Optimisation and characterisation of Pheroid® entrapped radio tracer formulations

Abstract

Pheroid® is a lipid-based drug delivery system which has been shown to entrap, protect and deliver hydrophilic, hydrophobic and amphiphilic compounds across biological membranes. As a result Pheroid® has the advantage of increasing gastrointestinal absorption of drugs. Pheroid® could provide a means of oral delivery for drugs such as 99mTc-MDP (99mTechnetium methylene diphosphonate), an intravenously administered radiotracer used during bone scans. Oral administration of Pheroid® 99mTc-MDP was shown in rats. This suggested Pheroid® may enhance bioavailability of radiopharmaceuticals potentially allowing an alternative to intravenous administration. Thus increasing safety for medical staff as well as patient compliance. Due to instability and loss of activity, formulations containing radiotracers are prepared shortly before administration. For clinical applications an optimised formulation with99mTc-MDP entrapment/association in Pheroid® within 2-6 hrs. is required. Formulation parameters of Pheroid® were systematically altered and the effects studied using particle size distribution and ?-potential as indication of stability. Formulations showing early stability were selected for further investigation. Light microscopy was used to confirm morphology while reflectance microscopy was used to qualitatively assess entrapment within or association of Sn-MDP with Pheroid®. Formulations displaying suitable entrapment were selected for quantitative determination of entrapment efficiencies utilizing a novel methodology. Entrapment efficiency was measured by incubating Pheroid-99mTc-MDP formulations on hydroxyapatite (HAP), as in vitro bone model. In the absence of metabolic function any amount of 99mTc-MDP unable to adsorb to the synthetic hydroxyapatite media may be considered to be associated with/entrapped within Pheroid® structures. Although stable formulations are necessary for pharmaceuticals, this study highlighted inadequacies of standard stability measurements as an indication of entrapment efficiency. Insight into the effect of certain formulation parameters on formulation stability as well as an alternative method for determining entrapment efficiencies for certain classes of compounds in Pheroid® formulations are highlighted. Two formulations were identified for further investigation for clinical application: Pheroid® vesicles with 99mTc-MDP added to aqueous phase (N2O-H2O) before preparation and Pre-prepared Pheroid® vesicles; 99mTc-MDP added 4 days after formulation. The latter having the advantage of ease of preparation within clinical settings. All experiments were conducted in triplicate and one way analysis of variance (ANOVA) was used to determined reproducibility, applying a significance level α = 0.05.