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3.13.3Organic Chemistry

Action of Anticancer Drugs — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 48 of 55

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Part 1 of 5Cisplatin Structure

3.13.3 Action of Anticancer Drugs
Cisplatin Structure
Formula
  • Cisplatin is [Pt(NH₃)₂Cl₂], a platinum(II) complex with two ammonia and two chloride ligands.
Shape
  • It is a four-coordinate square planar complex with bond angles of 90° between ligands.
Cis isomer
  • The two chloride ligands lie next to each other (cis);
    • the isomer with them opposite (trans) is not an effective drug.
Bonding
  • Each ligand donates a lone pair to platinum to form a dative covalent bond.

Part 2 of 5Action on DNA

1
In the cell, where the chloride concentration is low, the two chloride ligands are replaced by water molecules in ligand substitution reactions.
2
The nitrogen atoms on guanine bases of DNA then replace the water ligands, forming dative bonds to platinum.
3
Cisplatin bonds to two adjacent guanine bases on the same DNA strand, because the two vacant cis positions are close together.
4
This cross-link bends the DNA, so the strand can no longer unwind and be copied or read.
5
Cell division is blocked and the damaged cancer cell dies, so tumour growth is slowed.

Part 3 of 5Benefits and Drawbacks

Targets fast growth
  • Cancer cells divide rapidly, so they are damaged by DNA binding more than most healthy cells.
Cancers treated
  • Cisplatin treats testicular, ovarian, cervical, lung and some other cancers, and is often used with other drugs.
Side effects
  • Healthy fast-dividing cells are also hit, causing hair loss and nausea, and the drug can damage the kidneys.
Trans isomer
  • The trans isomer cannot bridge adjacent guanine bases because its chlorides are too far apart, so it is not effective.
Development
  • Related platinum drugs such as carboplatin were developed to reduce side effects.

Part 4 of 5Worked Example

Question

  • Calculate the percentage by mass of platinum in cisplatin, [Pt(NH₃)₂Cl₂].
  • (Ar: Pt 195.1, N 14.0, H 1.0, Cl 35.5)

  1. Mr = 195.1 + 2 × (14.0 + 3 × 1.0) + 2 × 35.5 = 195.1 + 34.0 + 71.0 = 300.1.
  2. Percentage of Pt = (195.1 ÷ 300.1) × 100 = 65.0%.

Answer 65.0% platinum by mass.

Part 5 of 5Exam Checklist & Tips

3.13.3 – Key Points for the Exam

  • Cisplatin The platinum(II) complex cis-[Pt(NH₃)₂Cl₂] used as an anticancer drug.
  • Ligand substitution Replacement of one ligand in a complex by another.
  • Square planar A four-coordinate shape with 90° bond angles in one plane.
  • Dative covalent bond A bond where both electrons come from the same atom.

Common Mistakes

  • Describing cisplatin as octahedral; it is square planar with four ligands.
  • Saying cisplatin attacks the phosphate backbone; it binds to nitrogen atoms in guanine.
  • Forgetting that the trans isomer is inactive because it cannot link adjacent bases.
  • Claiming cisplatin has no side effects, when it harms healthy dividing cells.

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