HoAcAc Microspheres: In Vitro and In Vivo Stability Studies RESULTS AND DISCUSSION Microsphere characteristics of the HoAcAcMS used in these experiments were in accordance with previous work (12). The microspheres had a holmium load of 45 ± 1%, and a size of 15 μm (97% of the microspheres between 10 and 20 μm) after sieving. As previously observed, no differences in size distribution were observed before and after neutron irradiation, irrespective of irradiation time (12). The in vitro release profile of holmium from the microspheres incubated in phosphate buffer is shown in Fig. 1. As can be observed, the release of holmium was below 0.5% after 6 months. This is comparable to the holmium release (0.7±0.2%) reported for holmium loaded poly(L-lactic acid) microspheres (HoPLLAMS) in the same buffer (16). The HoPLLAMS are considered radiochemically stable and are currently under clinical investigation in a Phase I study for treatment of liver malignancies (20). Electron microscopic inspection of the HoAcAcMS after incubation in phosphate buffer showed clusters of 833 microspheres amidst buffer. Since the surface charge of the HoAcAcMS is expected to be near neutral (21), the microspheres aggregate upon settling due to gravitational forces. The phosphate buffer filled the inter-microsphere spaces which turned into buffer salts upon drying, thereby encapsulating the microspheres. The microspheres remained intact, irrespective of the neutron irradiation time and incubation time (Fig. 2). The surface of the microspheres was still smooth after 26 weeks in buffer. This finding is in contradistinction to neutron irradiated HoPLLAMS incubated in a similar buffer (16) which showed a rough surface after 12 weeks of incubation and started to disintegrate after 24 weeks incubation. This disintegration was attributed to hydrolysis of the poly(L-lactic acid), leading to the formation of holmium lactate (16). The HoAcAcMS were subjected to elemental analysis to determine their composition after incubation in phosphate buffer (Table I). Interestingly, a quantitative exchange of acetylacetonate by phosphate was observed within 4 days of incubation. To confirm the exchange of acetylacetonate by phosphate, IR analysis was performed. The characteristic Fig. 5 Serum enzyme levels (ASAT, ALAT, γ-GT, ALP and albumin) of rabbits that received non-radioactive HoAcAcMS (solid line) and radioactive 166HoAcAcMS (dotted line) in time. Bars represent SD.