Chemotherapy: capecitabine activation: practice question

A Pharmacology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

Try it

Pharmacology · Chemotherapy: capecitabine activation

Sample item

Capecitabine is an oral prodrug converted to fluorouracil in three steps, and the final step occurs preferentially in tumour tissue. Which enzyme catalyses this final step?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Thymidine phosphorylase

Capecitabine is first hydrolysed by carboxylesterase in the liver, then converted by cytidine deaminase to 5-deoxy-5-fluorouridine, and finally to fluorouracil by thymidine phosphorylase. Tumours often have higher thymidine phosphorylase levels than normal tissue, which gives some selectivity.

Option by option

  • A. Dihydropyrimidine dehydrogenase

    Dihydropyrimidine dehydrogenase degrades fluorouracil and does not activate it.

  • B. Cytidine deaminase

    Cytidine deaminase catalyses the second step, forming deoxyfluorouridine.

  • C. Carboxylesterase

    Carboxylesterase catalyses the first step in the liver.

  • D. Thymidine phosphorylase (correct)

    Correct. Thymidine phosphorylase forms fluorouracil in the last step and is raised in many tumours.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Pharmacology questions

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, Thymidine phosphorylase. Capecitabine is first hydrolysed by carboxylesterase in the liver, then converted by cytidine deaminase to 5-deoxy-5-fluorouridine, and finally to fluorouracil by thymidine phosphorylase.

Why is "Dihydropyrimidine dehydrogenase" not the answer?

Dihydropyrimidine dehydrogenase degrades fluorouracil and does not activate it.

Why is "Cytidine deaminase" not the answer?

Cytidine deaminase catalyses the second step, forming deoxyfluorouridine.

Why is "Carboxylesterase" not the answer?

Carboxylesterase catalyses the first step in the liver.

What concept does this question test?

It tests Chemotherapy: capecitabine activation in Pharmacology. Tumours often have higher thymidine phosphorylase levels than normal tissue, which gives some selectivity.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Chemotherapy: capecitabine activation?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.