Pharmacodynamic and pharmacokinetic characterization of poly(ethylene glycol) conjugation to met-enkephalin analog [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE)

Poly(ethylene glycol), or PEG, conjugation to proteins and peptides is a gr owing technology used to enhance efficacy of therapeutics. This investigati on assesses pharmacodynamic and pharmacokinetic characteristics of PEG-conj ugated [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE), a met-enkephalin analog, in rodent (in vivo, in situ) and bovine (in vitro) systems. PEG-DPDPE showed i ncreased analgesia (i.v.) compared with nonconjugated form (p < 0.01), desp ite a 172-fold lower binding affinity for the <delta>-opioid receptor. [I-1 25]PEG-DPDPE had a 36-fold greater hydrophilicity (p < 0.01) and 12% increa se in the unbound plasma protein fraction (p < 0.01), compared with [I-125] DPDPE. [I-125] 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 [I-125]DPDPE. Time cou rse 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 co njugated and nonconjugated forms. Increased brain uptake of [I-125]PEG-DPDP E corresponded to analgesia data. [I-125]PEG-DPDPE in brain was shown to be 58.9% intact, with 41.1% existing as [I-125]DPDPE (metabolite), whereas [I -125]DPDPE was 25.7% intact in the brain (at 30 min). In vitro P-glycoprote in affinity was shown for [I-125]DPDPE (p < 0.01) but not shown for [I-125] PEG-DPDPE. In vitro saturable uptake, with 100 muM DPDPE, was shown for [I- 125]PEG-DPDPE (p < 0.05). In this study, PEG-conjugated DPDPE seems to act as a prodrug, enhancing peripheral pharmacokinetics, while undergoing hydro lysis in the brain and allowing nonconjugated DPDPE to act at the receptor.