GlaxoSmithKline (GSK) has revealed it plans to sell half of its 12.4% stake in South Africa's largest drug company, Aspen Pharmacare.Aspen shares currently trade at 40,650 South African rand, valuing it at roughly 185.3bn rand (£10.15bn)."GSK has a long and successful commercial partnership with Aspen and our investment in the company has grown in value significantly over time," said chief financial officer Simon Dingemans.The profit on the disposal will not be included in core operating profit and core earnings per share in 2015 and GSK said it will no longer account for Aspen as an associate after the sale.The sale, which it intends to complete via a private institutional placing priced in an accelerated bookbuild offering process, will leave GSK with a 6.2% stake, which it has agreed to hold for at least 180 days.Dingemans added: "As we continue to reshape the Group around our core franchises and drive the benefits from the Novartis transaction, optimizing our financial flexibility to invest behind these priorities is key."As a result we have decided now is the right time to realise further value from this successful relationship. We continue to believe in the strategy of Aspen and we remain committed to working together in the future."