Robert A. Branch

Clinical Implications of Prostaglandin and Thromboxane A2 Formation

(Second of Two Parts) Eicosanoids in the Regulation of the Circulation and of Renal Function The circulation is controlled by many distinct but interrelated regulatory mechanisms, including sympath...

Interethnic differences in genetic polymorphism of debrisoquin and mephenytoin hydroxylation between Japanese and Caucasian populations.

Interethnic differences in debrisoquin and mephenytoin hydroxylation have been compared between normal white (n = 183) and Japanese (n = 100) subjects with the 8‐hour urinary metaboUc ratio of debrisoquin and the urinary S/R enantiomeric ratio of mephenytoin to identify extensive (EM) and poor (PM) metabolizers. In white subjects the frequency of PMs was 8.7% and 2.7% for debrisoquin and mephenytoin, respectively. In contrast, in Japanese subjects no PMs of debrisoquin were identified, while the incidence of PMs of mephenytoin was 18%. These substantial differences (P < 0.001) in polymorphic distributions of oxidative drug metabolizing ability have implications for interethnic efficacy and toxicity of drugs and other xenobiotics that are metabolized by the involved cytochrome P‐450 isozymes.

Genetic polymorphism of S-mephenytoin hydroxylation

This review describes the pharmacological, biochemical and clinical studies that have led to a functional characterization of the mephenytoin polymorphism. In addition, attempts to understand the genetic basis of the metabolic defect at the biochemical and molecular levels are also described

Mephenytoin hydroxylation deficiency in Caucasians: Frequency of a new oxidative drug metabolism polymorphism

The ability of normal subjects to hydroxylate mephenytoin (100 mg) or debrisoquine (10 mg) after oral dosing was investigated in 156 unrelated Caucasians living in middle Tennessee. Urinary recovery of 4‐hydroxymephenytoin (4‐OH‐M) and the urinary S:R enantiomeric ratio of mephenytoin measured in an 8‐hr urine sample were investigated as phenotypic traits for mephenytoin, and the urinary metabolic ratio of debrisoquine was used to determine the debrisoquine hydroxylase phenotype. Both urinary 4‐OH‐M and the S:R ratio of mephenytoin discriminated between extensive (EM) and poor (PM) metabolizers of mephenytoin. The frequencies of PMs for mephenytoin and debrisoquine hydroxylation activity were 2.6% and 7.0%. These two defects in oxidative metabolism were not observed in the same subjects, which suggests that 4‐hydroxylation of mephenytoin is a new polymorphism independent of that for debrisoquine.

Validation of the five-drug "Pittsburgh cocktail" approach for assessment of selective regulation of drug-metabolizing enzymes.

To determine whether the probe drugs caffeine, chlorzoxazone, dapsone, debrisoquin (INN, debrisoquine), and mephenytoin can be simultaneously administered as a metabolic cocktail to estimate in vivo cytochrome P450 (CYP) and N‐acetyltransferase enzyme activities.

Prostacyclin production during pregnancy: Comparison of production during normal pregnancy and pregnancy complicated by hypertension

We investigated prostacyclin (PGI2) biosynthesis during pregnancy by measuring urinary excretion of 2,3-dinor-6-keto-prostaglandin F1 alpha (dinor) and 15-keto-13,14-dihydro-2,3-dinor-6-keto-prostaglandin F1 alpha (15 kd dinor) with the use of specific gas chromatography-mass spectrometry assays. Nine normotensive nonpregnant women, five normotensive women in the mid-trimester of pregnancy, eight normotensive women in the third trimester of pregnancy, and six women who developed hypertension during the third trimester provided 24-hour samples of urine. Normal pregnant women had a fivefold increase in urinary excretion of dinor in comparison to nonpregnant women (253 +/- 21 ng dinor/gm creatinine for controls vs. 1,224 +/- 110 and 1,127 +/- 152 for second and third trimesters) (mean +/- SEM). Pregnant subjects with hypertension had a significant (50%) reduction in urinary dinor excretion in comparison to normotensive pregnant subjects (561 +/- 105 ng dinor/gm creatinine). In subjects selected from each group, the ratio of dinor to 15 kd dinor remained constant. We conclude that PGI2 biosynthesis is increased during normal pregnancy, and that this increase is less in pregnancy-induced hypertension. This raises the possibility that PGI2 helps mediate hemodynamic changes during normal pregnancy, and that a relative decrease in production might be related to the pathogenesis of pregnancy-induced hypertension.

Propranolol's metabolism is determined by both mephenytoin and debrisoquin hydroxylase activities

