SYS6002 Plus JMT101 and Enlonstobart Injection in Participants With HNSCC

Inclusion Criteria:

* Aged 18 years or older, regardless of gender.
* Able to understand and voluntarily sign a written informed consent form (ICF).
* Confirmed recurrent or metastatic HNSCC that is not curable with local treatments. Primary tumor must be located in the oral cavity, oropharynx, hypopharynx, or larynx.
* Able to provide well-preserved or fresh tumor tissue.
* According to the Response Evaluation Criteria in Solid Tumors (RECIST) v1.1, at least one measurable lesion confirmed by CT or MRI.
* Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1.
* Life expectancy ≥ 3 months.
* Normal major organ function within 7 days prior to treatment.

Exclusion Criteria:

* Primary tumor located in the nasopharynx, nasal cavity, paranasal sinuses, salivary glands, thyroid or parathyroid glands, skin, or squamous cell carcinoma with an unknown primary site.
* Presence of leptomeningeal metastasis, spinal cord compression, symptomatic or progressive brain metastases.
* (Only for Stage 2 in Part 2) Prior systemic anti-tumor therapy for recurrent or metastatic HNSCC.
* Known allergy to any component of SYS6002, JMT101, or Enlonstobart injection.
* Participants who have previously received MMAE toxin ADC treatment (only for Part 1 and Stage 2 in Part 2).
* Adverse events related to prior anti-tumor treatment that have not resolved to ≤ Grade 1 according to NCI-CTCAE v5.0 (except for toxicities deemed low-risk by the investigator, such as Grade 2 alopecia).
* Washout period not met for anti-tumor medications or treatments.
* Any serious and/or uncontrolled comorbid conditions that may interfere with the participant's ability to participate in the study
* Pregnant or breastfeeding women; women of childbearing potential with a positive pregnancy test within 7 days before enrollment. Breastfeeding women may participate if they stop breastfeeding, but they cannot resume breastfeeding during the treatment and after the study ends.
* Any male or female participants of childbearing potential who refuse to use highly effective contraception during the study and for 6 months after the last dose of study drug.
* Unwilling or unable to comply with the study procedures and requirements, or deemed unsuitable for participation by the investigator.