**8. Inhibitors of γ-secretase in the clinical development**

with the substance faded away, at present, LY2811376 has become a lead structure, which could

The molecule marked with LY2886721 (**Figure 2**) represents the next evolutionary generation of orally acting BACE-1 inhibitors, which has entered into the second phase of clinical trials. Compared to its predecessor LY2811376, the novel drug LY2886721 did not exhibit any side effects in the area of the retina and any stroke. During the first phase of clinical trials on 47 healthy volunteers, no adverse effects were observed in 14 days (different dosing schemes repeated administration of 5, 15 and 35 or 70 mg single administration). The biological half-life fluctuated around 12 h, allowing the dosing once per day, when the drug holds the necessary biological effect even after substantial elimination from the body. Treatment with LY2886721 resulted in the reduction of the plasma and cerebrospinal levels of Aβ1–40 by up to 74% (i.e., after the highest dose of 70 mg). Similar decreasing changes were detected in the cerebrospinal levels of Aβ1–42 and sAPP-β, while the blood level of sAPP-*α* was increased, which is logi-

The second phase of clinical trials with LY2886721 was carried out in 130 patients with moderate to severe AD dementia type. This testing, however, was terminated because of liver abnor-

E2609 (**Figure 2**) is an orally available, nonpeptidic spirocyclic inhibitor of BACE-1, which induced a significant decline of brain levels of Aβ in preclinical studies. Based on this success, E2609 entered the first phase of clinical testing in which 73 volunteers, administered uniformly with increasing dose from 5 to 800 mg of the drug, and 50 volunteers, administered with different doses in the range of 25–400 mg, participated. The plasma half-life of E2609 is around 12–16 h, which again allows one-day dosing schedule. At the maximal single dose (400 mg), decrease of the cerebrospinal Aβ levels by up to 85% has been observed. The concentration of sAPP-β has been similarly reduced, while sAPP-*α* has been increased. Currently,

γ-Secretase is a member of aspartic protease family that cleaves glycoproteins of type I including APP. Unlike β-secretase, γ-secretase has a regulated intramembrane proteolytic activity (RIP), thus, it breaks down domains inside of the cytoplasmic membrane. It is known that it breaks down multiple substrates, and to this day more than 50 such substrates, including APP, have been identified. Among these substrates are Notch, Jagged and Nectin-1α. The signal transmission by RIP is implemented so that the released intracellular domain is moved into the nucleus, as it is in the case of Notch, which regulates specific gene expression. Notch is therefore cleaved to Notch intracellular domain, NICD, which causes in the nucleus the mentioned regulation. In relation to AD, this signal pathway is interesting from the perspec-

be administered orally and reach its biological target behind the blood-brain barrier.

8 Alzheimer's Disease - The 21st Century Challenge

cally explainable by relative excess of α-secretase in comparison with BACE-1.

malities, but, presumably, this is not associated with inhibition of BACE-1 [25].

the drug is in the third phase of clinical determinations [26].

tive of development and function of the nervous tissue.

**6.3. E2609**

**7. γ-Secretase**

In recent years, a series of potential inhibitors of γ-secretase has been designed and synthesized. Unfortunately, most of them are not specific to cleaving APP with γ-secretase, and, like in the case of BACE-1, they prevent processing of other γ-secretase substrates that do not have any or at least no obvious role in the pathogenesis of AD. For these reasons, the inhibition of γ-secretase has been associated with serious side effects, which adumbrated the end for most drug candidates in clinical testing.

Historically, the first inhibitor of γ-secretase that underwent clinical studies was BMS-299897 (**Figure 3**) compound prepared by Bristol-Myers Squibb. In 2001, clinical trials of this molecule

**Figure 3.** Inhibitors of γ-secretase in various stages of clinical testing.

began, but the results of this study have never been fully described. We only know that the next clinical trials have been terminated.

plaques in the brain of transgenic mice, as well as on their behavioral changes. This inhibitor has passed the phase II of clinical development, but further testing is currently not being performed

Amyloid Beta Hypothesis: Attention to β- and γ-Secretase Modulators

http://dx.doi.org/10.5772/intechopen.75629

11

PF-3084014 (**Figure 3**) is a new effective, aminotetraline based γ-secretase inhibitor developed by Pfizer, which does not affect Notch. In in vitro tests, the compound was evaluated as an uncompetitive and reversible inhibitor of human γ-secretase with IC50 = 6.2 nM. In studies on tissue cultures, it seems as a weak inhibitor of Notch with IC50 = 1915 nM. The ratio between the APP and Notch selectivity is roughly 1500. The merit of this compound is a sufficient penetration through the blood-brain barrier, long-term effect on reducing the Aβ levels and no rebound phenomenon for levels of Aβ in animal plasma. As in the case of BMS-708163, there is also a lack of data for this inhibitor about the storage of Aβ plaques in transgenic mice, as well as about their behavioral processes. PF-3084014 is currently introduced into the second

