In vitro evaluation of methotrexate and azathioprine for antipsoriatic activity.

The effects of methotrexate and azathioprine, two drugs used in antipsoriatic therapy, on oxygen consumption of surviving human skin and on enzymatic activities of human skin homogenates were investigated. In concentrations of 1 mM/1, both substances provoked a significant decrease in oxygen consumption of human skin; in this respect, there was practically no difference between methotrexate and azathioprine. In the enzyme assays, however, azathioprine was, by far, less effective than methotrexate. After an incubation of 120 min azathioprine (1mM/1) inhibited lactate and glucose-6-phosphate dehydrogenase activities by about 10 per cent only, whereas the corresponding values with methotrexate amounted to 80 and 70 per cent, respectively. Methotrexate revealed an immediate inhibitory effect on pure glucose-6-phosphate dehydrogenase whereas azathioprine produced no changes in this mode. Furthermore, only methotrexate inhibited "acid" phosphatase activity of human skin homogenates.--These data sustain the theory that the better clinical efficacy of methotrexate in patients with psoriasis might be due to the more pronounced inhibition of important enzymes such as the enzymes of the pentose phosphate shunt.