Action of anticancer drugs (A-level only) — AQA A-Level Chemistry
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Action of anticancer drugs (A-level only) explained
The drug is [Pt(NH₃)₂Cl₂]. Platinum sits at the centre with four monodentate ligands, two ammonia molecules and two chloride ions, arranged square planar with bond angles of 90°.
Read the full explanation
A square planar arrangement allows two isomers: one with the chlorides next to each other, one with them opposite. Only the first is the anticancer drug, so the cis in the name is doing chemical work rather than decorating it. Deducing the oxidation state is a routine mark: ammonia is a neutral ligand and the whole complex carries no charge, so the two chloride ligands must be balanced by platinum in the +2 state. Square planar complexes of this kind are not chiral, so there is no optical isomerism to discuss here.
Cisplatin prevents DNA replication in cancer cells by a ligand replacement reaction with DNA in which a bond is formed between platinum and a nitrogen atom on guanine.
Inside the cell it is the chloride ligands that leave. A nitrogen atom in the ring of guanine keeps a lone pair that is not being used for bonding, and that lone pair attacks the platinum, forming a coordinate bond in place of a platinum to chlorine bond. Because the two chlorides sit next to each other, the same platinum atom can pick up two guanine bases, tying them together and kinking the strand. The double helix can then no longer unwind and separate, so the enzymes that copy DNA cannot run along it: replication stops and the cell cannot divide. Note what is not happening. No covalent bond in the sugar-phosphate backbone is broken; the damage is a new bond made, not an old one destroyed.
Appreciate that society needs to assess the balance between the benefits and the adverse effects of drugs, such as the anticancer drug cisplatin.
Cisplatin is among the most effective anticancer drugs ever made; testicular cancer, once usually fatal, is now cured in the great majority of cases largely because of it. The harm comes from the same chemistry: platinum binds to guanine in whatever cell it reaches, not only in a tumour, so healthy tissues whose cells divide quickly suffer most. That is why patients lose hair, feel sick and become vulnerable to infection as the bone marrow makes fewer white cells, and why kidney damage and hearing loss are monitored. Weighing those costs against years of life gained is not a judgement chemistry can make alone; it involves the patient's informed consent, clinicians, regulators and the cost to a health service. Research into targeted delivery aims to cut the dose reaching healthy tissue.
Students should be able to: explain why cisplatin prevents DNA replication explain why such drugs can have adverse effects.
Two explain commands, and each needs a chain of reasoning rather than a fact. For the first, the chain runs: the chloride ligands are substituted, a lone pair on a nitrogen atom of guanine forms a coordinate bond to platinum, the adjacent arrangement lets one platinum grip two guanines, the strands can no longer separate, so the copying enzymes cannot work and the cell cannot divide. For the second, the chain runs: platinum binds guanine wherever it finds it, healthy cells contain the same base, and the tissues hit hardest are those dividing fastest, so bone marrow, hair follicles and the gut lining are damaged. Both chains end at a consequence, for the cell or for the patient, and that is where the final mark usually sits.
Your focus
- Draw the structure of the drug, showing square planar geometry with the chloride ligands adjacent.
- Deduce the oxidation state of platinum in the complex from the charges on its ligands.
- Distinguish the drug from its trans isomer in a diagram and state which one is used medically.
Show all 16 objectives
- State the coordination number of platinum here and justify it from the number of coordinate bonds.
- Write an equation for the reaction of the drug with two guanine bases, showing the two chloride ions released.
- Explain why a lone pair on a nitrogen atom of guanine is able to bond to platinum.
- Explain why the adjacent arrangement of the two chloride ligands is essential to the drug's action.
- Link the formation of the platinum to guanine bond to the failure of DNA replication and of cell division.
- Explain why an effective anticancer drug can still cause serious harm, using the non-selective binding of platinum.
- Name two healthy tissues damaged by the drug and state the adverse effect each produces.
- Given survival figures for a cancer treated with the drug and the incidence of a named side effect, argue for or against its use and quote numbers from the data in support.
- Suggest one way of reducing adverse effects and explain how it would work.
- Sequence the steps from the drug entering a cell to replication being blocked.
- Explain each of those steps in terms of bonding rather than description.
