TPS6099 Background: Both aggressive, radioactive iodine refractory thyroid cancers as well as high-grade salivary gland cancers respond poorly to chemotherapy, and there is no widely used standard of care. Targeted thearpies as well as cytotoxic chemotherapy (e.g. doxorubicin, or taxanes) are commonly used, but are only modestly effective. Both salivary gland and thyroid cancers have been shown to have tumor infiltrating lymphocytes, tumor inflammation, and PD-L1 expression ( Ayers, AACR 2015). Early data with anti-PD-1 immunotherapy using pembrolizumab (Keynote 28) show activity in ≤10a% of patients in a biomarker selected population (Keynote 28 thyroid and salivary gland cohorts). However, most patients are not PD-L1 positive and were not eligible. Recently synergy of anti-PD-1 checkpoint blockade with cytotoxic chemotherapy was reported in several studies (e.g. Langer et al, Keynote 21G) . Mechanistically chemotherapy may increase tumor inflammation, and eradicate immusupressive myeloid derived suppressor cells (MDSCs). Data suggest a significant increase in the response rate e. g. in KN21G from 29% to 55%. Furthermore the depths of responses and durability improve, including patients with PD-L1 negative tumors. Methods: We hypothesize that the combination of PD-1 checkpoint blockade and cytotoxic chemotherapy will show synergistic activity in aggressive thyroid cancers and salivary gland cancers. Eligible patients will have radioactive iodine refractory, aggressive thyroid cancer (cohort A, N = 25 pts), or progressive salivary gland cancers (cohort B, N = 25 pts). There will be no PD-L1 or other biomarker selection. Patients must have progressed on prior therapy. Patients will receive docetaxel at a dose of 75mg/m2 Q21 days as well as pembrolizumab 200mg flat-dose Q21 days intravenously. The primary outcome of this study is response rate. The addition of pembrolizumab to chemotherapy will increase the response rate from 20% (H0) to 40% (H1). A simon two-stage design will be used for each cohort (cohorts A and B) with an estimate 81% power in each arm. Patient screening and enrollment are expected to begin mid 2017. Clinical trial information: pending.