The present invention concerns methods and compositions for delivery of therapeutic agents to target cells, tissues or organisms. In preferred embodiments, the therapeutic agents are delivered in the form of therapeutic-loaded polymers that may comprise many copies of one or more therapeutic agents. In more preferred embodiments, the polymer may be conjugated to a peptide moiety that contains one or more haptens, such as HSG. The agent-polymer-peptide complex may be delivered to target cells by, for example, a pre-targeting technique utilizing bispecific or multispecific antibodies or fragments, having at least one binding arm that recognizes the hapten and at least a second binding arm that binds specifically to a disease or pathogen associated antigen, such as a tumor associated antigen. Methods for synthesizing and using such therapeutic-loaded polymers and their conjugates are provided.