Release

1.3.3 Release

Improvements in radiolabelling techniques have resulted in increasingly stable 90Y
microspheres. A new generation of glass (TheraSpheres®, Theragenics, Atlanta, GA,
USA) and resin microspheres (supplied by the Australian Nuclear Science and

Technology Organisation) have overcome the problem of leaching. Routine tests on
resin and glass microspheres showed that less than 0.1% of the activity leaches from
the microspheres. Clinical studies on metastatic liver cancer, using these newer glass
[59] and resin microspheres, have produced good results with very little toxicity [23].
Polymeric microspheres loaded with 166Ho showed a high stability. Mumper et al.
and also our group showed that more than 98% of 166Ho activity was retained in the
microspheres after 192h incubation at 37°C, in several physiological media [13,29].
On the one hand, this is a favourable characteristic of these microspheres for the
attempted application, but on the other hand this is surprising, since generally low
molecular weight compounds are released to a high extent and relatively rapidly from
PLLA microspheres [57,60-62]. We found that HoAcAc can be dispersed in the
amorphous PLLA phase up to 17% (w/w) indicating the presence of favourable
PLLA/HoAcAc interactions in the microspheres. Carbonyl groups of PLLA are likely
to interact with the holmium ion in the HoAcAc complex, by which this complex is
immobilized in the PLLA matrix. This interaction thus accounts for the high stability
(= low holmium release) found in HoAcAc loaded microspheres.