INHIBITION OF GROWTH OF DICTYOSTELIUM-DISCOIDEUM AMEBAS BY BISPHOSPHONATE DRUGS IS DEPENDENT ON CELLULAR UPTAKE

Purpose. The aim of the study was to determine whether bisphosphonates art internalised by Dictyostelium amoebae and whether cellular uptake is required for their growth-inhibitory effects. Bisphosphonates inhi bit growth of amoebae of the slime mould Dictyostelium discoideum, by mechanisms that appear to be similar to those that cause inhibition of osteoclastic bone resorption. Methods. Cell-free extracts prepared fr om amoebae that had been incubated with bisphosphonates were analysed by P-31-n.m.r. spectroscopy or ion-exchange f.p.l.c., to identify the presence of bisphosphonates or bisphosphonate metabolites respectively . The growth-inhibitory effect of bisphosphonates towards Dictyosteliu m amoebae was also examined under conditions in which pinocytosis was inhibited. Results. All of the bisphosphonates studied were internalis ed by Dictyostelium amoebae, probably by fluid-phase pinocytosis, and could be detected in cell-free extracts. Amoebae that were prevented f rom internalising bisphosphonates by pinocytosis were markedly resista nt to the growth-inhibitory effects of these compounds. In addition, b isphosphonates encapsulated within liposomes were more potent growth i nhibitors Dictyostelium owing to enhanced intracellular delivery of bi sphosphonates. Conclusions. All bisphosphonates inhibit Dictyostelium growth by intracellular mechanisms following internalisation of bispho sphonates by fluid-phase pinocytosis. It is therefore likely that bisp hosphonates also affect osteoclasts by interacting with intracellular, rather than extracellular, processes.