Pharmacodynamic Relationships between Duration of Action of JDTic-like Kappa-Opioid Receptor Antagonists and Their Brain and Plasma Pharmacokinetics in Rats

Evaluation of Specific Binding of [11C]RTI-97 to Kappa Opioid Receptor by Autoradiography and PET Imaging in Rat

Kappa opioid receptor (KOR) PET imaging remains attractive to understand the role of KOR in health and diseases and to help the development of drugs especially for psychiatric disorders such as depression, anxiety, and addiction. The potent and selective KOR antagonist RTI-97 labeled with carbon-11 was previously demonstrated to display specific KOR binding in mouse brain by ex vivo autoradiography studies. Herein, we evaluated [¹¹C]RTI-97 in rat by in vitro autoradiography and by in vivo PET imaging. The radiosynthesis of [¹¹C]RTI-97 was optimized to obtain high molar activities. Despite a low cerebral uptake, the overall results showed a heterogeneous repartition and specific KOR binding of [¹¹C]RTI-97 in brain and a high and specific accumulation of [¹¹C]RTI-97 in pituitary in accordance with KOR expression.

Frontal Cortical Monoamine Release, Attention, and Working Memory in a Perinatal Nicotine Exposure Mouse Model Following Kappa Opioid Receptor Antagonism

Perinatal nicotine exposure (PNE) produces frontal cortical hypo-dopaminergic state and attention and working memory deficits consistent with neurodevelopmental disorders such as attention deficit hyperactivity disorder (ADHD). Methylphenidate alleviates ADHD symptoms by increasing extracellular dopamine and noradrenaline. Kappa opioid receptor (KOR) antagonism may be another mechanism to achieve the same results because KOR activation inhibits frontal cortical dopamine release. We administered the selective KOR antagonist norbinaltorphimine (norBNI) (20 mg/kg; intraperitoneal) or methylphenidate (0.75 mg/kg; intraperitoneal) to PNE mouse model and examined frontal cortical monoamine release, attention, and working memory. Both compounds increased dopamine and noradrenaline release but neither influenced serotonin release. Both compounds improved object-based attention and working memory in the PNE group, with norBNI's effects evident at 2.5 h and 5.5 h but absent at 24 h. Methylphenidate's effects were evident at 0.5 h but not at 2.5 h. norBNI's effects temporally coincided with frontal cortical c-Jun N-terminal kinase phosphorylation. norBNI did not alter tissue dopamine content in the nucleus accumbens, offering preliminary support for lack of reinforcement.

Kappa Opioid Agonist-Induced Diuresis: Characteristics, Mechanisms, and Beyond

Activation of the kappa opioid receptor (KOR) induces antinociception, anti-pruritic activity, diuresis, sedation, and dysphoria. KOR agonist-induced diuresis is characterized as water diuresis, in which water excretion with urine is increased without altering electrolyte excretion. Both centrally and peripherally acting KOR agonists promote diuresis. KOR antagonists block KOR agonist-evoked diuresis suggesting that the diuretic effect is through activation of the KOR. Studies in different experimental animal species and in humans indicate that KOR agonists decrease antidiuretic hormone (ADH) secretion and release from the hypothalamus and posterior pituitary; decrease response to ADH in kidneys; increase renal sympathetic nerve activity; and increase adrenaline, noradrenaline, and dopamine release from the adrenal medulla. The therapeutic potentials of KOR agonists as water diuretics have been studied in animal models of cerebral edema due to ischemia and intracranial mass, hypertension, and cirrhosis. This chapter reviews characteristics, possible mechanisms, as well as therapeutic potentials of KOR agonist-induced diuresis.

The selective κ-opioid receptor antagonist JDTic attenuates the alcohol deprivation effect in rats

The mechanisms behind relapse to ethanol intake in recovering alcoholics are still unclear. The negative reinforcing effects contributing to ethanol addiction, including relapse, are considered to be partly driven by the κ-opioidergic system. As the κ-opioidergic system interacts with the mesolimbic reward pathway, the aim of the study was to clarify the role of nucleus accumbens shell κ-opioidergic mechanisms in relapse to ethanol intake by using the alcohol deprivation effect (ADE) paradigm. The ADE is defined as a transient increase in voluntary ethanol intake after a forced period of abstinence. Male Long-Evans rats were trained to voluntarily consume 10% (v/v) ethanol solution. Ethanol access and deprivation cycles were initiated after stable ethanol intake baselines had been reached and bilateral guide cannulas had been implanted above the nucleus accumbens shell. One cycle consisted of 10 days of 90 min access to ethanol followed by 6 days of ethanol deprivation. The ADE was measured in the beginning of a new cycle. Rats received JDTic, a selective κ-antagonist, either subcutaneously (10 mg/kg) or intra-accumbally (15 µg/site) or, as a reference substance, systemic naltrexone (0.3 mg/kg) before ethanol re-access, and the effects on the ADE were evaluated. Systemic and intra-accumbal JDTic significantly attenuated the ADE on the first day of ethanol re-access, as did systemic naltrexone. Additionally, naltrexone decreased ethanol intake levels. These results suggest that nucleus accumbens shell κ-opioidergic mechanisms may have a role in mediating relapse to ethanol intake. Additionally, κ-antagonism could be a valuable adjunct in ethanol relapse prevention.

