Resistance to apoptosis is a hallmark of many solid
tumors, including
pancreatic cancers, and may be the underlying basis for the suboptimal response to chemoradiation
therapies. Overexpression of a family of
inhibitor of apoptosis proteins (IAP) is commonly observed in pancreatic
malignancies. We determined the therapeutic efficacy of recently described small-molecule antagonists of the X-linked IAP (XIAP) in preclinical models of
pancreatic cancer. Primary
pancreatic cancers were assessed for XIAP expression by immunohistochemistry, using a
pancreatic cancer tissue microarray. XIAP small-molecule antagonists ("XAntag"; compounds 1396-11 and 1396-12) and the related compound 1396-28 were tested in vitro in a panel of human
pancreatic cancer cell lines (Panc1, Capan1, and BxPC3) and in vivo in s.c. xenograft models for their ability to induce apoptosis and impede neoplastic growth. In addition,
pancreatic cancer cell lines were treated with XAntags in conjunction with either
tumor necrosis factor-related apoptosis-inducing
ligand (TRAIL) or with radiation to determine potential synergy for such dual targeting of the apoptotic machinery. XIAP was overexpressed in 14 of 18 (77%) of primary
pancreatic cancers. The XAntags1396-11 and 1396-12, but not the inactive isomer 1396-28, induced profound apoptosis in multiple
pancreatic cancer cell lines tested in vitro, with a IC(50) in the range of 2 to 5 mumol/L. Mechanistic specificity of the XAntags for the baculoviral IAP repeat-2 domain of XIAP was shown by preferential activation of downstream "
effector" caspases (caspase-3 and caspase-7) versus the upstream "initiator"
caspase-9. S.c. BxPC3 xenograft growth in athymic mice was significantly inhibited by monotherapy with XAntags; treated xenografts showed marked apoptosis and increased cleavage of
caspase-3. Notably, striking synergy was demonstrable when XAntags were combined with either TRAIL or
radiation therapy, as measured by growth inhibition in vitro and reduced colony formation in soft
agar of
pancreatic cancer cell lines, at dosages where these therapeutic modalities had minimal to modest effects when used alone. Finally, XAntags in combination with the standard-of-care agent for advanced
pancreatic cancer,
gemcitabine, resulted in significantly greater inhibition of in vitro growth than
gemcitabine alone. Our results confirm that pharmacologic inhibition of XIAP is a potent therapeutic modality in
pancreatic cancers. These antagonists are independently capable of inducing
pancreatic cancer cell death and also show synergy when combined with proapoptotic
ligands (TRAIL), with radiation, and with a conventional
antimetabolite,
gemcitabine. These preclinical results suggest that targeting of the apoptotic machinery in
pancreatic cancers with XAntags is a promising therapeutic option that warrants further evaluation.