An AE was defined as any unfavorable and unintended change in the structure, function, or chemistry of the body temporally associated with the use of the Sponsor's product, whether or not considered related to the use of the product. Any worsening (i.e., any clinically significant adverse change in frequency and/or intensity) of a pre-existing condition which is temporally associated with the use of the Sponsor's product, was also an AE. The number of participants who experienced an AE was reported for each dose level group.

DLTs were AEs considered related to study drug that prevented escalation of the drug dose. Hematologic DLTs included any Grade 5 hematologic toxicity, Grade 4 neutropenia lasting for ≥7 days in duration, Grade 3 or Grade 4 neutropenia with fever \>38.5°C and/or infection requiring antibiotic or anti-fungal treatment, and Grade 4 thrombocytopenia (≤25.0 x 10\^9/L). Non-hematologic DLT was defined as any Grade 3, 4, or 5 non-hematologic toxicity, with the specific exceptions of: Grade 3 nausea or Grade 3 vomiting, Grade 3 diarrhea, or Grade 3 dehydration occurring in the setting of inadequate compliance with supportive care and lasting for \<48 hours, alopecia, inadequately treated hypersensitivity reactions, or Grade 3 elevated transaminases of ≤1 week in duration. Any drug-related AE leading to a dose modification of MK-5108, or any unresolved drug-related toxicity persisting\>6 weeks, was also considered a DLT.