A 38-year-old man with a history of recurrent episodes of acute pancreatitis presents with severe abdominal pain. Investigations reveal markedly elevated serum lactate (8 mmol/L) and pyruvate levels, with normal liver function tests. Genetic testing confirms a deficiency in pyruvate dehydrogenase complex (PDHC). Which drug is the first-line treatment to bypass the enzymatic defect and promote entry of substrates into the TCA cycle?