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

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 δ-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.