Effects of chemotherapeutic agents 5-fluorouracil and methotrexate alone and combined in a mouse model of learning and memory

The effectiveness of anti-inflammatory agents in reducing chemotherapy-induced cognitive impairment in preclinical models - A systematic review

Chemotherapy-induced cognitive impairment (CICI) is a debilitating condition resulting from chemotherapy administration for cancer treatment. CICI is characterised by various cognitive impairments, including issues with learning, memory, and concentration, impacting quality of life. Several neural mechanisms are proposed to drive CICI, including inflammation, therefore, anti-inflammatory agents could ameliorate such impairments. Research is still in the preclinical stage; however, the efficacy of anti-inflammatories to reduce CICI in animal models is unknown. Therefore, a systematic review was conducted, with searches performed in PubMed, Scopus, Embase, PsycInfo and Cochrane Library. A total of 64 studies were included, and of the 50 agents identified, 41 (82%) reduced CICI. Interestingly, while non-traditional anti-inflammatory agents and natural compounds reduced impairment, the traditional agents were unsuccessful. Such results must be taken with caution due to the heterogeneity observed in terms of methods employed. Nevertheless, preliminary evidence suggests anti-inflammatory agents could be beneficial for treating CICI, although it may be critical to think beyond the use of traditional anti-inflammatories when considering which specific compounds to prioritise in development.

The effect of doxorubicin or cyclophosphamide treatment on auditory brainstem response in mice

Clinical studies suggest that chemotherapy is associated with long-term cognitive impairment in some patients. Several underlying mechanisms have been proposed; however, the etiology of chemotherapy-related cognitive dysfunction remains relatively unknown. There is evidence that oligodendrocytes and white matter tracts within the CNS may be particularly vulnerable to chemotherapy-related damage and dysfunction. Auditory brainstem responses (ABRs) have been used to detect and measure functional integrity of myelin in a variety of animal models of autoimmune disorders and demyelinating diseases. Limited evidence suggests that increases in interpeak latencies, associated with disrupted impulse conduction, can be detected in ABRs following 5-fluorouracil administration in mice. It is unknown if similar functional disruptions can be detected following treatment with other chemotherapeutic compounds and the extent to which alterations in ABR signals represent robust and long-lasting impairments associated with chemotherapy-related cognitive impairment. Thus, C57BL/6 J mice were treated every 3rd day for a total of 3 injections with low or high dose cyclophosphamide, or doxorubicin. ABRs of mice were assessed on days 1, 7, 14, 56 and 6 months following completion of chemotherapy administration. There were timing and amplitude differences in the ABRs of the doxorubicin and the high dose cyclophosphamide groups relative to the control animals. However, despite significant toxic effects as assessed by weight loss, the changes in the ABR were transient.

Neuroprotective properties of chrysin on decreases of cell proliferation, immature neurons and neuronal cell survival in the hippocampal dentate gyrus associated with cognition induced by methotrexate

Methotrexate (MTX) is a drug widely used for chemotherapy and can reduce cancer cell production by inhibiting dihydrofolate reductase and decreasing cancer cell growth. MTX has a neurotoxic effect on neural stem and glial cells, leading to memory deficits. Chrysin is a natural flavonoid that contains essential biological activities, such as neuroprotective and cognitive-improving properties. Therefore, the aim of the present study was to investigate the protective effect of chrysin against MTX-induced memory impairments related to hippocampal neurogenesis. Seventy-two male Sprague Dawley rats were divided into six groups: control, MTX, chrysin (10 and 30 mg/kg), and MTX+ chrysin (10 and 30 mg/kg) groups. Chrysin (10 and 30 mg/kg) was administered by oral gavage for 15 days. MTX (75 mg/kg) was administered by intravenous injection on days 8 and 15. Spatial and recognition memories were evaluated using the novel object location (NOL) and novel object recognition (NOR) tests, respectively. Moreover, cell proliferation, neuronal cell survival, and immature neurons in the subgranular zone of the hippocampal dentate gyrus were quantified by Ki-67, bromodeoxyuridine/neuronal nuclear protein (BrdU/NeuN), and doublecortin (DCX) immunohistochemistry staining. The results of the MTX group demonstrated that spatial and recognition memories were both impaired. Furthermore, cell division reduction, neuronal cell survival reduction, and immature neuron decreases were detected in the MTX group and not observed in the co-administration groups. Therefore, these results revealed that chrysin could alleviate memory and neurogenesis impairments in MTX-treated rats.

The Effects of 5-Fluorouracil/Leucovorin Chemotherapy on Cognitive Function in Male Mice

5-Fluorouracil (5-Fu) and leucovorin (LV) are often given in combination to treat colorectal cancer. 5-Fu/LV prevents cell proliferation by inhibiting thymidylate synthase, which catalyzes the conversion of deoxyuridine monophosphate to deoxythymidine monophosphate. While 5-Fu has been shown to cause cognitive impairment, the synergistic effect of 5-Fu with LV has not been fully explored. The present investigation was designed to assess how the combination of 5-Fu and LV affect cognition in a murine model. Six-month-old male mice were used in this study; 15 mice received saline injections and 15 mice received 5-Fu/LV injections. One month after treatment, the elevated plus maze, Y-maze, and Morris water maze behavioral tasks were performed. Brains were then extracted, cryosectioned, and stained for CD68 to assay microglial activation and with tomato lectin to assay the vasculature. All animals were able to locate the visible and hidden platform locations in the water maze. However, a significant impairment in spatial memory retention was observed in the probe trial after the first day of hidden-platform training (first probe trial) in animals that received 5-Fu/LV, but these animals showed spatial memory retention by day 5. There were no significant increases in inflammation as measured by CD68, but 5-Fu/LV treatment did modulate blood vessel morphology. Tandem mass tag proteomics analysis identified 6,049 proteins, 7 of which were differentially expressed with a p-value of <0.05 and a fold change of >1.5. The present data demonstrate that 5-Fu/LV increases anxiety and significantly impairs spatial memory retention.

Animal models of chemotherapy-induced cognitive decline in preclinical drug development

RATIONALE

Chemotherapy-induced cognitive impairment (CICI), chemobrain, and chemofog are the common terms for mental dysfunction in a cancer patient/survivor under the influence of chemotherapeutics. CICI is manifested as short/long term memory problems and delayed mental processing, which interferes with a person's day-to-day activities. Understanding CICI mechanisms help in developing therapeutic interventions that may alleviate the disease condition. Animal models facilitate critical evaluation to elucidate the underlying mechanisms and form an integral part of verifying different treatment hypotheses and strategies.

OBJECTIVES

A methodical evaluation of scientific literature is required to understand cognitive changes associated with the use of chemotherapeutic agents in different preclinical studies. This review mainly emphasizes animal models developed with various chemotherapeutic agents individually and in combination, with their proposed mechanisms contributing to the cognitive dysfunction. This review also points toward the analysis of chemobrain in healthy animals to understand the mechanism of interventions in absence of tumor and in tumor-bearing animals to mimic human cancer conditions to screen potential drug candidates against chemobrain.

RESULTS

Substantial memory deficit as a result of commonly used chemotherapeutic agents was evidenced in healthy and tumor-bearing animals. Spatial and episodic cognitive impairments, alterations in neurotrophins, oxidative and inflammatory markers, and changes in long-term potentiation were commonly observed changes in different animal models irrespective of the chemotherapeutic agent.

