Shares in GlaxoSmithKline (GSK) edged lower on Friday due to the possible negative readacross from US pharmaceuticals peer Eli Lilly, which warned about headwinds to its revenues in 2014.Nevertheless, Panmure Gordon maintained its upbeat stance on London-listed GSK, saying it would use any weakness in the stock as a 'buying' opportunity.Eli Lilly warned on Thursday that it may not be able to meet its previous guidance of generating at least $20bn in revenues next year due to "challenging" market conditions and foreign-exchange effects in Japan and the emerging markets."From our coverage universe, GSK is most exposed to Japan, where it generates some 8.4% of revenues (FY2012A)," said Panmure analyst Savvas Neophytou."That said, we remain 'buyers' of the stock because of the excellent option value provided by the pipeline."Neophytou said that GSK has the best pipeline of respiratory pharmaceuticals with a number of late-stage assets in development. These should provide "mitigation" to increased risk to its biggest and best-selling product 'Advair' which is likely to face heightened competition from generics in the medium term following a Food and Drug Administration ruling last month. Panmure estimates that Advair generates a quarter of GSK's group profits currently.The analyst said that the stock's valuation - trading at 13.7 times 2013 earnings and 12.5 times 2014 earnings - is now "undemanding" and in line with the sector despite the strong pipeline."The company has been through the majority of its patent expiries, big liability settlements and boasts a strong balance sheet and very little M&A risk. With shareholder returns remaining strong, we remain 'buyers' of the stock with a price target of 1,850p."The stock was down 0.8% at 1,558.5p by mid-morning on Friday.BC