SILVER SPRING — United Therapeutics Corp. said Monday that its board of directors approved a plan to buy back an additional $420 million of the drug developer’s stock.

The Silver Spring company said the program will become effective March 4 and remain open for up to a year.

United Therapeutics finished a $100 million share repurchase program by buying nearly 1.2 million shares for $57.1 million during last year’s fourth quarter.

The company had 50.8 million shares outstanding in late October. Its stock closed at $53.79 on Friday and has climbed 2 percent so far this year.

United Therapeutics products include Remodulin, Tyvaso and Adcirca, which treat pulmonary arterial hypertension, a potentially fatal condition in which patients have abnormally high blood pressure in the arteries of the lungs.

Its shares finished at $53.79 on Friday. Its shares have traded in a 52-week range of $40.34 to $59.

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