Abstract Melanoma is the most aggressive form of skin cancer with a dismal 5-year survival rate of 16% at the metastatic stage of the disease. Exosomes have recently emerged as a critical mediator of cancer therapy resistance by regulating epithelial to mesenchymal transition (EMT) and drug efflux. Exosomes are extracellular vesicles (diameters 30-150nm) released from almost all types of cells including tumor cells which package and transfer proteins, RNA, and lipids (often called cargo) from one cell to another. This intercellular communication via exosomes has been reported to induce a drug-resistant phenotype across the tumor by transporting ABC multidrug transporters and mediators of EMT. Multiple studies have established that pituitary-, or tumor- or tumor microenvironment-derived growth hormone (GH) promotes EMT and drug resistance in cancer by binding to GHR-expressing tumor cells. A question arises: Does GH exert this effect across the tumor population via modulating tumor-derived exosomal cargo? No study has yet directly assessed the effect of GH on exosomes in terms of modulating EMT or drug resistance. We isolated exosomes from human melanoma cells with differential levels of GHR expression, following treatment with combinations of GH, anti-cancer drugs, and the GHR antagonist, pegvisomant. We observed no change in the quantity of tumor-derived exosomes with changes in GH action. However, we found significant upregulation of multidrug efflux pumps ABCB1, ABCC1, ABCC2, and ABCG2 in GH-treated tumor exosomes. Importantly, treatment of naïve melanoma cells with these exosomes increased their drug efflux rate and reduced drug retention. Exosomes derived from pegvisomant treated melanoma cells showed markedly reduced ABC transporter contents and did not increase the efflux rate of naïve tumor cells. A similar trend in matrix metalloproteinases (MMP2, MMP9) and TGFb levels in isolated exosomes as well as TGFb signaling and invasion capacity in naïve tumor cells treated with isolated exosomes were observed. These results present a unique effect of GH on tumor-derived exosomal content modulation towards propagating a drug-resistant tumor phenotype in naïve cancer cells. Acknowledgment: This work was supported in part by the State of Ohio’s Eminent Scholar Program that includes a gift from Milton and Lawrence Goll, by the AMVETS, and Ohio University’s Student Enhancement Award and Edison Biotechnology Institute. Citation Format: Prateek Kulkarni, Reetobrata Basu, John J. Kopchick. Growth hormone upregulates tumor-derived exosomal distribution of EMT mediators and multi-drug efflux pumps in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1004.