Shares of Akebia Therapeutics (AKBA) - Get Report jumped 24.9% to $11.70 in premarket trading on Wednesday on the heels of yesterday's announcement of the Cambridge, Mass.-based company's expanded partnership with Otsuka Pharmaceuticals to develop and market its anemia pill Vadadustat in Europe, China and other territories.

The original pact between the two companies, inked last December, covered U.S. commercial rights.

Under the new agreement, Akebia will receive $208 million from Tokyo-based Otsuka, including $73 million upfront and $135 million in development funding. Akebia is also eligible to receive up to $657million in future milestone payments. If Vadadustat is eventually approved in Europe, China and other territories, Otsuka will make tiered, double-digit royalty payments up to 30% on net sales.

Meanwhile, shares of Flexion Therapeutics (FLXN) - Get Report were down 12.9% to $22.60. The Burlington, Mass.-based firm after the market close on Tuesday announced a plan to offer $125 million principal amount of convertible senior notes due 2024 in a private offering. Flexion said it expects to give the initial buyers of the notes a 30-day option to purchase up to an additional $18.75 million aggregate principal amount of notes, solely to cover over-allotments.

The company plans to use part of the net proceeds for the commercialization and manufacture of Zilretta if approved. Flexion also intends to use the proceeds for product pipeline development, as well as working capital and general corporate purposes. A portion of the net proceeds could also be used to in-license, acquire or invest in complementary businesses, technologies or assets.

Among the other biotech stock movers include MediciNova (MNOV) - Get Report , whose shares were up 16.2% to $6.47. The La Jolla, Calif.-based company on Tuesday announced  exploratory interim clinical outcomes data from a clinical study of MN-166 (Ibudilast) in amyotrophic lateral sclerosis.

—Adam Feuerstein contributed to this article