The present invention provides a novel composition of matter useful for the treatment of neoplastic diseases. The novel composition is synergistic and comprised of galectin-3C in combination with a proteosome inhibitor, the combination having a pharmacologic activity greater than the expected additive effect of its individual components. Other embodiments of the invention provide novel synergistic compositions of galectin-3C with a proteasome inhibitor capable of reducing or overcoming resistance that develops to the proteasome inhibitor or reducing the adverse side effects from the proteasome inhibitor through increasing the therapeutic efficacy of lower doses.