The present invention provides peptides (including analogs and derivatives thereof) corresponding to residues 96-110 and 35-47 of ras-p2 1, which peptides have attached thereto a membrane-penetrating leader sequence. The subject peptides, analogs and derivatives thereof are useful in treatment of cancers and have been shown to induce phenotypic reversion of pancreatic cancer cells to non-cancerous cells. Pharmaceutical compositions comprising one or more subject peptides are also provided by the present invention. The present invention further provides replication incompetent Adenovirus (AdV) vectors comprising a promoter sequence and a nucleotide sequence encoding a subject peptide. Methods of treating cancer by administering one or more subject peptides, pharmaceutical compositions, and/or AdV vectors are also provided.