The present invention relates to substituted fused tricyclic compounds of formula (I) or (Ia), their tautomers, polymorphs, stereoisomers, prodrugs, solvates, co-crystals, pharmaceutically acceptable salts, pharmaceutical compositions containing them and methods of treating conditions and diseases that are mediated by JAK activity. The compounds of the present invention are useful in the treatment, prevention or suppression of diseases and disorders mediated by JAK activity. Such conditions include, but not limited to, arthritis, Alzheimer's disease, autoimmune thyroid disorders, cancer, diabetes, leukemia, T-cell prolymphocytic leukemia, lymphoma, myleoproliferation disorders, lupus, multiple myeloma, multiple sclerosis, osteoarthritis, sepsis, psoriatic arthritis, prostate cancer, T-cell autoimmune disease, inflammatory diseases, chronic and acute allograft transplant rejection, bone marrow transplant, stroke, asthma, chronic obstructive pulmonary disease, allergy, bronchitis, viral diseases, or Type I diabetes, complications from diabetes, rheumatoid arthritis, asthma, Crohn's disease, dry eye, uveitis, inflammatory bowel disease, organ transplant rejection, psoriasis and ulcerative colitis. The present disclosure also relates to process for the preparation of such compounds, and to pharmaceutical compositions containing them.