SKF-83959 is not a highly-biased functionally selective D1 dopamine receptor ligand with activity at phospholipase C

Abstract

SKF-83959 [6-chloro-7,8-dihydroxy-3-methyl-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine] is reported to be a functionally selective dopamine D1 receptor ligand with high bias for D1-mediated phospholipase C (PLC) versus D1-coupled adenylate cyclase signaling. This signaling bias is proposed to explain behavioral activity in both rat and primate Parkinson’s disease models, and a D1-D2 heterodimer has been proposed as the underlying mechanism. We have conducted an in-depth pharmacological characterization of this compound in dopamine D1 and D2 receptors in both rat brain and heterologous systems expressing human D1 or D2 receptors. Contrary to common assumptions, SKF-83959 is similar to the classical, well-characterized partial agonist SKF38393 in all systems. It is a partial agonist (not an antagonist) at adenylate cyclase in vitro and ex vivo, and is a partial agonist in D1-mediated β-arrestin recruitment. Contrary to earlier reports, it does not have D1-mediated effects on PLC signaling in heterologous systems. Because drug metabolites can also contribute, its 3-N-demethylated analog also was synthesized and tested. As expected from the known structure-activity relationships of the benzazepines, this compound also had high affinity for the D1 receptor and somewhat higher intrinsic activity than the parent ligand, and also might contribute to in vivo effects of SKF-83959. Together, these data demonstrate that SKF-83959 is not a highly-biased functionally selective D1 ligand, and that its reported behavioral data can be explained solely by its partial D1 agonism in canonical signaling pathway(s). Mechanisms that have been proposed based on the purported signaling novelty of SKF-83959 at PLC should be reconsidered.

Dopamine D1Receptor Signaling: Does GαQ–Phospholipase C Actually Play a Role?

“…In some cases, the published data actually agree with the findings of Lee et al (2014). For example, Gnanalingham et al (1995b) reported that SKF-83959 did not cause significant stimulation of adenylate cyclase, yet their data clearly show measurable (∼50%) intrinsic activity, with some data points marked as significant.…”

“…and with similar potency as seen with adenylate cyclase activation (Lee et al, 2014). Importantly, its potency in both D 1 -mediated adenylate cyclase and b-arrestin assays was in the nanomolar range, similar to its affinity (Lee et al, 2014).…”

“…The functional effects were inhibited by SCH23390 [(R)-8-chloro-3-methyl-5-phenyl-2,3,4,5-tetrahydro-1H-benzo[d]azepin-7-ol], as well as by structurally dissimilar D 1 antagonists, and SKF-83959 had no effect in untransfected cells. As noted above, while D 1 -selective, SKF-83959 also had micromolar affinity for D 2 receptors, where it was an antagonist at cAMP signaling (Lee et al, 2014).…”

“…Because of the continuing use of this ligand for its "novel" properties, we recently did a rigorous evaluation of SKF-83959 pharmacology in a variety of heterologous and native D 1 systems, studying both the parent ligand and a potential N-demethylated metabolite, 6-chloro-1-(m-tolyl)-2, 3,4,5-tetrahydro-1H-benzo[d]azepine-7,8-diol, that might contribute to pharmacological effects in vivo (Lee et al, 2014). Both in vitro and ex vivo, SKF-83959 was a partial agonist at D 1 -mediated adenylate cyclase in two different cell lines transfected with the human D 1 receptor, as well as in rat striatal homogenates.…”

“…Yet the increasing numbers of papers whose data depend in large part on the highly biased signaling of SKF-83959 have made it important to re-examine the pharmacology of this compound. In evaluating the evidence for SKF-83959 being a highly biased ligand, one must also consider the role of D There is universal agreement that SKF-83959 has nanomolar affinity for both rat and human D 1 receptors, heterologously expressed or in situ (Arnt et al, 1992;Chun et al, 2013;Lee et al, 2014), and also that the ligand has micromolar affinity for D 2 receptors (Arnt et al, 1992;Chun et al, 2013;Lee et al, 2014), where it is an antagonist at cAMP signaling and a low-intrinsic-activity partial agonist at b-arrestin recruitment (Lee et al, 2014). Similar D 1 -D 2 selectivity with a wide range of D 1 intrinsic activities has been shown for other ligands with similar benzazepine structures [i.e., ligands with substituents on a backbone of 1-phenyl-2,3,4,5-tetrahydro-1H-benzo [d]azepine] (Setler et al, 1978;Iorio et al, 1983;Weinstock et al, 1985;Neumeyer et al, 2003).…”