On the day the top drug regulator at the Food and Drug Administration resigned amid an investigation into his conduct, he also faced an explosive lawsuit that includes a series of incendiary texts and emails he is said to have sent to a former business associate.

George Tidmarsh, who led the FDA’s Center for Drug Evaluation and Research, stepped down Sunday after being placed on leave in recent days, STAT reported Sunday . At the same time, however, a lawsuit was filed against him in federal court by the Canadian drugmaker Aurinia Pharmaceuticals, alleging he defamed the company and had used his perch to advance a “longstanding personal vendetta” against Kevin Tang, once a Tidmarsh business partner and currently chairman of Aurinia.

Tidmarsh, who only took over the position this summe

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