Radioactive holmium acetylacetonate microspheres for interstitial microbrachytherapy: an in vitro and in vivo stability study

Abstract

Purpose: The clinical application of holmium acetylacetonate microspheres (HoAcAcMS) for the intratumoral radionuclide treatment of solid malignancies requires a thorough understanding of their stability. Therefore, an in vitro and an in vivo stability study with HoAcAcMS was conducted.

Methods: HoAcAcMS, before and after neutron irradiation, were incubated in a phosphate buffer at 37°C for 6 months. The in vitro release of holmium in this buffer after 6 months was 0.5%. Elemental analysis, scanning electron microscopy, infrared spectroscopy and time of flight secondary ion mass spectrometry were performed on the HoAcAcMS.

Results: After 4 days in buffer the acetylacetonate ligands were replaced by phosphate, without altering the particle size and surface morphology. HoAcAcMS before and after neutron irradiation were administered intratumorally in VX2 tumor-bearing rabbits. No holmium was detected in the faeces, urine, femur and blood. Histological examination of the tumor revealed clusters of intact microspheres amidst necrotic tissue after 30 days.

Conclusion: HoAcAcMS are stable both in vitro and in vivo and are suitable for intratumoral radionuclide treatment.

Figures

Fig. 1

Release of holmium from HoAcAcMS (diamonds) and 166HoAcAcMS irradiated for either 3 h (triangles) or 6 h (squares) during incubation in phosphate buffer for 6 months. The data are presented as the mean of two measurements.

Fig. 2

Scanning electron micrographs of HoAcAcMS and 166HoAcAcMS after incubation for different times in phosphate buffer. (a–c) HoAcAcMS, (a) after 1 month, (b) after 3 months and (c) after 6 months of incubation. (d–f) 166HoAcAcMS neutron irradiated for 3 h at a neutron flux of 5 × 1012 n cm−2 s−1 (d) after 1 month, (e) after 3 months and (f) after 6 months of incubation. (g–i) 166HoAcAcMS neutron irradiated for 6 h at a neutron flux of 5 × 1012 n cm−2 s−1 (g) after 1 month, (h) after 3 months and (i) after 6 months of incubation. The magnification in all images is 1000x, the bar represents 50 μm.

Fig. 3

TOF-SIMS spectra from control HoAcAcMS (HoAcAcMS) and buffer incubated HoAcAcMS (HoAcAcMS buffer). The micrographs show total ion count images in the negative (−ve) and the positive (+ve) secondary ion mode. The peaks of interest are indicated with an arrow.

Fig. 4

(a) Schematic outline of rabbit anatomy on X-ray CT image (L=liver, T=tumor, S=stomach, I=intestine). (b) X-ray CT image of rabbit before intratumoral administration of 166HoAcAcMS showing the tumor, (c) X-ray CT image of rabbit after intratumoral administration of 166HoAcAcMS showing the selective deposition in the tumor as white artifacts, (d) X-ray CT image 4 weeks after administration, before termination, the white area in the tumor is still present as a cluster, although the shape has changed.

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.

Fig. 6

Light micrographs at 400x magnification of a H&E stained section of tumor tissue showing microspheres amidst necrotic tissue. (a) non-radioactive HoAcAcMS. (b) 166HoAcAcMS. The microspheres in red are clustered amidst necrotic tissue. The bar represents 20 μm.