The Federal Circuit has upheld a New Jersey district court's ruling against Immunomedics that Cytogen did not literally infringe a patent (US 4,460,559) relating to tumour detection and localization by targeting intracellular marker substances, but disagreed with the ruling that there was no infringement according to the doctrine of equivalents.

Immunomedics sued Cytogen for patent infringement over their prostate cancer diagnostic product ProstaScint, which relies on the marker prostate-specific membrane antigen (PSMA). PSMA is a transmembrane antigen, with intracellular and extracellular domains, and the molecule binds to the interior side of the cell wall.

In considering the literal infringement, the court dismissed Cytogen's proposed broad-claim construction interpretation that an intracellular marker substance is “an antigen or a portion thereof that is located inside a cell”, arguing that the prosecution history of the patent showed that the inventor associated the term with a fully intracellular antigen. By a majority ruling, the appellate court agreed with the district court that the marker substance must be fully internal to the cell. Judge Prost disagreed with the majority's claim construction and, siding with Immunomedics, believed that 'marker substance' bears a broader meaning.

Although the Federal Court agreed with the district court that PSMA is not an intracellular marker substance, they criticized the lower court for viewing the world of antigens as consisting of either intracellular or cell-surface antigens. This assumption led the district court to conclude as a matter of law that antigens falling in two halves could not be equivalents as a matter of law. The Federal Court reversed the judgement on no infringement of equivalents and remanded the issue back to the district court because the factual dispute of categorizing transmembrane antigens remains to be resolved.