DOI
https://doi.org/10.25772/KNYJ-KX22
Author ORCID Identifier
https://orcid.org/0000-0001-7491-2860
Defense Date
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Pharmacology & Toxicology
First Advisor
William Dewey
Second Advisor
Matthew Halquist
Abstract
The ongoing opioid crisis constitutes a dire public health threat that has resulted in staggering loss of life. In 2021, opioids were implicated in over 75% of the approximately 107,000 deaths attributed to drug overdose (CDC), and synthetic opioids such as fentanyl and its analogues are key drivers of the surge in opioid fatalities in recent years. Although fentanyl is a μ opioid receptor agonist, it has several distinct attributes compared to other drugs in this category, such as morphine. These include enhanced lipophilicity, heightened potency to induce respiratory depression, more rapid entry into the central nervous system, reduced sensitivity to naloxone rescue after overdose, reduced cross-tolerance to fentanyl even with a previous history of opioid exposure, and promotion of skeletal muscle rigidity, or “wooden chest syndrome,” which increases risk of overdose death. However, there are relatively few extensive comparisons of potential differences in biodistribution between fentanyl and classical opioids such as morphine in mouse models, despite the fact that mice are often used in preclinical studies of parameters relevant to the opioid crisis, i.e. respiratory depression. Therefore, the objective of the present dissertation was to compare acute biodistribution of fentanyl and morphine in blood and 12 murine tissues at doses demonstrated to cause respiratory depression and to gauge potential differences in biodistribution following repeated administration of these opioids. To this end, whole-body plethysmography studies were run to evaluate doses of fentanyl and morphine that produced comparable respiratory depression in male Swiss Webster mice. Then, an LC/MS-MS method was developed to quantify fentanyl, morphine, and select metabolites (norfentanyl and 4-ANPP, or despropionyl fentanyl, and morphine-3-β-D-glucuronide, respectively) in mouse whole blood, whole brain, lung, heart, kidney, small intestine, large intestine, spleen, stomach, muscle, fat, and skin. Afterwards, mice received acute doses of subcutaneous (sc) fentanyl (0.3 mg/kg) or morphine (30 mg/kg) selected based on whole-body plethysmography studies, and samples were collected at 5, 15, 60, and 240 min. A separate cohort received repeated daily injections of these doses for 5 days prior to sample collection 60 min after the last treatment.
The data indicate that, after acute administration, time course of drug distribution varied by tissue, with fentanyl and morphine demonstrating similar time courses in tissues like lung, stomach, and small intestine, but differing in others, like brain and spleen. Moreover, fentanyl exhibits greater distribution out of the blood and into brain, liver, lung, and heart than morphine early after administration and accumulates out of blood into fat at later time points after administration to a greater extent than morphine. Ratios of total drug distribution (expressed as area under the curve) in tissue and blood over the observed acute administration time course suggest that fentanyl accumulation in tissue relative to blood in several regions of the body, such as lung, heart, kidney, spleen, fat, and small intestine, is greater than morphine. These findings indicate that, even though fentanyl’s fatal effects are largely centrally-mediated, this synthetic opioid could potentially have deleterious effects on several organs to a larger degree than morphine, both those involved in respiration and those not directly involved in respiration. The data also suggest that temporary storage of fentanyl in adipose tissue is greater relative to opiates like morphine. Repeatedly-treated mice did not demonstrate tolerance or altered biodistribution compared to drug-naïve mice, implying that repeated fentanyl or morphine exposure that does not induce tolerance is not sufficient to modify tissue distribution. Broadly speaking, this body of work provides an assessment of fentanyl, morphine, and associated metabolite levels in diverse matrices of a model organism widely used for studying physiological and behavioral effects of synthetic opioids. In addition, a useful bioanalytical method was generated for measuring fentanyl concentration in various mouse tissues, which could be applied to other preclinical studies conducting work related to the opioid crisis.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-8-2024