In vivo antitumor activity and induction of insulin-like growth factor-1-resistant apoptosis by SC-alphaalphadelta9

Abstract

We previously showed that SC-alphaalphadelta9 (4-(benzyl-(2-[(2, 5-diphenyl-oxazole-4-carbonyl)-amino]-ethyl)-carbamoyl)-2-decanoylami no butyric acid) is a novel antiphosphatase agent that selectively inhibits the growth of transformed cells in culture and affects elements of insulin-like growth factor-1 (IGF-1) signaling. We now show that SC-alphaalphadelta9 induces IGF-1-resistant apoptosis and kills tumor cells in vivo. In cultured murine 32D cells, SC-alphaalphadelta9 induced concentration-dependent apoptosis that was blocked by ectopic Bcl-2 expression. No apoptosis was detected in 32D cells treated with the congener SC-alpha109, which lacks the ability to disrupt IGF-1 signaling. After interleukin-3 withdrawal or etoposide treatment, exogenous IGF-1 prevented apoptosis and elevated levels of Cdc2, a biochemical indicator of a functional IGF-1 receptor pathway. In contrast, exogenous IGF-1 did not prevent apoptosis or loss of Cdc2 expression caused by SC-alphaalphadelta9. Furthermore, IGF-1 receptor overexpression failed to protect cells against SC-alphaalphadelta9-induced apoptosis. Kinetic analyses demonstrated that Cdc2 down-regulation after SC-alphaalphadelta9 treatment preceded both apoptosis and loss of the IGF-1 receptor, indicating that loss of Cdc2 was a direct effect of SC-alphaalphadelta9 treatment and not secondary to cell death. IGF-1 receptor autophosphorylation studies indicated that SC-alphaalphadelta9 did not interact directly with the IGF-1 receptor nor bind to the growth factor itself, suggesting a site of action distal to the IGF-1 receptor. In the SCCVII murine tumor model, a single i.p. injection of SC-alphaalphadelta9 caused a dose-dependent decrease in clonogenic cell survival. The IC(50) of SC-alphaalphadelta9 was 35 mg/kg, comparable to 25 mg/kg carboplatin. The ability to induce IGF-1-resistant apoptosis distinguishes SC-alphaalphadelta9 from other apoptosis-inducing agents and suggests compounds of this class deserve further study as potential anticancer agents.