From an EMA guideline:
This guideline lays down the non-clinical and clinical requirements for monoclonal antibody-containing medicinal products claimed to be similar to another one already authorised, i.e. similar biological medicinal products (biosimilars). The studies described here should be planned with the intention to detect any potential differences between biosimilar and reference medicinal product and to determine the relevance of such differences, should they occur. A biosimilar mAb should be similar to the reference mAb in physicochemical and biological terms. Any observed relevant difference would have to be duly justified and could contradict the biosimilar principle.
Is it acceptable to write biosimilar principle? I think that "biosimilarity principle" is the correct form, and I use "biosimilarity principle" in my translations.