The relative contributions of the debrisoquin and mephenytoin isozymes of hepatic cytochrome P‐450 to the stereoselective metabolism of propranolol have been studied in a panel of volunteers of known oxidative phenotypes. Six subjects were extensive metabolizers of both debrisoquin and mephenytoin (EM). Four subjects were poor metabolizers of debrisoquin but rapid for mephenytoin (PMD). Five subjects were poor metabolizers of mephenytoin but rapid for debrisoquin (PMM), and one individual had a deficiency for both test compounds (PMD/M). Partial metabolic clearances of each propranolol enantiomer to 4‐hydroxypropranolol (4‐OH‐P), the sulfate, and glucuronide conjugates of 4‐OH‐P, naphthoxylactic acid (NLA) and propranolol glucuronide, were estimated after a single oral dose of racemic propranolol (80 mg). The partial metabolic clearance of both enantiomers to total 4‐OH‐P in the PMD group was 75% less than in the EM and PMM groups, indicating the contribution of the debrisoquin isozyme to this route of metabolism. The R/S ratios for the clearance to 4‐OH‐P were similar between EM and PMD (2.5 ± 0.5 vs 2.5 ± 0.4, respectively), implying that the different enzymes involved in ring hydroxylation (i.e., the debrisoquin isozyme and other hydroxylases) have similar stereoselective preferences. The partial metabolic clearance to NLA was 55% less in the PMM group than in the EM and PMD groups, indicating that S‐mephenytoin 4‐hydroxylase contributes to the metabolic conversion of propranolol to NLA. The R/S ratios for the clearance to NLA were close to unity in all groups. The partial metabolic clearance to propranolol glucuronide also did not exhibit stereoselectivity and was similar in all groups. The one subject with deficiencies in both debrisoquin and S‐mephenytoin hydroxylase activities had reduced partial clearances of both 4‐OH‐P and NLA and was the only individual to have a reduced total propranolol clearance. Thus two independent isozymes of cytochrome P‐450, previously identified as being responsible for debrisoquin and S‐mephenytoin hydroxylation, contribute to the two separate oxidative routes of metabolism of propranolol. Deficiencies of both routes result in impaired total clearance of propranolol.

Amphotericin B Nephrotoxicity

SummaryThe frequency of fungal infections is increasing. Amphotericin B remains the antifungal drug of choice for most systemic infections, but a limiting factor for its use is the development of nephrotoxicity. Amphotericin B-induced nephrotoxicity is manifested as azotaemia, renal tubular acidosis, impaired renal concentrating ability and electrolyte abnormalities like hypokalaemia and sodium and magnesium wasting. All these abnormalities occur to varying degrees in almost all patients receiving the drug. Upon withdrawal of therapy renal function gradually returns to baseline, although in some instances permanent damage is sustained, especially when the cumulative dose exceeds 5g. Salt depletion enhances the development of nephrotoxicity. The mechanism of nephrotoxicity involves direct cell membrane actions to increase permeability, as well as indirect effects secondary to activation of intrarenal mechanisms (tubuloglomerular feedback) and/or release of mediators (thromboxane A2). The latter effects are presumably responsible for the observed acute decreases in renal blood flow and filtration rate, responses that are inhibited by several physiological and pharmacological interventions. Changes in intracellular calcium levels may also contribute to the observed effects.In the clinical situation, and in long term models of nephrotoxicity in the rat, salt loading protects against deterioration in renal function; recommendations are made for the optimisation of amphotericin B therapy by salt loading. New preparations of the drug, such as liposomal amphotericin B, may also prove useful in minimising nephrotoxicity while maintaining antifungal activity, but further research is needed with both salt loading and liposomal amphotericin B to confirm or deny their protective effect on kidney function.

Effects of age and cigarette smoking on propranolol disposition

The effects of age and cigarette smoking on the disposition of propranolol have been investigated in 27 normal men, aged 21 to 73 yr. The drug was administered orally (80 mg) every 8 hr and 40 µCi intravenous 3H‐propranolol were administered simultaneously with the seventh dose. Labeled and unlabeled propranolol concentrations were determined in serial blood samples obtained over the next 8 hr. Subgrouping the oral blood concentration/time data according to age (younger or older than 35 yr) showed that the mean levels in the older group were as much as twofold those in the younger group, and that the terminal half‐life (t½β) was prolonged with age (4.19 ± 1.38 vs. 5.05 ± l.36 hr, mean ± SD; p = 0.03). Substratification of cigarette smokers (>10 cigarettes daily) from nonsmokers indicated that the mean levels in the nonsmokers were 200% higher but there was no t½β difference. Despite the considerable interindividual variability, there was a trend for the weight‐normalized, average steady‐state levels to be lower in smokers than in nonsmokers, with the difference becoming smaller with increasing age. Analysis of the data indicated that the intrinsic total clearance of propranolol decreased with age only in the smokers, and toward the constant value of the nonsmokers, while apparent liver blood flow declined equally with age in both groups. Consequently, systemic‐clearance correlated negatively with age in the smokers and in the group as a whole. No age relationships were found in the volume of distribution and, thus, correlations of age and t½β were of the same order as those of systemic clearance. No changes were observed in systemic availability or plasma binding with aging in either group. It, therefore, appears that cigarette smoking habits are important in the altered steady‐state kinetics of propranolol that develops with aging. The results are consistent with a decreased induction of drug‐metabolizing enzyme with aging.

Normal Metabolism of Morphine in a Cirrhosis

Morphine disposition and elimination was studied in 6 healthy male subject s and 6 male patients with cirrhosis, to assess the role of differences, if any, on the reported intolerance of morphine in cirrhosis. In addition the elimination of indocyanine green was studied in the same subjects on a separate occasion. The elimination half-life of indocyanine green was increased and its plasma clearance was markedly reduced in patients with cirrhosis as compared with controls (p less than 0.05). In contrast the disposition and elimination of morphine were unaffected by moderate to severe cirrhosis. Furthermore, while marked sedation was observed in normal subjects, the cirrhotics demonstrated mild sedation with no clinical evidence of hepatic coma. The normal elimination of morphine in cirrhosis is in contrast to the decreased elimination of high clearance drugs metabolized by oxidation, such as lidocaine and meperidine. Morphine is also normally a high clearance drug that is detoxified by conjugation with glucuronic acid. Since intra- or extrahepatic shunting, or both, in cirrhosis do not significantly impair morphine clearance, we postulate that significant extrahepatic morphine conjugation may occur in both normal subjects and in patients with cirrhosis. Furthermore, the reported morphine intolerance to the central effects of morphine cannot be explained by impaired drug elimination and increased availability of morphine to cerebral receptors.