Pharmacology and Therapeutic Potential of Benzothiazole Analogues for Cocaine Use Disorder

Abstract

Pharmacological targeting of the dopamine D₄ receptor (D₄R)─expressed in brain regions that control cognition, attention, and decision-making─could be useful for several neuropsychiatric disorders including substance use disorders (SUDs). This study focused on the synthesis and evaluation of a novel series of benzothiazole analogues designed to target D₄R. We identified several compounds with high D₄R binding affinity (K_i ≤ 6.9 nM) and >91-fold selectivity over other D₂-like receptors (D₂R, D₃R) with diverse partial agonist and antagonist profiles. Novel analogue 16f is a potent low-efficacy D₄R partial agonist, metabolically stable in rat and human liver microsomes, and has excellent brain penetration in rats (AUC_brain/plasma > 3). 16f (5-30 mg/kg, i.p.) dose-dependently decreased iv cocaine self-administration in rats, consistent with previous results produced by D₄R-selective antagonists. Off-target antagonism of 5-HT_2A or 5-HT_2B may also contribute to these effects. Results with 16f support further efforts to target D₄R in SUD treatment.

Figure 1

Selected structures of notable D₄R ligands, including the atypical antipsychotics clozapine (5) and nemonapride (6). Following the discovery of the relatively high affinity of 5 for D₄R, several pharmaceutical companies developed novel ligands targeting D₄R, including 1–4. A recent resurgence in D₄R ligand discovery has identified a diverse array of novel agonists (7), partial agonists (8), and antagonists (9–13) including lead compound 16a from this study.

Scheme 1. Synthesis of Series 1 of Unsubstituted Benzothiazole Analogues

Reagents and conditions: (a) toluene, 5-chloro pentanoyl chloride or 4-chloro butanoyl chloride, RT; (b) CH₃CN, KI, K₂CO₃, reflux, appropriate arylpiperazine or arylpiperidine.

Scheme 2. Synthesis of Series 2 of Substituted or Unsubstituted Benzothiazole Analogues

Reagents and conditions: (a) toluene, 4-chloro butanoyl chloride, RT; (b) CH₃CN, KI, K₂CO₃, reflux, appropriate arylpiperazine or arylpiperidine.

Figure 2

Lead compound 16f (CAB-01–019) demonstrated excellent D₄R selectivity in functional assays and is a competitive antagonist at D₄R. (A) 16f is a potent full D₄R antagonist for β-arrestin recruitment with no D₄R agonist activity detected. At D₂R and D₃R, 16f is a low-potency antagonist that shows 391- and 859-fold selectivity for the D₄R, respectively (Table 3). D₃R exhibits partial agonist activity with 16f, while D₂R has very low partial agonist activity. (B) 16f potently antagonizes D₄R-mediated cAMP inhibition and is 97-fold more potent at D₄R than at D₂R (Table 2). Furthermore, 16f has low-efficacy D₄R agonism and is a low-potency partial agonist at D₂R. (C) With increasing concentrations of 16f, dopamine concentration–response curves are shifted to the right with no decrease in E_max, indicating that 16f is a competitive orthosteric ligand. Furthermore, the Schild plot (inset) of these data had a slope of 1.09 and K_b = 11.0 nM. All data are presented as means ± SEM from at least three independent experiments run in triplicate.

Figure 3

Phase I metabolic stability of 16f in rat (A) and human (B) liver microsomes. 16f shows time-dependent degradation in human and rat liver microsomes. 16f was modestly stable in human liver microsomes. Data are expressed as mean ± SEM, n = 3. As a positive control for phase I metabolism, metabolic stability of buprenorphine in rat (C) and human (D) liver microsomes is also presented.

Figure 4

(A) Time-dependent in vivo pharmacokinetic analysis of 16f (CAB-01–019) in Sprague–Dawley (SD) rats following intraperitoneal (ip) administration of 10 mg/kg 16f. Data expressed as mean ± SEM, n = 3 for each time point. (B) Calculated pharmacokinetic parameters of 16f in rats.

Figure 5

Effect of D₄R antagonist 16f on cocaine self-administration and food-maintained responding. (A). Number of infusions for each cocaine dose session at baseline, and following vehicle, 5, 15, and 30 mg/kg (i.p.) of 16f. 16f dose-dependently decreased intake at each cocaine dose in male Fisher F344 rats (n = 8 per group). (B). Number of food reinforcers for each component at baseline, and following vehicle, 5, 15, and 30 mg/kg (i.p.) of 16f. 16f dose-dependently decreased food-maintained responding in male Fisher F344 rats (n = 7–9 per group). Data expressed as mean ± SEM.