The present invention provides compositions and methods for preparing protein drug conjugate and its application to stem cell therapy and treating cancer cells. The composition is an antibody-binding-therapeutic drug conjugate that binds antibodies targeted to cancer or stem cells. The method of using the composition includes the steps of combining an antibody that targets the cancer cells or stem cells with a sample for a time sufficient to allow said antibody to bind the targeted cells present in said sample and adding the antibody-binding-drug conjugate for a time sufficient to allow binding to the antibody bound to the targeted cells. The presence of the drug reduces the number of or eliminates the targeted cancer cells or stem cells. A similar method is disclosed for selecting antibodies that may be effective antibody drug conjugates.
