Study of Neoadjuvant Imprime PGG and Pembrolizumab for Stage III, Resectable Melanoma

Inclusion Criteria: 1. Signed informed consent form 2. ≥18 years of age 3. Histologically confirmed diagnosis of resectable\* AJCC (8th edition) Stage IIIB, IIIC or IIID cutaneous or unknown primary melanoma (except for any in-transit or satellite metastases) (\*Resectable Stage III disease is defined as disease that is amenable to complete tumor resection (anticipated to be an R0 resection) as judged as judged by the responsible surgeon. Criteria to judge resectability include, but are not limited to, lesions located in anatomically inaccessible areas, or invading vascular or neural structures, or technical or other reasons preventing their complete removal) 4. No prior systemic treatment for melanoma (subjects who were previously resected, relapsed and are once again resectable are eligible) 5. RECIST 1.1 measurable disease: a.) ≥ 10mm in the longest diameter for primary (if applicable) lesions or lymph node and/or ≥ 15mm in the shortest diameter for lymph nodes b) Sufficient nodal +/1 primary lesions amenable to ≥ 2 excisional/ core biopsies 6. No prior radiotherapy to nodal basin 7. Subject consents to provide 2 newly obtained core or excisional biopsies from non-nodal, non-bone lesions (within 28 days prior to C1D1 and during Wk 5 of treatment), the use of the resected surgical specimen and additional blood samples for translational research correlative studies 8. Have peripheral blood levels of IgG anti-β-glucan antibody (ABA) of ≥ 20 mcg/mL as determined by an ELISA test prior to (within 90 days) start of study treatment 9. ECOG PS 0-1 (within 7 days of starting treatment) 10. Estimated life expectancy of ≥12 weeks, in the opinion of the Investigator 11. Adequate organ function, including all of the following within 15 days before Day 1: a.) Hematological: i.) Absolute neutrophil count (ANC) ≥ 1.5×109/L (\> 1,500/mm3) (subject may not use G-CSF or GM-CSF to achieve this level) ii.) Platelets ≥ 100×109/L (\>100,000 per mm3) iii.) Hemoglobin level \>9 gm/dL. Packed red blood cell transfusion is acceptable, as long as the subject has a stable result of \>9 gm/dL for at least 1week post-transfusion. Erythropoietin should not be used to achieve this level iv.) Adequate coagulation function at screening as determined by prothrombin time (PT) International Normalized Ratio (INR) \< 1.5 times the upper limit of normal (ULN) and partial thromboplastin time (PTT) \< 1.5 times the ULN v.) Lymphocyte count \>1500 cells/mL b.) Intact immune system as demonstrated by CD4 count \>500 cells/mm3 and CD8 count \>150 cells/mm3 c.) Renal: i.) Serum creatine or measured and calculated creatinine clearance (GFR can also be used in place of creatinine or CrCl) ≤1.5 × ULN and creatinine clearance ≥30 mL/min, per Cockcroft Gault formula d.) Hepatic: i.) Serum total bilirubin ≤1.5× ULN or direct bilirubin ≤ ULN for a subject with total bilirubin levels \>1.5× ULN ii.) AST/ALT \< 2.5 x ULN iii.) Albumin \>3 g/dL 12. Have a negative PCR test at screening for SARS-COV-2 RNA 13. Women of childbearing potential (WOCBP) must have 2 negative serum or urine pregnancy tests during Screening, the second within 24 hours prior to the first administration of study drug, and must agree to use highly effective physician-approved contraception from Screening to a minimum of 90 days following the last study drug administration 14. Willing and able to comply with all protocol-specified assessments and the study visit schedule. Exclusion Criteria: 1. Prior therapy for melanoma (resection of a previous melanoma lesion is acceptable) 2. Subjects with uveal or mucosal melanoma 3. Pregnant, lactating or not practicing adequate contraception (premenopausal women), or expecting to conceive or father children within the duration of the trial, starting from Screening to 90 days following the last dose of drug administration 4. Prior radiotherapy in the previous 2 weeks. Radiotherapy to presenting tumor is prohibited 5. Administration of a live, attenuated vaccine within 28 days prior to Day 1 or anticipation that such a live attenuated vaccine will be required during the study 6. History of autoimmune disease, including but not limited to inflammatory bowel disease, systemic lupus erythematosus, and autoimmune hepatitis, requiring systemic treatment in previous 12 months 7. Treatment with systemic corticosteroids or other systemic immunosuppressive medications (including but not limited to prednisone \[\>10mg\], dexamethasone, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor \[TNF\] agents) within 15 days prior to Day 1, or anticipated requirement for systemic immunosuppressive medications during the trial 8. Immunodeficiency, natural or iatrogenic (steroids, immunosuppressants) 9. History of malignancy within the last 3 years. Subjects with prior history of in situ cancer or basal or squamous cell skin cancer are eligible. Subjects with other malignancies are eligible if they were cured by surgery alone or surgery plus radiotherapy and have been continuously disease-free for at least 5 years 10. Known CNS metastasis of leptomeningeal disease 11. Known history of HIV, Hepatitis B, active Hepatitis C or tuberculosis 12. History of pneumonitis including interstitial lung disease 13. Has a known hypersensitivity to any component of protocol therapy, or their vehicle(s) 14. Uncontrolled concurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure (NYHA Grade 2), unstable angina pectoris, cardiac arrhythmia, any Class 3 or 4 cardiac disease as defined by the New York Heart Association Functional Classification or psychiatric illness 15. Has a fever \>38oC within 3 days before the first dose of study treatment 16. Had previous exposure to Imprime PGG 17. Received transfusion of blood products (including platelets or red blood cells) or administration of colony stimulating factors (including G-CSF, GM-CSF, or recombinant erythropoietin) within 4 weeks prior to Study Day 1 18. Any condition which could interfere with, or the treatment for which might interfere with the conduct of the study or which would, in the opinion of the Investigator, unacceptably increase the subject's risk by participating in the study 19. Subject is under legal custodianship 20. First-degree relatives of the investigator, study staff or the sponsor.