A Study of XMT-2056 in Advanced/Recurrent Solid Tumors That Express HER2

Inclusion Criteria:

• Participant has recurrent or metastatic solid tumors with HER2 expression and hasdisease progression after treatment, is intolerant to treatment, or iscontraindicated with available anti-cancer therapies known to confer benefit, basedon investigator's judgement. Note: Participants must have HER2 positivity per theresults of their most recent tumor tissue testing, defined as IHC 3+ or IHC 2+ incombination with in situ hybridization (ISH)+. Participants with ERBB2-activatingmutations or ERBB2 gene amplification in the absence of HER2 positivity areconsidered ineligible.

• Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.

• Participant must have measurable disease as defined by RECIST version 1.1.

• Participant has fresh tumor biopsy tissue available for submission to centrallaboratory. If obtaining fresh tumor tissue is medically contraindicated, archivaltumor tissue can be submitted following written approval of the request by the studyMedical Monitor. Samples must be obtained after the participant's most recentHER2-targeting therapy unless determined to be medically contraindicated afterdiscussion with the medical monitor.

Exclusion Criteria:

• • Participant is receiving immunosuppressive doses of systemic medications, (doses >10 mg/day prednisone or equivalent) that cannot be discontinued for at least 2weeks before the first dose and during study drug treatment administration. Note:physiologic hormone replacement therapy is an exception.

• Participant has received prior treatment targeting STING pathway.

• Diagnosis of additional malignancy that required active treatment (includingsurgery, systemic therapy, and radiation) within the last 2 years, expect foradequately treated basal cell or squamous cell skin cancer or carcinoma in situ ofthe breast or the cervix. Participants with an additional malignancy that has a lowrisk for recurrence may be eligible after discussion with the study Medical Monitor.

• Participants have untreated CNS metastases (including new and progressive brainmetastases), history of leptomeningeal metastasis, or carcinomatous meningitis.

1. Participants are eligible if CNS metastases are adequately treated andparticipants are neurologically stable for at least 2 weeks prior toenrollment.

2. In addition, participants must be either off corticosteroids, or on astable/decreasing dose of ≤ 10 mg prednisone daily (or equivalent).