** Shares of therapy developer Prothena Corporation PRTA.O are down nearly 2% at $4.77 in pre-market trading

** Jefferies downgrades the stock's rating to "hold" from "buy"; slashes PT to $6 from $32, a 8.8% discount to the last close

** PRTA becomes stock with lowest PT, per Jefferies, on Wall Street

** We see uncertainty around the pipeline given the failure of the AL amyloidosis (a disease where abnormal proteins damage organs) drug - brokerage

** Adds it finds "risk" in the co's Alzheimer's drug PRX012 phase 1 trial data

** Co says phase 3 clinical trial of Birtamimab in patients with AL amyloidosis did not meet primary endpoint

** Avg rating of 10 analysts is "buy"; median PT $29.5— data compiled by LSEG

** PRTA falls ~52.49% YTD, up to last close

(Reporting by Akriti Shah in Bengaluru)

((akriti.shah@thomsonreuters.com))