The relationship of patient age to the pathobiology of the clonal myeloid diseases.

The incidence of the major clonal myeloid diseases, clonal cytopenias, acute, subacute (oligoblastic), and chronic myelogenous leukemia, polycythemia vera, thrombocythemia, and idiopathic myelofibrosis increases in a log-linear manner from young adulthood through advanced age. In older patients, diseases requiring cytotoxic treatment are more difficult and less successful to manage because comorbid conditions and poor performance status are more prevalent, decreasing the tolerance to therapy and increasing the frequency of side effects. This age effect is highlighted by the dramatically less favorable outcome in older than younger patients with acute myeloid leukemia with similar "favorable" cytogenetic changes. In addition, in acute and subacute myeloid leukemia in older patients, the disease is intrinsically more resistant to therapy. Overexpression of drug resistance genes and unfavorable genetic mutations are more prevalent in older patients and provide evidence that acute myeloid leukemia is often qualitatively different in these patients. The gradient of age effects is continuous; the frequency of poor outcome increasing by decade (or less). The decline in survival becomes especially steep as quinquagenarians (50-year-olds) age to nonagenarians (90-year-olds). Although improved drug schedules have led to significant improvements in event-free survival in younger patients, these improvements have been far less evident in older patients. New approaches, especially the development of drugs aimed at new targets, will be required to obtain a high frequency of long-term remissions in older patients. Agents that reverse inherent cellular drug resistance, farnesyltransferase inhibitors, BCL-2 inhibitors, and FLT3 inhibitors are early examples of such approaches.