Treatment of two mastocytosis patients with a histidine decarboxylase inhibitor.

Alpha-fluoromethylhistidine, alpha-FMH (L-641.575, Merck, Sharp and Dohme), a specific irreversible inhibitor of the mammalian histidine decarboxylase, was investigated for its effect on possible histamine symptoms in mastocytosis. Two patients were treated for 28 and 22 weeks respectively. The first patient had systemic mastocytosis and a severe malabsorption causing weight loss, excessive fecal fat losses and electrolyte disturbances, the main symptom being frequent diarrhoea. The second patient had mainly skin manifestations diagnosed as urticaria pigmentosa and the main symptom was pruritus. There were no side effects of the drug. The first patient produced formed stools after one week of treatment, concomitant with a decrease in plasma histamine and in urinary excretion of the main histamine metabolite tele-methylimidazoleacetic acid (MeImAA). In the second patient definite benefit of the treatment was observed after two months. However, changes in histamine parameters occurred earlier. Plasma histamine and urinary MeImAA were reduced after one week and in two or three weeks reached a steady-state level of about 25% of pretreatment values. The results indicate an effective inhibition of histamine synthesis in both patients but only the diarrhoea seemed to be causally related to the change in histamine metabolism.