Targeting the cyclin dependent kinase and retinoblastoma axis overcomes standard of care resistance in BRAF V600E -mutant melanoma

// Antoneicka L. Harris 1 , Samantha E. Lee 2 , Louis K. Dawson 2 , Laura A. Marlow 1 , Brandy H. Edenfield 1 , William F. Durham 2 , Thomas J. Flotte 3 , Michael Thompson 4 , Daniel L. Small 2 , Aidan J. Synnott 2 , Svetomir N. Markovic 4 and John A. Copland 1 1 Department of Cancer Biology, Mayo Clinic, Jacksonville, FL, USA 2 Charles River Discovery Services, Morrisville, NC, USA 3 Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA 4 Hematology/Oncology Department, Mayo Clinic, Rochester, MN, USA Correspondence to: John A. Copland, email: copland.john@mayo.edu Keywords: melanoma; patient-derived tumor xenograft (PDTX); cyclin dependent kinase 4/6 (CDK4/6) inhibitors; retinoblastoma (Rb); mutant BRAF Received: December 13, 2017      Accepted: December 18, 2017      Published: December 23, 2017 ABSTRACT Patient-derived tumor xenograft (PDTX) mouse models were used to discover new therapies for naive and drug resistant BRAF V600E -mutant melanoma. Tumor histology, oncogenic protein expression, and antitumor activity were comparable between patient and PDTX-matched models thereby validating PDTXs as predictive preclinical models of therapeutic response in patients. PDTX models responsive and non-responsive to BRAF/MEK standard of care (SOC) therapy were used to identify efficacious combination therapies. One such combination includes a CDK4/6 inhibitor that blocks cell cycle progression. The rationale for this is that the retinoblastoma protein (pRb) is 95% wildtype in BRAF mutant melanoma. We discovered that 77/77 stage IV metastatic melanoma tissues were positive for inactive phosphorylated pRb (pRb-Ser780). Rb is hyperphosphorylated and inactivated by CDK4/6:cyclin D1 and when restored to its hypophosphorylated active form blocks cell cycle progression. The addition of a CDK4/6 inhibitor to SOC therapy was superior to SOC. Importantly, triple therapy in an upfront treatment and salvage therapy setting provided sustained durable response. We also showed that CDK4/6 blockade resensitized drug resistant melanoma to SOC therapy. Durable response was associated with sustained suppression of pRb-Ser780. Thus, reactivation of pRb may prove to be a clinical biomarker of response and the mechanism responsible for durable response. In light of recent clinical trial data using this triple therapy against BRAF V600E -mutant melanoma, our findings demonstrating superior and prolonged durable response in PDTX models portend use of this therapeutic strategy against naive and SOC resistant BRAF V600E -mutant metastatic melanoma coupled with pRB-Ser780 as a biomarker of response.