genotoxicity

Genotoxicity is a drug's ability to damage DNA, the cell's instruction manual. Corrupting that manual can scramble the genetic code, and if a damaged cell survives and keeps dividing, the resulting mutations may eventually seed cancer. For this reason genotoxicity is one of the most serious red flags a candidate drug can carry.

Genotoxic damage takes several forms: a reactive molecule may chemically bond to DNA bases (forming an adduct), wedge itself between base pairs (intercalation), or break the DNA strands outright. Some compounds act directly, while many become genotoxic only after metabolic activation into a reactive electrophile, which is why testing systems usually include a metabolic-activation step.

Regulators require a standard battery of genotoxicity tests before a drug can be given to humans, typically a bacterial mutation assay plus mammalian-cell tests for chromosome damage. A clearly genotoxic compound is almost always abandoned for chronic use, though deliberately DNA-damaging agents are still used in cancer chemotherapy, where killing tumor cells is the goal.