You understand the request, agree that you need to perform additional experiments, and know a valid alternative to the suggested animal experiment.
Offer an alternative option to the suggested animal experiment with a nonanimal methodology and describe the plan and the reasons why this approach will provide results that are more relevant.
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, but we believe we can achieve results that are more relevant to the research question by using nonanimal methods. Broadly speaking, the translatability of experiments on animals to findings in humans is much lower than was previously thought. Fundamental differences in physiology and genetics among species, critical disparities between human disease and the animal models created to study them, inherent effects of inbreeding and life in a laboratory, and the inability of animals to mimic human life histories and comorbidities severely limits the applicability of experiments on animals to the human-relevant questions we sought to answer. Specifically, [add information about poor translatability of animal models in the focus area of the study; check our Reference Library for potentially useful studies].
Additionally, there are legal and regulatory mandates to reduce and replace animal use where possible. For these reasons, we plan to add the following experiment instead of performing an experiment on animals: [describe the nonanimal experiment you will perform].
Therefore, we accept the reviewer’s request to [validate/verify/replicate] our results but will do so by the methods described above.