In vivo fate of unimers and micelles of a poly(ethylene glycol)-block-poly(caprolactone) copolymer in mice following intravenous administration

Abstract

Methoxy poly(ethylene glycol)-b-poly(caprolactone) (MePEG-b-PCL) copolymers with varying PEG block lengths and a constant PCL block length were synthesized by cationic ring-opening polymerization and used to form nano-sized micelles. Due to their small size and superior in vitro stability, the MePEG₅₀₀₀-b-PCL₅₀₀₀ micelles were selected for further in vitro characterization and an in vivo evaluation of their fate and stability following intravenous (i.v.) administration. Specifically, ₃H-labelled MePEG₅₀₀₀-b-PCL₅₀₀₀ micelles were i.v. administered to Balb/C mice at copolymer doses of 250, 2 and 0.2 mg/kg in order to examine the distribution kinetics of (1) copolymer assembled as thermodynamically stable micelles, (2) copolymer assembled as thermodynamically unstable micelles and (3) copolymer unimers, respectively. Overall, it was found that when the copolymer is assembled as thermodynamically stable micelles the material is effectively restricted to the plasma compartment. Interestingly, the copolymer was found to have a relatively long circulation half-life even when administered at a dose that would likely fall to concentrations below the CMC following distribution. Analysis of plasma samples from this group revealed that even 24 h post-administration a significant portion of the copolymer remained assembled as intact micelles. In this way, this study demonstrates that the hydrophobic and semi-crystalline nature of the PCL core imparts a high degree of kinetic stability to this micelle system.

Introduction

In aqueous media, amphiphilic block copolymers self-assemble to form micelles when the copolymer concentration is at or above the critical micelle concentration (CMC). At concentrations below the CMC the copolymer is present in solution as unimers [1]. Hydrophobic (e.g. paclitaxel) or amphiphilic (e.g. amphotericin B) drugs may be incorporated into the copolymer micelles as a means to formulate or deliver these drugs [2], [3], [4], [5], [6]. These formulations have been studied quite extensively with many reports on the in vivo fate of the drug following i.v. administration in micelles [3], [6], [7], [8], [9], [10], [11]. However, only a few studies have examined the in vivo fate of the copolymer micelles [6], [12], [13], [14], [15], [16] and unimers [6], [13], [15]. Specifically, Burt et al. studied the in vivo fate of paclitaxel and MePEG-b-poly(d,l-lactide) (MePEG-b-PDLLA) copolymers in rats following i.v. administration of a MePEG-b-PDLLA micelle formulation of this drug [6]. The in vivo pharmacokinetics and biodistribution for micelles prepared from tyrosine (Tyr)-PEG-b-PDLLA and tyrosyl-glutamic acid (Tyr-Glu)-PEG-b-PDLLA copolymers were also investigated by Kataoka’s group [13].

To this point, the effect of the administered copolymer dose on the in vivo behavior of micelles as well as the fate of copolymer unimers and micelles remains relatively unexplored. One of the only studies that has examined the influence of copolymer dose on the distribution kinetics of block copolymers was recently reported by Kabanov’s group [15]. Their study evaluated the pharmacokinetics and biodistribution of the Pluronic^® triblock copolymer P85 following administration of copolymer concentrations ranging from below to above the CMC of the material after the dilution that occurs upon administration [15].

Additional studies on the in vivo behavior and fate of other copolymer unimer and micelle systems are necessary as they will ensure full exploitation of block copolymer micelles as a viable drug formulation strategy. In this study, MePEG-b-PCL copolymers of varying PEG block lengths (i.e. M_n = 20,000, 10,000 and 5000 g/mol) and a constant PCL block length (i.e. M_n = 5000 g/mol) were synthesized, characterized and the physico-chemical properties of micelles formed from this series of copolymers were evaluated. The MePEG₅₀₀₀-b-PCL₅₀₀₀ micelle system was selected for an in vivo evaluation of copolymer fate and stability following i.v. administration due to its small size and superior in vitro stability. Specifically, the pharmacokinetics and biodistribution profiles of the copolymer were evaluated in mice following i.v. administration of three distinct doses of the copolymer.