Peptides and methods of their use for inhibiting drug and radiation-therapy resistance in cancerous cells in which efficacy of chemotherapy and/or radiotherapy of a patient is enhanced by administration of an effective amount of a peptide that inhibits cell adhesion mediated drug resistance (CAM-DR). Preferably, the peptide comprises D-amino acids having the sequence: kmviywkag (RZ-3) or is a variant or modified version thereof. The peptide is preferably administered to the patient prior to chemotherapy and/or radiation therapy. Inhibition of cell adhesion mediated drug resistance (CAM-DR) by RZ-3 in multiple myeloma cells is disclosed.