Pfizer's Allergan deal teeters on new U.S. inversion curbs

By Caroline Humer and Ransdell Pierson NEW YORK (Reuters) - U.S. drug maker Pfizer Inc's $160 billion agreement to acquire Botox maker Allergan Plc was on the brink of being abandoned on Tuesday, after the U.S. Treasury issued new rules on how tax "inversion" deals can be structured. Allergan's shares were hit considerably hard on Tuesday, showing how the rules that were issued on Monday targeted the biggest inversion attempted to date. The federal government has grappled with a wave of recent i