EFFECTS OF L-690,488, A PRODRUG OF THE BISPHOSPHONATE INOSITOL MONOPHOSPHATASE INHIBITOR L-690,330, ON PHOSPHATIDYLINOSITOL CYCLE MARKERS

In order to enhance the entry into cells of L-690,330, a bisphosphonate inhibitor of inositol monophosphatase (IMPase; a key, enzyme in the phosphatidylinositol (Pl) cell signaling pathway), the tetrapivaloyloxymethyl ester prodrug, L-690,488 [tetrapivaloyloxymethyl 1-(4-hydroxyphenoxy)ethane-1,1-bisphosphonate], was synthesized. The effects of L-690,488 were studied in cholinergically (carbachol)-stimulated rat cortical slices and Chinese hamster ovary cells stably transfected with the human muscarinic m1 receptor (m1 CHO cells). The accumulation of [3H]inositol monophosphates or [3H]cytidine monophosphorylphosphatidate ([3H]CMP-PA) after [3H]inositol or [3H]cytidine prelabeling, respectively, and inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate mass were measured. In rat cortical slices and m1 CHO cells, the maximum response and time course of accumulation of [3H]inositol monophosphates for L-690,488 and lithium were similar. However, the concentrations of L-690,488 required to produce these effects (EC50 values of 3.7 +/- 0.9 and 1.0 +/- 0.2 microM in cortical slices and m1 CHO cells, respectively) were much lower than with lithium (0.3-1.5 mM). Likewise, the time course and maximum accumulation of [3H] CMP-PA in L-690,488-treated m1 CHO cells was similar to lithium but L-690,488 was again much more potent (EC50 values = 3.5 +/- 0.3 microM and 0.52 +/- 0.03 mM for L-690,488 and lithium, respectively). In addition, L-690,488 attenuated the carbachol-induced elevation of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in m1 CHO cells, an effect reported previously with lithium. These results are all consistent with L-690,488 and lithium both depleting intracellular inositol as a consequence of inhibition of IMPase. That these effects of L-690,488 on the PI cycle are indeed due to inositol depletion is shown by the observation that the effects of L-690,488 on CMP-PA accumulation could be overcome by addition of exogenous myo-inositol (EC50 = 1.7 +/- 0.5 mM). These data show that inhibition of IMPase produces effects on the PI cycle comparable to lithium. As a corollary, the effects of lithium on the PI cycle are therefore consistent with its major mechanism of action being inhibition of IMPase.