Piper Jaffray analyst Edward Tenthoff downgraded Acceleron Pharma (XLRN) to Neutral and lowered his price target for the shares to $52 from $55.

Acceleron and partner Celgene (CELG) last night announced positive Phase III Believe results for luspatercept in transfusion dependent beta thalassemia, Tenthoff tells investors in a research note. The stock in premarket trading is up $6.10, or 13%, to $53.50.

This is a nice second win for luspatercept, but Acceleron shares are not a buy at current valuation levels, the analyst contends. He believes luspatercept does not command the same market opportunity in beta-thalassemia due to the expected approval of curative gene therapy by 2020. As such, Tenthoff lowered his penetration rates for luspatercept in beta thal to account for gene therapy competition. He says, however, that partner Celgene could acquire Acceleron, which presents a risk to his downgrade.