This is a Phase 1b Extension Trial to allow repeat intracerebroventricular injections of RB-ADSCs in subjects previously treated in and successfully completed RBI Protocol RB-ADSC-02. In the previous Phase 1 clinical trial, RB-ADSC-02, subjects with mild to moderate Alzheimer's disease (AD) received a single intraventricular injection of RB-ADSC. RB-ADSC will be delivered intracerebroventricularly every 2 months via the previously implanted Ommaya reservoir for up to 6 injections in total. The primary objective of safety is performed 2 months after the last dose administration at the month 12 follow-up visit. The secondary objective endpoint evaluations of efficacy are performed at the month 6 and 12 visits.