Cancer chemotherapy and cognitive function in rodent models: memory impairment induced by cyclophosphamide in mice.

To the Editor: The study by Lee et al. (1) is the first report on the effects of cancer chemotherapy on cognitive function in rodent models. Given the increasing concern about cognitive dysfunction in patients receiving chemotherapy, the development of animal models to characterize chemotherapy-induced cognitive impairment has been proposed as a priority for future research (2, 3). Unexpectedly, Lee et al. (1) found an enhancement of both memory and hippocampal synaptic plasticity following several weeks of treatment with cyclophosphamide in rats. We too have used rodent models to investigate the cognitive effects of cyclophosphamide. In contrast to the findings reported by Lee et al. (1), we have observed a transient memory impairment following cyclophosphamide administration in mice. In our experiments, male CF1 mice (70-90 days of age) were trained and tested in step-down inhibitory avoidance conditioning, a type of emotionally motivated, hippocampus-dependent memory task where animals learn to associate a location in the training apparatus with a footshock. Inhibitory avoidance training was carried out as described previously (4). Either 1 day or 1 week before behavioral training, animals were given a systemic injection of cyclophosphamide (8, 40, or 200 mg/kg, i.p.). Control animals were injected with saline. Mice treated with cyclophosphamide at 40 or 200 mg/kg 1 day before training showed significant impairment of 24-hour memory retention when compared with control animals [mean F SE retention test latencies (seconds) were 61.30 F 20.93 in the control group, 80.91 F 25.02 in the group treated with 8 mg/kg cyclophosphamide, 22.0 F 12.02 in the group treated with 40 mg/kg cyclophosphamide, and 12.36 + 2.87 in the group treated with 200 mg/kg cyclophosphamide; both Ps < 0.01 compared with the control group with two-tailed Mann-Whitney U tests; n = 10-11 animals per group]. There was no significant difference among groups in training performance [overall mean F SE training trial latency (seconds) was 12.77 F 1.46; P = 0.16]. A control experiment showed that cyclophosphamide did not affect open field behavior (4), indicating that the impairing effects of cyclophosphamide on inhibitory avoidance could not be attributed to drug-induced alterations in locomotion, motivation, or anxiety (data not shown). Systemic administration of cyclophosphamide (8, 40, or 200 mg/kg, i.p.) did not affect inhibitory avoidance memory when given 1 week before training (data not shown). Our results show that a single administration of cyclophosphamide induces memory impairment in a mice model of aversive conditioning. Further studies are required to characterize cognitive deficits induced by cancer chemotherapy in animal models and investigate the mechanisms underlying the differential effects of cyclophosphamide on memory in different experimental paradigms.