GSK525762 is a novel inhibitor of bromodomain and extraterminal (BET) proteins. Trametinib is a potent inhibitor of the mitogen-activated protein kinase proteins (MEK1 and MEK2). GSK525762 and trametinib are critical for growth and survival of tumor cells. This will be the first study demonstrating the synergistic effect of BET inhibitor and MEK inhibitor administered together against tumor cell growth. This study aims to evaluate the safety, tolerability, PK, PD, and preliminary efficacy of combination of GSK525762 and trametinib when administered concomitantly to subjects with small cell lung cancer (SCLC) and rat sarcoma virus oncogene homolog (Ras) mutated solid tumors. The study will be conducted in two parts; part 1 will consists of dose escalation and dose expansion cohorts and part 2 will consists of four disease specific cohorts (SCLC, Ras-mutated adenocarcinoma [RMAC] of the colon [Ras-mutated colorectal cancer {RMCRC}] and/or rectum, Ras-mutated non small cell lung cancer [RMNSCLC], Ras-mutated pancreatic adenocarcinoma [RMPAC]) and an optional "basket" cohort (Ras-pathway activated solid tumors [RAST]). Part 1 will focus on selection of the Part 2 dose based on safety/tolerability, PK, PD, and efficacy. Part 2 will investigate the overall response rate and clinical response. The total duration of study will be approximately three years (nine to twelve months for part 1 and two years for part 2). Approximately 138-156 subjects will be enrolled in the study.