Diphenylhydramine (A) is an antihistamine. Incorporation of a methyl group, such

Question

Diphenylhydramine (A) is an antihistamine. Incorporation of a methyl group, such as in ortho-methyl diphenylhydramine (B) or para-methyldiphenylhydramine (C) is often done in order to give the human metabolism a "handle" by which it can degrade and eliminate the drug, so that the drug does not accumulate and become toxic CH CH3 CH3 CH3 CH CH 3 CH3 (a) Suggest, therefore, how the body would most likely begin to metabolize compounds (B) and (C). (2 marks) (b) Compound (B) in fact exhibits no antihistamine activity, while (C) is 3.7 times more active than (A). Suggest a reason for each of these results. (4 marks) (c) In your opinion, is it more likely that this drug functions as an agonist or as an antagonist? Explain your choice briefly. (4 marks)

Explanation / Answer

a. The methyl group attached to the aromatic ring metabolized to carbolic acid via the process of metabolic oxidation by the body. The carboxylic acid make this compound easily excretable and thus eleiminate from the body easily and hence accumulation of this compound within the body is minimized.

b. Compound B and C have methyl group which provides lipophilicity to the compound. Here in case of compound C the activity increases because drug absrob and disstribute within the body efficiently due to increased lipophilicity and thus reaches to their target sufficiently. In case of compund B, lipophilicity also increases but the methyl group on ortho position causes steric hindrance between hydrogen atom of methyl group and lone pair electrons of oxygen atom and this hindrance inhibit the interaction between drug and its target which results in inactive compound.

c. Antihistaminic drugs act as antagonist as these drugs block the histamine receptors (H1, H2 and H3) due to which histamine does not reaches to the receptor which results in antihistminic activity.

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