Inter partes reexamination offers an attractive supplement (or even alternative) to litigation for challenging the validity of a patent. Unlike ex parte reexamination, inter partes reexamination advantageously enables a third party requester to participate in the prosecution, including any appeal.

Inter partes reexamination, however, is not without its risks. The estoppel aspect of the procedure is “the most frequently identified inequity that deters third parties from filing requests for inter partes reexamination of patents.”[1] Specifically, under 35 U.S.C. § 315(c), a third party requester may be estopped in litigation from challenging patent claims on invalidity grounds that were or could have been raised in the course of an inter partes reexamination.

Accordingly, as part of the decision to file an inter partes reexam, a party should carefully consider potential estoppels that could attach in a later district court proceeding. Such an inquiry can be difficult, as the statutes, rules, regulations, and related case law do not clearly define Section 315(c)'s estoppel provisions. And, although the USPTO has admitted that “there is widespread agreement that the estoppel provisions should be better defined,”[2] very little progress has been made so far.

Nevertheless, a party with an understanding of the language and history of Section 315(c) may be able to mitigate some of the risk. To assist with this inquiry, the following discussion identifies specific categories of prior art that may pose a relatively low risk of triggering Section 315(c) estoppel and, thus, may be preserved for future litigation.

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