CONCLUSION

Dyscognition exists as one of the serious side effects of cancer chemotherapy. Due to differing mechanisms of chemotherapeutic agents with differing tendencies to alter behavioral and biochemical parameters, chemotherapy may present a significant risk in resulting memory impairments in healthy as well as tumor-bearing animals.

Chemotherapy-induced cognitive impairment (CICI): An overview of etiology and pathogenesis

Many cancer patients treated with chemotherapy develop chemotherapy-induced cognitive impairment (CICI), often referred to as chemo-brain, which manifest during or post-treatment with variable degrees, onset and duration thereby affecting the patients' quality of life. Several chemotherapeutic agents have been studied to determine its possible association with cognitive impairment and to fully comprehend their contribution to CICI. A vast number of studies have emerged proposing several candidate underlying mechanisms and etiologies contributing to CICI such as direct neurotoxicity, BBB disruption, decreased hippocampal neurogenesis, white matter abnormalities, secondary neuro-inflammatory response and increased oxidative stress; however, the exact underlying mechanisms are still not well defined. This review summarizes CICI associated with most commonly used chemotherapeutic agents with emphasizes the possible underlying pathogenesis in both animal and clinical studies.

Neuroprotective effects of hesperidin against methotrexate-induced changes in neurogenesis and oxidative stress in the adult rat

Methotrexate (MTX) induces the formation of reactive oxygen species (ROS) and leads to neurotoxicity. The drug also negatively impacts neurogenesis and memory. Hesperidin (Hsd) is a major flavanoid with multiple beneficial pharmacological effects such as anti-oxidation, anti-inflammation, and neuroprotective effects. The aim of our study was to investigate the neuroprotective effects of Hsd against MTX-induced alterations in oxidative stress and neurogenesis. Sprague Dawley rats were divided into four groups: 1) a vehicle group, which received saline and propylene glycol, 2) an Hsd group, which was orally administered with Hsd (100 mg/kg) for 21 days, 3) an MTX group, which received MTX (75 mg/kg) by intravenous injection on days 8 and 15, and 4) an MTX + Hsd group, which received both MTX and Hsd. After treatment with MTX, p21-positive cells had increased significantly and doublecortin (DCX) expression in the hippocampus had decreased significantly. Treatment with MTX also increased malondialdehyde (MDA) in both the hippocampus and prefrontal cortex and decreased levels of brain-derived neurotropic factor (BDNF) and nuclear factor erythroid 2-related factor 2 (Nrf2) in the hippocampus and prefrontal cortex. Additionally, there were significant decreases in superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT) in the hippocampus and prefrontal cortex in the MTX group. However, co-treatment with Hsd ameliorated the negative effects of MTX on neurogenesis, oxidative stress, and antioxidant enzymes. These findings suggest that Hsd may be able to prevent neurotoxic effects of MTX by reducing oxidative stress and enhancing hippocampal neurogenesis.

Spatial memory deficits after vincristine-induced lesions to the dorsal hippocampus

Vincristine is a commonly used cytostatic drug for the treatment of leukemia, neuroblastoma and lung cancer, which is known to have neurotoxic properties. The aim of this study was to assess the effects of vincristine, injected directly into the dorsal hippocampus, in spatial memory using the spatial cone field discrimination task. Long Evans rats were trained in the cone field, and after reaching training criterion received bilateral vincristine infusions into the dorsal hippocampus. Vincristine-treated animals presented unilateral or bilateral hippocampal lesions. Animals with bilateral lesions showed lower spatial working and reference memory performance than control animals, but task motivation was unaffected by the lesions. Working and reference memory of animals with unilateral lesions did not differ from animals with bilateral lesions and control animals. In sum, intrahippocampal injection of vincristine caused profound tissue damage in the dorsal hippocampus, associated with substantial cognitive deficits.

Potential learning and memory disruptors and enhancers in a simple, 1-day operant task in mice

The objective of this study was to develop a rapid, 1-day learning and memory assay in mice that is sensitive to the effects of compounds that could impair or enhance acquisition and retrieval. Swiss-Webster, male mice were placed in experimental chambers for a 1-h acquisition session with an intermittent, audible tone. If a nose-poke response occurred during the tone, an Ensure water solution was presented. After 1 h, the mice returned to the chambers for 2 h. Drugs were injected before or after sessions to determine the effects on acquisition and/or retrieval. Mice injected with saline learned a nose-poke response as measured by decreased latencies to earn 10 reinforcers, increased reinforced response rates, and decreased nonreinforced response rates. Scopolamine and acetazolamide impaired retrieval of the nose-poke response, whereas ketamine only modestly impaired retrieval. Doses of 8-OH-DPAT or the novel carbonic anhydrase activator, MAI27, either had no effect or impaired some measures of responding. Neither 8-OH-DPAT nor MAI27 were able to prevent the modest impairments produced by ketamine. The simple, 1-day operant task is a rapid assay that can be used as an initial screen to test the effects of learning and memory disruptors and potentially enhancers.

Development of a Human APOE Knock-in Mouse Model for Study of Cognitive Function After Cancer Chemotherapy

Cancer-related cognitive impairment in breast cancer patients exposed to multi-agent chemotherapy regimens is associated with the apolipoprotein E4 (APOE4) allele. However, it is difficult to determine the effects of specific agents on cognitive impairment in human studies. We describe the development of a human APOE knock-in congenic C57BL/6J mouse model to study cancer-related cognitive impairment. Female APOE3 and APOE4 homozygous mice were either left untreated or treated with the most commonly used breast cancer therapeutic agent, doxorubicin. APOE3 and APOE4 mice had similar behaviors in exploratory and anxiety assays, which were affected transiently by doxorubicin treatment. Spatial learning and memory were measured in a Barnes maze: after 4 days of training, control APOE3 and APOE4 mice were able to escape with similar latencies. In contrast, doxorubicin-treated APOE4 mice had markedly impaired learning compared to doxorubicin-treated APOE3 mice at all time points. Voxel-based morphometry of magnetic resonance images revealed that doxorubicin treatment caused significant changes in the cortex and hippocampus of in both APOE3 and APOE4 mouse brains, but the differences were significantly greater in the APOE4 brains. The results indicate that doxorubicin-exposed APOE4 mice recapitulate key aspects of human cancer-related cognitive impairment. These data support the usefulness of this novel preclinical model for future elucidation of the genetic and molecular interactions of APOE genotype with chemotherapy; this model can also allow extension to prospective studies of older mice to study these interactions in the context of aging.

