The procedure known as inter partes reexamination is a proceeding which takes place in the United States Patent and Trademark Office and allows any person to challenge the validity of United States patents issuing from original applications filed in the United States on or after November 29, 1999 on the basis of published prior art applicable to the patentability of the claims.
To file a request for an inter partes reexamination, the following requirements set forth in the United States Patent Law, 35 U.S.C. 311, must be met:

1) the request must be in writing, including the identity of the real party in interest, and be accompanied by payment of an inter partes reexamination fee; and
2) the request must set forth the pertinency and manner of applying the cited prior art to every claim for which reexamination is requested.

One major difference between the ex parte reexamination and the inter partes reexamination is readily apparent from 1) above; that is, the identity of the actual person requesting the inter partes reexamination, referred to as the third party requester, cannot be concealed.

The request may be filed at any time during the life of the patent. The fee for filing a request for an inter partes reexamination is $8,800.00 (compared to the ex parte reexamination fee, which is $2,520.00). The request can only set forth a challenge to patentability in regards of prior art publications that would be applicable to an anticipation or obviousness rejection.

After the request is filed, the Director of the United States Patent and Trademark Office will determine whether “a substantial new question of patentability affecting any claim of the patent is raised.” The decision of the Director as to whether the reexamination should proceed is final and can not be appealed. The United States Patent and Trademark Office will refund a portion of the filing fee if the Director determines that there are no new questions of patentability raised in the request filed by the third party requester.

If the request is approved by the Director, an inter partes reexamination will be ordered. The procedure of the inter partes reexamination is similar to an ex parte reexamination and a patent owner will be able to amend the claims by amendment. However, it is not possible to enlarge the scope of the claims of the patent. An inter partes reexamination, however, allows the third party requester to file comments after the patent owner responds to an Office Action from the United States Patent and Trademark Office. The third party requester will receive copies of all documents issued by the United States Patent and Trademark Office. Each time documents are filed by the patent owner, the third party requester will obtain copies of the patent owner’s response, and the third party requester has 30 days to file written comments.

The third party requestor also is able to participate in any appeal proceeding filed by the patent owner. Also, the third party requester can file an appeal if the decision of the PTO is favorable to the patent owner.

Inter partes reexamination, because it is a relatively new procedure, has not yet been extensively used in the United States in comparison to litigation in the federal court system. The reason for the limited use of inter partes reexamination thus far is likely due to the lack of discovery and the potential estoppel issues associated with inter partes reexamination. In federal court litigation each party can resort to the discovery process to seek information about the other party’s case. Inter partes reexamination does not permit discovery to take place.
One form of estoppel is defined in 35 U.S.C. 315, which explains that a third party requester is prohibited from asserting, at a later time in a civil action in a federal court, the invalidity of any claim finally determined to be valid and patentable on any ground which the third party requester raised or could have raised during the inter partes reexamination proceeding.

Another potential estoppel is explained in 35 U.S.C. 317, which details that a third party requester is prohibited from requesting an inter partes reexamination on issues that were raised or could have been raised in the previously filed litigation or inter partes reexamination. Both of these estoppels can be overcome if there is newly discovered prior art that was unavailable to the third party requester and the United States Patent and Trademark Office at the time of the initial inter partes proceeding.

Originally, inter partes reexamination was criticized because a third party requester was not permitted to appeal the results of the inter partes reexamination or have the United States Patent and Trademark Office reconsider prior art that was previously cited or considered.

To address some of the perceived disadvantages of inter partes reexamination, the procedure has been amended (Patent and Trademark Authorization Act of 2002, Pub. L. 107-373) so that a third party requester can now appeal any final decision to the Court of Appeals for the Federal Circuit, and so that new questions of patentability are not precluded by a patent or printed publication being previously cited or considered by the United States Patent and Trademark Office. Hence, the procedure may be attractive for the situation where the anticipated costs of litigation are high and there is published prior art that was either misconstrued or not considered by the United States Patent and Trademark Office.

There is another potential amendment to the law being considered (H.R. 2231) which would expand the possible basis for challenging the validity of the patent to include any type of prior art including evidence of prior knowledge, use, or sale of the invention. Further, the potential amendment would allow challenges to be based on 35 U.S.C. 112 issues (lack of written description, enablement, utility, indefiniteness, etc., but excluding best mode violations). This potential amendment would include a time limit for filing the request for inter partes reexamination. The time limit is proposed to be 12 months from the date a patent issues. If this potential amendment is implemented by Congress, inter partes reexamination will become more similar to the opposition procedure in Europe.

In summary, inter partes reexamination provide a potentially low cost option for challenging the validity of a patent at any time. However, the inter partes reexamination proceeding can only be based on published prior art and creates certain estoppel issues, which may have an impact on future infringement actions. The inter partes reexamination procedure is more restrictive than European oppositions, but efforts may be made in the future to make it become more similar to the European opposition system.