Influence of verapamil on tumour cell growth in vitro following treatment with antiblastic drugs.

The effect of antiblastic drugs with different mechanisms of action has been investigated in LoVo cells (on the resistant line LoVo R and the parental LoVo S) for 48 h and for further 24 h in drug-free medium (recovery time) in order to evaluate drug action after its removal. Our results show that the drug effect persists in time, particularly in LoVo R cells. The inhibitory effect on cell growth was enhanced when the drugs were associated with verapamil, while no difference was detected with cisplatin and 4-hydroperoxycyclophosphamide, drugs which are not involved in P-glycoprotein-mediated multidrug resistance. These results demonstrate that verapamil is an effective agent in association with those drugs which are actively transported out of the tumour cells, while it is ineffective on drugs which can induce resistance by different mechanisms.