**3. Apolipoproteins effects on secondary drug response**

#### **3.1. Tamoxifen**

256 Lipoproteins – Role in Health and Diseases

chemotherapy treatment regiment [5].

disease (AD) particularly in females [9].

mechanism to help body fight carcinomas.

**2. Chemotherapy,** *ApoE* **and memory** 

higher triglyercide levels, higher VLDL levels, higher total cholesterol, higher total lipoproteins levels and elevated LDL or "bad cholesterol" levels all of which contribute to hypertension and diabetes confounding factors that have to be considered when designing a

In rodent models, the lack of *ApoE* or ApoE4 protein expression leads to destabilization of cell membranes, increased apoptosis, and heightened sensitivity to neuronal trauma; whereas, ApoE3 and ApoE2 protein expression allow for healthy cell functioning and neuroprotection [4]. ApoE4 has detrimental effects in transgenic mice, including behavioral abnormalities, such as deficits in spatial learning and memory using Morris water maze (MWM) [6], as well as significant alterations in the hippocampus and cortex [4,7]. The studies in mice are consistent with clinical studies indicating reduced spatial learning and memory in those who carry the *ApoE4* allele [8]. Experiments have also demonstrated that the three isoforms of human *ApoE* gene have different effects on the development of neurodegenerative diseases. Those individuals that are *ApoE4* carriers have an increased risk of age-related mild cognitive impairments (MCI) and the development of Alzheimer's

In ovarian cancer, ApoE protein levels act as a potential tumor-associated marker as found in serous carcinomas, but not in serious borderline or normal ovarian surface epithelium cells [10]. Up-regulation of ApoE protein levels is also seen in breast carcinomas, pancreatic cancer, stomach carcinomas, colon carcinomas and prostate carcinomas [10]. Blockage of ApoE expression in the serous carcinoma cell lines leads to cell cycle arrest and apoptosis. Women infused with ApoE protein at the time of diagnosis showed significantly higher survival rates [10]. This data suggest that upregulation of ApoE expression may be a defense

Over the last 20 years it has become apparent that chemotherapy drugs not only attack cancer cells, but also cross the blood brain barrier (BBB) leading to negative effects on cognitive processing which is known as Chemo-brain or Chem-fog [11,12]. Methotrexate, 5- Flourouracil (5-FU) the most common chemotherapy drugs used to treat breast, colorectal, head and neck cancers been shown in both rodent models and clinical studies to lead to neurocognitive deficits in a variety of domains including visual memory and visuospatial functioning [11,13,14]. In clinical studies, there is considerable variability between studies as to the extent and frequency of such impairments heretofore mentioned. However, rodent studies are clearly show that the drugs in the CMF (cyclophosphamide, methotrexate, 5-FU) regiment lead to decreased hippocampus cell proliferation and induce MWM memory impairments [15,16]. In addition, cytarabine (cytosine arabinoside) and ifosfamide among other chemotherapy have been shown to lead to memory impairments, hemiparesis, aphasia and progressive dementia [17]. However, there is significant lack of pre-clinical testing of

most chemotherapeutic agents and their long-term effects on memory.

Other confounding factors that can affect cognitive status after chemotherapy include other medications patients are taking to control comorbid conditions or to treat the tumor itself. Tamoxifen is used as an estrogen receptor modulator (SERM) in estrogen receptor (ER) positive breast cancer carcinomas. Tamoxifen is a pure estrogen receptor blocker. As with other drugs, tamoxifen and its metabolites can cross the BBB affecting ER in various brain regions including the cerebral cortex, hippocampus and amygdala [23]. In combination with chemotherapy, tamoxifen appears to intensify the cognitive impairments, particularly in visual memory, verbal working memory and visuospatial ability [24]. The Anatrozole, Tamoxifen Combined (ATAC) trial, also found verbal memory and processing speed impairments post-chemotherapy treated only with tamoxifen compared to women only on a combined ATAC treatment (Table 1) [25]. This suggests that tamoxifen has confounding effects when given as part of the chemotherapy regiment. Tamoxifen also appears to have both agonist and antagonist properties in the brain with reported up-regulation of proinflammatory cytokines shown to be related to cognitive dysfunction [26]. Positron emission tomography (PET) imaging of survivors does show higher hypometabolism with dual chemotherapy and tamoxifen, not seen in women treated only with tamoxifen [27]. Animal models using repeated tamoxifen or combinations of methotrexate and 5-FU injections both produced deficits in acquisition and retention in an operant learning paradigm (Table 1) [28]. These studies were conducted when women were still in treatment, leaving the question of potential long-term effects. There is a study that examined women who used tamoxifen for <4 years compared to >6 years of exposure. The study found that the current exposure led to greater memory deficits compared to non-users (Table 1) [29]. This suggests that while on therapy, patients may have acute memory impairments and that alternative drugs should be seriously considered.