This inhibitor, known also as begacestat, is a potent thiophene-related sulfonamide developed by Weyth. It is able to suppress the production of Aβ in low nanomolar concentrations in vitro (IC50 = 8 nM) and in vivo (Aβ1–42: IC50 = 15 nM). Cellular studies on the Notch cleavage showed 15× higher selectivity of this molecule to inhibit preferably the cleavage of APP. It was found that GSI-953 (**Figure 3**) improves the memory functions in transgenic mice; however, it does not diminish the level of Aβ1-40 in the CSF in people suffering from AD. This drug completed the first phase of the clinical trials, but the lack of its efficacy caused it to no longer be

The inhibitor ELND-006 (**Figure 3**) developed by Elan Pharmaceuticals shows increased selectivity for inhibition of the APP cleavage (IC50 = 0.34 nM) with regard to Notch cleavage (IC50 = 5.3 nM). Therefore, it does not significantly affect Notch; it has a good penetration through the blood-brain barrier and can reduce the level of Aβ in the brain in transgenic mice. The disadvantage of this drug is the rebound phenomenon in the plasma of animals and lack of data on behavioral processes in animal models of AD. Clinical studies of the drug have been terminated because of severe hepatic adverse reactions, which presumably are not

Presenilins (PSs) are membrane proteins encoded by two genes: PS1 and PS2. PS, nicastrin, anterior pharynx-defective (aph-1) and presenilin enhancer 2 (pen-2) form an active part of

related to the mechanism of γ-secretase inhibition by the drug [36].

the γ-secretase complex, while PS form the catalytic core of the complex [18].

[33].

**8.5. PF-3084014**

phase of clinical testing [34].

a subject of follow-up studies [35].

**9. Presenilins 1 and 2**

**8.6. GSI-953**

**8.7. ELND-006**

Six other inhibitors of γ-secretase are currently in various stages of clinical testing involving patients suffering from AD. As for the compounds LY-450139, MK-0752, BMS-708163, PF-3084014, GSI-953 and ELND-006, animal studies indicated that these substances reduced the brain levels of Aβ after oral or parenteral administration (**Figure 3**) [28].

### **8.1. LY-450139**

LY-450139 (**Figure 3**), also known as semagacestat, is an inhibitor of γ-secretase developed by Eli-Lilly. It is a derivative of benzoazepinone with triple selectivity to inhibit the cleavage of APP with respect to the cleavage of Notch (APP: IC50 = 15 nM and Notch: EC50 = 49 nM). This derivative has undergone all three phases of clinical trials. When tested on experimental animals, it was found that LY-450139 has an effect on the level of Aβ in the brain, cerebrospinal fluid (CSF) and the plasma in mice, guinea pigs and dogs. A similar positive effect was achieved in the cerebrospinal fluid (CSF) of AD sufferers. However, due to the neurotoxicity detected in transgenic mice, gastrointestinal problems and an increased risk of developing skin cancer in humans, clinical testing was abandoned [29, 30].

#### **8.2. MK-0752**

This substance developed by Merck is a non-selective inhibitor of APP and Notch formation. In healthy volunteers, MK-0752 (**Figure 3**) administration led to reduction of Aβ1–40 levels in the CSF. However, the drawback was the mentioned non-selectivity toward Notch cleavage and significant toxicity in humans. MK-0752 has reached only the first phase of clinical testing [31].

#### **8.3. E2012**

The drug E2012 (**Figure 3**) was developed by Eisai in cooperation with Torrey-Pines Therapeutics with the aim to reduce the levels of Aβ by modulating the γ-secretase without affecting the Notch. In mid-2006, the first phase of clinical testing has started, but in February 2007 it has been suspended due to the lenticular opacity observed in preclinical studies with rats. In the time of the study suspension, however, no health problems in humans were observed. In addition, the lenticular opacity has not appeared in later studies in monkeys. During a subsequent study, no eye toxicity was observed in rats, and, thus, the suspension of testing was repealed in April 2008. Currently, the drug is no longer the subject of research interests, anyway [32].

#### **8.4. BMS-708163**

The drug identified as BMS-708163 (**Figure 3**) is a benzene sulfonamide developed by Bristol-Myers Squibb. This molecule exhibits nearly 200× lower selectivity to Notch cleavage (Aβ1–40: IC50 = 0.3 nM and Notch: EC50 = 58 nM). Animal studies, specifically in rats and dogs, have shown the ability of BMS-708163 to reduce the levels of Aβ in the brain and the CSF without the Notchrelated gastrointestinal and lymphoid toxicity. Despite the fact that reduction of the Aβ level in the CSF has been observed in healthy volunteers, there is insufficient information on storing Aβ plaques in the brain of transgenic mice, as well as on their behavioral changes. This inhibitor has passed the phase II of clinical development, but further testing is currently not being performed [33].

