Morphologic and immunohistochemical (IHC) changes in metastatic melanoma (MM) tissue and associations with clinical outcome in patients (pts) on BRAF inhibitors (BRAFi).

8542 Background: Drugs that target oncogenic mutant BRAF have impressive response rates in MM pts, however resistance usually develops within 12 months. In pts on clinical trials of BRAFi, we investigated mechanisms of action and resistance by analysing MM tissue excised before and after treatment, and on progression. Methods: Pts with V600E or V600K mutant MM enrolled on the Ph 1/2 study of GSK2118436 or the Ph 2 study of RO5185426 had biopsies (bx) ≤7 days prior to (PRE) and 7 days +/-4 after commencing BRAFi (POST), and on progression (PROG). Morphologic and IHC (p27, pAKT, pERK, Ki67, TUNNEL assay) tissue bx features were correlated with clinical outcome data including time to progression (TTP) and overall survival (OS), and measures of response; FDG PET change in SUV max, CT-determined RECIST response and calliper measurement of subcutaneous lesions. Results: 12 pts had paired PRE and POST bx, 7 also had PROG bx. 3 pts had paired PRE and PROG bx only. Of the 15 pts, 2 were treated with RO5185426 and 13 with GSK2118436. Comparing paired PRE and POST bx, there was a marked decrease in Ki67 and mitotic rate in 11/12 pts (p<0.05) regardless of clinical response, and a systematic change in cell type (round to spindle), cell size (decreased), necrosis (increased), intratumoral lymphocytes (ITL) (increased) and apoptosis (increased) on exposure to BRAFi (p<0.05). Peritumoral lymphocytes (PTL) increased (p=0.07). Preliminary data suggest there was no systematic change in p27 or pAKT, however a higher PRE nuclear p27 level maybe associated with a poorer OS (HR 1.5, 95%CI 1.03-2.27, p=0.03) and TTP (HR 1.29, 95%CI 1.06-1.59, p=0.01). Change in calliper measurement of the POST lesion from baseline correlated negatively with changes in necrosis and ITL (p<0.05). For POST to PROG paired bx, apoptosis, ITL and PTL decreased (p<0.05). Conclusions: Specific morphological changes occur in melanoma tissue on exposure to BRAFi, some of which may be associated with clinical outcome. Baseline level of p27 may be prognostic in BRAF mutant pts. Changes in tumoral lymphocytic infiltrates in POST and PROG bx suggest that an immune response may have a role in BRAFi activity.