| pA2 online © Copyright 2004 The British Pharmacological Society |
223P
GKT, University of London Winter Meeting December 2003 |
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Assessment of CYP2D6 in vivo activity: relative sensitivity of urinary metabolic ratios of debrisoquine (DB), dextromethorphan (DM) and metoprolol (MP) to urine pH M. Özdemir1, Y. Aktan1, A. Rostami-Hodjegan2, H. K. Crewe2 & G. T. Tucker21Dept. Pharmacology, Faculty of Medicine, Osmangazi University, Eskisehir, Turkey ; 2Molecular Pharmacology & Pharmacogenetics, University of Sheffield, UK. |
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Print abstractUrinary drug/metabolite ratios (MR) have been used to assess specific enzyme activity non invasively in vivo. These indices are potentially confounded by the effect of renal function1. Although DB, DM and MP have different ionisation characteristics they are all common in vivo probe substrates of cytochrome P450 2D6 (CYP2D6). We have investigated the relative sensitivity of MR from these three probes to changes in urinary pH.
Following approval by the Ethics Committee, three groups of healthy volunteers each comprising 12 individuals were given either DB (10mg), DM (25mg) or MP (100mg). In each group probe drug was administered on 3 occasions separated by 10 day intervals such that on one occasion urine was acidified by taking oral ammonium chloride a priori, on another occasion it was alkalinised by ingestion of sodium bicarbonate. On the third occasion the urine pH was uncontrolled. The three arms of the study were randomised and cross over. After measuring urine pH samples were frozen at -20oC until analysis. Urinary MP/a-hydroxy-MP and DM/dextrorphan (DOR) were measured by HPLC with fluorescence detection. DB and 4-hyroxy-DB were measured by GLC with nitrogen selective detection. Differences between log transformed MR values (8 hr collection) in the study arms were assessed using the paired t test. Correlation of urinary pH with MR for each probe drug was investigated using Spearman rank test.
The meangeo MR (95% CI for differences), mean pH and correlation between MR and pH are shown in Table. MR for DB was not significantly different in any of the study arms while the corresponding values for MP and DM were significantly different in acidified or alkalinised urine compared to that of uncontrolled urine. Urinary pH correlated significantly with inverse MR for DM and MP but not DB.
| MR |
r
(p)
|
StudyArm
|
Urine
pH
|
Mean
MR Log(MR)
|
95%
CI(difference)
|
p
value
|
|
MP
/  -OH MP |
0.639
(0.00)
|
Placebo
|
6.5
|
0.82
|
-0.80
|
||
|
Acidic
|
4.8
|
1.83
|
0.11
|
-1.26,
-0.57
|
0.00
|
||
|
Alkaline
|
8.3
|
0.26
|
-2.02
|
0.39,
2.05
|
0.01
|
||
|
|
|||||||
| DB
/ 4-OH DB
|
0.065
(0.719)
|
Placebo
|
6.7
|
0.89
|
-0.49
|
||
|
Acidic
|
4.1
|
0.71
|
-0.74
|
-0.13,
0.63
|
0.18
|
||
|
Alkaline
|
8.8
|
0.96
|
-0.69
|
-0.18,
0.58
|
0.26
|
||
|
|
|||||||
|
DM / DOR |
0.833
(0.00)
|
Placebo
|
6.6
|
0.0017
|
-7.13
|
||
|
Acidic
|
4.7
|
0.0120
|
-5.13
|
-2.97,
-1.04
|
0.00
|
||
|
Alkaline
|
8.8
|
0.0003
|
-8.65
|
0.87,
2.16
|
0.00
|
||
The results from the current study suggest that, in the absence of control on urine pH, there is a high possibility of assigning false relative in vivo CYP2D6 metabolic activity to many individuals using DM and MP but not DB. Although this may not pose any problem for separation of CYP2D6 extensive metabolisers from poor metabolisers, the confounding effect of urine pH may play a role in apparent failure to establish a link between divergent genotypes within CYP2D6 extensive metabolisers and the individual in vivo enzyme activity (e.g. see Saschse et al.2).
(1) Tucker GT et
al (1998) Xenobiotica 28: 1255-73.
(2) Saschse C et al (1997) Am J Hum Genet 60: 284-95.