957 are D1 agonist radioligands that can potentially be used to study in vivo the functional high-affinity state of D1 receptors using PET.

Dopamine-induced reduction in the density of guanine nucleotide-sensitive D1 receptors in human postmortem brain in the absence of apparent D1: D2 interactions

The effects of dopamine and guanine nucleotides on the binding of the D1 dopamine receptor antagonist ligand [3H]SCH 23390 were examined in membranes prepared from putamen, caudate and nucleus accumbens of human postmortem brain. Dopamine induced a concentration-dependent decrease in the apparent maximum number of binding sites (Bmax) in each brain region studied, and displaced binding in a biphasic manner consistent with the presence of both high and low affinity states of the D1 receptor; the GTP analogue Gpp(NH)p transformed this biphasic displacement to a monophasic pattern consistent with a shift of high affinity sites to a low affinity state. However, the selective D2 antagonist eticlopride did not reverse the action of dopamine to decrease Bmax. These data suggest that dopamine decreases Bmax for D1 receptors through a high affinity, guanine nucleotide-sensitive agonist binding site, but fail to reveal D1:D2 interactions at this synaptic level.

The kinetics of [3H]SCH 23390 dissociation from rat striatal dopamine D1 receptors: effect of dopamine

The present study investigated possible allosteric interactions between dopamine and [3H]SCH 23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepi n-7-ol)- labelled dopamine D1 receptors in rat striatum. As previously described, dopamine prevented [3H]SCH 23390 binding in a mixed competitive/non-competitive manner, causing both a loss of ligand affinity and a decrease in Bmax. The effect of dopamine was largely reversed following pretreatment of the membranes with 100 microM Gpp(NH)p (5'-guanylylimidodiphosphate) and was significantly enhanced by omission of Na+ from the incubation buffer. In dissociation kinetic studies, two methods of initiating ligand dissociation were used: dilution into 100-fold volume excess of buffer or addition of a molar excess of drug. Both methods yielded similar rates of [3H]SCH 23390 dissociation. Inclusion of dopamine in the volume excess of buffer did not alter the k-1 for [3H]SCH 23390 dissociation. However, when 100 microM dopamine was used instead of 1 microM piflutixol to initiate dissociation, a significant slowing of the rate of dissociation of [3H]SCH 23390 occurred. This effect of dopamine on k-1 was Na(+)-dependent since in the absence of Na+ the dopamine-induced rate of dissociation was only slightly slower than control values. Under neither condition did dopamine accelerate the rate of ligand dissociation, indicating that dopamine does not interact allosterically with [3H]SCH 23390 binding sites. These data, therefore, preclude an allosteric mechanism to explain the dopamine-induced decrease in dopamine D1 receptor density and provide direct evidence that dopamine masks ligand binding by binding to a high affinity site which can be modulated by Gpp(NH)p and Na+.

Chronic antagonist treatment does not alter the mode of interaction of dopamine with rat striatal dopamine receptors

Chronic treatment with the D1 and D2 dopamine receptor antagonists SCH 23390 (0.5 mg/kg) and haloperidol decanoate (25 mg/kg) caused an up-regulation in D1 and D2 receptor densities, respectively, with no change in KD. Dopamine (20 microM) interacted with both receptor subtypes in a mixed competitive/non-competitive manner, causing a reduction in ligand binding affinity and an apparent decrease in receptor density. In the presence of dopamine, both vehicle-treated and SCH 23390-treated striatal preparations showed a significant loss in affinity for 3H-SCH 23390 binding to D1 receptors and a decrease in D1 receptor density of approximately 26%. Similarly, dopamine caused a substantial loss in 3H-spiperone binding affinity to D2 receptors and a 46% decrease in Bmax in both vehicle-treated and haloperidol-treated membranes. Thus, receptor up-regulation does not appear to alter the mode of interaction of dopamine with rat striatal dopamine receptors.