The main aim of this study is to check if people with advanced solid tumors have side effects from dazostinag, and to check how much dazostinag they can receive without getting significant side effects from it when given alone and in combination with pembrolizumab. The study will be conducted in two phases including a dose escalation phase and a dose expansion phase. In the dose escalation phase, escalating doses of dazostinag are being tested alone and in combination with pembrolizumab to treat participants who have advanced or metastatic solid tumors. In the dose expansion phase, dazostinag will be studied with pembrolizumab with or without chemotherapy in participants with untreated metastatic or recurrent, unresectable squamous cell carcinoma of head and neck (SCCHN) and in combination with pembrolizumab in third-line or later recurrent locally advanced or metastatic microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) and third-line recurrent locally advanced or metastatic microsatellite stable/mismatch repair proficient (MSS/pMMR) colorectal cancer (CRC).

The drug being tested in this study is called dazostinag. Dazostinag is being tested to treat people who have advanced or metastatic solid tumors.

The study will enroll approximately 374 participants. Part 1 consists of an initial Safety Lead-in to Dose Escalation Phase; Part 2 and Part 3 compose the Expansion Phase in 2 specific indications namely, previously untreated metastatic or recurrent, unresectable SCCHN (Part 2) and third-line or later recurrent locally advanced or metastatic MSI-H/dMMR and third-line recurrent locally advanced or metastatic MSS/pMMR CRC (Part 3). Participants will be assigned to the following treatment groups in the respective Phases of the study:

• Part 1 (Dose Escalation Phase): Safety Lead-in + Dazostinag single agent (SA) [Part 1A] Dazostinag 0.1 milligram (mg) in the Safety Lead-in followed by Dazostinag as escalating doses (0.2 mg and above) in Part 1A
• Part 1B (Combination Dose Escalation Phase): Dazostinag as escalating doses (0.2 mg and above) + Pembrolizumab
• Japan Safety Lead-in: Dazostinag SA 5.0 mg in the Safety Lead-in± Pembrolizumab in Arms A and B. Additional dose levels of Dazostinag (such as 14.0 mg) in combination with pembrolizumab may be explored during the Japan safety lead-in considering recommended dose for expansion (RDE1) as 5.0 mg and dose optimization.

Once a safe dose is recommended from Part 1, participants of select advanced or metastatic solid tumors will receive dazostinag in below defined cohorts in the expansion phase:

• Part 2A (SCCHN Combined Positive Score [CPS] ≥ 1 Dose Expansion Phase): Dazostinag + Pembrolizumab*
• Part 2B (SCCHN Dose Expansion Phase): Dazostinag + Pembrolizumab + Chemotherapy
• Part 3A (Expansion Phase in CRC): Dazostinag + Pembrolizumab in MSI-H/dMMR CRC
• Part 3B (Expansion Phase in CRC): Dazostinag + Pembrolizumab in MSS/pMMR CRC

This multi-center trial will be conducted worldwide. The overall time to participate in this study is 68 months. Participants will make multiple visits to the clinic, including 30 days after last dose of study drug for a follow-up assessment. Participants in Parts 2 and 3 will be followed for survival for up to 12 months after the last dose of study drug.

Inclusion Criteria:

1. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1.

2. Dazostinag SA (dose escalation Part 1A):

   o With histologically confirmed (cytological diagnosis is acceptable) advanced or metastatic solid tumors that have no standard therapeutic options or are intolerant to these therapies.

3. Dazostinag in combination with pembrolizumab (dose escalation Parts 1B and Japan safety lead-in):

   • With histologically confirmed (cytological diagnosis is acceptable) advanced or metastatic solid tumors that have no standard therapeutic options or are intolerant to them, including:
   • Tumors that have relapsed or are refractory to anti-programmed cell death ligand protein 1 (anti PD-(L)-1) therapy.
   • Tumors that are naive to anti-PD-(L)-1 therapy.

