Neurochemical and behavioral comparisons of contingent and non-contingent methamphetamine exposure following binge or yoked long-access self-administration paradigms.

RATIONALE: Abuse of the psychostimulant methamphetamine (METH) can cause long-lasting damage to brain monoaminergic systems and is associated with profound mental health problems for users, including lasting cognitive impairments. Animal models of METH exposure have been useful in dissecting the molecular effects of the drug on cognition, but many studies use acute, non-contingent "binge" administrations of METH which do not adequately approximate human METH use. Long-term METH exposure via long-access (LgA) self-administration paradigms has been proposed to more closely reflect human use and induce cognitive impairments. OBJECTIVE: To better understand the role of contingency and patterns of exposure in METH-induced cognitive impairments, we analyzed behavioral and neurochemical outcomes in adult male rats, comparing non-contingent "binge" METH administration with contingent (LgA) METH self-administration and non-contingent yoked partners. RESULTS: Binge METH (40 mg/kg, i.p., over 1 day) dramatically altered striatal and hippocampal dopamine, DOPAC, 5-HT, 5-HIAA, BDNF, and TrkB 75 days after drug exposure. In contrast, 6-h LgA METH self-administration (cumulative 24.8-48.9 mg METH, i.v., over 16 days) altered hippocampal BDNF in both contingent and yoked animals but reduced striatal 5-HIAA in only contingent animals. Neurochemical alterations following binge METH administration were not accompanied by cognitive deficits in Morris water maze, novel object recognition, or Y-maze tests. However, contingent LgA METH self-administration resulted in impaired spatial memory in the water maze. CONCLUSIONS: Overall, substantial differences in neurochemical markers between METH exposure and self-administration paradigms did not consistently translate to deficits in cognitive tasks, highlighting the complexity of correlating METH-induced neurochemical changes with cognitive outcomes.

Novel and High Affinity 2-[(Diphenylmethyl)sulfinyl]acetamide (Modafinil) Analogues as Atypical Dopamine Transporter Inhibitors.

The development of pharmacotherapeutic treatments of psychostimulant abuse has remained a challenge, despite significant efforts made toward relevant mechanistic targets, such as the dopamine transporter (DAT). The atypical DAT inhibitors have received attention due to their promising pharmacological profiles in animal models of cocaine and methamphetamine abuse. Herein, we report a series of modafinil analogues that have an atypical DAT inhibitor profile. We extended SAR by chemically manipulating the oxidation states of the sulfoxide and the amide functional groups, halogenating the phenyl rings, and/or functionalizing the terminal nitrogen with substituted piperazines, resulting in several novel leads such as 11b, which demonstrated high DAT affinity (Ki = 2.5 nM) and selectivity without producing concomitant locomotor stimulation in mice, as compared to cocaine. These results are consistent with an atypical DAT inhibitor profile and suggest that 11b may be a potential lead for development as a psychostimulant abuse medication.

Novel Analogues of (R)-5-(Methylamino)-5,6-dihydro-4H-imidazo[4,5,1-ij]quinolin-2(1H)-one (Sumanirole) Provide Clues to Dopamine D2/D3 Receptor Agonist Selectivity.

Novel 1-, 5-, and 8-substituted analogues of sumanirole (1), a dopamine D2/D3 receptor (D2R/D3R) agonist, were synthesized. Binding affinities at both D2R and D3R were higher when determined in competition with the agonist radioligand [(3)H]7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT) than with the antagonist radioligand [(3)H]N-methylspiperone. Although 1 was confirmed as a D2R-preferential agonist, its selectivity in binding and functional studies was lower than previously reported. All analogues were determined to be D2R/D3R agonists in both GoBRET and mitogenesis functional assays. Loss of efficacy was detected for the N-1-substituted analogues at D3R. In contrast, the N-5-alkyl-substituted analogues, and notably the n-butyl-arylamides (22b and 22c), all showed improved affinity at D2R over 1 with neither a loss of efficacy nor an increase in selectivity. Computational modeling provided a structural basis for the D2R selectivity of 1, illustrating how subtle differences in the highly homologous orthosteric binding site (OBS) differentially affect D2R/D3R affinity and functional efficacy.

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

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.

Using click chemistry toward novel 1,2,3-triazole-linked dopamine D3 receptor ligands.

