Iminosugars: potential inhibitors of liver glycogen phosphorylase

The first synthesis of the single isomers (3R,4R,5R); (3S,4S,5S): (3R,4R,5S) and (3S,4S,5R) of 5-hydroxymethyl-piperidine-3,4-diol from Arecolin is reported, including the synthesis of a series of N-substituted derivatives of the (3R,4R,5R)-isomer (Isofagomine). The inhibitory effect of these isomers as well as of a series of N-substituted derivatives of the (3R,4R,5R)-isomer and selected hydroxypiperidine analogues on liver glycogen phosphorylase (GP) showed that the (3R,4R,5R) configuration was essential for obtaining an inhibitory effect at submicromolar concentration. The results also showed that all three hydroxy groups should be present and could not be substituted, nor were extra OH groups allowed if sub-micromolar inhibition should be obtained. Some inhibitory effect was retained for N-substituted derivatives of Isofagomine; however, N-substitution always resulted in a loss of activity compared to the parent compound, IC50 values ranging from 1 to 100 microM were obtained for simple alkyl, arylalkyl and benzoylmethyl substituents. Furthermore, we found that it was not enough to assure inhibitory effect to have the (R,R,R) configuration. Fagomine, the (2R,3R,4R)-2-hydroxymethylpiperidine-3,4-diol analogue, showed an IC50 value of 200 microM compared to 0.7 microM for Isofagomine. In addition, Isofagomine was able to prevent basal and glucagon stimulated glycogen degradation in cultured hepatocytes with IC50 values of 2-3 microM.