Timing the filing of an inter partes review for companies in the early stage of developing a biosimilar product should be carefully considered based on the recent Federal Circuit case, Momenta Pharmaceuticals, Inc. v Bristol-Myers Squibb Co., Appeal No. 2017-1694 (February 7, 2019). Momenta filed an IPR before the PTAB to challenge the validity of a BMS patent covering Orencia® (abatacept) while in its Phase I trials for its own biosimilar product. The PTAB instituted review, conducted trial, and sustained the validity of the BMS patent claims. During this period, however, Momenta’s own product failed its Phase I trials and subsequently decided to terminate its pursuit of the biosimilar to the abatacept product at that time. As an Article III court, the Federal Circuit held that Momenta’s decision to no longer pursue its biosimilar product rendered its standing to appeal the PTAB’s decision moot as there was no judiciable injury. Nevertheless, the question of when to file an IPR remains largely unanswered for a biosimilar manufacturer. Given the expense, duration, and uncertainty of getting approval, biosimilar companies will want to retain its right to appeal an adverse IPR decision. In contrast, an adverse IPR decision for the brand company is retained regardless of the status of the biosimilar. While the Court did not specifically address whether Momenta could have met its standing requirement if it remained actively pursuing its biosimilar, it is clear that the position of brand vs. biosimilar companies vastly contrast as the amici clearly established in this case.  According to BIO and PhRMA, the earliest time for a biosimilar company to have standing is upon the filing of its Biologics Licensing Application under the Biologics Price Competition and Innovation Act. Irrespective of the holding of Momenta, timing the filing of an IPR by a biosimilar company is a decision that may be best addressed by future cases that are directly on point.