Inclusion Criteria:

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

          -  Subjects ≥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
                  (eg, RAS/BRAF wildtype)

               -  An anti-VEGF agent, unless contraindicated (eg, bevacizumab) Note: Subjects with
                  known microsatellite instability-high (MSI-H) status must have received treatment
                  with an immune checkpoint inhibitor unless contraindicated.

          -  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 required archival and pre-treatment tumor biopsy for assessment of
             HER3 expression levels by IHC and exploratory biomarkers, defined as:

               1. Pre-treatment tumor biopsy. Subjects 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. Archival tissue must be available and of sufficient quantity, as defined above,
                  at the time of screening. If archival tissue 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-study tumor biopsy. When at least 10 on-study 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:

        The following Parameters / Laboratory values:

        Platelet count: ≥100,000/mm3 or ≥100 × 109/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/mm3 or ≥1.5 × 109/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
                  pneumonectomy.

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

          -  Evidence of leptomeningeal disease.

          -  Has 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. Immune checkpoint inhibitor therapy <21 days;

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

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

               6. 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. Participants with
             chronic Grade 2 toxicities may be eligible per the discretion of the Investigator
             after consultation with the Sponsor (Medical Monitor or designee).

          -  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. Participants will be eligible for
             enrollment only if the viral load, according to local standards of detection, is
             documented to be low or undetectable in the absence of anti-viral therapy and during
             the previous 12 weeks prior to the viral load evaluation.

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