#### **8.5. PF-3084014**

began, but the results of this study have never been fully described. We only know that the

Six other inhibitors of γ-secretase are currently in various stages of clinical testing involving patients suffering from AD. As for the compounds LY-450139, MK-0752, BMS-708163, PF-3084014, GSI-953 and ELND-006, animal studies indicated that these substances reduced

LY-450139 (**Figure 3**), also known as semagacestat, is an inhibitor of γ-secretase developed by Eli-Lilly. It is a derivative of benzoazepinone with triple selectivity to inhibit the cleavage of APP with respect to the cleavage of Notch (APP: IC50 = 15 nM and Notch: EC50 = 49 nM). This derivative has undergone all three phases of clinical trials. When tested on experimental animals, it was found that LY-450139 has an effect on the level of Aβ in the brain, cerebrospinal fluid (CSF) and the plasma in mice, guinea pigs and dogs. A similar positive effect was achieved in the cerebrospinal fluid (CSF) of AD sufferers. However, due to the neurotoxicity detected in transgenic mice, gastrointestinal problems and an increased risk of developing

This substance developed by Merck is a non-selective inhibitor of APP and Notch formation. In healthy volunteers, MK-0752 (**Figure 3**) administration led to reduction of Aβ1–40 levels in the CSF. However, the drawback was the mentioned non-selectivity toward Notch cleavage and significant toxicity in humans. MK-0752 has reached only the first phase of clinical testing [31].

The drug E2012 (**Figure 3**) was developed by Eisai in cooperation with Torrey-Pines Therapeutics with the aim to reduce the levels of Aβ by modulating the γ-secretase without affecting the Notch. In mid-2006, the first phase of clinical testing has started, but in February 2007 it has been suspended due to the lenticular opacity observed in preclinical studies with rats. In the time of the study suspension, however, no health problems in humans were observed. In addition, the lenticular opacity has not appeared in later studies in monkeys. During a subsequent study, no eye toxicity was observed in rats, and, thus, the suspension of testing was repealed in April 2008. Currently, the drug is no longer the subject of research interests, anyway [32].

The drug identified as BMS-708163 (**Figure 3**) is a benzene sulfonamide developed by Bristol-Myers Squibb. This molecule exhibits nearly 200× lower selectivity to Notch cleavage (Aβ1–40: IC50 = 0.3 nM and Notch: EC50 = 58 nM). Animal studies, specifically in rats and dogs, have shown the ability of BMS-708163 to reduce the levels of Aβ in the brain and the CSF without the Notchrelated gastrointestinal and lymphoid toxicity. Despite the fact that reduction of the Aβ level in the CSF has been observed in healthy volunteers, there is insufficient information on storing Aβ

the brain levels of Aβ after oral or parenteral administration (**Figure 3**) [28].

skin cancer in humans, clinical testing was abandoned [29, 30].

next clinical trials have been terminated.

10 Alzheimer's Disease - The 21st Century Challenge

**8.1. LY-450139**

**8.2. MK-0752**

**8.3. E2012**

**8.4. BMS-708163**

PF-3084014 (**Figure 3**) is a new effective, aminotetraline based γ-secretase inhibitor developed by Pfizer, which does not affect Notch. In in vitro tests, the compound was evaluated as an uncompetitive and reversible inhibitor of human γ-secretase with IC50 = 6.2 nM. In studies on tissue cultures, it seems as a weak inhibitor of Notch with IC50 = 1915 nM. The ratio between the APP and Notch selectivity is roughly 1500. The merit of this compound is a sufficient penetration through the blood-brain barrier, long-term effect on reducing the Aβ levels and no rebound phenomenon for levels of Aβ in animal plasma. As in the case of BMS-708163, there is also a lack of data for this inhibitor about the storage of Aβ plaques in transgenic mice, as well as about their behavioral processes. PF-3084014 is currently introduced into the second phase of clinical testing [34].

#### **8.6. GSI-953**

This inhibitor, known also as begacestat, is a potent thiophene-related sulfonamide developed by Weyth. It is able to suppress the production of Aβ in low nanomolar concentrations in vitro (IC50 = 8 nM) and in vivo (Aβ1–42: IC50 = 15 nM). Cellular studies on the Notch cleavage showed 15× higher selectivity of this molecule to inhibit preferably the cleavage of APP. It was found that GSI-953 (**Figure 3**) improves the memory functions in transgenic mice; however, it does not diminish the level of Aβ1-40 in the CSF in people suffering from AD. This drug completed the first phase of the clinical trials, but the lack of its efficacy caused it to no longer be a subject of follow-up studies [35].

### **8.7. ELND-006**

The inhibitor ELND-006 (**Figure 3**) developed by Elan Pharmaceuticals shows increased selectivity for inhibition of the APP cleavage (IC50 = 0.34 nM) with regard to Notch cleavage (IC50 = 5.3 nM). Therefore, it does not significantly affect Notch; it has a good penetration through the blood-brain barrier and can reduce the level of Aβ in the brain in transgenic mice. The disadvantage of this drug is the rebound phenomenon in the plasma of animals and lack of data on behavioral processes in animal models of AD. Clinical studies of the drug have been terminated because of severe hepatic adverse reactions, which presumably are not related to the mechanism of γ-secretase inhibition by the drug [36].
