Pfizer has taken a 15% equity stake in French privately held biotech Vivet Therapeutics, with an exclusive option to acquire the remaining shares.

Vivet is focused on developing gene-therapy treatments for inherited liver disorders with a high medical need. Its strategy is to target the liver using a liver tropic adeno associated virus (AAV) to introduce therapeutic genes to hepatocytes, correcting the genetic disorder.

The companies will collaborate on developing VTX-801, Vivet’s proprietary treatment for Wilson disease, which is a rare, chronic and potentially life-threatening liver disorder that causes serious copper poisoning. Existing therapies for patients suffering from Wilson disease have either sub-optimal results or significant side effects.

“VTX-801 could provide a potentially transformative therapeutic option for patients with Wilson disease by directly addressing the underlying cause of the disease – the inability to excrete copper owing to a mutation in the gene that codes for that function,” said Seng Cheng, senior vice president and chief scientific officer of Pfizer’s rare disease research unit.

Under the terms of the transaction, Pfizer paid around $51 million upon signing and may pay up to nearly $636 million (including the option exercise payment), depending on certain milestones.  Monika Vnuk, Pfizer’s vice president, worldwide business development, will also join Vivet’s board of directors as part of the deal.

Pfizer can exercise its option to acquire all Vivet’s outstanding shares following the delivery of certain data from a Phase 1/2 clinical trial for VTX-801.

“The potential of of VTX-801 has already been demonstrated in preclinical models and our partnership with Pfizer will help accelerate development of VTX-801 and expand our other innovative technologies,” said Vivet co-founder and chief scientific officer, Gloria Gonzalez-Aseguinolaza.

Vivet is also advancing liver-directed gene therapy programs for progressive familial intrahepatic cholestasis for bile excretion defects and citrullinemia for defects in the urea cycle, which leads to the build-up of ammonia and other toxic substances in the blood.