Abstract Application of the HIV protease inhibitor saquinavir (Saq) to cancer chemotherapy is limited by its numerous side effects. To overcome this toxicity, we modified the original compound by covalently attaching a nitric oxide (NO) group. We compared the efficacy of the parental and NO-modified drugs in vitro and in vivo. The novel compound saquinavir-NO (Saq-NO) significantly reduced the viability of a wide spectrum of human and rodent tumor cell lines at significantly lower concentration than the unmodified drug. In contrast to Saq, Saq-NO had no effect on the viability of primary cells and drastically reduced B16 melanoma growth in syngeneic C57BL/6 mice. In addition, at the equivalent of the 100% lethal dose of Saq, Saq-NO treatment caused no apparent signs of toxicity. Saq-NO blocked the proliferation of C6 and B16 cells, up-regulated p53 expression, and promoted the differentiation of these two cell types into oligodendrocytes or Schwann-like cells, respectively. Although it has been well documented that Saq decreases tumor cell viability by inhibiting Akt, the anticancer properties of Saq-NO were completely independent of the phosphatidylinositol 3-kinase/Akt signaling pathway. Moreover, Saq-NO transiently up-regulated Akt phosphorylation, delivering a protective signal that could be relevant for primary cell protection and the absence of drug toxicity in vivo. It was unlikely that released NO was independently responsible for these drug effects because Saq-NO treatment increased intracellular and secreted NO levels only slightly. Rather, the chemical modification seems to have produced a qualitatively new chemical entity, which may have a unique mode of action against cancer cells.[Mol Cancer Ther 2009;8(5):1169–78]