United Healthcare will buy drug benefits manager Catamaran in a deal worth almost $13 billion, USAToday reports.

The acquisition will help United Healthcare, the nation's largest medical insurer, keep the prices of pharmaceutical drugs more manageable.

Pharmacy Benefit Managers (PBM) help deliver drug benefits for employers and health plans. It also runs large mail order pharmacies, which helps the company get better prices from drugmakers, Fox Business reports.

Employers are looking to cut prescription costs on expensive drugs. By merging with Catamaran, United Healthcare's pharmacy benefits unit, OptumRx, will be better equipped to negotiate prices in the PBM market. OptumRx fills about 600 million prescriptions per year.

Catamaran was formed when SXC Health Solutions and PBM Catalyst Health Solutions merged in 2012.

United Healthcare offered $61.50 per share of Catamaran. That is a 27 percent higher than Catamaran's closing price Friday on the Nasdaq.

In early trading, Catamaran's shares are up more than 24 percent from Friday's closing. It is currently trading at $60.01 per share.

United Healthcare's shares are also trending higher Monday, up about $3.00 or about 2.6 percent to $121.03.

Jefferies analyst Brian Tanquilut said in a research note that the deal "makes sense to us, but admittedly came much earlier than we expected."

"We had always viewed Catamaran as a compelling asset for companies looking for scale in the PBM sector such as Optum or Walgreens but expected Catamaran to grow the business much further before pursuing a sale," he said.

After the deal is completed, United Healthcare expects to fill more than 1 billion prescriptions per year.

The transaction between United Healthcare and Catamaran is expected to close sometime in the fourth quarter of 2015. Once the deal closes, Catamaran Chief Executive Mark Thierer will become the CEO of OptumRx. OptumRx's current CEO, Timothy Wicks, will become the president.