ABSTRACT The dementia of Alzheimer's disease (AD) is caused by the deposition of A¿ amyloid in brain over many years. Once the amyloid plaque forms in brain, it is permanent in the absence of plaque disaggregation therapy. The most potent form of plaque diasaggregation therapy is a monoclonal antibody (MAb) against the A¿ amyloid peptide of AD, and passive immune therapy of AD is currently being tested in clinical trials. However, the anti- A¿ MAb does not cross the blood-brain barrier (BBB). Consequently, very high doses of MAb must be administered to lower plaque in brain of AD transgenic mice, and these high doses causes brain microhemorrhage. The present research will genetically engineer a new form of anti-A¿ MAb that is enabled to cross the BBB via receptor- mediated transport in both the blood-to-brain and brain-to-blood directions. The PI has previously genetically engineered a chimeric MAb against the mouse transferrin receptor (TfR) that crosses the BBB in the blood-to-brain direction via receptor-mediated transport on the mouse BBB TfR, and also crosses the BBB in the brain-to-blood direction on the BBB Fc receptor (FcR). In addition, the PI has genetically engineered, and expressed a single chain Fv (ScFv) antibody against the amino terminal portion of the A¿ peptide. The present research will produce a new fusion protein, wherein the anti-A¿ ScFv is fused to the carboxyl terminus of the chimeric MAb against the mouse TfR, and this new fusion protein is designated the TfRMAb-A¿ScFv bi-specific antibody (BSA). Following the genetic engineering of the new BSA, the protein will be transiently expressed in COS cells, and the bi- functionality of the BSA will be demonstrated with a mouse TfR binding assay and an A¿ binding assay. The BSA will then be permanently expressed in Chinese hamster ovary (CHO) cells, followed by selection, and dilutional cloning, and purification with affinity and cation exchange chromatography. The purified BSA will then be used to treat APPswe/PSEN1(dE9) double transgenic mice over 3 month period. Control mice will be treated with either saline or with the conventional high dose anti-A¿ MAb that does not cross the BBB. The mice will be evaluated for neurobehavior, plasma A¿ levels, brain A¿ plaque content, and brain micro-hemorrhage using Prussian blue histochemistry. This research will provide the necessary pre-clinical pharmacology to support an IND filing for the treatment of humans with AD using genetically engineered fusion antibodies that both cross the BBB via receptor-mediation and bind and disaggregate A¿ amyloid plaque of AD. RELEVANCE TO PUBLIC HEALTH The dementia of Alzheimer's disease (AD) is caused by the deposition of A¿ amyloid in brain over many years, and once the amyloid plaque forms in brain, it is permanent in the absence of plaque disaggregation therapy. The most potent form of plaque diasaggregation therapy is a monoclonal antibody (MAb) against the A¿ amyloid peptide of AD, and passive immune therapy of AD is currently being tested in clinical trials; however, the anti- A¿ MAb does not cross the blood-brain barrier (BBB). The present research will test in AD transgenic mice the efficacy of a new genetically engineered fusion antibody that both crosses the BBB via receptor-mediation and binds and disaggregates A¿ amyloid plaque of AD.