- Explain adverse effects by applying the same binding chemistry to healthy tissue.
- Suggest why the dose given to a patient must be controlled carefully, using the fact that platinum binds guanine in healthy cells as readily as in cancer cells.
Action of anticancer drugs (A-level only) exam tips
Marking Points
- one mark for the formula [Pt(NH₃)₂Cl₂], with two ammonia and two chloride ligands
- one mark for describing the shape as square planar, with bond angles of 90°
- one mark for deducing the oxidation state of platinum as +2, because ammonia is neutral and the complex carries no overall charge
- one mark for drawing the two chloride ligands adjacent to each other, which is the cis isomer
- one mark for stating that the chloride ligands, not the ammonia ligands, are the ones replaced
- one mark for a coordinate bond formed from a lone pair on a nitrogen atom of guanine to the platinum
- one mark for the two binding sites being adjacent, so one platinum bonds to two guanine bases
- one mark for explaining that the strands can no longer separate, so DNA cannot be replicated and the cell cannot divide
- one mark for stating that the drug also affects healthy cells and not only cancer cells
- one mark for naming a rapidly dividing healthy tissue such as bone marrow, hair follicles or the lining of the gut
- one mark for naming a specific adverse effect and linking it to the tissue affected
- one mark for a balanced judgement that names a benefit as well as a risk
- one mark for naming ligand substitution of the chloride ligands as the first step
- one mark for the coordinate bond from a nitrogen atom of guanine to the platinum
- one mark for the consequence that the strands cannot separate, so replication is prevented
- one mark for explaining adverse effects by the same binding happening in healthy cells
- one mark for identifying rapidly dividing healthy cells as the ones most affected
Examiner Tips
- 💡Draw a square with platinum at the centre and a ligand at each corner; a sketch earns the shape mark that words often miss.
- 💡State the oxidation state with its sign and show the reasoning from the charges on the ligands.
- 💡If the question names the drug, the geometry is already given to you, so check your drawing agrees with the name.
- 💡Use the phrases ligand substitution and coordinate bond; both are specification vocabulary and both are usually marking points.
- 💡Name guanine and name nitrogen. Binds to a base is too vague to credit.
- 💡Two marks means two ideas, the bond formed and the consequence for replication, so write both.
- 💡A question that says discuss or evaluate wants both sides, so write at least one benefit and one adverse effect explicitly.
- 💡Tie each adverse effect to a tissue whose cells divide quickly, which is the chemistry-based reason being looked for.
- 💡Keep the answer specific to this drug; general remarks that all medicines carry risks earn nothing.
- 💡Write the mechanism as numbered steps; explain marks are awarded for the links between steps.
- 💡Check that you have answered both halves of a two-part explain question; students routinely answer the first and forget the second.
- 💡Reuse the specification's own words, ligand replacement and coordinate bond, because the mark scheme is written from them.
Common Mistakes
- drawing the complex as tetrahedral, which is not the shape adopted by platinum in this oxidation state
- writing the formula with an overall charge, when two chloride ligands and platinum in the +2 state make it neutral
- putting the two chloride ligands opposite each other, which draws the trans isomer that is not used as a drug
- calling the cis and trans forms optical isomers
- giving the coordination number as two because only the chloride ligands are charged
- saying the ammonia ligands are replaced, when it is the chlorides that are substituted
- calling the new platinum to nitrogen bond ionic, or calling it a hydrogen bond, rather than coordinate
- claiming the drug breaks the sugar-phosphate backbone
- saying it bonds to the sugar or the phosphate instead of to a nitrogen atom on guanine
- stopping at binds to DNA without saying that replication and cell division are prevented
- answering only that the drug has side effects, without saying why healthy cells are attacked
- claiming the drug can tell a cancer cell from a healthy one
- listing adverse effects with no benefit at all when the question asks for a balance
- writing about cost or ethics in general terms without any chemistry of the drug
- describing what the drug does but never reaching the consequence that the cell cannot divide
- explaining adverse effects as an allergic reaction rather than as binding to DNA in healthy cells
- writing that it attacks the cancer, which explains nothing
- muddling the two explanations, so the answer about side effects repeats the replication mechanism