Ariad Pharmaceuticals Inc. lost two-thirds of its market value Wednesday, but the damage didn’t stop there: On Thursday, the shares fell more than 7 percent. The trigger was news that the Cambridge biotechnology company stopped enrolling patients in ongoing trials of its leukemia drug. New data show too many had blood clots and heart problems in an earlier study. Ariad must change the doses and eligibility rules and the warning label for the drug, Iclusig, will be modified. The drug remains on the market.