P-065 - (Poster #65) Fentanyl’s Physicochemistry Influences its Unique Pharmacology and Abuse

Background: Fentanyl has emerged as the most widely-abused opioid.  It seems appropriate and important to ask the question why fentanyl, of the large number of other opioids, has evolved to this status. One explanation could be that the affinity or intrinsic activity of fentanyl at the mu-opioid receptor (MOR) is greater than other opioids – but that is not the case. In in vitro assays using membrane preparations, the affinity of fentanyl for MOR (± Na+ ion in buffer) is about the same as the affinity of morphine, and in assays testing for MOR activation (GTPgS binding) the potency of fentanyl is less than 2-fold that of morphine. In contrast, fentanyl is significantly more potent than is morphine in intact cells and in in vivo tests. It has recently been reported (Sutcliffe et al., 2022) that the physiochemistry of fentanyl allows it to access the orthosteric MOR site via a novel – intramembrane – route.

Purpose/Objectives: To review recent publications that have hypothesized that the lipophilic physiochemistry of fentanyl allows it, in contrast to most other opioids, which are lipophobic, to access the orthosteric MOR binding site via an intramembrane route, and to assess the potential impact of this characteristic on fentanyl’s clinical pharmacology and potential for abuse.  

Methods: A search was conducted of published literature using Internet sites such as PubMed, MEDLINE and other sources dating back to the discovery of xylazine. Search terms included “fentanyl” alone and in combinations with “pharmacology”, “receptor(s)”, “in vitro”, “affinity”, “intrinsic activity”, “efficacy”, “potency”, “MOR”, “lipophilicity”, “in vivo”, “orthosteric”, “allosteric”, etc. in order to identify publications that contained possibly relevant basic science information. Studies published in the English language (or translations where available) were included. Additional sources were obtained or suggested from the references within the primary sources. Only peer-reviewed publications were used for integration into the review.

Results: The chemistry and molecular actions of opioids are typically quite straightforward.  Standard opiates and opioids are analogs of 4,5-epoxymorphinans (e.g., the opiates morphine and codeine, and the opioids oxycodone, hydrocodone, oxymorphone, hydromorphone, and buprenorphine).  Although fentanyl’s chemical structure belongs to a different structural class (phenylpiperidines), it has long been assumed that its molecular pharmacology is similar to other opioids.  However, its potency in comparison to other opioids increases when intact cells are used in in vitro assays, and in in vivo tests, where it is up to 100-fold more potent.  One contributor to the difference relates to fentanyl’s high lipid solubility, which allows for rapid transport across the blood-brain-barrier.  A recent publication (Sutcliffe et al., 2022) proposed that its high lipid solubility and flexible elongated chemical structure (six rotatable bonds), in addition to a central protonatable nitrogen facilitates a novel binding process to MOR.  The total thermodynamic free-energy change is nearly the same for fentanyl and morphine.  Of note, the difference from morphine as reference is evidenced by: different partition into the lipid bilayer; reassertion of action after washout of extracellular ligand molecules; binding to MOR via two separate access routes (both from the extracellular phase, and from the lipid phase); and resistance to reversal by naloxone, which blocks the “canonical” pathway (binding of agonist directly via the extracellular pathway).  The transmembrane route of access to MOR provides a mechanistic explanation of the unique pharmacology of fentanyl compared to other opioids (Kelly et al., 2021).

Conclusions/Implications for future research and/or clinical care: The molecular pharmacology of fentanyl is usually described as being straightforward: it is a full agonist at MOR.  And its abuse and adverse effects (including overdose) are typically described with this simple mechanism of action in mind. The surprising finding revealed in our review is that fentanyl, far from having the typical interaction with MOR, has an additional transmembrane access to the orthosteric MOR binding pocket due to its atypical physicochemical property of high lipophilicity. Thus, fentanyl’s physicochemistry underlies its unique in vivo pharmacology, including lower susceptibility to reversal by naloxone (Hill et al., 2020).