Acetylator Phenotype

Perhaps one of the best known and fully described genetic factors in drug disposition and metabolism is the acetylator phenotype. It has been known for more than 30 years that for certain drugs which are acetylated, and isoniazid (figure 7.15) was the prototype, in human populations there is variation in the amount of this acetylation. This variation was found to have a genetic basis and did not show a normal Gaussian distribution, but one interpreted as bimodal, suggesting a genetic polymorphism (figure 5.17). The two groups of individuals, were termed rapid and slow acetylators because of the difference in both the amount and rate of acetylation. There are now a number of drugs which show this polymorphism in humans including isoniazid, hydralazine, procainamide,

TABLE 5.14 Some toxicities and diseases proposed to be associated with acetylator phenotype

Foreign compound

Adverse effect

Higher incidence in

Isoniazid

Peripheral neuropathy

Slow acetylators

Isoniazid

Hepatic damage

Slow acetylators

Hydralazine

Lupus erythematosus

Slow acetylators

Procainamide

Lupus erythematosus

Slow acetylators

Isoniazid in combination with phenytoin

Central nervous system toxicity

Slow acetylator

Sulphasalazine

Haemolytic anaemia

Slow acetylators

*Aromatic amines

Bladder cancer

Slow acetylators

Aromatic amines

Mutagenesis/carcinogenesis

Rapid acetylators

Food pyrolysis products?

Colo-rectal cancer

Rapid acetylators

^Controversial.

tHas yet to be fully substantiated.

^Controversial.

tHas yet to be fully substantiated.

dapsone, phenelzine, aminoglutethimide, sulphamethazine, products of caffeine metabolism and probably aromatic amines such as the carcinogen benzidine. The acetylation polymorphism is an important genetic factor in a number of toxic reactions (table 5.14), and both isoniazid and hydralazine are discussed in more detail in Chapter 7. The trait has now been observed in other species, including the rabbit, mouse, hamster and rat.

Studies in human populations suggest that the acetylator phenotype is a single gene trait, with two alleles at a single autosomal genetic locus with slow acetylation a simple Mendelian recessive trait. The dominant-recessive relationship is unclear however. Genetic studies in the rabbit, mouse and hamster indicate that there is codominant expression of these alleles. Thus there are three possible genotypes with the following arrangement of the slow (r) and rapid (R) alleles: rr, rR and RR. These would be manifested as slow, intermediate and rapid acetylators. However the ability to distinguish the three phenotypes in human populations is dependent on the method used and the particular drug administered. The heterozygous group, rR, may therefore be indistinguishable from the homozygous rapid group. The genetic trait governs the forms of the A-acetyltransferase enzyme, which catalyses the acetylation of amine, hydrazine and sulphonamide groups (see Chapter 4). In rabbits the activity of liver A-acetyltrans-ferase was studied in vitro and found to show trimodal or bimodal distributions.

However, some aromatic amino compounds are not polymorphically acetylated, such as p-aminobenzoic acid andp-aminosalicylic acid. This is clearly illustrated by procainamide (figure 5.18) which is itself polymorphically acetylated (figure 5.19a) whereas the hydrolysis product p-aminobenzoic acid (figure 5.18) is monomorphically acetylated in the same individuals (figure 5.19b). Studies in rabbits showed that there were only small differences in acetylation in vitro between liver from rapid and slow acetylator phenotypes for monomorphic substrates such as p-aminobenzoic acid (2-2.5-fold) compared with large differences for polymorphic substrates (10-100-fold). However, using polymorphic substrates with the isolated enzyme, no correlation could be found between Km

Slow Acetylators Drug Metabolism
FIGURE 5.18 Metabolism of procainamide. Procainamide and the hydrolysis product p-aminobenzoic acid are both acetylated.
Procainamide Dose Table
FIGURE 5.19 Frequency distribution for the acetylation ofprocainamide (a) and p-aminobenzoic acid (b) in human subjects. Data represents excretion of acetylated product in the urine 6 h after dosing. Data from De Souich and Erill

and acetylator status; only when monomorphic substrates were used was this apparent (table 5.15). It is notable that both the substrates used are negatively charged at physiological pH. Thus the Km and Fmax data obtained for the isolated rabbit A-acetyltransferase enzyme using p-aminobenzoic acid and p-aminosalicylic acid show striking differences between genotypes, and support the hypothesis that the enzymes from slow and rapid acetylators are structurally different (table 5.15). The enzyme from heterozygous animals was intermediate between the homozygous values. The slow acetylator enzyme thus has a much greater affinity for these particular substrates, but a lower capacity than the rapid acetylator enzyme. It seems that in the rabbit the A-acetyltransferase from the slow phenotype is probably saturated in vivo at normal concentrations of drug and is therefore operating at maximum efficiency, whereas the rapid enzyme has such a high Km that it would remain unsaturated and would be below maximum capacity. For monomorphic substrates therefore the overall

TABLE 5.15 Characteristics of rabbit N-acetyltransferase with various substrates

Apparent Km («M)

Apparent Fmax

(nmol/min/mg)

Substrate*rr

RR

rr

RR

PABA

<5

105

0.24

9.3

PAS

<5

74

0.31

5.0

PA

200

67

0.35

4.4

SMZ

160

90

0.38

4.8

*PABA, p-aminobenzoic acid; PAS, /»-aminosalicylic acid; PA, procainamide; SMZ, sulphamethazine. Data from Weber (1987) The Acetylator Genes and Drug Response (New York: Oxford University Press).

