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Inhibition of Cancer Cell Growth by Calcium Channel Antagonists in the Athymic Mouse¹

Department of Pharmacology [J. M. T., R. U. S.] and The Center for Molecular Genetics [R. U. S.], The University of Michigan, Ann Arbor, Michigan 48109-0626

The calcium channel antagonists (CCAs) amlodipine, diltiazem, and verapamil inhibited HT-39 human breast cancer cell proliferation in a concentration-dependent manner. The apparent 50% inhibitory dose values were 1.5 µM for the dihydropyridine amlodipine, 5 µM for the benzothiazapine diltiazem, and 10 µM for the phenylalkylamine verapamil. Amlodipine treatment caused a rapid concentration-dependent decrease of intracellular calcium concentration in the HT-39 cell line. Addition of 1 µM amlodipine had no effect on intracellular calcium levels, 3 µM amlodipine lowered intracellular calcium levels in the HT-39 cells by 13.7%, and 10 µM amlodipine lowered intracellular calcium levels by 33.2%. Also, lowering medium calcium levels from 2.0 mM to 0.5 µM resulted in a rapid 41.3% decrease in intracellular calcium and a concomitant 60% inhibition of HT-39 cell DNA synthesis. When HT-39 cells were transplanted into athymic mice, marked hypercalcemia developed. Serum calcium levels from control mice were 8.3 ± 0.6 mg/dl (mean ± SE; n = 4); those from tumor-bearing mice were 11.3 ± 0.08 mg/dl (mean ± SE; n = 17). Blood calcium levels correlated directly with tumor size (r = 0.91, P < 0.01). We examined the capacity of three CCAs to specifically inhibit HT-39 tumor growth in vivo. One week after inoculation of HT-39 cells, mice were acclimated to vehicle or 0.1 mg/day amlodipine, 1.0 mg/day diltiazem, or 1.0 mg/day verapamil, in their drinking water, for 7 days. Oral administration of the dihydropyridine amlodipine (0.35 mg/day) for 10 days inhibited HT-39 breast tumor growth by 83.5 ± 20.1% (mean ± SE). Oral administration of diltiazem (3.5 mg/day) inhibited HT-39 breast tumor growth rate by 46.5 ± 6.6% over a 2-week measurement period, and verapamil (3.5 mg/day) inhibited tumor growth rate by 68.2 ± 9.7% (mean ± SE). The CCAs had no effect on mouse body weight or gross organ morphology at the concentrations used. Lack of depolarization-induced calcium fluxes in the HT-39 cell line suggests that these cells do not express voltage-operated calcium channels. Thus, our study correlates an effect of amlodipine to lower intracellular calcium levels, by a mechanism not known at present, with its effect to inhibit HT-39 cell proliferation. These findings are important since they demonstrate that amlodipine and other CCAs with known pharmacodynamics and side effects act to blunt breast tumor progression in vivo.

¹ This work was supported by grants from NIH (CA-43859), the Children's Leukemia Foundation of Michigan, and the Breast Cancer Research Institute of the University of Michigan.

² To whom requests for reprints should be addressed, at Department of Pharmacology, University of Michigan Medical School, 6322 Medical Science Building I, Ann Arbor, MI 48109-0626.

Received 1/21/92. Accepted 2/26/92.

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