Discovery of 4-(4-(2-((5-Hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)(propyl)amino)ethyl)piperazin-1-yl)quinolin-8-ol and its analogues as highly potent dopamine D2/D3 agonists and as iron chelator: in vivo activity indicates potential application in symptomatic and neuroprotective therapy for Parkinson's disease

Abstract

The role of iron in the pathogenesis of Parkinson's disease (PD) has been implicated strongly because of generation of oxidative stress leading to dopamine cell death. In our overall goal to develop bifunctional/multifunctional drugs, we designed dopamine D2/D3 agonist molecules with a capacity to bind to iron. Binding assays were carried out with HEK-293 cells expressing either D2 or D3 receptor with tritiated spiperone to evaluate inhibition constants (K(i)). Functional activity of selected compounds was carried out with GTPgammaS binding assay. SAR results identified compounds (+)-19a and (-)-19b as two potent agonists for both D2 and D3 receptors (EC(50) (GTPgammaS); D2 = 4.51 and 1.69 nM and D3 = 1.58 and 0.74 nM for (-)-19b and (+)-19a, respectively). In vitro complexation studies with 19b demonstrated efficient chelation with iron. Furthermore, the deoxyribose assay with 19b demonstrated potent antioxidant activity. In PD animal model study, (-)-19b exhibited potent in vivo activity in reversing locomotor activity in reserpinized rats and also in producing potent rotational activity in 6-OHDA lesioned rats. This reports initial development of unique lead molecules that might find potential use in symptomatic and neuroprotective treatment of PD.

Figure 1

Molecular structures of D3 preferring compounds and iron chelators.

Figure 2

UV-visible absorption spectra of complex formation between racemic 19b (0.6 mM) and FeCl₃ in water at various pH.

Figure 3

Chelating potency of 19b via displacement of ferrozine complexed to FeSO₄ and is expressed as percent of control. Each point represents value from experiment done in triplicate and is expressed as mean ± SEM.

Figure 4

Molecular ion peaks of complexes formed from racemic 19b and FeCl₃ at pH 7.4. Peaks at m/z 975 and 1434 correspond to 19b-Fe complex stoichiometry 2:1 and 3:1.

Figure 5

Hydroxyl radical scavenging capacity of 19b in deoxyribose-containing solution. Values are reported as percentage versus a blank ± SD

Figure 6

Effect of different drugs upon reserpine (5.0 mg/Kg, s.c.)-induced hypolocomotion in rats. Data are means ± S.E.M, n = 4 per value. Horizontal activity was measured as described under materials and methods. Panel A is the representation of horizontal locomotor activity at discrete 30-min intervals after the administration of (−)19b (i.p.) and ropinirole (s.c.) at the dose of 10 µMol/kg compared to control rats in 18 h post reserpine treatment. One way ANOVA analysis demonstrates significant effect among treatments F (3,95) = 31.36 (P< 0.0001). Dunnett’s analysis following ANOVA showed that the effects of (−)19b (P< 0.01) and ropinirole (P< 0.05) were significantly different compared to reserpine control.

Figure 7

Effect on turning behavior of two different doses of (−)19b (i.p.) and vehicle in lesioned rats studied for maximum 12 h. Each point is the mean ± SEM of 3–4 rats. The drugs were administered i.p. One way ANOVA analysis demonstrates significant effect among treatments: F (4,95) = 21.12 (P< 0.0001). Dunnett’s analysis showed that the effect of (−)19b on rotations at two doses was significantly different compared to vehicle (P< 0.01) and the effect of ropinirole was significant compared to vehicle (p<0.05).

Scheme 1

a. n-Propylamine, NaCNBH₃, CH₃COOH, dichloroethane, RT, overnight; b. (+)-chlocyphos, EtOH; c. (−)-chlocyphos, EtOH; d. chloroacetyl chloride, TEA, dichloromethane, 0 °C, 30 min; e. (Boc)₂O, CH₂Cl₂, 0 °C, 2 h; f. K₂CO₃, CH₃CN, 80 °C, 2 h; g. TFA/DCM (1/1), RT, overnight; h. LiAlH₄, THF, reflux, 2 h; i. BBr₃, −40 °C, CH₂Cl₂, overnight

Scheme 1

a. n-Propylamine, NaCNBH₃, CH₃COOH, dichloroethane, RT, overnight; b. (+)-chlocyphos, EtOH; c. (−)-chlocyphos, EtOH; d. chloroacetyl chloride, TEA, dichloromethane, 0 °C, 30 min; e. (Boc)₂O, CH₂Cl₂, 0 °C, 2 h; f. K₂CO₃, CH₃CN, 80 °C, 2 h; g. TFA/DCM (1/1), RT, overnight; h. LiAlH₄, THF, reflux, 2 h; i. BBr₃, −40 °C, CH₂Cl₂, overnight

Scheme-2

a. HCl (32% water), HCHO (37% in water), 0 °C, RT, 8 h; b. N-substitued piperazine (9a or 9b), (Me₂CH)₂-NEt, CHCl₃, RT, 1 day.

Scheme-3

a. Trimethylorthoformate, reflux, 1 h; b. o-anisidine, DMF (cat. amt), relux, 2 h; c. Diphenyl ether, 300 °C, 15 min; d. POCl₃, reflux, 2h.

Scheme-4

a. DIPEA, 2-propanol, relux, overnight; b. 48% aqueous HBr, reflux, overnight