4. For expansion phase only:

   • SCCHN (Part 2):
   • Participants with histologically confirmed (cytological diagnosis is acceptable) metastatic or recurrent, unresectable SCCHN that is considered incurable by local therapies. Participants should not have had prior systemic therapy administered in the recurrent or metastatic setting. Systemic therapy which was completed more than 6 months before signing consent if given as part of multimodal treatment of locally advanced disease is allowed.
   • Anatomic subsites to be included are oral cavity, oropharynx, hypopharynx, larynx, nasal cavity, and paranasal sinuses (maxillary, ethmoid, sphenoid, and frontal). The exception to this is nasopharyngeal cancer and salivary gland tumors, which will not be included.
   • Participants with oropharyngeal cancer or tumors arising in the paranasal sinuses (maxillary, ethmoid, sphenoid, and frontal) must agree to provide archival tissue for human papilloma virus (HPV) testing or if known, HPV testing results (using CINtec® p16 Histology assay is preferred but not required) and a 70% cutoff point must be provided. Alternatively, archival tissue or a fresh excisional or core needle biopsy (≥ 2 cores) is required for the determination of HPV status. If HPV status was previously tested using this method (CINtec® p16 Histology assay is preferred but not required), no additional testing is required. Archival tissue can be obtained up to 90 days prior to screening. Samples that are older than 90 days at screening may be used after consultation with the sponsor.
   • For Part 2A, tumors must have a PD-L1 CPS ≥ 1. Participants must agree to provide fresh tumor biopsy for analysis from a core or excisional biopsy (fine needle aspirate is not sufficient) at screening for PD-L1 CPS assessment by a central laboratory. This specimen may be the diagnostic sample for participants with a new diagnosis of metastatic SCCHN. Participants for whom newly obtained samples cannot be obtained (eg, inaccessible or participant safety concern) may submit an archived specimen only upon agreement from the Sponsor. Archival tissue can be obtained up to 90 days prior to screening provided there was no other treatment from the time of biopsy until the start of study treatment. For Part 2B, any CPS is eligible but fresh or archival tissue is required for confirmation of CPS status. Collection of the tissue samples for PD-L1 assessments for Part 2B can be discontinued by the sponsor if sufficient data has been collected or dazostinag activity does not justify further collection.
   • For Part 2B, participants must be eligible to receive treatment with either cisplatin or carboplatin in combination with 5-fluorouracil (5-FU) per the treating physician.

5. CRC (Part 3):

   • Third-line or later MSI-H/dMMR CRC (Part 3A): Participants with histologically confirmed (cytological diagnosis is acceptable) recurrent locally advanced or metastatic MSI-H/dMMR CRC whose disease has progressed on or following therapy with 1) an anti-PD-1 or PD-L1 antibody (i.e., pembrolizumab) and 2) at least one line of combination chemotherapy including a fluoropyrimidine and irinotecan OR oxaliplatin with or without an anti-epidermal growth factor receptor (EGFR) or anti-vascular endothelial growth factor (VEGFR) monoclonal antibody (i.e., cetuximab or bevacizumab). MSI-H/dMMR CRC participants must have received at least 6 weeks of prior treatment with an anti-PD-(L)-1 antibody. Only one line of anti-PD-(L)-1 is permitted.
   • Third-line MSS/pMMR CRC (Part 3B): Participants with histologically confirmed (cytological diagnosis is acceptable) recurrent locally advanced or metastatic MSS/pMMR CRC whose disease has progressed on or following therapy with 2 different lines of combination chemotherapy, including therapy with a fluoropyrimidine and irinotecan AND therapy with a fluoropyrimidine and oxaliplatin. Both lines of therapy may be given with or without an anti-EGFR or anti-VEGFR monoclonal antibody (i.e., cetuximab or bevacizumab).

   Participants with MSS/pMMR CRC must have progressed on or after combination chemotherapy regimens containing BOTH irinotecan AND oxaliplatin.

   • Participants with MSI-H/dMMR or MSS/pMMR CRC must have documented MSI/MMR status assessed by a Clinical Laboratory Improvements Amendment-certified (United States [US] sites or an accredited (outside of the US) local laboratory using immunohistochemistry (IHC) and/or polymerase chain reaction (PCR) or next generation sequencing (NGS) assay.
   • Adequate tumor tissue available for central laboratory confirmation of MSI/MMR status. Note: confirmation of central test positivity is not required before treatment.
   • Participants with MSI-H/dMMR or MSS/pMMR CRC must have been treated with 2 prior lines of therapy in the recurrent locally advanced or metastatic setting.

6. Adequate bone marrow, renal, hepatic and cardiac functions.

7. Left ventricular ejection fraction (LVEF) > 50%, as measured by echocardiogram or multiple-gated acquisition (MUGA) scan within 4 weeks before receiving the first dose of study drug.

8. Clinically significant toxic effects of previous therapy have recovered to Grade 1 (per NCI CTCAE Version 5.0) or baseline, except for alopecia, Grade 2 peripheral neuropathy, and/or autoimmune endocrinopathies with stable endocrine replacement therapy.

9. In dose escalation Part 1, (not applicable for the Japan safety lead-in) once peripheral evidence of dazostinag pharmacodynamic stimulation of the innate and/or adaptive immune system is observed in the blood and/or an imaging response/partial response (CR/PR) is observed in at least 1 participant, subsequent participants must:

   • Have at least 1 lesion amenable for biopsy.
   • Agree to have 2 tumor biopsies: 1 during the screening period and 1 while on dazostinag treatment.

