Question

Methotrexate, used as an antimetabolite, is a product of N-methylation of para-aminobenzoic acid and pyridine hydroxyl. The isosteric group is

• A. SH
• B. CH_3
• C. NH_2
• D. CF_3

Hint:

Isosteres are useful in drug design for improving activity or metabolic stability by structural mimicry.

Answer 1

The Correct Option is B

- Methotrexate is a folic acid analog and acts as a competitive inhibitor of the enzyme dihydrofolate reductase (DHFR), which is essential for DNA synthesis.
- In structure-activity relationships (SAR), bioisosteric replacement is a concept where functional groups with similar physical or chemical properties are substituted without significantly altering the biological activity.
- The N-methyl group (CH\textsubscript{3}) acts as a classical isosteric group in methotrexate, mimicking structural features of naturally occurring folates to retain binding affinity and metabolic stability.
- Among the given options, CH\textsubscript{3} serves as a non-polar, small group that can mimic hydrogen or methyl moieties in bioisosterism.
- Therefore, the correct isosteric group relevant to methotrexate’s activity is CH\textsubscript{3}.

Answer 2

Methotrexate is made from folic acid by replacing the 4-hydroxyl group on the pteridine ring with an amino group and adding a methyl group on the nitrogen that links to the para-aminobenzoic acid unit. The question asks which group is considered isosteric in this modification. Let's weigh each option as a possible isostere for the added methyl substituent.

1. SH: A thiol group is a classical bioisostere for -OH because sulfur and oxygen belong to the same periodic group and share similar bonding behavior, but it is much larger and more polarizable than a methyl group, so it does not match a methylation-based isosteric replacement.
2. CH3: A methyl group is a well known isostere for a hydrogen atom, since replacing H with CH3 changes the size only slightly according to the classical isosteric replacement rules, while often improving lipophilicity and metabolic stability. Methotrexate's N-methyl substitution on the para-aminobenzoic acid nitrogen is exactly this kind of hydrogen to methyl isosteric change, and it also blocks oxidation at that nitrogen, which folic acid itself does not have.
3. NH2: An amino group replacing the 4-hydroxyl of the pteridine ring is indeed part of how methotrexate differs from folic acid, but that particular change is a bioisosteric replacement of -OH with -NH2, not the isostere associated with the N-methylation step the question is asking about.
4. CF3: A trifluoromethyl group is a common isostere for a methyl group in modern medicinal chemistry because of its similar size but very different electronic effect, but it is not the group actually used or referenced in methotrexate's structure.

The isosteric substitution tied to the N-methylation step in methotrexate is the hydrogen to methyl replacement.

So the correct answer is CH3.