Influence of formulation variables on the biodistribution of multifunctional block copolymer micelles

The physico-chemical characteristics and composition of block copolymer micelles (BCMs) may influence the pharmacokinetics and consequently, the desired delivery characteristics. In this study the influence of formulation variables such as size, density of targeting ligand [i.e. epidermal growth factor (hEGF)] and the bifunctional chelator (BFC) used for labelling the BCMs with (111)In, on the pharmacokinetics and biodistribution in mice were evaluated. BCMs were prepared from Me-PEG(x)-b-PCL(y) (x=2.5 k, y=1.2 k for 15 nm BCMs and x=5 k, y=5 k for 60 nm BCMs) with (targeted, 1 or 5 mol% hEGF) or without (non-targeted) hEGF-PEG(x)-b-PCL(y). To investigate the effect of the BFC on the pharmacokinetics, the BCMs were labelled with (111)In using p-SCN-Bn-DOTA (Bn-DOTA-PEG(x)-b-PCL(y)), H(2)N-DOTA (DOTA-PEG(x)-b-PCL(y)), DTPA anhydride (DTPA-PEG(x)-b-PCL(y)) or p-SCN-Bn-DTPA (Bn-DTPA-PEG(x)-b-PCL(y)). The resulting 15 nm or 60 nm non-targeted or targeted (1 or 5 mol% hEGF) were injected via a tail vein to mice bearing MDA-MB-468 human breast cancer xenograft that overexpress EGFR, followed by pharmacokinetics and biodistribution studies. Pharmacokinetic parameters were determined by fitting the blood concentration vs time data using a two compartment model with i.v. bolus input. Pharmacokinetic parameters were found to depend on BCM size, the BFC used as well as the density of hEGF on the surface of the BCMs. BCMs labelled with p-SCN-Bn-DTPA ((111)In-Bn-BCMs) showed improved pharmacokinetics (i.e. extended circulation lifetime) and tumor uptake compared to those labelled with DOTA-PEG(x)-b-PCL(y), p-SCN-Bn-DOTA or DTPA dianhydride. Formulations with a high density of hEGF (5 mol% hEGF) had short circulation half-lives. BCMs labelled with (111)In via p-SCN-Bn-DTPA showed highest accumulation in the liver and spleen and slower whole body elimination. Smaller sized BCMs were rapidly cleared from the circulation. Increasing the density of hEGF on the surface did not improve tumor uptake due to faster clearance from the circulation. To achieve improved pharmacokinetics and in turn effective exploitation of the EPR effect, p-SCN-Bn-DTPA emerged as the optimal BFC for radiolabelling BCMs while a lower density of hEGF gave more favourable organ distribution.