Mice bearing human breast (MDA-MB-231) tumors receive intravenous injections of NSC 74859 (S3I-201) or a vehicle solution every two or three days over a period of two weeks, with tumor sizes measured every 2-3 days. In contrast to the control group receiving vehicle solution, whose tumors continue to grow, mice treated with S3I-201 exhibit significant inhibition of tumor growth. Further observations post-treatment cessation reveal no signs of tumor regrowth, indicating the potential for a lasting impact of S3I-201 on inhibiting tumor growth^[1].

In comparison to control tumors treated with vehicle solution (n=15), which exhibited continuous growth, S3I-201 treatment on somatotroph tumor xenografts (n=15) markedly reduces tumor growth throughout the study period. Tumors from NSC 74859-treated rats are notably smaller than those in the untreated group (220±16 mm3 vs. 287±16 mm3, P<0.01) as soon as five days following the injection of NSC 74859. By the fifteenth day post-treatment, the average tumor volume in NSC 74859-treated rats is only 64% of that in control animals (449±40 mm3 vs. 708±83 mm3, P<0.01). The experiment concludes with the rats being euthanized and the tumors collected 15 days after beginning treatment, revealing an average tumor weight in NSC 74859-treated rats of 78±8 mg, compared to 114±13 mg in tumors from control rats, a 32% reduction (P<0.05)[3].

NSC 74859 (S3I-201) distinctly suppresses Stat3’s ability to bind DNA more effectively than Stat1, with IC50 values of 86±33 μM for Stat3•Stat3, 160±43 μM for Stat1•Stat3, and more than 300 μM for Stat1•Stat1. It also inhibits Stat5 activity with an IC50 of 166±17 μM. NSC 74859 significantly reduces the number of viable cells and curtails the growth of transformed NIH 3T3/v-Src mouse fibroblasts and breast carcinoma cell lines (MDA-MB-231, MDA-MB-435, and MDA-MB-468). At concentrations of 30-100 μM, NSC 74859 prompts substantial apoptosis in MDA-MB-435 and NIH 3T3/v-Src cells, both of which exhibit constitutively active Stat3. MDA-MB-435 cells show greater sensitivity to 30 μM NSC 74859. Conversely, MDA-MB-453 cells and normal mouse fibroblasts (NIH 3T3), which do not exhibit abnormal Stat3 activity, show less sensitivity to NSC 74859 at concentrations of 100 μM or lower. A dose of 300 μM or higher of NSC 74859 induces broad, nonspecific cytotoxicity regardless of the Stat3 activation status^[1].

Huh-7 cells, lacking expression of β2SP or TBGFR2, are susceptible to STAT3 inhibition by NSC 74859, with an IC50 of 100 μM, irrespective of CD133+ status. The IC50 values for NSC 74859 are 150 μM for Huh-7 and SNU-398 cells, 15 μM for SNU-475 cells, and 200 μM for SNU-182 cells. NSC 74859 inhibits the growth of breast carcinoma cell lines MDA-MB-435, MDA-MB-453, and MDA-MB-231, with an IC50 approximately close to 100 μM^[2].