“The repeat-dose toxicity studies for relugolix were conducted in mice (up to 13 weeks), rats (up to 26-weeks) and monkeys (up to 39 weeks) using the intended oral route of administration.”
✓ VerifiedFDA multi-discipline review, p. 33Source ↗
Why that species
Monkey: high GnRH receptor homology (97.5%) and similar affinity and potency at the GnRH receptor
“The cynomolgus monkey was selected as the non-rodent species because of the high sequence homology with the human GnRH receptor (97.5% compared to 92.1% in dog) and the similar affinity and potency at the GnRH receptor relative to dogs.”
“In a 39-week study, relugolix (1.5, 5, 15, or 50 mg/kg/day) was administered by oral gavage to cynomolgus monkeys followed by a 13-week recovery period.”