(Reuters) — Specialty pharmacy Park Irmat Drug Corp. is suing OptumRx, saying the pharmacy benefit unit of UnitedHealth Group plans to remove Irmat from its network as of the end of the month.

In the suit, filed late on Thursday in New York Supreme Court, Irmat said it received notice that it will be cut off from reimbursement by OptumRx, which operates a mail-order pharmacy that competes with Irmat, particularly for medicines that treat dermatological ailments.

Irmat is the latest specialty, mail-order pharmacy to be dropped by a payer after drugmaker Valeant Pharmaceuticals International Inc was accused of using its close ties with Philidor Rx Services to improperly inflate revenue. Valeant has denied the allegations and cut ties with Philidor, prompting other payers to follow suit.

OptumRx did not have an immediate comment on the lawsuit.

Several pharmacy benefit managers, including OptumRx, have removed Philidor from their networks and said they are evaluating other specialty pharmacies.

Express Scripts Holding Co., the largest U.S. pharmacy benefit manager, has cut Linden Care pharmacy from its network, citing close ties with drugmaker Horizon Pharma LLC.

Irmat is seeking an injunction barring Optum from terminating or excluding it. Irmat said it was in danger of going out of business as the result of Optum's action.

Irmat's mail-order business has been an unmitigated success. From 2012 to 2015, the pharmacy's business grew exponentially, both in revenue and geographic scope.

By way of example, Irmat's revenue from Optum members increased from approximately $1.99 million in 2012, to $3.8 million in 2013, to $15.3 million in 2014, and was projected to grow to $33 million in 2015.

In its lawsuit, Irmat says it began in 2013 to participate in programs sponsored by drugmakers Galderma SA and Aqua Pharmaceuticals under which the manufacturers covered patient co-payments on leading dermatological drugs.

Galderma is owned by Nestle SA. Aqua is owned by Almirall SA