. Concentration-dependent modulation of amyloid-beta in vivo and in vitro using the gamma-secretase inhibitor, LY-450139. J Pharmacol Exp Ther. 2006 Nov;319(2):924-33. PubMed.

Recommends

Please login to recommend the paper.

Comments

Make a Comment

To make a comment you must login or register.

Comments on this content

  1. The observations by Lanz et al. on time- and concentration-dependent changes of Aβ peptide in plasma, CSF, and brain of guinea pigs treated with the γ-secretase inhibitor LY450139 extend our knowledge of a well-recognized, yet still poorly understood property of this enigmatic enzyme. This property is the elevation of Aβ above baseline following acute (and now also chronic) treatment in-vivo with low doses of inhibitor. This effect is most easily detected in plasma, and somewhat more variably in CSF. Lanz et al. also report elevation of plasma Aβ above baseline at later time points (preceded by reduction below baseline at early time points), following acute treatment with high/inhibitory doses of LY411575. Mass spectrophotometric analysis of secreted Aβ species from H4 cells expressing APPsw revealed differential dose-dependent effects on elevation of Aβ1-40, 1-42, and 11-40 species. Elevation of plasma Aβ has been observed with the structurally related LY411575 in Tg2576 by the same authors (Lanz et al., 2004).

    The observations of Lanz et al. have been noted in humans with LY450139 as well (Siemers et al., 2005), and are relevant to Aβ as a biomarker for γ-secretase inhibitors in the clinic. In addition to the mechanistic basis underlying elevation of Aβ at low concentrations of γ-secretase inhibitor, another question is whether elevation of Aβ is model-specific, or compound-specific. Treatment with low doses of LY411575 did not elevate plasma Aβ in TgCRND8 mice (Wong et al., 2004), suggesting the model may be a factor. A structurally unrelated γ inhibitor, the sulfonamide BMS-299897, did not elevate plasma Aβ at low doses in Tg2567 (Barten et al., 2005), nor in guinea pig (Anderson et al., 2005). The benzodiazepene γ inhibitor, compound E, also did not elevate plasma Aβ in guinea pigs at low doses (Grimwood et al., 2005). Together, the latter data suggest elevation of plasma Aβ may be specific to certain classes of γ inhibitors in specific models.

    Effects on CSF Aβ following γ inhibitor treatment have also proven to be both compound- and model-dependent. In the current study, Lanz observed elevation of CSF Aβ under limited conditions in guinea pig CSF with LY450139. The dibenzocaprolactam LY411575 elevated CSF Aβ in Tg2576 (Lanz et al., 2004), but not in rats (Best et al., 2005). The sulfonamide MRK-560 also did not elevate CSF Aβ in rats at low doses (Best et al., 2006). No changes in CSF Aβ in humans treated with LY450139 have been reported (Siemers et al., 2005; Siemers et al., 2006). Elucidating the mechanistic basis for Aβ stimulation at low doses of γ inhibitors will further add to the list of novel catalytic mechanisms discovered from study of this unique enzyme.

    References:

    . Studies of Abeta pharmacodynamics in the brain, cerebrospinal fluid, and plasma in young (plaque-free) Tg2576 mice using the gamma-secretase inhibitor N2-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N1-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]az. J Pharmacol Exp Ther. 2004 Apr;309(1):49-55. PubMed.

    . Safety, tolerability, and changes in amyloid beta concentrations after administration of a gamma-secretase inhibitor in volunteers. Clin Neuropharmacol. 2005 May-Jun;28(3):126-32. PubMed.

    . Chronic treatment with the gamma-secretase inhibitor LY-411,575 inhibits beta-amyloid peptide production and alters lymphopoiesis and intestinal cell differentiation. J Biol Chem. 2004 Mar 26;279(13):12876-82. PubMed.

