A multipartite peptide that inhibits release of exosomes in a cell, comprising an N-terminal end and a C-terminal end and comprising at least one secretion modifying region (SMR) peptide from HIV-1 Nef and at least one Clusterin (Clu)-binding peptide (Clu-BP). Pharmaceutical compositions comprising these peptides alone or in synergistic combinations with other active agents in methods for treating cancers and/or infectious diseases are further provided herein.