Strengthening animal welfare: monitoring humane endpoints in a rat model of mammary tumorigenesis undergoing a ladder resistance training protocol.

Breast cancer is the most common cancer in women worldwide, underscoring the need for ethical and effective preclinical models to advance disease management. This study aimed to highlight the importance of humane endpoints in a rat model of mammary carcinogenesis undergoing ladder resistance training to evaluate and ensure the absence of animal suffering. Twenty-eight female Wistar rats were assigned randomly to four groups of seven animals per group: sedentary, sedentary induced (with N-methyl-N-nitrosourea (MNU)), exercised and exercised MNU-induced. At 7 weeks of age, the animals in the induced groups were injected intraperitoneally (i.p.) with the carcinogenic agent (N-methyl-N-nitrosourea; i.p; 50 mg/kg). The animals' welfare was monitored using a set of biological characteristics. The resistance exercise training, started at 9 weeks old and comprised climbing a ladder three times per week for 18 weeks, gradually increasing the load throughout the protocol. Of 14 MNU-induced rats, 9 (64%) developed mammary cancer. Exercised rats tended to show delayed tumor ulceration despite larger tumor volumes. No significant differences in bodyweight were observed, although weight loss appeared earlier in sedentary-induced animals. Exercised rats exhibited more stress signs, including reduced grooming and one case of chromodachryorrhea, yet had lower tumor-related humane endpoint scores. Three rats were euthanised early due to tumor-related endpoints. Minor tail wounds occurred but did not affect exercise or welfare. These results indicate that, with careful endpoint monitoring, the exercise protocol is well tolerated and may improve performance in tumour-bearing rats, supporting ethical standards while allowing accurate investigation of resistance training effects.