    . Dynamics of {beta}-amyloid reductions in brain, cerebrospinal fluid, and plasma of {beta}-amyloid precursor protein transgenic mice treated with a {gamma}-secretase inhibitor. J Pharmacol Exp Ther. 2005 Feb;312(2):635-43. PubMed.

    . Reductions in beta-amyloid concentrations in vivo by the gamma-secretase inhibitors BMS-289948 and BMS-299897. Biochem Pharmacol. 2005 Feb 15;69(4):689-98. PubMed.

    . Determination of guinea-pig cortical gamma-secretase activity ex vivo following the systemic administration of a gamma-secretase inhibitor. Neuropharmacology. 2005 Jun;48(7):1002-11. PubMed.

    . Quantitative measurement of changes in amyloid-beta(40) in the rat brain and cerebrospinal fluid following treatment with the gamma-secretase inhibitor LY-411575 [N2-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N1-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-di. J Pharmacol Exp Ther. 2005 May;313(2):902-8. PubMed.

    . In vivo characterization of Abeta(40) changes in brain and cerebrospinal fluid using the novel gamma-secretase inhibitor N-[cis-4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560) in the rat. J Pharmacol Exp Ther. 2006 May;317(2):786-90. PubMed.

    . Safety, tolerability, and changes in amyloid beta concentrations after administration of a gamma-secretase inhibitor in volunteers. Clin Neuropharmacol. 2005 May-Jun;28(3):126-32. PubMed.

    . Effects of a gamma-secretase inhibitor in a randomized study of patients with Alzheimer disease. Neurology. 2006 Feb 28;66(4):602-4. PubMed.

  2. This thorough study from researchers at Pfizer demonstrates that a γ-secretase inhibitor elevates Aβ levels at low doses in a guinea pig model (with endogenous Aβ that is identical to human). This phenomenon has been reported for some other γ-secretase inhibitors. However, the compound in the current investigation is in phase 2 clinical trials by Eli Lilly and is the most advanced γ inhibitor to date, so it is particularly important to know its specific effects. The obvious concern is that low doses in humans may have the opposite of the desired effect and may actually exacerbate AD. Although the elevation of Aβ at low doses of inhibitor are seen most clearly for Aβ40 in the guinea pig plasma, elevation of
    Aβ42 in the CSF also appears elevated. However, due to problems with variability in measuring Aβ42 in the CSF, it is difficult to say whether these elevations reached statistical significance.

    The mechanism of these mysterious and paradoxical increases remains unclear. Nevertheless, this study shows the Aβ elevating effects in cell culture as well, suggesting that the effect is due to direct interaction with the protease complex. Perhaps partial inhibition raises substrate levels so that more Aβ is produced by the remaining (uninhibited) protease complexes. γ-secretase likely has further secrets to reveal before this phenomenon can be truly understood.

  3. LY-450139 did not have an easy start. There was always the issue of Notch selectivity. One serious case of gastrointestinal bleeding occured in the phase Ib/IIa trials, but that was assigned to Barrett oesephagus and not to impaired Notch signaling. The phase IIa trials revealed a discrepancy between LY-450139 concentration and Abeta levels. Now, Lanz et al. report a concentration dependent modulatory, not full inhibitory, effect on Abeta production in a guinea pig model, which avoids transgenes and overexpression.

    The concentration-dependent mode of action as judged by plasma and CSF levels is reminescent of the Scios inhibitors, the Abbenante tripeptide and other early, unselective peptidomimetics. The critical issues of sampling timepoint and the known Abeta overshoot 6-9 h after administration was overcome by steady-state sampling.
    The steady-state administration, which will be not be easy to achieve in humans, result in increased plasma Abeta levels at low doses and Abeta reduction at high doses. These data rule out stockpiled substrate under low dosage regime as the reason for the Abeta overshoot at later time points. The 2 binding site model proposed by Mark Shearman at the Madrid Meeting 2006 may hold the key for this observation: At low dosage, one site is occupied only. This results in modulation; whereas at higher dosage both site are occupied, which results in full inhibition.