Doxorubicin-induced elevated oxidative stress and neurochemical alterations in brain and cognitive decline: protection by MESNA and insights into mechanisms of chemotherapy-induced cognitive impairment ("chemobrain")

Chemotherapy-induced cognitive impairment (CICI) is now widely recognized as a real and too common complication of cancer chemotherapy experienced by an ever-growing number of cancer survivors. Previously, we reported that doxorubicin (Dox), a prototypical reactive oxygen species (ROS)-producing anti-cancer drug, results in oxidation of plasma proteins, including apolipoprotein A-I (ApoA-I) leading to tumor necrosis factor-alpha (TNF-α)-mediated oxidative stress in plasma and brain. We also reported that co-administration of the antioxidant drug, 2-mercaptoethane sulfonate sodium (MESNA), prevents Dox-induced protein oxidation and subsequent TNF-α elevation in plasma. In this study, we measured oxidative stress in both brain and plasma of Dox-treated mice both with and without MESNA. MESNA ameliorated Dox-induced oxidative protein damage in plasma, confirming our prior studies, and in a new finding led to decreased oxidative stress in brain. This study also provides further functional and biochemical evidence of the mechanisms of CICI. Using novel object recognition (NOR), we demonstrated the Dox administration resulted in memory deficits, an effect that was rescued by MESNA. Using hydrogen magnetic resonance imaging spectroscopy (H1-MRS) techniques, we demonstrated that Dox administration led to a dramatic decrease in choline-containing compounds assessed by (Cho)/creatine ratios in the hippocampus in mice. To better elucidate a potential mechanism for this MRS observation, we tested the activities of the phospholipase enzymes known to act on phosphatidylcholine (PtdCho), a key component of phospholipid membranes and a source of choline for the neurotransmitter, acetylcholine (ACh). The activities of both phosphatidylcholine-specific phospholipase C (PC-PLC) and phospholipase D were severely diminished following Dox administration. The activity of PC-PLC was preserved when MESNA was co-administered with Dox; however, PLD activity was not protected. This study is the first to demonstrate the protective effects of MESNA on Dox-related protein oxidation, cognitive decline, phosphocholine (PCho) levels, and PC-PLC activity in brain and suggests novel potential therapeutic targets and strategies to mitigate CICI.

Chemotherapy and cognition: International cognition and cancer task force recommendations for harmonising preclinical research

Cancer survivors who undergo chemotherapy for non-CNS tumours often report substantial cognitive disturbances that adversely affect quality of life, during and after treatment. The neurotoxic effects of anti-cancer drugs have been confirmed in clinical and pre-clinical research. Work with animals has also identified a range of factors and underlying mechanisms that contribute to chemotherapy-induced cognitive impairment. However, there is a continuing need to develop standard cognitive testing procedures for validation and comparison purposes, broaden the search for biological and neurochemical mechanisms, and develop improved animal models for investigating the combined effects of treatment, the disease, and other potential factors (e.g., age, stress). In this paper, a working group, formed under the auspices of the International Cognition and Cancer Task Force, reviews the state of pre-clinical research, formulates strategic priorities, and provides recommendations to guide animal research that meaningfully informs clinical investigations.

Asiatic acid protects against cognitive deficits and reductions in cell proliferation and survival in the rat hippocampus caused by 5-fluorouracil chemotherapy

The chemotherapy drug, 5-fluorouracil (5-FU), has been reported to cause cognitive impairments in cancer patients. The drug also reduces cell proliferation and survival in the brain. Asiatic acid (AA) is a triterpene compound found in Centella asiatica that can protect against reduction of neurogenesis in the hippocampus and memory deficits induced by valproic acid (VPA). In the present study, we investigated the preventive effects of AA on the deficits in spatial working memory and cell proliferation and survival caused by 5-FU chemotherapy in a rat model. Male Sprague Dawley rats received 5-FU (5 i.v. injections, 25 mg/kg) on day 8, 11, 14, 17 and 20 of the study. This was co-administered with AA (30 mg/kg, oral gavage tube) either 20 days before receiving 5-FU (preventive), after receiving 5-FU (recovery), or for the entire period of the experiment (throughout). Spatial working memory was determined using the novel object location (NOL) test and hippocampal cell proliferation and survival of dividing cells were quantified using immunohistochemistry. Rats in the 5-FU alone and recovery groups showed memory deficits in the NOL test and reductions in cell proliferation and cell survival in the subgranular zone (SGZ) of the hippocampal dentate gyrus. Rats in the control, AA alone, and both preventive and throughout co-administration groups, however, did not exhibit these characteristics. The results showed that 5-FU chemotherapy impaired memory and reduced cell proliferation and cell survival in the SGZ of the hippocampal dentate gyrus. However, these impairments in the animals receiving 5-FU chemotherapy were restored to control levels when AA was co-administered before and during 5-FU treatment. These data demonstrate that AA can prevent the spatial working memory and hippocampal neurogenesis impairments caused by 5-FU chemotherapy.

KU32 prevents 5-fluorouracil induced cognitive impairment

Chemotherapy induced cognitive impairment (i.e. chemobrain) involves acute and long-term deficits in memory, executive function, and processing speed. Animal studies investigating these cognitive deficits have had mixed results, potentially due to variability in the complexity of behavioral tasks across experiments. Further, common chemotherapy treatments such as 5-fluorouracil (5-FU) break down myelin integrity corresponding to hippocampal neurodegenerative deficits and mitochondrial dysfunction. There is little evidence, however, of pharmacological treatments that may target mitochondrial dysfunction. Using a differential reinforcement of low rates (DRL) task combining spatial and temporal components, the current study evaluated the preventative effects of the pharmacological agent KU32 on the behavior of rats treated with 5-FU (5-FU+Saline vs. 5FU+KU32). DRL performance was analyzed the day after the first set of injections (D1), the day after the second set of injections (D7) and the last day of the experiment (D14). The 5FU+KU32 group earned significantly more reinforcers on the DRL task at D7 and D14 than the 5FU+Saline group. Further, the 5FU+KU32 group showed significantly better temporal discrimination. The 5FU+KU32 showed within-group improvement in temporal discrimination from D7 to D14. No significant differences were observed in spatial discrimination, however, those in the 5FU+Saline group responded more frequently on T3 compared to the 5FU+KU32 group, highlighting temporal discrimination differences between groups. The current data suggest that KU32 shows promise in the prevention of chemotherapy induced impairments in temporal discrimination.

Long-lasting impairments in adult neurogenesis, spatial learning and memory from a standard chemotherapy regimen used to treat breast cancer

The negative impact of chemotherapy on cognitive function in cancer patients has gained increasing attention in the last decade. Whilst the short-term acute effects on cognition are expected following chemotherapy, the persistence of such impairments in the long-term is still in question. This is despite clinical evidence indicating cognitive difficulties may persist well beyond treatment and affect quality of life. In the present study, we assessed the long-term (3 months) cognitive impact of chemotherapy in a mouse model intended to mimic the human female post-menopausal population receiving chemotherapy for breast cancer. Ovariectomized, female, C57BL/6J mice received two doses of Doxorubicin, Cyclophosphamide, and 5-Fluorouracil or saline vehicle (control), separated by one week. During this interval, mice received BrdU injections to label dividing cells. Results indicate a persistent impairment in learning and recall (1h, 24h and 48h) on the Morris water maze, reduced survival and differentiation of new neurons (BrdU+/NeuN+), and a persistent decline in proliferation of new cells (Ki67(+)) in the dentate gyrus. Locomotor activity, motor performance, and anxiety-like behavior were unaffected. We further evaluated the efficacy of a diet enriched in omega-3-fatty acids (DHA+EPA+DPA), in reversing long-term chemotherapy deficits but no rescue was observed. The model described produces long-term cognitive and cellular impairments from chemotherapy that mimic those observed in humans. It could be useful for identifying mechanisms of action and to test further the ability of lifestyle interventions (e.g., diet) for ameliorating chemotherapy-induced cognitive impairments.

