The CAFC in the Eisai ruling -

Inequitable conduct in prosecuting a patent application before the United States Patent & Trademark Office may take the form of an affirmative misrepresentation of material fact, a failure to disclose material information, or the submission of false material information, but in every case this false or misleading material communication or failure to communicate must be coupled with an intent to deceive. Innogenetics, 512 F.3d at 1378 (citations omitted). Materiality, defined as "what a reasonable examiner would have considered important in deciding whether to allow a patent application," and intent are both questions of fact, and require proof by clear and convincing evidence. Id. To satisfy the "intent" prong for unenforceability, "the involved conduct, viewed in light of all the evidence, including evidence indicative of good faith, must indicate sufficient culpability to require a finding of intent to deceive." Kingsdown Med. Consultants, Ltd. v. Hollister Inc., 863 F.2d 867, 876 (Fed. Cir. 1988) (en banc) (citing Norton v. Curtiss, 433 F.2d 779 (CCPA 1970)). Gross negligence is not sufficient. Id. This is a high bar.

Aventis was a 2-1 ruling, with the focus on intent to deceive, particularly reminding that the more material the omission, the less needed to prove intent.

"To satisfy the intent to deceive element of inequitable conduct, 'the involved conduct, viewed in light of all the evidence, including evidence indicative of good faith, must indicate sufficient culpability to require a finding of intent to deceive.'" Impax Labs., Inc. v. Aventis Pharms. Inc., 468 F.3d 1366, 1374-75 (Fed. Cir. 2006) (quoting Kingsdown, 863 F.2d at 876). Given that direct evidence is often unavailable, intent is generally inferred from surrounding facts and circumstances. Id. at 1375. The district court, upon finding materiality and intent, shall "balance the equities to determine whether the patentee has committed inequitable conduct that warrants holding the patent unenforceable." Id. (quoting Monsanto Co. v. Bayer Bioscience N.V., 363 F.3d 1235, 1239 (Fed. Cir. 2004)). "The more material the omission or misrepresentation, the less intent that must be shown to elicit a finding of inequitable conduct." Id.

In a recent IP Today, Debra Zuckerman Anderson at Meyer Unkovic & Scott provided a checklist for prosecutors to guard against inequitable conduct.

1. Remind your client of the duty to disclose relevant prior art, including related copending applications (and make sure to file this prior art with the United States Patent and Trademark Office). Make sure that the examiner at the patent office is aware of activity in related cases, if handled by other examiners.
2. Ask about attendance at conferences, posters, and abstracts submitted to journals.
3. Remember to file prior art uncovered in foreign searches.
4. Ask about offers for sale or other public disclosures through use or publication.
5. If you are filing a declaration, make certain that it is clear to the PTO where the interests of the declarant lie. Do not conceal from the PTO facts known to you that are adverse to your client's position (such as negative data), in order to advance your client's case.
6. Discuss the date of invention and related evidence with your client.