Reply

We appreciate Dr Albright and Dr Raybon’s interest in the analysis of “A Gamma-Secretase Inhibitor Decreases AmyloidBeta Production in the Central Nervous System.” In their letter, they have challenged 2 aspects related to how the data from our study were analyzed. The first is related to how area under the curve (AUC) calculations are used to quantify the level of inhibition of amyloid-beta (A ) synthesis, and the second is how the placebo data from the study should be interpreted. We agree that A synthesis and clearance are occurring simultaneously, even for the newly synthesized and labeled A . However, the process that is dominant, and thus primarily affects the shape of the labeled A concentration versus time curve, changes over the 36-hour period that data were collected. During the early portion of the 36-hour period the dominant process is synthesis, and during the later portion of the 36-hour period the dominant process is clearance. We have excluded the middle 12-hour period from our analyses for exactly the reason Albright and Raybon have raised; during this time period, the synthesis and clearance of labeled A are competing processes, and it would be difficult with simple AUC analyses to separately quantify the 2 processes. Our justifications for using the first 12 hours to quantify inhibition of A synthesis are (1) LY450139 is only present at significant inhibitory concentrations during this time period; and (2) labeled leucine was only infused for 9 hours, and therefore would not be available to be incorporated into newly synthesized A for times much longer than 9 hours. Albright and Raybon have used an analogy to traditional pharmacokinetics to justify their AUC analysis approach, and we can extent this analogy to justify why their approach is not appropriate for our study. Pharmacokineticists sometimes use the AUC of the initial portion of a concentration time curve to better understand the rate of absorption, and they use the full AUC of the concentration time curve to understand extent of absorption. A simple example of this is for drugs where the rate of absorption is significantly affected by food, but the extent of absorption and thus total AUC is only slightly affected. In this situation, only the initial AUC is significantly altered, and using the total AUC to interpret how much food inhibited the rate of absorption would lead to erroneous conclusions. The same is true for our study as it relates to using the most appropriate AUC to quantify inhibition of the rate of A synthesis. The analysis methods that we have used in our study have been established over many years of isotope tracer research. Newer methodologies that apply more complex analysis methods, such as mechanistic and physiologically based modeling, will likely be used in the future to better quantify complex processes such as A trafficking. The second aspect of the study challenged by Albright and Raybon is how the placebo data were utilized. The study was designed and implemented as a blinded placebo-controlled study of the effects of treatment on A production and concentration. The ad hoc proposal to ignore the placebo group and compare A changes to baseline is not compatible with the study design. As demonstrated in this and other studies, TABLE: Summary of C6-Leucine and A Changes after Administration of LY450139 Using Several Methods