Polymers

1.2.1.4 Polymers

Polymer-based microspheres have many advantages over other materials, in particular
their near-plasma density, biodegradability and biocompatibility. However, their
major disadvantage is their inability to withstand high thermal neutron fluxes [16].
Additives [13,28] and adjustment of irradiation-parameters [29] can overcome this
problem.
Polymer-based microspheres used for internal radionuclide therapy are mainly
prepared by a solvent evaporation technique. In the solvent evaporation process, the
polymer is dissolved in a suitable water immiscible volatile solvent, and the
medicament is dispersed or dissolved in this polymeric solution. The resulting
solution or dispersion is then emulsified by stirring in an aqueous continuous phase,
thereby forming discrete droplets. In order that the microspheres should form, the
organic solvent must first diffuse into the aqueous phase and then evaporate at the
water/air interface. As solvent evaporation occurs the microspheres harden, and free
flowing microspheres can be obtained after suitable filtration and drying [30]. The
solvent evaporation method has been used for preparation of poly(L-lactic acid)
(PLLA) microspheres containing 166Ho, 90Y and 186Re/188Re.
Mumper et al. [13,28,31] and also our group [29] prepared PLLA microspheres
with holmium-165-acetylacetonate (HoAcAc) [32]. HoAcAc complex and PLLA
were dissolved in chloroform and the solution was added to a polyvinyl alcohol
(PVA) solution and stirred until the solvent had evaporated. Microspheres were
graded and collected according to size, on stainless steel sieves of 20-50 μm. These
microspheres can be dispensed in patient-ready doses, that only need to be activated
by neutron bombardment to a therapeutic amount of radioactivity in a nuclear reactor
[29]. These holmium loaded microspheres are currently being tested by intrahepatic
arterial administration to rat liver tumours (Fig. 1a). A seven-fold increase of the
166Ho-microspheres in and around the tumour compared with normal liver is found
(Fig. 1b), based on distribution of radioactivity.