PHARMACEUTICAL company GlaxoSmithKline has made a strong start to the year after a healthy jump in sales in the US.

The success of new asthma drug Advair helped sales of GSK's pharmaceutical products soar 15 per cent in the US in the first three months of the year.

The group, which is cutting 400 jobs from a workforce of 1,500 in Barnard Castle over the next two years, said the treatment was already its third largest product after being on the US market for less than a year.

Group-wide sales in the first quarter to March 31 rose seven per cent to £5.11bn as pre-tax profits jumped 15 per cent to £1.59bn.

Chief executive Dr Jean-Pierre Garnier said the results demonstrated the breadth and strength of GSK's product portfolio.

He added that the quarterly performance provided "a strong platform to build on for the rest of the year".

GSK said it was on track to record earnings growth in the mid-teens this year with low-teens or better next year.

Analysts have voiced concerns that challenges to GSK's patents on both its antibiotic Augmentin and anti-depressant Paxil could hit its performance.

Last month, a US judge declared that three patents protecting Augmentin from generic competition until 2017 were invalid.

Dr Garnier said there were "too many moving parts" to judge the impact an unsuccessful defence of the patents would have on GSK's earnings growth.

He said: "We have a very good chance of defending Augmentin in appeal as well as Paxil so that's where I'll leave it."

The group, the second biggest stock in the City, was formed by the merger of Glaxo Wellcome and SmithKline Beecham two years ago.

It said savings resulting from the merger had also bolstered earnings growth in the first quarter.

Advair registered £363m of sales in the first quarter, placing it just behind Augmentin and Paxil as GSK's top-sellers.

Close to £200m of those sales came in the US, which accounted for 55 per cent of the group's pharmaceutical sales in the three months.

Dr Garnier said Sir Roger Hurn's decision to resign as chairman of Prudential yesterday did not affect his role as GSK's deputy chairman.

He said: "Sir Roger Hurn has made a valuable contribution to Glaxo Wellcome and now GSK as non-executive deputy chairman and we look forward to him continuing on the board in this capacity."