An adverse event (AE) is any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. A serious adverse event (SAE) is an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly. The TEAEs are defined as events present at baseline that worsened in intensity after administration of study drug or events absent at baseline that emerged after administration of study drug.

A DLT is defined as any toxicity not clearly and directly related to the primary disease or to another etiology and included: any Grade 3 or higher toxicity that occurs during the DLT evaluation period, aspartate aminotransferase or alanine aminotransferase \>= 3 × upper limit of normal (ULN) together with total bilirubin \>= 2 × ULN, where no other reason other than the study drugs, can be found to explain the combination of increases, Grade ≥ 2 myocarditis, Grade 2 non infectious pneumonitis that does not resolve to Grade ≤ 1 within 7 days of the initiation of maximal supportive care, any Grade \>= 2 MEDI5395 related toxicity that prevents administration of more than 1 dose of MEDI5395 within the 18-day dosing period, in sequential dosing cohorts, any Grade \>= 2 MEDI5395 related toxicity that prevents administration of the first dose of durvalumab, and any AE that, after consultation with the Sponsor and investigators, is deemed a DLT.

Number of participants with TEAEs resulting in permanent discontinuation of MEDI5395 are reported.

Number of participants with TEAEs resulting in permanent discontinuation of durvalumab are reported.

Number of participants with abnormal vital signs (blood pressure, pulse rate, respiration rate, oxygen saturation, and body temperature) reported as TEAEs are reported.

Laboratory assessment included hematology, clinical chemistry, coagulation, cardiac parameters, and urinalysis. Participants with abnormal laboratory parameters reported as TEAEs are reported.

Number of participants with abnormal ECG parameters reported as TEAEs are reported.

Change from baseline in LVEF values are reported.

Maximum reduction from baseline in GLS values are reported. The GLS \< 16% is abnormal, GLS \> 18% is normal, and GLS 16% to 18% is borderline.

ECOG performance status is used by doctors and researchers to assess how a participant's disease is progressing, assess how the disease affects the daily living activities of the participant and determine appropriate treatment and prognosis. The scores are: 0 = Fully Active, able to carry out all pre-disease performance without restrictions; 1 = Restricted activity but ambulatory and able to carry out light work or work of a sedentary nature; 2 = Ambulatory and capable of self-care but unable to carry out work activities; 3 = Capable of only limited self-care, confined to bed or chair more than 50% of waking hours; 4 = Completely Disabled, unable to carry out any self-care and totally confined to bed or chair; 5 = Dead.