It was listed in the 10-K as "available for partnering". Evidently no partners appeared and they dropped it from current plans.
ARO-F12
ARO-F12 is in development as a potential treatment for factor 12 (F12) mediated diseases such as hereditary angioedema (HAE) and thromboembolic disorders. Factor 12 initiates the intrinsic coagulation pathway and reducing its production using Arrowhead’s RNAi technology may present opportunities in both disease areas.
Thrombosis and HAE
Thrombosis is the formation of blood clots that can obstruct blood flow. Broadly speaking, thrombosis can occur in veins or arteries and may cause serious repercussions if not treated. HAE is a rare genetic disorder with a prevalence of approximately 1/5,000-1/10,000. Patients with HAE can experience recurrent and dangerous acute inflammatory attacks in multiple tissues, with attacks of laryngeal edema being particularly serious and potentially fatal.