A 58-year-old male with no prior psychiatric history is referred by his primary care provider for evaluation of progressive personality change over the past 18 months. His wife reports he has become increasingly disinhibited, making inappropriate sexual comments to strangers and spending excessively on items he does not need. He shows reduced empathy, has lost interest in his grandchildren, and his diet now consists almost exclusively of sweets. On mental status examination, he displays blunted affect, poor insight into his behavioral changes, and perseverative speech patterns. He scores 22/30 on the MMSE, losing points primarily on executive function tasks. MRI reveals asymmetric frontal and temporal lobe atrophy. Which of the following is the most likely diagnosis?