Tumor volume of MCF-7 (a) and triple-negative MDA-MB-231 (b) human breast cancers xenografted into nude mice. The mice were treated without (control 1), with 25 nmol of GnRH-I agonist Triptorelin (control 2) or with 25 nmol of GnRH-II antagonist [Ac-D2Nal1, D-4Cpa2, D-3Pal3,6, Leu8, D-Ala10]GnRH-II. Treatment was repeated every 2 days. Tumor volumes were measured on days 7, 11, 16, and 21 of treatment (MCF-7; (a) or on days 4, 8, 12, and 16 of treatment (MDA-MB-231; (b)). The mice were killed after 21 (MCF-7 (a)) or 16 (MDA-MB-231 (b)) days. Each experimental group consisted of five animals. Vertical bars represent SEM. (a) P < 0.001 versus control; (b) P < 0.05 versus control.