Neurobiological changes by cytotoxic agents in mice

Cognitive deficit is a frequently reported side-effect of adjuvant chemotherapy. A large number of animal studies has been performed to examine the neurobiological mechanisms underlying this phenomenon, however, definite conclusions from these studies are restricted due to differences in experimental set-up. We systematically investigated the effects of 6 cytotoxic agents on various neurobiological parameters. C57Bl/6J mice were treated with cyclophosphamide, docetaxel, doxorubicin, 5-fluorouracil, methotrexate, or topotecan. The animals were sacrificed 3 or 15 weeks after treatment and the effect on neurogenesis, blood vessel density, and neuroinflammation was analyzed using immunohistochemistry. None of the cytostatic agents tested affected neurogenesis (cell survival or cell proliferation). Blood vessel density was increased in the hippocampus and prefrontal cortex 3 weeks after treatment with docetaxel and doxorubicin compared with control animals. A decrease in the number of microglial cells was observed in the prefrontal cortex after treatment with cyclophosphamide, docetaxel, 5-FU, and topotecan compared with control mice. The observed decrease in microglia cells is indicative of inflammation that occurred after treatment. Overall, the magnitude of the effects was relatively modest. Therefore, we conducted a similar study with topotecan in Abcg2;Abcb1a/b knock out and wildtype FVB mice. Animals were sacrificed 3 weeks after treatment and no notable effect was seen in hippocampal cell differentiation (DCX), microglia activation, or blood vessel density. Perhaps the FVB strain is more resistant to the neurotoxic effects of topotecan which makes this not the correct model to study the mechanism of chemotherapy-induced cognitive impairment.

Clavulanic acid enhances glutamate transporter subtype I (GLT-1) expression and decreases reinforcing efficacy of cocaine in mice

The β-lactam antibiotic ceftriaxone (CTX) reduces cocaine reinforcement and relapse in preclinical assays through a mechanism involving activation of glutamate transporter subtype 1 (GLT-1). However, its poor brain penetrability and intravenous administration route may limit its therapeutic utility for indications related to CNS diseases. An alternative is clavulanic acid (CA), a structural analog of CTX that retains the β-lactam core required for GLT-1 activity but displays enhanced brain penetrability and oral activity relative to CTX. Here, we tested the hypothesis that CA (1, 10 mg/kg ip) would enhance GLT-1 expression and decrease cocaine self-administration (SA) in mice, but at lower doses than CTX. Experiments revealed that GLT-1 transporter expression in the nucleus accumbens of mice treated with repeated CA (1, 10 mg/kg) was enhanced relative to saline-treated mice. Repeated CA treatment (1 mg/kg) reduced the reinforcing efficacy of cocaine (0.56 mg/kg/inf) in mice maintained on a progressive-ratio (PR) schedule of reinforcement but did not affect acquisition of cocaine SA under fixed-ratio responding or acquisition or retention of learning. These findings suggest that the β-lactamase inhibitor CA can activate the cellular glutamate reuptake system in the brain reward circuit and reduce cocaine's reinforcing efficacy at 100-fold lower doses than CTX.

Higher cardiorespiratory fitness levels are associated with greater hippocampal volume in breast cancer survivors

As breast cancer treatment is associated with declines in brain and cognitive health, it is important to identify strategies to enhance the cognitive vitality of cancer survivors. In particular, the hippocampus is known to play an important role in brain and memory declines following cancer treatment. The hippocampus is also known for its plasticity and positive association with cardiorespiratory fitness (CRF). The present study explores whether CRF may hold promise for lessening declines in brain and cognitive health of a sample of breast cancer survivors within 3 years of completion of primary cancer treatment. We explored the role of cardiovascular fitness in hippocampal structure in breast cancer survivors and non-cancer female controls, as well as performed a median split to compare differences in hippocampal volume in relatively higher fit and lower fit cancer survivors and non-cancer controls. Indeed, CRF and total hippocampal volume were positively correlated in the cancer survivors. In particular, higher fit breast cancer survivors had comparable hippocampal volumes to non-cancer control participants (Cohen's d = 0.13; p > 0.3), whereas lower fit breast cancer survivors showed significantly smaller hippocampal volumes compared to both lower fit and higher fit control participants (Cohen's d = 0.87, p < 0.05). These results are the first to identify that CRF may protect the brain health of breast cancer survivors within 3 years of treatment. The present study uniquely contributes to the field of cancer and cognition and emphasizes the importance of investigating how individual differences in CRF play a role in brain changes of breast cancer survivors.

Chemotherapy, cognitive impairment and hippocampal toxicity

Cancer therapies can be associated with significant central nervous system (CNS) toxicity. While radiation-induced brain damage has been long recognized both in pediatric and adult cancer patients, CNS toxicity from chemotherapy has only recently been acknowledged. Clinical studies suggest that the most frequent neurotoxic adverse effects associated with chemotherapy include memory and learning deficits, alterations of attention, concentration, processing speed and executive function. Preclinical studies have started to shed light on how chemotherapy targets the CNS both on cellular and molecular levels to disrupt neural function and brain plasticity. Potential mechanisms include direct cellular toxicity, alterations in cellular metabolism, oxidative stress, and induction of pro-inflammatory processes with subsequent disruption of normal cellular and neurological function. Damage to neural progenitor cell populations within germinal zones of the adult CNS has been identified as one of the key mechanisms by which chemotherapy might exert long-lasting and progressive neurotoxic effects. Based on the important role of the hippocampus for maintenance of brain plasticity throughout life, several experimental studies have focused on the study of chemotherapy effects on hippocampal neurogenesis and associated learning and memory. An increasing body of literature from both animal studies and neuroimaging studies in cancer patients suggests a possible relationship between chemotherapy induced hippocampal damage and the spectrum of neurocognitive deficits and mood alterations observed in cancer patients. This review aims to briefly summarize current preclinical and neuroimaging studies that are providing a potential link between the neurotoxic effects of chemotherapy and hippocampal dysfunction, highlighting challenges and future directions in this field of investigation.

The long-term impact of oxaliplatin chemotherapy on rodent cognition and peripheral neuropathy

Chemotherapy treatment is associated with cognitive dysfunction in cancer survivors after treatment completion. The duration of these impairments is unclear. Therefore this paper aims to evaluate the lasting impact of varying doses of the chemotherapy oxaliplatin (OX) on cognition and peripheral neuropathy. In Experiment 1 rats were treated once a week for 3 weeks with either physiological saline (control) or 6 mg/kg OX i.p. and were assessed for peripheral neuropathy, using von Frey filaments, and cognitive function, using novel object and location recognition, up to 2 weeks after treatment completion. For Experiment 2 rats received 3 weekly i.p. injections of either physiological saline (control), 0.6 mg/kg, 2mg/kg or 6 mg/kg OX and assessed for peripheral neuropathy and cognitive function up to 11 months after treatment completion. Systemic OX treatment induced lasting effects on cognitive function at 11 months after treatment, and peripheral neuropathy at 1 month after treatment and these were dose dependent; higher doses of OX resulted in worse cognitive outcomes and more severe peripheral neuropathy.

Cognitive impact of cytotoxic agents in mice

RATIONALE AND OBJECTIVES

Adjuvant chemotherapy is associated with changes in cognition in a subgroup of cancer patients. Chemotherapy is generally given as a combination of cytotoxic agents, which makes it hard to define the agent responsible for these observed changes. Literature on animal experiments has been difficult to interpret due to variance in experimental setup.

