You understand and agree with the request, but there is no alternative to the animal experiment suggested.
Explain how the suggested experiment would be valuable but that due to the lack of an appropriate model, it is infeasible.
For example:
A suggestion that we would like to take great care to address is the request that we perform experiments on animals in order to [validate/verify/replicate] our findings. The reviewer suggested that we conduct in vivo experiments in animals for this purpose. Being cautious of the legal and regulatory mandates to reduce and replace animal use where possible, and the increased availability and scientific relevance of alternative methods, we have been thorough in looking for any alternative to the animal experiment suggested by the reviewer. Unfortunately, we did not find an alternative and thus believe this request to be infeasible.
For this reason, we respectfully decline the reviewer’s request to perform experiments on animals and hope that the editors will agree with this decision.