Poly(L-lactic Acid) Microspheres Containing Neutron-Activatable Holmium-165: A Study of the Physical Characteristics of Microspheres Before and After Irradiation in a Nuclear Reactor

The solvent evaporation technique was employed to prepare poly(L-lactic acid) (PLA) microspheres with ¹⁶⁵Ho acetylacetonate (Ho-AcAc). Particle size, percentage Ho-165, percent residual solvent, and retentive ability of the spheres were found to be strongly affected by preparatory conditions. Differential scanning calorimetry (DSC) thermograms suggested that the Ho-AcAc existed in the PLA matrix as a molecular dispersion. High neutron flux irradiations of the PLA spheres in a nuclear reactor produced Ho-166, a therapeutic radionuclide that emits high-energy negatrons (E_max = 1.84 MeV; half-life = 26.9 hr). The gamma radiation dose (53– 75 Mrad) from the core of the reactor provided an overkill of all bioburdens in the PLA spheres. Gel permeation chromatography (GPC) analysis showed that these irradiations caused a reduction in PLA molecular weight. Infrared spectra, ¹³C NMR spectra, ¹H NMR spectra, and DSC thermograms further confirmed the presence of lower molecular weight PLA but proved the overall maintenance of PLA structure.