Evidence for a phase transition in the early development of prehension

The genotoxicity of twenty one clinically used (or discontinued) antihistamines is reviewed. New results are also presented from an evaluation of selected antihistamines in the V79 in vitro micronucleus assay. For two antihistamines, no genotoxicity data is available. Of the remaining nineteen, nine have been reported as positive and one equivocal in at least one genotoxicity assay despite the fact that none possess structural alerts for genotoxicity. Ethidium displacement and bleomycin amplification studies in V79 cells indicate that nine of these ten antihistamines are capable of intercalative DNA binding. Further, nine of the ten positive compounds, but none of the tested compounds which also intercalate but are reported to be negative in gene-tox assays (e.g. triprolidine, chlorcyclizine, clemastine), possess a dimethylamino substituent suggesting the requirement for this cationic function in the genotoxicity. It is proposed that the apparent genotoxicity of antihistamines and possibly many other pharmaceuticals derives from a hitherto unappreciated propensity of these drugs for stabilized intercalative DNA binding. It is further proposed that the bleomycin amplification assay may provide a widely applicable means for assessing functional intercalative drug/DNA interaction in intact mammalian cells.