Inclusion Criteria:

   - Participant has recurrent or metastatic solid tumors with HER2 expression and has
   disease progression after treatment, is intolerant to treatment, or is contraindicated
   with available anti-cancer therapies known to confer benefit, based on investigator's
   judgement. Note: Participants must have HER2 positivity per the results of their most
   recent tumor tissue testing, defined as IHC 3+ or IHC 2+ in combination with in situ
   hybridization (ISH)+. Participants with ERBB2-activating mutations or ERBB2 gene
   amplification in the absence of HER2 positivity are considered 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 central
   laboratory. If obtaining fresh tumor tissue is medically contraindicated, archival
   tumor tissue can be submitted following written approval of the request by the study
   Medical Monitor. Samples must be obtained after the participant's most recent
   HER2-targeting therapy unless determined to be medically contraindicated after
   discussion 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 2 weeks
   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 (including surgery,
   systemic therapy, and radiation) within the last 2 years, expect for adequately
   treated basal cell or squamous cell skin cancer or carcinoma in situ of the breast or
   the cervix. Participants with an additional malignancy that has a low risk for
   recurrence may be eligible after discussion with the study Medical Monitor.

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

      1. Participants are eligible if CNS metastases are adequately treated and
      participants are neurologically stable for at least 2 weeks prior to enrollment.

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