1. The inhibition of the human liver phenol sulphotransferase (HL-PST) and
catechol sulphotransferase (HL-CST) by five fenamates has been studied and
the activities of HL-PST and I-IL-CST were measured with 4-nitrophenol and
dopamine as substrates, respectively.
2. The IC50 for inhibition of HL-PST were 0.02 mu M (mefenamic acid); 0.12
mu M (tolfenamic acid); 0.28 mu M (niflumic acid); 0.87 mu M (meclofenamic
acid) and 1.50 mu M (mefenamic acid).
3. HL-CST was less susceptible than HL-PST to the inhibition by fenamates a
nd the IC50 for HL-CST were 36 mu M (tolfenamic acid); 70 mu M (flufenamic
acid); 76 mu M (mefenamic acid); 180 mu M (niflumic acid) and 185 mu M (mec
lofenamic acid).
4. The ratios of the IC50 for HL-CST:HL-PST were drug-dependent and ranged
from 47 (flufenamic acid) to 3800 (mefenamic acid). Mefenamic acid is a rel
atively potent and selective inhibitor of HL-PST.
5. The IC50 for HL-PST obtained with mefenamic acid was three orders of mag
nitude lower than the peak plasma concentration of this drug after an oral
dose of 0.5 g. Accordingly, mefenamic acid should impair sulphation in vivo
.