Role for the Propofol Hydroxyl in Anesthetic Protein Target Molecular Recognition

Abstract

Propofol is a widely used intravenous general anesthetic. We synthesized 2-fluoro-1,3-diisopropylbenzene, a compound that we call “fropofol”, to directly assess the significance of the propofol 1-hydroxyl for pharmacologically relevant molecular recognition in vitro and for anesthetic efficacy in vivo. Compared to propofol, fropofol had a similar molecular volume and only a small increase in hydrophobicity. Isothermal titration calorimetry and competition assays revealed that fropofol had higher affinity for a protein site governed largely by van der Waals interactions. Within another protein model containing hydrogen bond interactions, propofol demonstrated higher affinity. In vivo, fropofol demonstrated no anesthetic efficacy, but at high concentrations produced excitatory activity in tadpoles and mice; fropofol also antagonized propofol-induced hypnosis. In a propofol protein target that contributes to hypnosis, α1β2γ2L GABAA receptors, fropofol demonstrated no significant effect alone or on propofol positive allosteric modulation of the ion channel, suggesting an additional requirement for the 1-hydroxyl within synaptic GABAA receptor site(s). However, fropofol caused similar adverse cardiovascular effects as propofol by a dose-dependent depression of myocardial contractility. Our results directly implicate the propofol 1-hydroxyl as contributing to molecular recognition within protein targets leading to hypnosis, but not necessarily within protein targets leading to side effects of the drug.

Common general anesthetic propofol impairs kinesin processivity

“…1J), but it contains the 1-hydroxyl that has been linked to molecular recognition within targets that contribute to anesthesia end points. Woll et al (32) synthesized a compound named fropofol, in which the 1-hydroxyl was substituted with fluoride, dramatically reducing the ability to hydrogen bond (Fig. 1K).…”

“…1K). This analog maintains a similar molecular volume as propofol with a small increase in hydrophobicity, and fropofol also binds some of the molecular targets of propofol, such as apoferritin (32). With the 1-hydroxyl substitution, fropofol failed to induce loss of mobility end points in Xenopus laevis tadpoles and mice and does not enhance GABA A receptor activity.…”

“…With the 1-hydroxyl substitution, fropofol failed to induce loss of mobility end points in Xenopus laevis tadpoles and mice and does not enhance GABA A receptor activity. However, fropofol does retain the propofol-like ability to depress myocardial contractility (8,32). Therefore, fropofol can be used to separate the desired from some undesired end points of anesthesia.…”

GABA_A Receptor Modulation by Phenyl Ring Compounds Is Associated with a Water Solubility Cut-Off Value

“…Hence, propofol only modestly inhibits NMDA receptors at an anesthetic EC 50 because the drug is delivered at low concentrations under clinical conditions, resulting in relative receptor specificity. Presumably, other low-affinity receptor interactions with other receptors are possible as well, some of which may result in undesirable effects during anesthesia [6] .…”

“…Although only some alkylphenols bind GABA A receptors with high affinity, many other phenyl ring compounds are still capable of augmenting GABA-induced currents at high concentrations, presumably at low-affinity binding sites [6][7][8] . If low-affinity GABA A binding pockets are functionally analogous to NMDA low-affinity binding pockets, then GABA A receptors should exhibit a molar water solubility cut-off effect just as NMDA receptors do.…”