In vitro and in vivo inhibition of inositol monophosphatase by the bisphosphonate L-690,330.
We have previously described the synthesis of bisphosphonate-containing inhibitors of inositol monophosphatase. In the present study, a more detailed examination of the in vitro and in vivo properties of one of these compounds, L-690,330, is described. L-690,330 is a competitive inhibitor of inositol monophosphatase with a Ki, depending on the source of IMPase, of between 0.2 and 2 microM. Although approximately 1,000-fold more potent in vitro than lithium, in muscarinic ml receptor-transfected Chinese hamster ovary cells prelabelled with [3H]inositol, L-690,330 only produced 40% of the accumulation of [3H]inositol monophosphates achieved by lithium at the same concentration (10 mM), suggesting that the ability of L-690,330 to cross the cell membrane is limited. Nevertheless, under conditions of cholinergic stimulation (100 mg/kg of pilocarpine s.c.), high doses of L-690,330 were able to increase brain inositol(l)phosphate levels in vivo to three- to fourfold control levels. This effect was dose dependent (ED50 = 0.3 mmol/kg s.c.) and was maximal after 1 h. In peripheral tissues, the effects of L-690,330 on inositol(l)phosphate levels mimicked those of lithium both qualitatively and quantitatively. However, in the brain, the effects of L-690,330 were much less than seen with lithium, consistent with the blood-brain barrier restricting access of the polar L-690,330 into the CNS, thereby further limiting entry of compound into cells in the brain. In the future, it may be possible to develop prodrugs of this compound, which circumvent many of the cell permeability problems inherent in bisphosphonate compounds.