In this work, novel series of pyran, thiophene and thienopyrimidine derivatives based on 2-acetamide-thiadiazole scaffold were designed and synthesized for evaluation as selective COX-2 inhibitors in-vitro and investigated in-vivo as anti-inflammatory and analgesic agents against carrageenan-induced rat paw oedema model in irradiated rats, since its well-known that ionizing radiation plays an important role in exaggerating the inflammatory responses and in enhancing the release of inflammatory mediators in experimental animals. Toxicological studies were carried out to evaluate the ulcerogenic activity, acute toxicity and kidney and liver functions for the most potent compounds. In order to understand the binding mode of the synthesized compounds into the active site of COX-2, docking study was performed. Most of the tested compounds showed high inhibitory ability to COX-2. Among them, thiadiazole derivatives bearing thiophene and thienopyrimidine moieties were the most active derivatives, compound 26 showed extremely high selectivity index (SI) of >555.5μM which is nearly two folds better than celecoxib (>277.7μM), in addition to compounds 3, 16, 17, 21 and 26 with SI in the range of >308.6- >384.6μM. The 4-chlorothieno[2.3-d]pyrimidine derivative of thiadiazole 21 showed the highest anti-inflammatory activity in this study having 24.49% of oedema compared to celecoxib (18.61%) in addition to compounds 17 and 26 with 24.70 and 25.40% of oedema, respectively, while the thiadiazol-2-acetamide derivative 2 was the most potent analgesic compound with the highest nociceptive threshold (85.72g) very close to that of celecoxib (90.23g). These compounds showed high safety margin on gastric mucosa with no ulceration effect. Also the most active in-vivo anti-inflammatory compounds 17, 21 and 26 were found to be non-toxic in experimental rats with normal kidney and liver functions. Docking study of the synthesized compounds showed similar orientation as celecoxib within the active site of COX-2 enzyme and similar ability to emerge deeply in the additional pocket and binding with Arg513 and His90 the key amino acids responsible for selectivity.