*PABA, p-aminobenzoic acid; PAS, /»-aminosalicylic acid; PA, procainamide; SMZ, sulphamethazine. Data from Weber (1987) The Acetylator Genes and Drug Response (New York: Oxford University Press).

rates of metabolism in vivo would be similar. For polymorphic substrates (sulphamethazine and procainamide, for example), the Km values are much greater and more similar in the two phenotypes but the Fmax values are markedly different and therefore the metabolism of such compounds in vivo is different in the two phenotypes (table 5.15).

It should be noted that in the hamster the situation is the reverse, with p-aminobenzoic acid and p-aminosalicylic acid being polymorphically acetylated, and sulphamethazine and procainamide monomorphically acetylated.

Thus, in the rabbit, hamster and mice the basis for the acetylator polymorphism is a qualitative difference in the A-acetyltransferase enzyme. In mice recent studies have indicated that other factors, such as modifier genes, may also affect the expression of acetyltransferase activity. In the human there are two A-acetyltransferases, NAT1 and NAT2. NAT1 is an enzyme with a wide tissue distribution which has a high affinity for monomorphic substrates such as p-aminobenzoic acid. NAT2 is mainly found in liver and has a high affinity for polymorphic substrates. However, there is post translational modification of the primary gene product. This has been related to several mutant alleles which have been identified in human slow acetylator populations. Thus, the human slow acetylators mainly have two mutations which results in less functional NAT2.

As can be seen from table 5.14, the acetylator phenotype is a factor in a number of toxic effects due to foreign compounds including the carcinogenicity of aromatic amines. Generally the slow acetylators are more at risk, probably because acetylation protects the amino or hydrazine group from metabolic activation. Thus in the case of carcinogenic aromatic amines such as benzidine, it has been suggested that slow acetylators are more susceptible to bladder cancer. (Thus one study showed a relative risk of slow: rapid of 1.36 and for individuals exposed to aromatic amines in industry a relative risk of slow: rapid of 1.7.) However, this is by no means certain, as one recent study of the role of the acetylator phenotype in bladder cancer in Chinese workers exposed to benzidine found no association of increased risk with the slow acetylator phenotype which, it was suggested, may even have a protective effect. Contrary to this, another very recent study in Taiwanese individuals did find an association between the slow acetylator (NAT2) genotype and bladder cancer. Conversely evidence is accumulating suggesting rapid acetylators may be more at risk from colo-rectal cancer, possibly as a result of exposure to aromatic amines. Pyrolysis of food during cooking produces

TABLE 5.16 Acetylatorphenotype distribution in various ethnic groups (INH, Isoniazid; SMZ, Sulphamethazine)

Ethnic group

Rapid acetylators (%)

Drug

Eskimos

95-100

INH

Japanese

88

INH

Latin Americans

70

INH

Black Americans

52

INH

White Americans

48

INH

Africans

43

SMZ

South Indians

39

INH

Britons

38

SMZ

Egyptians

18

INH

Data from Lunde et al. (1977) Clin. Pharmacokin., 2, 182.

Data from Lunde et al. (1977) Clin. Pharmacokin., 2, 182.

various mutagenic and carcinogenic amines which are known to be acetylated. Studies have shown that rapid acetylator mice have greater DNA adduct formation with 2-aminofluorene than slow acetylator mice. The role of acetylation in carcinogenicity is discussed in more detail in Chapter 7 with regard to aminofluorene derivatives.

There are examples where several factors, including the acetylator phenotype, operate together. Hydralazine toxicity is one such example which is discussed in detail in Chapter 7. Another is the haemolytic anaemia caused by the drug thiozalsulphone (Promizole) which occurs particularly in those individuals who are both glucose-6-phosphate dehydrogenase deficient and slow acetylators. Promizole is acetylated, and studies in rapid and slow acetylator mice confirmed that acetylation was a factor as well as extent of hydroxylation. The latter may also be another factor in humans as is discussed below. The acetylator polymorphism also exhibits an interesting ethnic distribution in humans, as shown in table 5.16, which may have important implications for the use of drugs in different parts of the world. As well as being a factor in the toxicity of several drugs, the acetylation polymorphism may influence the efficacy of treatment. For example, the plasma half-life of isoniazid is two to three times longer and the concentration higher in slow acetylators compared with rapid acetylators. Therefore the therapeutic effect will tend to be greater in the slow acetylators as the target microorganism is exposed to higher concentrations of drug.

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Responses

  • savanna
    What is acetylator phenotype?
    2 years ago
  • sebhat
    What idls INH phenotype acetylator?
    2 years ago
  • Eden
    What is rapid acetylators?
    2 years ago
  • meriadoc
    What does slow acetylator mean and drug disposition?
    2 years ago
  • NICOLE
    Can you be both rapid and slow acetylator?
    10 months ago
  • fastred zaragamba
    How to find acetylator status?
    19 days ago

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