Polymorphs and Prodrugs and Salts (Oh My!): An Empirical Analysis of 'Secondary' Pharmaceutical Patents

11 Pages Posted: 20 Dec 2012 Last revised: 6 Oct 2023

See all articles by Amy Kapczynski

Amy Kapczynski

Yale University - Law School

Chan Park

Medicines Patent Pool

Bhaven N. Sampat

Columbia University - Mailman School of Public Health; National Bureau of Economic Research (NBER)

Date Written: October 10, 2012


While there has been much discussion by policymakers and stakeholders about the effects of “secondary patents” on the pharmaceutical industry, there is no empirical evidence on their prevalence or determinants. Characterizing the landscape of secondary patents is important in light of recent court decisions in the U.S. that may make them more difficult to obtain, and for developing countries considering restrictions on secondary patents.

We read the claims of the 1304 Orange Book listed patents on all new molecular entities approved in the U.S. between 1988 and 2005, and coded the patents as including chemical compound claims (claims covering the active molecule itself) and/or one of several types of secondary claims. We distinguish between patents with any secondary claims, and those with only secondary claims and no chemical compound claims (“independent” secondary patents).

We find that secondary claims are common in the pharmaceutical industry. We also show that independent secondary patents tend to be filed and issued later than chemical compound patents, and are also more likely to be filed after the drug is approved. When present, independent formulation patents add an average of 6.5 years of patent life (95% C.I.: 5.9 to 7.3 years), independent method of use patents add 7.4 years (95% C.I.: 6.4 to 8.4 years), and independent patents on polymorphs, isomers, prodrug, ester, and/or salt claims add 6.3 years (95% C.I.: 5.3 to 7.3 years). We also provide evidence that late-filed independent secondary patents are more common for higher sales drugs.

Policies and court decisions affecting secondary patenting are likely to have a significant impact on the pharmaceutical industry. Secondary patents provide substantial additional patent life in the pharmaceutical industry, at least nominally. Evidence that they are also more common for best-selling drugs is consistent with accounts of active “life cycle management” or “evergreening” of patent portfolios in the industry.

Keywords: patents, pharmaceuticals, innovation, intellectual property, secondary patents, international intellectual property law

Suggested Citation

Kapczynski, Amy and Park, Chan and Sampat, Bhaven N., Polymorphs and Prodrugs and Salts (Oh My!): An Empirical Analysis of 'Secondary' Pharmaceutical Patents (October 10, 2012). PLOS ONE, December 2012, Volume 7, Issue 12, Yale Law School, Public Law Working Paper No. 271, Yale Law & Economics Research Paper No. 462, Available at SSRN: https://ssrn.com/abstract=2191728

Amy Kapczynski

Yale University - Law School ( email )

P.O. Box 208215
New Haven, CT 06520-8215
United States

Chan Park

Medicines Patent Pool ( email )

150 Route de Ferney
P.O. Box 2100

Bhaven N. Sampat (Contact Author)

Columbia University - Mailman School of Public Health ( email )

600 West 168th St. 6th Floor
New York, NY 10032
United States

National Bureau of Economic Research (NBER) ( email )

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