METHODS

We examined the effects of cytotoxic agents administered separately on various cognitive measures in a standardized animal model. Male C57Bl/6 mice received cyclophosphamide, docetaxel, doxorubicin, 5-fluorouracil, methotrexate, or topotecan. These agents represent different compound classes based on their working mechanism and are frequently prescribed in the clinic. A control group received saline. Behavioral testing started 2 or 15 weeks after treatment and included testing general measures of behavior and cognitive task performance: spontaneous behavior in an automated home cage, open field, novel location recognition (NLR), novel object recognition (NOR), Barnes maze, contextual fear conditioning, and a simple choice reaction time task (SCRTT).

RESULTS

Cyclophosphamide, docetaxel, and doxorubicin administration affected spontaneous activity in the automated home cage. All cytotoxic agents affected memory (NLR and/or NOR). Spatial memory measured in the Barnes maze was affected after administration with doxorubicin, 5-fluorouracil, and topotecan. Decreased inhibition in the SCRTT was observed after treatment with cyclophosphamide, docetaxel, and topotecan.

CONCLUSIONS

Our data show that, in mice, a single treatment with a cytotoxic agent causes cognitive impairment. Not all cytotoxic agents affected the same cognitive domains, which might be explained by differences in working mechanisms of the various agents.

Cannabidiol inhibits paclitaxel-induced neuropathic pain through 5-HT(1A) receptors without diminishing nervous system function or chemotherapy efficacy

BACKGROUND AND PURPOSE

Paclitaxel (PAC) is associated with chemotherapy-induced neuropathic pain (CIPN) that can lead to the cessation of treatment in cancer patients even in the absence of alternate therapies. We previously reported that chronic administration of the non-psychoactive cannabinoid cannabidiol (CBD) prevents PAC-induced mechanical and thermal sensitivity in mice. Hence, we sought to determine receptor mechanisms by which CBD inhibits CIPN and whether CBD negatively effects nervous system function or chemotherapy efficacy.

EXPERIMENTAL APPROACH

The ability of acute CBD pretreatment to prevent PAC-induced mechanical sensitivity was assessed, as was the effect of CBD on place conditioning and on an operant-conditioned learning and memory task. The potential interaction of CBD and PAC on breast cancer cell viability was determined using the MTT assay.

KEY RESULTS

PAC-induced mechanical sensitivity was prevented by administration of CBD (2.5 - 10 mg·kg⁻¹) in female C57Bl/6 mice. This effect was reversed by co-administration of the 5-HT(1A) antagonist WAY 100635, but not the CB₁ antagonist SR141716 or the CB₂ antagonist SR144528. CBD produced no conditioned rewarding effects and did not affect conditioned learning and memory. Also, CBD + PAC combinations produce additive to synergistic inhibition of breast cancer cell viability.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that CBD is protective against PAC-induced neurotoxicity mediated in part by the 5-HT(1A) receptor system. Furthermore, CBD treatment was devoid of conditioned rewarding effects or cognitive impairment and did not attenuate PAC-induced inhibition of breast cancer cell viability. Hence, adjunct treatment with CBD during PAC chemotherapy may be safe and effective in the prevention or attenuation of CIPN.

The impact of sustained and intermittent docetaxel chemotherapy regimens on cognition and neural morphology in healthy mice

RATIONALE

A subset of cancer survivors demonstrates impairments in cognition long after chemotherapy completion. At present, it is unclear whether these changes are due to direct neurotoxic effects of chemotherapy.

OBJECTIVES

This study examined the impact of variable docetaxel (DTX) chemotherapy dosing on brain DTX exposure via analyses of neural morphology and changes in cognition.

METHODS

Male CD-1 mice were treated with DTX either intermittently (8 mg/kg i.p. weekly) or via a sustained delivery system (DTX-PoLigel), which continuously releases DTX. Both groups received total DTX doses of 32 mg/kg. Mice were assessed on the novel object recognition (NOR) task and the Morris water maze (MWM) shortly after treatment.

RESULTS

Post-treatment behavioral testing demonstrated impaired NOR in mice treated with either dosing schedule relative to controls. No differences were observed between groups in MWM training and initial testing, though control mice performed better than chance while DTX-treated mice did not. Appreciable amounts of DTX were found in the brain after both treatment regimens. DTX treatment did not significantly increase levels of apoptosis within the CNS. However, some elevation in neural autophagy was observed following DTX treatment. Analysis of astrocytic activation demonstrated that intermittent DTX treatment resulted in an elevation of GFAP-positive astrocytes for 48 h after administration. Sustained chemotherapy demonstrated prolonged but lower levels of astrocyte activation over 9 days following implantation.

CONCLUSIONS

DTX treatment induced cognitive impairment shortly after treatment. Further, these findings suggest an association between DTX dosing, neurotoxicity, and cognitive effects.

Consequences of cancer treatments on adult hippocampal neurogenesis: implications for cognitive function and depressive symptoms

The human brain is capable of generating new functional neurons throughout life, a phenomenon known as adult neurogenesis. The generation of new neurons is sustained throughout adulthood due to the proliferation and differentiation of adult neural stem cells. This process in humans is uniquely located in the subgranular zone of the dentate gyrus in the hippocampus. Adult hippocampal neurogenesis (AHN) is thought to play a major role in hippocampus-dependent functions, such as spatial awareness, long-term memory, emotionality, and mood. The overall aim of current treatments for cancer (such as radiotherapy and chemotherapy) is to prevent aberrant cell division of cell populations associated with malignancy. However, the treatments in question are absolutist in nature and hence inhibit all cell division. An unintended consequence of this cessation of cell division is the impairment of adult neural stem cell proliferation and AHN. Patients undergoing treatment for cancerous malignancies often display specific forms of memory deficits, as well as depressive symptoms. This review aims to discuss the effects of cancer treatments on AHN and propose a link between the inhibition of the neurogenetic process in the hippocampus and the advent of the cognitive and mood-based deficits observed in patients and animal models undergoing cancer therapies. Possible evidence for coadjuvant interventions aiming to protect neural cells, and subsequently the mood and cognitive functions they regulate, from the ablative effects of cancer treatment are discussed as potential clinical tools to improve mental health among cancer patients.

Cytokines as mediators of chemotherapy-associated cognitive changes: current evidence, limitations and directions for future research

Objectives

While various clinical and pharmacological determinants for chemotherapy-associated cognitive impairment have been identified, conflicting evidence suggests that cytokines might play an intermediary role. The objective of this systematic review was to evaluate the current evidence pertaining to the associations among chemotherapy, cytokines induction and cognitive impairment in cancer patients.

Methods

A literature search with PubMed and SciVerse Scopus was conducted in March 2013 to gather relevant articles and abstracts that fulfilled the inclusion and exclusion criteria. This review included studies that had performed objective and/or subjective cognitive assessments and cytokine measurements on defined populations of cancer patients who received chemotherapy.

Results

High methodological heterogeneity existed among the selected studies which differed in cancer populations, subject characteristics, cognitive endpoints, types of cytokines tested and their measurement methods. Weak to moderate correlations were observed between IL-1β, IL-6, TNF-α levels, and different degrees of cognitive impairment. Different types of chemotherapy treatments might lead to varying presentations and severities of cytokine-induced cognitive impairment. Notably, the time concordance between the onset of cytokine induction and occurrence of cognitive impairment was not well elucidated. A number of confounding factors was identified to interfere with the expression levels of cytokines; these confounders included subjects' cancer types, ages, genders, genetics and psychosocial characteristics such as anxiety, depression and fatigue.

