Inclusion Criteria:

Part A1 dose escalation and A2 dose optimization: Participants must meet all of the following criteria:

• Have a histologic or cytologic diagnosis of unresectable or metastatic solid tumor malignancy.
• Have previously received at least one prior therapy targeting programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), and/or another T-cell co-stimulatory or immune checkpoint pathway, in any treatment setting (including neoadjuvant or adjuvant).
• Have received prior SOC therapy appropriate for their disease type and stage and have no remaining available treatment options with established clinical benefit; or, in the opinion of the investigator, are unlikely to tolerate or derive clinically meaningful benefit from appropriate SOC therapy; or have declined SOC therapy.
• Participants must have demonstrated radiographic progression and have at least 1 measurable lesion per RECIST v1.1 that has not been previously irradiated or has demonstrated progression of disease since radiation therapy.
• ECOG PS 0 or 1 or equivalent. Participants with ECOG PS 2 may be considered upon discussion with the Sponsor Medical Monitor.
• Participants with adequate organ function.

Exclusion Criteria:

Exclusion Criteria (Part A)

• Active brain metastases (new lesions identified on imaging, existing lesions showing progression per Response Evaluation Criteria in Solid Tumors, version 1.1 (RECIST v1.1)/imaging characteristics or by clinical criteria, lesions requiring active interventions for symptom control).
• Carcinomatous meningitis.
• Known or suspected hypersensitivity to ARV-6723 or pembrolizumab or any of its excipients.
• Active autoimmune disease or history of autoimmune diseases that may relapse.
• History of severe immune-related adverse events (irAE) attributed to prior anti-PD-1/anti-CTLA-4/anti-LAG-3 therapy.
• Prior treatment with any HPK1-targeting agent
• Systemic anti-cancer therapy or radiation therapy within 14 days prior to study treatment start.
• Current use of any prohibited concomitant medication(s) or herbal supplements which cannot be discontinued, prior to start of study intervention and for the duration of the study.
• Baseline (screening) standard 12-lead electrocardiogram (ECG) that demonstrates clinically relevant abnormalities that may affect participant safety or interpretation of study results.