Tetracycline in the treatment of malignant effusions: evidence for a cytostatic action of the decomposed drug.

During the in vitro growth of human carcinoma cell lines an inhibition of cell growth in media containing tetracycline was observed at a concentration which was about 1000 times below the one reached in the pleural fluid of malignant effusions after tetracycline instillation. During short time drug-cell contact only previously heated tetracycline showed growth inhibition like doxorubicin, a substance of similar structure and origin. Freshly prepared tetracycline inhibited cell growth only after several minutes of drug cell contact. Our observations in vitro suggest that decomposed tetracycline plays an important role in the control of malignant effusions.