Cambridge-based Idera Pharmaceuticals Inc. reported its lead drug candidate, for moderate to severe plaque psoriasis, was more effective than a placebo in a Phase 2a clinical trial. Idera said 48 percent of patients with the condition (12 of 25) treated with IMO-3100 demonstrated improvement. The trial involved two dose levels of the drug administered for four weeks, with a four-week follow-up. None of the 12 patients who got placebos showed significant improvement.