Investing.com - Enough is enough.After an 80% spike since Brian Niccol was named CEO in February, Chipotle shares were downgraded to sell by Mizuho.In a note to clients, the Wall Street firm said investors "should reduce their risk" because the stock price already reflects an "aggressive recovery" in sales and profit margins.Niccol recently outlined his turnaround plan for the burrito chain, but analysts called it short on details.Given that, Mizuho said there are no clear catalysts to "justify significant earnings upside." Mizuho, however, did acknowledge Niccol's "impressive track record" as CEO of Taco Bell, calling him the "best choice to lead Chipotle."Niccol replaced founder Steve Ells, who struggled to guide the company back from a series of food safety incidents dating back to 2015. The stock traded as high as $750 a share back then.