Discrepancies in Kappa Opioid Agonist Binding Revealed through PET Imaging

Kappa opioid receptor (KOR) modulation has been pursued in many conceptual frameworks for the treatment of human pain, depression, and anxiety. As such, several imaging tools have been developed to characterize the density of KORs in the human brain and its occupancy by exogenous drug-like compounds. While exploring the pharmacology of KOR tool compounds using positron emission tomography (PET), we observed discrepancies in the apparent competition binding as measured by changes in binding potential (BP_ND, binding potential with respect to non-displaceable uptake). This prompted us to systematically look at the relationships between baseline BP_ND maps for three common KOR PET radioligands, the antagonists [¹¹C]LY2795050 and [¹¹C]LY2459989, and the agonist [¹¹C]GR103545. We then measured changes in BP_ND using kappa antagonists (naloxone, naltrexone, LY2795050, JDTic, nor-BNI), and found BP_ND was affected similarly between [¹¹C]GR103545 and [¹¹C]LY2459989. Longitudinal PET studies with nor-BNI and JDTic were also examined, and we observed a persistent decrease in [¹¹C]GR103545 BP_ND up to 25 days after drug administration for both nor-BNI and JDTic. Kappa agonists were also administered, and butorphan and GR89696 (racemic GR103545) impacted binding to comparable levels between the two radiotracers. Of greatest significance, kappa agonists salvinorin A and U-50488 caused dramatic reductions in [¹¹C]GR103545 BP_ND but did not change [¹¹C]LY2459989 binding. This discrepancy was further examined in dose-response studies with each radiotracer as well as in vitro binding experiments.

Potent and Selective Tetrahydroisoquinoline Kappa Opioid Receptor Antagonists of Lead Compound (3 R)-7-Hydroxy- N-[(1 S)-2-methyl-1-(piperidin-1-ylmethyl)propyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (PDTic)

Past studies have shown that it has been difficult to discover and develop potent and selective κ opioid receptor antagonists, particularly compounds having potential for clinical development. In this study, we present a structure-activity relationship (SAR) study of a recently discovered new class of tetrahydroisoquinoline κ opioid receptor antagonists which led to (3 R)-7-hydroxy- N-{(1 S)-2-methyl-1-[(-4-methylpiperidine-1-yl)methyl]propyl}-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (12) (4-Me-PDTic). Compound 12 had a K_e = 0.37 nM in a [³⁵S]GTPγS binding assay and was 645- and >8100-fold selective for the κ relative to the μ and δ opioid receptors, respectively. Calculated log BB and CNS (central nervous system) multiparameter optimization (MPO) and low molecular weight values all predict that 12 will penetrate the brain, and pharmacokinetic studies in rats show that 12 does indeed penetrate the brain.

Potent and Selective Tetrahydroisoquinoline Kappa Opioid Receptor Antagonists of Lead Compound (3 R)- N-[1 R)-1-(Cyclohexylmethyl)-2-methylpropyl]-7-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (CDTic)

Animal pharmacological studies suggest that potent and selective κ opioid receptor antagonists have potential as pharmacotherapies targeting depression, anxiety, and substance abuse (opiates, alcohol, nicotine, cocaine). We recently reported lead compound 1 as a new class of κ opioid receptor antagonists with only one basic amine group. Analogues were synthesized and evaluated for their in vitro opioid receptor antagonist properties using a [³⁵S]GTPγS binding assay. All analogues were pure opioid receptor antagonists with no agonist activity. Compounds 1, 8, 9, 13, and 14 ( K_e values 0.058-0.64 nM) are highly potent and highly selective for the κ relative to the μ and δ opioid receptors. Favorable calculated physiochemical properties were confirmed in rat PK studies, demonstrating brain penetration for selected compounds 1, 9, and 13. High κ opioid receptor potency and selectivity and highly favorable calculated physiochemical and PK properties for brain penetration suggest these compounds should be considered for further development.