Veloxis Pharmaceuticals has filed a lawsuit demanding the FDA grant final approval of its extended-release immunosuppression drug Envarsus, which is being held up by the exclusivity of a rival product.

The company received tentative approval of Envarsus XR (tacrolimus) in October, but the FDA said it must wait to launch until a patent on Astellas’ tacrolimus product, Astagraf XL, expires in July 2016.

In its complaint, filed this week in D.C. federal court, Veloxis alleges the FDA ignored significant clinical differences between Envarsus XR and Astagraf XL when granting its product conditional approval.

The most obvious difference is that Envarsus XR is a tablet while Astagraf XL is a capsule, said William Polvino, president and CEO of Veloxis. But he added the drugs also behave very differently pharmacokinetically, which required the company to conduct an extensive clinical development program.

Veloxis did not rely upon any data in the Astagraf submission, Polvino said. Veloxis’ clinical program was based on the FDA’s recognition of the differences between Envarsus XR and Astellas’ immediate-release capsule Prograf (tacrolimus), as well as Astagraf XL.

That is part of the reason why the company is utterly confused by the FDA’s tentative approval, Polvino said.

The company says it met with the FDA to avoid litigation. On Dec. 5, the FDA advised that if Veloxis amended its package insert to limit Envarsus XR’s use solely to patients being converted from Prograf, it would grant immediate final approval of the Envarsus NDA. But Veloxis declined to revise its NDA, noting that doing so would artificially limit the patient population that would benefit from the drug.

Veloxis also argues that Astagraf XL was not entitled to exclusivity because the law prohibits granting exclusivity to drugs that contain antibiotics approved prior to 1997, unless the application for approval was submitted after October 2008.

The complaint says Astellas initially submitted its NDA for Astagraf XL in 2005. The company withdrew its pending NDA after the FDA sent an “approvable letter,” and then resubmitted it in 2012. Veloxis says the 2012 NDA was filed as a separate NDA, but that Astellas did not complete any new studies necessary for the approval between withdrawing and resubmitting the NDA.

An Astellas spokeswoman said the company agrees with the FDA’s decisions to grant and uphold market exclusivity for Astagraf XL. The FDA declined to comment on pending litigation. — Jonathon Shacat