|Lintuzumab is a monoclonal antibody directed against CD33, a surface antigen found on myeloid leukemia cells. A radiolabelled version of this antibody has been developed, ACTIMAB-A®, which is labelled with actinium-225 (225Ac) to increase the antibody's potency (see Patent US6670456 ). 225Ac emits α particles and is used as these cause less nonspecific cytotoxicity than β particles (ie decreased killing of bystander cells). 225Ac has been termed a 'nano-generator' as for each decay producing a high-energy α particle, a series of daughter atoms generate additional α particles, effectively amplifying the radioactive dose at the target site. Lintuzumab is a humanized version of the murine anti-CD33 antibody, M195 .