Metiamide

Metiamide is an H2-receptor antagonist developed from another H2 antagonist, burimamide. After discovering that burimamide is largely inactive at physiological pH, due to the presence of its electron donating side chain, the following steps were undertaken to stabilse burimamide: more...

• The addition of an electronegative sulphur atom close to the imidazole ring
• Addition of methyl group to the 4 position on the imidazole ring

The result is metiamide:

Metiamide

• is 10 times more potent than burimamide
• inhibited histamine-stmulated relase of gastric acid
• increased healing rate of peptic ulcers
• provided sysmptomatic relief for ulcerous patients.

However,

• in clinical trials, an unacceptable number of patients dosed with metiamide developed agranulocytosis
• Greatly decreased number of circulating granulocytes
• Clinical trials aborted.

It was determined that the thiourea group was the cause of the agranulocytosis. Therefore replacement of the sulphur atom in the thiourea group was suggested,

• with O or NH resulted in a compound with much less activity (20 times less than metiamide)
• with CHNO₂ and NCN - leads to formation of cimetidine

The nitro and nitrile groups are sufficiently electronegative to reduce the pK_a of the neighbouring nitrogens to the same acidity of the thiourea group, hence preserving the activity of the drug in a physiological environment.

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