Arrowhead Pharmaceuticals, 502 S. Rosa Road, says its hefty drug development agreement with Janssen Pharmaceuticals is now official.

Arrowhead Pharmaceuticals has finalized its agreement with Janssen Pharmaceuticals that’s worth $250 million for Arrowhead, just to start.

The waiting period under the Hart-Scott Rodino Antitrust Act expired with no problems expressed by regulators, the companies said Wednesday.

Janssen — part of the Johnson & Johnson conglomerate — will give Arrowhead $175 million upfront for the worldwide exclusive license for Arrowhead’s ARO-HBV drug to treat chronic hepatitis B and options to work with Arrowhead on drugs to fight up to three more diseases.

Johnson & Johnson Innovation will also make a $75 million investment in Arrowhead.

Milestone payments on ARO-HBV and potential additional drugs could raise the total payment to Arrowhead to $3.7 billion.

Arrowhead is based in Pasadena, California but its research and development are conducted in Madison and about 90 of its 110 employees are here.

The company’s platform is based on RNA interference, a process often called gene silencing, to fight a variety of diseases.

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Judy Newman is a business reporter for the Wisconsin State Journal.