The purpose of this study is to determine the safety, tolerability, and recommended dose of ELVN-002 in combination with trastuzumab in participants with advanced-stage HER2-positive tumors and in combination with trastuzumab, and chemotherapy in participants with advanced-stage HER2-positive colorectal cancer and breast cancer.

Parts 1 and 3 of this study are designed to evaluate preliminary safety, tolerability, and pharmacokinetics (PK) of ELVN-002 in combination with trastuzumab in participants with advanced stage HER2 positive solid tumors. In addition, Part 3 will evaluate the preliminary efficacy of ELVN-002 in combination with trastuzumab in participants with advanced-stage HER2-positive solid tumors.

Part 2 of this study will evaluate the preliminary safety, tolerability, and PK of ELVN-002 in combination with trastuzumab and chemotherapy; capecitabine and oxaliplatin(CAPEOX) or 5-fluorouracil (5-FU), leucovorin (LCV) and oxaliplatin (mFOLFOX6) in participants with advanced stage HER2 positive colorectal cancer, or eribulin or capecitabine in participants with advanced-stage HER2-positive breast cancer, or paclitaxel in participants with advanced stage solid tumors.

In part 4, the preliminary safety, tolerability, PK, and efficacy of ELVN-002 in combination with trastuzumab and CAPEOX or mFOLFOX6 will be evaluated in participants with HER2-positive colorectal cancer.

Inclusion Criteria:

• Pathologically or histologically documented solid tumor.

• Locally advanced or relapsed/refractory disease or unresectable metastatic disease.

• HER2-positive disease based on the following local testing:

  • Colorectal cancer: IHC3+, IHC2+/ISH+, NGS amplification by tissue (no RAS or BRAF mutation allowed)
  • Breast cancer: IHC3+ or IHC2+/ISH+ by tissue
  • Gastric cancer: IHC3+ or IHC2+/ISH+ by tissue
  • Other cancers: IHC3+, IHC2+/ISH+, NGS amplification by tissue or ctDNA

• Prior therapies for Part 1 (Dose Escalation ELVN-002 + trastuzumab):

  • Colorectal cancer: treated with prior fluoropyrimidine, oxaliplatin, irinotecan-based regimens, anti-epidermal growth factor receptor (EGFR) treatment (if clinically indicated), anti-vascular endothelial growth factor (VEGF) treatment (if clinically indicated), and an anti-programmed death ligand 1 (PD-(L)-1) treatment (if the tumor is microsatellite instability (MSI)-high/deficient mismatch repair (dMMR)
  • Breast cancer: treated with prior taxane, pertuzumab, trastuzumab, and fam-trastuzumab deruxtecan (T-DXd) if available and appropriate based on local standard of care and investigator's assessment
  • Gastric cancer: treated with trastuzumab/platinum fluorouracil containing regimen and T-DXd.
  • Other cancers: progressed during or after ≥ 1 prior line of systemic therapy for locally advanced unresectable or metastatic disease
  • Prior HER2 targeted therapy is allowed

• Prior therapies for Part 2 (Phase 1a Dose Escalation ELVN-002 + trastuzumab + chemotherapy):

  • Colorectal cancer: candidate for CAPEOX (capecitabine and oxaliplatin) or mFOLFOX6 (5-FU, LCV and oxaliplatin), and treated, if clinically indicated, with an anti-programmed death ligand 1 (PD-(L)-1) treatment (if the tumor is microsatellite instability (MSI)-high/deficient mismatch repair (dMMR). Prior HER2 targeted therapy is allowed.
  • Breast cancer: candidate for capecitabine, paclitaxel or eribulin, and treated with prior taxane, pertuzumab, trastuzumab, and T-DXd, if available and appropriate, based on local standard of care and investigator's assessment. No prior HER2 targeted tyrosine kinase inhibitor therapy (antibody-drug conjugates and antibodies are allowed), no prior capecitabine (for the capecitabine cohort), no prior eribulin (for the eribulin cohort), and no taxane as immediate prior therapy (paclitaxel cohort).

• Prior therapies for Part 3 (Phase 1b Dose Expansion ELVN-002 + trastuzumab):

  • Colorectal cancer: treated with prior fluoropyrimidine, oxaliplatin, irinotecan-based regimens, anti-epidermal growth factor receptor (EGFR) treatment (if clinically indicated), anti-vascular endothelial growth factor (VEGF) treatment (if clinically indicated), and an anti-programmed death ligand 1 (PD-(L)-1) treatment if the tumor is microsatellite instability (MSI)-high/deficient mismatch repair (dMMR). No prior HER2 targeted therapy.
  • Breast cancer: treated with prior taxane, pertuzumab, trastuzumab, and T-DXd if available and appropriate based on local standard of care and investigator's assessment. No prior HER2 targeted tyrosine kinase inhibitor therapy (antibody-drug conjugates and antibodies are allowed).
  • Gastric cancer: treated with prior trastuzumab/platinum fluorouracil containing regimen and T-DXd. No prior HER2 targeted therapy.
  • Other cancers: Progressed during or after ≥ 1 prior line of systemic therapy for locally advanced unresectable or metastatic disease. No prior HER2 targeted therapy.

• Prior therapies for Part 4 (Phase 1b Dose Expansion ELVN-002 + trastuzumab + chemotherapy):

  * Colorectal cancer: candidate for CAPEOX or mFOLFOX6 and not a candidate for first-line anti-programmed death ligand 1 (PD-(L)-1) treatment (if the tumor is microsatellite instability (MSI)-high/deficient mismatch repair (dMMR). No prior therapy for metastatic disease (1 cycle of mFOLFOX6 or 1 cycle of CAPEOX allowed). No prior HER2 targeted therapy.

• At least 1 measurable lesion based on RECIST v 1.1 within 6 weeks before the first dose of ELVN-002 (Part 3 and Part 4 only; Phase 1b Dose Expansion cohorts)

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1

• Adequate hematological, hepatic, renal, and cardiac function

Exclusion Criteria:

• Treatment with anticancer therapy within a specific time before the first dose:

  • Chemotherapy (including ADC) ≤ 3 weeks
  • Immunotherapy ≤ 4 weeks
  • Hormonal therapy ≤ 2 weeks
  • TKI ≤ 2 weeks
  • Any experimental therapy ≤ 3 weeks or 5 half-lives, whichever is longer
  • Radiotherapy-wide therapy ≤ 3 weeks
  • Radiotherapy limited field (including stereotactic brain) ≤ 2 weeks
  • Antibody ≤ 3 weeks

• Any brain lesion requiring immediate local therapy

• Ongoing use of corticosteroids for central nervous system (CNS) symptoms at a dose of > 2 mg daily of dexamethasone (or equivalent)

• Leptomeningeal disease

• Uncontrolled seizures

• Participants for any chemotherapy cohort: ongoing Grade 2 or higher neuropathy of any cause

• Inability to swallow pills or any significant gastrointestinal disease that would preclude adequate oral absorption of medications.

• Ongoing adverse effects from prior treatment > CTCAE Grade 1 except for Grade 2 alopecia

• Corrected QT interval (QTc) of >470 milliseconds (ms) for females or >450 ms for males