This is an open-label, nonrandomized, multicenter, Phase 1 study to evaluate the safety, tolerability, pharmacokinetics, and preliminary antitumor activity of MK-6204 as monotherapy in participants with advanced solid tumors.

This is an open-label, nonrandomized, multicenter, Phase 1 study to evaluate the safety, tolerability, pharmacokinetics, and preliminary antitumor activity of MK-6204 as monotherapy in participants with advanced solid tumors. The primary objectives are to evaluate the safety and tolerability of MK-6204. The secondary endpoints include PK parameters, ORR, DOR, and immunogenicity of MK-6204. The study will include a Screening Phase, a Treatment Phase, and a Post-treatment Follow up Phase.

Inclusion Criteria:

1. ≥ 18 years of age
2. Have a histologically or cytologically confirmed advanced/metastatic solid tumor by pathology report and have failed or do not have available standard treatments. The tumor types will be limited to: CRC; Gastric carcinoma or gastroesophageal junction (GEJ) adenocarcinoma; Esophageal carcinoma; Pancreatic cancer; NSCLC; Cervical carcinoma; Head and Neck squamous cell carcinoma.
3. Have measurable disease by RECIST 1.1 as assessed by the local site investigator/radiology. Target lesions situated in a previously irradiated area are considered measurable if progression has been shown in such lesions.
4. Have a performance status of 0 or 1 on the ECOG Performance Scale.
5. The participant has provided documented informed consent for the study.
6. Participants who agree to provide archival tumor tissue sample or newly obtained core, incisional, or excisional biopsy of a tumor lesion not previously irradiated.
7. Participants who have AEs due to previous anticancer therapies must have recovered to ≤Grade 1 or baseline. Participants with endocrine-related AEs who are adequately treated with hormone replacement or participants who have ≤Grade 2 neuropathy are eligible.

Exclusion Criteria:

1. Active severe digestive disease, including but not limited to complete or incomplete gastric outlet obstruction, persistent/recurrent vomiting, severe gastrointestinal hemorrhage, gastric or duodenal ulcers, acute gastrointestinal perforation, acute necrotizing pancreatitis, ulcerative enteritis, congenital megacolon, or Crohn's disease.
2. Participants with a history of interstitial lung disease (ILD) or a history of noninfectious pneumonitis that required steroids, have current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at screening.
3. Received strong cytochrome P450 (CYP3A4) inhibitors or inducers within 2 weeks prior to the first dose of study intervention or within 5 half-lives of drug elimination, whichever is longer.
4. Received strong breast cancer resistance protein (BCRP) inhibitors within 2 weeks prior to the first dose of study intervention or within 5 half-lives of drug elimination, whichever is longer.
5. Received prior systemic anticancer therapy including investigational agents within 4 weeks or 5 half-lives, whichever is shorter, before intervention allocation.
6. Known additional malignancy that is progressing or has required active treatment within the past 2 years.
7. Known active CNS metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are radiologically stable (i.e., without evidence of progression) for at least 4 weeks as confirmed by repeat imaging performed during the study screening, are clinically stable and have not required steroid treatment for at least 14 days before the first dose of study intervention.