10. Must have at least 1 RECIST version 1.1-evaluable (measurable) lesion. For the dose escalation phase (Part 1) only, nonmeasurable only disease is acceptable.

11. Pharmacokinetic (PK)/pharmacodynamic blood must be drawn on a peripherally-inserted catheter. Dazostinag is preferentially administered through a central line, but peripheral infusion is acceptable. If a peripheral line is used for dazostinag and/or pembrolizumab infusion, it must be separate than the one used for PK/pharmacodynamic collection.

Exclusion Criteria:

1. Corrected QT interval by Fredericia (QTcF) greater than (>) 450 milliseconds (men) or > 475 milliseconds (women) on a 12-lead electrocardiogram (ECG) during the screening period.

2. Grade greater than or equal to (≥) 2 hypotension (that is, hypotension for which nonurgent intervention is required) at screening or during Cycle 0 Day 1 (C0D1) [for Japan safety lead-in only] and Cycle 1 Day 1 (C1D1) predose assessment.

3. Oxygen saturation less than (<) 92 percent (%) on room air at screening or during C0D1 (for Japan safety lead-in only) and C1D1 predose assessment.

4. Treated with other STING agonists/antagonist and toll-like receptors agonists within the past 6 months.

5. Current history of pneumonitis, interstitial lung disease, severe chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, other restrictive lung diseases, acute pulmonary embolism, or Grade ≥ 2 pleural effusion or ascites not controlled by tap or requiring indwelling catheters.

6. History of brain and leptomeningeal metastasis unless:

   • Brain metastases are clinically and radiologically stable or improved (that is, ≥ 4 weeks) following surgery, whole-brain radiation, or stereotactic radiosurgery, AND
   • Off corticosteroids.

7. Ongoing Grade ≥ 2 infection or participants with Grade ≥ 2 fever of malignant origin.

8. Chronic, active hepatitis (example: participants with known hepatitis B surface antigen seropositive and/or detectable hepatitis C virus [HCV]- ribonucleic acid [RNA]).

9. For participants in the dose escalation SA Part 1A only: refusal of standard therapeutic options.

10. For participants receiving pembrolizumab only: contraindication and/or intolerance to the administration of pembrolizumab.

11. For participants receiving chemotherapy in Part 2B: contraindication and/or intolerance to the administration of both platinum agents (cisplatin and carboplatin) and/or 5-FU.

12. Participant has had any other prior or concurrent malignancy within 2 years prior to enrollment with the following exceptions: adequately treated localized basal cell or squamous cell carcinoma, or curatively treated in situ carcinoma of the cervix or breast. Other exceptions may be considered upon sponsor consultation.

13. Concurrent chemotherapy (except for Part 2B), immunotherapy (except for pembrolizumab in Part 1B, Part 2, and Part 3), biologic, or hormonal therapy (except for adjuvant endocrine therapy for a history of breast cancer). Concurrent use of hormones for noncancer-related conditions is acceptable (except for corticosteroid hormones) unless allowed per exclusion criterion 16.

14. Radiation therapy within 14 days (42 days for radiation to the lungs) and/or systemic treatment with radionuclides within 42 days before C1D1 of study drug(s). Participants with clinically relevant ongoing pulmonary complications from prior radiation therapy are not eligible.

15. Use of systemic corticosteroids or other immunosuppressive therapy, concurrently or within 7 days of C1D1 of study drug(s), with the following exceptions:

    • Topical, intranasal, inhaled, ocular, intra-articular, and/or other non-systemic corticosteroids.
    • Premedications required for computed tomography (CT) or magnetic resonance imaging (MRI) scans.
    • Physiological doses of replacement steroid therapy (example: for adrenal insufficiency).
    • For participants enrolled in Part 2B, chemotherapy premedication with steroids can be administered according to local standards of care practice.

16. Use of medications that are known clinical organic anion-transporting polypeptide B1 (OATP1B1) and/or OATP1B3 inhibitors, concurrently or within 14 days of C1D1 of study drug(s).

17. Receipt of live attenuated vaccine (eg, tuberculosis Bacillus Calmette-Guerin vaccine, oral polio vaccine, measles, rotavirus, yellow fever) within 28 days of C1D1 of study drug(s).

18. Recipients of allogeneic or autologous stem cell transplantation or organ transplantation.

    For Part 2 SCCHN only:

19. Has progressive disease within 6 months of completion of curatively intended systemic treatment for locoregionally advanced SCCHN.

20. Has a life expectancy of less than 3 months and/or has rapidly progressive disease (eg, tumor bleeding, uncontrolled tumor pain) in the opinion of the treating investigator.

21. Has received prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2 or anti-cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) agent.

22. Participants with known dihydropyrimidine dehydrogenase (DPD) deficiency or thymidine phosphorylase gene (TYMP) mutations (Part 2B only).

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.