Sacituzumab Govitecan in Metastatic Salivary Gland and Thyroid Cancers (SG-SGTC)

Inclusion Criteria: 1. The participants (or legally acceptable representative if applicable) provide written consent for the trial. 2. At least 21 years of age on the day of signing informed consent. 3. Have histologically or cytologically confirmed diagnosis of advanced or unresectable salivary gland cancer or thyroid cancer, not amenable to local treatment. 4. Have measurable disease based on RECIST 1.1. 5. Patients with metastatic or recurrent unresectable advanced salivary gland cancers must have progressive disease within the previous 6 months and have seen at least 1 prior line of systemic treatment, unless the patients had contraindications to the systemic treatment considered standard of care or there was no standard of care for that particular salivary gland histological subtype. 6. Patients with unresectable metastatic or recurrent RAI refractory differentiated thyroid carcinoma, must have progressive disease within the previous 13 months and have seen at least 1 prior line of tyrosine kinase inhibitor (TKI), unless the patients had contraindications to the TKIs. Prior next generation sequencing is not mandated, but if known to harbour somatic pathogenic RET alteration or NTRK gene fusion, then the patient must have received 2 prior lines of TKIs, unless the patients had contraindications to the TKIs or declined the 2nd line of TKIs. 7. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. 8. Have an adequate organ function. Specimens must be collected within 10 days prior to the start of study treatment. 9. Provision of blood and tumour tissue samples (newly obtained biopsy if clinically feasible or archival specimen) to support exploratory biomarker analysis. Exclusion Criteria: 1. Has untreated brain metastases or leptomeningeal metastases. 2. Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks prior to first dose of study treatment. 3. Has received prior radiotherapy within 2 weeks of the start of study treatment. Participants must have recovered from all radiation-related toxicities, not require corticosteroids, and not have had radiation pneumonitis. 4. Has a known additional malignancy that is progressing or has required active treatment within the past 3 years. 5. Known severe hypersensitivity reaction to Sacituzumab govitecan. 6. Has an active infection requiring systemic treatment. This includes TB (Bacillus Tuberculosis). 7. Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. 8. Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. 9. Recent cardiovascular thromboembolic event, such as the following: 1. Cardiac chest pain, defined as moderate pain that limits instrumental activities of daily living, ≤ 4 weeks before enrolment 2. Symptomatic pulmonary embolism ≤ 4 weeks before enrolment 3. Any history of acute myocardial infarction ≤ 6 months before enrolment 4. Any history of heart failure meeting New York Heart Association (NYHA) Classification III or IV ≤ 6 months before enrolment 5. Any event of ventricular arrhythmia ≥ Grade 2 in severity ≤ 6 months before enrolment 6. Any history of cerebrovascular accident ≤ 6 months before enrolment 10. Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial 11. A WOCBP who has a positive urine pregnancy test within 72 hours prior to first dose. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. Note: In the event that 72 hours have elapsed between the screening pregnancy test and the first dose of study treatment, another pregnancy test (urine or serum) must be performed and must be negative for subject to start receiving study medication. 12. Is not willing to practice contraception during the study, and for up to a period of at least 6 months after the last dose of SG: 1. A woman of childbearing potential who does not agree to follow the contraceptive guidance on the use of effective contraception during the treatment period and for at least 6 months after the last dose of study medication. 2. A male participant who does not agree to the use of effective contraception during the treatment period and for at least 3 months after the last dose of study treatment, and to refrain from donating sperm during this period. 13. History of having received a live virus vaccination (e.g., yellow fever, MMR, nasal flu, chicken pox or Zostavax) within 4 weeks prior to the first dose of trial treatment.