Analogs and homologs of
PDMP were synthesized, based on its structure (D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol). This compound had previously been found to block the synthesis of GlcCer (
glucosylceramide). Increasing the acyl chain length from 10 to 16
carbon atoms greatly enhanced the efficacy of the
enzyme inhibitor, as did the use of a less polar cyclic
amine, especially a
pyrrolidine instead of a
morpholine ring. Replacement of the phenyl ring by a chain corresponding to
sphingosine also yielded a strongly inhibitory material. By using a chiral synthetic route, we showed that the isomers active against
GlcCer synthase had the R,R-(D-threo)-configuration. However, strong inhibition of the growth of human
cancer cells in plastico was produced by both the threo and erythro racemic compounds, showing involvement of an additional factor (beyond simple depletion of cell
glycosphingolipids by blockage of GlcCer synthesis). The growth arresting effects could be correlated with increases in cellular
ceramide and diglyceride levels. The aliphatic pyrrolidino compound was strongly inhibitory toward the
glucosyltransferase and produced almost complete depletion of
glycolipids, but did not inhibit growth or cause an accumulation of
ceramide. Attempts were made to see whether the differences in growth effects could be attributed to the influence of the inhibitors on related
enzymes (
ceramide and
sphingomyelin synthase and
ceramidase and
sphingomyelinase). While some stimulation of
enzyme activity was noted, particularly at high inhibitor concentrations (50 microM), these findings did not explain the differing effects of the different inhibitors. The best inhibitors of
GlcCer synthase compared favorably in efficacy with some
cancer chemotherapeutic drugs in current use when tested with a battery of human
cancer cells.