The metabolism of debrisoquine in man: (1) regioselectivity of hydroxylation and (2) aberrant oxidative metabolism in two sibling patients with carbimazole-induced agranulocytosis

Regioselective hydroxylation of debrisoquine by cytochrome P4502D6: implications for active site modelling

1. Debrisoquine, a prototypic probe substrate for human cytochrome P4502D6 (CYP2D6), is hydroxylated at the alicyclic C4-position by this enzyme. Phenolic metabolites of debrisoquine (5-, 6-, 7- and 8-hydroxydebrisoquine) have also been reported as in vivo metabolites, but the role of CYP2D6 in their formation is unclear. 2. As part of studies to develop a predictive model of the active site of CYP2D6 using pharmacophore and homology modelling techniques, it became important to determine the precise regioselective hydroxylation of debrisoquine by CYP2D6. 3. Data from studies with human liver microsomes and yeast microsomes containing cDNA-derived CYP2D6 demonstrated unequivocally that debrisoquine was hydroxylated by CYP2D6 at each aromatic site in the molecule, as well as at the alicyclic 4-position. The four phenolic metabolites amounted to > 60% of the total identified products and the pattern of regioselective hydroxylation (4-HD > 7-HD > 6-HD > 8-HD > 5-HD) was similar in both in vitro systems. 4. A pharmacophore model for CYP2D6 indicated that while the hydroxylation of debrisoquine at alternative positions could arise from the substrate adopting multiple binding orientations, the energy constraints for the aromatic hydroxylations were unfavourable. An alternative proposal involving essentially a single binding orientation and a mechanism of hydroxylation based on benzylic radical spin delocalization could satisfactorily rationalize all the hydroxylations of debrisoquine. 5. This latter proposal demonstrates the need to consider the mechanism of oxidation as well as the spatial orientation of the substrate in the development of a predictive model of the active site of CYP2D6.

Demonstration of mRNA for five species of cytochrome P450 in human bone marrow, bone marrow-derived macrophages and human haemopoietic cell lines

The expression of mRNA for five cytochrome P450s (CYP1A1, 2A6/7, 2D6, 2E1 and 3A4) was studied in human bone marrow, bone-marrow-derived macrophages and blood monocyte-derived macrophages. Reverse transcriptase polymerase chain reaction (RT-PCR) detected expression of all five CYPs in each of these cell populations. All five CYPs were also expressed in the haemopoietic cell lines HL-60 and HEL and in Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cell lines. The data suggest that bone marrow macrophages and probably other bone marrow cell types are capable of metabolizing xenobiotics. This metabolic potential may play a role in the bone marrow damage induced by some drugs and chemicals.

Human variability and noncancer risk assessment- An analysis of the default uncertainty factor

A 10-fold uncertainty factor is used for noncancer risk assessments to allow for possible interindividual differences between humans in the fate of the chemical in the body (kinetics) and target organ sensitivity (dynamics). Analysis of a database on the variability in each of these aspects is consistent with an even subdivision of the 10-fold factor into 10(0.5) (3.16) for kinetics and 10(0.5) (3.16) for dynamics. Analysis of the number of subjects in a normally and log-normally distributed population which would not be covered by factors of 3.16 supports this subdivision and also the use of a 10-fold factor to allow for both aspects. Analysis of kinetic data for subgroups of the population indicates that the standard default value of 3.16 for kinetics will not be adequate for all routes of elimination and all groups of the population. A scheme is proposed which would allow the selection of appropriate default uncertainty factors based on knowledge of the biological fate and effects of the chemical under review. Copyright 1998 Academic Press.

Debrisoquine hydroxylation in a Polish population

The genetic polymorphism of drug oxidation mediated by cytochrome P450IID6 (CYP2D6) was determined in 154 Polish volunteers using debrisoquine as the test substance. The results showed a bimodal distribution of the debrisoquine metabolic ratio (MR). Nine persons (5.8%) with MR > 12.6 were classified as poor metabolisers (gene frequency 0.242), which is in substantial agreement with the data reported for other Caucasian populations.

Data-derived safety factors for the evaluation of food additives and environmental contaminants

A safety factor of 100-fold is commonly applied to animal data to derive the acceptable daily intake (ADI) of food additives; other factors have been used in some cases and higher values are used more frequently for determining the tolerable daily intake (TDI) of environmental chemicals. The 100-fold factor is considered to represent the product of a 10-fold factor to allow for species differences between the test animal and humans and a 10-fold factor to allow for inter-individual differences. A scheme is proposed whereby data relevant to the safety assessment of a compound, e.g. species differences in toxicokinetics, can contribute quantitatively to the safety factor and therefore to the ADI or TDI. For this to be possible, it is necessary to subdivide each of the 10-fold factors into two separate factors to allow for differences in toxicokinetics and toxicodynamics. For any compound, data on one particular aspect may be used to derive a specific data-derived factor for that aspect. The overall safety factor will then be calculated as the product of the known data-derived factor(s) and default values for the remaining unknown factors. In this way the derivation of the safety factor would be clearly defined and the potential impact of additional data on other aspects identified. Additional safety factors (over and above the 100-fold or overall data-derived factor) are also proposed to allow for the nature or severity of the toxicity and the adequacy of the database. These factors are consistent with previous evaluations and will allow the logical derivation of factors greater than either 100 or the appropriate data-derived factor. These additional factors will be of greatest value in the derivation of safety factors for the calculation of the TDIs of environmental contaminants but may also be applied if necessary to the safety assessment of food additives. In such cases the rationale and logic for a safety factor in excess of 100 will be clearly defined.

Debrisoquine oxidation polymorphism in a Tasmanian population

The debrisoquine hydroxylation phenotype was studied in 152 unselected healthy Tasmanian subjects, who were mostly Caucasians of British ancestry. Following a 10 mg oral dose of debrisoquine (D), the ratio of D/4-hydroxydebrisoquine excreted in 8-h urine (metabolic ratio, MR) was determined. MR values were bimodally distributed. Thirteen subjects (8.6%) had MR values from 13.8 to 93.3 and were considered to be poor metabolisers of D, while the others were extensive metabolisers with MR values of 0.04 to 5.4. The D hydroxylation phenotype was not associated with sex. These findings confirm the constancy of D polymorphism in a Caucasian population even after migration to another country.

Safety factors and establishment of acceptable daily intakes

The acceptable daily intake (ADI) of food additives and other foreign compounds is derived usually by the application of a 100-fold safety factor to the 'no observable adverse effect' level in animal experiments. This safety factor is recognized as comprising two 10-fold factors to allow for possible differences between animals and humans, and for possible differences between individual humans. This paper reviews the adequacy of these safety factors in relation to published data on inter-species and inter-individual differences in both the kinetics and dynamics of foreign compounds. Although the 100-fold factor is reasonable, situations can be envisaged where the scientific data base indicates that considerably higher or lower factors would be more appropriate. A scheme is proposed that would allow structured flexibility to be available in the choice of safety factor to be used in the regulation of foreign compounds.