Distinguish between therapeutic window and therapeutic index in humans. 

Therapeutic window:

Therapeutic index:

State one advantage of using morphine as an analgesic.

Explain why diamorphine (heroin) is more potent than morphine using section 37 of the data booklet.

Therapeutic window:
range of dosage «over which a drug» provides the therapeutic/desired effect without causing adverse/toxic effects [✔]

Therapeutic index:
toxic dose of drug for 50 % of population divided by minimum effective dose for 50 % of population
OR
$\frac{\text{TD50}}{\text{ED}50}$  [✔]

Note: M1 may be scored from a correctly labelled diagram.

Accept “difference between ED50/minimum effective/therapeutic dose «for 50% of population» AND TD50/toxic dose «for 50% of population»” for M1.

Do not accept reference to lethal dose used in therapeutic index in animal studies.

blocks pain impulses/binds with «opioid» receptors in brain/CNS
OR
effective against strong pain
OR
sedate patients to reduce trauma [✔]

Note: Accept “effective against pain after surgery/cancer/following serious injury”.

Accept “relieves anxiety/stress associated with severe/terminal illness”.

morphine has «two» hydroxyl groups AND diamorphine has «two» ester/ethanoate/acetate groups
OR
molecule of diamorphine is less polar than morphine
OR
groups in morphine are replaced with less polar/non-polar groups in diamorphine  [✔]

«less polar molecules» cross the blood–brain barrier faster/more easily
OR
diamorphine is more soluble in non-polar environment of CNS/central nervous system than morphine [✔]

Note: Accept “alcohol/hydroxy” for “hydroxyl” but not ”hydroxide”.

Accept “fats” for “lipid”.

Accept “heroin” for “diamorphine”.

Most students receive one mark for this question and therapeutic window was probably the more successfully mark. Teachers need to remind students not to refer to lethal dose for therapeutic index. Some students forgot to mention the 50/50 % when explaining therapeutic index or inverted the ration as ED50/TD50.

This was not a particularly well answered question even though it has been seen in previous exams. Many students confused it with the idea of solubility and passing through the blood brain barrier which was 1(b) (ii).

This question was reasonably well answered with many students receiving at least one of the two marks.