Piroxicam and other nonsteroidal anti-inflammatory drugs (NSAID) have antitumor activity against naturally acquired cancer in dogs and human beings, and against experimentally induced tumors in rodents. We are investigating potential mechanisms of NSAID anti-tumor activity. The direct cytotoxicity of piroxicam, indomethacin, and aspirin against 4, canine tumor cell lines (transitional cell carcinoma, squamous cell carcinoma, melanoma, and soft tissue sarcoma) was determined in short-term growth rate assays and in clonogenic assays. Piroxicam was evaluated alone and in combination with the lipoxygenase inhibitor zileuton, and in combination with the chemotherapeutic agents cisplatin and carboplatin. The 50% inhibitory concentrations (IC50) against melanoma cells in short-term growth rate assays were: 530 micromolar piroxicam, 180 micromolar indomethacin, and greater than 1 mM aspirin. These IC50 values were over 10 times greater than serum concentrations of these drugs that could safely be achieved in vivo. The IC50 of zileuton combined with piroxicam (280 micromolar) was not different from the IC50 of zileuton alone (230 micromolar; ANOVA P = 0.47) in melanoma cells. Similarly, addition of piroxicam did not alter the IC50 of either cisplatin (1.6 micromolar) or carboplatin (6.1 micromolar). These results suggest that NSAID, at serum concentrations achievable in vivo, do not have direct cytotoxicity against canine tumor cells tested. It is unlikely that the in vivo antitumor activity of NSAID is attributable to a direct cytotoxic effect.

The effect of oxytetracycline (OTC) on bovine blood mononuclear cells and neutrophil functions was examined in vitro. Neutrophil functions tested include respiratory burst, peroxidase, and antibacterial activities. Neutrophils were treated with OTC (10 to 1,500 > microgram/ml) before exposure to either opsonized zymosan or bacteria. A dose-response inhibition of antibacterial activity to high concentrations of OTC (500 to 1,000 microgram/ml) was observed. Beginning at a concentration of 15 microgram/ml, OTC treatment of neutrophil Iysates resulted in decreased peroxidase activity. A dose response was not observed. In contrast, respiratory burst, measured by nitroblue tetrazolium dye reduction, increased after OTC exposure, but only at high concentrations (500 and 1,000 microgram/ml) of OTC. Mitogen-induced proliferation of blood mononuclear cells cocultured with OTC and concanavalin A, phytohemagglutinin-P, or pokeweed mitogen was inhibited at an OTC concentration of 100 microgram/ml at 48 and 72 hours of culture. These results indicate that blood mononuclear cells are more sensitive to the inhibitory effects of OTC than are neutrophils. Furthermore, the OTC-mediated inhibition of neutrophil antimicrobial activity is inversely related to the increase in nitroblue tetrazolium reduction. This suggests that OTC is uncoupling the hexose monophosphate shunt from production of secreted oxygen radicals. These results also suggest that the peroxidase enzyme system has a large biological reserve capacity.