Age affects temporal response, but not durability, to serial ketamine infusions for treatment refractory depression

Abstract Background: Ketamine is a rapid-acting treatment for patients with treatment refractory depression (TRD), however treatment responses are often transient and ketamine9s antidepressant action lacks robust clinical durability. Little is known about which patient characteristics are associated with faster or more durable ketamine responses. Ketamine9s antidepressant mechanism is proposed to involve modulation of glutamatergic signaling leading to long term potentiation (LTP) and synaptogenesis, and these neuroplasticity pathways have been shown to be attenuated with older age. We therefore investigated the impact of patient age on the speed and durability of ketamine9s antidepressant effects in veterans receiving serial intravenous ketamine infusions for TRD. Methods: Beck Depression Inventory (BDI-II) scores from 49 veterans receiving six ketamine infusions (twice weekly) were examined from a retrospective case series. Percent change in BDI-II scores across the infusion series were assessed with respect to patient age using a mixed-linear model. Follow-up analyses examined the age x infusion number interaction effect at each assessment time point. To assess treatment durability, BDI-II change scores three weeks following the sixth infusion were correlated with age. Results: There was a significant age x infusion number interaction (F=3.01, p=.0274) across the six infusions. Beta estimates at each infusion showed a significant effect of age at infusion #4 (B=.88% +/-.29%, t=3.02, p=.004) and a trend towards significance at infusion #5 (B=.62% +/-.31%, t=1.95, p=.057). There was no significant correlation between percent change in BDI-II and age at three-week follow-up. Conclusions: Older age is associated with an altered trajectory of antidepressant response across serial ketamine infusions, with a model-predicted difference of 8.8% less improvement in BDI-II score for each decade in age mid-way through the infusion course. In contrast, antidepressant durability at three-week follow-up was not related to age. These data suggest age is an important moderating factor of patient response to ketamine, and that differing mechanisms may underlie speed and durability of ketamine9s antidepressant activity.