* GSK weighing disposal of established drugs in Western markets
* Part of wider reshaping of British drugmaker's business
* Other pharma firms also reviewing mature drug portfolios
* GSK said to be in "no rush" to divest older non-core assets
(Adds more detail on valuation, background on other companies)
LONDON, May 29 (Reuters) - GlaxoSmithKline has invited private equity firms to consider making offers for a range of its older drugs sold in Western markets, according to three people with direct knowledge of the matter.
The move is part of a reshaping of the drugmaker's business, which also involves a major asset swap deal struck last month with Novartis.
Chief Executive Andrew Witty said last month that Britain's biggest pharmaceuticals company could dispose of individual medicines or a broader portfolio of older established products.
A GSK spokesman had no further comment on the potential disposals on Thursday.
Private equity firms approached by GSK include Advent International, Blackstone and KKR, the sources said, and the drugmaker is being advised by Lazard. Officials at Lazard and the three private equity firms declined to comment.
GSK said at its first-quarter results on April 30 that it was evaluating options to "maximise the value of our portfolio and currently reviewing our Established Products Portfolio". The portfolio includes products such as the antidepressant Paxil, migraine treatment Imitrex and Zantac for stomach acid.
The company has started breaking out sales results for these mature medicines for the first time this year as a prelude to a potential divestment of at least some of them.
"You should not be surprised if we were able to transact a disposal of some of that established product portfolio in the next year or two," Witty told reporters last month. "That is not part of our future."
One person familiar with GSK's thinking said it was in "no rush" to dispose of the 50 or so older medicines in the division and it also planned to retain rights to most of the products in emerging markets, where they form an important part of GSK's business.
Sales of established products totalled 814 million pounds ($1.36 billion) in the first quarter of 2014, down 11 percent on a year earlier, reflecting increased competition from cheaper generic copies. Around half of those sales were made in emerging markets.
The drugmaker's approach to private equity firms was first reported by Sky News.
WIDER INDUSTRY TREND
GSK has already disposed of some non-core products, having agreed last September to sell thrombosis medicines Arixtra and Fraxiparine to South Africa's Aspen Pharmacare for 700 million pounds ($1.2 billion), or around two times their annual sales.
Assuming a similar sales multiple, the Western-markets half of the current established drugs portfolio might be worth around 3.25 billion pounds, based on the level of sales reported in the first quarter.
GSK's decision to review its portfolio of older prescription drugs is part of a wider industry trend. Once companies divest their mature drugs, the remaining faster-growing and newer products can have a bigger impact on financial results.
Pfizer, the biggest U.S. drugmaker, has created a similar established products unit as a possible prelude to eventually divesting its mature drugs, and other firms are considering their options.
France's Sanofi and U.S. drugmakers Merck & Co and Abbott Laboratories are all exploring selling off some of their older drugs that have lost patent protection, Reuters reported last month.
Finding a keen buyer, however, may not be so easy. The appetite among rival drugmakers for such ageing assets is limited, leaving private equity firms as the most obvious purchasers, since they can milk the cash flow without having to worry about the expense of drug development.
($1 = 0.5982 British Pounds)
(Additional reporting by Anjuli Davies; Editing by Pravin Char)