RBS2418 (investigational product) is a specific immune modulator, working through ectonucleotide pyrophosphatase/phosphodiesterase I (ENPP1), designed to lead to anti-tumor immunity by increasing endogenous 2'-3'-cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) and adenosine triphosphate (ATP levels) and reducing adenosine production in the tumors. RBS2418 has the potential to be an important therapeutic option for subjects both as monotherapy and in combination with other cancer treatments including monotherapy and in combination with other cancer treatments including immunotherapy or chemotherapy. This study is an open-label, multi-site Phase 1a/1b study of RBS2418, a selective ENPP1 inhibitor, in combination with pembrolizumab or other approved anticancer therapies or as a monotherapy in subjects with advanced unresectable, recurrent or metastatic tumors. The phase 1a (dose escalation phase) has been completed. The Phase 1b expansion phase of the study has been increased in size and scope.