UPDATE 2-Mallinckrodt to buy Questcor for its lucrative MS drug

(Adds details on valuation, Acthar sales)

April 7 (Reuters) - Specialty pharmaceuticals company Mallinckrodt Plc will buy drugmaker Questcor Pharmaceuticals Inc for about $5.6 billion to gain access to its lucrative drug to treat multiple sclerosis, Acthar Gel.

This acquisition, Mallinckrodt's second in less than two months, highlights its efforts to tap into the fast-growing specialty drugs market, which focuses on complex and chronic diseases.

The Irish company bought Cadence Pharmaceuticals Inc for $1.3 billion in February.

Questcor's shareholders will receive $30 in cash and 0.897 Mallinckrodt shares for each Questcor share held, for a total value of about $86.10 per Questcor share, the companies said in a statement on Monday.

Questcor's shares were trading at $91 before the bell on Monday as Mallinckrodt's shares jumped 11 percent.

The offer represents a 27 percent premium to Questcor's Friday close.

The deal values Questcor at 12 times forward earnings, well below the median of 18.4 for the broader pharmaceutical industry, according to Thomson Reuters StarMine data.

In the 12 months to Friday's close, Questcor's stock had risen more than 125 percent.

Acthar Gel is approved by the U.S. Food and Drug Administration for 19 conditions, many of which are associated with autoimmune and inflammatory diseases, including multiple sclerosis and infantile spasms.

Almost all of Questcor's revenue comes from the drug, which had annual sales of about $760 million in 2013, an increase of about 50 percent from 2012. The company acquired the drug from Aventis in 2001.

Mallinckrodt, which traces its roots back to 1840 in St. Louis, Missouri, makes drugs for pain management, cerebral and spinal spasticity, inflammatory diseases and depression. It also makes generic drugs and active pharmaceutical ingredients.

The company had revenue of $2.2 billion in 2013.

Dublin-based Mallinckrodt, which was spun off from Covidien Plc in July last year, said it expected the deal to add to adjusted earnings in 2014 and significantly boost adjusted profit in 2015.

Mallinckrodt will fund the deal through cash on hand and debt financing from Barclays, which also advised the company.

Wachtell, Lipton, Rosen & Katz and Arthur Cox were Mallinckrodt's legal advisers in Ireland.

Centerview Partners was Questcor's financial adviser, and Latham & Watkins LLP and Matheson were its legal advisers in Ireland.

(Reporting by Esha Dey in Bangalore; Editing by Kirti Pandey)