Cell experiment:

The antiproliferative activity is evaluated after 72 hours of drug exposure by cell counting. Drug concentrations able to inhibit cell proliferation by 50% (IC50) and 20% (IC20) are calculated from dose-response curves.

Animal experiment:

To generate tumor xenografts, exponentially growing cells (A431 and A431/topotecan, 107 cells/mouse; SiHa 2.5 × 107 cells/mouse, Caski 107 cells/mouse) are s.c. injected into the mice flanks. For antitumor activity studies, groups of four/five mice bearing tumor implanted in both flanks are used. Tumor fragments are implanted on day 0, and tumor growth is followed by biweekly measurements of tumor diameters with a Vernier caliper. Tumor volume (TV) is calculated according to the formula TV (mm3) = d2 × D/2, in which d and D are the shortest and the longest diameter, respectively. Treatment starts 5 to 13 days after implant, when the tumors are just palpable, but established (TV = 80-90 mm3). Namitecan, irinotecan, and cetuximab are administered every fourth day for four times. Cetuximab is given 1 hour after each administration of the camptothecin.

Namitecan is a potent topoisomerase I inhibitor, with antitumor property.

Namitecan and cetuximab cooperate in inhibiting EGFR expression. Namitecan induces a dose-dependent decrease in EGFR expression in the different cell lines[1]. ST1968 induces a comparable level of apoptosis in A431 and A431/TPT cells with IC50 of 0.21 and 0.29 μM[2].

Namitecan (10 mg/kg) in combination with cetuximab (1 mg/mouse) induces synergistic antitumor effects in SCC models as a function of EGFR gene copy number[1]. ST1968 (25 mg/kg) causes acceptable body weight loss and no toxic deaths. ST1968 produces a 100% complete response rate in the mice bearing the A431 tumor, and retains a relevant activity in the topotecan-resistant tumor[2].

[1]. De Cesare M, et al. Synergistic antitumor activity of cetuximab and namitecan in human squamous cell carcinoma models relies on cooperative inhibition of EGFR expression and depends on high EGFR gene copy number. Clin Cancer Res. 2014 Feb 15;20(4):995-1006. [2]. Zuco V, et al. Efficacy of ST1968 (namitecan) on a topotecan-resistant squamous cell carcinoma. Biochem Pharmacol. 2010 Feb 15;79(4):535-41.