Eli Lilly and Co., maker of the antipsychotic Zyprexa and impotence pill Cialis, sued Dutch drugmaker Synthon BV’s U.S. unit to prevent it from selling a generic version of the lung treatment Adcirca in the U.S.

Synthon Pharmaceuticals is seeking U.S. Food and Drug Administration approval to sell a copy of the medicine. Lilly and its Icos unit contend the copy would infringe two patents that expire in 2020.

Adcirca was approved in May to improve exercise ability in people with pulmonary arterial hypertension, a potentially deadly lung disorder that can cause shortness of breath, chest pain and eventually heart failure.

The medicine is a once-a-day tablet that contains the same active ingredient, tadalafil, as Cialis. The impotence pill was Lilly’s fifth-biggest drug last year with global sales of $1.56 billion, according to the company’s annual report.

The federal court complaint, filed March 15 in Lilly’s hometown of Indianapolis, seeks a court order that FDA approval can’t be granted before the patent expires.

Officials with Synthon in the Netherlands couldn’t immediately be reached for comment.