UPDATE 3-United Therapeutics starts litigation with FDA over rival Liquidia's drug application

(Adds FDA's response in paragraph 6)

Feb 21 (Reuters) - United Therapeutics said on Wednesday it had filed a case with the U.S. Food and Drug Administration, alleging that due procedure was not followed in allowing rival Liquidia's application for a blood pressure drug.

United Therapeutics alleged that Liquidia skirted long standing FDA rules, precedents and procedures when it filed its application for an inhaled dry powder of treprostinil for pulmonary hypertension associated with interstitial lung disease (PH-ILD).

PH-ILD is a group of disorders that cause scarring in the lungs.

United Therapeutics has also sued Liquidia for patent infringement of its drug Tyvaso.

Liquidia said last month the FDA was still reviewing its drug, decision on which was due on Jan. 24.

Both the FDA and Liquidia declined to comment on the ongoing litigation. (Reporting by Leroy Leo and Christy Santhosh in Bengaluru; Editing by Shinjini Ganguli)