A First-in-Human, Open-label, Phase 1/2 Clinical Trial to Assess the Safety, Tolerability, Pharmacokinetics and Preliminary Efficacy of HMBD-501 in Patients With Advanced-Stage, Relapsed/Refractory HER3-Expressing Solid Tumors

Inclusion Criteria: * Body weight ≥ 40 kg. * Willing and able to provide signed written informed consent before any study-related screening procedures are performed. * Patients with histologically or cytologically confirmed diagnosis of advanced-stage or metastatic HER3+ solid tumors that are relapsed or refractory to or ineligible for standard therapy, or for whom no standard therapy is available; or the patient has documented their refusal of standard of care therapies. These include the following: 1. Unresectable or metastatic cutaneous melanoma (HER3+) 2. Locally advanced or metastatic mutated EGFR (mEGFR) NSCLC (HER3+) 3. Unresectable, locally advanced or metastatic breast cancer 4. Relapsed or refractory solid tumors, with documented HER3+ expression such as Pancreatic Ductal Adenocarcinoma (PDAC) and gastric cancers, may be allowed in the protocol following sponsor approval on a case-by -case basis. * If molecular pathology report to confirm HER3+ status is not available, willingness to undergo fresh tumor biopsy for retrospective assessment of HER3+ status following enrollment.. * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-2. * Contraceptive requirements: 1. Women of childbearing potential (WOCBP) must use contraception from at least 28 days prior to study start, during the study, and for at least 6 months after the last dose of study drug. 2. Males who are sexually active with partner(s) who are WOCBP must agree to use a male condom with spermicide beginning at study start, during the study and for at least 6 months after the last dose of study drug. * Females must: 1. Agree to not donate eggs (ova, oocytes) for the purposes of assisted reproduction during the study and for at least 6 months after the last dose of study drug. 2. Agree to not breastfeed and do not plan to become pregnant during the study and for at least 6 months after the last dose of study drug. * Males must: 1. Agree to not donate sperm beginning at study start, during the study, and for at least 6 months after the last dose of study drug. 2. Agree to not plan to father a child beginning at study start, during the study, and for at least 6 months after last dose of study drug. * Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. Exclusion Criteria: * Any of the following treatment interventions within the specified time frame prior to study drug administration at study start: 1. Any anti-tumor-directed drug therapy within 21 days or 5 times the elimination half-life (whichever is shorter). 2. Treatment with investigational drugs within 21 days. 3. Major surgery within 21 days. 4. Radiation therapy ≤4 weeks or radiotherapy that included \>30% of the bone marrow. 5. Autologous or allogeneic stem cell transplantation or allogeneic tissue/organ transplant within 3 months. 6. CYP3A4 strong inhibitor (including any prescription or non-prescription drugs or herbal supplements) ≤4 half-lives. 7. CYP3A4 strong inducer ≤4 half-lives. 8. OATP1B inhibitor (including any prescription or non-prescription drugs or herbal supplements) ≤4 half-lives. * Prior treatment with a HER3-targeted ADC or any exatecan- or exatecan-derivative-conjugated ADC inhibitor as last line of therapy. * Prior treatment with a topoisomerase I inhibitor as last line of therapy. * Primary immune deficiency (e.g. congenital syndromes). * Active and uncontrolled infections requiring intravenous antibiotic or antiviral treatment within 2 weeks prior to study start. * Known/suspected hypersensitivity against ENV-501, human or humanized immunoglobulin Gs (IgGs), or their ingredients. * History of noninfectious or drug-induced pneumonitis or interstitial lung disease (ILD). * Known seropositivity (except after vaccination or confirmed cure for hepatitis) for human immunodeficiency virus (HIV), hepatitis B virus (HBV), or hepatitis C virus (HCV). * Leptomeningeal disease, symptomatic or uncontrolled (active) brain metastasis (note: brain metastases not requiring steroids or anti-epileptic therapy are allowed if stable for ≥4 weeks prior to study start and patient is neurologically stable). * Pregnant or WOCBP who have a positive b-human chorionic gonadotropin (HCG) test result at Screening or within 7 days prior to study start. * Patients with second malignancies that are active (uncontrolled, metastatic) or requiring therapy. * Patient who is an immediate family member (spouse, parent, child, or sibling; biological or legally adopted) of personnel directly affiliated with the study site or the Sponsor.