Compositions and methods are provided in embodiments directed to maintaining a desired concentration range of one or more drugs in a subject, and in particular embodiments to maintaining a desired drug concentration in a body compartment in a subject, based on the surprising discovery that antibodies persist in solution in body compartments such that antibody-antigen equilibrium principles can counteract drug clearance mechanisms. One or more antibodies are selected that have a dissociation constant KD that is similar to the desired drug concentration, wherein KD is independent of the affinity of the drug for a specific drug target (receptor) in the subject.