Discovery of drugs to treat cocaine dependence: behavioral and neurochemical effects of atypical dopamine transport inhibitors

Abstract

Stimulant drugs acting at the dopamine transporter (DAT), like cocaine, are widely abused, yet effective medical treatments for this abuse have not been found. Analogs of benztropine (BZT) that, like cocaine, act at the DAT have effects that differ from cocaine and in some situations block the behavioral, neurochemical, and reinforcing actions of cocaine. Neurochemical studies of dopamine levels in brain and behavioral studies have demonstrated that BZT analogs have a relatively slow onset and reduced maximal effects compared to cocaine. Pharmacokinetic studies, however, indicated that the BZT analogs rapidly access the brain at concentrations above their in vitro binding affinities, while binding in vivo demonstrates apparent association rates for BZT analogs lower than that for cocaine. Additionally, the off-target effects of these compounds do not fully explain their differences from cocaine. Initial structure-activity studies indicated that BZT analogs bind to DAT differently from cocaine and these differences have been supported by site-directed mutagenesis studies of the DAT. In addition, BZT analog-mediated inhibition of uptake was more resistant to mutations producing inward conformational DAT changes than cocaine analogs. The BZT analogs have provided new insights into the relation between the molecular and behavioral actions of cocaine and the diversity of effects produced by dopamine transport inhibitors. Novel interactions of BZT analogs with the DAT suggest that these drugs may have a pharmacology that would be useful in their development as treatments for cocaine abuse.

Figure 1:

Chemical structures of cocaine, GBR 12909 and the BZT analogs referred to in the present manuscript. For a more complete description of the chemistry of the BZT analogs see Newman and Katz (2009).

Figure 2:

Dose-dependent effects of 4-F- and 4-Cl-substituted BZT analogs on locomotor activity in mice. Ordinates: horizontal activity counts after drug administration. Abscissae: dose of drug in μmol/kg, log scale. Each point represents the average effect determined in eight mice. The data are from the 30-min period during the first 60 min after drug administration, in which the greatest stimulant effects were obtained. Note that the 4-F-substituted compounds (left panel) were generally more efficacious than the 4-Cl-substituted compounds, and no members of either group had efficacy comparable to that of cocaine. Left panel symbols: Filled circles: cocaine; open circles: 4’-F-BZT; squares: 4’,4”-diF-BZT; triangles: 3’,4’-diCl,4”-F-BZT; downward triangles: 3’,4’-diF-BZT; diamonds: 3’,4”-diF-BZT; hexagons: 4’-Br,4”-F-BZT. Right panel symbols: Filled circles: cocaine; open circles: 4’-Cl-BZT; squares: 4’-Cl-BZT (with the diphenyl-ether system at the asymmetric C3 of the tropane ring, in the equatorial, β, configuration); triangles: 4’,4”-diCl-BZT; downward triangles: 3’,4’-diCl-BZT. See Figure 1 for compound structures. Modified from Katz et al. (1999).

Figure 3:

Effects of BZT analogs in rats trained to discriminate injections of cocaine from saline. Ordinates for top panels: percentage of responses on the cocaine-appropriate key. Ordinates for bottom panels: rates at which responses were emitted (as a percentage of response rate after saline administration). Abscissae: drug dose in μmol/kg (log scale). Each point represents the effect in four to six rats. The percentage of responses emitted on the cocaine-appropriate key was considered unreliable and not plotted if fewer than half of the subjects responded at that dose. Note that the fluoro-substituted compounds (left panels) were generally more effective in substituting for cocaine than the Cl-substituted compounds. See Figure 1 for compound structures. Modifed from Katz et al. (1999).

Figure 4:

Substitution of different doses of cocaine or other monoamine uptake inhibitors and BZT analogues in rats trained to self-administer cocaine (0.32 mg/kg/injection). Ordinates: responses per second. Abscissae: injection dose (mg/kg/injection). Each point represents the mean (vertical bars represent S.E.M.) of from 6 to 11 subjects. Panel A: Cocaine (filled circles) and methylphenidate (open circles). Panel B: Cocaine (filled circles) and citalopram (open circles) or nisoxetine (triangles). Panel C: Cocaine (filled circles) and AHN 1–055 (open circles), AHN 2–055 (triangles up), or JHW 007 (triangles down). Modified from Hiranita et al. (2009).

