Kinetics of cyclocreatine and Na⁺ cotransport in human breast cancer cells: Mechanism of activity

The growth-inhibitory effect of cyclocreatine (CCr) and the kinetics of CCr and Na⁺ cotransport were investigated in MCF7 human breast cancer cells and its adriamycin-resistant subline with use of ³¹P- and ²³Na-NMR spectroscopy. The growth-inhibitory effect in the resistant line occurred at a lower CCr concentration and was more pronounced than in the wild-type line. This correlated with an ~10-fold higher affinity of CCr to the transporter in the resistant line. The passive diffusion coefficient of CCr was also higher in the resistant line by three- to fourfold. The transport of CCr was accompanied by a rapid increase in intracellular Na⁺. This increase was found to depend on the rate of CCr transport and varied differently with CCr concentration in the two cell lines. It is proposed that the cotransport of CCr and Na⁺ followed by increased Na⁺ concentration, together with the accumulation of the highly charged phosphocyclocreatine, are responsible for cell swelling and death.