A Dose Escalation Study to Investigate the Safety, Pharmacokinetics (PK), Pharmacodynamics (PD), and Clinical Activity of GSK525762 Plus Trametinib in Subjects With Solid Tumors

Inclusion Criteria:

* Written informed consent provided.
* Males and females 18 years old and greater, at the time of signing the informed consent.
* Histologically- or cytologically confirmed diagnosis of one of the following; Part 1 dose escalation cohorts: Advanced (metastatic or non-resectable) SCLC with any mutational status, or any solid malignancy that demonstrates an activating mutation in Harvey rat sarcoma viral oncogene homolog (HRAS), Kirsten rat sarcoma viral oncogene homolog (KRAS), or neuroblastoma RAS viral (v-ras) oncogene homolog (NRAS); Part 1 dose expansion (PD cohort\[s\]) and part 2: Advanced (metastatic or non-resectable) SCLC with any mutational status, or adenocarcinoma of the colon or rectum, or NSCLC or adenocarcinoma of the pancreas, all of which must demonstrate an activating mutation in HRAS, KRAS, or NRAS; Part 2 "basket" cohort: Advanced (metastatic or non-resectable) solid malignancy that demonstrates Ras pathway activation (including but not limited to activating BRAF/HRAS/KRAS/NRAS mutation, inactivating neurofibromin (NF1) mutation, or evidence of Ras pathway activation by gene expression analysis).
* Disease that did not respond to, or progressed after, at least 1 prior line of therapy, or has no generally-accepted standard therapy (dose escalation cohorts and optional "basket" cohort only).
* Measurable disease during part 1, measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) version (v) 1.1 is recommended but not required. Subjects enrolled in part 2 must demonstrate measurable disease per RECIST v1.1.
* PD expansion subjects only: Subjects must consent to pre-dosing and on-therapy tumor biopsies and additional sample collection procedures.
* All prior treatment-related toxicities must be National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE) v4 \<=Grade 1 (except alopecia \[permitted at any grade\]) and peripheral neuropathy (permitted at \<=Grade 2) at the time of screening.
* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 to 1.
* Must have adequate organ function as defined by the following values: Absolute neutrophil count (ANC) \>=1.5 x 10\^9/liter (L); hemoglobin \>=9 grams per deciliter (g/dL) subjects that required transfusion or growth factor need to demonstrate stable hemoglobin for 7 days of 9 g/dL; platelets \>=100 x 10\^9/L; prothrombin time (PT)/International normalized ration (INR) and partial thromboplastin time (PTT) \<=1.5 x upper limit of normal (ULN); albumin \>=2.5 g/dL; total bilirubin \<=1.5 x ULN; aspartate transaminase (AST) \<=2.5 x ULN; alanine transaminase (ALT) \<=2.5 x ULN OR \<5 x ULN; estimated glomerular filtration rate \>=50 milliliter (mL)/minute/1.73 m\^2; ejection fraction\>= lower limit of normal (LLN); troponin \<=ULN
* Able to swallow and retain orally administered medication.
* A female subject is eligible to participate if she is of: Non-childbearing potential; Childbearing potential and agrees to use one of the contraception methods from the time of the screening pregnancy test until at least 7 months after the last dose of study treatment; All female subjects of childbearing potential must have a negative serum pregnancy test \<=7 days prior to first dose of study treatment; Female subjects who are lactating must discontinue nursing prior to the first dose of study treatment and must refrain from nursing throughout the treatment period and for 5 half-lives of GSK525762/trametinib or at least 28 days (whichever is longer) following the last dose of study treatment.
* Male subjects with female partners of childbearing potential must agree to abide by the reproductive guidelines from the first dose of study treatment and for at least 16 weeks after the last dose of study treatment.

Exclusion Criteria:

* Primary malignancy of the central nervous system or malignancies related to human immunodeficiency virus (HIV) or solid organ transplant.
* Prior therapy with any BET inhibitor.
* Recent prior therapy, defined as follows: Any non-biologic anti-cancer drug (either investigational or approved) within 14 days or 5 half-lives, whichever is longer, prior to the first dose of GSK525762 and trametinib; any nitrosoureas or mitomycin C within 42 days prior to the first dose of GSK525762 and trametinib; any biologic anti-cancer agent within 28 days prior to the first dose of GSK525762 and trametinib; any radiotherapy within 14 days or major surgery within 28 days prior to the first dose of GSK525762 and trametinib.
* Therapeutic-dose anticoagulation (e.g., warfarin, heparin) must be discontinued and coagulation parameters must be normalized prior to the first dose of GSK525762 and trametinib. Low dose (prophylactic) low molecular weight heparin or other anticoagulants are permitted.
* Current or planned use of a prohibited medication during treatment with GSK525762 and trametinib.
* Evidence of severe or uncontrolled systemic diseases (e.g., unstable or uncompensated respiratory, hepatic, renal, cardiac disease, or clinically significant bleeding episodes). Any serious and/or unstable pre-existing medical (aside from malignancy) condition, psychiatric disorder, or other conditions that could interfere with subject's safety, obtaining informed consent or compliance to the study procedures, in the opinion of the investigator.
* Symptomatic or untreated leptomeningeal or brain metastases or spinal cord compression. Subjects with a history of central nervous system (CNS) involvement may be enrolled so long as all of the following requirements are met: Subjects must have received definitive therapy for the CNS involvement (e.g., surgery, radiotherapy, or stereotactic radiosurgery \[i.e., gamma knife or equivalent\]); At least 28 days must have elapsed since the CNS-directed therapy; All symptoms and AEs from the CNS-directed therapy must have resolved to \<=Grade 1; Lesion stability must be demonstrated by serial imaging spaced at least 28 days apart; If the subject remains on corticosteroids, the dose must be stable to decreasing for the 28-day interval prior to study Day 1; The subject does not receive any enzyme-inducing anticonvulsants (EIACs) from 14 days prior to study Day 1 until the End of Treatment.
* Cardiac abnormalities as evidenced by any of the following: Baseline QTcF interval \>450 millisecond (msec); Clinically significant conduction abnormalities or arrhythmias; Presence of cardiac pacemaker or defibrillator with a paced ventricular rhythm limiting ECG analysis; History or evidence of current \>=Class II congestive heart failure as defined by New York Heart Association (NYHA); History of acute coronary syndromes (including unstable angina and myocardial infarction), coronary angioplasty, or stenting within the past 3 months. Subjects with a history of stent placement requiring ongoing antithrombotic therapy (e.g., clopidogrel, prasugrel) will not be permitted to enroll.
* Current active liver or biliary disease (with the exception of Gilbert's syndrome or asymptomatic gallstones, liver metastases, or otherwise stable chronic liver disease per investigator assessment).
* Presence of hepatitis B surface antigen (HBsAg) or positive hepatitis C antibody test result at screening or within 3 months prior to first dose of study treatment.
* History of known HIV infection or positive HIV test at screening.
* Any serious known immediate or delayed hypersensitivity reaction(s) to GSK525762 or trametinib, or idiosyncrasy to drugs chemically related to the investigational drugs.
* Subjects with a history of known bleeding disorder(s) or history of clinically significant (as judged by the investigator and medical monitor) hemorrhage (e.g., GI, neurologic, pulmonary) within the past 6 months.
* History of retinal vein occlusion.
* History of pneumonitis or interstitial lung disease.
* Any clinically significant gastrointestinal abnormalities that may alter absorption of oral medications, (e.g., malabsorption syndrome).