Fig. 1: Concept of the AG10-based approach to limit BBB penetration of therapeutic molecules.

Lipophilic mu-opioid receptor antagonists (e.g., naloxone) are hypothesized to reverse opioid-induced constipation (OIC) caused by the interaction of morphine (opioid agonist) with mu-opioid receptors in the gastrointestinal (GI) tract. However, these antagonists can also cross the blood-brain barrier (BBB) and displace morphine from the mu-opioid receptors in the brain, which results in reversing the intended analgesic effect of morphine. By conjugating naloxone to the hydrophilic derivative of AG10, we generated an AG10-Naloxone conjugate that has limited penetration across the BBB. The high peripheral selectivity of the AG10-Naloxone conjugate to the mu-opioid receptors in the GI tract is attributed to the high polarity of the AG10 ligand and reversible binding to the abundant plasma protein, transthyretin (TTR). Increasing the hydrophilicity of molecules is typically associated with fast renal excretion and shorter in vivo half-life. Unlike other approaches, the binding of our hydrophilic AG10 conjugates to TTR in plasma (TTR half-life is ~2 days) results in an extended circulation half-life. Because of their high selectivity to the peripheral tissues, the AG10-Naloxone conjugate should reverse the action of opioid agonists (e.g., morphine) in the GI tract without compromising the analgesic effect of opioid agonists in the brain or precipitating opioid-related withdrawal symptoms. In addition, direct intracerebroventricular administration of the AG10-Naloxone conjugate in the brain will allow for evaluating the role of mu-opioid receptors in the brain in causing analgesia and constipation, without any effect on mu-opioid receptors in the periphery.