Opioid substitution and antiretroviral therapies have steadily increased the life spans of AIDS patients with opioid addiction, while the adverse drug-drug interactions and persistence of HIV-associated neurocognitive disorders still require new strategies to target opioid abuse and HIV-1 comorbidities. A bivalent ligand 1 with a 21-atom spacer was thus synthesized and explicitly characterized as a novel pharmacological probe to study the underlying mechanism of opioid-enhanced NeuroAIDS. The steric hindrance generated from the spacer affected the binding affinity and Ca2+ flux inhibition function activity of bivalent ligand 1 at the chemokine receptor CCR5 more profoundly than it did at the mu opioid receptor (MOR). However, the CCR5 radioligand binding affinity and the Ca2+ flux inhibition function of the ligand seemed not necessarily to correlate with its antiviral activity given that it was at least two times more potent than maraviroc alone in reducing Tat expression upon HIV-1 infection in human astrocytes. Furthermore, the ligand was also about two times more potent than the simple mixture of maraviroc and naltrexone in the same viral entry inhibition assay. Therefore bivalent ligand 1 seemed to function more effectively by targeting specifically the putative MOR–CCR5 heterodimer in the viral invasion process. The results reported here suggest that a properly designed bivalent ligand may serve as a useful chemical probe to study the potential MOR–CCR5 interaction during the progression of NeuroAIDS.

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The bivalent ligand approach has been utilized not only to study the underlying mechanism of G protein-coupled receptors dimerization and/or oligomerization, but also aimed to enhance ligand affinity and/or selectivity for potential treatment of a variety of diseases by targeting this process. Substance abuse and addiction have made both the prevention and the treatment of human immunodeficiency virus (HIV) infection more difficult to tackle. It has been extensively studied that morphine, a mu opioid receptor (MOR) agonist, can accelerate HIV infection through up-regulating the expression of the chemokine receptor CCR5, a well-known co-receptor for HIV invasion to the host cells. Meanwhile, two research groups have described the putative MOR/CCR5 heterodimers in their independent studies. The purpose of this paper is to report the design and synthesis of a bivalent ligand to explore the biological and pharmacological process of the putative MOR/CCR5 dimerization phenomenon. The developed bivalent ligand thus contains two distinct pharmacophores linked through a spacer; ideally one of which will interact with the MOR and the other with the CCR5. Naltrexone and Maraviroc were selected as the pharmacophores to generate such a bivalent probe. The overall reaction route to prepare this bivalent ligand was convergent and efficient, and involved sixteen steps with moderate to good yields. The preliminary biological characterization showed that the bivalent compound 1 retained the pharmacological characteristics of both pharmacophores towards the MOR and the CCR5 respectively with relatively lower binding affinity, which tentatively validated our original molecular design.

1 The opioid antagonists, naloxone, GPA 2163, levallorphan and Mr-2266 reduced the latency of the jumping reaction of mice in the hot plate test. The (+)-isomers of levallorphan and Mr-2266 which are devoid of antagonistic activity did not increase this latency. 2 In the same nociceptive reaction test, the enhancing effect of naloxone progressed in a dose-range similar to that required for the antagonism by naloxone of the depressive action of morphine. 3 The facilitatory effect of naloxone was not blocked by the previous administration of morphine or etorphine but it was prevented by pretreatment with a high dose of buprenorphine. 4 The antagonism by naloxone of morphine and of buprenorphine did not follow the same pattern. 5 The factors which are or may be involved in the efficacy of naloxone in enhancing nociceptive reactions are discussed. 6 The enhancing effect of naloxone may be due to an antagonism of endogenous ligands for the opiate receptor. If so, these ligands would be involved in reaction to but not in perception of nociceptive stimuli which need not be harmful ones.

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This study compared the development of tolerance to two orally bioavailable prescription opioids, oxycodone and hydrocodone, to that of morphine, and the reversal of this tolerance by ethanol. Oxycodone (s.c.) was significantly more potent in the mouse tail-withdrawal assay than either morphine or hydrocodone. Oxycodone was also significantly more potent in this assay than hydrocodone when administered orally. Tolerance was seen following chronic subcutaneous administration of each of the three drugs and by the chronic administration of oral oxycodone, but not following the chronic oral administration of hydrocodone. Ethanol (1 g/kg i.p.) significantly reversed the tolerance to the subcutaneous administration of each of the three opioids that developed when given 30 minutes prior to challenge doses. It took twice as much ethanol, when given orally, to reverse the tolerance to oxycodone. We investigated whether the observed tolerance to oxycodone and its reversal by ethanol were due to biodispositional changes or reflected a true neuronal tolerance. As expected, a relationship between brain oxycodone concentrations and activity in the tail-immersion test existed following administration of acute oral oxycodone. Following chronic treatment, brain oxycodone concentrations were significantly lower than acute concentrations. Oral ethanol (2 g/kg) reversed the tolerance to chronic oxycodone, but did not alter brain concentrations of either acute or chronic oxycodone. These studies show that there is a metabolic component of tolerance to oxycodone; however, the reversal of that tolerance by ethanol is not due to an alteration of the biodisposition of oxycodone, but rather is neuronal in nature.

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