Amphetamine Decreases α2C-Adrenoceptor Binding of [11C]ORM-13070: A PET Study in the Primate Brain

Background:

The neurotransmitter norepinephrine has been implicated in psychiatric and neurodegenerative disorders. Examination of synaptic norepinephrine concentrations in the living brain may be possible with positron emission tomography (PET), but has been hampered by the lack of suitable radioligands.

Methods:

We explored the use of the novel α_2C-adrenoceptor antagonist PET tracer [¹¹C]ORM-13070 for measurement of amphetamine-induced changes in synaptic norepinephrine. The effect of amphetamine on [¹¹C]ORM-13070 binding was evaluated ex vivo in rat brain sections and in vivo with PET imaging in monkeys.

Results:

Microdialysis experiments confirmed amphetamine-induced elevations in rat striatal norepinephrine and dopamine concentrations. Regional [¹¹C]ORM-13070 receptor binding was high in the striatum and low in the cerebellum. After injection of [¹¹C]ORM-13070 in rats, mean striatal specific binding ratios, determined using cerebellum as a reference region, were 1.4±0.3 after vehicle pretreatment and 1.2±0.2 after amphetamine administration (0.3mg/kg, subcutaneous). Injection of [¹¹C]ORM-13070 in non-human primates resulted in mean striatal binding potential (BP _ND) estimates of 0.65±0.12 at baseline. Intravenous administration of amphetamine (0.5 and 1.0mg/kg, i.v.) reduced BP _ND values by 31–50%. Amphetamine (0.3mg/kg, subcutaneous) increased extracellular norepinephrine (by 400%) and dopamine (by 270%) in rat striata.

Conclusions:

Together, these results indicate that [¹¹C]ORM-13070 may be a useful tool for evaluation of synaptic norepinephrine concentrations in vivo. Future studies are required to further understand a potential contribution of dopamine to the amphetamine-induced effect.     

Chronic Δ9-Tetrahydrocannabinol Exposure Induces a Sensitization of Dopamine D2/3 Receptors in the Mesoaccumbens and Nigrostriatal Systems

Δ⁹-Tetrahydrocannabinol (THC), through its action on cannabinoid type-1 receptor (CB₁R), is known to activate dopamine (DA) neurotransmission. Functional evidence of a direct antagonistic interaction between CB₁R and DA D₂-receptors (D₂R) suggests that D₂R may be an important target for the modulation of DA neurotransmission by THC. The current study evaluated, in rodents, the effects of chronic exposure to THC (1 mg/kg/day; 21 days) on D₂R and D₃R availabilities using the D₂R-prefering antagonist and the D₃R-preferring agonist radiotracers [¹⁸F]fallypride and [³H]-(+)-PHNO, respectively. At 24 h after the last THC dose, D₂R and D₃R densities were significantly increased in midbrain. In caudate/putamen (CPu), THC exposure was associated with increased densities of D₂R with no change in D₂R mRNA expression, whereas in nucleus accumbens (NAcc) both D₃R binding and mRNA levels were upregulated. These receptor changes, which were completely reversed in CPu but only partially reversed in NAcc and midbrain at 1 week after THC cessation, correlated with an increased functionality of D_2/3R in vivo, based on findings of increased locomotor suppressive effect of a presynaptic dose and enhanced locomotor activation produced by a postsynaptic dose of quinpirole. Concomitantly, the observations of a decreased gene expression of tyrosine hydroxylase in midbrain together with a blunted psychomotor response to amphetamine concurred to indicate a diminished presynaptic DA function following THC. These findings indicate that the early period following THC treatment cessation is associated with altered presynaptic D_2/3R controlling DA synthesis and release in midbrain, with the concurrent development of postsynaptic D_2/3R supersensitivity in NAcc and CPu. Such D_2/3R neuroadaptations may contribute to the reinforcing and habit-forming properties of THC.     

An Unexpected Degradation Pathway of a 1,2,4-Triazolo[4,3-a]pyridine Derivative: The Formation of 2 Cationic Pseudodimers of an 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitor Drug Candidate in a Stressed Capsule Formulation

Degradation of an active pharmaceutical ingredient (API), a 2-(3-(1-(4-chlorophenyl)cyclopropyl)-[1,2,4]triazolo[4,3-a]pyridin-8-yl)propan-2-ol hydrochloride salt, was observed in a capsule formulation stressed at 50°C or 40°C/75% relative humidity conditions for 1 month. Two unknown degradants were identified as cationic pseudodimers of the API via accurate mass liquid chromatography-mass spectrometry and 1- and 2-dimensional NMR analyses. A plausible degradation pathway of the API was postulated which led to the identification of 2 key N-oxide degradants in the stressed capsule formulation at trace levels. It was hypothesized that the N-oxide degradants could be protonated and undergo further transformation so as to react with another API free base to form pseudodimeric N-oxide intermediates, followed by protonation/dehydration to yield the cationic pseudodimers of the API. The proposed degradation pathway was further supported by formulation screening studies: (1) the removal of magnesium stearate (base/lubricant) from the formulation to reduce the formation of API free base, which is susceptible to oxidation to form N-oxides; (2) the replacement of API hydrochloride salt by its free base form to eliminate the proton source for protonation of the N-oxides so as to prevent their further transformation; and (3) the addition of anti-oxidants to minimize the oxidation of API free base to N-oxides.     

Structure-activity relationship study of N⁶-(2-(4-(1H-Indol-5-yl)piperazin-1-yl)ethyl)-N⁶-propyl-4,5,6,7-tetrahydrobenzo[d]thiazole-2,6-diamine analogues: development of highly selective D3 dopamine receptor agonists along with a highly potent D2/D3 agonist and their pharmacological characterization

In our effort to develop multifunctional drugs against Parkinson's disease, a structure-activity-relationship study was carried out based on our hybrid molecular template targeting D2/D3 receptors. Competitive binding with [(3)H]spiroperidol was used to evaluate affinity (K(i)) of test compounds. Functional activity of selected compounds in stimulating [(35)S]GTPγS binding was assessed in CHO cells expressing either human D2 or D3 receptors. Our results demonstrated development of highly selective compounds for D3 receptor (for (-)-40K(i), D3 = 1.84 nM, D2/D3 = 583.2; for (-)-45K(i), D3 = 1.09 nM, D2/D3 = 827.5). Functional data identified (-)-40 (EC(50), D2 = 114 nM, D3 = 0.26 nM, D2/D3 = 438) as one of the highest D3 selective agonists known to date. In addition, high affinity, nonselective D3 agonist (-)-19 (EC(50), D2 = 2.96 nM and D3 = 1.26 nM) was also developed. Lead compounds with antioxidant activity were evaluated using an in vivo PD animal model.