Conclusion

Although existing studies observed cognitive impairment and cytokine dysregulation in patients who receive chemotherapy, our results suggest that the intermediary role of cytokines in post-chemotherapy cognitive impairment is still controversial and requires further evaluation. A list of methodological recommendations is proposed to harmonize future studies of this subject matter.

Effects of early chemotherapeutic treatment on learning in adolescent mice: implications for cognitive impairment and remediation in childhood cancer survivors

PURPOSE

Among children diagnosed with acute lymphoblastic leukemia (ALL) and given chemotherapy-only treatment, 40% to 70% of survivors experience neurocognitive impairment. The present study used a preclinical mouse model to investigate the effects of early exposure to common ALL chemotherapeutics methotrexate (MTX) and cytarabine (Ara-C) on learning and memory.

EXPERIMENTAL DESIGN

Preweanling mouse pups were treated on postnatal day (PND) 14, 15, and 16 with saline, MTX, Ara-C, or a combination of MTX and Ara-C. Nineteen days after treatment (PND 35), behavioral tasks measuring different aspects of learning and memory were administered.

RESULTS

Significant impairment in acquisition and retention over both short (1 hour) and long (24 hours) intervals, as measured by autoshaping and novel object recognition tasks, was found following treatment with MTX and Ara-C. Similarly, a novel conditional discrimination task revealed impairment in acquisition for chemotherapy-treated mice. No significant group differences were found following the extensive training component of this task, with impairment following the rapid training component occurring only for the highest MTX and Ara-C combination group.

CONCLUSIONS

Findings are consistent with those from clinical studies suggesting that childhood cancer survivors are slower at learning new information and primarily exhibit deficits in memory years after successful completion of chemotherapy. The occurrence of mild deficits on a novel conditional discrimination task suggests that chemotherapy-induced cognitive impairment may be ameliorated through extensive training or practice.

Cognitive impairments and cancer chemotherapy: translational research at a crossroads

Cancer chemotherapy is often associated with cognitive deficits which may remain after the treatment has ended. As more people survive cancer, concern is increasing about the impact of these problems with memory and executive function when they return to everyday life. When chemotherapeutic drugs are administered to healthy animals in dosing regimens modeling those used in humans, cognitive deficits also occur, and these preclinical studies can provide information about the biological mechanisms by which the cancer fighting drugs affect the brain. Evidence from animal studies points to damage to hippocampus, particularly a disruption of neurogenesis, whereas human studies emphasize cognitive deficits associated with impairments in frontal cortical function. This discrepancy may be due more to the tasks selected by researchers, and the choice of biochemical endpoints than inherently different effects of chemotherapy in humans and rodents. These differences in approach must be reconciled if common underlying mechanisms are to be identified, with the hope of leading to novel drug or non-pharmacological treatments. This may be achieved by broadening the scope of human and animal studies, and by looking outside the topic of chemotherapy-induced cancer deficits to learn from the advances being made by studying the effects of stress and somatic disease on brain function, and the cognitive impairments now recognized to result from a wide range of mental and physical illnesses.

Establishing a model for assessing DNA damage in murine brain cells as a molecular marker of chemotherapy-associated cognitive impairment

AIMS

Chemotherapy-associated cognitive impairment often follows cancer chemotherapy. We explored chemotherapy-induced DNA damage in the brain cells of mice treated with 5-fluorouracil (5FU), an antineoplastic agent, to correlate the extent of DNA damage to behavioral functioning in an autoshaping-operant mouse model of chemotherapy-induced learning and memory deficits (Foley et al., 2008).

MAIN METHODS

Male, Swiss-Webster mice were injected once with saline or 75 mg/kg 5FU at 0, 12, and 24h and weighed every 24h. Twenty-four h after the last injection, the mice were tested in a two-day acquisition and the retention of a novel response task for food reinforcement. Murine brain cells were analyzed for the presence of single- and double-strand DNA breaks by the single cell gel electrophoresis assay (the Comet assay).

KEY FINDINGS

We detected significant differences (p<0.0001) for all DNA damage characteristics (DNA "comet" tail shape, migration pattern, tail moment and olive moments) between control mice cohort and 5FU-treated mice cohort: tail length - 119 vs. 153; tail moment - 101 vs. 136; olive moment - 60 vs. 82, correspondingly. We found a positive correlation between increased response rates (r=0.52, p<0.05) and increased rate of errors (r=0.51, p<0.05), and DNA damage on day 1. For all 15 mice (saline-treated and 5FU-treated mice), we found negative correlations between DNA damage and weight (r=-0.75, p<0.02).

SIGNIFICANCE

Our results indicate that chemotherapy-induced DNA damage changes the physiological status of the brain cells and may provide insights to the mechanisms for cognitive impairment after cancer chemotherapy.

Pharmacokinetic application of a bio-analytical LC-MS method developed for 5-fluorouracil and methotrexate in mouse plasma, brain and urine

In the past we have reported significant cognitive deficits in mice receiving 5-fluorouracil in combination with low-dose methotrexate. To explain such interactions, a pharmacokinetic study was designed. A sensitive bio-analytical method was therefore developed and validated for 5-fluorouracil and methotrexate in mouse plasma, brain and urine with liquid chromatography coupled to a single quadrupole mass spectrometer. Chromatographic separation was accomplished by Agilent® Zorbax® SB-C18 column, with isocratic elution (5 mM ammonium acetate and methanol, 70:30, %v/v) at a flow rate of 300 μL/min. The limit of quantitation for both drugs was 15.6 ng/mL (plasma and brain) and 78.1 ng/mL (urine), with interday and intraday precision and accuracy ≤15% and a total run time of 6 min. This bio-analytical method was used for the pharmacokinetic characterization of 5-fluorouracil and methotrexate in mouse plasma, brain and urine over a period of 24 h. This method allowed characterization of the brain concentrations of 5-fluorouracil over a period of 24 h.

The effects of chemotherapy on cognitive function in a mouse model: a prospective study

PURPOSE

Clinical studies indicate that up to 70% of patients with cancer who receive chemotherapy experience cognitive impairment. The present study used a prospective longitudinal design to assess short- and long-term effects of commonly used anticancer drugs on cognitive performance in a mouse model.

EXPERIMENTAL DESIGN

Normal mice received three weekly injections of a combination of methotrexate + 5-fluorouracil (CHEMO group) or an equal volume of saline (SAL group). Cognitive tests, measuring different aspects of learning and memory, were administered before treatment, immediately after treatment, and three months later. Structural MRI scanning was conducted at each stage of cognitive testing.

RESULTS

The CHEMO group exhibited deficits on cognitive tasks acquired pretreatment [spatial memory, nonmatching-to-sample (NMTS) learning, and delayed NMTS], as well as impaired new learning on two tasks (conditional associative learning, discrimination learning) introduced posttreatment. Consistent with clinical evidence, cognitive deficits were pronounced on tests that are sensitive to hippocampal and frontal lobe dysfunction, but the CHEMO group's poor performance on the discrimination learning problem suggests that impairment is more widespread than previously thought. Cognitive deficits persisted for at least three months after treatment but some recovery was noted, particularly on tests thought to be under frontal lobe control. The MRI tests did not detect brain changes that could be attributed to treatment.

CONCLUSIONS

Chemotherapeutic agents can have adverse effects on information acquired pretreatment as well as new learning and memory and, despite some recovery, impairment is long lasting.