In breast cancer survivors undergoing tamoxifen treatment, their total cholesterol, VLDL, high density lipoproteins (HDL) and Apolipoprotein B (ApoB) protein levels have been shown to decrease in both *ApoE4* carriers and non-*ApoE4* carriers (Table 1) [30,31]. Breast cancer patients who are *ApoE4* carriers have higher plasma triglyceride levels and altered ApoA-1/ApoB ratio. Both are risk factors for cardiovascular events, after tamoxifen treatment (Table 1) [30,31]. In non-*ApoE4* carriers, there were lower levels of lipoprotein (a) after treatment, but no effect on triglycerides or the ApoA-1/ApoB ratio (Table 1). This suggests that non-*ApoE4* carriers have a more positive response to tamoxifen with respect to lipid profiles and risk for cardiovascular complications [31]. Considering tamoxifen and *ApoE4* both have a deleterious effect on cognition and tamoxifen has an *ApoE* genotype dependent effect on lipid profiles, suggest further investigations are warranted to understand the mechanistic relationship.

#### **3.2. Anatrozole**

Anatrozole also known as arimidex is an aromatase inhibitor that lowers estrogen levels and is used as a treatment in estrogen positive breast cancer patients post-surgery. Results of the ATAC trial of 9399 women indicated that those only on arimidex had better clinical outcomes including vascular events and gynecological problems compared to the tamoxifen group with no differences seen in cognitive outcomes [25,32]. There data suggests that arimidex was the preferred initial treatment by women treated for breast cancer [32]. However, other studies indicate that women on arimidex treatment had greater cognitive decline than tamoxifen treatment in verbal and visual memory (Table 1) [33,34]. At this point no effects have been seen on cholesterol, lipoproteins or apolipoprotein levels in both animal models and clinical studies (Table 1) [35,36]. Although, there are not alternations in lipids levels, the cognitive side effects are of concern with this medication and should be examined with respect to *ApoE* genotype.

#### **3.3. Letrozole**

Letrozole, a potent aromatase interfering with adrenal steroid biosynthesis, has also been assessed as a replacement for tamoxifen or as secondary maintenance treatment after tamoxifen as part of the Breast International Group (BIG 1-98) trial [37,38]. In the BIG 1-98 study, better overall cognitive outcomes were seen in women on letrozole treatment compared to those on a tamoxifen treatment (Table 1) [37,38]. In the tamoxifen only group, increased endometrial cancer and vaginal bleeding where found [38]. Participants given letrozole only did displayed more incidences of skeletal and cardiac events and hypercholesterolemia compared to the tamoxifen group [37]. After letrozole treatment, unfavorable effects have been seen including increased serum total cholesterol, LDL and ApoB with atherogenic ratio risk of total cholesterol/HDL and LDL/HDL levels (Table 1) [39]. Therefore, the better cognitive outcomes may not out-weigh negative effects on lipoprotein levels, particularly in those with or at risk for hypertension and/or diabetes. The potential confounding factor of *ApoE* genotype has not been reported.

#### **3.4. Exemestane**

258 Lipoproteins – Role in Health and Diseases

drugs should be seriously considered.

understand the mechanistic relationship.

examined with respect to *ApoE* genotype.

**3.2. Anatrozole** 

**3.3. Letrozole** 

[28]. These studies were conducted when women were still in treatment, leaving the question of potential long-term effects. There is a study that examined women who used tamoxifen for <4 years compared to >6 years of exposure. The study found that the current exposure led to greater memory deficits compared to non-users (Table 1) [29]. This suggests that while on therapy, patients may have acute memory impairments and that alternative

In breast cancer survivors undergoing tamoxifen treatment, their total cholesterol, VLDL, high density lipoproteins (HDL) and Apolipoprotein B (ApoB) protein levels have been shown to decrease in both *ApoE4* carriers and non-*ApoE4* carriers (Table 1) [30,31]. Breast cancer patients who are *ApoE4* carriers have higher plasma triglyceride levels and altered ApoA-1/ApoB ratio. Both are risk factors for cardiovascular events, after tamoxifen treatment (Table 1) [30,31]. In non-*ApoE4* carriers, there were lower levels of lipoprotein (a) after treatment, but no effect on triglycerides or the ApoA-1/ApoB ratio (Table 1). This suggests that non-*ApoE4* carriers have a more positive response to tamoxifen with respect to lipid profiles and risk for cardiovascular complications [31]. Considering tamoxifen and *ApoE4* both have a deleterious effect on cognition and tamoxifen has an *ApoE* genotype dependent effect on lipid profiles, suggest further investigations are warranted to

