ELVN-002 With Trastuzumab +/- Chemotherapy in HER2+ Solid Tumors, Colorectal and Breast 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