Inclusion Criteria:

          -  Participant has provided written informed consent prior to the start of any study
             specific procedures.

          -  Participants ≥18 years (follow local regulatory requirements if the legal age of
             consent for study participation is >18 years old).

          -  Pathological/histological confirmation of advanced or metastatic colon or rectal
             adenocarcinoma.

          -  Must be resistant, refractory, or intolerant to at least 2 prior lines of systemic
             therapy, that must include all of the following agents:

               -  Fluoropyrimidine

               -  Irinotecan

               -  Platinum agents (e.g, oxaliplatin)

               -  An anti-epidermal growth factor receptor (EGFR) agent, if clinically indicated

               -  An anti-VEGF agent, if clinically indicated (eg, bevacizumab)

               -  An immune checkpoint inhibitor (eg, microsatellite instability-high [MSI-H]
                  status)

               -  A BRAF inhibitor, if clinically indicated (eg, BRAF V600E positive)

          -  Has at least 1 measurable lesion confirmed by blinded independent central review
             (BICR) as per Response Evaluation Criteria in Solid Tumors (RECIST) Version (v) 1.1.

          -  Willing to provide a required pre-treatment tumor biopsy and an additional archival
             tissue sample for the assessment of HER3 expression levels by immunohistochemistry and
             exploratory biomarkers, defined as:

               1. Pre-treatment tumor biopsy. Participants may be exempted from the requirement to
                  provide a pre-treatment tumor biopsy if archival tumor tissue was collected
                  within 3 months of screening during or after treatment with the last prior cancer
                  treatment and is of sufficient quantity (2 cores or 20 slides with adequate tumor
                  tissue content).

               2. An additional archival tissue sample collected greater than 3 months prior to
                  screening must be available and of sufficient quantity, as defined above, at the
                  time of screening. If an archival tissue sample (collected greater than 3 months
                  prior to screening) is not available, a subject may be included provided the
                  pre-treatment tumor biopsy is obtained and after discussion and agreement from
                  Sponsor (Medical Monitor or designee).

               3. Consent to provide on-treatment tumor biopsy. When at least 10 treatment tumor
                  biopsies have been collected, the Sponsor will provide written notification of a
                  change to the requirement.

          -  Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1.

          -  Life expectancy ≥3 months.

          -  Has adequate bone marrow reserve and organ function at baseline based on local
             laboratory data defined as follows within 14 days prior to Cycle 1 Day 1:

               -  Platelet count: ≥100,000/mm^3 or ≥100 × 10^9/L (platelet transfusions are not
                  allowed up to 14 days prior to Cycle 1 Day 1 to meet eligibility)

               -  Hemoglobin: ≥9.0 g/dL (transfusion and/or growth factor support is allowed)

               -  Absolute neutrophil count: ≥1500/mm^3 or ≥1.5 × 10^9/L

               -  Serum creatinine (SCr) OR creatinine clearance (CrCl): SCr ≤ 1.5 × upper limit of
                  normal (ULN), OR CrCl ≥ 30 mL/min as calculated using the Cockcroft- Gault
                  equation or measured CrCl; confirmation of CrCl is only required when creatinine
                  is >1.5 × ULN

               -  Alanine aminotransferase /aspartate aminotransferase: ≤3 × ULN (if liver
                  metastases are present, ≤5 × ULN)

               -  Total bilirubin: ≤1.5 × ULN if no liver metastases (<3 × ULN in the presence of
                  documented Gilbert's syndrome [unconjugated hyperbilirubinemia] or liver
                  metastases)

               -  Serum albumin: ≥2.5 g/dL

               -  Prothrombin time (PT) or PT-international normalized ratio (INR) and activated
                  partial thromboplastin time (aPTT) / partial thromboplastin time (PTT): ≤1.5 ×
                  ULN except for subjects on coumarin- derivative anticoagulants or other similar
                  anticoagulant therapy, who must have PT-INR within therapeutic range as deemed
                  appropriate by the Investigator

        Exclusion Criteria:

          -  Any history of interstitial lung disease (including pulmonary fibrosis or radiation
             pneumonitis), has current interstitial lung disease (ILD), or is suspected to have
             such disease by imaging during screening.

