Cambridge’s Ariad Pharmaceuticals Inc. fell after saying its experimental cancer medicine ponatinib caused serious side effects in 17 percent of patients in a clinical trial. About 15 percent of the patients in the trial, designed to study the drug’s effect on chronic myeloid leukemia, discontinued taking the therapy, according to a statement. Ponatinib, a treatment for hematological cancers, is in the second of three trials typically needed for regulatory approval. Ariad recently reported third-quarter net income of $13.9 million or 10 cents per share.