Poly(amine-co-ester) polymers, methods of forming active agent-load nanoparticles therefrom, and methods of using the nanoparticles for drug delivery are disclosed. The nanoparticles can be coated with an agent that reduces surface charge, an agent that increases cell-specific targeting, or a combination thereof. Typically, the loaded nanoparticles are less toxic, more efficient at drug delivery, or a combination thereof compared to a control other transfection reagents. In some embodiments, the nanoparticles are suitable for in vivo delivery, and can be administered systemically to a subject to treat a disease or condition.