Figure 5:

Effects of presession treatment with methylphenidate and N-substituted BZT analogs on cocaine self-administration. Ordinates: responses per second. Abscissae: injection dose (mg/kg/injection). Each point represents the mean with S.E.M. (n = 6 to 10). Methylphenidate, AHN 1–055, AHN 2–005, or JHW 007 were administered orally at 60, 180, 240, or 300 min before sessions, respectively. Modified from Hiranita et al. (2009).

Figure 6:

Time course of effects of systemic administration of cocaine (Panel A) or 4-Cl-BZT (Panel B) on extracellular levels of DA in dialysates from the NAC shell. Results are means (with vertical bars representing S.E.M.) of the amount of DA in 10-min dialysate samples, expressed as percentage of basal values, uncorrected for probe recovery. Modified from Tanda et al. (2005).

Figure 7:

Effects of combinations of cocaine and JHW 007 at 10 min after injection on extracellular levels of DA. Panel A: Dose-dependent maximal effects of JHW 007 administered 10 min before compared with those of cocaine administered immediately before assessments. Ordinates: change in extracellular DA levels as a percentage of basal values before injection. Abscissae: dose of drug in mg/kg, log scale. Panel B: Observed effects of the combinations. Ordinates: change in extracellular DA levels as a percentage of basal values during the 30-min period after cocaine administration. Abscissae: dose of cocaine in mg/kg, log scale. Panel C: Observed effects compared with the predicted effects of the combinations. Ordinates and abscissae are as in Panel B. The calculated (predicted) additive dose-effect curve for cocaine in the presence of 3.0 (top) or 10.0 (bottom) mg/kg of JHW 007 is shown by the dashed straight line and the effects of cocaine alone are shown by the dotted line. The experimental (obtained) values are shown by the connected circles. Modified from Tanda et al. (2009).

Figure 8:

Time course of displacement of specific [¹²⁵I]RTI-121 accumulation in striatum of mice following IP injection of cocaine, AHN 1–055, AHN 2–005, or JHW 007. Ordinates: specific [¹²⁵I]RTI-121 binding as a percentage of that obtained after vehicle injection. Abscissae: time. For each point the number of replicates was from 5 to 10 or 13. Note that maximal displacement of [¹²⁵I]RTI-121 was obtained with cocaine at 30 min after injection, and at later times with the other compounds. Modified from Desai et al. (2005a, b).

Figure 9:

Relationship between dopamine transporter occupancy, determined in studies of displacement of [¹²⁵I]RTI-121 in striatum, and locomotor-stimulant effects of cocaine, AHN 1–055, AHN 2–005 or JHW 007. Ordinates: difference between mean horizontal activity counts after drug and after saline. Abscissae: percent displacement of [¹²⁵I]RTI-121. For each drug, the solid line represents the linear regression of percent occupancy of the DA transporter and horizontal locomotor activity when the line is forced to intersect the origin, the point representing no occupancy and no effect. Dashed lines represent 95% confidence limits for the regression lines. Note that the locomotor-stimulant effects of cocaine are less strongly related to DA transporter occupancy than are the effects of AHN 1–055, AHN 2–005, or JHW 007. Modified from Desai et al. (2005a, b).

Figure 10:

Time courses for the effects of pretreatments with increasing doses of telenzepine (TZP) on cocaine-stimulated DA extracellular levels in dialysates from the NAc shell (Panel A), and NAc core (Panel B). Cocaine (3.0 mg/kg i.p.) was administered at time=0. Each point represents means (with vertical bars representing SEM) of the amount of DA in 10-min dialysate samples, expressed as percentage of basal values, uncorrected for probe recovery. Modified from Tanda et al. (2007).

Figure 11:

The relationship between dopamine uptake potency ratios of various dopamine transporter ligands in COS-7 cells transfected with the Y335A mutant and WT DAT (affinity ratio) and behavioral activity. Top Panel: Correlation of affinity ratio and the degree to which the drugs substituted for cocaine in rats trained to discriminate cocaine from vehicle injections. The correlation coefficient (r²) was 0.74 (p<0.0001). Bottom Panel: Correlation of affinity ratio and the maximal locomotor-activity stimulation in mice. The correlation coefficient (r²) was 0.59 (p<0.0005). The affinity ratio was calculated from the IC₅₀ values for inhibition of [³H]dopamine uptake by the compound in COS-7 cells transiently expressing either DAT WT or Y335A. Modified from Loland et al. (2008).