          -  Clinically severe pulmonary compromise (based on Investigator's assessment) resulting
             from intercurrent pulmonary illnesses including, but not limited to:

               1. any underlying pulmonary disorder (e.g., pulmonary emboli, severe asthma, severe
                  chronic obstructive pulmonary disease, restrictive lung disease, pleural
                  effusion)

               2. any autoimmune, connective tissue or inflammatory disorder with pulmonary
                  involvement (eg, rheumatoid arthritis, Sjögren's syndrome, sarcoidosis)

                    -  OR prior complete pneumonectomy.

          -  Is receiving chronic systemic corticosteroids dosed at >10 mg prednisone or equivalent
             anti-inflammatory activity or any form of immunosuppressive therapy prior to Cycle 1
             Day 1. Participants who require use of bronchodilators, inhaled or topical steroids,
             or local steroid injections may be included in the study.

          -  Evidence of leptomeningeal disease.

          -  Evidence of clinically active spinal cord compression or brain metastases

          -  Inadequate washout period prior to Cycle 1 Day 1 of U3-1402:

               1. Whole brain radiation therapy <14 days or stereotactic brain radiation therapy <7
                  days;

               2. Any cytotoxic chemotherapy, investigational agent or other anticancer drug(s)
                  from a previous cancer treatment regimen or clinical study <14 days or 5
                  half-lives, whichever is longer;

               3. Monoclonal antibodies other than immune checkpoint inhibitors, such as
                  bevacizumab (anti-VEGF) and cetuximab (anti-EGFRs) <28 days;

               4. Immune checkpoint inhibitor therapy <21 days;

               5. Major surgery (excluding placement of vascular access) <4 weeks;

               6. Radiotherapy treatment to >30% of the bone marrow or with a wide field of
                  radiation <28 days or palliative radiation therapy <14 days;

               7. Chloroquine/hydroxychloroquine ≤14 days.

          -  Prior treatment with an anti-HER3 antibody and/or antibody drug conjugate (ADC) that
             consists of an exatecan derivative that is any topoisomerase I inhibitor (e.g,
             trastuzumab deruxtecan).

          -  Has unresolved toxicities from previous anticancer therapy, defined as toxicities
             (other than alopecia) not yet resolved to National Cancer Institute Common Terminology
             Criteria for Adverse Events (NCI-CTCAE) v5.0 Grade ≤1 or baseline.

          -  Had primary malignancies other than CRC within 3 years prior to Cycle 1 Day 1, except
             adequately resected non-melanoma skin cancer, curatively treated in-situ disease, or
             other solid tumors curatively treated.

          -  Uncontrolled or significant cardiovascular disease prior to Cycle 1 Day 1.

          -  Known Hepatitis B and/or Hepatitis C infection, such as those with serologic evidence
             of viral infection within 28 days of Cycle 1 Day 1.

               1. Participants with past or resolved hepatitis B virus (HBV) infection are eligible
                  if:

                    -  Hepatitis B surface antigen (HBsAg) negative and hepatitis B core antibody
                       (anti-HBc) positive; OR

                    -  HBsAg positive and HBV deoxyribonucleic acid (DNA) viral load is documented
                       to be ≤2000 IU/mL in the absence of anti-viral therapy and during the
                       previous 12 weeks prior to the viral load evaluation with normal
                       transaminases values (in the absence of liver metastasis); OR

                    -  HBsAg positive and HBV DNA viral load is documented to be ≤2000 IU/mL in the
                       absence of anti-viral therapy and during the previous 12 weeks prior to the
                       viral load evaluation for participants with liver metastasis and abnormal
                       transaminases with a result of AST/ALT <3 × ULN.

               2. Participants with a history of hepatitis C infection will be eligible for
                  enrollment only if the viral load according to local standards of detection is
                  documented to be below the level of detection in the absence of anti-viral
                  therapy during the previous 12 weeks (ie, sustained viral response according to
                  the local product label but no less than 12 weeks, whichever is longer).

          -  Participant with any human immunodeficiency virus (HIV) infection.

          -  Any evidence of severe or uncontrolled systemic diseases (including active bleeding
             diatheses, active infection), psychiatric illness/social situations, geographical
             factors, substance abuse, or other factors which in the Investigator's opinion makes
             it undesirable for the participant to participate in the study or which would
             jeopardize compliance with the protocol. Screening for chronic conditions is not
             required.