Alexion Pharmaceuticals Inc. and Affibody AB are jointly developing ABY-039 to treat rare Immunoglobulin G-mediated autoimmune diseases.

Affibody will receive $25 million up front from Boston-based Alexion. Affibody could also receive up to an additional $625 million upon achieving certain development and sales milestones as well as tiered low double-digit royalties.

Immunoglobulin G is the most common type of antibody found in blood circulation. ABY-039 is a bivalent antibody-mimetic — artificial peptides that behave like antibodies — that targets the neonatal Fc receptor, also called the Brambell receptor, a human protein encoded by the FCGRT gene.

ABY-039 was specifically designed to combine Affibody's protein therapeutics platform and Albumod technology to achieve an extended half-life compared to other similar therapies and potential for less frequent, self-administered therapy.

Solna, Sweden-based Affibody, a biotechnology company, has the option to co-promote ABY-039 in the U.S. and will lead its clinical development for an undisclosed indication.

The transaction is expected to close in the second quarter, subject to clearance under the Hart-Scott Rodino Act.