Horizon Pharma has gone hostile with an all-stock offer for Depomed valued at about $2.1 billion after its target rejected friendly overtures to talk about a deal.

The Dublin-based company said Tuesday that it will take an offer of $29.25 of its stock for each Depomed share directly to that company's shareholders. The tax-free bid represents about a 42 percent premium to Depomed's closing price Monday.

Shares of Depomed jumped $7.51, or 36.4 percent, to $28.15 in Tuesday morning trading.

Horizon said that it has been trying to discuss a deal with Depomed Inc. since March, but that company's board has rejected its proposal and declined to start discussions.

Depomed said in a written statement that it has received Horizon's offer, which it believes substantially undervalues the company.

Horizon CEO Timothy P. Walbert said in a letter to his Depomed counterpart, James A. Schoeneck, that the combined company would be significantly larger and more diversified. It would have 13 marketed drugs, which would nearly double Horizon's current lineup.

Horizon Pharma Plc has completed four deals in the past two years. They include the acquisitions of Hyperion Therapeutics and the U.S. rights to the rheumatoid arthritis treatment Vimovo. Walbert said in his letter that these deals show Horizon can blend new medicines and businesses into its operations while delivering strong results.

Depomed, which is based in Newark, California, has a portfolio that includes the migraine drug Cambia and the painkiller Nucynta ER.

The deal value adds up to about $3 billion counting debt. Depomed shareholders would end up with a stake of about 25 percent in the combined company.

U.S.-trade shares of Horizon declined $1.58, or 4.6 percent, to $32.96.