Tenidap modulates cytoplasmic pH and inhibits anion transport in vitro. I. Mechanism and evidence of functional significance.

Tenidap is a novel anti-inflammatory and antiarthritic agent that in clinical studies of rheumatoid arthritis patients, displays symptomatic efficacy superior to nonsteroidal anti-inflammatory drugs (NSAIDs) and equivalent to combinations of NSAIDs and second line agents. Clinical and preclinical biochemical studies have demonstrated that tenidap combines cytokine modulation with suppression of prostaglandin biosynthesis. To better understand tenidap's mechanism of action, in vitro studies of intracellular pH (pHi) were conducted. In cells loaded with the pH-sensitive fluorescence dye 2',7'-bis-(2-carboxyethyl)-5-(and -6) carboxyfluorescein, tenidap, but not NSAIDs, caused a rapid and sustained acidification of the cytoplasmic compartment. Tenidap did not act as a proton ionophore, as it did not dissipate the low pH within lysosomes. Mammalian cells regulate pHi through the concerted action of a number of specific transport proteins, including sodium-proton antiporters and chloride-bicarbonate exchangers. Tenidap did not alter pHi via inhibition of the sodium-proton antiporter, but inhibited activity of chloride-bicarbonate exchangers, as did UK5099, a known anion-transport inhibitor that also lowers pHi. This similar activity suggests that the pHi change is coupled to anion transport inhibition. As a result of the pHi change, tenidap affected pH-dependent cellular activities. Tenidap inhibited mannose 6-phosphate receptor-mediated endocytosis, inhibited protein synthesis, and stimulated accumulation of the amino acid leucine. Effects on these cellular processes rapidly reversed when tenidap was removed from the culture medium. Tenidap's in vitro activities were highly dependent on the medium composition; protein content, pH, and bicarbonate concentration all were important factors that influenced activity. These results indicate that tenidap is a potent anion-transport inhibitor and modulator of pHi. Within the appropriate cell or tissue microenvironment, these activities may contribute to tenidap's novel therapeutic profile.