Generation of lead-212 and radioimmunoconjugates for use in cancer therapy and the ‘emanating power’ of stearates

The emanating power of stearates (i.e. their ability to release radioactive gas generated within them) combined with the diffusivity of this gas, permit the spatial separation of Pb-212 from its longer lived and thus undesirable parents. A generator for the α-particle emitter Pb-212 is described. It features two chambers separated by a porous septum. A small chamber is loaded with Th-228 carried by barium stearate and a large one contains a powder, which retains recoiling products of radon. The Pb-212 could be harvested daily for therapeutic use with an initial efficacy of over 60% but radiation damage to the stearate reduced its emanating power after about 3 month of continuos operation. The generator, described here, allows for harvesting ultrapure short-lived products and relatively simple workup procedures. An important assumption concerning the feasability of a Pb-212 complexed radioimmunoconjugate is that the entrapped Pb-212 can survive the Bi-212 recoil and the 2.24 MEV β-emission during the Pb–Bi transition. To test this we produced a monoclonal antibody modified organic chealate intermediate to which the generator produced Pb-212 has been complexed. We were able to show selective tumor cell cytotoxicity and stability of the conjugate under ordinary cell culture conditions.