CollaGenex Pharmaceuticals Inc. said today it stopped a 40-milligram mid-stage study of its acne treatment incyclinide, after discovering a second patient had a severe adverse reaction to the drug.

On May 2, the Newtown company said it suspended enrollment of new patients after one patient showed significant phototoxicity - a severe reaction to sunlight that can cause blisters and second-degree burns. The company said today it subsequently has learned of another case, and, as a result, has stopped treatment of all patients in the 40-milligram study.

CollaGenex said it was evaluating its options for continuing development of incyclinide, a modified tetracycline, for acne at lower 10-milligram and 20-milligram doses, where no adverse side effects have been seen.

The company is continuing a mid-stage trial of incyclinide to treat rosacea, a skin condition associated with inflammatory lesions and facial redness. CollaGenex said it will meet with the Food and Drug Administration to discuss the drug's development.

Shares fell 31 cents, or 2.78 percent, to $10.85 on the Nasdaq.