Cognitive impairments caused by oxaliplatin and 5-fluorouracil chemotherapy are ameliorated by physical activity

RATIONALE

Studies in women with breast cancer, and in animal models, have demonstrated that chemotherapy can have a negative impact on cognitive function. Which chemotherapy agents cause problems with cognition and the aetiology of the impairment is unknown. Furthermore, there is no proven treatment.

OBJECTIVES

This study aimed to evaluate the effects of 5-fluorouracil (5FU) and oxaliplatin (OX) chemotherapy agents commonly used to treat colorectal cancer on cognition in laboratory rodents. Furthermore, we assessed physical activity as a potential remedy for the observed chemotherapy-induced cognitive deficits.

RESULTS

In rodents, treatment with 5FU and OX alone impairs memory as measured by novel object recognition. But combined treatment appears to have greater detrimental effects on hippocampal-dependent tasks, contextual fear recall and spatial reference memory (water maze), yet had no effect on cued fear recall, a non-hippocampal task. These impairments were prevented by 4 weeks of wheel running overnight after 5FU/OX treatment. We found a significant interaction between chemotherapy and exercise: rats receiving both 5FU/OX and exercise had improved cognition relative to non-exercising 5FU/OX rats on novel object recognition and spatial reference memory.

CONCLUSIONS

The combination 5FU/OX had a significant impact on cognition. However, rats treated with 5FU/OX that exercised post chemotherapy had improved cognition relative to non-exercising rats. This suggests that physical activity may prove useful in ameliorating the cognitive impairments induced by 5FU/OX.

Fluoxetine counteracts the cognitive and cellular effects of 5-fluorouracil in the rat hippocampus by a mechanism of prevention rather than recovery

5-Fluorouracil (5-FU) is a cytostatic drug associated with chemotherapy-induced cognitive impairments that many cancer patients experience after treatment. Previous work in rodents has shown that 5-FU reduces hippocampal cell proliferation, a possible mechanism for the observed cognitive impairment, and that both effects can be reversed by co-administration of the antidepressant, fluoxetine. In the present study we investigate the optimum time for administration of fluoxetine to reverse or prevent the cognitive and cellular effects of 5-FU.

Male Lister-hooded rats received 5 injections of 5-FU (25 mg/kg, i.p.) over 2 weeks. Some rats were co-administered with fluoxetine (10 mg/kg/day, in drinking water) for 3 weeks before and during (preventative) or after (recovery) 5-FU treatment or both time periods (throughout). Spatial memory was tested using the novel location recognition (NLR) test and proliferation and survival of hippocampal cells was quantified using immunohistochemistry. 5-FU-treated rats showed cognitive impairment in the NLR task and a reduction in cell proliferation and survival in the subgranular zone of the dentate gyrus, compared to saline treated controls. These impairments were still seen for rats administered fluoxetine after 5-FU treatment, but were not present when fluoxetine was administered both before and during 5-FU treatment. The results demonstrate that fluoxetine is able to prevent but not reverse the cognitive and cellular effects of 5-FU. This provides information on the mechanism by which fluoxetine acts to protect against 5-FU and indicates when it would be beneficial to administer the antidepressant to cancer patients.

Preserved learning and memory following 5-fluorouracil and cyclophosphamide treatment in rats

Some patients experience enduring cognitive impairment after cancer treatment, a condition termed "chemofog". Animal models allow assessment of chemotherapy effects on learning and memory per se, independent of changes due to cancer itself or associated health consequences such as depression. The present study examined the long-term learning and memory effects of a chemotherapy cocktail used widely in the treatment of breast cancer, consisting of 5-fluorouracil (5FU) and cyclophosphamide (CYP). Eighty 5-month old male F344 rats received contextual and cued fear conditioning before treatment with saline, or a low or high dose drug cocktail (50mg/kg CYP and 75 mg/kg 5FU, or 75 mg/kg CYP and 120 mg/kg 5FU, i.p., respectively) every 30 days for 2 months. After a 2-month, no-drug recovery, both long-term retention and new task acquisition in the water maze and 14-unit T-maze were assessed. Neither dose of the CYP/5FU cocktail impaired retrograde fear memory despite marked toxicity documented by enduring weight loss and 50% mortality at the higher dose. Acquisition in the water maze and Stone maze was also normal relative to controls in rats treated with CYP/5FU. The results contribute to a growing literature suggesting that learning and memory mediated by the hippocampus can be relatively resistant to chemotherapy. Future investigation may need to focus on assessments of processing speed, executive function and attention, and the possible interactive contribution of cancer itself and aging to the post-treatment development of cognitive impairment.

Recent and remote spatial memory in mice treated with cytosine arabinoside

Clinical studies suggest that chemotherapy is associated with long-term cognitive impairment in some patients. A number of underlying mechanisms have been proposed, however, the etiology of chemotherapy-related cognitive dysfunction remains relatively unknown. As part of a multifaceted approach, animal models of chemotherapy-induced cognitive impairment are being developed. Thus far, the majority of animal studies have utilized a rat model, however, mice may prove particularly beneficial in studying genetic risk factors for developing chemotherapy-induced cognitive impairment. Various chemotherapy agents, including cytosine arabinoside (Ara-C), have been found to impair remote spatial memory in rats in the Morris water maze. The present study evaluated the effects of Ara-C on remote (30 d) spatial memory in mice. In addition, the possibility that time relative to chemotherapy treatment may modulate the effect of chemotherapy on spatial learning and/or recent (1 d) memory was explored. Male C57BL/6J mice received either Ara-C (275 mg/kg i.p. daily for 5 days) or saline. Spatial learning and memory was assessed using the Morris water maze. Half the mice performed a remote (30 d) memory version of the task and the other half performed a recent (1 d) memory version of the task. The experiment was designed such that the probe trial for the recent memory version occurred on the same day relative to chemotherapy treatment as the remote memory version. Despite significant toxic effects as assessed by weight loss, Ara-C treated mice performed as well as control mice during acquisition, recent memory, and remote memory portions of the task. As are some humans, C57BL/6J mice may be resistant to at least some aspects of chemotherapy induced cognitive decline.

Effects of repeated administration of chemotherapeutic agents tamoxifen, methotrexate, and 5-fluorouracil on the acquisition and retention of a learned response in mice

RATIONALE

A number of cancer chemotherapeutic agents have been associated with a loss of memory in breast cancer patients although little is known of the causality of this effect.

OBJECTIVES

To assess the potential cognitive effects of repeated exposure to chemotherapeutic agents, we administered the selective estrogen receptor modulator tamoxifen or the antimetabolite chemotherapy, methotrexate, and 5-fluorouracil, alone and in combination to mice and tested them in a learning and memory assay.

METHODS

Swiss-Webster male mice were injected with saline, 32 mg/kg tamoxifen, 3.2 or 32 mg/kg methotrexate, 75 mg/kg 5-fluorouracil, 3.2 or 32 mg/kg methotrexate in combination with 75 mg/kg 5-fluorouracil once per week for 3 weeks. On days 23 and 24, mice were tested for acquisition and retention of a nose-poke response in a learning procedure called autoshaping. In addition, the acute effects of tamoxifen were assessed in additional mice in a similar procedure.

