What percentage of new compounds investigated may show suitable biological activity?

• A. more than 50%
• B. about 25%
• C. less than 2%
• D. exactly 10%

Answer: (C)

In drug discovery, the chance that a newly screened compound shows meaningful biological activity against a specific target is incredibly small. Large libraries are tested, but only a tiny fraction bind with enough affinity and specificity to be considered “hits.” This is because binding to a target depends on precise shape, charge, and functional groups, and most random molecules don’t fit well or produce the desired effect. Even among those that show some activity, many fail later due to poor potency, lack of selectivity, or undesirable drug-like properties, so the pool of viable leads remains very small. That’s why the typical hit rate is very low—often well under 2%—making less than 2% the most accurate reflection of how many new compounds may show suitable biological activity. Higher percentages would imply screening yields hits far more readily than is observed in practice.