Anatrozole also known as arimidex is an aromatase inhibitor that lowers estrogen levels and is used as a treatment in estrogen positive breast cancer patients post-surgery. Results of the ATAC trial of 9399 women indicated that those only on arimidex had better clinical outcomes including vascular events and gynecological problems compared to the tamoxifen group with no differences seen in cognitive outcomes [25,32]. There data suggests that arimidex was the preferred initial treatment by women treated for breast cancer [32]. However, other studies indicate that women on arimidex treatment had greater cognitive decline than tamoxifen treatment in verbal and visual memory (Table 1) [33,34]. At this point no effects have been seen on cholesterol, lipoproteins or apolipoprotein levels in both animal models and clinical studies (Table 1) [35,36]. Although, there are not alternations in lipids levels, the cognitive side effects are of concern with this medication and should be

Letrozole, a potent aromatase interfering with adrenal steroid biosynthesis, has also been assessed as a replacement for tamoxifen or as secondary maintenance treatment after tamoxifen as part of the Breast International Group (BIG 1-98) trial [37,38]. In the BIG 1-98 study, better overall cognitive outcomes were seen in women on letrozole treatment compared to those on a tamoxifen treatment (Table 1) [37,38]. In the tamoxifen only group, increased endometrial cancer and vaginal bleeding where found [38]. Participants given letrozole only did displayed more incidences of skeletal and cardiac events and There is another alternative for estrogen suppression therapy exemestane, an aromatase inhibitor, general used after tamoxifen is not working in post-menopausal women. Exemestane has been examined as part of the randomized Tamoxifen and Exemestane Adjunctive Multinational (TEAM) trial [24]. The preliminary data from the TEAM trial found that tamoxifen is associated with lower verbal and executive function, while exemestane did not seem to alter cognitive performance levels (Table 1) [24]. Analysis of data from 72 patients as part of the EORTC trail 10958 indicates that treatment with exemestane resulted in reduced triglyceride levels and tamoxifen treatment increased triglyceride levels [40]. All other lipid parameters including HDL, ApoA-1, ApoB or Lip (a) levels at 8, 24 and 48 weeks were unchanged by either treatment [40]. Further studies will be needed to determine if stable cognitive performance and lipid levels are effect of *ApoE* genotype.

#### **3.5. Raloxifene**

Raloxifene, also a SERM and is used as a hormone replacement therapy, has the positive effects of estrogen on the skeletal system and is an antagonist of estrogen in breast or endometrial tissues [41]. Raloxifene treatment also appears to be less detrimental to cognitive function assessed by Modified Mini-Mental State (3MS) compared to tamoxifen [42]. Used to prevent osteoporosis, it has been shown that after three years of treatment raloxifene did not affect overall cognitive scores [43]. The Multiple Outcomes of Raloxifene Evaluation (MORE) study of 7478 women, reported finding that raloxifene treatment lowered the risk of cognitive decline in word list recall test and there was no overall effect on cognitive function (Table 1) [44]. A subset of the National Surgical Adjuvant Breast and Bowel Project (NSABP) Study of Tamoxifen and Raloxifene (STAR), the CoSTAR study for women at high risk for breast cancer did not find any cognitive effects of either drug (Table 1) [45]. Together the evidence supports that raloxifene treatment does not impair cognitive function the way that tamoxifen treatment and in fact may even lower the risk of cognitive decline [34].

With respect to lipid and lipoprotein levels post hoc analysis of 2659 women in the MORE study, found that raloxifene treatment in women with or without high triglycerides lead to reduced cholesterol levels with healthier lipoprotein parameters (Table 1) [46]. Studies in Greek women, found that LDL cholesterol levels were lower in women treated with raloxifene [47]. Raloxifene also appears to raise HDL levels and ApoA-1 while decreasing ApoB protein levels and improving ratios of total cholesterol to lipoproteins (Table 1). After one year of treatment with raloxifene women had reduced fat mass and trunk and central regions along with decreased adiposity in their truck and abdominal regions (Table 1) [48]. Overall, raloxifene treatment improves cholesterol health and alters fat distribution in a positive manner to help prevent obesity, making it a better candidate for overall health compared to tamoxifen. Its effects in relation to *ApoE* genotype have not been reported.

#### **3.6. Estradiol**

In post-menopausal estradiol (also known as 17β-estradiol or oestradiol) treatment is used for estrogen replacement therapy. Healthy post-menopausal women given estradiol display improved visuospatial abilities measured by a mental rotation task (Table 1) [49]. Other non-randomized studies in women with surgically induced amenorrhea or those with AD indicate that estrogen replacement treatment may help to improve or minimize cognitive deficits [50]. Even in men those given estradiol performed better on visual memory after treatment (Table 1) [51]. These results are consistent with improved memory in mice given other replacement estrogens treatments [52]. Over half of randomized clinical studies find significant improvements in cognition and attention after estrogen replacement therapy (Table 1) [53]. Estradiol has been shown to increase levels of ApoE in the brain, proposed to be beneficial for neuronal reorganization and repair [7]. In a health study of 3,393 women, results suggest that estrogen replacement reduces the risk of agerelated cognitive decline in non-*ApoE4* women, but not in *ApoE4* carriers (Table 1) [54]. Another study with 181 post-menopausal women, also found the best learning and memory performance after estrogen replacement is seen in non-*ApoE4* carriers [55]. This suggests that knowing *ApoE* genotype may be helpful to assess potential response to estrogen replacement therapy.


estrogen replacement therapy.

