The synthesis of various pyrazino[1′,2′:1,5]pyrrolo[2,3-b]-indole-1,4-dione analogs has been successfully implemented in the present application. From these efforts, compounds having the structure of Formula I-c: or a pharmaceutically acceptable salt, tautomer, or stereoisomer thereof, wherein R1, R2, R4-R8, R3′, R6′, and n are as defined herein, are provided. These biologically active derivatives have been further used to prepare cell-specific drug conjugates effective in treating various diseases including cancer.