Ancrod (current brand name: Viprinex) is a defibrinogenating agent derived from the venom of the Malayan pit viper.

It was found that ancrod's actions are FAD dependent and that the substance has interesting apoptotic properties (causing programmed cell death), which remain to be explored.

Due to its special mode of action (see below) and its price, Arwin has never been used as 'normal' anticoagulant such as heparin, but only for the symptomatic treatment of moderate to severe forms of peripheral arterial circulatory disorders such as those resulting from years of heavy smoking and/or arteriosclerosis.

The substance is intended for subcutaneous injection and intravenous infusion, and indirectly inhibits aggregation, adhesion, and release of thrombocytes mediated through the action of a fibrinogen degradation product (FDP).

The decreased viscosity is directly attributable to lowered fibrinogen levels and leads to important improvements in blood flow and perfusion of the microcirculation.

The rheological changes are readily maintained and the viscosity approaches pretreatment values very slowly (within about 10 days) after stopping ancrod.

[8][9] The failure of ancrod in the 6-hour window for ischemic stroke trial in 2008 led to cuts in staff, an effort to sell off the company's assets, and finally to the dissolution of NTI in August 2009.

For the treatment of established deep vein thrombosis; central retinal and branch vein thrombosis; priapism; pulmonary hypertension of embolic origin; embolism after insertion of prosthetic cardiac valves; rethrombosis after thrombolytic therapy and rethrombosis after vascular surgery.

[15] An RCT called "STAT" was published in 2000; it included 500 subjects and ancrod or placebo was administered within three hours of the stroke.

[17] Another trial was launched to explore the 6 hour window, but it was halted early in 2008 when an independent review committee looked at the interim data and found no signal of benefit.