Tau neurofibrillary tangles are found in the brains of patients suffering from Alzheimer's disease and other tauopathies. The progressive spreading of tau pathology from one brain region to the next is believed to be caused by extracellular transsynaptic transmission of misfolded tau between neurons. Preclinical studies have shown that antibodies against tau can prevent this transfer of misfolded tau between cells. Thus, antibodies against tau have the potential to stop or slow the progression of tau pathology observed in human tauopathies. To test this hypothesis, a humanized anti-tau antibody (ABBV-8E12) was developed and a phase 1 clinical trial of this antibody has been completed. The double-blind, placebo-controlled phase 1 study tested single doses of ABBV-8E12 ranging from 2.5 to 50 mg/kg in 30 patients with progressive supranuclear palsy (PSP). ABBV-8E12 was found to have an acceptable safety profile with no clinically concerning trends in the number or severity of adverse events between the placebo and dosed groups. Pharmacokinetic modelling showed that the antibody has a plasma half-life and cerebrospinal fluid:plasma ratio consistent with other humanized antibodies, and there were no signs of immunogenicity against ABBV-8E12. Based on the acceptable safety and tolerability profile of single doses of ABBV-8E12, AbbVie is currently enrolling patients into two phase 2 clinical trials to assess efficacy and safety of multiple doses of ABBV-8E12 in patients with early Alzheimer's disease or PSP.