A Study of TransCon TLR7/8 Agonist With or Without Pembrolizumab in Patients With Advanced or Metastatic Solid Tumors

Inclusion Criteria:

   - At least 18 years of age.

   - Participants must have histologically confirmed locally advanced, recurrent or
   metastatic solid tumor malignancies that cannot be treated with curative intent
   (surgery or radiotherapy).

   - Participants must have progressed on or be intolerant of available standard of care
   treatment options or have disease for which there is no standard of care treatment
   available, with the exception of participants enrolling to the neoadjuvant cohorts.

   - At least 2 lesions of measurable disease, unless specified otherwise in the selection
   criteria.

   - Willingness to undergo biopsies.

   - Demonstrated adequate organ function within 28 days of Cycle 1 Day 1 (C1D1).

   - Life expectancy >12 weeks as determined by the Investigator.

   - Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, or 2.

   - Participants who have undergone treatment with anti-PD-1, anti-PD-L1, or antiCTLA 4
   antibody must have at least 4 weeks from the last dose of antibody and evidence of
   disease progression per investigator assessment before enrollment.

   - Participants who have previously received an immune checkpoint inhibitor prior to
   enrollment must have any immune related toxicities resolved to ≤Grade 1 or baseline
   (prior to the checkpoint inhibitor) to be eligible.

   - Female and male participants of childbearing potential who are sexually active must
   agree to use highly effective methods of contraception.

Exclusion Criteria:

   - Participants who have been previously treated with a TLR agonist (excluding topical
   agents for unrelated disease) are not eligible.

   - Other active malignancies within the last 2 years are excluded.

   - Active autoimmune diseases, regardless of need for immunosuppressive treatment at the
   time of screening, with the exception of patients well controlled on physiologic
   endocrine replacement.

   - Systemic immunosuppressive treatment with the exception for patients on corticosteroid
   taper (for example, for chronic obstructive pulmonary disease exacerbation).
   Participants cannot start dosing on study until steroid dose is at or lower than 10 mg
   per day prednisone or equivalent.

   - Women who are breastfeeding or have a positive serum pregnancy test during screening
   or within 72 hours prior to C1D1 are not eligible.

   - Vaccination with live, attenuated vaccines within 4 weeks of enrollment.

   - Symptomatic central nervous system metastases.

   - Known bleeding disorder that is deemed to place the patient at unacceptable risk for
   bleeding complications from intratumoral injections or biopsies.

   - Known hypersensitivity to any component of TransCon TLR7/8 Agonist or pembrolizumab.

   - Any uncontrolled bacterial, fungal, viral, or other infection.

   - Treatment with any other anti-cancer systemic treatment (approved or investigational)
   or radiation therapy within 4 weeks of first dosing on study is not allowed.

   - Significant cardiac disease

   - A marked baseline prolongation of QT/QTc (corrected QT) interval (e.g., repeated
   demonstration of a QTc interval >480 ms (National Cancer Institute NCI) Common
   Terminology Criteria for Adverse Events [CTCAE] grade 1) using Fredericia's QT
   correction formula.

   - A history of additional risk factors for Torsades de Pointes (TdP) (e.g., heart
   failure, clinically significant hypokalemia, family history of Long QT Syndrome).

   - The use of concomitant medications that prolong the QT/QTc interval within 14 days of
   enrollment.

   - Positive for HIV or with active hepatitis B or C infection.