(ShareCast News) - WPP's wholly-owned operating company Grey Healthcare Group has acquired a minority interest in US-based OptimizeRX Corp (OPRX) for an undisclosed sum.WPP said the investment by GHG, a healthcare communications services company, continues its strategy of investing in digital and important markets such as the US.OPRX manages a digital platform that allows pharmaceutical manufacturers to communicate with physicians and consumers to promote their products. Its products allow doctors and staff access to sample vouchers, co-pay coupons and other patient support to search, print or electronically dispense drug samples directly to patients as well as a network of pharmacies.At 1311 BST, WPP shares were up 3.7% at 1,374.00p.