The filing of a patent application in a single country or with a multicountry patenting organization sets a priority date. This allows the applicant to file an application on the same invention in most other countries under the provisions of the Paris Convention for the Protection of Industrial Property. The Paris Convention gives the applicant a year to file applications in all member countries, claiming an invention disclosed in the b Markush structures are named for an early patentee, Eugene Markush, whose patent application was the subject of a landmark decision by the U.S. Patent and Trademark Office.

We claim:

1. A compound of the formula:


Ri is H, C!-C3 alkyl, C3-C5 cycloalkyl or C^ perfluoroalkyl;

R2 is H, Ci-C6 alkyl optionally substituted by OH, Cj—C3 alkoxy or C3-C6 cycloalkyl, or C1-C3 perfluoroalkyl;

R3 is Ci-C6 alkyl, C3-C6 alkenyl, C3-C6 alkynyl, C3-C7 cycloalkyl, Cj-C6 perfluoroalkyl or (C3-C6 cycloalkyl) C]-C6 alkyl;

R4 taken together with the nitrogen atom to which it is attached completes a 4-N(R6)-piperazinyl group;

R5 is H, C1-C4 alkyl, C]-C3 alkoxy, NR7R8, or CONR7R8;

R6 is H, Cx-C6 alkyl, (C1-C3 alkoxy) C2-C6 alkyl hydroxy C2-C6 alkyl, (R7R8N)C2-C6 alkyl, (R7RSNC0)C1-C6 alkyl, CONR7R8, CSNR7R8 or C(NH)NR7R8

R7 and R8 are each independently H, Q-C4 alkyl, (Ci-C3 alkoxy)C2-C4 alkyl or hydroxy C2-C4 alkyl; and

Pharmaceutically acceptable salts thereof.

Figure 3 Generic patent claim covering sildenafil and its salts, US 5,250,534, Claim 1.

original application and indicating that priority is claimed. As of 2004, most industrialized countries are members of the Paris Convention. There are exceptions, the most significant being Taiwan. For each patent application received, the patent examiner attempts to determine whether the claimed invention was patentable at the time of the priority filing. A multicountry patent application may be filed under the PCT either as the priority application itself or simply as an additional application within the priority year. A patentability search and an optional preliminary examination are then conducted by the World Intellectual Property Organization (WIPO), and national applications are filed 19 or 31 months after the priority application date. Most patent offices publish the patent application 18 months after the priority date. This first publication is not a granted patent; examination proceeds afterward and a granted patent may or may not be published after examination is complete. When the patent examiner finds relevant prior art, the applicant may try to amend the claims to exclude the scope covered or suggested by the prior art. If the applicant's amendments and arguments are unsuccessful, the application will be rejected. A rejection can be appealed to the patent office or to the national courts. The patent will issue if any of the claims are deemed to be patentable over the art discovered by the examiner. Some patenting organizations allow third parties to oppose the grant of the patent. A competitor of the patentee can submit prior art to the issuing patent office, alleging that the claimed invention is unpatentable. If the opposer prevails, the patent will be withdrawn or reissued in amended form. Oppositions are common in the European Patent Office (EPO) and in Japan; U.S. law does not have a provision for opposition ofpatents but does allow third parties to request re-examination of a granted patent. The owner of a patent can enforce its rights by suing others for infringement of a granted patent. If the court finds for the patentee, the infringer must cease from making, using, or selling the invention claimed in the patent. A common defense for a charge of infringement is a countersuit on the basis that the patent is invalid or unenforceable. Even without a judgment of invalidity, a patentee can lose the right to the claimed invention before the end of the statutory patent term if proper maintenance fees are not paid periodically to keep the patent in force. At the end of the statutory term (20 years from filing in most cases), the patent will expire.

Under certain circumstances, patent coverage for approved drugs may be extended or restored. As the meaningful life ofphar-maceutical patent coverage may be shortened by lengthy regulatory review procedures, agencies may agree to extend patent coverage in many countries for some approved drugs. In European countries, the patent term extension is accomplished through a Supplementary Protection Certificate, documented by the patent offices. In the United States, patent rights are extended through the provisions of the Hatch-Waxman Drug Price Competition and Patent Term Restoration Act of 1984. Only one patent can beextended foreachapproveddrug, andwhenanextendedpatent covers more than one marketed drug, only one drug can be extended. Term extensions are determined by the Food and Drug Administration (FDA) and documented in The Orange Book (Approved Drug Products with Therapeutic Equivalents).

The Hatch-Waxman Act includes provisions for both patent term extensions and the development of generic drugs for marketing shortly after the expiration of the patents on the approved drug. The Orange Book lists all of the patents covering the approved drug that are registered with the FDA. More than one patent can be listed for each approved drug. Some drugs have listings for the patent that claims the compound per se, methods for making it, dosage forms, and methods for treating diseases. The generic manufacturer can certify that the listed patent either does not cover the product it intends to produce or that the patent is invalid or unenforceable, triggering the filing of a lawsuit by the patentee and a 30 months stay on issuance of an ANDA approval. Until a law change in 2003 restricted the types of patents that could be listed, patentees were also listing patents on pro-drugs, metabolites, and chemical intermediates. This often triggered excessive litigation and sequential 30-month stays.

Until 1995, U.S. patents had a term of 17 years from the initial grant of the patent. When the term was changed to 20 years in 1995, patents then in pending or in force were given the longer of 17 years from grant or 20 years from filing. U.S. patents issued since 2001 may also be unintentionally extended by virtue of processing delays by the U.S. Patent Trademark Office. Where the grant of a patent is delayed beyond three years from the filing date, the amount of delay attributable to the applicant is subtracted from the amount of delay caused by the government, and an extension is granted to the patent term. This type of extension is printed on the face of the patent.

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