Welcome to the Analysis of Medicines

Imagine picking up a packet of pain relief tablets or an antibiotic suspension from the pharmacy. How do we know that each tablet contains the exact dose of the active drug? How do scientists verify that a life-saving vaccine actually triggers the intended biological response?

In this chapter of AS 6: Medicine, Drugs and Clinical Trials, we explore two fundamental methods used in pharmaceutical quality assurance: chemical quantitative analysis (such as titrations and calibration curves) and bioassays (measuring biological activity in living systems). Don't worry if quantitative chemistry or biological testing seems daunting at first; we will break down each method step-by-step!

Part 1: Quantitative Chemical Analysis

Chemical analysis involves measuring the exact amount or concentration of an active pharmaceutical ingredient (API) in a medicine using standard laboratory reactions.

1. Titrations (Volumetric Analysis)

Titration is a technique where a solution of known concentration (a standard solution) is added gradually to a solution of unknown concentration until the reaction reaches its neutralisation or endpoint.

• Acid-Base Titrations: Used to determine the concentration of acidic or basic active ingredients. For example, testing the amount of Aspirin (acetylsalicylic acid) in a tablet by titrating it against a standardised base such as sodium hydroxide.
• Redox Titrations: Used when the active ingredient undergoes oxidation or reduction. For example, determining the concentration of Vitamin C (ascorbic acid) in a medicine or supplement.

What is a Primary Standard?

A primary standard is a reagent that is extremely pure, stable, has a high molar mass, and does not absorb water or gases from the air. Because of its known high purity, a primary standard can be weighed accurately to prepare a solution of known, precise concentration, which is then used to standardise other solutions.

Essential Formulae and the "Golden Rule" of Volume

In your CCEA calculations, the key formula linking moles, concentration, and volume is:

\(n = C \times V\)

Where:
• \(n\) = number of moles (\(mol\))
• \(C\) = concentration (\(mol/dm^3\))
• \(V\) = volume (\(dm^3\))

The Golden Rule of Volume: Laboratory burettes and pipettes measure volume in cubic centimetres (\(cm^3\)), but concentration is given in cubic decimetres (\(dm^3\)). You must divide \(cm^3\) by \(1000\) before calculating:

\(V\ (dm^3) = \frac{V\ (cm^3)}{1000}\)

CCEA Exam Conventions for Titration Data

Examiners look for specific laboratory recording conventions:

• Precision: Burette readings must be recorded to one decimal place (or consistent with the instrument resolution specified).
• Titration Table Format: Your results table should clearly show three rows or columns:
  1. Initial Burette Reading (\(cm^3\))
  2. Final Burette Reading (\(cm^3\))
  3. Titre (\(cm^3\)) (calculated as Final \(-\) Initial)
• Concordant Results: Titres that are within \(0.10\ cm^3\) of each other are considered concordant. Only concordant titres should be averaged to find your mean titre value; rough/initial trial runs that fall outside this range must be discarded.

2. Colorimetry and Calibration Curves

When an active ingredient is coloured (or can react to produce a coloured complex), scientists use colorimetry or spectrophotometry to find its concentration.

How it works:
1. Prepare a series of known standard solutions of the drug across a range of concentrations.
2. Measure the light absorbance of each standard solution using a colorimeter.
3. Plot a calibration curve with Absorbance on the vertical y-axis and Concentration on the horizontal x-axis.
4. Measure the absorbance of the unknown medicine sample.
5. Use the calibration curve to find the unknown absorbance on the y-axis, draw a line across to the curve, and read down to the x-axis to determine its exact concentration.

Part 2: Bioassays (Biological Analysis)

What happens if a medicine cannot be analysed using standard chemical titration or colorimetry? This is where bioassays become essential.

What is a Bioassay?

A bioassay is an analytical method used to estimate the concentration or potency of a substance by measuring its biological effect on living cells, living tissues, or whole organisms.

Why Use a Bioassay Instead of Chemical Testing?

• Some medicines are complex biological macromolecules (e.g., vaccines, insulin, or antibiotics) whose chemical structure is too intricate for simple chemical assays.
• Chemical tests measure the mass or amount of a substance, but they cannot prove whether that substance is biologically active. Bioassays confirm that the medicine produces the required therapeutic effect in a living biological system.

Types of Bioassays: Graded vs. Quantal

Bioassays fall into two main categories:

1. Graded Bioassays:
• A graded bioassay measures a continuous, progressive response that increases proportionally as the dose increases.
• Examples: Measuring the degree of contraction in an isolated strip of muscle tissue as drug concentration rises, or measuring the diameter of the clear zone of inhibition on an agar plate around an antibiotic disc.

2. Quantal Bioassays (All-or-Nothing):
• A quantal bioassay measures whether an individual organism shows a specific, predefined response or not (yes/no, all-or-nothing). It tracks the percentage of a population showing that response at various doses.
• Key Example: Determining the \(LD_{50}\) (Lethal Dose 50), which is the dose of a drug required to kill \(50\%\) of a test animal population. Quantal assays can also measure the effective dose required to produce a specific symptom relief in \(50\%\) of subjects.

International Standards and Units

Because living tissues and organisms naturally vary, bioassays do not report drug amounts solely in milligrams. Instead, the test sample is always tested alongside an official International Standard (IS).

The activity of the medicine is then expressed in International Units (IU), which quantify the specific biological potency compared to that recognised standard reference.

Part 3: Common Pitfalls and Key Distinctions

Ensure you avoid these common exam mistakes:

• Purity vs. Potency:
  - Purity is a chemical measure indicating the absence of contaminants or unwanted by-products.
  - Potency is a biological measure indicating the amount of biological activity or effect produced per unit dose relative to an International Standard.
  Do not use these words interchangeably!

• Units Slip-ups: Always check your volumes. If a titration question gives volumes in \(cm^3\), divide by \(1000\) before using \(n = C \times V\).

• Concordancy Check: Only average titres that are within \(0.10\ cm^3\) of each other. Never include your rough/initial overshoot trial in the final mean calculation.

Quick Review Summary

• Titrations: Used for chemical quantitative analysis (e.g., acid-base for Aspirin, redox for Vitamin C). Requires primary standards and concordant titres within \(0.10\ cm^3\).
• Calibration Curves: Colorimeter measures light absorbance of known standard concentrations; unknown concentration is found by interpolation.
• Bioassay: Measures potency through biological effects on living systems. Used for complex biologicals like insulin, vaccines, and antibiotics.
• Graded Bioassay: Measures a continuous scale response (e.g., muscle contraction, zone of inhibition).
• Quantal Bioassay: Measures an all-or-nothing response in a population (e.g., \(LD_{50}\)).
• Standards: Bioassays compare unknown samples against an International Standard (IS) and report activity in International Units (IU).