AP1903, also known as Rimiducid, is a lipid-permeable tacrolimus analogue with homodimerizing activity. Dimerizer drug AP1903 homodimerizes an analogue of human protein FKBP12 (Fv) which contains a single acid substitution (Phe36Val) so that AP1903 binds to wild-type FKBP12 with 1000-fold lower affinity. This agent is used to homodimerize the Fv-containing drug-binding domains of genetically engineered receptors such as the iCD40 receptor of the autologous dendritic cell vaccine BP-GMAX-CD1, resulting in receptor activation.