The dopamine D3 receptor (D3R) is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, Parkinson's disease, restless leg syndrome, and drug addiction. A common molecular template used in the development of D3R-selective antagonists and partial agonists incorporates a butylamide linker between two pharmacophores, a phenylpiperazine moiety and an extended aryl ring system. The series of compounds described herein incorporates a change to that chemical template, replacing the amide functional group in the linker chain with a 1,2,3-triazole group. Although the amide linker in the 4-phenylpiperazine class of D3R ligands has been previously deemed critical for high D3R affinity and selectivity, the 1,2,3-triazole moiety serves as a suitable bioisosteric replacement and maintains desired D3R-binding functionality of the compounds. Additionally, using mouse liver microsomes to evaluate CYP450-mediated phase I metabolism, we determined that novel 1,2,3-triazole-containing compounds modestly improves metabolic stability compared to amide-containing analogues. The 1,2,3-triazole moiety allows for the modular attachment of chemical subunit libraries using copper-catalyzed azide-alkyne cycloaddition click chemistry, increasing the range of chemical entities that can be designed, synthesized, and developed toward D3R-selective therapeutic agents.

Structure-Metabolism Relationships in the Glucuronidation of d-Amino Acid Oxidase Inhibitors.

Representative d-amino acid oxidase (DAAO) inhibitors were subjected to in vitro liver microsomal stability tests in the absence or presence of uridine diphosphate glucuronic acid (UDPGA). While carboxylate-based DAAO inhibitors displayed little glucuronidation, most DAAO inhibitors containing α-hydroxycarbonyl moiety exhibited nearly complete glucuronidation within 30 min. The one exception was 6-[2-(3,5-difluorophenyl)ethyl]-4-hydroxypyridazin-3(2H)-one 10, which exhibited some degree of resistance to glucuronidation by liver microsomes from mice, rats, and humans.

Beyond small-molecule SAR: using the dopamine D3 receptor crystal structure to guide drug design.

The dopamine D3 receptor is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, restless leg syndrome, and drug addiction. The high protein sequence homology between the D3 and D2 receptors has posed a challenge to developing D3 receptor-selective ligands whose behavioral actions can be attributed to D3 receptor engagement, in vivo. However, through primarily small-molecule structure-activity relationship (SAR) studies, a variety of chemical scaffolds have been discovered over the past two decades that have resulted in several D3 receptor-selective ligands with high affinity and in vivo activity. Nevertheless, viable clinical candidates remain limited. The recent determination of the high-resolution crystal structure of the D3 receptor has invigorated structure-based drug design, providing refinements to the molecular dynamic models and testable predictions about receptor-ligand interactions. This chapter will highlight recent preclinical and clinical studies demonstrating potential utility of D3 receptor-selective ligands in the treatment of addiction. In addition, new structure-based rational drug design strategies for D3 receptor-selective ligands that complement traditional small-molecule SAR to improve the selectivity and directed efficacy profiles are examined.

High-resolution structures and orientations of antimicrobial peptides piscidin 1 and piscidin 3 in fluid bilayers reveal tilting, kinking, and bilayer immersion.

While antimicrobial peptides (AMPs) have been widely investigated as potential therapeutics, high-resolution structures obtained under biologically relevant conditions are lacking. Here, the high-resolution structures of the homologous 22-residue long AMPs piscidin 1 (p1) and piscidin 3 (p3) are determined in fluid-phase 3:1 phosphatidylcholine/phosphatidylglycerol (PC/PG) and 1:1 phosphatidylethanolamine/phosphatidylglycerol (PE/PG) bilayers to identify molecular features important for membrane destabilization in bacterial cell membrane mimics. Structural refinement of (1)H-(15)N dipolar couplings and (15)N chemical shifts measured by oriented sample solid-state NMR and all-atom molecular dynamics (MD) simulations provide structural and orientational information of high precision and accuracy about these interfacially bound α-helical peptides. The tilt of the helical axis, τ, is between 83° and 93° with respect to the bilayer normal for all systems and analysis methods. The average azimuthal rotation, ρ, is 235°, which results in burial of hydrophobic residues in the bilayer. The refined NMR and MD structures reveal a slight kink at G13 that delineates two helical segments characterized by a small difference in their τ angles (<10°) and significant difference in their ρ angles (~25°). Remarkably, the kink, at the end of a G(X)4G motif highly conserved among members of the piscidin family, allows p1 and p3 to adopt ρ angles that maximize their hydrophobic moments. Two structural features differentiate the more potent p1 from p3: p1 has a larger ρ angle and less N-terminal fraying. The peptides have comparable depths of insertion in PC/PG, but p3 is 1.2 Å more deeply inserted than p1 in PE/PG. In contrast to the ideal α-helical structures typically assumed in mechanistic models of AMPs, p1 and p3 adopt disrupted α-helical backbones that correct for differences in the amphipathicity of their N- and C-ends, and their centers of mass lie ~1.2-3.6 Å below the plane defined by the C2 atoms of the lipid acyl chains.