UPDATE 1-Glaxo starts late-stage tests with "Son of Advair"

* New combination inhaler targets COPD lung disease

* Clinical trials programme to enrol 6,000 patients

(Adds details, background)

LONDON, Oct 27 (Reuters) - An experimental lung drug that GlaxoSmithKline (GSK.L: Quote, Profile, Research) hopes will be a successor to top-seller Advair has started final-stage clinical tests, the British drugmaker and its partner Theravance (THRX.O: Quote, Profile, Research) said on Tuesday.

Treatment has commenced as part of a Phase III programme to develop the next-generation combination medicine for patients with chronic obstructive pulmonary disorder (COPD), also known as "smoker's lung".

So far, dosing is limited to two 12-month studies but patient screening is underway in a further three studies within the so-called Horizon programme, which will involve 6,000 patients in all, the companies said in a statement.

The clinical tests will assess a new once-daily long-acting beta agonist (LABA), known by the code '444, in combination with a daily corticosteroid, fluticasone furoate.

The project is important because Glaxo's existing blockbuster inhaler Advair could encounter generic competition in the key U.S. market in 2011. It is also facing increased competition from new arrivals.

Advair is expected to generate sales of around 4.9 billion pounds ($8.04 billion) worldwide this year, according to Thomson Reuters I/B/E/S consensus forecasts.

Advair, like its successor, also combines two ingredients -- a steroid to fight inflammation and a beta agonist to open airways by relaxing the muscles that tighten during an asthma attack.

Both those elements will be replaced in the follow-on product, which is widely known as "son of Advair". (Reporting by Ben Hirschler; Editing by David Cowell)