Pharmacogenetic perspectives: genes, drugs and disease

Immunologic mechanisms of cutaneous drug reactions

Idiosyncratic reactions (type B) are a major complication of drug therapy, because they are related to both the drug and to individual factors in the host. In comparison with other organs, the skin is quite frequently a target of allergic reactions, which are mainly elicited by small molecular weight compounds. This is the case in allergic contact dermatitis as well as in drug allergic reactions. In contrast to allergic contact dermatitis however, drug-induced hypersensitivity reactions of the skin have enormous variability with regard to their pathophysiological pathways, clinical signs of symptoms, severity, and the drugs which can elicit these reactions. Allergic reactions are mediated either by specific antibodies or a cellular immunocompetent immune response. About 25% to 30% of type B reactions are estimated to be allergic drug reactions, which are classified by the latency period between the ingestion of the responsible allergen and the onset of clinical symptoms. Studying the mechanisms of these hypersensitivity reactions improves our understanding of these diseases in general, and shows the importance of the skin as a signaling organ in these reactions.

Genetic influences in caries and periodontal diseases

Deciphering the relative roles of heredity and environmental factors ("nature vs. nurture") in the pathogenesis of dental caries and diseases of the periodontium has occupied clinical and basic researchers for decades. Success in the endeavor has come more easily in the case of caries; the complex interactions that occur between host-response mechanisms and putative microbiologic pathogens in periodontal disease have made elucidation of genetic factors in disease susceptibility more difficult. In addition, during the 30-year period between 1958 and 1987, only meager resources were targeted toward the "nature" side of the nature/nurture dipole in periodontology. In this article, we present a brief history of the development of genetic epistemology, then describe the three main research mechanisms by which questions about the hereditary component of diseases in humans can be addressed. A critical discussion of the evidence for a hereditary component in caries susceptibility is next presented, also from a historical perspective. The evolution of knowledge concerning possible genetic ("endogenous", "idiotypic") factors in the pathogenesis of inflammatory periodontal disease is initiated with an analysis of some foreign-language (primarily German) literature that is likely to be unfamiliar to the reader. We identify a turning point at about 1960, when the periodontal research community turned away from genetics in favor of microbiology research. During the past five years, investigators have re-initiated the search for the hereditary component in susceptibility to common adult periodontal disease; this small but growing body of literature is reviewed. Recent applications of in vitro methods for genetic analyses in periodontal research are presented, with an eye toward a future in which persons who are at risk--genetically predisposed--to periodontal disease may be identified and targeted for interventive strategies. Critical is the realization that genes and environment do not act independently of each other; the appearance or magnitude of heritability may differ with various environments.

Acetylator phenotype and congenital malformations

The hypothesis has been tested that an unusual maternal acetylator phenotype can predispose to congenital malformations in the fetus. The acetylator phenotype of normal caucasian control women and of mothers of malformed children was established by measuring urinary sulphadimidine and its acetylated metabolite. A further control group was the fathers of the malformed newborn. The malformations studied were facial-cleft, spina-bifida and congenital heart disease. The acetylator phenotype was shown not be modified by pregnancy. 49 of 100 (49%) control women were rapid acetylators. Amongst the 108 mothers of malformed babies, 56 (52.8%) were slow acetylators and 52 (47.2%) were fast acetylators, 42 out of 83 (50.5%) of the fathers of malformed were slow acetylators and 41 (49.5%) were fast acetylators. Thus, the acetylator phenotype of the mothers of malformed children is no different from the acetylator phenotype of controls.

No evidence of a genetic polymorphism in the oxidative metabolism of midazolam

The benzodiazepine midazolam is rapidly eliminated by oxidative metabolism. In young healthy volunteers elimination half-life (t1/2) is about 2.4 hours. A recent study showed a prolonged t1/2 from 8 to 22 hours in 6.5% of surgical patients, and a genetic polymorphism of midazolam's metabolism has been suggested. Therefore, we measured in 168 surgical patients the elimination of midazolam and its major hydroxylated metabolite (alpha-OH-midazolam) in blood and urine. Co-medication, disease status, smoking habits and alcohol intake were recorded; normal liver and kidney functions were assessed by routine laboratory tests. Midazolam was administered intravenously (0.1 to 0.2 mg/kg) for the induction of anaesthesia. Blood was drawn 1.5, 3, 4.5 and 6 hours after application and urine was collected for 6 hours. Plasma protein binding of midazolam was determined by equilibrium dialysis. Midazolam and alpha-OH-midazolam were measured in plasma by specific gas-liquid chromatography and in urine by high performance liquid chromatography. Data for the dose-corrected area under plasma-level curve of midazolam (AUC-midazolam/dose: 1.23 +/- 961 x 10(5) h/ml; mean +/- SD) and for the metabolic plasma ratio (AUC of alpha-OH-midazolam/AUC-midazolam: 0.52 +/- 0.28) demonstrated a log-normal distribution. Likewise, the percentage of the unbound fraction of midazolam in plasma (5.0 +/- 2.4%), urinary excretion of alpha-OH-midazolam (55.9 +/- 22.7% of dose) and the values for t1/2 (2.9 +/- 1.1 hours) did indicate a unimodal distribution. Age, comedication and smoking habits did not affect the disposition of midazolam. However, patients with regular intake of alcohol had a higher (p less than 0.05) metabolic ratio. Only in 3 patients could a prolonged t1/2 of midazolam from 7.5 to 10.2 hours be detected, but plasma levels and urinary excretion of alpha-OH-midazolam in those individuals were found to be normal. Therefore it is very unlikely that the oxidative metabolism of midazolam exhibits a genetic polymorphism.

