VPT 311: General and Systemic Veterinary Pharmacology (2+1)

ASSAY OF DRUGS 

• Assay of drugs refers to the quantitative estimation of a drug / its active component in a formulation.
• It is carried out for the following purposes:
• To test the purity of a drug preparation.
• To compare the potency of a drug with that of a standard.
• To assess the efficiency of preparation while purifying the drug from a mixture / natural product.

Assays are of different types

• Chemical assay
• In this the concentration of drug is determined by chemical methods suah as spectrophotometry, fluorometry, HPLC or mass spectroscopy. These methods use one of the physiochemical properties of the compounds such as absorbance, fluorescence, polarity, charge to mass ratio etc. These are the most commonly used methods.
• Bioassay
• In this the concentration of biologically active component in a unit quantity of the test drug is compared to the response produced by the standard or reference drug. Bioassays are more cumbersome but less accurate when compared to chemical assays. Eg: assay of histamine in isolated guinea pig ileum, assay of acetylcholine in frog rectus abdominis muscle and assay of antibiotics using bacteria.
• Immunoassay
• This is commonly used for the assay of hormones. The method depends on the binding between the radiolabelled hormone and the corresponding antibody. The method is highly sensitive to detect minute quantities of the hormones in body fluids.