High Affinity Dopamine D3 Receptor (D3R)-Selective Antagonists Attenuate Heroin Self-Administration in Wild-Type but not D3R Knockout Mice

Abstract

The dopamine D3 receptor (D3R) is a promising target for the development of pharmacotherapeutics to treat substance use disorders. Several D3R-selective antagonists are effective in animal models of drug abuse, especially in models of relapse. Nevertheless, poor bioavailability, metabolic instability, and/or predicted toxicity have impeded success in translating these drug candidates to clinical use. Herein, we report a series of D3R-selective 4-phenylpiperazines with improved metabolic stability. A subset of these compounds was evaluated for D3R functional efficacy and off-target binding at selected 5-HT receptor subtypes, where significant overlap in SAR with D3R has been observed. Several high affinity D3R antagonists, including compounds 16 (Ki = 0.12 nM) and 32 (Ki = 0.35 nM), showed improved metabolic stability compared to the parent compound, PG648 (6). Notably, 16 and the classic D3R antagonist SB277011A (2) were effective in reducing self-administration of heroin in wild-type but not D3R knockout mice.

Chart 1. Selected Lead D₃R-Selective Antagonists and Partial Agonists

Scheme 1. Synthesis of 4-(4-Arylpiperazine)butylamine Intermediates

Reagents and conditions: (a) 3-chloro-2-methoxyaniline or naphthalen-1-amine, K₂CO₃, bis(2-chloroethyl)amine HCl, diglyme, reflux, 48 h; (b) appropriate 4-arylpiperazine, 2-propanol, reflux, overnight; (c) hydrazine (anhydrous), EtOH, reflux, 2–3 h.

Scheme 2. Synthesis of Target Compounds 14–37

Reagents and conditions: (a) (i) 1,1′-carbonyldiimidazole (CDI), THF, room temperature, 2 h; (ii) appropriate 4-arylpiperazine amine, THF, 0 °C to room temperature, overnight.

Figure 1

(a) Phase I metabolism data for all compounds with an unsubstituted linker chain. (b) Phase I metabolism data for all compounds with a 3-OH substituted linker chain. (c) Phase I metabolism data for all quinoline compounds. (d) Phase I metabolism data for all indole compounds.

Figure 2

Effects of compound 2 on heroin self-administration in WT and D₃KO mice. All the data are expressed as mean ± SEM. (A) Total numbers of heroin infusions after injection of each dose of compound 2. (B) Normalized data showing drug-induced % change in heroin self-administration over new basal levels immediately before each test day. One-way ANOVA with repeated measures over drug dose revealed a significant drug treatment main effect in WT mice ((A) F_3,24 = 10.93, p < 0.001; (B) F_3,24 = 13.06, p < 0.001) but not in D₃KO mice ((A) F_3,12 = 2.21, p > 0.05; (B) F_3,12 = 1.46, p > 0.05). *p < 0.05, ***p < 0.001, compared to vehicle.

Figure 3

Effects of compound 16 on heroin self-administration in WT and D₃R-KO mice. All the data are expressed as mean ± SEM. (A) Total numbers of heroin infusions after injection of each dose of compound 16. (B) Normalized data showing drug-induced % change in heroin self-administration over new baseline immediately before each test day. One-way ANOVA with repeated measures over drug dose revealed a statistically significant drug treatment main effect in WT mice ((A) F_3,18 = 2.13, p > 0.05; (B) F_3,18 = 9.09, p < 0.001) but not in D₃KO mice ((A) F_3,18 = 0.63, p > 0.05; (B) D₃KO, F_3,18 = 1.78, p > 0.05). We note that one-way ANOVA did not reveal a significant treatment main effect for the data shown in (A) WT mice. However, the direct two group comparison between the vehicle and 10 mg/kg 16 groups revealed a statistically significant reduction ((A) WT, paired t test, q = 5.07, p < 0.05). These may be related to the relatively smaller group size and/or the relatively variable basal levels of heroin self-administration in different subjects. Therefore, the renormalized data (% change over baseline) are provided ((B) in this figure; the same as in Figures 2B and 4B) in this study. *p < 0.05, ***p < 0.001, compared to vehicle.

Figure 4

Effects of compound 32 on heroin self-administration in WT and D₃KO mice. All the data are expressed as mean ± SEM. (A) Total numbers of heroin infusions after injection of each dose of compound 32. (B) Normalized data showing drug-induced % change in heroin self-administration over new baseline immediately before each test day. One-way ANOVA with repeated measures over drug dose revealed a statistically significant drug treatment main effect in WT mice ((A) F_3,18 = 0.83, p > 0.05; (B) F_3,18 = 3.34, p < 0.05) and D₃KO mice ((A) F_3,15 = 3.63, p < 0.05; (B) F_3,15 = 4.39, p < 0.05) *p < 0.05, compared to vehicle.