Malignant mesothelioma is an aggressive and lethal pleural cancer that overexpresses transforming growth factor β (TGFβ). We investigated the efficacy of a novel small-molecule TGFβ type I receptor (ALK5) kinase inhibitor, SM16, in the AB12 syngeneic model of malignant mesothelioma. SM16 inhibited TGFβ signaling seen as decreased phosphorylated Smad2/3 levels in cultured AB12 cells (IC₅₀, ∼200 nmol/L). SM16 penetrated tumor cells in vivo, suppressing tumor phosphorylated Smad2/3 levels for at least 3 h following treatment of tumor-bearing mice with a single i.p. bolus of 20 mg/kg SM16. The growth of established AB12 tumors was significantly inhibited by 5 mg/kg/d SM16 (P < 0.001) delivered via s.c. miniosmotic pumps over 28 days. The efficacy of SM16 was a result of a CD8⁺ antitumor response because (a) the antitumor effects were markedly diminished in severe combined immunodeficient mice and (b) CD8⁺ T cells isolated from spleens of mice treated with SM16 showed strong antitumor cytolytic effects whereas CD8⁺ T cells isolated from spleens of tumor-bearing mice treated with control vehicle showed minimal activity. Treatment of mice bearing large tumors with 5 mg/kg/d SM16 after debulking surgery reduced the extent of tumor recurrence from 80% to <20% (P < 0.05). SM16 was also highly effective in blocking and regressing tumors when given p.o. at doses of 0.45 or 0.65 g/kg in mouse chow. Thus, SM16 shows potent activity against established AB12 malignant mesothelioma tumors using an immune-mediated mechanism and can significantly prevent tumor recurrence after resection of bulky AB12 malignant mesothelioma tumors. These data suggest that ALK5 inhibitors, such as SM16, offer significant potential for the treatment of malignant mesothelioma and possibly other cancers. [Cancer Res 2007;67(5):2351–9]