Vol. 298, Issue 2, 848-856, August 2001

Pharmacodynamic and Pharmacokinetic Characterization of Poly(Ethylene glycol) Conjugation to Met-Enkephalin Analog [D-Pen²,D-Pen⁵]-enkephalin (DPDPE)

Ken A. Witt, Jason D. Huber, Richard D. Egleton, Michael J. Roberts, Michael D. Bentley, Lihong Guo, Hongbing Wei, Henry I. Yamamura and Thomas P. Davis

Department of Pharmacology, University of Arizona, College of Medicine, Tucson, Arizona (K.A.W., J.D.H., R.D.E., H.W., H.I.Y., T.P.D.); and Shearwater Corporation, Huntsville, Alabama (M.J.R., M.D.B., L.G.)

Poly(ethylene glycol), or PEG, conjugation to proteins and peptides is a growing technology used to enhance efficacy of therapeutics. This investigation assesses pharmacodynamic and pharmacokinetic characteristics of PEG-conjugated [D-Pen²,D-Pen⁵]-enkephalin (DPDPE), a met-enkephalin analog, in rodent (in vivo, in situ) and bovine (in vitro) systems. PEG-DPDPE showed increased analgesia (i.v.) compared with nonconjugated form (p < 0.01), despite a 172-fold lower binding affinity for the -opioid receptor. [¹²⁵I]PEG-DPDPE had a 36-fold greater hydrophilicity (p < 0.01) and 12% increase in the unbound plasma protein fraction (p < 0.01), compared with [¹²⁵I]DPDPE. [¹²⁵I]PEG-DPDPE had a 2.5-fold increase in elimination half-life (p < 0.01), 2.7-fold decrease in volume of distribution (p < 0.01), and a 7-fold decrease in plasma clearance rate (p < 0.01) to [¹²⁵I]DPDPE. Time course distribution showed significant concentration differences (p < 0.01) in plasma, whole blood, liver, gallbladder, gastrointestinal (GI) content, GI tract, kidneys, spleen, urine, and brain (brain, p < 0.05), between the conjugated and nonconjugated forms. Increased brain uptake of [¹²⁵I]PEG-DPDPE corresponded to analgesia data. [¹²⁵I]PEG-DPDPE in brain was shown to be 58.9% intact, with 41.1% existing as [¹²⁵I]DPDPE (metabolite), whereas [¹²⁵I]DPDPE was 25.7% intact in the brain (at 30 min). In vitro P-glycoprotein affinity was shown for [¹²⁵I]DPDPE (p < 0.01) but not shown for [¹²⁵I]PEG-DPDPE. In vitro saturable uptake, with 100 µM DPDPE, was shown for [¹²⁵I]PEG-DPDPE (p < 0.05). In this study, PEG-conjugated DPDPE seems to act as a prodrug, enhancing peripheral pharmacokinetics, while undergoing hydrolysis in the brain and allowing nonconjugated DPDPE to act at the receptor.