We synthesized a series of thienopyrimidine derivatives and examined their cytotoxic effects on several cell lines. One of the derivatives, NSL-1406, was shown to exert potent cytotoxic effects on leukemia cell line including P388 cells and J774 cells. It was also inhibitory on mouse osteoclasts and suppressed the in vitro bone resorption by osteoclasts at nanomolar concentrations.