Special pharmacokinetic considerations in children

Pediatric patients have greater degrees of pharmacokinetic variability and unpredictability than adults. This variability results from the effects of pharmacogenetics, age and growth, prior and current comedication, and disease. Newborns with seizures have the least predictable dosage requirements, and their needs change as drug-eliminating mechanisms mature in the neonatal period. Infants have the highest relative capacities to eliminate antiepileptics of any age group and require the largest relative doses. In addition to age-related trends, children demonstrate the same drug-specific, pharmacokinetic phenomena that adults do, including nonlinear phenytoin elimination, nonlinear valproate binding, and autoinduction of carbamazepine. Intercurrent illness and drug interactions further modify the age-related pharmacokinetic patterns in children and make dosage requirements even more unpredictable. Recent studies have shown that febrile illness can affect drug elimination, sometimes decreasing drug levels by 50% or more. Intermittent treatment with benzodiazepines administered either orally or rectally can be an important adjunct and help minimize this type of problem for children with marginally controlled epilepsy. Intermittent benzodiazepines are also helpful for children who have febrile seizures and who need only occasional antiepileptic protection.

Altered distribution of debrisoquine oxidation phenotypes in patients with systemic lupus erythematosus

Oxidative metabolism in patients with systemic lupus erythematosus (SLE) was studied using the antihypertensive drug, debrisoquine. The metabolism of this drug to its principal metabolite, 4-hydroxydebrisoquine, is catalyzed by a discrete isozyme of cytochrome P-450. The extent of this reaction exhibits genetic polymorphism, with 2 phenotypes, "poor metabolizers" and "extensive metabolizers," discernible in the normal population. We observed the poor metabolizer debrisoquine phenotype in 9 of 42 patients with idiopathic SLE (21%), in contrast with 12 of 147 healthy volunteers (8%), which is a significant difference in frequency (P less than 0.04). These data provide further evidence for altered oxidative metabolism in SLE and support the concept that genetic differences in oxidative metabolism of endogenous compounds, such as sex steroid hormones, or of xenobiotics might influence susceptibility to SLE.

Genetics of drug transformation

Biotransformations of drugs are controlled or strongly affected by genetic factors. During the past few years several genetic deficiencies of drug-metabolizing reactions catalyzed by members of the family of cytochrome P-450 were observed. Choice of the appropriate drug to study and attention to urinary metabolites have been the essential ingredients for the recent discovery of genetic deficiencies of drug metabolism in man which include recessive deficiency of debrisoquine/sparteine metabolism and of mephenytoin metabolism. The clinical significance of these defects is discussed. Ethanol after metabolism to acetaldehyde is further metabolized to acetic acid by aldehyde dehydrogenase. Numerous isozymes of aldehyde dehydrogenase exist, one of which possesses a high affinity for acetaldehyde. Approximately 40% of the Oriental population lack this high affinity isozyme so that in these individuals who may have symptoms of flushing and other unpleasant effects the acetaldehyde formed is destroyed only at high plasma concentrations.

Recent developments in chemotherapy of ocular diseases

The profile of organisms responsible for ocular infections has changed over the past few decades. Although nonresistant and resistant staphylococci still lead the list, gram-negative, mixed infections and anaerobic organisms have become more evident. In spite of more potent and broader spectra antibiotics, resistant organisms still emerge. The therapy of intraocular infections requires an awareness of the latest antibiotics, particularly their advantages, limitations, toxicities, and administration. Iontophoresis through the sclera overlying the pars planum area with small diameter applicators may offer another route of administration which may reduce or eliminate the value of direct intravitreal injections of antibiotics. Reduction of the harmful effects of endotoxins of organismal origin might minimize the adverse effects of intraocular infection, particularly after gram-negative invasion. The choice of antibiotic may influence the quantities of endotoxins released.