Effect of Oral Cimetidine in the Protoporphyrias

Erythropoietic protoporphyria (EPP) and X-linked protoporphyria (XLP) are genetic defects of heme biosynthesis that cause life-long, painful cutaneous sensitivity to light. EPP and XLP, collectively called the protoporphyrias, result in the accumulation of the light-sensitive molecule protoporphyrin IX initially in the bone marrow during hemoglobin synthesis and secondarily in erythrocytes, plasma, and the liver. In addition to photosensitivity, protoporphyria can also result in anemia, gallstones, and liver failure. No therapy has been demonstrated to reduce protoporphyrin levels or prevent the potentially life-threatening complications of EPP. Cimetidine has gained attention as a possible treatment for human porphyrias because of a potential off-target effect of inhibition of delta-aminolevulinate synthase (ALAS), the first enzyme of heme biosynthesis. This inhibition was first described in vitro; however, case reports of benefit in EPP have been anecdotal and uncontrolled. Therefore, the objective of this study is to determine the efficacy and safety of oral cimetidine administration in the protoporphyrias. Efficacy will be based on protoporphyrin levels, photosensitivity, and quality of life questionnaires. If the results are positive, this would be the first study providing quality evidence for an agent acting as a disease-modifying therapy for EPP. The study is also attractive because it repurposes an already approved drug with few side effects to treat a rare human disease.

The study design is a prospective, blinded, randomized, 2x2 cross-over design comparing cimetidine to placebo in patients with protoporphyria. Eligible protoporphyria patients will be randomized with equal allocation to one of two treatment sequences that will be administered over two 3-month study periods. Randomization will be stratified by site and permuted block randomization will be used to prevent chronological bias. Patients randomized to sequence 1 will receive placebo during period 1 and cimetidine during period 2. Patients randomized to sequence 2 will receive cimetidine during period 1 and placebo during period 2. Between periods, to eliminate any carry-over effects from the treatment administered in period 1, a wash-out period of 3 months will occur in which all patients receive neither cimetidine nor placebo. Three months was selected for each study period and for the wash-out period because of the rapid decline in protoporphyrin in red cells over the lifespan of the red cell (120 days), as well as to account for the time frame needed to measure light sensitivity in EPP.