The present invention relates to combination therapy with drugs, such as microtubule inhibitors, PARP inhibitors, Bruton kinase inhibitors or PI3K inhibitors, with antibodies or immunoconjugates against HLA-DR or Trop-2. Where immunoconjugates are used, they preferably incorporate SN-38 or pro-2PDOX. The immunoconjugate may be administered at a dosage of between 1 mg/kg and 18 mg/kg, preferably 4, 6, 8, 9, 10, 12, 16 or 18 mg/kg, more preferably 8 or 10 mg/kg. The combination therapy can reduce solid tumors in size, reduce or eliminate metastases and is effective to treat cancers resistant to standard therapies, such as radiation therapy, chemotherapy or immunotherapy. Preferably, the combination therapy has an additive effect on inhibiting tumor growth. Most preferably, the combination therapy has a synergistic effect on inhibiting tumor growth.