A Study to Evaluate the Safety of GSK2398852 When Co-administered With GSK2315698 in Patients With Systemic Amyloidosis

Inclusion Criteria:

* Subject has been medically diagnosed with systemic amyloidosis and falls into one of the patient groups (small to moderate amyloid load involving the spleen for Part A; moderate to large amyloid load involving the spleen (to a moderate/large extent) for Part A (following agreement from external safety committee); moderate to large amyloid load involving the spleen and liver (spleen involved to a moderate/large extent) for Part A extension (if required); and moderate to large amyloid load involving the spleen (and liver in subset of subjects only) for Part B).
* Alanine aminotransferase (ALT) \<3x upper limit of normal (ULN) and bilirubin \<1.5x ULN (isolated bilirubin \>1.5x ULN is acceptable if bilirubin is fractionated and direct bilirubin \<35%).
* Male or female between 18 and 70 years of age inclusive, at the time of signing the informed consent.
* Subject is ambulant and capable of attending for the study visit schedule.
* Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form.
* A female subject is eligible to participate if she is of non-childbearing potential; or females on hormone replacement therapy (HRT) and whose menopausal status is in doubt will be required to use one of the approved contraception methods.
* Male subjects with female partners of child-bearing potential must agree to use one of the approved contraception methods.
* Smokers (\<10 /day) are permitted but must be willing to abstain for the duration of residential study sessions

Exclusion Criteria:

* A positive pre-study Hepatitis B surface antigen or positive Hepatitis C antibody result within 3 months of screening
* The subject has participated in a clinical trial and has received an investigational therapeutic product (unlicensed) within the following time period prior to the first dosing day in the current study: 3 months, 5 half-lives or twice the duration of the biological effect of the investigational product (whichever is longer). This timeframe will not apply to short term administration of GSK2315698 in study CPH114527.
* Pregnant females as determined by positive serum or urine human chorionic gonadotropin (hCG) test at screening or prior to dosing.
* Lactating females.
* Estimated glomerular filtration rate (GFR)\<30 milliliter (mL)/minute (min) \[\<60 mL/min for the first 4 subjects to be enrolled\]
* Evidence of an active urinary sediment on microscopy as evidenced by the presence of red cell casts
* Decompensated cardiac failure or a recent history of syncope associated with cardiac disease.
* In a subject in whom there is a clinical suspicion of cardiac amyloid, an echocardiogram is consistent with significant cardiac amyloid, whether symptomatic or not.
* Clinically significant anaemia- hemoglobin (Hb) \<9 gram (g)/deciliter (dL).
* Use of prohibited medications.
* Poor or unsuitable venous access.
* Subjects with a QT interval corrected using Fridericia's formulas (QTcF) of \>480 ms or other electrocardiogram (ECG) abnormalities which, in the opinion of the investigator are clinically significant and may increase safety risk.
* Uncontrolled hypertension with systolic blood pressure (BP) \>170 mmHg and /or diastolic \>100 mmHg
* Presence of any co-morbid condition (e.g. severe or unstable coronary artery disease; moderate-severe chronic obstructive pulmonary disease) which in the opinion of the investigator would increase the potential risk to the subject.
* Subjects with active vasculitis
* Exclusions from Equilibrium contrast Magnetic Resonance Imaging (EqMRI) scanning \[Contraindications to Magnetic Resonance Imaging (MRI) scanning including, but not limited to: Intracranial aneurism clips (except Sugita); History of intra-orbital metal fragments that have not been removed by an MD (as confirmed by orbital X-Ray); Pacemakers and non-MR compatible heart valves; Inner ear implants; History of claustrophobia; estimated GFR \<30 mL/min (gadolinium exclusion)\]
* Subjects with dementia or a diagnosis of cerebral amyloid angiopathy.