Pharmacogenetic aspects of therapy with cholinesterase inhibitors: The role of CYP2D6 in Alzheimer's disease pharmacogenetics

Recent studies demonstrate that the therapeutic response in Alzheimer's disease (AD) is genotype-specific. More than 200 genes are potentially associated with AD pathogenesis and neurodegeneration, and approximately 1,400 genes distributed across the human genome account for 20 to 95% of variability in drug disposition and pharmacodynamics. Cytochrome P450 enzymes encoded by genes of the CYP superfamily, such as CYP1A1 (5q22-q24), CYP2A6 (19q 13.2), CYP2C8 (10q24), CYP2C9 (10q24), CYP2C 19 (10q24. 1-q24.3), CYP2D6 (22q13. 1), CYP2E1 (10q24.3-qter), and CYP3A5 (7q22.1), acting as terminal oxidases in multicomponent electron transfer chains which are called P450-containing monooxygenase systems, metabolize more than 90% of drugs. Some of the enzymatic products of the CYP gene superfamily can share substrates, inhibitors and inducers whereas others are quite specific for their substrates and interacting drugs. Some cholinesterase inhibitors (tacrine, donepezil, galantamine) are metabolized via CYP-related enzymes, especially CYP2D6, CYP3A4, and CYP1A2. The distribution of CYP2136 genotypes in the Spanish population is the following: (a) Extensive Metabolizers (EM)(51.61%): *1/*1, 47.10%; and *1/*10, 4.52%; (b) Intermediate Metabolizers (IM)(32.26%): *1/*3, 1.95%; *1/*4, 17.42%; *11*5, 3.87%; *1/*6, 2.58%; *]/*7, 0.75%; *10/*10, 1.30%; *4/*10, 3.23%; *6/*10, 0.65%; and *7/*10, 0.65%; (b) Poor Metabolizers (PM)(9.03%): *4/*4, 8.37%; and *5/*5, 0.65%; and (c) Ultrarapid Metabolizers (UM)(7.10%): *1xN/*1, 4.52%; *IxN/*4, 1.95%; and CYP2136 gene duplications, 0.65%. PMs and UMs also accumulate genotypes of risk associated with APOE-, PS-, ACE-, and PRNP-related genes. Approximately, 15% of the AD population may exhibit an abnormal metabolism of cholinesterase inhibitors; about 50% of this population cluster would show an ultrarapid metabolism, requiring higher doses of cholinesterase inhibitors to reach a therapeutic threshold, whereas the other 50% of the cluster would exhibit a poor metabolism, displaying potential adverse events at low doses. In AD patients treated with a multifactorial therapy, including cholinesterase inhibitors (e.g., donepezil), the best responders are the CYP2D6-related EMs and IMs, and the worst responders are PMs and UMs. In addition, the presence of the APOE-4 allele in genetic clusters integrating CYP2D6 and APOE genotypes contributes to deteriorate the therapeutic outcome. From these data, it can be postulated that pharmacogenetic and pharmacogenomic factors are responsible for 7545% of the therapeutic response in AD patients treated with conventional drugs.