Structure and activity of cimetidine

The finding that metiamide and cimetidine are both good H2 antagonists of similar activity shows that the cyanoguanidine group is a good bioisostere for the thiourea group. 

Amino Tautomer Imino Tautomer Amino Tautomer 

Since tautomer II is favoured, the guanidine group does in fact bear a close structural similarity to the thiourea group. Both groups have a planar pi electron system with similar geometries (equal C-N distances and angles). They are polar and hydrophilic with high dipole moments and low partition coefficients. They are weakly basic and also weakly acidic such that they are un-ionized at pH 7.4. 

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