Aromatic fatty acids such as phenylbutyrate (PB) and its metabolite phenylacetate (PA) induce growth arrest, differentiation and apoptosis in solid tumor cells. Despite their antiproliferative action they were reported to exhibit a synergistic effect in combination with cytotoxic drugs like topotecan, and others. Since the activity of the camptothecines (CPTs) depends on local pH conditions, we investigated, whether PB/PA modulate CPT effects indirectly by affecting intracellular pH in SW620 and SW480 colon cancer cells. The results for the colon carcinoma cells show an antagonistic interaction for the combination of CPT and 0.25-5 mM PA in viability assays, resulting in an approximately 3-fold increase in IC50 (control: 20±7 nM). A synergistic effect with significantly increased numbers of late apoptotic/necrotic cancer cells (difference+ 21±4%) and 1.4-fold sensitization were detected upon inclusion of 2.5 mM PA during a 4-h CPT (10 μM) loading phase. In response to 0.25-1 mM PA/PB the cells exhibit a reversible decrease of pHi (0.1-0.31 pH units) in HEPES-or bicarbonate-buffered media. Dose-dependent acidification and pHi-recovery occurred following addition of PA and PB after an acid load and inhibition of the Na+/H+-antiporter and bicarbonate exchangers, pointing to a possible intracellular mechanism of cytoplasmic acidification. It is concluded that the synergistic modulation of CPT toxicity by short-term PA/PB treatment in colon carcinoma cells is caused by changes in intracellular pH, possibly affecting quantity and localization of the active closed lactone form of this drug.