RESULTS

The chemotherapeutic agents alone and in combination reduced body weight relative to saline treatment over the course of 4 weeks. Repeated treatment with tamoxifen produced both acquisition and retention effects relative to the saline-treated group although acute tamoxifen was without effect except at a behaviorally toxic dose. Repeated treatment with methotrexate in combination with 5-fluorouracil produced effects on retention, but the magnitude of these changes depended on the methotrexate dose.

CONCLUSIONS

These data demonstrate that repeated administration of tamoxifen or certain combination of methotrexate and 5-fluorouracil may produce deficits in the acquisition or retention of learned responses which suggest potential strategies for prevention or remediation might be considered in vulnerable patient populations.

Preserved learning and memory in mice following chemotherapy: 5-Fluorouracil and doxorubicin single agent treatment, doxorubicin-cyclophosphamide combination treatment

Clinical studies suggest that chemotherapy is associated with long-term cognitive impairment in some patients. A number of underlying mechanisms have been proposed, however, the etiology of chemotherapy-related cognitive dysfunction remains relatively unknown. As part of a multifaceted approach, animal models of chemotherapy induced cognitive impairment are being developed. Thus far, the majority of animal studies have utilized rats, however, mice may prove particularly beneficial in studying genetic risk factors for developing chemotherapy induced cognitive impairment. Thus, C57BL/6J mice were treated once a week for three weeks with saline, doxorubicin and cyclophosphamide (D&C), doxorubicin (Dox), or 5-fluorouracil (5-FU). Recent and remote contextual fear conditioning and novel object recognition (NOR) was assessed. Despite significant toxic effects as assessed by weight loss, the chemotherapy treated mice performed as well as control mice on all task. As are some humans, C57BL/6J mice may be resistant to at least some aspects of chemotherapy induced cognitive decline.

An update on cancer- and chemotherapy-related cognitive dysfunction: current status

The purpose of this review is to summarize the current literature on the effects of cancer treatment-related cognitive difficulties, with a focus on the effects of chemotherapy. Numerous patients have cognitive difficulties during and after cancer treatments and, for some, these effects last years after treatment. We do not yet fully understand which factors increase susceptibility to cognitive difficulties during treatment and which cause persistent problems. We review possible contributors, including genetic and biological factors. Mostly we focus is on cognitive effects of adjuvant chemotherapy for breast cancer; however, cognitive effects of chemotherapy on the elderly and brain tumor patients are also discussed.

The effects of cyclophosphamide on hippocampal cell proliferation and spatial working memory in rat

Cyclophosphamide (CP) is a chemotherapy used in combinations that are associated with cognitive impairment. In the present study male Lister-hooded rats (n = 12) were used to investigate the effects of chronic administration of CP (30mg/kg, 7 i.v. doses, or an equivalent volume of saline) on performance in the novel location recognition (NLR) task and on the proliferation and survival of hippocampal cells. The survival of hippocampal cells dividing at the beginning of treatment was significantly reduced by CP. However, no difference was seen between CP treated and control groups for the number of cells proliferating 7 days after the final injection and both groups performed equally well in the NLR task. These results indicate that the given dose of CP acutely reduces the survival of newly born hippocampal cells. However, it does not have a longer term effect on spatial working memory or hippocampal proliferation, suggesting that CP is less neurotoxic than other chemotherapies with which it is used in combination.

A proposed mechanism for chemotherapy-related cognitive impairment ('chemo-fog')

WHAT IS KNOWN AND OBJECTIVE

Chemotherapeutic drugs for cancer treatment are, of necessity, cytotoxic. Unintended damage to normal central nervous system neuronal structure or function might lead to deleterious adverse effects on cognitive function, a mild form of which is reported by some cancer survivors. Understanding the physiologic connection between cancer chemotherapy and the reported cognitive dysfunction, could help inform choice of drugs, treatment regimens and new drug development. Our objective is to comment on a proposed mechanism for 'chemo-fog'.

COMMENT

An increasing number of patients are surviving cancer and are generating a new and rapidly growing category within the healthcare system. Some of these cancer survivors are reporting that they are experiencing residual and lingering effects from the cancer, or from its treatment, and that they now need care as survivors. This has given rise to the new field of 'survivor care'. Control of chemo-fog is an important aspect and understanding its mechanism, the basis for more rationale therapy. Such insight would also help direct drug-discovery efforts.

WHAT IS NEW AND CONCLUSION

New evidence suggests that 'chemo-fog' may be due to excessive cytokine release by the cytotoxic agents. Control of the elevated levels of the blood-brain-barrier-permeable pro-inflammatory cytokines, may help minimize this adverse effect.

Chemotherapy-induced structural changes in cerebral white matter and its correlation with impaired cognitive functioning in breast cancer patients

A subgroup of patients with breast cancer suffers from mild cognitive impairment after chemotherapy. To uncover the neural substrate of these mental complaints, we examined cerebral white matter (WM) integrity after chemotherapy using magnetic resonance diffusion tensor imaging (DTI) in combination with detailed cognitive assessment. Postchemotherapy breast cancer patients (n = 17) and matched healthy controls (n = 18) were recruited for DTI and neuropsychological testing, including the self-report cognitive failure questionnaire (CFQ). Differences in DTI WM integrity parameters [fractional anisotropy (FA) and mean diffusivity (MD)] between patients and healthy controls were assessed using a voxel-based two-sample-t-test. In comparison with healthy controls, the patient group demonstrated decreased FA in frontal and temporal WM tracts and increased MD in frontal WM. These differences were also confirmed when comparing this patient group with an additional control group of nonchemotherapy-treated breast cancer patients (n = 10). To address the heterogeneity observed in cognitive function after chemotherapy, we performed a voxel-based correlation analysis between FA values and individual neuropsychological test scores. Significant correlations of FA with neuropsychological tests covering the domain of attention and processing/psychomotor speed were found in temporal and parietal WM tracts. Furthermore, CFQ scores correlated negatively in frontal and parietal WM. These studies show that chemotherapy seems to affect WM integrity and that parameters derived from DTI have the required sensitivity to quantify neural changes related to chemotherapy-induced mild cognitive impairment.

Disruption of learning processes by chemotherapeutic agents in childhood survivors of acute lymphoblastic leukemia and preclinical models

OBJECTIVE

With the survival rate of acute lymphoblastic leukemia (ALL) surpassing 90 percent within this decade, new research is emerging in the field of late effects. A review of the research investigating the relationship of treatment regimens for ALL to specific late effect deficits, underlying mechanisms, and possible remediation is warranted to support continued studies.

METHODS

The clinical literature was briefly surveyed to describe the occurrence and topography of late effects, specifically neurocognitive deficits. Additionally, the preclinical literature was reviewed to uncover potential underlying mechanisms of these deficits. The advantages of using rodent models to answer these questions are outlined, as is an assessment of the limited number of rodent models of childhood cancer treatment.

RESULTS

The literature supports that childhood survivors of ALL exhibit academic difficulties and are more likely to be placed in a special education program. Behavioral evidence has highlighted impairments in the areas of attention, working memory, and processing speed, leading to a decrease in full scale IQ. Neurophysiological and preclinical evidence for these deficits has implicated white matter abnormalities and acquired brain damage resulting from specific chemotherapeutic agents commonly used during treatment.

CONCLUSIONS

The exact role of chemotherapeutic agents in learning deficits remains mostly unknown. Recommendations for an improved rodent model of learning deficits in childhood cancer survivors are proposed, along with suggestions for future directions in this area of research, in hopes that forthcoming treatment regimens will reduce or eliminate these types of impairments.