Tamoxifen Estrogen receptor

function

function

Anatrozole Aromatase inhibitor

**Treatment Function Effects on** 

modulator (SERM) lowers estrogen

lowers estrogen

been reported.

**3.6. Estradiol** 

raloxifene [47]. Raloxifene also appears to raise HDL levels and ApoA-1 while decreasing ApoB protein levels and improving ratios of total cholesterol to lipoproteins (Table 1). After one year of treatment with raloxifene women had reduced fat mass and trunk and central regions along with decreased adiposity in their truck and abdominal regions (Table 1) [48]. Overall, raloxifene treatment improves cholesterol health and alters fat distribution in a positive manner to help prevent obesity, making it a better candidate for overall health compared to tamoxifen. Its effects in relation to *ApoE* genotype have not

In post-menopausal estradiol (also known as 17β-estradiol or oestradiol) treatment is used for estrogen replacement therapy. Healthy post-menopausal women given estradiol display improved visuospatial abilities measured by a mental rotation task (Table 1) [49]. Other non-randomized studies in women with surgically induced amenorrhea or those with AD indicate that estrogen replacement treatment may help to improve or minimize cognitive deficits [50]. Even in men those given estradiol performed better on visual memory after treatment (Table 1) [51]. These results are consistent with improved memory in mice given other replacement estrogens treatments [52]. Over half of randomized clinical studies find significant improvements in cognition and attention after estrogen replacement therapy (Table 1) [53]. Estradiol has been shown to increase levels of ApoE in the brain, proposed to be beneficial for neuronal reorganization and repair [7]. In a health study of 3,393 women, results suggest that estrogen replacement reduces the risk of agerelated cognitive decline in non-*ApoE4* women, but not in *ApoE4* carriers (Table 1) [54]. Another study with 181 post-menopausal women, also found the best learning and memory performance after estrogen replacement is seen in non-*ApoE4* carriers [55]. This suggests that knowing *ApoE* genotype may be helpful to assess potential response to

**Lipids/Apolipoproteins** 

levels.

Decreased total cholesterol, VLDL, HDL and ApoB protein

No effects seen in rodent or

clinical studies.

**Effects on Cognition** 

Reduced verbal and visual memory compared to tamoxifen. Deficits

Leads to impairments in visual memory, verbal working memory and visuospatial ability.


**Table 1.** Hormone treatments effects of lipids/apolipoproteins levels and cognition.

#### **3.7. Tibolone**

Tibolone is another drug used in hormonal replacement therapy having estrogenic, progestogenic, and androgenic effects. Long-term treatment does appear to decrease anxiety, improve semantic memory and overall quality of life; however, one study reported that those in treatment did score worse on attention task compared to women not on treatment (Table 1) [56,57]. Tibolone appears to be the most beneficial with respect to reducing total cholesterol, triglyercide, HDL and ApoA-1 levels compared to raloxifene and estradiol (Table 1) [47,58-60]. This suggests that hormone replacement therapy with medications such as tibolone in post-menopausal women are beneficial to cognitive health and lipid profiles.

### **3.8. Cetrorelix**

Cetrorelix an antagonist of hypothalamic luteinizing hormone-releasing hormone (LHRH), is used in treatment of prostate carcinoma, benign prostatic hyperplasia, and ovarian cancer to reduce gonadotrophins and sex steroids [61]. In mice, a study suggests that it is anxiolytic, anti-depressive and able to correct beta-amyloid 25-35 associated memory consolidation impairments (Table 1) [61]. Injection of cetrorelix into *ApoE* deficient mice (*ApoE-/-*) mice suggests that the associated suppression of testosterone leads to increased atherosclerosis despite lower cholesterol levels in the male mice [62]. In female *ApoE-/-* mice, the reduction in testosterone also leads to reduction in estradiol, insulin and HDL levels without effects on atherosclerosis [62]. In a pilot study conducted in men, treatment with cetrorelix resulted in increased ApoA-1, HDL, insulin and leptin consistently (Table 1) [63]. Therefore, when this drug is used within a chemotherapy treatment regiment it is important to carefully monitor lipid levels. Additional studies are needed to examine if *ApoE* genotype has any effect on response and potential long-term cognitive side effects of this drug.

#### **4. Recommendations**

