the invention relates to particular substituted heterocycle fused gamma - carbolines , their prodrugs , in free , solid , pharmaceutically acceptable salt and / or substantially pure form as described herein , pharmaceutical compositions thereof , and methods of use in the treatment of diseases involving 5 - ht2a receptor , serotonin transporter and / or pathways involving dopamine d1 / d2 receptor signaling systems , and / or the treatment of residual symptoms .